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Definition Exploring Rocks: Minerals and Rocks
Minerals are crystalline chemical compounds found in nature. Every
mineral has a specific arrangement of atoms, or structure, unique
to that mineral. Some minerals are always made up of the same
chemical elements; others may have varieties with slightly
different compositions. The structure and composition of a mineral
determine its properties, such as crystal shape, cleavage,
hardness, color, and the way it interacts with polarized light.
Rocks are aggregates of many grains of one or more minerals. Rocks
may also contain fossils, glass, and fragments of other rocks.
To help understand the distinction between minerals and rocks,
think of cloth made of colored threads woven together. Each color
of thread corresponds to a mineral, the cloth to a rock. Even if
the threads are all one color, it is still cloth, just as marble is
a rock even though it contains only one mineral.
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This rock, amphibolite, is made of three minerals; biotite,
hornblende, and plagioclase.
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