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What is a Mineral?

by Chuck DeFlorin
Minnesota Mineral Club

A mineral, from the "Manual of Mineralogy," by Klien and Hurlburt, is defined as the following: "naturally occurring homogenous solid with a definite (but generally not fixed) chemical composition and a highly ordered atom arrangement. It is usually formed by an inorganic process."

This definition sounds complicated but when broken down piece by piece it starts to make a lot of sense. Naturally occurring means nothing made by man. It must occur naturally in the earth's crust. If you break apart a piece of pyrite and analyze it, we would have two pieces of pyrite. However, if you break apart a piece of granite the pieces would vary from piece to piece. Solid excludes gases and liquids and defined chemical composition means that whatever the appearance, a mineral should always have the same composition, expressed in a chemical formula. They should also have well defined physical properties. Highly ordered atomic structure is a way of saying that a mineral is a crystalline solid.

Minerals have an internal structure of atoms linking together in a perfect ordered lattice which is what forms a crystal. Usually formed by an inorganic process, which is saying that all living organisms are excluded from the mineral kingdom. However, like the work "usually" notes, there are some exceptions to the rule. So substances like bones, shells, pearls and ivory are not minerals but amber, aragonite, melanophogite, mellite, oxammite and wheatleyite are. The key here is that minerals are the result of geological activities and not the results and not directly the products the products of organisms.

All minerals have physical properties which are determined by their chemical and structural compositions. There are exceptions to this rule also. Many minerals exhibit certain properties that others do not, such as fluorescence, radioactivity and magnetism. There are nine physical properties that all minerals possess: hardness, tenacity, color, specific gravity, cleavage and fracture, crystal form, diaphansity and streak.

Hardness is the resistance of a crystal surface to scratching. The geologic community has been using "Moh's Hardness Scale" for the past 150 years. This scale consists of ten minerals of varying hardnesses:

The Moh's scale is only a relative hardness scale and cannot be used as a truly scientific test because it is too inexact. But it is close enough for identifying hardness in the field for determining minerals.

Tenacity is the resistance of a mineral to breaking, crushing, bending or tearing. Minerals may be brittle malleable, ductile, flexible or elastic. Some minerals may also be sectile - capable of being cut.

To identify their color you don't need any complicated devices, just some real good light. Colors will vary from sample to sample depending on the impurities that are present. Some minerals, like fluorite, range from colorless, white, pink, yellow, brown, blue and blue-black, so can't be identified by their color alone. Other minerals have an oxidation layer, so be sure you are looking at the true color.

The cleavage and fracture are the form of the fragments obtained by breaking the mineral with a hammer. The fragments tend to break along planes of weak bonds in the crystal structure. Galena, for example, will break up into small cubes.

Luster originates from the light reflected at the surface of the specimen. There are eight different kinds of luster a mineral can have:

Minerals are made of crystals and these crystals are categorized into crystal systems:

All minerals have a specific gravity or density. This is the weight of the mineral in relation to the weight of the same volume of water. For example, quartz with a specific gravity of 2.65, is 2.65 times as heavy as the same volume of water. The specific gravity of minerals varies between one and twenty. Values under two are considered light (amber) and anything over 2.9 are considered heavy minerals (galena).

Diaphansity or transparency is the degree to which a specimen allows light to pass through it. There are minerals which are transparent (semi-transparent), and non-transparent, which is known as opaque. For most precious stones, transparency is the key factor which influences their value.

Last of the physical properties of minerals is streak. Whereas the recognizable coloring in a mineral is caused by slight traces if impurities, the streak color always produces a unique color to the whole mineral. For example, for fluorite the streak is always white, regardless of whether the fluorite looks yellow, blue, green or black. In order to obtain the streak color, the specimen has to be rubbed on an unglazed porcelain plate which is called a streak plate. For the streak test, always use fresh fracture surfaces and avoid oxidation coatings and tarnished material.

If you check each specimen for the different physical properties, you should be able to identify what mineral it is or where to look it up in your reference books.

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