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Gems and Minerals: Just what is a Mineral?

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Just What Is a Mineral?

A mineral is a naturally occurring, usually inorganic, homogenous solid with a definite chemical composition that is variable within fixed limits and has a highly ordered atomic arrangement. Beryl (Emerald):

Naturally Occurring: These words distinguish between materials formed by natural processes and those artificially created by man.  A man-made substance is not a mineral, even if it has the same chemical composition and structure as a non-man-made substance that is considered to be a mineral.  For example, a crystal of beryl chiseled from the rock of a Brazilian or Columbian emerald mine is a mineral, but a crystal of beryl synthesized in a laboratory is not.

Inorganic:

Almost all minerals are formed through inorganic processes - that is, not through the activities of living organisms.  Several materials that are formed organically have been recognized as minerals; these biogenic minerals include aragonite, the orthorhombic calcium carbonate that composes pearls and the shells of molluscs, brachiopods, and other invertebrate organisms, the opal in diatom shells, and sulfur and sulfide minerals Native Sulfur and Calcite precipitated by bacteria Petroleum, coal, amber, and jet are not minerals - nor are your teeth (although they are composed of material very similar to the mineral apatite).  However, if you were to be subducted under the South American continent, and the apatite in your teeth recrystallized as temperatures and pressures increased, your teeth could then be classified as minerals and would be eligible for display on these pages.  Needless, to say, the organic material in the rest of your body would go through some major changes, but ultimately your body could be reduced to carbon in the form of the mineral graphite, the low temperature and pressure polymorph of the mineral diamond.

Homogenous Solids: 

Homogeneous materials cannot be physically subdivided into simpler chemical compounds.  The requirement that a mineral be a solid eliminates all gases and liquids.  A crystal of glacial ice (a solid) is therefore a mineral; a drop of water or the mercury that was once mined in Terlingua, Texas (which is a liquid), are not.

Definite Chemical Composition: 

The chemical composition of a mineral can vary, but only within fixed limits.  This means that a mineral always has the same basic "recipe", at least as far as major components or, in baking terminology, ingredients, are concerned.  The principle is much the same as making a cake:  The proportions of basic ingredients (such as flour, sugar, butter, and baking powder) can only be varied slightly without making it inedible, while changing the special ingredients that are typical of one type of cake will turn it into a different type.

Quartz

The structure of a mineral determines how much variation is possible.  For instance, the mineral quartz (SiO2) has very little compositional variation; it is essentially composed of one atom of silicon (Si) for every two oxygen (O) atoms, and its structure makes it difficult for other sorts of atoms to become included.  Other minerals are much more variable, and some materials, such as limonite, are so extremely variable that they are considered to be "mineraloids", rather than true minerals.

The chemical formulae of minerals with variable composition tends to be very complex.  One example is tourmaline, whose formula is: (Na, Ca)(Li, Mg, Al)(Al, Fe3+, Mn3+)6(BO3)3 (Si6O18)(OH)4.  A detailed explanation of the the meaning of this formula and what it tells about tourmaline's structure and composition can be found on a separate page.

Variations in mineral composition can produce differences in structure, and are reflected in differences in the color, density, and other physical and optical properties.  These differences are used to define different "varieties" of a particular mineral.  For example, red or pink tourmaline is called rubellite, blue tourmaline is indicolite and iron-rich and black tourmaline is schorl.

Ordered Atomic Structure: 

This means that the atoms (or ions) in a mineral are arranged in a regular, repeated, three-dimensional array (this is what defines a crystalline solid).  Solids such as common opal and chrysocolla that do not have an ordered atomic arrangement (referred to as "amorphous") thus are not minerals.  In between crystalline and amorphous solids lie the "glasses" (including man-made glass and naturally-occurring obsidian).  The atoms of these are partly ordered, usually due to very rapid cooling from a molten state, creating what are called "supercooled liquids".  Supercooled liquids tend to flow under the effects of gravity, but far too slowly to be visible to the human eye.  For example, each pane of the multi-storied windows in the United Nations building in New York City (installed in the 1960s) is now much thicker at its base than at its top.

Experts currently recognize approximately 3,000 distinct minerals, and a few new ones are discovered each year.  These minerals are classified into chemical groups.  Minerals may also be classified by their affiliation with certain chemical elements or compounds.

Precious OpalMineraloids meet most, but not all, of the criteria required to be classified as minerals.  For example, opals (SiO2 . nH2O) are composed of layers of closely packed silica spheres, with H2O molecules and additional silica filling the void spaces between the silica spheres.  Common, or amorphous, opal is a mineraloid because the silica spheres have highly variable sizes and thus lack a predictable three-dimensional structure.  Precious opal, however, is composed of silica spheres  of approximately the same size, producing a predictable three-dimensional array (as shown in this SEM image made at the Caltech mineral spectroscopy lab).  Precious opals formed through the low temperature inorganic precipitation of colloidal silica from silica-rich waters are classed as minerals; precious opals formed through the accumulation of siliceous tests of silica-secreting organisms are not.

Quartz variety AmethystGemstones are minerals, mineraloids, or non-minerals that can be faceted, polished, or otherwise modified in shape to enhance their appearance.  In addition to being attractive due to its color, or the ability to reflect or transmit light, a gemstone typically is fairly hard and durable.  The most valuable gemstones, referred to as "precious" gemstones, are extremely hard (and extremely rare!).  Softer, less durable minerals usually are less valuable and are classified as "semi-precious".

Quartz variety Tiger Eye

Diamonds, emeralds (beryl), rubies, and sapphires (red and all other colors of gem-quality corundum) are referred to as "precious" minerals; the softer, less durable jade (nephrite and jadeite), garnet, amethyst, citrine, tiger eye, rose quartz (quartz), peridot (olivine), tourmaline, and turquoise are called "semi-precious".  Two exceptions to this rule are precious opals and pearls, which are classed as precious, but are soft compared to other precious stones.

 

 

Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY


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