Scientists have identified over 4,000 different minerals. A small group of these minerals make up almost 90% of the rocks of Earth’s crust. These minerals are known as the common rock-forming minerals.

To be considered a common rock-forming mineral, a mineral must: A) be one of the most abundant minerals in Earth’s crust; B) be one of the original minerals present at the time of a crustal rock’s formation; and, C) be an important mineral in determining a rock’s classification.

Minerals that easily meet these criteria include: plagioclase feldspars, alkali feldspars, quartz, pyroxenes, amphiboles, micas, clays, olivine, calcite and dolomite.