Minerals from European fumaroles.
The fumarolic mineralogy of the Icelandic active volcanoes, the Tyrrhenian volcanic belt (Italy) and the Aegean active arc (Greece) is investigated, and literature data surveyed in order to define the characteristics of the European fumarolic systems. They show broad diversity of mineral associations, with Vesuvius and Vulcano being also among the world localities richest in mineral species. Volcanic systems, which show recession over a longer period, show fumarolic development from the high-temperature alkaline halide/sulphate, calcic sulphate or sulphidic parageneses, synchronous with or immediately following the eruptions, through medium-temperature ammonium minerals, metal chlorides, or fluoride associations to the late low-temperature paragenesis dominated by sulphur, gypsum, alunogen, and other hydrous sulphates. The situation can be different in the systems that are not recessing but show fluctuations in activity, illustrated by the example of Vulcano where the high-temperature association appears intermittently. A full survey of the mineral groups and species is given in respect to their importance and appearance in fumarolic associations.
- fumarolic minerals
- Iceland fumaroles
- Tyrrhenian volcanic belt fumaroles
- Aegean active arc fumaroles
- mineral sublimates
At present, there are three active volcanic provinces in Europe (Figure 1). The first one is situated in NW corner, on the spreading Mid-Atlantic Ridge, nurtured by the Iceland hot spot and causing intermittent activation of numerous volcanoes. The second is the Tyrrhenian volcanic belt with a very active volcanism. It comprises the Roman comagmatic province, which is related to the extension of the Tyrrhenian basin and the rollback of the Apennine chain, and the Aeolian Arc, the genesis of which is generally ascribed to the subduction of the Ionian microplate under Calabria. The third one is the Aegean Active Volcanic Arc, related to the eastern Mediterranean lithosphere subduction under the Aegean-Anatolian microplate. It is characterized by a dormant or recessing volcanism. Is this remarkable variety in causes of volcanism and its activity reflected in the products of volcanic emanations, and in which way? This chapter tries to summarize the mineralogical evidence and give the answer to this question.
There is no process in geology that, on closer inspection, can be called simple. Compared to the processes on the similar scale, the mineralization of a fumarole or a fumarolic field might well be the most complex one. It happens in an open system with high kinetic energy, high mass transport rate, and subject to constantly fluctuating conditions. It involves reactions of gases, fluids, and solids at the boundary between the atmosphere, volcanic gases, meteoric water, hydrothermal solutions, lava, and country rock. Minerals in fumaroles form either as direct sublimates due to changes in temperature or composition of gases, as accumulations of aerosols carried by gas, through gas-solid reactions either by the assimilation of gas components or by leaching of solid, through temperature-induced reactions (mineral reactions, phase transitions, dehydration), through the action of water or vapour (hydration or dissolution) or by crystallization from solution. Inspection of the literature about fumaroles reveals that authors use the following two distinct attributions: sublimates and encrustations. The term sublimate is used for the minerals formed by deposition from gases, and encrustations are formed from fumarolic fluids, but the study reveals many inconsistencies. The reason is that the processes mentioned earlier happen simultaneously at the same place and are entangled. Moreover, most of the minerals can be formed through more than one process. Here, we will consider all minerals in fumaroles that are formed under whatsoever influence of volcanic gases as fumarolic minerals and avoid making classifications that would have too many exemptions and in many cases be misleading. Another question is the definition of a fumarole itself. In their work on Icelandic fumaroles, Jakobsson et al.  distinguished the following two types of fumarolic associations: volcanogenic, which are formed by short-lived, shallow-rooted thermal systems and characterized by no discharge of water and by encrustations that are primarily product of magmatic degassing; and solfataric hydrothermal systems, which are long-lived with a deep-rooted source and surface exposures of high-temperature hydrothermal activity with extensive water-rock interaction. We keep our choice of the European localities as close as possible to the former type (volcanogenic), although it, in the context broader than specific Icelandic conditions, becomes more arbitrary because the fumaroles of a quiescent volcano closely resemble solfataras in their process of formation and mineralogy. The fumaroles on the Solfatara crater, which gave the name to the second category, are not identical in all their characteristics to those encountered on Iceland and illustrate how difficult it is to draw a border line. We are aware that our choice of what to include and what to exclude among the European places with gas emanations can be disputed, but we think that it illustrates well all stages of surface pneumatic processes connected to volcanism from its birth to its old stage and the last breaths of a volcano.
Measured with geological scales, fumaroles are short-lived phenomena. They are a surface or close to the surface feature of a volcano and much more sensitive to weathering than surrounding lava. The formation and dissolution and erosion of fumarolic minerals are contemporary processes, and when the gas emanations ultimately stop, the mineral content in the vents of fumaroles disappears in a short time and just the roots of a fumarolic system can be found in a fossilized form. Besides high chemical reactivity and instability, the fumarolic minerals often appear in microcrystalline aggregates and in the mixtures of many phases. The methods of research therefore require specific approaches and strategies.
2.1. Sample collection and preparation
In this work, the samples from 12 European volcanoes and other fumarolic systems have been investigated (Krafla, Askja, Hekla, Fimmvörðuháls, Eldfell, Surtsey, Vulcano, Etna, Soussaki, Milos, Santorini, and Nisyros). The data for Campi Flegrei and Vesuvius, plus additional data for investigated volcanoes, have been taken from the literature.
The samples have been left to cool down at atmospheric conditions after extraction. The fragile samples were mounted in plastic boxes fixed to avoid the physical damage. The less sensitive ones or those that were extracted in crushed form were sealed in plastic bags. After separating the chosen parts under the microscope, the samples for the X-ray diffraction (XRD) were crushed in agate mortar, and the samples for the scanning electron microscope with energy-dispersive spectrometer (SEM-EDS) analyses were sputtered with a 30-nm-thick carbon film using an Edwards Auto 306 thermal evaporator. The fumarolic samples are mostly composed of intimate mixtures of several minerals with micrometer-sized crystals. Even the most painstaking separation work is many times not in stand to produce a pure one-phase sample for powder X-ray diffraction (PXRD), and hence, the separation of multiple seemingly different portions of a sample was almost always performed in order to get the best overview of the phase composition.
2.2. X-ray diffraction
PXRD is the method of choice for the identification of minerals, alone or in mixtures, and for the quantitative phase analysis. Essential for handling complex mixtures is a diffractometer with high resolution, and therefore, the D8ADVANCE Bruker-AXS powder diffractometer with the primary Ge111 monochromator and the LinxEye silicon strip detector in reflection geometry was used for the measurement of bulk samples (at the Department of Geosciences, University of Copenhagen). The wavelength of X-ray used for measurement was 1.54059 Å. The separated portions of samples for PXRD were usually in very small quantities (≤20 mm3). They were therefore mounted as thin layers on specially cut single-crystal quartz plates that produce no scattering inside the instrument's measuring range that was 5° to 70° or 90° 2θ, with measurement steps of 0.02° and sufficient measuring time for producing good diffraction patterns (usually 4 s per step). The identification of minerals was done with the help of the JCPDS powder diffraction database (ICDD product) and the own set of calculated patterns from the ICSD database (FIZ Karlsruhe product) plus use of the Rietveld method (Topas 4.1 program, Bruker-AXS product). Modern single-crystal diffractometers enable fast analyses of small grains (only several tens of micrometer in diameter). The full crystal structure characterisation they offer is essential in the definition of new mineral species encountered in fumaroles and builds the basis for many of results reported here.
The use of a SEM equipped with EDS (Si(Li), Ge or silicon drift) gives a possibility to obtain very good images by secondary and backscattered electrons together with a chemical microanalysis. The ED detector has the advantage of providing a full spectrum in a very short time that can be very helpful for quick, preliminary identification of minerals. The high counting efficiency of the solid-state ED detectors allows a complete analysis of a mineral in very short time (50 s or less) and with very low probe currents of the beam (about 500 pA). The low probe current involves the generation of a small X-ray escape volume and therefore accurate analysis of minerals even in the case of crystals of extremely small size and in the co-presence of other mineral phases, without the risk of evaporating the sample. Moreover, an ED detector that does not require critical positioning of the sample [2, 3] can give good quantitative chemical data analysing directly the natural faces of the crystals without a need of polished surfaces required for microprobe, but mostly impossible for fumarolic samples, which as a rule consist of loose aggregates of tiny crystals. The last generation of solid-state silicon drift detectors (SSD), normally equipped with a very thin polymeric SuperAtmosphere Thin Window©, gives an output signal with much higher count rates that guarantee also a better sensitivity for light elements even at very low probe currents. Their use in conjunction with the newest software for the correction of the matrix effects  is highly recommended for the samples of fumarolic minerals.
Our investigations were carried out with two SEM instruments: the Stereoscan 360 of Cambridge Instruments and the EVO-50XVP of Zeiss-Cambridge. Microanalyses were carried out with Oxford-Link Ge ISIS energy dispersive spectrometer or Oxford X-max (80 mm2) silicon drift detector, both equipped with a SuperAtmosphere Thin Window©. The operating conditions were as follows: 15 kV accelerating potential, 500 pA probe current, counting time 100 s. The software used was Z AF4/FLS and eXtended Pouchou and Pichoir (XPP) correction scheme, respectively, both Oxford-Link Analytical products.
3. The minerals
More than 200 different minerals have been described from European fumaroles. With the rest of the world, the number might be over 300. Most of them are rare or extremely rare but can be important as indicators of specific conditions in the fumarole where they have been found. Table 1 gives the chemical and crystallographic parameters of the scientifically confirmed European fumarolic minerals. In the following text, we describe the main features of the mineral groups and individual minerals in fumaroles. The exposition of species follows the mineral classification of Strunz  with small modifications. Jakobsson et al.  listed in their work a number of supposed new minerals and labelled them each with a two-letter symbol. For those that still do not have mineral names, we used the same notation. The labels for the localities are as follows: A (Askja), CF (Campi Flegrei), El (Eldfell), Et (Etna), F (Fimmvörðuháls), H (Hekla), Kr (Krafla), M (Milos), N (Nisyros), Sa (Santorini), So (Soussaki), Su (Surtsey), Ve (Vesuvius), and Vu (Vulcano).
|Minerals||Formula||Sp.gr.||Unit cell parameters|
|Sulphur-α*||S8||Fddd||10.465 12.866 24.49||63082|
|Sulphur-β||S8||P21/c||10.926 10.855 10.790 95.9||870|
|rosickýite||S8||P2/c||8.455 13.052 9.267 124.9||66517|
|Selenium-α||Se8||P21/n||9.054 9.083 11.601 90.8||2718|
|pyrrhotite||Fe11S12||Cc||6.897 11.954 17.702 101.3||166063|
|realgar*||As4S4||P21/n||9.325 13.571 6.587 106.4||15238|
|pararealgar||As4S4||P21/c||9.909 9.655 8.502 97.29||80125|
|alacranite1||As8S9||P2/c||9.942 9.601 9.178 101.9||98792|
|alacranite2||As4S4||C2/c||9.943 9.366 8.908 102.0||95290|
|dimorphite-α (HT)||As4S3||Pnma||9.158 8.033 10.200||188058|
|dimorphite-β (LT)||As4S3||Pnma||11.217 9.922 6.607||188059|
|demicheleite||BiS(Br,Cl,I)||Pnam||8.042 9.851 4.033||161637|
|bismuthinite*||Bi2S3||Pnma||11.269 3.972 11.129||153946|
|mozgovaite||PbBi4(S,Se)7||Bbmm?||13.18 37.4 4.05?|||
|galenobismutite*||PbBi2S4||Pnam||11.802 14.569 4.076||158392|
|cannizzarite*||Pb48Bi56(S1-xSex)132||P21/m||38.87 4.090 39.84 102.3||169960|
|cosalite||Pb2Bi2S5||Pnma||23.89 4.062 19.143||169944|
|lillianite||Pb3Bi2S6||Bbmm||13.540 20.64 4.110||246062|
|heyrovskyite||Pb6Bi2S9||Bbmm||13.719 31.39 4.132||180078|
|kirkiite||Pb10Bi3As3S19||P21/m||8.700 26.24 8.774 119.7||156249|
|vurroite||Pb8(Pb,Bi)2(Sn,Bi)(As,Bi,Pb)(Bi,As)10S27Cl3||C2/c||8.371 45.50 27.27 98.8||160401|
|carnallite||KMgCl3(H2O)6||Pnna||16.119 22.47 9.551||64691|
|eriochalcite||CuCl2(H2O)2||Pbmn||7.414 8.089 3.746||40290|
|ammineite||CuCl2(NH3)2||Cmcm||7.688 10.645 5.736||180189|
|ferruccite||NaBF4||Cmcm||6.837 6.262 6.792||36067|
|avogadrite||KBF4||Pnma||8.659 5.480 7.030||9875|
|barberiite||NH4BF4||Pnma||9.077 5.679 7.279||9918|
|pachnolite||NaCaAlF6H2O||Fd||12.117 10.414 15.680 90.4||40132|
|gearksutite||CaAlF4(OH)H2O||P-1||4.94 6.81 6.978 101.1 94.9 110.1||89800|
|jakobssonite*||CaAlF5||C2/c||8.601 6.290 7.219 114.6||188924|
|leonardsenite*||MgAlF5(H2O)2||Imma||7.064 10.131 6.774||411650|
|thermessaite||K2AlF3SO4||Pbcn||10.810 8.336 6.822||161272|
|thermessaite-(NH4)||(NH4)2AlF3SO4||Pbcn||11.301 8.612 6.850|||
|HH||Ca3Al2F10(OH)2(H2O)3?||C2/m?||6.257 22.19 6.311 115.5 (?)||unp|
|malladrite*||Na2SiF6||P1||8.859 8.859 5.038 90 90 120||40917|
|heklaite*||KNaSiF6||Pnma||9.339 5.503 9.796||183232|
|knasibfite||K3Na4(SiF6)3BF4||Imm2||5.522 17.106 9.175||160430|
|erythrosiderite||K2FeCl5H2O||Pnma||13.75 9.92 6.93||30321|
|kremersite||(NH4)2FeCl5H2O||Pnma||13.706 9.924 7.024||200322|
|melanothallite||Cu2OCl2||Fddd||7.469 9.597 9.700||96610|
|atacamite*||Cu2Cl(OH)3||Pnma||6.030 6.865 9.120||61252|
|cotunnite||PbCl2||Pnam||7.619 9.043 4.534||202130|
|challacolloite*||KPb2Cl5||P21/c||8.849 7.918 12.472 90.1||416430|
|hephaistosite||TlPb2Cl5||P21/c||9.003 7.972 12.569 90.05||166293|
|brontesite||(NH4)3PbCl5||Pnma||8.435 15.773 8.445||166092|
|steropesite||Tl3BiCl6||Cc||26.69 15.127 13.014 108.1||163661|
|tenorite||CuO||C2/c||4.684 3.423 5.129 99.5||16025|
|pseudobrookite||Fe2TiO5||Ccmm||9.793 3.730 9.976||51225|
|tridymite||SiO2||Cc||18.494 4.991 23.76 105.8||1109|
|akaganeite||FeOOHClx||I2/m||10.600 3.034 10.513 90.2||69606|
|gibbsite||Al(OH)3||P21/n||8.684 5.078 9.736 94.5||6162|
|doyleite||Al(OH)3||P-1||5.00 5.168 4.983 97.4 118.7 104.7||50581|
|hydromagnesite||Mg5(CO3)4(OH)2(H2O)4||P21/c||10.105 8.954 8.378 114.4||920|
|aragonite||CaCO3||Pmcn||4.961 7.970 5.742||166085|
|cerussite||PbCO3||Pmcn||5.180 8.492 6.134||6178|
|azurite||Cu3(CO3)2(OH)2||P21/c||5.011 5.850 10.353 92.4||158577|
|thermonatrite||Na2CO3H2O||P21ab||6.472 10.724 5.259||1852|
|natron||Na2CO3(H2O)10+x||Cc||12.750 9.001 12.590 115.8||97924|
|trona||Na3H(CO3)2(H2O)2||C2/c||20.41 3.493 10.333 106.5||192710|
|sassolite*||H3BO3||P-1||7.039 7.053 6.578 92.6 101.2 119.8||24711|
|clinometaborite||HBO2 (LT)||P21/a||7.127 8.842 6.773 93.2||183581|
|ameghinite||NaB3O3(OH)4||C2/c||18.428 9.882 6.326 104.4||4219|
|chalcocyanite||CuSO4||Pnma||8.409 6.709 4.833||71017|
|vanthoffite||Na6Mg(SO4)4||P21/c||9.797 9.217 8.199 113.5||16607|
|eldfellite||NaFe(SO4)2||C2/m||8.043 5.139 7.115 92.1||166768|
|yavapaiite||KFe(SO4)2||C2/m||8.150 5.162 7.855 94.9||26004|
|thenardite*||Na2SO4||Fddd||5.860 12.304 9.817||2895|
|metathenardite*||Na2SO4 (HT)||P63/mmc||5.326 7.126||63077|
|mascagnite||(NH4)2SO4||Pnam||7.782 10.636 5.993||34257|
|mercallite||KHSO4||Pbca||8.415 9.796 18.967||249738|
|FB||Na3H(SO4)2||P21/c||8.644 9.641 9.139 108.8||249553|
|therasiaite||(NH4)3KNa2Fe2(SO4)3Cl5||Cc||18.284 12.073 9.535 108.1||5296|
|anhydrite*||CaSO4||Amma||6.991 6.996 6.238||15876|
|gypsum*||CaSO4(H2O)2||C2/c||6.277 15.181 5.672 114.1||161622|
|bassanite||CaSO4(H2O)0.5||C2||17.559 6.962 12.070 133.4||380286|
|omongwaite||Na2Ca5(SO4)6(H2O)3||C2||12.089 6.903 6.354 90.1||88942|
|glauberite||CaNa2(SO4)2||C2/c||10.129 8.306 8.533 112.2||16901|
|barite||BaSO4||Pbnm||7.154 8.879 5.454||76926|
|celestite||SrSO4||Pbnm||6.867 8.354 5.346||92608|
|anglesite||PbSO4||Pbnm||6.955 8.472 5.397||92609|
|dolerophanite||Cu2OSO4||C2/m||9.370 6.319 7.639 122.3||61513|
|antlerite||Cu3SO4(OH)4||Pnma||8.289 6.079 12.057||96348|
|euchlorine*||KNaCu3O(SO4)3||C2/c||18.41 9.43 14.21 113.7||69451|
|chlorothionite||K2CuSO4Cl2||Pnma||7.732 6.078 16.292||22364|
|linarite||PbCuSO4(OH)2||P21/m||9.701 5.650 4.690 102.6||68173|
|baliczunicite||Bi2O(SO4)2||P-1||6.739 11.184 14.175 80.1 88.5 89.5|||
|leguernite||Bi38O42(SO4)15||P2||11.249 5.657 11.914 99.2|||
|kieserite||MgSO4H2O||C2/c||6.891 7.624 7.645 117.7||68345|
|rozenite*||FeSO4(H2O)4||P21/n||5.979 13.648 7.977 90.4||23912|
|chalcanthite||CuSO4(H2O)5||P-1||6.116 10.716 5.961 82.4 107.3 102.6||20657|
|pentahydrite||MgSO4(H2O)5||P-1||6.314 10.565 6.030 81.1 109.8 105.1||2776|
|hexahydrite*||MgSO4(H2O)6||C2/c||10.110 7.212 24.41 98.3||16546|
|nickelhexahydrite||NiSO4(H2O)6||C2/c||9.880 7.228 24.13 98.4||65018|
|römerite*||Fe3(SO4)4(H2O)14||P-1||6.463 15.309 6.341 90.5 101.1 85.7||15207|
|halotrichite*||FeAl2(SO4)4(H2O)22||P21/c||6.195 24.26 21.26 100.3||96598|
|pickeringite||MgAl2(SO4)4(H2O)22||P21/c||6.184 24.27 21.23 100.3||90028|
|alunogen*||Al2(SO4)3(H2O)17||P-1||7.42 26.97 6.062 89.6 97.3 91.5||12129|
|mirabilite||Na2SO4(H2O)10||P21/c||11.474 10.356 12.788 107.8||411348|
|campostriniite||(NH4,K)2Bi2.5Na2.5(SO4)6.H2O||C2/c||17.748 6.982 18.221 114.0|||
|rhomboclase*||FeH(SO4)2(H2O)4||Pnma||9.742 18.333 5.421||183662|
|syngenite||K2Ca(SO4)2H2O||P21/m||9.771 7.145 6.247 104.0||157072|
|kröhnkite||Na2Cu(SO4)2(H2O)2||P21/c||5.518 12.666 5.808 108.4||422593|
|cyanochroite||K2Cu(SO4)2(H2O)6||P21/a||9.066 12.13 6.149 104.4||2925|
|bloedite||Na2Mg(SO4)2(H2O)4||P21/a||11.135 8.248 5.542 100.8||151453|
|polyhalite||K2Ca2Mg(SO4)4(H2O)2||F-1||11.690 16.330 7.600 91.6 90 91.9||6304|
|tamarugite*||NaAl(SO4)2(H2O)6||P21/a||7.353 25.22 6.097 95.2||15187|
|picromerite||MgK2(SO4)2(H2O)6||P21/a||9.072 12.212 6.113 104.8||26772|
|kalinite||KAl(SO4)2(H2O)11||C2/c||19.92 9.27 8.304 98.8|||
|kainite||KMgSO4Cl(H2O)2.75||C2/m||19.72 16.23 9.53 94.9||26003|
|SH||Na2Mg3(SO4)2(OH)2(H2O)4||Cmc21||19.735 7.223 10.028||425875|
|phoenicochroite||Pb2OCrO4||C2/m||14.001 5.675 7.137 115.2||34831|
Metallic sulphides are confined to deeper parts of a volcanic system with its high-temperature hydrothermal conditions and rarely appear as sublimates and then in small amounts on the surface of the fumaroles. Here, their formation depends on the persistence of reducing conditions in parts of these largely fluctuating systems. These were especially characteristic for Vesuvius in the periods immediately following the eruptions and for Vulcano during the thermal “crisis” (temperature increase in the fumaroles). Semimetals form stronger covalent bonds with sulphur and are more volatile, and therefore, they can appear in fumaroles in significant quantities either as simple sulphides or, when combining with metals, as sulphosalts (see below). Observed in the investigated localities are As and Bi, whereas Sb did not appear in quantities sufficient to form its minerals. Arsenic forms discrete molecules with sulphur and its simple sulphides are confined to low-temperature hydrothermal deposits and low-temperature fumaroles where they can appear in important quantities (CF). On Vulcano, however, it is mostly a constituent of sulphosalts together with Bi. The transport of the latter is supposed to occur as Bi-Cl complexes, so the abundance of its sulphides, sulphosalts and other minerals in the fumaroles of Vulcano is interpreted as a combined action of both elements .
Sulphosalts, sulphides containing thioarsenide, thioantimonide, or thiobismuthite group(s), are relatively rare constituents of fumaroles. Among the fumaroles reported here, they appear only on Vulcano where they were abundant during the “thermal crises” when the fumarolic temperatures exceeded approximately 450°C. Recent analysis of the roots of paleofumarole at El Indio, Chile, revealed subsurface formation of sulphosalts crystallized from a melt condensing from hot fumarolic gases . We could expect the same situation in the roots of the fumaroles on Vulcano during the periods of lower gas dynamics. When it increased during the thermal crisis, the hot front of the sulphosalt formation reached the surface and this established the new thermodynamical conditions where their crystals formed as sublimates at fumarole vents through a quenching process, producing generally very small crystals, homogeneous and lacking traces of decomposition . They are silver-grey in colour, with metallic lustre and acicular (the latter with the exceptions of kirkiite and cannizzarite). A simple sulphide bismuthinite is here described together with sulphosalts because of its close structural and genetic relation to these minerals.
Sulphosalts belonging to the PbS-Bi2S3 system predominate on Vulcano due to high concentration of Pb and Bi in the gases, transported as volatile chlorides or chlorosulphide complexes .
Halides are abundant in some fumaroles and some of them appear commonly in fumaroles (e.g. halite and salammoniac). Where the halides are abundant, they usually appear with a number of species many of which are unique for the fumarolic environment or have it as the main mode of occurrence. As compounds with predominately ionic bonding, they mostly are colourless or light colour and are relatively soft. Characteristic for fluorides in fumaroles is the partial substitution of F by OH. In chlorides, the substitution of OH for Cl is rare or non-existing due to the difference in ionic sizes. When present together, Cl, OH, and O play distinct structural roles and form oxy- or hydroxychlorides. Br and I often partly substitute Cl. Among complex fluorides, the groups of borofluorides, aluminofluorides, and silicofluorides can be defined. Finally, halides in combination with other anions, usually of complex type (like [SO4]2−) also exist in fumaroles. Degassing dynamics of Cl, F and other halogens is a complex function of physical and chemical magma conditions  and can vary largely between different volcanoes. Cl generally degasses faster than F  and the abundance of H2O enhances it further , which can produce a temporal segregation of the major amounts of chlorides and fluorides in the fumaroles after volcanic eruptions. This, however, can be modified if other sources than magma contribute to gas production. Significant amounts of fluorides appear in most of the Icelandic volcanoes. They are less abundant, but still in important quantities on Vulcano, whereas they are rarer on Vesuvius and Etna. Aluminofluorides are characteristic for all Icelandic volcanoes (except Askja that does not have registered fluorides). Fumaroles on Hekla contain also significant silicofluorides. They are found on Vulcano as well, but here more characteristic are borofluorides. Contrary to fluorides, chlorides are more important in Italian fumaroles (especially Vesuvius and Vulcano) than in Icelandic ones. On Icelandic volcanoes, they were present in important quantities in the first fumaroles after the eruptions, but a decrease with time is also observed while fumaroles still were producing significant amounts of fluorides (El, H). An important thermodynamic distinction exists between the most abundant chlorides in fumaroles, where those of alkali elements represent the high-temperature product, whereas salammoniac characterizes the low-temperature fumaroles. The border line is approximately at 300°C.
3.5. Oxides and hydroxides
Oxides are mostly products of reaction of gases with scoria, lava or other rocks in which fumaroles form and in lesser amount form as sublimates. The leaching of lava by aggressive fumarolic gases can lead to material consisting exclusively of oxides of Si, Al, Fe, and Ti, opal-cristobalite, corundum, hematite, and anatase, respectively, mixed with anhydrite (in Icelandic fumaroles) or alunite (in Greek fumaroles).
Carbonates are rare constituents in fumaroles. It might look strange considering how abundant CO2 is in volcanic gases, but the scarcity of solid products is due to the typical high acidity of the fumarolic environments making most carbonates unstable. Where they appear in significant quantities, specific conditions are present, for example, low acidic or even basic environments and high water activity, like in the case of occurrences of Na carbonates on Vesuvius or calcite on Surtsey and Etna.
Over 210 boron minerals are known. Among them only three borofluorides mentioned earlier and four borates are observed in fumaroles. The boric acid,
Together with halides sulphates are the group of minerals with the largest number of various species in fumaroles. Calcium sulphates, such as gypsum and in lesser grade anhydrite, are common and in many fumaroles very abundant. Sodium sulphate, thenardite, and K-Al sulphate, alunite, can also appear in large quantities in some types of fumaroles. Specific for sulphates is existence of often several hydrous forms related to the anhydrous ones. They possess different stability fields that depend on the humidity and temperature, and can, therefore, appear in various zones of the same fumarole. The anhydrous sulphates of fumarolic origin are generally instable and readily hydrate under atmospheric conditions (anhydrite and barite group minerals are exemptions).
4. European fumaroles
4.1. Icelandic volcanoes
Iceland is one of the most active terrestrial volcanic regions, with eruption frequencies of ≥20 events per century. It is also one of the most productive with magma output rates of ∼8 km3 per century in historic time . Volcanism in Iceland is caused by the interaction of the Mid-Atlantic Ridge and the Iceland mantle plume. This volcanism can be traced back to the opening of the North Atlantic at 61 Ma as evidenced by massive volcanism in East Greenland, Ireland, Scotland, and the Faroe Islands. The oldest volcanic rocks exposed in Iceland are about 16 Ma . During Late-Pleistocene and Holocene times, some 41 volcanic systems have been active in Iceland and its insular shelf. They are confined to volcanic zones, which are either rift zones or non-rifting flank zones. Volcanic systems in the rift zones produce rocks belonging to the tholeiitic series, while rocks of the mildly alkalic and transitional alkalic series are confined to the flank zones . The majority of volcanic rocks are basalts, but significant amounts of silicic and intermediate rocks have also been produced. Encrustations have been studied from the products of seven eruptions in five volcanic systems in Iceland (Figure 11).
4.1.1. The Vestmannaeyjar volcanic system
The Vestmannaeyjar archipelago represents the southernmost volcanic system in the eastern volcanic zone of Iceland. It is a partly submarine system-producing basaltic to intermediate rocks of the alkalic series. Encrustation samples from two separate eruptions were examined, the 1963–1967 Surtsey eruption and the 1973 Eldfell eruption.
The surface encrustations on Surtsey were mainly deposited in two types of environments, as sublimates deposited directly from a gaseous state on lava and scoria at relatively high temperatures, and in a vapour-dominated system in lava craters and shallow lava caves, where steam emanation was vigorous . Encrustation samples were collected in 13 expeditions from 1965 to 1998 . One can recognize the following main mineral associations: 1. gypsum with thenardite, calcite, and fluorite; 2. halite with anhydrite, glauberite, thenardite, and Na-K-Mg hydrous sulphates; 3. ralstonite with other fluorides (Figure 12).
The Eldfell magma was more evolved than the magma erupted at Surtsey and as a result released a larger amount of volatiles. After the cessation of the Eldfell eruption, the extrusives and the feeder dikes have continued to release a considerable amount of gases, especially at the Eldfell scoria cone. The Eldfell lava is blocky, although large parts of it are covered with an apron of scoria. Early on, volcanic gases formed extensive encrustations on the surface of the lava and the first encrustations were already collected in February 1973 . The lava encrustations on Eldfell were deposited directly as sublimates from a gaseous phase discharged from the cooling lava at a range of temperatures. Due to its thickness, the lava has been cooling down at a considerably slower rate than on Surtsey. The Eldfell crater is made up of coarse scoria mixed with volcanic bombs and lava fragments. When the crater was visited in June 2012, a narrow section of the crater rim was still very hot, reaching maximum temperatures of 290°C at 15 cm depth. Encrustation specimens were collected on the lava in 1973 and 1975, and at Eldfell crater between 1988 and 1995, and in 2009, all from the areas that were not affected by the cooling operations undertaken on the island during the eruption. The observed mineral associations are as follows: (1) (early) salammoniac, cryptohalite, gypsum, jarosite; (2) anhydrite, bassanite, gypsum; (3) anhydrite with ralstonite (later oskarssonite), jakobssonite and other fluorides; (4) anhydrite with Na-K-Al-Mg anhydrous and hydrous sulphates and (5) (late) anhydrite, hematite, corundum, anatase, opal/cristobalite.
4.1.2. The Eyjafjallajökull volcanic system
The Eyjafjallajökull volcanic system is located in the southern part of the eastern volcanic zone. It produces rocks belonging to the transitional alkalic series and consists of a ridge-shaped central volcano elongated E-W, reaching an altitude of 1651 m. It has a large summit crater flanked by mainly E-W-oriented eruptive fissures.
Encrustations were sampled at Fimmvörðuháls during the eruption, and at three separate times in the following months of 2010, and once in 2011. Very high gas temperatures (up to 800°C) were recorded during sampling. Volcanic ash from the Eyjafjallajökull eruption was cemented into a crust on the hot surfaces of craters and lava. This provided favourable conditions for the formation and preservation of volcanogenic encrustations beneath the crust. During the eruption, thin white coatings were observed on the fresh lava. Such coatings also appeared on samples collected while hot. The coatings were found to be thenardite, a readily soluble mineral that is quickly washed away . The mineral associations determined through the analysis of samples are as follows: (1) halite and sulphates (Na-K-Mg-Ca-Al) and (2) ralstonite with other fluorides (Figure 13).
4.1.3. The Hekla volcanic system
The Hekla central volcano is one of the most active volcanoes in Iceland, with more than 18 eruptions recorded in historical time. It is the production centre of the Hekla volcanic system in south Iceland. It produces basaltic to intermediate rocks of the transitional alkalic series and intermediate to silicic rocks belonging to the tholeiitic series.
The observed mineral associations in fumaroles are as follows: (1) (early) salammoniac, cryptohalite, gypsum, sulphur; (2) ralstonite with jakobssonite and other alumino- and silicofluorides and (3) thenardite, anhydrite or gypsum, glauberite.
4.1.4. The Askja volcanic system
The Askja volcanic system is located in the Northern Volcanic Zone in the central highland of Iceland. It consists of a central volcano with nested calderas bisected by a volcanic fissure swarm. The youngest caldera was formed following a plinian eruption in 1875. The system produces mainly tholeiitic basalts, with minor rhyolite, dacite and hybrid intermediate rocks.
Temperatures of 420°C were still measured at a depth of 20 cm at the crater in June 1962 ; however, in August 1962, all soluble salts had been washed away and, apparently, no new encrustations were being formed. Encrustations formed both at the lava surface and in caves but were apparently not widespread . Óskarsson  studied samples collected in June 1962 and found encrustations of sulfur, sal ammoniac and an NH-Fe-Cl-compound at surface, and stalactites of metathenardite in lava caves. The 1961 Askja samples available to us, all of which were presumably collected during the summer of 1962, derive from two shallow lava caves. The observed mineral associations are as follows: (1) thenardite with halite and Na-Mg-K (chloro)sulphates and (2) salammoniac.
4.1.5. The Krafla volcanic system
The Krafla volcanic system is located in the northern part of the northern volcanic zone. It consists of a central volcano with rhyolite formations around the rim of a caldera. A volcanic fissure swarm bisects the central volcano. The rocks belong to the tholeiitic series ranging from olivine tholeiites to rhyolites.
4.2. Italian volcanoes
Italy is one of the most volcanically active countries in Europe. Its recent and active volcanism is mainly connected with the convergence of the Eurasian and the African plate and the extension induced by the formation of the Tyrrhenian basin, which produced volcanism along the Tyrrhenian Sea, in Sicily and Sicily Channel, and on the Tyrrhenian Sea floor. This Plio-Quaternary magmatism exhibits an extremely variable composition and permits to distinguish various magmatic provinces, which differ in major and/or trace elements and/or isotopic compositions. The most important Italian localities showing a fumarolic activity belong to the Neapolitan area (Mt.Vesuvius and Campi Flegrei), to the Aeolian Arc (La Fossa Crater at Vulcano) and to the Sicily Province (Mt. Etna). In addition to these, a number of interesting areas with actual hydrothermal activity are present in Italy, but they have not been considered in this paper because nowadays they do not produce sublimates (Figure 14).
4.2.1. The Campanian Plain
The Neapolitan Province is located in the Campania Region, along the Tyrrhenian coastline, in one of the most densely populated active volcanic areas of the Earth. The geology of the area is prevalently represented by volcanics erupted from the Upper Pleistocene to present by Mt. Somma-Vesuvius on the east, by Ischia and Campi Flegrei volcanic fields on the west and by different ignimbrite eruptions (Campanian Ignimbrites) connected to fissural volcanism along fractures activated in the Campanian Plain. Campanian Plain is a waste graben bordered by Mesozoic carbonate platforms, which began to form in Late Pliocene but largely developed in Quaternary times. Its origin is related to the counter-clockwise rotation of the Italian Peninsula and the contemporaneous opening of the Tyrrhenian Sea, which generated a stretching and thinning of the continental crust accompanied by the consequent subsidence of the carbonate platform along most of the Tyrrhenian coast. The formation of the Campanian Plain was accompanied by the uplift of the central part of the Southern Appennines, in a regional stress regime generating NW-SE and NE-SW-trending faults and establishing the ideal conditions for magma to form and rise to the surface. With the exception of oldest volcanic rocks with calc-alkaline composition, erupted magmas younger than 400 ka are alkaline, with high potassium compositions erupted only at Vesuvius .
4.2.2. The Phlegrean Fields
The Phlegrean Fields (Campi Flegrei: “The Burning Plain") are an area of active volcanism located about 25 km west of Vesuvius and 5 km west-southwest of Naples and consists of a large caldera formed about 35,000 years ago with the eruption of 80 km3 of ash (the Campanian Tuff). The caldera is about 12 km in diameter and includes a series of mostly monogenetic pyroclastic vents. After the caldera formation, with a progressive decrease in volume of the emitted products, the eruptive centres migrated towards the centre of the caldera. There were two historic eruptions in the area. The 1198 phreatic eruption was at Solfatara, a pyroclastic cone formed about 4000 years ago and still in a fumarolic stage. The last one in 1538 produced the new pyroclastic cone of Monte Nuovo. It was explosive and generated pyroclastic flows, but also produced a lava lake and lava flows. After that, the Phlegrean area is characterized by fumarolic activity and periodic episodes of unrest involving seismic activity and slow ground motion (bradyseism) with uplifts. Spectacular fumaroles, with temperatures rising to 160°C, are mainly located at the Solfatara cone, but occur also along active fractures throughout the area both on land and in the Gulf of Pozzuoli. They belong to a widespread geothermal system, which is characterized by a geothermal gradient of up to 170°/km, numerous superimposed aquifers, and a zoning typical of hydrothermal mineral deposition . The predominance of magmatic volatiles over the meteoric water in the volcanic fluids is suggested by isotopic compositions, as well as by the modelling of the fluxes of CO2 and S. The mineralogy of Solfatara is generally characterized by sulphur and a sulphate paragenesis (alunite and alunogen with other sulphates) . At La Bocca Grande vent, where the vent temperature reaches 160°C, arsenic sulfides (realgar and pararealgar) are deposited  (Figure 15).
4.2.3. Mt. Somma-Vesuvius
The present Somma-Vesuvius is a moderate size composite central volcano rising more than 1200 m asl. The activity of Monte Somma has been characterized by a series of sub-Plinian and Plinian eruptions alternating with long quiescent periods lasting from centuries to millennia, followed by periods of semipersistent activity characterized by lava effusions and low-energy explosive eruptions in the past 4 kyr. The Plinian AD 79 eruption  broke a long period of quiescence and represented one of better-known devastating events in the Mt. Vesuvius history. The recent Vesuvius cone has grown within the oldest Mt. Somma caldera possibly after this eruption. After it, the volcano featured two sub-plinian events in AD 472 and 1631 . Throughout the following centuries, Vesuvius activity was characterized by periods of open-conduit activity with the alternating strombolian activity with violent eruptions. The eruption of 1906 is the largest explosive eruption of the twentieth century. The climax of the eruption was reached on the April 7th, when impressive lava fountains, accompanied by earthquakes, rose from the crater. At the end of the eruption, the top of the cone was truncated to form a vast crater. After this explosive event, the Vesuvius area was characterized by quiescent periods alternated by prolonged volcanic activity, effusions of lava and low energy explosions. The last eruption occurred in March 1944.
Mt. Vesuvius is presently affected by relatively low level volcanic-hydrothermal activity, which is mainly characterized by the following: (a) widespread fumarolic emissions that are accompanied by diffuse soil CO2 degassing in the crater area ; (b) CO2-rich groundwaters along the southern flank of Vesuvius and in the adjacent plain and (c) seismic activity with epicentres clustered inside the crater . Fumarolic fluids discharging by the crater rim fumaroles are of relatively low temperatures (<75°C) and are mainly composed of atmospheric components. Fumaroles from the crater bottom have H2O and CO2 as the major components, followed by H2, H2S, N2, CH4, CO, and He (in order of decreasing content), and discharge temperature of about 95°C . CH4 and NH3 contents suggest that the origin of these fluids could be from a high-temperature hydrothermal system located below the Vesuvius’ crater . Lacroix classified older Vesuvian fumaroles in four different types : (1) HT (>300°C) with halite, sylvite, thenardite, Na-K carbonates, aphthitalite, sulphides, Cu-oxide and chlorides (alteration); (2) (“acid”,
4.2.4. Vulcano Island
Vulcano is the southernmost of the seven islands that form the Aeolian archipelago. Its activity dates back about 120 Ka. The four main eruptive centres of the island (Vulcano Primordiale, Lentia, La Fossa cone, and Vulcanello) are formed by a progressive migration of the volcanic activity from SSE to NNW. In the middle of La Fossa caldera sits La Fossa cone, the active volcanic centre of the island, which formed during the last 5.5 ka through recurrent hydromagmatic to volcanic explosive phases . Since its last eruption in 1888–1890, Vulcano has remained in a fumarolic stage of varying intensity with shallow seismicity. Presently, active fumarole fields are concentrated in the northern section of the La Fossa crater and in the neighbourhood of the coast at Baia di Levante. The former show high variability in temperature and fluid compositions as a function of the volcanic activity, the latter are typical hydrothermal emissions.
Over the last two decades, several models were proposed to explain the genesis of the Vulcano fumarole fluids, each of them involving the presence of a deep and a shallow component in the gas phase. Compositionally, the crater fumaroles are rich in CO2 as the main component and have significant concentrations of HCl, SO2, H2S, HF, and CO . The diffuse emissions at Baia di Levante are more typical of hydrothermal fluids, with higher CH4 and H2S contents than the crater fumaroles, lower CO concentrations, and no measurable amount of SO2 . Strong variations of the chemicophysical features of the gas output at the crater have been observed periodically. During these so-called “crisis”, the crater fumaroles demonstrated a substantial increase in temperature from the usual range between 330 and 400°C and in the magmatic component of the total gas flux. The interest in the collection and analysis of the fumarolic minerals especially increased during the last thermal crisis, which started in 1988 and reached a maximum in 1993. A systematic research was started, supported by the Italian National Group of Volcanology. This led to the discovery of a large variety of rare phases and new minerals [8, 12, 13, 26–29, 31–34, 40, 49, 55, 66, 90, 91]. La Fossa crater became a mineralogical attraction and the large number of already described minerals from the locality  increases constantly making it one of the most prolific type locality in the world. The fumarolic mineral association observed on La Fossa Crater are (1) (LT + MT) sulphur, borates, borosilicates, sulphates (mainly hydrous, only in oxidizing conditions), halogenides and sulfohalogenides and (2) (HT >400–450°C) sulphides, sulphosalts, sulfochlorides (reducing conditions); anhydrous sulfates (oxidizing conditions) (Figure 16).
4.2.5. Mt. Etna
Mount Etna is one of the largest European volcanoes, world famous for its spectacular and frequent eruptions. Its main feature is the voluminous lava emission, occasionally associated with explosive activity from its four summit craters. Mt. Etna is a basaltic composite apparatus, with a basal diameter of 40 km and 3350 m of altitude, situated on the eastern border of Sicily. The Etnean volcanism is still not definitely understood in its geological context. Mt Etna is situated on the crossing of important regional fault systems trending NW-SE, NE-SW, and WSW-ESE and this probably facilitates the uprise of magma in this place. There is some evidence that Etna is but the most recent manifestation of volcanism fed from a very long-lived mantle source, having caused numerous earlier phases of mafic volcanism in the Monti Iblei, SE Sicily, from the late Triassic to the early Pleistocene. Mt. Etna has erupted many times in historical time and presently is constantly active with spectacular summit and flank eruptions, interspersed by periods of intermittent activity. The recent volcanic activity has been characterized by almost continuous summit eruptions of effusive and moderate explosive activity. Its most powerful historical eruption occurred in 122 BC . This plinian summit eruption produced a large volume of pyroclastics (ash and lapilli), which fell in a sector on the southeast flank of the volcano, causing devastation in the city of Catania. Presently, the frequent eruptive activity heavily affects the morphology of the summit area that consists of a central crater (Voragine) surrounded by three active cones (Bocca Nuova, NE Crater, and SE Crater) and is cut by N-S-oriented fracture system, mainly related to the extensional stress produced by magma ascent . From the four active craters in historical times, a large number of summit and flank eruptions have occurred  producing numerous composite lava fields and more than 300 scoria and spatter cones. The chemical composition of magmas produced in historical times is rather uniform, ranging from alkali basalt to basic mugearite.
The fumarolic activity at the crater area of Mt. Etna is variable and largely influenced by the activity of the volcano, which is continuous and changes frequently. Consequently, as the morphology of the summit area undergoes significant variations over time, also the localization and amplitude of gas discharging and temperature of fumaroles change. Generally, the fumaroles are aligned along dynamic fractures produced by extensional stress phenomena, but also around hornitos. Intense fumarolic activity is often present due to residual degassing during the cooling of erupted lava. The most of the gas discharging is from the surface of lava flows, but it also happens in the inner parts of volcanic lava tubes and caves in which the cooling of the gases often produces interesting deposition of sublimates and encrustations. After exsolving from magmas, the gases ascend along rock fractures sometimes reaching directly the surface (high-temperature fumarole). In other cases (low-temperature fumarole), they interact with peripheral hydrothermal systems and surficial acquifers, undergo contamination and change their pristine composition . The observed main mineral associations are as follows: (1) (surface, HT) thenardite, halite and sylvine; (2) (surface, LT) sulphur, salammoniac, hydrous sulphates and (3) (in caves) halite, sylvite with thenardite, and hydrous sulphates.
4.3. Greek volcanoes
4.3.1. The Aegean (Hellenic) active volcanic arc
The Aegean volcanic arc is one of the tectonically most active regions of the Mediterranean area, extending from the mainland of Greece through the volcanic centers of Soussaki, Aegina, Methana, Poros, Milos, Santorini, Kos, Yali, and Nisyros, to the Bodrum peninsula in Turkey. In that area, the eastern Mediteranian lithosphere subducts under the Aegean and the Aegean microplate overrides the eastern Mediterranean . The volcanism started 3.5 Ma ago and is still continued up today in the form of post-magmatic activity [132–135]. The Pliocene-Quaternary volcanic arc of the Aegean arose from the subduction of the African plate beneath the microplate of the Aegean-Anatolian  with simultaneous destruction of intermediate-Tethys oceanic crust. The Pliocene-Pleistocene volcanism in the arc of the Aegean Sea is dominated by andesites and dacites, while in the central and eastern parts of the arc, the lower Quaternary volcanism is characterized by dacitic or rhyolitic composition (Figure 17).
4.3.2. Soussaki volcano
The Soussaki area represents the NW end of the active Aegean volcanic arc. The volcanic activity took place between 4.0 and 2.3 Ma ago. The lithological types occurring in the area are rocks of the dacitic and rhyolitic composition, remnants of the late Pliocene to Quaternary volcanic activity [132–135, 137]. Gas manifestations display typical geothermal compositions with CO2 and water vapour as the main components and CH4 and H2S as minor species. The volcanic activity observed today manifests itself by emanation of warm fluids, while widespread fumarolic alteration, vapours, and warm (35–45°C) gas emissions are still observed. Reaction between the fluids and pre-existing rocks, consisting mainly of serpentinite, chert, marlstone, limestone, and subordinate rhyodacitic lava, has resulted in the formation of gypsum, sulphur, silica polymorphs, and Fe and Mg sulphates. Kaolinite, anhydrite, carbonates, and Ni, and K-Al sulfates are also present in some cases (, this work). The observed main mineral associations can be summarized as follows: (1) gypsum with quartz and carbonates and (2) Fe-Mg hydrous sulphates.
4.3.3. Milos Island
The Milos volcanic district is a wide volcanic archipelago comprising the islands of Milos, Kimolos, Antimilos, and Poliegos. The volcanic activity started about 5 million years ago and is now considered to be extinct. Milos Island is located at the western end of the Aegean Volcanic Arc. It is built mainly of calcalkaline volcanic rocks (tuffs, pumice flows, ignimbrites, pyroclastic flows, domes, and lava flows of andesitic-dacitic, and rhyolitic composition). The volcanic sequence is built on Miocene-Pliocene clastic and carbonate platform sediments, which unconformably overly a metamorphic basement. Following their emplacement, volcanic rocks were involved in an intense hydrothermal activity. The volcanism in the area developed with submarine activity character and was followed by a sub-aerial effusive phase. The distribution of the hydrothermal minerals derived basing on cores and cuttings from the two drill sites to a depth of about 1100 m  indicates the dilution of the K-, Na-, Cl-rich hydrothermal fluid of the deep reservoir by a Ca-, Mg-rich cold water at a shallower level. In some places, the hydrothermal activity is expressed by the occurrence of many hot springs (30–85°C), fumaroles (98–102°C), hot grounds (100°C at a depth of 30–40 cm) and submarine gas emissions, widespread on and around the island. Fyriplaka volcano is the expression of the most recent volcanic activity on the island and includes fumaroles, solfataras, and hot grounds. Where gas emanations are very high, the soils are altered and covered with thin layers of secondary minerals as alunite, magnesite, and sulphur.
We examined samples from the area of Amygdales and in the neighbourhood of Paleochori. The substrate rock is argillaceous, composed of SiO2 polymorphs and kaolinite mixed with alunite, which is the dominating fumarolic mineral. The main fumarolic associations are as follows: (1) alunite with sulphur (plus quartz, opal, kaolinite) and (2) alunite and alunogen with Fe-hydrous sulphates and barite (plus sulphur, pyrite, quartz, opal, muscovite).
4.3.4. Santorini volcano
The volcanic field of Santorini consists of various islands (Thera, Therasia, Aspronisi, Palea Kameni, and Nea Kameni), the Christiania islands around 20 km to the southwest and the submerged Columbus volcano 7 km to the northeast. It is the most active part of the Aegean Volcanic Arc. Between Thera, Therasia, and Aspronisi, exists a sea-flooded caldera of about 13 km diameter. According to various geochronological data, the volcanic activity in the Santorini volcano started about 2 Ma ago. The field has had 12 major (1–10 km3 or more of lava), and numerous minor, explosive eruptions over the last ∼200 ka. A hypothesis the eruption mechanism  supposes a large water-filled crater, an extensional environment to facilitate downward penetration of water, and a hot silicic magma.
Nea Kameni fumaroles have a distinct hydrothermal nature with compositions dominated by H2O, CO2 and H2 with minor H2S, and SO2 typically absent . We investigated samples from the fumaroles at the Nea Kameni central crater. The altered rock consists of cristobalite, opal, plagioclase (around An50), gypsum, alunite or natroalunite and sporadic quartz, trydimite, hematite, and kaolinite. The sublimate is mostly sulphur that makes yellowish crusts, aggregates of isometric crystals or spear-like dendritic crystals. Hydrated sulphates are alunogen (mixed with the substrate or as thin platy crystals), rhomboclase, coquimbite, and tamarugite. Jarosite is also found in small quantities (Figure 18).
4.3.5. Nisyros volcano
Nisyros is the newest of the major active volcanoes of Greece, composed almost exclusively of volcanic rocks, with the oldest of them being little less than 160,000 years and the youngest reaching the limits of prehistory, about 20,000 years ago. In the wider area of Nisyros at the eastern margin of the volcanic arc, the first eruptions date 3.4 million years. Since then, small or large eruptions built up Nisyros and the islets Pirgousa, Pachia, Strogyli, and Giali. None of the eruptions of the volcano recorded in historical sources produced molten rock, as they are hydrothermal and originate from the overheated steam in the underground of the island. Seawater and rainwater penetrate the rocks of the island, are heated by magma and converted to superheated steam, which exerts tremendous pressure and causes a hydrothermal explosion when overcomes the weight and consistency of the caprock. Such explosions were recorded in Nisyros in historical times. In the southern part of the caldera floor, there are traces of 20 craters, 10 of them being well preserved. The largest and most striking crater is Stephanos. The latest hydrothermal craters are concentrated in the area of Lofos (=hill), a small post-dome that largely has been destroyed by hydrothermal explosions. Here are situated six well-preserved craters, the creation of three of them being recorded in historical sources. In October or November 1871, a powerful earthquake caused the beginning of a series of hydrothermal explosions, which until 1873 created two small craters: Polyvotis and Alexandos (or Flegethron). The last recorded hydrothermal explosion in Nisyros is the one of 1887, which created the crater of Small Polyvotis. Various amounts of volcanic gases come out from the intra-caldera soil area including also the main fumarolic craters of Kaminakia, Ramos, Stefanos, Lofos, and Flegethron. The gas species are H2S, CO2, CH4, and CO [141–143](Figure 19).
The only sublimate mineral observed at the vents at the edges of the Stefanos and Polyvotis craters is sulphur. It develops as granular aggregates or acicular spear-like crystals. On the floor of the Stefanos crater, numerous circular formations surround the “hot spots” of the fluid, vapour, and gas emanations. The central part of these formations consists of the reddish to yellowish sand formed by grains of quartz, labradoritic plagioclase, alunite, alunogen, and gypsum. They are surrounded by white rims containing dendritic crystals of alunogen and metaalunogen. On their outer part is a blackish zone with additional voltaite and tamarugite. The outermost rim consists of aggregates of microscopic yellowish globules of rhomboclase overgrown by silky needles of halotrichite.
The list of the minerals found in European fumaroles impresses by the number of different species, especially taking in account that some prolific mineral groups (like silicates and phosphates) do not contribute to the list or contribute very little. Among the European fumarole localities, Vesuvius and Vulcano stand as the two volcanoes with the richest mineralogy. This status is due to the exceptional abundance of otherwise rare elements in their emanations: Cr, Mn, Ni, Cu, B, Tl, Pb, As, Se on Vesuvius, Ba, Au, Zn, B, Tl, Sn, Pb, As, Bi, Se, Te, Br, I on Vulcano. The presence of the rare elements that form rare minerals is, however, not the only reason for the diversity of fumarole mineralogy. Although they do not contain minerals of rare elements, the three Icelandic volcanoes Surtsey, Eldfell, and Hekla each have around 30 different minerals in their fumaroles. Moreover, their dominating mineral associations are different from the main associations of both Vesuvius and Vulcano, and are differing among themselves, although all five fumarolic systems are formed by the same combination of main gaseous species (H2O, CO2, HCl, HF, SO2, and H2S). This illustrates the importance of various factors that can influence the processes and through that the mineral world of fumaroles mentioned in the introduction. The intensive parameters, composition and temperature of volcanic gases and lava, can decisively be supplemented by other factors in forming or modifying the minerals in fumaroles. We will try to examine them in the following comparison of different fumarolic systems.
In general, the fumaroles can be classified according to their temperature of formation and mineral stability. Already in the nineteenth century, Lacroix made a classification of Vesuvius fumaroles  after their mineralogy, reflecting the temperature of formation. He distinguished the high-temperature “dry” fumaroles characterized by the K and Na salts and also some sulphides and oxides, the medium-temperature “acid” fumaroles characterized by the chlorides of Fe, Mg, Al, and Mn, fumaroles of even lower temperature characterized by salammoniac and other ammonium minerals, and the low-temperature fumarole producing sulphuric acid and steam with sulphur and gypsum as typical minerals. We shall try to classify the fumaroles investigated in this work in three categories: (1) HT, the high temperature (>400°C), (2) MT, medium temperature (200–400°C) and (3) LT, low temperature (<200°C). The given temperature ranges are approximate. HT fumaroles appear with the eruption of the volcano and are short living after its finish. Alternatively, they can be products of temperature increase in the volcanic system not leading to eruption, but again short-lived (Vulcano). MT and LT fumaroles might be active at the same time as the HT fumaroles, but at different places in the system where fumarolic gases travelled longer and got cooled and/or diluted by the atmosphere before they come to the surface. MT fumaroles prevail in the period of recession after the paroxysm and last roughly for decades, maybe centuries, while the shallow intruded magma cools down, or the degassing surface of the magma retreats to greater depths. LT fumaroles are the only ones present in the quiescence period with a deep thermal source and may transform eventually to a solfatara (mofeta), if the volcanic cycle is finished or very much prolonged.
Inspecting the mineralogy of European fumaroles, we can see that specific three (or four) minerals are typical in most cases for the HT fumaroles. These are halite, (meta) thenardite (with or without aphthitalite), and anhydrite. The mentioned four minerals are all stable to over 800°C. The fumarolic systems can be classified according to relative abundances of these minerals reflecting the Na(+K)/Ca and Cl/S proportions in the fumarolic paragenesis. In this classification, the alcalic fumaroles are those of Vesuvius (dominantly chloridic), Fimmvörðuháls and Askja (chloridic-sulphatic) and Krafla (sulphatic). The mixed alcalic-calcic are Surtsey and Etna (chloridic-sulphatic) and Hekla (sulphatic), the calcic is Eldfell (sulphatic). Outside the classification stands Vulcano, with a high-temperature association composed of sulphides/sulphosalts in combination with sulphochlorides. It can be locally expanded with an association of anhydrous sulphates where and when the atmospheric influence is large and the general reducing conditions change to oxidative.
The MT fumaroles reflect more the individual nature of each fumarolic system. First of all, the range of temperature, the relatively slow flux rates and the chemistry of fumarolic systems permit the deposition and the growth of a larger number of mineral phases, including products of reaction between gas, steam, or liquid and minerals previously formed. In addition, the often longer time of their existence and the longer and slower ascent of the fluid phase allows for a larger contribution of other influences on magmatic exhalations, that is, interaction with the wall rock and surficial fluids. Here, we can recognize the following associations: A, salammoniac in association with other ammonium minerals; B, metal chlorides (Fe-Al-Mg plus others); C, fluoride association with ralstonite as the dominating mineral, typically mixed with other aluminofluorides. Type A was registered on all recently active volcanoes except Surtsey and Fimmvörðuháls. On Eldfell and Hekla, it was forming only early following the eruption. On Vesuvius and especially Vulcano, it is a persistent feature, where the latter also demonstrates a rich variety of ammonium minerals not found on other places. The situation on Icelandic volcanoes suggests that nitrogen in the volcanic gases is one of the first species to be exhausted upon an eruptive episode. If it is true, it would mean that the shallow-degassing magmas of Vesuvius and Vulcano are constantly replenished in nitrogen or that it is there supplied from some other sources. Type B is found on Vesuvius and Vulcano. On the former, the chlorides are mixed with fluorides, on the latter with fluorides (boro- and silicofluorides), bromides and even an iodate. The type C is characteristic for a number of Icelandic volcanoes (Surtsey, Eldfell, Fimmvörðuháls, Hekla, Krafla). In Hekla fumaroles, silicofluorides appear together with aluminofluorides.
The dominating minerals in the LT fumaroles are sassolite, gypsum, alunogen, and sulphur (listed with decreasing thermal stability). The differences in thermal stability are practically of no importance in this group because all minerals form at temperatures that are around 100°C or even lower in this type of fumaroles. Here, also alunite can be named as an abundant component. It is a product of feldspar decomposition through action of sulphuric gases and solutions in solfataras and low-temperature fumaroles, together with opal, and in this respect does not represent a simple sublimate. It is stable to around 500°C and could be a component of high-temperature fumarole as well, but the process of formation makes it a typical representative of the low-temperature one. The LT fumaroles can be categorized by the contents of the above-mentioned minerals. The sulphur dominated fumaroles are found on Nisyros, Etna, Campi Flegrei, Vesuvius, Vulcano, and Krafla. Gypsum dominated are those of Soussaki, Surtsey, Eldfell, and Hekla. Vulcano and Vesuvius have low-temperature fumaroles that contain also abundant sassolite together with sulphur and gypsum. The second type of fumaroles on Nisyros and fumaroles on Milos are dominated by alunogen, and finally, the fumaroles on Santorini have comparable amounts of sulphur, gypsum and alunogen. In all of them, various kinds of hydrous sulphates appear, which can be used as fine-tuning indicators of temperature and humidity conditions.
The classifications given above can serve a general orientation purpose and understanding of the actual conditions in the fumarolic system under investigation. They are in no way clearly delimited categories and mixing of associations is a frequent phenomenon. It can be produced as a consequence of the gradual lowering of temperature and the flux rate, but often also a simultaneous formation of closely separated minerals at quite different temperatures occurs due to the high fluctuation of conditions on a small scale at the border between the volcanic gases and the atmosphere. On Vulcano, the high-temperature anhydrous sulphates that form under the locally oxidizing conditions are found in close association with sulphides and sulphosalts formed under the reducing conditions. Our detailed investigation of fumarolic profiles on Eldfell shows that within less than 20 cm the low-temperature mineral association of fluorides changed to the high-temperature association of anhydrous sulphates. The mixed associations can also be an artefact produced by the mineral instability at atmospheric conditions, and laboratory analyses taken alone can give misleading results. Our analyses of Eldfell material showed always hydrous Mg-Al sulphates accompanying the anhydrous ones taken from the levels featuring temperatures over their stability ranges. Their formation as atmospheric hydration products was confirmed by the field observations of the deliquescent behaviour of exposed excavated samples and through laboratory analyses of freshly crushed massive ones. The caveat about the above classification is that it presents a general picture from which numerous important details have been removed in order to give as clear as possible an overview. For a full characterization of individual fumarolic systems, the details are indispensable, and this emphasizes the importance of detailed studies.
The realm of fumarolic mineralogy presents us with interesting and important still unanswered questions. For example, the seeming contradiction of the experimental results that show sulphuric species to be the ones with the fastest and earliest devolatilization from silicic melts, and the observation that sulphates and elemental sulphur are the most persistent parts of fumaroles, leads to important enquiries into the sub-surface volcanogenic sulphuric cycle (see e.g. ). Another interesting largely unsolved problem is the understanding of reasons why sometimes large differences in mineral compositions of seemingly similar fumarolic systems exist. During the last century, the appearance of powerful methods of research on microscale has revealed a great number of new mineral species from fumarolic samples, typically rare and exotic, and contributed to the solid state science and the general mineralogical knowledge. We believe that the application of these methods to investigation of both minor and main components of fumaroles and integrated with research in other fields (like gas evolution and composition and gas-magma-rock interactions) can also bring answers to the most intriguing questions of the genesis and origins of particular fumaroles. Accomplishing this, they can become another important tool in the surveillance of active volcanic systems.
The authors are grateful to researchers and students with whom they collaborated in the investigations of European fumaroles: Filippo Vurro, Daniela Pinto, Donatella Mitolo, Eric Leonardsen, Sigurður S. Jónsson, Níels Óskarsson, Sigmundur Einarsson, Anna Katerinopoulou and Morten J. Jacobsen. We thank Helene Almind for the technical assistance during XRD measurements and Dora Balić Žunić for technical support during the field work and manuscript preparation. Parts of the research work were financially supported by the Danish Agency for Science Technology and Innovation and the National Group for Volcanology, Italy.