![]() These are much less common and extensive than the calcareous, and are formed under exceptional conditions. Rock salt is often very pure, and then it is transparent and colourless but it is frequently stained by iron, or mingled with dust blown into the lake or lagoon which deposited the salt, or mixed with clay and other mechanical sediments. One such mass, near Berlin, exceeds 4000 feet in thickness. Again, it may occur in enormously thick masses, the result of long-continued precipitation. ![]() The salt may be present only as an ingredient of shale (saline shale), or may form thin layers, indicating brief periods of deposition, followed by freshening of the water. Rock Salt (NaCl) is precipitated by evaporation from the dense brine of salt lakes and lagoons, following the deposition of gypsum, which explains the very common association of the two rocks in successive beds. Piece of banded travertine polished, natural size. The mineral sometimes occurs in the form of anhydrite (CaS04), but it is not known under what conditions the anhydrous sulphate has been deposited from solution.įig. It forms compact, crystalline, or fibrous beds, looking like limestone, but much softer and not effervescing with acid portions of the beds may consist of transparent selenite crystals. When pure, gypsum is white, but it is often coloured gray, brown, or red, by iron stains, and it may even be black. Gypsum (CaS04.2H20) is deposited from solution in salt lakes find lagoons, in which evaporation balances the influx of water (p. When the spheres are larger, resembling peas in size and shape, the rock is called pisolite. 264) is made in this fashion, and calcareous sinter often has a similar structure. Oolite is a limestone composed of minute spherules of carbonate of lime, cemented into a more or less compact mass, somewhat resembling fish-roe, whence is derived the name, meaning "egg rock." The spherules are made up of concentric layers of carbonate of lime, deposited from solution around some nucleus, it may be a particle of sand or dust, or a calcareous fragment. The so-called "Mexican onyx" or "onyx marble" is a beautifuly banded travertine derived from ancient spring deposits. In structure they are banded and show rings of growth, which distinguishes them from the organic limestones. These deposits are made of cal-cite (or aragonite), are often very pure, and usually white, and more or less translucent, though they may be stained by other substances dissolved with the lime. Precipitates Of The Alkalies And Alkaline EarthsĬalcareous Tufa or Sinter, Travertine, Stalactite, Onyx Marbles, are all forms of carbonate of lime deposited from solution, either around the vents of springs, or by percolating waters in limestone caverns, or in lakes and streams. The chemical precipitates may be classed under the following heads: a, Precipitates of the alkalies and alkaline earths b, siliceous precipitates c, ferruginous precipitates. 191), and in closed bodies of water without outlet and subject to evaporation. This arises from the fact that the chemical processes occur in a conspicuous way only around the mouths of certain classes of springs (p. The silica that formed the flint derives from silica fixed in the skeletal structure of many marine organisms.Rocks which have been principally or entirely formed by chemical processes are, for the most part, of locally restricted extent, and are not at all comparable to the great masses of mechanical and organic sediments. Where the dissolved oxygen and hydrogen sulphide meet, acidic conditions are generated, carbonate is dissolved and silica precipitated to form flint. Bacterial activity in the sediment within the seabed generates hydrogen sulphide which, when migrating towards the seabed/seawater interface, meets dissolved oxygen migrating down in the sediment. The carbonate has to be dissolved with silica precipitated in its place.Īs Chalk is an alkaline rock an acid has to be generated to dissolve the Chalk in order to enable the silica to precipitate. The silica replaces the original Chalk carbonate grain by grain. Reply by Amanda Lane (Chair of Solent Regional group)įlint is a microcrystalline rock made of silica and is considered to have begun forming soon after the deposition of Chalk. ![]() Q: How is flint formed in Chalk on the southern coast of England? ![]()
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