Mineral Forms and Calcination
Mineral Forms of Rock Gypsum
The chemical composition of gypsum is CaSO4-2H2O - calcium sulfate dihydrate - a combination of elements that make it important to farming and industry.
In the natural state, gypsum contains two molecules of chemically combined water. During calcination, the process of heating mined gypsum at temperatures exceeding 212F, three-fourths of this water (one and one-half molecules) is driven off. The result is a hemi-hydrate of calcium sulfate commonly called Plaster of Paris, which after further processing is suitable for commercial applications.
When the Plaster of Paris is mixed with water to a pourable state and allowed to set, it reverts back to a solid mass. This process is called recrystallization. The material heats and expands until it reaches the chemical composition of natural gypsum-containing two molecules of water. Certain modifiers can be added to the plaster to accelerate or retard the setting process.
Typically rock gypsum occurs naturally in several mineral forms.
- Gypsite is found as a sand contaminated with clay and is not an important source of gypsum.
- Alabaster, which is used for art carvings, generally occurs as inclusions in deposits of rock gypsum.
- Satin spar, named for its silky luster, is a minor form of gypsum that appears in some quantity at our Kansas quarry.
- Selenite contains mica-like structures and is generally not suitable for making industrial gypsum plasters.
- Anhydrite, or anhydrous calcium sulfate, is a hard, dense rock with the same chemical composition as gypsum – except it contains no water of crystallization.