Ask GeoMan...

I am taking Geology 100 at the University of Maryland. Could you tell me any five common rock-forming minerals, and one characteristic feature of each.


Great question. I'm assuming you are referring to minerals which form igneous rocks. The vast majority of the common igneous rock-forming minerals belong to the silicate group, which means that they are based on silicon and oxygen for their basic elemental components. The silicates can be further divided into mafic and felsic - 2 very broad categories that relate to the other elements which occur along with the ever-present SiO2. In any event, from mafic to felsic, the common igneous rock forming minerals would include the following:

Olivine: olive green to black, translucent, with a conchoidal fracture. Olivine phenocrysts are relatively common in some basaltic rocks (like those found in Hawaii), and make an extremely pretty contrast with the black groundmass of the basalt. A semi-precious variety occurs (peridot), which can be cut and faceted like any other gemstone.

Pyroxene: green to black, nearly opaque, 2 cleavages at approx. 90°. Enstatite is a common member of the pyroxene family, and can be found in gabbro and mafic diorites. Pyroxenite, an igneous rock composed totally of pyroxene minerals, is related to ultramafic terrains and is therefore relatively rare at the surface of the earth's crust.

Amphibole: mostly black, forms long, slender crystals with 2 cleavages at 60° and 120°. The most common member of the amphibole family is hornblende, which is easy to identify in diorite, granodiorite, and some granites. Amphibolite is a metamorphic equivalent of basalt, and can contain extremely coarse grained specimens of hornblende.

Feldspar: all have 2 cleavages at approx. 90° and a hardness of 6. Approximately 60% of the earth's crust is composed of feldspar, and I tell my Geology 101 students that it's probably a pretty good idea to be able to identify the various members of the family. The mafic variety (plagioclase) may have striations (very fine "razor-cut" grooves on selected cleavage faces), but not always. The felsic variety (orthoclase) can often be pink and has no striations. Both can be white, which can make a specific determination of which feldspar a bit awkward (especially if there are no visible striations).

Mica: translucent to black (felsic to mafic), with one (1) perfect cleavage, causing it to easily break into thin sheets. The mafic mica is called biotite, with the more felsic member of the family affectionately referred to as muscovite.

Quartz: hard, durable, relatively inert, and no cleavage (but a great conchoidal fracture). Quartz is the last mineral to form in a felsic (granitic) rock, and can generally be found filling in between all of the other minerals. When allowed to cool and crystallize in open space, quartz commonly forms 6-sided (hexagonal) crystals which are highly prized and sought after by many people for a variety of natural (and super-natural) uses.

Click here for additional information on Bowen's Reaction Series, which describes the progression of minerals which are formed as magma is cooled and undergoes a phase change from liquid to solid.


Click here to ask GeoMan a question

Return to Ask GeoMan's Index of Questions

Return to GeoMan's Home Page


You are GeoManiac number since April 1, 1997