In geology, a mineral is a naturally occurring solid that has a well-defined chemical composition and crystal structure. Most minerals are inorganic, although some mineralogists allow for minerals that are organic compounds or else made by organisms rather than geological processes. The word mineral comes from the Medieval Latin word minera, which means ore or mine. Several thousand minerals are known. Of these, around one hundred are components of rocks.
- A mineral is a natural solid with a defined chemical composition and crystalline structure.
- Most minerals are inorganic and form from geological processes.
- A mineral can be a pure elements or a compound. It is not a mixture of two or more substances.
Examples of Minerals
Minerals include elements that occur in relatively pure form as solids in nature (native elements) and compounds. However, unlike most compounds in chemistry, minerals often contain two or more elements that occupy the same positions in a mineral’s crystal structure. For example, the formula for the mineral mackinawite is (Fe,Ni)9S8, so FeNi8S8 and Fe2Ni6S8 both are possible.
Here are some examples of minerals and their chemical formulas:
- Apatite [Ca5(PO4)3(OH,Cl,F)]
- Calcite (CaCO3)
- Copper (Cu)
- Corundum (Al2O3)
- Diamond (C)
- Fluorite (CaF2)
- Gold (Au)
- Graphite (C)
- Olivine [(Mg,Fe)2SiO4]
- Orthoclase feldspar (KAlSi3O8)
- Quartz (SiO2)
- Silicon (S)
- Sulfur (S)
- Topaz [Al2SiO4(OH,F)2]
Difference Between a Rock and a Mineral
The terms “rock” and “mineral” don’t mean the same thing. A mineral is a natural solid with a definite composition and structure. A rock consists of one or more minerals or mineraloids. So, a rock can be a mineral, and vice versa. But, some samples of minerals are not rocks. Most rocks consist of multiple minerals and/or mineraloids. For example, granite (a rock) mainly contains the minerals quartz, feldspar, and plagioclase. An ore is a rock that is rich in certain minerals.
Difference Between a Mineral and a Gem
Most gems consist of minerals, but not all minerals are gems. A gem or gemstone is a cut and polished solid that is usually a mineral crystal. While all minerals have a crystalline structure, sometimes it is only visible as tiny grains in the overall solid. For example, the gemstones ruby and sapphire consist of the mineral corundum.
Mineraloids
Some natural solids are similar to minerals, but don’t meet all of the criteria for the definition. Mineraloids are natural mineral-like substances that don’t have the constant chemical composition of a mineral or else lack a crystal structure. Obsidian and opal are good example of mineraloid that are amorphous rather than crystalline. Pearl is a mineraloid because it contains organic matter bonded to calcite or aragonite in no definite proportion.
How to Identify a Mineral
Identifying a mineral is a complex task, as two minerals may look similar at first glance. Here are some the properties that distinguish different minerals.
- Chemical composition: Chemical composition gives the identity and ratio of elements in a mineral.
- Crystal structure: Crystal structure is the geometric arrangement of atoms in space.
| Crystal Structure | Lengths of Axes | Angles | Examples |
|---|---|---|---|
| Isometric | a = b = c | α = β = γ = 90° | garnet, halite, pyrite |
| Tetragonal | a = b ≠ c | α = β = γ = 90° | andalusite, rutile, zircon |
| Orthorhombic | a ≠ b ≠ c | α = β = γ = 90° | aragonite, olivine |
| Hexagonal | a = b ≠ c | α = β = 90°, γ = 120° | calcite, quartz, tourmaline |
| Monoclinic | a ≠ b ≠ c | α = γ = 90°, β ≠ 90° | gypsum, orthoclase |
| Triclinic | a ≠ b ≠ c | α ≠ β ≠ γ ≠ 90° | albite, kyanite |
- Hardness: Hardness is a mineral’s resistance to being scratched.
- Luster: Luster describes how light reflects from a sample’s surface.
- Diaphaneity: Diaphaneity is a measure of how transparent a mineral is.
- Color: Color actually isn’t that helpful, in part because trace impurities affect it.
- Streak: The streak test indicates the color left behind when you rub a mineral against a hard surface.
- Other Optical Properties: These include asterism, iridescence, chatoyancy, pleochroism, and tarnish.
- Cleavage: Cleavage describes the characteristic way a mineral breaks.
- Specific gravity: This is a measure of a mineral’s density.
- Other properties: Other characteristics include radioactivity, magnetism, acid resistance, odor, and flavor.
Types of Minerals
The two major types of minerals are silicates and non-silicates. The reason is because most of the Earth’s crust consists of silicates, which are minerals containing the elements silicon and oxygen.
Silicates
Mineralogists classify silicates largely by their crystal structure, based on the [SiSO4]4- tetrahedron.
- Orthosilicates: Tetrahedra share no corners. Examples of orthosilicates include pyrope garnet, zircon, and topaz.
- Disilicates: Two tetrahedra share one oxygen atom. Epidote is a disilicate.
- Inosilicates: Single silicate chains share two corners, while double chains share two or three corners. The pyroxenes and amphiboles are inosilicates.
- Phyllosilicates: A sheet structure forms with three shared oxygen atoms. Mica is an example of a phyllosilicate mineral.
- Tectosilicates (framework silicates): Tetrahedra share all four corners. Examples include quartz, feldspar, and zeolite.
- Cyclosilicates (ring silicates): Tetrahedra share two corners, forming a cylinder. Beryl and tourmaline are cyclosilicates.
Non-Silicates
Some of the important classes of non-silicate minerals are carbonates, halides, oxides, phosphates, sulfates, and sulfides.
- Carbonates: Carbonates contain the carbonate anion (CO3) combined with one more other elements. An example of a carbonate is calcite (CaCO3).
- Halides: Halides all contain a halogen element (e.g., F, Cl, I). Halite (NaCl, salt) is an example of a halide.
- Oxide: The main element in an oxide is oxygen. An example of an oxide is chromite (FeCr2O4).
- Phosphates: Phosphates contain the phosphate anion (PO43-). An example of a phosphate mineral is fluoroapatite [Ca5(PO4)3F].
- Sulfates: Sulfates contain the sulfate anion (SO42-). These minerals occur in both hydrous (with water) and anhydrous forms. Examples include celestine (SrSO4) and gypsum (CaSO4⋅2H2O).
- Sulfides: Sulfides contain one or more metals or semimetals, bound to sulfur. Sphalerite (ZnS), galena (PbS), and cinnabar (HgS) are examples of sulfide minerals.
The native elements are another broad class of non-silicates. These include diamond, graphite, copper, iron, and gold.
The organic minerals are in their own class. These substances contain organic carbon (i.e., carbon bonded to hydrogen), yet form via geologic processes. An example of an organic mineral is whewellite (CaC2O4⋅H2O). Whewellite deposits from hydrothermal vents.
References
- Chesterman, C.W.; Lowe, K.E. (2008). Field Guide to North American Rocks and Minerals. Toronto: Random House of Canada. ISBN 978-0394502694.
- Dyar, M.D.; Gunter, M.E. (2008). Mineralogy and Optical Mineralogy. Chantilly, VA: Mineralogical Society of America. ISBN 978-0939950812.
- Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993). Manual of Mineralogy (21st ed.). New York: Wiley. ISBN 047157452X.
- Lowenstam, H.A. (1981). “Minerals formed by organisms”. Science. 211 (4487): 1126–31. doi:10.1126/science.7008198
- Rafferty, John P., ed. (2011): Minerals. In the series Geology: Landforms, Minerals, and Rocks. Rosen Publishing Group. ISBN 978-1615304899.