MINERALS

MINERAL – a naturally occurring, inorganic solid with an orderly internal arrangement of atoms (crystalline structure) and a definite (but not fixed) chemical composition. The distinctive crystalline structure and chemical composition of a mineral give it a unique set of physical properties such as luster, hardness, and how it breaks.

Minerals, especially gems and precious metals, are among the oldest objects used and treasured by society. Today our industrial society relies heavily upon metal extracted from many minerals. Of the nearly four thousand known minerals, only a few hundred have any current economic value. No more than a few dozen are abundant.

PHYSICAL PROPERTIES OF MINERALS LUSTER - describes the manner in which light is reflected from the surface of a mineral. Metallic – metal-like appearance Non-metallic – do not have metallic luster Vitreous – glassy Pearly – like a pearl Earthy – dull Waxy – like waxed paper

TRANSMISSION OF LIGHT Opaque – no light is transmitted Translucent – light but not the image is transmitted Transparent – the image is visible through the mineral

SPECIFIC GRAVITY – the ratio of the weight on a mineral to the weight of an equal volume of water DENSITY - mass divided by volume HARDNESS – measure of the resistance of a mineral to abrasion or scratching

HARDNESSDESCRIPTION Less than 2.5A mineral that can be scratched by your fingernail 2.5 to 5.5A mineral that cannot be scratched by your fingernail and cannot scratch glass Greater than A mineral that scratches 5.5glass

COLOR – most unreliable, slight impurities in a mineral result in samples having different colors STREAK – the color of the fine powder of a mineral obtained by rubbing a corner across a piece of unglazed porcelain. The streak may not be the same color as the mineral.

CLEAVAGE AND FRACTURE CLEAVAGE – the tendency of some minerals to break along regular planes of weak bonding When broken, minerals that exhibit cleavage produce smooth, flat surfaces called cleavage planes. FRACTURE – minerals that do not exhibit cleavage when broken. Fracturing can be irregular, splintery, or conchoidal (smooth curved surfaces resembling broken glass)

CRYSTAL FORM – the external shape of a mineral that results from the internal, orderly arrangement of atoms. The flat external surfaces of a crystal are called crystal faces ISOMETRIC OR CUBIC – three axes of equal length intersect at 90 degrees, Examples – galena, halite, pyrite TRICLINIC – three axes of unequal length are oblique to one another, Examples – plagioclase feldspars, turquoise

HEXAGONAL – three horizontal axes the same length intersect at 60 degrees. The vertical axis is shorter or longer than the horizontal axes. Examples – calcite, hematite, quartz ORTHORHOMBIC – three axes of different lengths intersect at 90 degrees Examples – olivine, topaz MONOCLINIC – three axes of different lengths, two intersect at 90 degrees. The third is oblique to the others Examples – micas, gypsum, augite TETRAGONAL – three axes intersect at 90 degrees. The two horizontal are of equal length. The vertical axes is longer or shorter than the horizontal axes. Examples – zircon, chalcopyrite