MOHS Scale

Source: Kingzett's Chemical Encyclopedia

The Mohs Scale is used to determine the hardness of solids, especially minerals. It is named after the German mineralogist Friedrich Mohs. As indicated in the chart above, moissanite has a Mohs Hardness rating of 9.25. The scale reads as follows, with the hardness and mineral given from softest to hardest:

• 1 - Talc: easily scratched by the fingernail
• 2 - Gypsum: just scratched by the fingernail
• 3 - Calcite: scratches and is scratched by a copper coin
• 4 - Fluorite: not scratched by a copper coin and does not scratch glass
• 5 - Apatite: just scratches glass and is easily scratched by a knife
• 6 - Orthoclase: easily scratches glass and is just scratched by a file
• 7 - Quartz (Amethyst, Citrine, Tiger's Eye and Aventurine): not scratched by a file
• 8 - Topaz
• 9 - Corundum (Sapphires & Rubies)
• 10 - Diamond: cuts glass

Hardness

The hardness of a mineral is a measure of its ability to resist abrasion or scratching by other minerals or by an object of known hardness. A simple scale based on empirical tests has been developed and is called the Mohs Hardness Scale. The scale consists of 10 minerals arranged in increasing hardness with 1 being the softest. The 10 minerals selected to form the scale of comparison are listed above.  Objects with higher values on Mohs' scale are capable of scratching objects with lower values. For example, a rock specimen that can be scratched by a copper coin but not by the fingernail is said to have a hardness of about 3.  A rock specimen with a hardness of 5 or more is considered hard.

Cleavage

Cleavage is the tendency of a mineral to split or separate along preferred planes when broken. It is fairly consistent from sample to sample for a given mineral and is a valuable aid in the mineral's identification. Cleavage is described by noting the direction, the degree of perfection, and (for two or more cleavage directions) the angle of intersection of cleavage planes. Some minerals have one cleavage direction; others have two or more directions with varying degrees of perfection.  Some minerals, such as quartz, form crystals but do not cleave.

Fracture

Fracture is the way in which a mineral breaks when it does not cleave along cleavage planes. It can be helpful in field identification.  The common kinds of fracture are:

• Conchoidal. This fracture surface exhibits concentric, bowl-shaped structures like the inside of a clam shell (for example, chert or obsidian).
• Fibrous or splintery. This fracture surface shows fibers or splinters (for example, some serpentine).
• Hackly. This fracture surface has sharp, jagged edges (for example, shist).
• Uneven. This fracture surface is rough and irregular (for example, basalt).

Luster and Color

The appearance of a mineral specimen in reflected light is called its luster. Luster is either metallic or nonmetallic. Common nonmetallic lusters are--

• Vitreous (having the appearance of glass).
• Adamantine (having the brilliant appearance of diamonds).
• Pearly (having the iridescence of pearls).
• Silky (having a fibrous, silk like luster).
• Resinous (having the appearance of resin).

For some minerals, especially the metallic minerals, color is diagnostic. Galena (lead sulphide) is steel gray, pyrite (iron sulphide) is brass yellow, and magnetite (an iron ore) is black. However, many nonmetallic minerals display a variety of colors. The use of color in mineral identification must be made cautiously since it is a subjective determination.

Streak

The color of a powdered or a crushed mineral is called the streak. The streak is obtained by rubbing the mineral on a piece of unglazed porcelain, called a streak plate. The streak is much more consistent in a mineral than the color of the intact specimen. For example, an intact specimen of the mineral hematite (an iron ore) may appear black, brown, or red, but the streak will always be dark red. The streak is most useful for the identification of dark-colored minerals such as metallic sulfides and oxides. Minerals with hardness 6.5 will not exhibit a streak, because they are harder than a piece of unglazed porcelain.

Specific Gravity

The specific gravity of a substance is the ratio of its weight (or mass) to the weight (or mass) of an equal volume of water. In field identification of minerals, the heft, or apparent weight, of the specimen is an aid to its identification. Specific gravity and heft are controlled by the kinds of atoms making up the mineral and the packing density of the atoms. For example, ores of lead always have relatively high specific gravity and feel heavy.