Rock-Forming Minerals Comprise the Majority of Earth's Rocks

Svartifoss waterfall, Skaftafell, Vatnajokull National Park, Iceland
Iceland's Svartifoss waterfall is an example of igneous rock. arnaudmaupetit / Getty Images
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Amphibole (Hornblende)

Blackest of the bunch
The Rock-Forming Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

A handful of very abundant minerals account for the great majority of the Earth's rocks. These rock-forming minerals are the ones that define the bulk chemistry of rocks and how rocks are classified. Other minerals are called accessory minerals. The rock-forming minerals are the ones to learn first. The usual lists of rock-forming minerals contain anywhere from seven to eleven names. Some of those represent groups of related minerals. 

The amphiboles are important silicate minerals in granitic igneous rocks and metamorphic rocks. Learn more about them in the amphibole gallery.

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Biotite Mica

Black mica
The Rock-Forming Minerals. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Biotite is black mica, an iron-rich (mafic) silicate mineral that splits in thin sheets like its cousin muscovite. Learn more about biotite in the mica gallery.

03
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Calcite

No limestones without it
The Rock-Forming Minerals. Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)

Calcite, CaCO3, is the foremost of the carbonate minerals. It makes up most limestone and occurs in many other settings. Learn more about calcite here.

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Dolomite

Magnesian calcite
The Rock-Forming Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Dolomite, CaMg(CO3)2, is a major carbonate mineral. It is usually created underground where magnesium-rich fluids meet calcite. Learn more about dolomite.

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Feldspar (Orthoclase)

Top mineral in the whole crust
The Rock-Forming Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Feldspars are a group of closely related silicate minerals that together make up the majority of the Earth's crust. This one is known as orthoclase

The compositions of the various feldspars all blend together smoothly. If the feldspars can be considered a single, variable mineral, then feldspar is the most common mineral on Earth. All feldspars have a hardness of 6 on the Mohs scale, so any glassy mineral that's slightly softer than quartz is very likely to be a feldspar. A thorough knowledge of the feldspars is what separates geologists from the rest of us.

Learn more about the feldspar minerals. See the other feldspar minerals in the ​feldspars gallery.



 

06
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Muscovite Mica

White mica
The Rock-Forming Minerals. Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)

Muscovite or white mica is one of the mica minerals, a group of the silicate minerals known by their thin cleavage sheets. Learn more about muscovite.

07
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Olivine

Common in basalt
The Rock-Forming Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)

Olivine is a magnesium-iron silicate, (Mg, Fe)2SiO4, a common silicate mineral in basalt and the igneous rocks of the oceanic crust. Learn more about olivine.

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Pyroxene (Augite)

One of the dark minerals
The Rock-Forming Minerals. Photo courtesy Krzysztof Pietras of Wikimedia Commons

Pyroxenes are dark silicate minerals that are common in igneous and metamorphic rocks. Learn more about them in the pyroxene gallery. This pyroxene is augite.

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Quartz

The continents' top mineral
The Rock-Forming Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Quartz (SiO2) is a silicate mineral and the most common mineral of the continental crust. Learn more about it in the ​quartz picture gallery.

Quartz occurs as clear or cloudy crystals in a range of colors. It's also found as massive veins in igneous and metamorphic rocks. Quartz is the standard mineral for hardness 7 in the Mohs hardness scale.

This double-ended crystal is known as a Herkimer diamond, after its occurrence in a limestone in Herkimer County, New York.