Defining rocks and minerals


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Gemstone tourmaline in quartz. Photo courtesy Krause Publications

This exclusive excerpt is from the new book Collecting Rocks, Gems & Minerals, Identification, Values, Lapidary Uses by Patti Polk (Krause Publications, 2010). Learn more at shop.collect.com.

What are rocks and minerals, and what is the difference between the two? A mineral, by definition, is any naturally occurring inorganic substance generally characterized by a definitive crystal structure that is classified according to the way the atoms of the mineral are arranged. A mineral’s chemical composition is determined by the combination, or singularity, of the 103 known chemical elements. All minerals are arranged into groups according to their chemical composition and their crystal structure. Basic elements that occur naturally are also considered minerals.

All minerals belong to a chemical group, which describes their affiliation with certain elements or compounds. The eight main chemical groups are known as: native elements, sulfides, oxides, halides, nitrates, sulfates, phosphates, and silicates. Some of these chemical groups have sub-categories, which may be categorized in some mineral references as separate groups.

Minerals also have distinctive properties, such as color, hardness, crystal habit, specific gravity, luster, fracture, and tenacity. Some minerals exhibit certain properties that others do not, such as fluorescence and radioactivity.

There are currently about 3,000 different types of known minerals, and new ones are constantly being discovered. Basically, minerals are the individual crystalline substances that are the essential building blocks of all the rocks on Earth.

What is a rock? The best way to define a rock is to say that it is a mixture of two or more naturally occurring mineral elements, and its composition is determined by the combination of minerals and organic materials present when it was formed. Rocks fall into three distinct categories depending on how they were formed: igneous, sedimentary, and metamorphic. Igneous rocks are formed through volcanic activity, both extrusive and intrusive; sedimentary, through weathering and depositing in streambeds and oceans; and metamorphic, which are igneous rocks that have been transformed into a new mineral composition by extreme heat and pressure over time.

Rocks may range in size from tiny pebbles to entire mountainsides and can be composed of tiny microscopic grains of minerals all the way up to large coarse agglomerates of different minerals that are clearly visible to the naked eye.

In less technical terms, rocks are simply aggregates of minerals. Minerals are the individual elements that bind together to form rocks, much like salt, pepper, meat, and vegetables mix together to create a bowl of stew. In other words, rocks are always made of minerals, but minerals are never made of rocks. ?



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More Images:

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Gold leaf in a bottle.
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A sedimentary fossiliferous limestone formation.

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