Mineral of the Month--December
by Ken Casey
Pyrite is found in many forms, mostly crystalline and massive. Crystals range from pyrite cubes to pyritohedrons, and rarely octahedrons. The massive variety can permeate rock as ore veins. Crystals tend to form in limestone and lapis lazuli, for example, making for decorative building material and gem-quality rough for lapidary work.
Often, fossils may be formed with pyrite as a constituent, such as those found in metamorphosed marine sediments. Nature can create wondrous beauty from a simple iron sulfide, sculpting it into sand dollars, fern leaves, or sea shell replacements.
The pyrite family is composed of minerals that vary slightly from the typical iron sulfide chemistry, such as, chalcopyrite, which contains copper (CuFeS2). Another variant is arsenopyrite, which has arsenic (FeAsS) in it's makeup. Our most common form is called, just plain 'pyrite' (FeS2).
Another name is "fool's gold", because it's shiny yellow, metallic luster resembles that of gold (Au). A good field test to rule out gold as a candidate is to tap hard on a small, broken specimen with a rock hammer. If it breaks, it is most likely pyrite. If it flattens, it may contain gold, or copper. The giveaway to copper's presence is it's brownish, or coppery, color. Both gold and pyrite are associated with each other.
Other pyrite associations include: quartz, calcite, limestone, dolomite (from metamorphic rocks), and with other sulfide minerals, such as sphalerite and galena (from hydrothermal veins).
Sometimes pyrite is left behind by weathering, and leaves a characteristic cube-form laden in the surrounding rock. A 'rusty' color may denote a 'pseudomorph' (false form) of 'limonite after pyrite'. Limonite is essentially rusty iron.
Pyrite, and it's relatives, are found all around the world, perhaps on other planets. We are fortunate to have several collecting locales in our area to find our own representatives of this shiny wonder.
On recent DMS fieldtrips, we have found nice specimens of the pyrite family. To see them, visit the links below.
by: Karissa Hendershot, Ken Casey
This page last updated: February 19, 2011 10:14:42 AM