Cassiterite, tin dioxide, is the mineral that serves as the chief source
of tin. Tin does not corrode and tarnish, and one of its main uses is in
the making of tin cans. (Actually, our tin cans are made from thin
sheets of steel that have been coated with a protective layer of tin.)
Cassiterite has either a nonmetallic or a submetallic luster. Some
specimens are brilliant and shiny; others are dull. Cassiterite may be
translucent to transparent. It may be black, brown, gray, reddish brown,
or yellowish brown. When rubbed across a streak plate, this mineral
leaves a pale brown, a pale yellow, or a white streak. Cassiterite is
quite heavy—it has a specific gravity of 6.8 to 7.1. It is too hard to
be scratched by an average pocket knife.
Sometimes, prospectors use a chemical test to help them identify
cassiterite. They put small pieces of metallic zinc into a jar or test
tube containing dilute hydrochloric acid. Then they add a few fragments
of the mineral that they suspect is cassiterite. If the fragments are
cassiterite, they become covered with a pale gray coating of metallic
tin.
Cassiterite’s most common crystal shape is a short, 8-sided prism with
pyramids at each end, but perfect crystals are not often found. Most
Texas cassiterite does not show a crystal shape. Instead, it occurs as
crystalline masses in igneous rocks and as loose pebbles that have
weathered out of these rocks.
Cassiterite occurs in a number of places in the United States but not in
large quantities. A small amount of cassiterite has been found in quartz
veins in Precambrian granite in both central Texas and west Texas. In El
Paso County, the cassiterite is found on the east side of the Franklin
Mountains a few miles north of El Paso, where some of it has been mined.
In central Texas, cassiterite occurs in the Streeter area of Mason
County.
When the granite rocks in these areas were formed, probably not all of
the hot magmas cooled and hardened at the same time. The fluids given
off by the remaining magmas contained tin and several other elements. It
is believed that these fluids moved up into cracks in the granite rocks
and formed the cassiterite.