Anhydrite, calcium sulfate, is a rather soft mineral that you can
scratch with a pocket knife, although not with a fingernail. It has a
glassy or a pearly luster and is transparent or translucent. Most
anhydrite is white, but impurities cause it to be grayish, bluish, or
reddish. When rubbed across a streak plate, anhydrite gives a white
streak. This mineral has an uneven fracture, and it cleaves in three
directions that are at right angles to each other. It commonly occurs as
rectangular cleavage fragments or as sugary crystalline masses.
Anhydrite resembles dolomite, limestone, or gypsum. You can use a
hardness test to distinguish it from gypsum (anhydrite is harder) and an
acid test to distinguish it from limestone and dolomite. A drop of
dilute hydrochloric acid will fizz when you put it on limestone or
powdered dolomite. On anhydrite, the acid does not fizz.
[Illustration: Massive anhydrite.]
Anhydrite occurs at several places in Texas. It is, for example, seen in
bluffs along the Double Mountain Fork and the Salt Fork of the Brazos
River in north-central Texas. Most of the Texas anhydrite, however,
occurs underground. In the Gulf Coastal Plain, the anhydrite is found
below the surface in salt domes. (Salt domes are described with halite
on p. 66 and with sulfur on p. 91.)
Another anhydrite locality is in the subsurface Permian basin of west
Texas. Oil wells drilled in this basin penetrate great, thick deposits
of massive anhydrite. The anhydrite was deposited during the Permian
Period from a sea that covered this area. As the sea gradually
evaporated, the mineral matter that was dissolved in it came out of
solution to form anhydrite, halite, and several other minerals.
Antigorite. _See_ Serpentine.
Argentite. _See_ Silver Minerals.