EAS-100-51: Introduction to Earth Science


    The word "mineral" may mean different things to different people - some may associate it with gemstones, with rocks and geology, or even vitamin pills. To a geologist, however, a mineral must have all of the following characteristics:
  1. Must be naturally occurring (not man-made)

  3. Must be a solid (not liquid or gas)

  5. Must have a fairly definite chemical composition

  7. Must be inorganic (not produced exclusively by living organisms or biological processes)

  9. Must have a crystalline structure (an orderly, internal, repeating arrangement of atoms)
    In summary, a mineral is a naturally occurring, inorganic, solid element or compound with a definite composition and a regular internal crystal structure.


    There are over 2000 known varieties of minerals. However, most of these are fairly rare, and are sought by mineral collectors for their gem quality. The more common minerals add up to about 200 or so varieties. The most important of the common minerals are silicates (compounds containing the elements silicon and oxygen, and usually a metallic element) and carbonates (carbonates containing carbon, oxygen, and a metallic element). The other common minerals may be collectively called non-silicates.


    The silicates are called rock-forming minerals because they make up most of the rocks in the earth's crust. They commonly originate as a result of crystallization from a molten material called magma, which exists in the earth's interior.

    During the latter 19th century, geologists were attempting to re-create the process by which igneous rocks in the earth's crust were made. Geochemists analyzed and calculated the "average" chemical composition of igneous rocks such as granite, basalt, and andesite. However, this technique only told them the percentage of each kind of element - it did not reveal how these elements were combined into individual minerals which made up these rocks.


    During the late 1800's, geologist Norman L. Bowen devised an experiment in which he mixed together the exact proportions of elements of basalt, according to its "recipe." Then, he heated the mixture to over 1100 C., until it was completely molten (liquid), and observed the formation of different minerals as the magma cooled; Bowen found that as each mineral crystallized, it settled by gravity to the bottom of the magma chamber and altered the chemistry of the remaining molten mixture.  The minerals which crystallized in this sequence are called the Bowen's Reaction Series (see the diagram in your textbook), consisting of a discontinous series of mineral crystallization (olivine crystallized first at the highest temperature, followed successively by pyroxene, amphibole, biotite mica, potassium feldspar, muscovite mica, and quartz; and a continuous series (the plagioclase feldspars, ranging from calcium-rich to sodium-rich).

    When these silicate minerals (except for quartz) are subject to weathering at the earth's surface, they change their chemical compositions and become clay minerals, which are similar to the micas, except they are much smaller (microscopic) in grain size. Clay minerals make up mud, which later turn into rocks called shales, which are the most common sedimentary rock.


    One way to classify silicates is to separate them into two broad groups which we may term felsic and mafic (or ferromagnesian). Felsic (fel from "feldspar" and sic from "silica") minerals are usually light in color, ranging from colorless to white to light gray or red. Mafic minerals rich in iron and magnesium are usually dark in color, ranging from dark gray to dark green to black. Igneous rocks may be classified in this manner, based upon their mineral content.


    A mineral may be identified by a combination of diagnostic physical properties; this information may be obtained by performing simple tests using instruments such as a steel file, pocket knife, glass plate, copper penny, fingernail, unglazed porcelain tablet, and dilute hydrochloric acid.  These techniques will be taught to you in the laboratory portion of these course.

    The hardness of a mineral is defined as its ability to resist scratching, and may be determined on the Mohs Scale between 1 (softer than your fingernail - like the mineral talc) to 10 (the hardness of diamond). The hardness of a copper penny is 3, a glass plate is about 5, and a pocket knife or hardened steel file about 6.

    The color of a mineral is usually unreliable as a diagnostic property (the mineral fluorite can be purple, blue, green, yellow, white, or colorless), but the color of its powdered form (called the streak) is usually more useful (the streak of fluorite is always white, no matter what color the whole mineral is; hematite has a characteristic reddish-brown streak).

    Sometimes a mineral's chemical composition may be revealed if it reacts with dilute hydrochloric acid. Carbonates such as calcite will fizz (release carbon dioxide gas), while sulfides such as galena will stink (release hydrogen sulfide, or "rotten egg gas.")

    Metallic elements are usually more dense (heavier) than non-metallic elements; consequently, metallic minerals are usually more dense than non-metallic ones. The mineral magnetite is magnetic, while other iron ore minerals such as hematite are usually non-magnetic.

Copyright © 1990 by William K. Tong