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1.32: What Are Minerals? and What Is the Difference Between a Rock and a Mineral?

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    Basic Geologic Principles

    Some basic geologic concepts are helpful for explaining the origin of rocks that formed in ocean basins or are observable in rocky outcrops along coastlines. Rocks form in many ways, and because the Earth is so old, rocks that may have formed in one location may have been altered or moved long distances from its place of origin.

    What Are Minerals? What Are Their Significance?

    A mineral is a naturally occurring, inorganic (never living) solid with a definite internal arrangement of atoms (crystal structure) and has a chemical formula that only varies over a limited range that does not alter the crystal structure.

    Currently there are about 4,000 known minerals of different chemical composition and internal atomic crystal arrangements (discussed below). However, slightly more than a dozen are considered "common minerals" because of their abundance on the earth surface. Figure 1.87 shows common rock-forming minerals.

    In contrast, minerals considered gems are, mostly, exceedingly rare. Most minerals are chemical compounds consisting of two or more elements, however, some elements naturally occur in mineral form including gold, copper, platinum, sulfur, and iron.

    Rock Forming MineralsFigure 1.87. Common rock-forming minerals are the most abundant minerals found on our planet Earth.

    What Is the Difference Between a Rock and a Mineral?

    A rock is a relatively hard, naturally formed aggregate of mineral matter or petrified matter. Rocks are mixtures and may consist of one or more minerals, but may include organic matter and other non-mineral substances, such as gases and water. Rocks are what makes up the materials of the solid Earth and other rocky planets and moons in the Solar System. The word stone is another common term used to describe rock.

    Rocks consist of one or more minerals. Figure 1.88 shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions.

    Minerals forming rocksFigure 1.88. Combinations of common minerals occur in different kinds of rocks. The kind of rock depends on the geologic setting where they form: igneous, sedimentary, or metamorphic.

    The mineral composition of a rock reflects the physical environment and geologic history where a rock formed. Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space. Most of the rocks we see on the surface of our planet formed by processes that happened long ago. However, we can see these processes that form rocks actively taking place in many places today. Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland. However, most rocks we see around us form very slowly in settings that may not be visible on the land's surface. Slow processes creating rocks can be inferred by observing reefs growing and accumulating in the oceans, or sediments being carried by flowing water in streams or moved by waves crashing on beaches. We can see sediments being deposited, but we cannot see them turning into stone because the process may take thousand or even millions of years.

    The mineral composition of a rock reflects the physical environment and geologic history where a rock formed.

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