MAGNETITE AND ILMENITE FROM THE ZONE OF UNDERWATER EROSION OF THE UPPER ALBIAN CARBONACEOUS SANDSTONE IN THE INTERFLUVE BODRAK–KACHA (MOUNTAIN CRIMEA)
Keywords:
magnetite, ilmenite, sandstone, underwater erosion, hydrothermal solutions, Upper Albian, Crimean Mountains.Abstract
We discovered magnetite and ilmenite in carbonaceous sandstones, which have been formed during underwater erosion of the Upper Albian deposits in the interfluve Bodrak–Kacha (Mountain Crimea). Magnetite is presented by octahedral-shaped crystals, ilmenite – by table-like and lamellar crystals of hexagonal and irregular shape. The size of grains is 0.10–0.25 mm. Magnetite crystals contain ultra- and micropores. The vertices and edges of magnetite crystals are slightly smoothed. On the surface of the crystals, due to etching by hydrochloric acid, you can see various depressions and cavities with partially preserved relics of other minerals. The main simple form on ilmenite crystals is pinacoid {0001}; the faces of one or two rhombo-hedra are much less developed. Various hollows and grooves of different shape and size are visi-ble on the surface of crystals after etching by HCl. They have the signs of minerals intergrowths and points of various dislocations emergence. Such hollows are often filled with remnants of the minerals that have been not completely dissolved under the action of HCl. There are also the growths of other minerals, and intergrowths of magnetite crystals and possibly also hematite (as the products of solid solution decay). Green clay mineral, common in sandstones, is represented, according to X-ray analysis, by smectite. Magnetite and ilmenite are typical minerals of igneous rocks and of their weathering products. Authigenous magnetite and ilmenite are very rare in sedimentary rocks. The content of Ti and Fe in sea water is very low; therefore, these minerals cannot be formed from sea water. Obviously, they have been formed from hydrothermal solutions flowing into the saltwater pool through the consedimentation dislocations. This is evidenced by the presence in sandstones of authigenous smectite in the form of margin around the rounded quartz grains.
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