Gypsum is a soft mineral which can be scratched easily with the finger
or thumb-nail. It may be glassy or transparent, or may grade into an
opaque white body, possibly stained by iron oxide, but it is always very
soft. Of the three varieties of gypsum—selenite, alabaster, and
satinspar—only the first two have been found in Missouri by the writer.
The chemical composition of gypsum is CaSO₄·2H₂O.
[Illustration: Transparent, flexible variety of gypsum (selenite).]
[Illustration: Fine-grained, white, opaque gypsum (alabaster).]
The chief use of gypsum is in the manufacture of plaster of paris,
during which it is pulverized and heated to drive off part of the water
of crystallization so that its composition corresponds to CaSO₄·½H₂O.
This powder, when mixed with water and poured into a mold, heats and
sets; that is, it hardens by taking up enough of the water to restore
its original composition.
Although thick, wide-spread beds of gypsum occur in other localities,
probably most of the gypsum in Missouri has been secondarily formed, as
from the reaction of sulphuric acid from oxidizing pyrite on calcite;
and its quantity is limited to small crystals, veins, and crusts in or
on other rocks. Gypsum may be an impurity in coal, and some beautiful
crystals a few inches long have been found in weathering clay deposits.
It therefore cannot be considered as a commercially valuable mineral of
this state.