Download The Mohs Hardness Scale

The Mohs Hardness Scale
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a
harder material to scratch a softer material. It was created in 1812 by the German mineralogist Friedrich Mohs and is
one of several definitions of hardness in materials science. The method of comparing hardness by seeing which
minerals can scratch others, however, is of great antiquity, having first been mentioned by Theophrastus in his treatise
On Stones, circa 300 BC. On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1.5; a fingernail, 2.2–2.5; a
copper penny, 3.2–3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.[10] A streak
plate (unglazed porcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple
way to approximate the position of a mineral on the scale. The minerals of the Mohs scale are listed in the lid of the
box containing them. The box also may contain a streak (porcelain) and glass plate, nail, penny (or copper plate) and
maybe a magnet. Your job will be to learn to recognize all those minerals (except diamond) by hardness, form, color,
luster and cleavage. Read about each one below.
Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine,
pyroxene, amphibole, olivine, in the presence of carbon dioxide and water with the chemical formula H2Mg3(SiO3)4 or
Mg3Si4O10(OH)2. In loose form, it is the widely-used substance known as talcum powder. It can be easily scratched by
a fingernail. It has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Talc is not soluble
in water, but it is slightly soluble in dilute mineral acids. Its color ranges from white to grey or green and it has a
distinctly greasy feel. Its streak is white. Soapstone is a metamorphic rock composed predominantly of talc.
Gypsum (Selenite is the transparent form) is a common mineral, with thick and extensive evaporite beds in
association with sedimentary rocks. Deposits are known to occur in strata from as early as the Permian age. Gypsum
is deposited in lake and sea water, as well as in hot springs, from volcanic vapors, and sulfate solutions in veins.
Gypsum occurs in nature as flattened and often twinned crystals and transparent cleavable masses called selenite. It
may also occur in a silky, fibrous form, in which case it is commonly called satin spar. Finally it may also be granular
or quite compact. In hand-sized samples, it can be anywhere from transparent to opaque. A very fine-grained white or
lightly tinted variety of gypsum is called alabaster, and is prized for ornamental work of various sorts. In arid areas,
gypsum can occur in a flower-like form typically opaque with embedded sand grains called desert rose. Gypsum is a
very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O.
Calcite is a carbonate mineral and the most stable form of calcium carbonate (CaCO3). Its luster is vitreous in
crystallized varieties. Color is white or none, though shades of gray, red, yellow, green, blue, violet, brown, or even
black can occur when the mineral is charged with impurities. Calcite is transparent to opaque. Calcite, like most
carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by
groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations.
Although calcite is fairly insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide
gas. Calcite is a common constituent of sedimentary rocks, limestone in particular, much of which is formed from the
shells of dead marine organisms. Approximately 10% of sedimentary rock is limestone. Calcite is the primary mineral
in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and it occurs in caverns as
stalactites and stalagmites.
Fluorite (also called fluorspar) is a halide mineral composed of calcium fluoride, CaF2. Fluorite may occur as a vein
deposit, especially with metallic minerals, where it often forms a part of the gangue (the surrounding "host-rock" in
which valuable minerals occur) and may be associated with galena, sphalerite, barite, quartz, and calcite. Many
samples of fluorite fluoresce under ultra-violet light, a property that takes its name from fluorite. Many minerals, as
well as other substances, fluoresce. Fluorite comes in a wide range of colors and has subsequently been dubbed "the
most colorful mineral in the world". The most common colors are purple, blue, green, yellow, or colorless.
Apatite is a group of phosphate minerals. Transparent to translucent, usually green, less often colorless, yellow, blue
to violet, pink, brown. The primary use of apatite is in the manufacture of fertilizer - it is a source of phosphorus. It is
occasionally used as a gemstone. Apatite is occasionally found to contain significant amounts of rare earth elements
and can be used as an ore for those metals.
Orthoclase (Microcline is one form of orthoclase) is an important silicate mineral found in igneous rock. The name is
from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate
names are alkali feldspar and potassium feldspar. The gem known as moonstone (see below) is largely composed of
orthoclase. Colorless, Greenish, Grayish yellow, White, Pink. it is vitreous, pearly on cleavage surfaces.
Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. It is made up of a
continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra,
giving an overall formula SiO2. Colorless through various colors to black. It is 6-sided prism ending in 6-sided pyramid
(typical), drusy, fine-grained to microcrystalline, massive. Luster is vitreous – waxy to dull when massive. Quartz is an
essential constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as
sandstone and shale and is also present in variable amounts as an accessory mineral in most carbonate rocks. It is
also a common constituent of schist, gneiss, quartzite and other metamorphic rocks. Because of its resistance to
weathering it is very common in stream sediments and in residual soils.
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4(F,OH)2.
Pure topaz is colorless and transparent but is usually tinted by impurities; typical topaz is wine, yellow, pale gray or
reddish-orange, blue brown. Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type.
It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows like those at Topaz Mountain in
western Utah.
Corundum is a crystalline form of aluminium oxide (Al2O3) with traces of iron, titanium and chromium. It is a rockforming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are
present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. It is
commonly used as an abrasive. Because of its hardness and resistance to weathering, it commonly occurs as a
detrital mineral in stream and beach sands.
Diamond has the highest hardness and thermal conductivity of any bulk material. Those properties determine the
major industrial application of diamond in cutting and polishing tools. Diamond has remarkable optical characteristics.
Because of its extremely rigid lattice, it can be contaminated by very few types of impurities, such as boron and
nitrogen. Combined with wide transparency, this results in the clear, colorless appearance of most natural diamonds.
Most natural diamonds are formed at high-pressure high-temperature conditions existing at depths of 140 to 190
kilometers (87 to 120 mi) in the Earth mantle. Carbon-containing minerals provide the carbon source, and the growth
occurs over periods from 1 billion to 3.3 billion years (25% to 75% of the age of the Earth). Diamonds are brought
close to the Earth surface through deep volcanic eruptions by a magma. Diamonds can also be produced synthetically
in a high-pressure high-temperature process which approximately simulates the conditions in the Earth mantle.
The rhyming peg mnemonic device is used to teach Mohs hardness scale.
See the sweating, slipping sun,
Rating talc as number one.
Buy a gypsy's gypsum shoe,
Or a scratched-up pair of two.
Trim the cave-rock calcite tree,
Always branching into three.
Shut the bulging fluorite door,
That the toothpaste labels "four."
Fear the crossed-bone killer hive
With an appetite for five!
Orthoclase-tipped music sticks,
Beat a rhythm pounding six.
Seven jars, all cloudy quartz
Hold crystals from heaven's parts
Then a heavy topaz skate,
Does a looping figure eight.
Sanded off corundum vine,
Had red ruby roses nine.
But the pecking diamond hen,
Gets a perfect score of "10"!
Or try mine: Two Giddy Chickens Found An Oddly Quiet Turtle Cooking Dinner