What is a rock?

Rocks are solid aggregates of minerals. Some rocks are mono-minerallic (eg limestone – calcite) but most consist of aggregates of several minerals. Granite has three main mineral constituents: quartz (SiO2), feldspar, and biotite mica. In fact feldspar is an example of one of the most common rock-forming minerals. Descriptively we can subdivide the minerals encountered in the field:

Rock-forming minerals

These are essential components of the rocks that contain them. They include: the olivine group, the pyroxene group, the amphibole group, the mica group, the feldspars, quartz, and calcite.

Accessory minerals

These tend to form small crystals and occur in limited quantities eg garnet and tourmaline.

Secondary minerals

These are formed by the alteration of pre-existing minerals and are usually of little mechanical strength. Examples are: chlorite, serpentine, talc, kaolin, and epidote.

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