Geology And Geophysics

Characteristics of Minerals



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Minerals are identified by their characteristics. Some of these characteristics may describe its appearance, others its hardness.

Color

The color of a mineral is dependent on what chemicals are in the mineral itself. Many have a specific color; however, if a mineral is opaque it is easier to discern one from another. There is more variety of the degree of color when a mineral is translucent to transparent. Of all the characteristics, color gives scientists the least information on classifying a mineral.

Luster

The description of the surface of a mineral in the light is luster. The brilliance of the light can change the luster. There are a series of words used to describe this characteristic: metallic, meaning it reflects light and is opaque; submetallic, meaning it is opaque, dark colored, and dull; nonmetallic, meaning it does not reflect light.

Within the nonmetallic classification, a mineral can further be described as being: waxy, pearly, silky, greasy, resinous, adamantine.

Transparency

Minerals have the ability to transmit light. Even if a mineral is opaque, when it is cut into slices it can become translucent. This characteristic is also known as diaphaneity.

Specific Gravity

The heft of a mineral, or how heavy it is, is called the specific gravity. To determine this, the weight of a mineral is compared to the weight of an equal amount of water. This is found by “the difference between the weight of the mineral in air and the weight of the mineral in water,” according to Rocks and Minerals.

Hardness

The hardness of a mineral is determined by what scratches the mineral and what it can scratch. The Mohs scale rates ten minerals from softest, at one, to the hardest, at ten. Talc is the softest mineral; then, in order, come gypsum, calcite, fluorite, apatite, orthoclase feldspar, quartz, topaz, corundum and then diamond.

Harder minerals, due to the force holding the crystals together, can scratch softer ones.

Tenacity

How well a mineral holds together, or its cohesiveness, is tenacity. This can be tested by attempting to crush, bend, break or tear a mineral. There are different reactions that a mineral may present under stress: Brittle turns to powder or crumbs when hammered; sectile can be separated with a knife; malleable can be flattened by a hammer; ductile can be stretched into a wire; flexible but inelastic can be bent but stays in the new bent position; flexible and elastic when bent springs back to its original position.

Crystal form

This is the shape of a crystal or the shape it would take if it were to grow in a cavity. It is possible for a mineral to have many different crystal shapes. The shape that a crystalline structure takes is determined by how the atoms, molecules or ions are arranged and joined. This is the crystal lattice. As the degree of the structure increases, the fibers of the crystal become harder to see with a human eye or with a hand lens. By using high magnification on a microscope, microcrystalline and cryptocrystalline structures can be seen. A mineral is called amorphous when there is not a crystalline structure. This is rare.

Cleavage or fracture

It is common for minerals to break along lines or along smooth surfaces if hit sharply. The way that it breaks shows the cleavage of the mineral.

There are two sets of criteria used to define cleavage. First is how easy it is to get cleavage. If it is easily obtained and the planes are easily seen, it is perfect. If there is some difficulty in obtaining cleavage but it still has defined planes, it is considered easy. If it takes a lot of effort to obtain cleavage and the planes are difficult to see, it is imperfect.

The direction of the surface is also used to define the cleavage. The shape formed by the surface is the same as the name given: cubic, rhombohedral, octahedral, dodecahedral, basal or prismatic.

Magnetism

The ability of a mineral to attract or repel magnetic materials is magnetism. There can be difficulty in determining the different magnetisms.

Odor

There are some minerals that react with an odor if they are moistened, heated, breathed on or rubbed.

Streak

The color that the mineral is when it is in powdered form is the streak. When a mineral is in solid form, the minerals can change colors by reflecting light in different ways. However, when the minerals are broken down, reflection rarely happens. Streak is used more reliably to identify color of a mineral.

By using these characteristics, and referring to a field guide, most people are able to go out to the field and successfully identify minerals.

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  • InfoBoxCallToAction ActionArrowhttp://www.rocksforkids.com/RFK/identification.html#Properties
  • InfoBoxCallToAction ActionArrowhttp://imnh.isu.edu/Exhibits/Online/RLO/GeoOutreach/index.php?r=mineral&s=luster
  • InfoBoxCallToAction ActionArrowhttp://www.sdnhm.org/archive/kids/minerals/howto-transparency.html
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