Overview
Minerals have specific physical properties that make them easier to identify in the field. Most of the minerals that are rock-forming contain silicon and oxygen, but there are some that contain calcium. Other types of minerals are used as ore, are mined for gems, or occur in small amounts.
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Physical Properties
Geologists tend to classify minerals on the basis of observable qualities, such as the way their crystals tend to form, the way they break, their hardness, and properties of color. Compare the cubic forms of ordinary salt crystals with the prisms of quartz with asbestos fibers. Some minerals, such as mica, break into sheets, while others fracture into pieces. Some other minerals cleave along distinct planes, such as precious gems that can be cut and shaped. Various minerals have different degrees of hardness, measured from a scale between 1 for talc and 10 for diamond. The color of a mineral is a result of its chemical composition. Rubies and sapphires are both the same mineral, a type of aluminum oxide called corundum. However, rubies also contain chromium, and sapphires also contain titanium or iron.
Rock-Forming Minerals
Geologists classify the most common minerals as rock-forming. The silicates contain silicon and oxygen and make up over 90 percent of the minerals in the Earth’s crust. Types of silicates include feldspar, quartz, pyroxene, amphibole, the clay minerals, and olivine. They differ in the way their chemical bonds form and the other elements that form chemical bonds with the silicon-oxygen combination. Some feldspars contain potassium, while others contain calcium and sodium.
Carbonate Rocks
Not all rock-forming minerals are silicates. Calcite and dolomite contain calcium. They are common in cliffs and other rocks near the sea, and are often a sign that the area was once under water. The carbonates react with acid, so geologists often carry a solution of weak HCl to test samples of rocks.
Ore, Gems, and Other Minerals
Some types of minerals are important sources of commercially valuable substances. While gold and silver are often mined as pure metals, minerals such as copper, lead, and zinc are mined from other compounds. Gems are especially rare crystals. Some diamonds are set as jewelry, while others, just as hard, have industrial applications. Types of quartz crystals include opal, agate, amethyst, jasper, and tiger’s eye. Some other common minerals include magnetite, garnet, and pyrite.
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