What Are Rock Forming Minerals?
What is 60% of the earth's crust composed of?
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Hi I’m Geologist John Swanson for About.com and today I’ll be talking about rock forming minerals. There are many minerals that form igneous rock. Here are five of the most common.Olivine has a olive green to black color and is translucent with conchoidal fracturing. Olivine phenocrysts are relatively common in "basaltic rocks (like those found in Hawaii), and are extremely stunning in contrast with the black groundmass of the basalt.Pyroxene is a green to black colored mineral. It is nearly opaque, and has two 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, is composed entirely of pyroxene minerals. Pyroxenite is related to"ultramafic terrains and is rare at the surface of the earth's crust.Amphibole is mostly black and forms in long, slender crystals with two cleavages at 60° and 120°. Hornblende is the most common member of the amphibole family. It 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 feldspars have two cleavages at approximately 90° and carries a hardness of six. Nearly 60% of the earth's crust is composed of feldspar.. The mafic variety may have striations but don’t expect every specimen to have striations. The felsic variety is often pink and has no striations. Both forms can also form in a white color, which makes it difficult to determine specifically which feldspar it is.Mica can be anywhere from translucent to black. Mica often has one perfect cleavage. This cleavage causes 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 is hard, durable, relatively inert, and has no cleavage. Quartz is the last mineral to form in a felsic rock and is generally 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. These quartz crystals are highly prized and sought after by many people for a variety of natural uses.Thanks for watching. If you’d like to learn more, please find us on the web at About.com.