What Is the Difference Between Rocks and Minerals?

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Difference Between Rocks and Minerals
A rock is a solid made of minerals and mineraloids. A mineral is an inorganic solid with a defined composition and crystalline structure.

A lot of people think rocks and minerals are the same thing, but they really are not. To a geologist, the simple difference between rocks and minerals is that rocks consist of minerals. For example, granite is a rock that contains the minerals feldspar, quartz, and sometimes mica, biotite, and hornblende. The minerals form an aggregate, meaning they interlock together much like puzzle pieces. Take a closer look at rocks versus minerals and get examples of both.

  • A rock consists of one or more minerals.
  • A mineral is an inorganic crystalline solid with a characteristic chemical composition.
  • Mineraloids are like minerals, but they either aren’t inorganic, aren’t crystalline, or else don’t have well-defined chemistry.
  • A single mineraloid or a mineral can be a rock. But, usually rocks contain multiple components that form an aggregate.

Rocks

A rock is a solid mass of one or more minerals or mineraloids. Most rocks are inorganic, but some have organic origins. Rocks are identified based on the minerals they contain and the way they form. The three main types of rocks are igneous rocks, metamorphic rocks, and sedimentary rocks.

  • Igneous rocks have volcanic origins. They forms from solidifying lava or magma. Examples of igneous rocks are obsidian, granite, basalt, and pumice.
  • Metamorphic rocks form when a rock (igneous, sedimentary, or other metamorphic rock) is subjected to temperature or pressure conditions that change it. Examples of metamorphic rocks include jade, gneiss, schist, marble, serpentine, and soapstone.
  • Sedimentary rocks form from the accumulation and cementation of bits of rocks, minerals, or organic matter. Some sedimentary rocks contain fossils. Examples of sedimentary rocks include shale, limestone, and sandstone.

Examples of Rocks and the Minerals They Contain

Examples of rocks include:

  • Andesite: Andesite is an igneous rock that contains the minerals feldspar, quartz, and feldspathoid.
  • Basalt is an igneous rock containing alkali feldspar, plagioclase, quartz, and feldspathoid.
  • Chert is a sedimentary rock containing microcrystalline quartz.
  • Coal is a sedimentary rock that mainly contains carbon.
  • Gneiss is a metamorphic rock rich in the minerals feldspar and quartz.
  • Slate is a metamorphic rock containing mica, chlorite, and quartz.

Two different rocks may contain approximately the same minerals, but their origin, relative abundance of the minerals, and properties distinguish them from each other.

Minerals

A mineral is a natural inorganic solid that has a defined chemical composition and crystal structure. Solid chemical elements are minerals, although mercury (a liquid) is also considered a mineral. Some minerals are pure chemical compounds, too. However, the chemical composition of a mineral usually is not quite as well-defined. For example, the chemical formula for the mineral machinawite is (Fe,Ni)9S8 or FexNi9-xS8. Trace elements cause considerable variation between minerals. For example, both ruby and sapphire are forms of the mineral corundum, but their composition makes ruby red and sapphire blue (or other colors).

Here are some examples of minerals:

  • Cinnabar (HgS)
  • Copper (Cu)
  • Corundum (Al2O3)
  • Gold (Au)
  • Halite (NaCl)
  • Mica [X2Y4–6Z8O20(OH, F)4], where X is usually K, Na, Ca; Y is Al, Mg, Fe; Z is Si or Al
  • Quartz (SiO2)

Note: Some mineral classification systems allow organic minerals, while other restrict the term to inorganic solids.

Mineraloids

A mineraloid is a mineral-like substance that lacks a crystalline structure and/or has an organic origin. For example, obsidian and opal are mineraloids because they have a glassy or amorphous structure. Jet is a mineraloid because it is made from decaying wood under high pressure. Pearl is another mineraloid. Pearl is organic. Although it has a crystalline structure, there is no well-defined chemical composition.

References

  • Blatt, Harvey; Tracy, Robert J. (1996). Petrology (2nd ed.). W.H. Freeman. ISBN 978-0-7167-2438-4.
  • Rafferty, John P., ed. (2011). “Minerals”. Geology: Landforms, Minerals, and Rocks. Rosen Publishing Group. ISBN 978-1615304899.
  • Newman, D.K.; Banfield, J.F. (2002). “Geomicrobiology: How Molecular-Scale Interactions Underpin Biogeochemical Systems”. Science. 296 (5570): 1071–77. doi:10.1126/science.1010716
  • Wenk, Hans-Rudolf; Bulakh, Andrei (2004). Minerals: Their Constitution and Origin. Cambridge University Press. ISBN 978-0-521-52958-7.