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

Svartifoss waterfall, Skaftafell, Vatnajokull National Park, Iceland
arnaudmaupetit / Getty Images

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. 

01
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Amphibole

Kaersutite on tuff matrix.

Marek Novotňák / Wikmedia Commons / CC BY-SA 4.0

The amphiboles are important silicate minerals in granitic igneous rocks and metamorphic rocks.

02
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Biotite Mica

Biotite mica

James St. John / Wikimedia Commons / CC BY 2.0

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

03
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Calcite

Calcite at 2012 Tucson Gem & Mineral Show

Simeon87 / Wikimedia Commons / CC BY-SA 3.0

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

04
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Dolomite

Dolomite and Talc

Didier Descouens / Wikimedia Commons / CC BY-SA 4.0

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

05
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Feldspar (Orthoclase)

feldspar var. moonstone

Parent Géry / Wikimedia Commons / Public Domain

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. Thorough knowledge of the feldspars is what separates geologists from the rest of us.

06
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Muscovite Mica

Muscovite Mica Oberpfalz, Germany

Hannes Grobe/AWI / Wikimedia Commons / CC BY 3.0

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

07
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Olivine

olivine crystals

Jan Helebrant / Flickr / CC BY-SA 2.0

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

08
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Pyroxene (Augite)

Augite, Ruhengeri Region, Northern Province, Rwanda

Didier Descouens / Wikimedia Commons / CC BY-SA 4.0

Pyroxenes are dark silicate minerals that are common in igneous and metamorphic rocks.

09
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Quartz

Herkimer quartz

Parent Géry / Wikimedia Commons / Public Domain

Quartz (SiO2) is a silicate mineral and the most common mineral of the continental crust.

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.