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Geology & Earth Science



Minerals are earth materials that have four main characteristics: they are solid, inorganic, naturally occurring, and have a definite chemical structure.


Mineral Properties

Minerals are identifiable based on a number of specific properties:

  1. Crystal form: The outward expression of a mineral’s chemical structure. For example, quartz has a hexagonal, or 6-sided, crystal form.

  2. Cleavage: Planes of weakness in the mineral’s crystal lattice along which the mineral tends to break. Cleavage faces are usually flat surfaces.

  3. Fracture: If a mineral lacks cleavage, it fractures in an irregular, jagged manner.

  4. Hardness: The resistance of a mineral to being scratched. Geologists use the Mohs scale to assign each mineral a hardness between 1 (softest) and 10 (hardest).

    The Mohs hardness scale

    Hardness   Mineral
    10   diamond
    9   corundum
    8   topaz
    7   quartz
    6   feldspar
    5   apatite
    4   fluorite
    3   calcite
    2   gypsum
    1   talc

  5. Streak: The color a mineral leaves when rubbed across a piece of unglazed porcelain.

    • A mineral’s visible color is not a reliable diagnostic property. A single mineral may vary in color from sample to sample, but its streak color does not. For example, quartz may be clear, gray, purple, or pink, but its streak is always colorless.

  6. Luster: The way light reflects off a mineral’s surface. Luster may be described as vitreous (glassy), metallic, pearly, silky, or dull.

  7. Specific gravity: The comparison of a mineral’s weight to the weight of an equal volume of water (water’s specific gravity is 1). The greater a mineral’s specific gravity, the greater its density.

  8. Other diagnostic properties: Some minerals are magnetic, some taste salty, and some fizz when hydrochloric acid is dropped on them.


Mineral Groups

  1. Silicates: The most common mineral group. Silicates have a framework of silicon (Si) and oxygen (O), the two most common elements in the Earth’s crust.

    1. Silicon-oxygen tetrahedron: The basic silicate structure, which consists of four oxygen atoms around a central silicon atom.

    2. Silicate minerals can form from:

      1. A single tetrahedron (e.g., olivine)

      2. Single chains (pyroxenes, e.g., augite)

      3. Double chains (amphiboles, e.g. hornblende)

      4. Sheets (micas, e.g., muscovite, biotite)

      5. Three-dimensional networks (e.g., feldspar, quartz)

  2. Nonsilicates: Less common but also important rock-forming minerals.

    1. Carbonates: Contain carbon and oxygen in a carbonate group (CO3). Calcite (CaCO3), which forms limestone and marble, is a common carbonate.

    2. Oxides: Usually consist of oxygen and another element. Common oxides include ice (H2O) and magnetite (Fe3O4).

    3. Sulfides: Contain sulfur ions. Pyrite, or “fool’s gold,” is a common sulfide.

    4. Sulfates: Contain sulfur and oxygen in a sulfate group (SO4). Gypsum, a material used in buildings, is a common sulfate.

    5. Halides: Contain a “salt” ion such as Na, Cl, or F. Halite, or common table salt (NaCl), is a halide.

    6. Native elements: Minerals that exist in pure elemental form. Native elements include gold (Au), silver (Ag), and copper (Cu).


Common Rock-Forming Materials

  1. Felsic minerals: Comprise over 50% of the Earth’s crust. Felsic minerals are silicates that are light in color, contain little iron and magnesium, and have abundant silica.

    1. Quartz (SiO2): Has vitreous luster; lacks cleavage but has conchoidal fracture (smooth, curved fracture like that of glass); lacks streak; and is usually gray in color but can be pink, purple, or black.

    2. Feldspars:Potassium feldspar (KAlSi3O8) and plagioclase ((Ca,Na)AlSi3O8) both have distinct cleavage planes that meet at about a 90° angle. Potassium feldspar usually is cream or pink in color, whereas plagioclase usually is in a range between white and light gray.

    3. Mica: A family of sheet silicates, including silvery muscovite and black biotite. Micas are important minerals and often give rocks a sparkly appearance.

  2. Mafic minerals: Contain iron and/or magnesium, making them dark.

    1. Olivine ((Fe, Mg)2SiO4): Has glassy luster, conchoidal fracture, and is usually dark green. Olivine is a major component of the upper mantle.

    2. Pyroxenes: Usually dark green to black, with distinctive cleavage planes that meet at right angles. Pyroxenes form a group of chemically complex minerals, the most common of which is augite, which are common in oceanic crust.

    3. Amphiboles: A complex group, distinguished from pyroxenes on sight by their cleavage planes, which meet at 60° and 120°. The most common is hornblende.