Dissociation is the name given to a special case of chemical decomposition, such that the products can recombine to give the original substance when the temperature is lowered or when the conditions are altered. The decomposition of calcium arbonate (chalk) into lime and carbon dioxide is an example of dissociation, since these products recombine to form calcic carbonate under slightly different conditions. The decomposition of water at high temperatures is another example. Research Dissociation
Gypsum is a common monoclinic mineral distributed in sedimentary rocks, often as thick beds. Gypsum is usually found under beds of rock salt as it's one of the first minerals to crystallize from evaporated saltwaters and is also produced in volcanic areas and in rock veins due to the action of sulphuric acid. Gypsum is used in the production of plaster of Paris. It is chemically a hydrated calcic sulphate and has the formulae CaSO4.2H2O and a relative hardness of 2.
Gypsum is found in a compact state as alabaster, or crystallized as selenite, or in the form of a soft chalky stone, which in a very moderate heat gives out its water of crystallization, and becomes a very fine white powder, extensively used under the name of plaster of Paris. This last is the most common, and is found in great masses near Paris, where it forms the hill of Montmartre, near Aix in Provence, and near Burgos in Spain.
Gypsum may be geologically of any age, but occurs abundantly in the more recent sedimentary formations, and is even now forming, either as a deposit from water holding it in solution, or from the decomposition of iron pyrites when the sulphuric acid combines with lime, or from the action of, sulphurous vapours in volcanic regions on calcareous rocks. When gypsum occurs without water it is called anhydrite, but in its most ordinary state it is combined with water. Research Gypsum
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Matt and Leela Probert
Abbreviations
Research Dissociation
