California Mineral Education Foundation

Rocks and Minerals

Mineral: a naturally occurring inorganic solid having a definite internal structure and a definite chemical composition that varies only within strict limits.  Chemical composition and internal structure determine its physical properties, including the tendancy to assume a particular geometric form (crystal form).

Rock: an aggregate of minerals.

Rock Types:

Igneous rocks form from molten rock material consisting of liquid, gas and crystals.  The best known examples of igneous activity are magmas from volcanic eruptions. Less obvious are enormous volumes of liquid rock that cools before reaching the surface.  Granite is the most common variety of this type of igneous rock.

Sedimentary rocks form from fragments derived from other rocks, by precipitation from water or organic materials formed by biochemical processes.  They typically occur in layers, or strate.  Approximately 85% to 90% of mineral products used by our society come from sedimentary rocks.

Metamorphic rocks are formed from preexisting rocks when the alteration of minerals and textures are changed by temperature and pressure, often a result of the constant motion of tectonic plates.  Metamorphic rocks can be formed from igneous, sedimentary, or even previously metamorphosed rocks.