It is well known that gypsum, CaSO42H2O, dehydrates at temperatures above 80°C. It first loses 1½ molecules of water to form the substance hemihydrate, CaSO4½H2O (plaster of Paris) and then with further heating at higher temperature it dehydrates virtually to completion to form ‘dead-burnt gypsum’.* Where pure, gypsum crystals are generally colourless, but hemihydrate and ‘dead-burnt gypsum’ are both chalky white.
The change in appearance which gypsum undergoes on heating provides a useful means of detecting it and assessing its abundance when it is present as small grains in soils and sediments, providing the grains are visible to the naked eye. The test can be carried out in the field by simply heating small samples of the soil or sediment on a metal plate. Grains of gypsum will turn white in a matter of a few minutes, whereas most other mineral grains remain unaltered. With the use of a hand lens the test can be applied to particles down to very fine sand-grade size. It is of course often necessary to remove silt and clay-grade materials from the sample and concentrate the sand grains by simple decantation before heating.
The test is usually easy to apply to siliciclastic sediments, because few of the other mineral grains are normally white in colour. It sometimes can be a little more difficult in the case of gypsum in carbonate sediments because of the inherent white colour of the most carbonate grains. However, the white colour of ‘dead-burnt gypsum’ is usually much more intense