Rock-Forming Minerals

minerals that are constant and intrinsic components of rocks. They are the most widely distributed minerals in the earth’s crust. Silicates are the most important rock-forming minerals and constitute at least 75 percent of the earth’s crust. The largest group of silicates are feldspars. Feldspathoids, pyroxenes, amphiboles, olivines, and micas are silicates that are found less frequently.

Each rock group characteristically has its own rock-forming minerals; for magmatic rocks they are quartz, feldspars, pyroxenes, amphiboles, micas, olivines, and feldspathoids; for sedimentary rocks they are calcite, dolomite, anhydrite, and clay minerals; and for metamorphic rocks they include kyanite, andalusite, sillimanite, cordierite, staurolite, chlorite, serpentine, certain garnets, wollastonite, and glaucophane. The accessory rock-forming minerals include zircon, apatite, magnetite, ilmenite, sphene, fluorite, monazite, and certain sulfide minerals.

The principal rock-forming minerals have been thoroughly studied. Their characteristic composition, texture, and properties have been identified, which facilitates the genetic analysis of rock masses. The crystalline structures of all the chief rock-forming minerals have been investigated, and detailed information has been obtained concerning their chemical characteristics, including the way cations are distributed according to different structural positions between coexisting rock-forming minerals. Data on impurity elements has also been obtained. The phase interrelationships of rock-forming minerals have been determined by means of experimentally obtained phase diagrams and the study of transitions at high and superhigh temperatures and pressures. The basis for determining the conditions of formation of magmatic and metamorphic rock-forming minerals is the physicochemical analysis of their paragenesis.

The physical properties of rocks used in petrophysical problem solving and geophysical prospecting methods are determined by the electrical, magnetic, elastic, thermal, and radioactive properties of the minerals forming the rocks.