ROCKS & MINERALS

Nature provides us with an abundance of rocks and minerals. Over 3,000 minerals have been recognized so far. By definition, a mineral is an inorganic compound composed of a specific ratio of chemical elements and a specific crystalline structure. Thus the mineral halite, or common table salt, is always composed of one atom of sodium and one atom of chlorine and crystallizes in the cubic form. Pyrite consists of one iron atom for each two sulfur atoms and it crystallizes in the cubic form. Marcasite has the same chemical composition as pyrite, but is a different mineral because it has a different crystal structure, orthorhombic. In some minerals, the chemical composition can vary slightly when one element replaces some of another similar element. A rock is composed of a certain set of minerals. For example, granite is composed of quartz, feldspar, and mica.

There are many uses for rocks and minerals. Industrially, rocks are used for roads, in concrete, for making steel and aluminum, for electronic devises, just to name but a few of the enormous number of uses. Many people collect rocks and minerals, some people tumble them for jewelry and gemstone trees, yet others facet gemstones for jewelry or display. The lists could go on and on. In all cases, minerals need to be identified to be useful. Minerals are identified by many physical properties which include hardness, specific gravity, streak, cleavage, and crystal structure.

Hardness is a useful property of a mineral. Friedrich Mohs first suggested, over a century ago, a hardness scale consisting of common minerals. He set his scale from 1 to 10, the values being arbitrary, but a higher number meant a harder mineral. The ten minerals are:

A given mineral can scratch any other mineral of the same or lower hardness. Thus, quartz can scratch another quartz specimen or any other mineral with a hardness of 7 or less.

Other common materials that can be added to the list include:

Many other hardness scales, including some absolute ones, have been devised since. See "Gemstone & Mineral Data Book" by John Sinkankas for other hardness scales and detailed lists of minerals and their hardnesses. A knowledge of mineral hardness is very useful in polishing rocks and minerals by tumbling. The basis for tumble polishing rocks is for the grit to scratch off the sharp edges and the surface of the rock or mineral. This is done in a succession of steps with finer and finer grit to make the scratches smaller and smaller until they are no longer seen by the naked eye and the rock or mineral appears polished. For guidelines on using a vibratory tumbler click here. A hardness ruler has been constructed as an easy reference and a way to stimulate student activity in the hobby since every student needs a ruler. To see the mineral hardness ruler click here.

Specific gravity or density is the weight of a substance relative to the same volume of water, where water is defined as 1. Thus the specific gravity of gold is 19.3 meaning that a cube of gold one inch on each side is 19.3 times as heavy as a cube of water one inch on each side.

The streak of a mineral is the color of the powdered mineral, which is usually produced by scratching the mineral on an unglazed porcelain plate. Since the porcelain plate has a hardness of about 7, minerals of higher hardness cannot be subjected to this test, but instead would have to be physically powdered. The streak color can be the same as the solid mineral or slightly different. For example, hematite which is black leaves a red streak.

Cleavage is the tendency of a crystallized substance to split along the crystalline planes to yield smooth surfaces. It is related to crystal structure which is a three dimensional geometric figure determined by the atomic arrangement of the given mineral. The crystal structures consist of six systems: isometric or cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic.

Optical properties such as color and luster can be useful in identifying minerals. The color of an object is the result of visible light being reflected from the object. Color can be helpful in identifying certain minerals where one color predominates such as blue for azurite and green for malachite. In other cases, color may not be as helpful, particularly where a given mineral may have many colors due to trace impurities. When the trace impurities give a certain color to a particular mineral then varieties are classified. For example, purple quartz is amethyst, yellow to orange quartz is citrine, green beryl is emerald, and so on. Luster is the brilliance of a mineral surface under reflected light. Terms such as pearly, silky, resinous, greasy, adamantine, vitreous, and metallic are used to describe a mineral surface.

For more details on the physical and optical properties above consult books such as:

C. Cipriani and A. Borelli "Simon & Schuster's Guide To Gems and Precious Stones" Simon & Schuster, New York.

"Simon & Schuster's Guide To Rocks and Minerals" Simon & Schuster, New York.

R. Pearl "How To Know the Minerals and Rocks" Signet Key Books, New York

F. Pough "A Field Guide to Rocks and Minerals" Houghton Mifflin Co., Boston

J. Sinkankas "Gemstone & Mineral Data Book" Van Nostrand Reinhold Co., New York

J. Sinkankas "Field Collecting Gemstones and Minerals" Geoscience Press, Prescott AZ

H. Zim and R. Perlman "Rocks and Minerals: A Golden Nature Guide" Simon & Schuster, New York


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