**4. Discussion**

*Materials at the Nanoscale*

**Figure 11.**

*in the area.*

*GNR5 R red sandstone from Ramon crater. a. Minerals observed with electron diffraction were: anatase and goethite surrounded by clays. b. Dark field image showing larger anatase crystal and small crystals of anatase* 

*GNKS1 sandstones exposed in Manara cliff. Minerals observed: a. clays and ilmenite; b. ilmenite electron diffraction 0.255 nm; c. cluster of hematite with electron diffraction of 0.37–0.38 nm and clays over a small* 

*quartz (SiO2) grain on the upper left side quartz grain and clays.*

**120**

**Figure 12.**

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 [4]. 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 [14], 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 [15].

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 [16]. 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 [17]. 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 [18]. 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 [19].

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 [20]. 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 [21]. Usually jarosite and alunite form a solid solution and they crystalize in saline lakes or acid sulphate soils [22]. In Utah (USA) they form cement in Jurassic sandstones and they precipitated in marginal marine to coastal dune [23]. Jarosite usually formed at low pH conditions and it requires an arid environment to prevent its decomposition into ferric oxyhydroxides [24]. It is possible that both jarosite and alunite were crystallized due to short transgression episodes of the Thetis Sea.
