Physical Properties of Minerals
The appearance of light reflected by a mineral
∑ metallic - has the brilliant appearance of a metal; often opaque
∑ vitreous - glassy, shiny and translucent
∑ earthy - dull, not shiny
∑ pearly - iridescent pearl-like sheen, usually on cleavage planes
A measure of a mineralís resistance to scratching.† A scale of relative hardness, Mohís hardness scale, lists the hardness of 10 minerals and some everyday objects.† The hardness of an unknown mineral is discovered by determining whether or not it will scratch the minerals and objects on the list.
7††††††† quartz†††††††††††††††††††††††††† easily scratches glass
6††††††† orthoclase feldspar††††††† barely scratches glass
5††††††† apatite††††††††††††††††††††††††† nail is about 5
2††††††† gypsum†††††††††††† fingernail is about 2.5
The tendency for a mineral to break along predefined planes of weakness, usually forming smooth surfaces.† The weak planes are controlled by the internal structure of the crystal.† Not all minerals possess cleavage, only those which break in specific directions on relatively smooth planes.† Some minerals have more than one direction of cleavage.
Minerals which do not break in a predictable fashion along smooth planes (cleavage) will break along an uneven surface called fracture.
This describes the external shape of a crystal.† This is the growth shape of a mineral, as opposed to cleavage and fracture which show the shape in which a crystal breaks.† Although size may vary, many, but not all, minerals have a distinctive crystal form:
††††††††††† cubic - halite, pyrite, fluorite, galena
††††††††††† rhombohedral - calcite
††††††††††† flat sheets - mica, clay minerals
††††††††††† prismatic - quartz
Although it is easy to recognize, color is often misleading; some minerals occur in a wide variety of colors (quartz, fluorite, calcite...).† Some minerals, however, do have fairly distinctive colors (olivine, malachite, amphibole...).† Sometimes color is not easy to determine, particularly if the mineral has a metallic luster; in these cases, streak (see below) is used instead.
The color of powdered particles of a mineral.† The powder is produced by rubbing the mineral on a white, porcelain plate.† This only works with minerals of hardness<7 since that is the hardness of the porcelain streak plate.
Related to the density, or weight per unit volume of a mineral.† Some minerals feel heavier than expected for their size.† These minerals have high specific gravity.
Calcite is the only mineral which fizzes (releases bubbles of CO2) in contact with dilute HCl (hydrochloric acid).
Clear crystals of calcite can be seen through, but produce a double image.
A few minerals (particularly halite) have a distinctive taste.