Texas Rocks and Minerals: An Amateur's Guide

by Roselle M. Girard

Pumicite. See [Volcanic Ash].

Pyrite

[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.

[Pyrite] veins in white [marble] from Llano County, Texas.

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.