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Normative Mineralogy Especially for Shales, Slates, and Phyllites

Written By

Hans Wolfgang Wagner

Submitted: December 7th, 2021 Reviewed: December 22nd, 2021 Published: March 17th, 2022

DOI: 10.5772/intechopen.102346

IntechOpen
Mineralogy Edited by Miloš René

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Mineralogy [Working Title]

Dr. Miloš René

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Abstract

First, an insight into normative mineralogy and the most important methods for calculating the standard or norm minerals, such as the CIPW norm, is given. This is followed by a more detailed explanation of “slatenorm” and “slatecalculation” for low and very low metamorphic rocks, such as phyllites, slates, and shales. They are particularly suitable for fine-grained rocks where the mineral content is difficult to determine. They enable the determination of a virtual mineral inventory from full chemical analysis, including the values of carbon dioxide (CO2), carbon (C), and sulfur (S). The determined norm or standard minerals include the minerals—feldspars, carbonates, micas, hydro-micas, chlorites, ore minerals, and quartz. The advantages of slatenorm and slatecalculation compared to other methods for calculating normal minerals of sedimentary rocks are discussed.

Keywords

  • slates
  • shales
  • phyllites
  • norm mineral calculation
  • CIPW
  • slatenorm
  • slatecalculation
  • micas
  • hydro-micas
  • illite
  • low- and very low-grade metamorphism

1. Introduction

A norm or standard mineral inventory is a fictitious inventory that, in contrast to the modal and in fact mineral inventory, is calculated from the oxides of a chemical analysis of rock. Each oxide is then divided by its molecular weight. These molecular numbers are then the basis for further calculations (Table 1).

Oxide symbolApproximate molecular weight
(AMU)
SiO260.1
TiO279.9
Al2O3102.0
Fe2O3159.7
FeO71.8
MnO70.9
MgO40.3
CaO56.1
Na2O62.0
K2O94.2
P2O5141.9
CO2
(Non-carbonate C)
44.0
12.0)
SO3 (or S)80.1 (or 32.1)

Table 1.

The full chemical analysis for standard mineral calculations.

The results of the calculations are given as standard or norm minerals. They are given as abbreviations and mostly in small letters such as q (or qu) = quartz or mu = muscovite. The standard minerals each have a clearly defined chemical composition. This distinguishes them from some minerals in nature that have a more complex composition, such as pyroxenes, amphiboles, or chlorites. In these cases, simplifications or end links of a mixture series are used [1, 2, 3].

Usually, standard minerals are frequently occurring rock-forming minerals. Only in exceptional cases do you have to use minerals that occur only rarely or not at all in nature. The norm mineral inventory should come as close as possible to the actual, modal mineral inventory, which is not always possible [1, 2, 3].

The CIPW (Cross, Iddings, Pirsson, Washington) norm [4] is the most common method for magmatites. The Rittman norm for igneous rocks or the Niggli norm for igneous rocks and metamorphic rocks are less common.

There are other methods for standard mineral calculations for sediments, such as SEDNORM [5], SEDMIN [6], PELNORM [7], and a linear program without a name (mainly for sandstones [8]).

The slatenorm and the slatecalculation norm are used for fine-grained sedimentary rocks and very low and low-grade metamorphic rocks, such as shales, slates, and phyllites [9]. As new methods, they are dealt with in great detail here.

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2. CIPW norm

The CIPW system is one of the best-known and best-elaborated chemical classifications of igneous rocks. It was developed as early as 1902. The CIPW system (CIPW norm) is based on a normative mineral inventory. This normative mineral inventory consists of a number of standard minerals (Table 2).

Normative mineral nameAbbreviationMineral formula
QuartzQSiO2
KorundCAl2O3
OrthoclaseorK2O Al2O3 6SiO2
AlbiteabNa2O Al2O3 6SiO2
AnorthiteanCaO Al2O3 2SiO2
LeuciteleK2O Al2O3 4SiO2
NephelineneNa2O Al2O3 2SiO2
KaliophilitekpK2O Al2O3 4SiO2
DiopsidediCaO (Mg, Fe)O 4SiO2
WollastonitewoCaO SiO2
Hypersthenehy(Mg, Fe)O SiO2
Olivineol2(Mg, Fe)O SiO2
AcmiteacNa2O Fe2O3 4SiO2
MagnetitemtFeO Fe2O3
HematitehmFe2O3
IlmeniteilFeO TiO2
Apatiteap3(3CaO P2O5)
PyriteprFeS2
CalciteccCaO CO2

Table 2.

The standard minerals in the CIPW norm [1, 2, 3].

With them, groups of substances of chemical analysis are summarized and thus the quite complex chemistry of the igneous rocks is made clearer. As before, however, the CIPW standard plays a major role for the purpose of comparing the chemical properties.

The calculation of the CIPW standard is carried out according to a specified scheme. It can be found in a number of publications [3, 4]. Computer programs have also been developed since 1965 [10]. Several websites now offer online facilities for calculating CIPW [11, 12].

There are now numerous suggestions for improvement and additions to the CIPW standard. In particular, a standard method that is consistent with a rock classification as recommended by the International Union of Geological Sciences IUGS has proven to be necessary. There will be detailed step-by-step instructions for a standard igneous norm (SIN) presented [13]. The review of the main computer programs for the classification of igneous rocks in the sense of the IUGS leads to the new program “Igneous Rock Classification System” (IgRoCS) [14].

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3. Niggli and Rittmann norm

The Niggli norm (or Niggli’s molecular catanorm) was developed by P. Niggli in 1933 [2, 3, 15]. It was later renamed the equivalent norm. It is in contrast to the CIPW standard, flexible in the choice of minerals to be considered. On the basis of the equivalent numbers standardized to 100%, the chemical rock analysis (Table 1) are the so-called base molecule groups developed. These are the simplest chemical compounds of the most common elements in the earth’s crust. A standard mineral inventory can now be calculated from these basic molecule groups. The standard minerals calculated in this way correspond to the minerals that are present at high temperatures and high-pressure conditions arise. This is why, this standard was given the name Katanorm. Niggli’s work was taken up by Barth (1952) [3, 16] and the norm was converted into the atomic equivalents. Barth developed the other methods Mesonorm and Epinorm for lower metamorphic rocks [17]. A computer program has also been developed for this purpose [18].

Even so, the Niggli norm never really caught on. The greater flexibility of the Katanorm compared to the CIPW norm brings one greater computational effort. In addition, the Katanorm minerals do not come from the modal mineral composition of rock closer than the standard minerals to the CIPW standard [3].

Rittmann (1973) [3, 19] developed a calculation method based on the Katanorm Niggli. It is closer to nature, but also much more complicated. The calculation method is particularly important for volcanites, in which the modal mineral inventory can only be partially recognized. The minerals are often submicroscopic, only partially or not at all crystallized. The Rittmann norm uses information that remain unused in the CIPW norm and the Niggli norm, such as the degree of oxidation, the H2O and CO2 content, or the geological situation, and the structure of the rock. The method, therefore, includes several different standard calculation methods that take this information into account. A computer program was developed at an early stage [20].

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4. SEDNORM, SEDMIN, PELNORM and other

SEDNORM [5] is a method for calculating standard minerals from unconsolidated sediments and coal ash from full chemical analysis. As with the Rittmann standard, different standard variants can be selected depending on the available data (e.g., clay chemistry). According to the authors, SEDNORM is suitable for use on sandstones, shales, and carbonate rocks. The calculation process of SEDNORM is described but is not available online as a program.

SEDMIN [6] is a standard mineral calculation for sediments that focuses on the minerals smectite, chlorite, kaolinite, illite, and ambiguous sericite. Within the full chemical analysis, TiO2 is used to calculate kaolinite. According to the author, SEDMIN is able to predict predominant clay minerals even with atypical sample data. A calculation program for SEDMIN is available online [21].

PELNORM [7] is a calculation method for clay minerals and pelitic rocks from data as in Table 1. The procedure differentiates between the variants with missing smectite (A) and existing smectite (B). K2O can be used to determine both orthoclase (K-feldspar, or) (A1, B1) or illite (ill) (A2, B2) plus orthoclase. According to the authors, the method achieves a good match between normative and modal mineral composition. The calculation steps of PELNORM are described together with program steps in FORTRAN. A computer program is not available online.

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5. Slatenorm and slatecalculation norm in general

In a research project from 1989 to 1991, an attempt was made to use a full chemical analysis similar to the CIPW norm for magmatic rocks to make a rather inaccurate norm mineral evaluation for roof and wall slates ([22] and Table 2 therein). Sericites (muscovite and paragonite), chlorites, quartz, and total carbonates were estimated as the main minerals. Ward and Gómez-Fernandez [23] used the Rietveld method-based Siroquant data processing system for X-ray powder diffraction analysis for the determination of the slate main minerals quartz, feldspar, micas, and chlorites. However, the application of the method was limited to low carbonate Spanish roofing slate. The determined feldspar (albite) values were higher than those of chlorite and likely to be too high. Jung and Wagner [24] created a calculation method similar to the CIPW norm that was ready for practical use. They managed to determine the mineral constituents, and in particular, the content of free quartz with sufficient accuracy—for the first time—to some essential practical statements.

The results of such norm calculations have already been used not only in test certificates but also in a manual [25]. Other authors cited such norm calculations together with results of other analyses and found good matches [26]. The results of more than 20 years of application of “slatenorm” [22, 25, 26, 27, 28, 29, 30] have shown that the inclusion of additional ore minerals, color-giving minerals, and hydro-micas (especially illite) in the new slatecalculation is reasonable.

The extended method slatecalculation presented here is based on a previous, unpublished program called “slatenorm” [24, 31] (Appendix A and B). The calculations are based on the full chemical analysis (Table 1). In the case of slatenorm and slatecalculation norm, a distinction is made between CO2 (= carbonated C) and C (= non-carbonated C, approximate atomic weight = 12.0). In addition, S (approximate atomic weight = 32.1) and not SO3 are being used as a basis. In a first step, the extended algorithm includes the distinction of sulfides. So far, pyrite was the only sulfide calculated in “slatenorm.” In many higher metamorphic slates (e.g., slates from Spain), pyrrhotite is predominant and should be included in the calculation because it is more susceptible to oxidation. Therefore, in the first step, the extended algorithm differentiates various sulfides.

The basic calculations of the algorithm, fundamental norm minerals, and chemical formulas are described in a very simplified form below (quoted from [9], detailed flowchart see Appendix A):

P2O5ap=apatite=3.3CaOP2O5E1
S(approx. Half,sofarmacroscopically or microscopically determined)pnpyrrhotiteFeSE2
ResidualSpt.=pyrite=FeS2E3

Fe2O3 and TiO2 → tm = titanomagnetite = FeO Fe2O3 TiO2, frequently occurring mixture mineral in slates (cf. [22, 32]) At the deficit of FeO in some cases a back calculation (28–31 or 49–51) tm in ru=rutile=TiO2is needed.

TiO2ilm=ilmenite=FeOTiO2E4
Fe2O3he=hematite=Fe2O3E5
Cgr=graphite=CE6
CO2carbonates(cc=calcite=CaOCO2,dol=dolomite=CaOMgO2CO2,sid=siderite=FeOCO2)E7
ResidualCaOan=anorthite=CaOAl2O32SiO2E8
K2Omu=muskovite=K2O3Al2O36SiO22H2OE9
Na2Opa=paragonite=Na2O3Al2O36SiO22H2OE10

The very variable minerals of the chlorite group require more complicated considerations with regard to their composition (see subsection 7).

MgOmc=3MgO 2SiO22H2O=mc/3serpentineE11
FeOfc=3FeO 2SiO22H2O=fc/3greenaliteE12
MgOandAl2O3mac=2MgOAl2O3SiO22H2O=mac/2amesiteE13
FeOandAl2O3fac=2FeOAl2O3SiO22H2O=fac/2daphniteE14

With the negative rest of Al2O3 → (ab, or) = feldspars:

instead ofpaNa2Oab=albite=Na2OAl2O36SiO2E15
instead ofmuK2Oor=orthoclase=K2OAl2O36SiO2E16

In case of high Al2O3 – contents the calculation of chloritoid might be necessary:

instead offacct=chloritoid=FeOAl2O3SiO2H2OResidualSiO2qz=quartz=SiO2ResidualH2OwateraqE17

The remaining H₂O = aq may have a positive or negative value, and this is the basis for an extended algorithm determining hydro-micas. In the input data, carbon compounds (e.g., CO2 and organic C) and elementary sulfur (S) are not included in the glow loss (or loss of ignition (LOI), but rather subtracted. Furthermore, in the case of predominant FeO compounds, the description of the total amount of Fe as Fe2O3 leads to an unrealistic oxidation gain at the expense of the LOI. Only a carefully corrected LOI can be incorporated in the norm calculation as H₂O = aq but will still be less precise. Thus, the calculated data will have a higher range of variation.

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6. The calculation of hydro-micas (ill = illite and br = brammallite) in slatecalculation

A positive aq value is a basis for the hydro-micas calculation (see steps of calculation 40–60 in Appendix A). The original values, as well as the results of calculation steps 9 (Al2O3 and SiO2) and 17 or 19 (in Appendix A) (MgO and FeO), are used as the starting point (Quoted from [9]):

aqshareK2Oill=illite=0.65K2O3Al2O36SiO22.35H2OE18
aqshareNa2Obr=brammallite=0.65Na2O3Al2O36SiO22.35H2OE19

The calculation of hydro-micas requires a recalculation of the remaining phyllosilicates, like micas (mu, pa, steps 41 and 42 in Appendix A) and chlorites (mac, mc, fac, fc, steps 47–59 in Appendix A).

If aq - contents >0, the calculation of limonite is necessary as well:

instead ofhelm=limonit=Fe2O3H2OE20
ResidualSiO2qz=quartz=SiO2E21
ResidualH2OaqE22

If the ill and br values are exceptionally high, in rare cases, MgO and FeO can remain after the calculation and lead to an Al2O3-deficit. In this case, the following varieties may be calculated as hydro-micas:

aqshareK2Omaill=0.65K2O1.05MgO1.95Al2O36SiO22.35×H2OMol.Wtof Mineral:705.21E23
aqshareK2Ofaill=0.65K2O1.05FeO1.95Al2O36SiO22.35×H2OMol.Wtof mineral:738.32E24

or in combination:

aqshareK2Omfaill=0.65K2O0.525MgO0.525FeO1.95Al2O36SiO2×2.35H2OMol.Wtof mineral:721.77E25
aqshareNa2Omabr=0.65Na2O1.05MgO1.95Al2O36SiO22.35×H2OMol.Wtof mineral:684.27E26
aqshareNa2Ofabr=0.65Na2O1.05FeO1.95Al2O36SiO22.35×H2OMol.Wtof mineral:717.39E27

or in combination:

aqshareNa2Omfabr=0.65Na2O0.525MgO0.525FeO1.95Al2O3×6SiO22.35H2OMol.Wtof mineral:700.83E28

If the first calculation steps yield negative values or additional information about other minerals is available, further calculations may be considered (e.g., Alta-Quartzite-schist and others):

CaOepi=epidote=4CaOFe2O32Al2O36SiO2H2OE29
CaOwo=wollastonite=CaOSiO2E30
TiO2tit=titanite=CaOTiO2SiO2E31
Fe2O3mt=magnetite=FeOFe2O3E32
MnOsp.=spessartine=3MnOAl2O33SiO2E33

As a final step the norm minerals are added up to minerals and/or mineral groups:

an+ab+or=feldsparsE34
cc+dol+sid=carbonatesE35
mu+pa=micasE36
ill+br=hydromicasE37
mac+fac+mc+fc=chloritesE38
pn+pt.=sulfidesE39
pn+pt.+tm+ilm+ru+he+mt=oremineralsE40
qz+an+ab+or+sp=rigid mineralsE41
mu+pa+ill+br+mc+mac+fc+fac=elastic mineralsE42
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7. The chlorites in both methods

Due to the great variability of the chlorites, 3 versions (I, II, and “Grundversion” = GV) were used as a basis for the calculation of slatenorm. Later only the “Grundversion” was used as a basis for slatenorm and slatecalculation. Jung & Wagner (1996–2000) [24] determined for chlorite porphyroblasts in roof slate relatively high iron and aluminum contents with Fe/(Fe + Mg) ratios between 0.6 and 0.7 and a replacement of silicon by aluminum [IV] of 35–40 (Weight-%). According to the classification of the chlorites, these are aphrosiderites in this case (Figure 1). The three versions concern the inherently variable proportion of aluminum that replaces tetravalent silicon (Si). The Mg-Al-chlorite (mac for short) and Fe-Al-chlorite (fac for short) are taken as the basis as standard minerals.

Figure 1.

Composition of chlorite porphyroblasts in Spanish roof slates, light—Field of variation of natural chlorites, further explanations in the text [33].

They are given here as simplified formulas without taking trivalent Fe into account:

Mg-Chlorite:

Serpentine 6MgO 4SiO2 4H2O in all versions slatenorm and slatecalculation for mc.

Clinochlore 5MgO 1Al2O3 3SiO2 4H2O in slatenorm version II for mac.

X 4,5MgO 1,5Al2O3 3SiO2 2,5H2O in slatenorm version I for mac.

Amesite 4MgO 2Al2O3 2SiO2 4H2O in slatenorm “Grundversion” (= GV) and slatecalculation for mac.

Fe-Chlorite:

Greenalite 6FeO 4SiO2 4H2O in all versions slatenorm and slatecalculation for fc.

Chamosite 5FeO 1Al2O3 3SiO2 4H2O in slatenorm version II for fac.

Y 4,5FeO 1,5Al2O3 3SiO2 2,5H2O in slatenorm version I for fac.

Daphnite 4FeO 2Al2O3 2SiO2 4H2O in slatenorm “Grundversion”(= GV) and slatecalculation for fac.

As an example, full chemical analyses of five different sample (Tables 3 and 4) were selected, including a shale, a phyllite, two low-carbonate roofing slates from Germany and Spain, and roofing slate with carbonate (Magog). The biggest difference between slatecalculation and the three versions of slatenorm (I, II, and “Grundversion” = GV) is when hydro-micas (illite and brammallite) are calculated. In this case, more SiO2 and Al2O3 are “consumed” than with micas and thus fewer chlorite minerals are calculated.

NigeriaGermanySpainUSAWorld
Shale: Abaka- biliRoof slates: AltlayRoof slates: MagogRoof slates: S. Pedro de T.Phyl-lites: Pol Lugo greenPierre shale C870 [7, 35]Ave-rage sedi- men-tary rock after [5, 34]
Ø 3Ø 8Ø 2Ø 2
SiO248.456.253.152.358.159.759.7
TiO21.71.00.81.21.00.60.0
Al2O331.520.616.122.520.015.414.6
Fe2O33.50.81.61.42.14.64.8
FeO0.06.64.77.05.01.0
MnO0.00.20.10.10.10.20.0
MgO0.62.73.42.93.42.12.6
CaO0.00.75.90.40.41.54.8
Na2O1.00.81.01.31.91.10.9
K2O7.83.93.43.64.32.53.2
P2O50.00.10.10.30.10.20.0
Non-carbonate C0.30.40.50.70.00.00.0
CO20.00.74.40.20.30.94.7
H2O+6.44.53.94.23.58.53.4
S0.00.20.40.10.10.60.0
Ʃ101.299.299.498.1100.298.798.7
Fe2O3/FeO0.120.330.200.424.75

Table 3.

Seven exemplary full chemical analyzes [5, 7, 33, 34, 35].

NigeriaGermany
Shales: AbakabiliRoof slates: AltlayRoof slates: Magog
slatenormslate- calc.slatenormslate- calc.slatenormslate- calc..
IIIGVIIIGVIIIGV
Quartz10.410.310.44.231.031.031.330.527.827.827.827.3
Feldspar0.20.20.215.10.00.00.00.00.60.60.63.4
Mica78.576.578.525.242.942.942.938.840.940.940.931.6
Hydro-mica0.00.00.047.40.00.00.06.20.00.00.07.8
Fe chlorite5.85.76.10.012.310.813.011.28.88.88.86.3
Mg chlorite1.51.41.51.36.86.97.37.07.97.97.97.9
Chlorite7.37.07.61.319.017.820.318.216.616.616.614.2
Carbonates0.00.00.00.01.41.41.41.410.110.110.110.1
Pyrrhotite0.00.00.00.00.00.00.00.00.00.00.00.0
Pyrite0.10.10.10.10.40.40.40.40.70.70.70.7
Sulfides0.10.10.10.10.40.40.40.40.70.70.70.7
Ti mineral1.71.71.76.51.91.91.92.71.41.41.42.9
Ore minerals1.71.71.76.52.32.32.33.02.12.12.13.7
Chloritoid0.0000.00.00.02.90.00.00.00.00.00.0
Aqua2.12.12.12.50.40.30.20.30.20.20.20.4
Ʃ100.5100.2100.9102.797.799.199.199.299.199.199.199.3
Spain
Roof slates:
San Pedro de T.
Phyllites: Pol Lugo green
slatenormslate- calc.slatenormslate- calc.
IIIGVIIIGV
Quartz24.724.625.025.823.723.723.724.8
Feldspar0.00.00.00.09.39.39.39.3
Mica46.746.746.746.746.946.946.946.9
Hydro-mica0.00.00.00.00.00.00.00.0
Fe chlorite13.612.214.311.910.310.310.37.05
Mg chlorite7.67.88.18.47.47.47.47.4
Chlorite21.120.022.520.317.717.717.714.5
Carbonates0.50.50.50.50.70.70.70.7
Pyrrhotite0.00.00.00.10.00.00.00.0
Pyrite0.10.10.10.10.20.20.20.2
Sulfides0.10.10.10.20.20.20.20.2
Ti mineral2.22.22.23.61.81.81.83.7
Ore minerals2.42.42.43.82.02.02.04.1
Chloritoid0.02.90.00.00.00.00.00.0
Aqua−0.3−0.4−0.50.3−0.6−0.6−0.6−0.3
Ʃ96.597.997.998.099.999.999.9100.2

Table 4.

Results of a comparison of all versions of slatenorm and slatecalculation (analysis from Table 3) [33].

In the case of shales (Table 3), there are only meaningful results with slatecalculation, i.e., when considering and calculating hydro-micas. The old slatenorm, on the other hand, does not lead to meaningful, sometimes negative results.

In the German roof slates, too, slatecalculation results in the hydro-micas illite and brammallite, which also lead to lower chlorite values. The appearance of chloritoid in slatenorm version II in the roof slates from Altlay/Germany and San Pedro de T./Spain, which actually do not contain this mineral, shows that this version is incorrect. There are differences in ore minerals including titanium minerals between the results of slatecalculation and all versions of slatenorm. Slatecalculation determines additional minerals, such as pyrrhotite pn and titanomagnetite tm, with the additional consideration of Fe2O3. This leads to higher ore mineral values. At the same time, the Fe chlorite values are lower (fc and fac).

Due to the natural variability of chlorites and the use of standard minerals, such as fc and mc, which do not actually occur as end links in nature (Figure 1), the results of both standard mineral calculations should only be given as sums (chlorite = mac + mc + fac + fc), possibly divided into Fe-chlorite (= fac + fc) or Mg-chlorite (= mac + mc).

The “Grundversion” of slatenorm works better than the other two versions I and II and is therefore preferable. The more complex calculation of slatecalculation leads to better values than slatenorm, especially when hydro-micas are calculated.

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8. Applications (slatenorm & slatecalculation)

Slatenorm (later slatecalculation) was originally developed as a method of determining the mineral content to assess the suitability of the rock as a roofing slate. For this purpose, more than 360 analyses were carried out similar to [20] based on practical experience in normal, oxidizing, occasional oxidizing and carbonate slate, and also in high carbon content-slate with carbonate and hard slate divided. There are also shale and phyllite/schist. For reasons of clarity, not all types in [9] are to be treated.

Additional results will be added in ref. [33]. The nine samples of the group “oxidizing slates” are actually only errors in the selection during the extraction or manufacture of “occasional oxidizing slates” (17 samples). As far as all other properties are concerned, it is a normal slate (see Figure 2). The 94 samples of the group “high carbon content- slates” are also normal slates, but with a no carbonate carbon content of over 1%. EN 12326 [36] excludes roofing slate with a content of more than 2%.

Figure 2.

Diagram showing the ratio of rigid to elastic minerals, calculated with slatecalculation, with changes according to [26].

Cardenes et al. [26] summarized the cleavage and perforability of roofing slate in a diagram of rigid (= standard minerals: qz + an + ab + or = Quartz and Feldspar) and elastic minerals (= standard minerals: mu + pa + ill + br + mac + mc + fac + fc = Mica, Hydro-mica and Chlorite). However, the classes “soft,” “medium hard,” “hard,” and “very hard” listed there are imprecise. The reasons for these properties are not always related to the rigid mineral content and especially the quartz content. The determination of the quartz content remains an important prerequisite for the evaluation of a roofing slate deposit.

The class boundaries in Figure 2 (gray lines) provided in ref. [26] should be corrected. Some slates with carbonate that can be processed normally fall into the wrong class there (there is medium-hard). Obviously, the good cleavability of the carbonates or, to a lesser extent, the feldspars or chloritoids have to be included in the assessment. This leads to new class boundaries in Figure 2 (black lines). There is a narrow field between the classes “normal” and “hard” in which slates of both classes occur. In Figure 2, some slates from the groups’ carbonate slate, high carbon content-slate, phyllite/schist, and shale have been assigned the properties hard or normal.

The phyllosilicates calculated in slatecalculation (after [9]) show a total mica content from usually above 40% (up to a maximum of 60%) and a chlorite content from more than 10% (up to a maximum of 25%) in normal slates (Figure 3). Only for samples with higher carbonate (“carbonate” and “with carbonate”) or higher carbon (“high carbon content”), are the proportions lower. The ratio of mica (mu + pa + ill + br) to chlorite (mac + mc + fac + fc) is 3 to 1. In “normal” slates, the Fe-chlorites content (fac + fc) outweighs the Mg-chlorites content (mac + mc). The calculated proportion of hydro-micas (ill and br) could reflect (in addition to the Kübler index or the organic matter reflectance [37, 38], Figure 4) the degree of metamorphism in most of the samples.

Figure 3.

Phyllosilicates as calculated by slatecalculation: Types of phyllosilicates (after [9]).

Figure 4.

Methods for determining the grade of metamorphosis (according to [9,33,38,39]).

That is why, phyllites always have hydro-mica values of 0% in the calculation outputs.

The slates of the Iberian Variscides also show very low positive percentages of hydro-micas, while slates from the Central European Variscides (Ardennes and Rhenohercynian zone) have a lower metamorphic grade and show higher values of hydro-micas (Figures 3 and 4).

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9. Discussion

Correspondences between the standard methods for sediments (SEDNORM, SEDMIN, PELNORM, subsection 4) on the one hand and slatenorm and slatecalculation, on the other hand, are to be expected. As Tables 5 and 6 shows, however, these are low. There are rather clear differences.

NigeriaGermany
Shales: AbakabiliRoof slates: AltlayRoof slates: Magog
Slate- normslate- calc.SED- MINSED- MIN **Slate- normslate- calc.SED- MINSED- MIN *Slate- normslate- calc.SED- MINSED- MIN *
Quartz10.44.2−11.10.231.331.027.032.727.827.328.428.4
Anorthite0.00.00.00.00.50.5
Albite0.05.18.68.60.00.06.96.90.12.98.18.1
K-spar0.09.7832.05.80.00.00.00.00.00.00.00.0
Feldspar0.20.20.215.10.00.06.96.90.63.48.18.1
Muskovite66.025.232.829.629.125.5
Paragonite12.60.010.19.115.915.9
Sericite78.525.20.049.742.938.830.630.640.931.624.624.6
Illite39.713.213.24.73.83.85.57.67.6
Brammallite7.71.52.3
Hydro-mica47.413.213.26.23.83.87.87.67.6
Mica+Hm.78.572.713.213.242.945.034.334.340.939.432.232.2
Chlorite7.61.30.70.720.318.29.19.116.714.212.212.2
Calcite0.00.00.90.90.40.40.00.010.110.18.88.8
Dolomite0.00.00.00.01.01.01.61.60.00.00.70.7
Ferro-Dolo.0.00.00.00.00.00.00.00.00.00.00.00.0
Gypsum0.00.21.11.12.02.0
Pyrite/Pn.0.10.10.00.00.40.00.00.00.70.70.00.0
Rutile1.70.00.80.60.00.00.81.00.00.00.80.8
Titanomagn.6.51.62.9
Ilmenite0.00.01.90.01.4
Hematite0.02.82.60.00.06.56.50.04.54.5
Limonite0.20.00.1
Apatite0.00.00–00.00.20.20.20.20.30.30.30.3
Smectite0.00.00.00.00.00.0
Kaolinite55.118.312.50.00.00.0
Phyllosilic,86.274.069.081.963.263.255.843.457.653.644.444.4
Aqua2.12.50.20.30.20.2
Ʃ100.4102.3103100.898.797.610093.498.698.898.098.0
Spain
Roof slates: S. Pedro de T.Phyllites: Pol Lugo green
Slate- normslate- calc.SED- MINSED- MIN *Slate- normslate- calc.SED- MINSED- MIN *
Quartz25.025.815.626.423.724.820.124.4
Anorthite0.00.00.00.0
Albite0.00.010.910.99.39.316.316.3
K-spar0.00.05.90.00.00.07.10.0
Feldspar0,00.016.910.99.39.323.416.3
Muskovite30.830.836.636.6
Paragonite15.915.910.210.2
Sericite46.746.719.628.146.946.922.032.2
Illite0.04.54.50.07.47.4
Brammallite0.00.0
Hydro-mica0.04.54.50.07.47.4
Mica+Hm.46.746.724.132.646.946.929.439.6
Chlorite22.520.310.610.617.714.512.612.6
Calcite0.00.00.00.00.00.00.00.0
Dolomite0.20.20.40.40.70.70.20.2
Ferro-Dolo.0.40.30.00.00.00.00.00.0
Gypsum0.40.40.50.5
Pyrite/Pn.0.10.10.00.00.20.20.00.0
Rutile0.00.00.81.20.00.00.81.8
Titanomagn.2.73.7
Ilmenite2.20.91.80.0
Hematite0.07.37.30.05.35.3
Limonite0.00.0
Apatite0.60.60.60.60.30.30.30.3
Smectite10.610.60.00.0
Kaolinite23.60.09.20.0
Phyllosilic.69.267.068.953.764.661.451.252.2
Aqua−0.5−0.3−0.6−0.3
Ʃ97.797.1110.5100.999.9100101.8100.9

Table 5.

Comparison of the results from slatenorm, slatecalculation and SEDMIN. * = TiO2 (kaolinite) = 0, ** = TiO2 (kaolinite) = 1.1. Empty fields = not calculated.

USAWorld
Pierre shaleAverage sedimentary rock after [5, 34]
Slate- normslate- calc.SED- MINPEL- NORM MPEL- NORM NSlate- normslate- calc.SED- MINSED- NORM *
Quartz39.034.841.923.426.238.137.039.335.3
Anorthite1.11.10.60.0
Albite0.00.08.68.60.00.64.87.6
K-spar0.00.00.00.03.70.0
Feldspar1.11.19.22.65.41.28.47.67.8
Muskovite21.10.027.111.8
Paragonite12.60.027.10.0
Sericite34.50.021.137.311.825.3
Illite31.30.030.427.815.03.033.0
Brammallite20.3
Hydro-mica51.630.427.821.23.033.0
Mica+Hm.34.551.621.130.427.837.333.028.333.0
Chlorite14.30.06.411.44.07.0
Calcite2.02.00.06.46.46.78.8
Dolomite0.00.02.43.93.93.51.8
Ferro-Dolo.0.00.00.00.00.00.0
Carbonate2.02.02.43.62.210.410.410.210.6
Gypsum3.02.62.20.0
Pyrite/Pn.1.01.00.00.00.00.0
Rutile0.60.00.60.00.00.0
Titanomagn.2.30.00.0
Ilmenite0.00.00.00.0
Hematite0.04.60.03.65.0
Limonite3.75.3
Apatite0.40.30–30.00.00.00.0
Smectite0.033.028.00.00.0
Kaolinite0.03.02.60.07.2
Clay mineral0.036.030.60.07.2
Phyllosilic,48.851.627.468.659.848.737.035.240.2
Aqua5.35.30.40.61.0
Ʃ96.1102.289.4100.895.898.898.795.899.9

Table 6.

Comparison of standard calculation results taken from the literature (SEDNORM [5] and PELNORM [7]) with own calculations according to SEDMIN [6], slatenorm, and slatecalculation. M = modal mineral inventory, N = calculated [7], * = 6, 7 in Table 6 in [5]. Analyzed from Table 3.

In Table 5, the five examples from Table 3 are calculated using the specified methods. Table 6 contains the standard mineral results of two further examples in Table 3 from the literature given there. Standard minerals remain, which are not calculated and are marked as gaps in Tables 5 and 6. In SEDNORM, SEDMIN, and PELNORM, the clay minerals kaolinite (2SiO2 Al2O3 0.05TiO2 2H2O, Mol. Wt of mineral: 262.15, according to ref. [6]) and smectite (4SiO2 Al2O3 0.1Na2O 0.1CaO 10.9 H2O: 550.46 of mineral: 550.46 of mineral, according to ref. [6]). In contrast to the other methods, a distinction is made in slatenorm and slatecalculation between Na and K mica or Na and K hydro-mica. In the other methods, on the other hand, only K mica (muskovite) or illite is calculated. PELNORM [7] only considers a small proportion of Na2O in the formula for illite (0.025Na2O 0.30K2O Al2O3 0.2MgO 0.125Fe2O3 3.40SiO2 Mol. Wt of mineral: 374.17). SEDMIN uses the very low TiO2 content in the above chemical formula (only 0.05 in 2SiO2 Al2O3 0.05TiO2 2H2O!) to calculate kaolinite. This leads to high kaolinite contents in phyllites and slates, where this mineral is not even present (Table 5).

As mentioned in ref. [9], the Abakabili shale (Table 5) may contain a small amount of clay minerals. In the literature [40], the main component is illite with 30–38%, smectite/montmorillonite with 20–30%, quartz with 28–30% (instead of 11% in Table 5), and kaolinite with only 15–25% (instead of 55% in Table 5).

There are often TiO2 minerals such as rutile (TiO2), ilmenite (FeO TiO2 or titanomagnetite FeO Fe2O3 TiO2) in the group of slates, which in slatenorm and slatecalculation are consequently calculated as standard minerals ru, ilm, and/or tm. Using the TiO2 in the SEDMIN standard calculation of kaolinite leads to the excessively high results in Table 5. For this reason, a corrected value of TiO2 (if used to calculate kaolinite: * = 0 ** = 1.1) was used there as an alternative.

SEDNORM and SEDMIN specify the Fe compounds to a large extent as hematite (Fe2O3). However, this mineral is rarely found in shales and slates. Slatenorm and slatecalculation, therefore, count Fe above all to be monovalent Fe sulfides. Hematite is only calculated if, in rare cases, the color of the rock is red and not black or green (see Figure 1 in [9]). In Table 6, the mean value of the worldwide sedimentary rocks is given from the chemical analysis in Table 3 (after [5, 34]). With SEDNORM, only calculation results are given for which, according to standard mineral calculations, there are no longer any excess MgO, Na2O, K2O, or CO2 (6, 7 in Table 6 in [5]). As far as the quartz, the carbonates and the sum of the phyllosilicates are concerned, the methods slatenorm, slatecalculation, SEDMIN, and SEDNORM show sufficient agreement.

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10. Conclusions

There are several suitable methods for calculating a standard mineral inventory of rock from the full chemical analysis. Among these, the CIPW standard is used, in particular, when the minerals in igneous rocks are particularly fine-grained or not crystallized at all.

The new standard mineral calculations presented here slatenorm and slatecalculation for fine-grained sediments and very low grade and low-grade metamorphic rocks have now been added. The method has already proven itself in assessing roof slates.

In addition to the Kübler index or the reflectance of coal substances with the calculated content of hydro-micas (illite and brammallite), the method also provides information on the degree (grade) of metamorphosis. Further research here seems to be worthwhile in the future.

A detailed comparison of slatenorm and slatecalculation with the standard mineral methods for sedimentary rocks, such as SEDNORM, SEDMIN, and PELNORM, show only a few matches in the results. As far as methodological differences are overcome, the methods for sediments could be appended to it as further calculation steps after the calculation of slatecalculation. The residual water aq should be used as a basis. This requires a new calculation method that has several variants to choose from.

Acknowledgments

The development of the computer program “slatenorm” was funded by Rathscheck Schiefer und Dach-Systeme - ZN der Wilh. Werhahn KG Neuss, D 56727 Mayen-Katzenberg. The publication here was permitted.

Conflict of interest

The author declares no conflict of interest.

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Written By

Hans Wolfgang Wagner

Submitted: December 7th, 2021 Reviewed: December 22nd, 2021 Published: March 17th, 2022