Most minerals you see while walking around an outcrop or a road cut reflect the local geology. The Sierra Nevada in California, which is primarily granitic, has an abundance of quartz, plagioclase feldspar, and micas. On the other hand, the granite at Pikes Peak, Colorado, also contains pink-to-orange orthoclase, a feldspar more common to the interior of the continent (Figure 1).
In the dark volcanic rocks of Hawaii and Iceland, you will see olivine, amphiboles, and pyroxenes in addition to quartz and feldspars.
Areas that have been subjected to intense heat or pressure, like the Sierra Nevada foothills or the Tibetan Plateau, commonly contain garnets, serpentine, talc, chlorite, epidote, actinolite, kyanite, staurolite, and andalusite (Figure 2). Less common (but more exciting) minerals found in such places are corundum (rubies and sapphires) and jade.
Cliffs in the Canadian Rockies are mainly limestone, so calcite, dolomite, aragonite, and siderite are common minerals. Limestone is identified by the way the calcite in it fizzes when diluting hydrochloric acid is dripped on it. Dolomite will fizz as well, but only after it has been scratched. West of Salt Lake City, you pass through the dried bed of the Great Salt Lake. The white minerals covering the ground are halite (salt) and anhydrite ( Figure 3)
In contrast to rocks that are formed more or less in place, sandstone and shale reflect the geology of their source or provenance. The reason is simply that these rocks are made up of grains that were transported. The white cliffs in Zion National Park in Utah, for example, contain dune sandstone of the Jurassic Navajo Formation. The Navajo Formation contains quartz, feldspar, zircon, and other minerals derived from an original source rock that may have been as far away as the Appalachian Mountains in the eastern United States.
Most sandstone and shale are made up of combinations of quartz, feldspar, other rock fragments, and sometimes shell fragments.
Occasionally, sandstone will contain uncommon minerals. The winnowing of beach sand by wave action leaves smears of black sand, the accumulation of heavy minerals like magnetite. Rarely, beach sandstones contain gold, diamonds, ilmenite, and garnet. These same heavy-mineral deposits can accumulate in river channel sandstones.
Mineral Collecting: Knowing Where to Look
Minerals are intimately related to local geology. Knowing the processes that formed an area means knowing which minerals you are likely to find there.
Many fine mineral and crystal specimens come from rock cavities where they were able to grow uninterrupted. These cavities can be a gap along a fault or fracture in granitic, volcanic, or metamorphic rocks. Cavities can also form in limestone and get filled with crystals, forming geodes (Figure 4). Gas pockets in lava can also produce geodes.
Mine dumps have especially good mineral pickings. Consider requesting a tour of an active mine.
A Sampling of Mineral Collecting Field Guides
A local gem and mineral clubs and societies are excellent sources of information on collecting locations. You can find them in a phone book or on the Internet.
Mineral and fossil collecting guides tell you where others have found minerals. Some examples include the following:
Crow, M. 1994. Rockhound’s Guide to Texas (and California, Colorado, Montana, New Mexico and Washington), Falcon Press Publishing Co., 166 pp.
Dake, H. C. 1962. Northwest Gem Trails, Mineralogist Publishing Co., 95 pp.
Johnson, C. 1992. Western Gem Hunters Atlas, Cy Johnson & Son, 96 pp.
Johnson, K., and R. Troll. 2007. Cruisin’ the Fossil Freeway, Fulcrum Publishing, 202 pp.
Johnson, L. 2014. Rockhounding Oregon (also California, Colorado, Idaho, Montana,
Nevada, Pennsylvania, New Jersey, Utah, and Wyoming), Falcon Guides.
Mitchell, J. R. 1997. Gem Trails of Colorado, Gem Guides Book Co., 144 pp.
National Audubon Society. 1979. Field Guide to Rocks and Minerals (North America), Knopf, 856 pp.
Pough, F. 1998. A Field Guide to Rocks and Minerals, Houghton Mifflin Co., 349 pp.
Ransom, J. 1964. A Range Guide to Mines and Minerals, Harper & Row.
Romaine, G. 2009. Gem Trails of Oregon (also Arizona, California, Idaho, Nevada, New Mexico, Texas, Utah, and Washington), Gem Guides Book Co.
G.L. Prost and B.P. Prost