Location, colors and mineral composition in the fine fractions in sandstones from Hatira crater, Ramon crater, and Manara cliff in Israel
Fine fraction in quartz arenite sandstones from Lower Cretaceous Hatira formation in Israel was observed by Transmission electron microscope (TEM). Samples were collected from Hatira and Ramon craters located in southern part of Israel and from Manara cliff from the northern part of Israel. The additional phases cause yellow, red, dark red and dark violet colors of the layered sandstones. The motivation was to identify the minerals of the fine factions that cause the variations in the colors. The minerals observed were clay minerals, mainly kaolinite (Al4Si4O20(OH)8), some illite (K0.65Al2.0[Al0.65Si3.35O10](OH)2) and smectite. Iron oxides were goethite (FeOOH) and hematite (Fe2O3), Titanium-iron oxides observed was ilmenite (FeTiO3), and Titanium-oxides were rutile (TiO2), and anatase (TiO2). Sulphates observed were jarosite (KFe3(SO4)2(OH)6) and alunite (KAl3(SO4)2(OH)6). Some of the hematite was formed by recrystallization of goethite. Ilmenite disintegrated into small iron oxides mainly hematite. Euhedral to sub-hedral rutile (TiO2) and anatase (TiO2) were preserved in clay-minerals. Crystals of alunite and jarosite were observed in sandstones in both craters. They probably crystallized due to some transgression of the Thetis Sea.
- clay minerals
During the Lower Cretaceous, siliciclastic sediments were deposited on terrestial terrestrial partly lacustrine environments. [1, 2, 3]. The sandstones are quartz (SiO2) arenite with rounded quartz grains, clay minerals and siltstones. Lower Chemical weathering of the Pan-African continental basement favored silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates which prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny . Cretaceous sandstone originates from Paleozoic sandstones, the first-cycle quartz-rich sandstones . The stratigraphic cycles showed progradational – retrogradational trends due to small global sea-level rises which inundated the continent for short periods and fossils were found . In Lebanon similar sandstones were deposited during the Lower Cretaceous . The composition of the sandstones was 85–95% quartz indicating well-sorted sandstone. Sedimentological observations suggest that the Chouf Formation deposited in fluvial, coastal plain and deltaic environments. The origin of the sandstone is from recycling of Paleozoic sandstones. Sandstones of the Lower Cretaceous were deposited and covered by sediments that were deposited after the Thetis Ocean covered the area during the Upper Cretaceous, hence, hard carbonate stones, limestones, dolomites and marls precipitated, overlying the sandstones. At the end of the Mesozoic era, closure of the Thetis Sea yielded formation of the Syrian arc with monoclines of the Hatira and Ramon areas. After regression of the Thetis Sea during the Oligocene, an erosion surface truncated the hard carbonate rocks of the fold’s crest and exposed the underlying friable Lower Cretaceous sandstone. For crater (Makhtesh) Hatira, such an erosion base level formed locally and for a limited period in the Early Miocene. The opening of the Ramon crater started in the Early Pliocene, when the Syrian Arc Fold Belt was uplifted and arched and the Dead Sea Rift was established as a deep intercontinental erosion base in the east . In the northern part of Israel, the Lower Cretaceous sandstones were exposed close to the Dead Sea transform fault and uplift of the Manara cliff and formation of Hula valley.
Composition and morphology of the nano-crystals that coat quartz grain reflect the environmental conditions in which they were formed as primary or secondary forms .
In this paper, we present Transmission Electron Microscopic (TEM) observation of the nano-sized minerals from Lower Cretaceous sandstones exposed in the Hatira and Ramon craters in southern Israel, and in the Manara cliff in the northern part of Israel (Figure 1).
The main motivation of this research is to identify nano-crystals in sandstone that cause the variability in their colors. By using TEM detrital or authigenic phases can be identified.
2. Sampling and methods
The samples that were collected were separated according to their colors: dark violet, violet, dark red, red and yellow (Table 1). Sand stone were put for few minutes into an ultrasonic bath, then the fine fraction was separated from the quartz grains and freeze-dried. Each sample was suspended in distilled water and placed on Cu supported holey carbon film and dried. Nano-sized (5–200 nm) particles were checked with Transmission Electron Microscopy (TEM) using a JEOL JEM-2100F analytical TEM operated at 200 kV, equipped with a JED-2300 T Energy Dispersive Spectrometer (EDS) for microprobe elemental analyses. All chemical analyses were obtained by point analyses with a beam width of 1 nm and are presented as atomic ratios. JEOL Analytical Station software, based on the Cliff-Lorimer ratio technique, with an accuracy of ~5%, was used for the calculations. The CuKa line was used to calibrate the spectrometer. Energy-filtered TEM (EFTEM) experiments were performed using a Gatan image filter. The titanium L-edge (456 eV), silicon L-edge (99 eV) and iron L-edge (708 eV) were used for elemental mapping using the three-window method.
|No.||Sample||Color||Location||Minerals identified with HRTEM|
|1||GNH4 YR||Yellow-red||Hatira crater, east||Kaolinite, illite, hematite, goethite|
|2||GNH4 DR||Dark red||Hatira crater, east||Quartz, clays, jarosite, anatase, goethite, calcite, ilmenite|
|3||GNH6 R||Red||Hatira crater, east||Kaolinite, ilmenite, hematite, quartz|
|4||GNR1 V||Violet||Ramon crater northern cliff highway 40||Anatase, kaolinite illite, smectite, goethite, magnetite, maghemite|
|5||GNR1 Y||Yellow||Ramon crater northern cliff highway 40||Hematite, illite, rutile, quartz|
|6||GNR2 DV||Dark violet||Ramon crater northern cliff highway 40||Hematite, goethite, kaolinite alunite, apatite|
|7||GNR2 R||Red||Ramon crater northern cliff highway 40||Goethite, hematite, anatase, illite, kaolinite|
|8||GNR2 VR||Violet, red||Ramon crater northern cliff highway 40||Hematite, goethite, kaolinite anatase|
|9||GNR3 R||Red||Ramon crater northern cliff highway 40||Goethite, hematite, kaolinite, illite|
|10||GNR5 R||Red||Ramon crater northern cliff highway 40||Goethite hematite, anatase, kaolinite, illite, smectite|
|11||GNKS1||Red||Manara cliff near Kiriat Shmona||Kaolinite, ilmenite, apatite, goethite, quartz|
Crystalline phases were identified, using Selected Area Electron Diffraction (SAED) in the TEM. With this method, a very high-energy electron beam (200 kV) transmits through the sample and the d-values obtained enabled their identification. The accuracy of the d-values determination was better than 0.005 nm. The smallest area of SAED was 100 nm. If a good lattice image was obtained in very small particles by High Resolution Transmission Electron microscopy, the use of Fast Fourier Transformation (FFT) enabled the identification of the minerals, using the program Digital Micrograph (Gatan).
3.1 Hatira crater sandstones
Samples from the Hatira crater were collected at the eastern side of the crater. The colors of the sandstones vary, with thin layers of red sandstone or dark red close to clay layers. Usually the yellow layers are thicker.
In the sample: yellow-red (GNH4 YR), from the Hatira crater (Figure 2), a cluster of goethite (FeOOH) with clay minerals, mainly kaolinite (Al4Si4O20(OH)8), was observed, and a cluster of hematite (Fe2O3) shows the previous morphology of goethite, indicating that hematite was formed by recrystallization of goethite preserving the acicular morphology of goethite. Crystal size of hematite is around 15 nm. Small crystals (25-50 nm) with Ti impurity might reflect ilmenite (FeTiO3) disintegration. The dominance of goethite crystals contributes to the yellow color and hematite contributes to the red color. The recrystallization of goethite into hematite might have happened due to thermal transformation, as the sandstones were covered by younger layers.
Sample GNH4 DR (Figure 3) had a dark red color and the minerals identified were clay minerals, small crystals of calcite (CaCO3), jarosite (KFe3(SO4)2(OH)6), goethite, ilmenite, and anatase (TiO2). The image obtained by TEM shows clay minerals with clusters of anatase and goethite. Jarosite appears with euhedral or sub-hedral morphology, indicating that it was formed in the sandstone. The crystal size of jarosite is 100-200 nm. Ilmenite crystal had Fe/Ti 0.85 and the crystal’s size is ≈200 nm.
Sample GNH6 R (Figure 4) was collected from a thin red layer and vein within a clay layer in the sandstone. Ilmenite was found as a cluster of small particles surrounded by clay minerals. The ilmenite point analysis had Fe/Ti 0.38, indicating that a larger crystal disintegrated and was preserved within the clay layer. EFTEM RGB color shows disintegration of the ilmenite mineral. Usually ilmenite is resistant to weathering processes and it probably arrived at the area along with the quartz grains. At the lower part of the image hematite formed a layer, causing the red color of the sandstone. Recrystalliztion of ilmenite into hematite was found also in sands along the Mediterranean coast. The precursor and the recrystallized tiny hematite crystals remain close to each other since they are all kept within clay minerals .
3.2 Ramon crater sandstones
Sandstones are exposed at the northern cliff of the Ramon crater close to highway 40. Volcanic eruption occurred in the area during the Lower Cretaceous, forming basanite flows and paleosol between the volcanic flows . The samples presented were collected above the basanitic flows. Lower cretaceous sandstones in the Ramon crater have various colors: dark violet, violet, dark red, red and yellow (Figure 1c).
The violet sample GNR1V (Figure 5) had kaolinite clusters, euhedral crystals of anatase (50-100 nm), magnetite (Fe3O4) and maghemite (Fe3O4) that was probably formed by magnetite oxidation. Kaolinite crystals had euhedral morphology, indicating that they crystallized in the area. Similar euhedral morphology of some of the anatase and rutile (TiO2) crystals indicates that they might also have been formed in the sandstone by recrystallization of other phases. Another option is that the euhedral morphology of the Ti-oxides results from their resistance to weathering. In sample GNR1 V, anatase crystals reached a size of 90 nm and Fe/Ti atomic ratios were 0.04–0.26.
Sample GNR1Y (Figure 6) presents additional phases of a yellow sandstone. Quartz grain was observed surrounded by a cluster of illite (K0.65Al2.0[Al0.65Si3.35O10](OH)2), hematite and a euhedral rutile crystal (250 nm) with Fe/Ti 0.03 atomic ratio. Hematite tiny crystals <100 nm were also observed close to the quartz grain and on the clays.
In sample GNR2 DV dark violet sandstone from the Ramon crater (Figure 7), the main clay mineral is kaolinite forming clusters with hematite and alunite (KAl3(SO4)2(OH)6). The largest crystal size of hematite is around 300 nm, contributing to the violet color of the sandstone. Point analysis in % atom of alunite yielded: Al 6.00–6.37, S 2.04–2.34, K 0.29–0.32 and Fe 0.01. Alunite was probably precipitated from acid rain or evaporation of sea water that might have entered the area during the Lower Cretaceous. Tiny crystals of goethite formed clusters with clays.
Sample GNR2 R from red sandstone in the Ramon crater (Figure 8) had clay minerals, illite and kaolinite with anatase, goethite and hematite. Anatase (~50 nm) had euhedral morphology, indicating that it might have crystallized in the sandstone. The goethite crystals had acicular morphology (~150 nm); some of the goethite partially recrystallized into hematite preserving the initial acicular goethite morphology. Tiny crystals of hematite were also observed on goethite crystals. A high resolution image was taken from a cluster of hematite preserving outer goethite morphology.
Sample GNR2 VR (Figure 9) is from violet-red sandstone from the Ramon crater. The minerals identified were kaolinite, goethite, hematite and anatase. High resolution of the sample yielded morphology of euhedral acicular crystals that were the initial stage of goethite, and the inner morphology was of small crystals of hematite. Goethite has been transformed into hematite preserving the acicular morphology. The cluster of hematite includes the clay mineral, kaolinite. Euhedral anatase crystal (~60 nm) was observed close to clay minerals with impurity of Fe/Ti 0.027 atomic ratio.
Sample GNR3 R (Figure 10) red sandstone from the Ramon crater had clusters of goethite with kaolinite, illite and smectite. Small crystals (<50 nm) that were observed on the clays had impurity of Ti with the atomic ratio of Ti/Fe 0.027. These crystals might result from disintegration of ilmenite.
Sample GNR5 R (Figure 11) red sandstone from the Ramon crater had clay minerals, goethite crystals and anatase (TiO2). The size of the goethite crystals reached 800 nm.The size of euhedral anatase crystal was 200 nm and impurity of iron was Fe/Ti 0.04 atomic ratio. From a dark field image obtained, tiny crystals of anatase are located in the upper part of the image.
3.3 Sandstone on Manara cliff
Sandstones of Lower Cretaceous were exposed in the northern part of Israel due to uplift along the Dead Sea transform fault. The main phases identified in the fine fraction were clay minerals, mainly kaolinite, ilmenite and hematite (Figure 12). The size of ilmenite crystals varies between 1000 nm and 50 nm. Most of the ilmenite crystals disintegrate into small crystals and Fe/Ti ratios decrease. The tiny ilmenite crystals are preserved in clays covering a quartz grain. Hematite tiny crystals that result from disintegration of ilmenite and recrystallize into hematite are preserved within clay minerals.
The Lower Cretaceous sandstones are mainly quartz arenite with rounded quartz grains that were separated from the fine fraction. These sandstones originated from Paleozoic sandstones that were the first-cycle quartz-rich sandstones and resulted from widespread chemical weathering of the Pan-African continental basement . The composition of the sandstones changes from the arkose at the lower layers to sub-arkose and the younger layers are mainly mature quartz arenite [12, 13]. The fine fraction includes minerals that were formed by disintegration of the initial phases or result from dust storms.
Clay minerals usually cover the quartz grains. In the samples studied the dominant clay mineral is mainly kaolinite, small amounts of illite and some smectite. Kaolinite is usually crystallized at permeable bedrock in warm, moist regions forming as a residual weathering product. Illite might result from weathering of muscovite or K-feldspar , and smectite was formed from weathering of other minerals like biotite or amphibole that were in the Paleozoic sandstones from the first cycle. The crystal size of kaolinite is around 200-500 nm. Quartz-arenite stones from the Lower Miocene Moghra in Egypt were studied showing similar results with smectite, illite and kaolinite as clay minerals and hematite as the iron oxide .
Iron oxides were goethite, hematite, and some magnetite and maghemite. The iron oxides are responsible for the colors of the sandstones. Similar results were found in sands on the Atlantic coastal plain as iron oxides coat sand grains . Goethite is usually yellow but impurities might change its color. Crystal’s size might also affect the color. Goethite crystal sizes of 300 nm–1000 nm cause a yellow color; with smaller crystals it becomes darker. Hematite is usually yellow-red but larger crystals cause the appearance of a purple color. Formation of the iron oxides depends on the environment in which they were crystalized. Goethite usually crystallizes at fast oxidation at lower pH . Hematite results from recrystallization of goethite close to a clay layer that adsorbs the OH of goethite, and hematite crystallizes (Figure 2a). Hematite and ilmenite (FeTiO3) form a solid solution. Disintegration of ilmenite formed hematite crystals with Ti impurity and the leftover of the ilmenite was enriched with Ti forming Fe/Ti < 1 ratios.
Rutile (TiO2) and anatase (TiO2) had euhedral morphology, indicating that they might have crystallized in the area or, due to their hardness, they preserved their initial morphology. Rutile was found in the Paleozoic sandstone along with tourmaline . Fe impurity in Ti-oxides might indicate that they were formed by in the area as ilmenite disintegrated and they were preserved within the surrounding clay minerals. It is also possible that rutile and anatase recrystallized from biotite from the magmatic and metamorphic rocks from the basement .
Jarosite (KFe3(SO4)2(OH)6) and alunite (KAl3(SO4)2(OH)6) were observed in dark violet or dark red sandstones. Jarosite was found in younger marl layers of Taqiya formation in the southern part of Israel, as a result of alteration of pyrite . It was also found in Jurassic sandstone in the Ramon crater, and two possible formation processes were suggested: acid rain or from transgression, for a short period, of the sea . Usually jarosite and alunite form a solid solution and they crystalize in saline lakes or acid sulphate soils . In Utah (USA) they form cement in Jurassic sandstones and they precipitated in marginal marine to coastal dune . Jarosite usually formed at low pH conditions and it requires an arid environment to prevent its decomposition into ferric oxyhydroxides . It is possible that both jarosite and alunite were crystallized due to short transgression episodes of the Thetis Sea.
Quartz arenite sandstones were formed during Lower Cretaceous in the southern and northern parts of Israel. The fine fraction observed by TEM includes clay minerals mainly kaolinite and small amounts of illite and smectite. Additional minerals that contribute to the colors of the sandstones are iron oxides, goethite and hematite. The initial phase of iron oxides that coat quartz grains or form clusters was goethite, and hematite crystals preserved the initial acicular goethite structure. Ilmenite preserved within clay minerals disintegrate into small crystals enriched with iron. The tiny euhedral crystals of Ti oxides were probably formed in the sand. Jarosite and alunite observed in Ramon and Hatira craters were crystallized in the sandstones due to short transgression of the Thetis Sea.
The project was sponsored by Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva, Israel 8410501.