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.


 

This rock, amphibolite, is made of three minerals; biotite, hornblende, and plagioclase.

 


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Amphibolite, Hamilton County, New York
Plane polarized light
 

This rock, marble, is made of grains of only one mineral, calcite.

 


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Marble, Naxos, Greece
Crossed polarizers



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