gypsum - teaching student specimens of transparent selenite gypsum - Unit of 5 specimens
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Gypsum - chemical formula: CaSO4・2H2O
Gypsum, calcium sulfate with two attached water molecules, is formed as a chemical precipitate in an evaporite environment. When Africa collided with Europe roughly 5.6 million years ago closing the Mediterranean's opening to the Atlantic, the sea completely dried up, leaving a thick gypsum bed on the what is now the sea floor. During the Permian, roughly 280 million years ago, gypsum formed in what is now Arizona, Nevada and Utah formed. This transparent selenite was mined in Mohave County, Arizona, not far from St. George, Utah. The story has it that early settlers used this selenite as window glass.
Selenite is the transparent form of gypsum that exhibits three unequal cleavages. It can easily be split between rows of atoms in one direction, yielding broad cleavage flakes. With some effort, it can be broken in the other two cleavage directions, yielding rhombohedral flakes that a student will confuse with calcite, though calcite has good rhombohedral cleavage and cannot be scratched by a fingernail. Gypsum, with a hardness of 2, can easily be scratched by a fingernail (hardness 2.5). A fibrous form of gypsum, called satin spar, is silky and not transparent. In desert dry lakes, gypsum crystallizes in the form of an interlocking rosette of small plates with included sand. This form of gypsum is often called "desert rose."
In arid areas, selenite can form in soils by evaporation of mineralized groundwater, by precipitation within the groundwater itself, and when sulfur from sulfur-rich minerals grains in the soil is transformed into sulfuric acid by weathering and oxidation. The sulfuric acid reacts with calcium carbonate in calcareous soils to form gypsum. Selenite formed under these conditions often has soil inclusions.
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