Minerals

A mineral can be defined as a naturally occurring inorganic solid that possesses an orderly internal structure and a definite chemical composition. Some people, like physicists, might be guilty of picking up a rock and calling it a mineral. The term "rock" is less specific, referring to any solid mass of mineral or mineral-like material. Common rocks are often made up of crystals of several kinds of minerals. There are some substances, like opal, which have the appearance of a mineral but lack any definite internal structure, are sometimes called "mineraloids". Lutgens and Tarbuck give the following list of essential characteristics of a "mineral":

1. It must occur naturally.
2. It must be inorganic
3. It must be a solid
4. It must possess an orderly internal structure, that is, its atoms must be arranged in a definite pattern.
5. It must have a definite chemical composition that may vary within specified limits."

The most common minerals are the silicates, as one would guess by looking at the abundances of the elements in the Earth's crust, but there is a great variety of minerals. Minerals are classified in many ways, including hardness, optical properties, crystal structure, etc. Shipman, et al. comment that over 2000 minerals have been found in the Earth's crust, but that about 20 of them are common and fewer than 10 account for over 90% of the crust by mass.

Non-silicates constitute less than 10% of the Earth's crust. The most common non-silicates are the carbonates, the oxides, and the sulfides. There are also naturally occuring phosphates and salts. There are a few elements which occur in pure form, including gold, silver, copper, bismuth, arsenic, lead and tellurium. Carbon is found in both graphite and diamond form. Some minerals are valued as gems because of their hardness, color and beauty.

Carbonates

The carbonate ion (CO₃^2-) can bond with a variety of other ions to produce the carbonate minerals. The bonding with calcium to form the mineral calcite produces one of the most abundant of the non-silicate minerals. All carbonates have the property of dissolving easily in acidic water.

Oxides

Oxygen, the most abundant element in the Earth's crust, bonds readily with a number of metallic ions to form the oxides. These oxides form important ores for the metal resources.

Sulfides and Sulfates

Sulfur ions (S^2- ) bind with a number of positive ions to form the sulfide minerals. Many of them are important ores for the ions to which they bind.

Phosphates

Phosphorous in the form of phosphate ions (PO₄^3- ) binds with positive ions to form the phosphate minerals.