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.