Mineral Identification Minerals combine with each other to form rocks. For example, granite consists of the minerals feldspar, quartz, mica, and amphibole in varying ratios. Rocks are thus distinguished from minerals by their HETEROGENEOUS composition. A mere 100 of the several thousand known types of minerals constitute the main components of rocks.

PHYSICAL PROPERTIES Approximately 3,000 minerals exist in nature. How do we identify them? Remember minerals differ from one another because each has a specific chemical composition and a unique three- dimensional arrangement of atoms within its structure. These differences result in a variety of physical properties, including the minerals' appearance, how they break, how well they resist being scratched, even how they smell, taste, and feel. But not all of these properties are equally useful. The physical properties we use to identify minerals 1) Color 2) Streak 3) Luster 4) Cleavage/Fracture 5) Hardness 6) Crystal Shape 7) Specific Gravity 8) Other Properties

Mineral Identification Color The color of a mineral is one of its most obvious attributes, and is one of the properties that is always given in any description. Color results from a mineral’s chemical composition, impurities that may be present, and flaws or damage in the internal structure. The Many Colors of Fluorite Unfortunately, even though color is the easiest physical property to determine, it is not the most useful in helping to characterize a particular mineral. The problem is shown to the right, in which the mineral fluorite (CaF 2 ) displays a rainbow of colors.

Color *Some minerals do have only a single color that can be diagnostic, as for instance the yellow of sulfur, the green of malachite or the blue of azurite. Also, although many minerals vary in color few span the spectrum of colors as fluorite does.

Streak *The color of a mineral when it is powdered is called the streak of the mineral. Crushing and powdering a mineral eliminates some of the effects of impurities and structural flaws, and is therefore more diagnostic for some minerals than their color. Streak can be determined for any mineral by crushing it with a hammer, but it is more commonly (and less destructively) obtained by rubbing the mineral across the surface of a hard, unglazed porcelain material called a streak plate.

Streak The color of the powder left behind on the streak plate is the mineral's streak. The streak and color of some minerals are the same. For others, the streak may be quite different from the color, as for example the red- brown streak of hematite, from a mineral that is often a gray to silver-gray mineral. The combination of luster, color, and streak may be enough to permit identification of the mineral. Examples of Streak

Luster *The luster of a mineral is the way its surface reflects light. Most terms used to describe luster are self- explanatory: metallic, earthy, waxy, greasy, vitreous (glassy), adamantine (or brilliant, as in a faceted diamond). *A metallic luster is a shiny, opaque appearance similar to a bright chrome bumper on an automobile *Other shiny, but somewhat translucent or transparent lusters (glassy, adamantine), along with dull, earthy, waxy, and resinous lusters, are grouped as non-metallic.

Cleavage In some minerals, bonds between layers of atoms aligned in certain directions are weaker than bonds between different layers. In these cases, breakage occurs along smooth, flat surfaces parallel to those zones of weakness. In some minerals, a single direction of weakness exists, but in others, two, three, four, or as many as six may be present. These breaking patterns are known as the mineral’s cleavage. Examples of Cleavage

Cleavage

Fracture When bonds between atoms are approximately the same in all directions within a mineral, breakage occurs either on irregular surfaces or along smooth, curved surfaces. This is described as the minerals fracture. Some examples of fracture are splintery or irregular or conchoidal (similar to those formed when thick pieces of glass are broken.

Hardness Hardness Testing * The Mohs Hardness Scale is a relative scale. This means that a mineral will scratch any substance lower on the scale and will be scratched by any substance with a higher number. Diamond is not 10 times harder than talc or 1.1 times harder than corundum, as would be the case with an absolute hardness scale. Most often we are able only to narrow down hardness to within a certain range; for example, if an unknown mineral scratches a copper penny but does not scratch a glass plate, its hardness must be greater than 3.0 and less than 5.5.