What does the term "rock forming minerals" mean?

Answer 1

Rock forming minerals are simply the minerals that make up rocks. The term is generally applied to igneous rocks much more than the other two types. These are the minerals that crystallize out of a melt as it cools.

The basic rock forming minerals are generally the minerals that appear on Bowen's reaction series. They usually come out of liquid in the order Bowen lists them, but its not always the case. Any how, rock forming minerals are usually olivine, pyroxenes, amphiboles, micas, quartz, and feldspars. Within those frames come a whole slew of indiviual minerals depending on melt composition, temperature, pressure, any multipressure history, water/volatile content, oxygen fugacity, and rate of cooling.

Answer 2

Rock forming minerals are the minerals whose presence leads to the classification of a rock, usually igneous in origin.

Quartz, for example, is one of the basic building blocks of many igneous rocks. Feldspar is another. Still another is hornblende. Minerals such as those of the mica family are generally not thought of as rock forming minerals, because their presence or absence in a rock does not change the type of rock it is. The garnet family is another common non-rock forming mineral, for the same reason.

Bear in mind that almost any mineral can form a rock in a specific thermochemical environment, but some are relatively unimportant in the grand scheme of things.

Answer 3

The "rock-forming minerals" are the ones that are so common they make up significant portions of real rocks.