Celestite is a strontium sulfate mineral. It is colorless, white,
yellow, or gray. Light blue specimens of this mineral also are found,
and it is because of this sky-like color that celestite gets its name.
The word celestite comes from the Latin word _caelestis_, meaning _of
the sky_.
Celestite has a glassy to a pearly luster, and it is either transparent
or translucent. It gives a white streak when rubbed across a streak
plate. Celestite has a specific gravity of 3.95 to 3.97. It is, however,
lighter than barite, a mineral that it resembles. Celestite is not very
hard—a knife will scratch it, although your fingernail will not. It
cleaves in three directions, and some of the fragments are flat and
slabby.
[Illustration: Celestite cleavage fragment from Lampasas County, Texas.]
Celestite occurs commonly either as prism-shaped or flat crystals and as
cleavable, granular, or fibrous crystalline masses. In Texas, it is
found in geodes, as rounded nodules, or as bedded or layer-like deposits
in limestones and other sedimentary rocks. In Real County, celestite
occurs on the walls of a cave in Cretaceous limestone.
Some celestite may be deposited by sea water, but much of the Texas
celestite is believed to have been deposited by underground water that
seeped through cracks and pores in the limestones and other sedimentary
rocks. This water picked up and dissolved strontium compounds that were
scattered in small amounts through the rocks. Then, it re-deposited the
strontium in the rocks as celestite.
In Texas, beds of celestite occur in Permian rocks in Coke, Fisher, and
Nolan counties and in Lower Cretaceous rocks in Brown, Comanche, and
Mills counties. Celestite geodes and nodules are found in Lower
Cretaceous limestone rocks in Lampasas, Travis, and Williamson counties,
and in Permian rocks in Coke, Fisher, Nolan, and Taylor counties.
Celestite is one of two minerals (the other mineral is _strontianite_,
strontium carbonate) used as a source of strontium. Strontium compounds
give a crimson-red color to a flame, so they are used in fireworks,
tracer bullets, and flares. Perhaps you have seen a red flare set out on
the highway at night to warn motorists that a truck has stalled. The
chances are good that the flare’s red flame was due to a strontium
compound. Some of the Texas celestite has been mined, but most of the
strontium minerals now used in the United States are imported from
England and Mexico.
Cerargyrite. _See_ Silver Minerals.
Chalcedony. _See_ Quartz.
Chalcocite. _See_ Copper Minerals.
Chalcopyrite. _See_ Copper Minerals.
Chalk. _See_ Limestone.
Chert (Flint). _See_ Quartz.
Chrysotile. _See_ Asbestos; Serpentine.