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Minerals

Minerals are the building blocks of the universe. The 3,500 or so named minerals are the components which form rocks, ores, and even meteorites. Minerals are homogeneous naturally occurring solids of inorganic origin. Each mineral species can be defined by its particular chemical composition and its internal molecular (crystalline) structure.

IDENTIFYING MINERALS

Although thousands of mineral species have been described in literature, most are known from very limited occurrences. The common minerals number only a few hundred. These are the rock-forming minerals, the minerals we are most likely to encounter. Elaborate equipment is used in the laboratory to identify minerals, but most common minerals may be identified by observing simple physical properties which are related to their chemistry and crystalline structure.

Color

Certain minerals, such as azurite, consistently exhibit a specific color which aid in identification. This is due to dominant chemical components in their composition. In azurite the blue color is due to copper. Many copper minerals exhibit a blue or green color. The color of a powdered sample of a mineral is generally more consistent than the color of a hand sample. This powdered sample is called a streak, normally made by scratching the sample against a white tile or “streak plate”, or by simply pulverizing a small fragment. Though many minerals, in hand sample, exhibit a wide range of colors, this color may be controlled by minor inclusions, contaminants, or trace-elements, and is not of tremendous help in identification.. The color of the streak is remarkably uniform for each mineral.

Transparency -Translucency – Opacity

Another characteristic helpful in identification of minerals is the degree to which light will pass through the mineral. If no light will pass through a thin sliver of the mineral, the specimen is termed opaque. When light will pass through and objects on the other side can be seen, as through glass, the specimen is termed transparent. When light can pass through but no image can be seen through the specimen, as with porcelain, it is termed translucent.

Luster

Luster is used to describe the reflectivity of light from a surface. This property is fairly consistent for each mineral because it is related to the structure and composition. Metallic luster requires a truly opaque mineral. Submetallic luster is present in mineral which reflects light like a metal but is translucent or transparent. Nonmetallic lusters include vitreous – like a piece of glass; adamantine – like a diamond; resinous – like a piece of resin; pearly – like mother-of-pearl in minerals with a plate-like structure; or silky – like the shimmer of silk or satin seen in minerals with a fibrous structure.

Hardness

Relative hardness is the resistance of a smooth surface of a mineral to being scratched by another object. Ten common minerals have been set in a scale with 1 the softest and 10 the hardest, to gauge their relative hardness. This is the Moh’s hardness scale. Talc, the softest mineral is hardness 1. Your fingernail has a hardness of about 2 1/2; a knife-blade about 5 1/2. Diamond, the hardest mineral, is hardness 10. A mineral will scratch those items with a lower Moh’s hardness and will be scratched by those higher on the scale.

Specific Gravity or Density

Every mineral, because of its composition and crystalline structure has a particular density or specific gravity. This is noticed as the difference in weight between two similar objects of different composition. Density is measured as weight divided by volume. For uniformity in mineralogy specific gravity is utilized which is the weight of a sample divided by the weight of an equal volume of water.

Cleavage and fracture

Cleavage and fracture are terms used to describe the manner in which a crystal breaks. If the crystal breaks irregularly, this is termed fracture. Cleavage is when a crystal will consistently break along certain crystallographic planes of weakness, resulting in a plane similar to a crystal face. The quality of the cleavage is described by terms such as perfect, imperfect, distinct, and indistinct. Peeling of mica into thinner sheets, or the initial cleaving of a diamond with a mallet and chisel are examples. Some minerals exhibit cleavage in more than one crystallographic direction. The quality in each direction may be the same or may range from perfect to indistinct. Fracture, in minerals without a characteristic cleavage, may also be distinctive. Types of fracture include conchoidal, like that of glass; splintery, even, and uneven, and other simple descriptive terms; and hackly, like the jagged edge of broken cast iron.