Pyrite is a shiny, pale golden-yellow or brassy-yellow metallic mineral.
This mineral, an iron disulfide, is so often mistaken for gold that it
is widely known by the nickname _fool’s gold_.
Except for their similar color and luster, pyrite and gold are really
very different. When you rub pyrite across a streak plate, it leaves a
black, a greenish-black, or a brownish-black streak, but the streak of
gold is gold-colored. Pyrite is too hard for the average pocket knife to
scratch, but a knife will scratch gold easily. Pyrite is brittle and
readily breaks, but gold is malleable and flattens out when hit with a
hammer. Pyrite is only about 5 times as heavy as an equal volume of
water, but pure gold is over 19 times as heavy. And pyrite may have a
brown or a multicolored tarnish on it, but gold never tarnishes.
[Illustration: Pyrite veins in white marble from Llano County, Texas.]
[Illustration: Cubic crystals of pyrite.]
Pyrite is a common mineral and is found in many of the igneous,
metamorphic, and sedimentary rocks of Texas. It may be scattered through
the rocks, or it may fill cracks and cavities in them. This mineral
occurs as granular and compact masses, as rounded masses, or as
crystals. The crystals are commonly cubes, pyritohedrons, or
octahedrons. In some crystals, the shapes are combined (such as a cube
with an octahedron or two pyritohedrons grown through each other). You
may notice that the sides of some cubes and pyritohedrons have fine,
parallel grooves (called _striae_ or _striations_) on them.
Pyrite originates in a number of different ways. Some of it forms, along
with other minerals in igneous rocks, from hot magmas. It also forms in
metamorphic rocks by the same processes that produce these rocks. Some
of the pyrite in limestone and other sedimentary rocks is formed when
the rocks themselves are deposited by seas or streams. Pyrite also is
deposited by the hot fluids that are given off by magmas. These fluids
travel up into cracks and other openings in rocks and then form pyrite
as well as other minerals. Much pyrite forms in still another way. As
water seeps through rocks, it dissolves some of the iron minerals that
they contain. When, under certain conditions, these iron solutions mix
with hydrogen sulfide (this is the gas that makes some water smell like
rotten eggs), pyrite is formed.
Pyrite alters easily. Because of this, most builders carefully check the
limestone, granite, marble, or whatever other building stone they plan
to use to be sure that it does not contain large amounts of pyrite. When
exposed to the weather, pyrite changes to limonite and causes an
unsightly rust stain.
Pyrite is used as a source of sulfur, and it is produced for this
purpose in several states. In Texas, however, no pyrite deposits have
been found that are large enough to be mined.
Pyrolusite. _See_ Manganese Minerals.