Encyclopedia of Earth and Physical Sciences - Vol. 7

By Joyce Tavolacci | Go to book overview

MOHS’ SCALE
The Molis’ scale rates the hardness
of minerals

The mineral talc is
composed of crystalline
magnesium silicate. It
is the softest mineral in
the Mohs’ scale.

CONNECTIONS
The hardness test is
just one of the many
methods used to identify
different minerals and
GEMSTONES
Scratching the
surface of a mineral
involves breaking
CHEMICAL
BONDS.

The harder a gemstone is, the less likely it is to be dulled by scratching and wear. How do scientists measure this hardness? This question was of great importance to jewelers over many centuries, as they searched for tests to distinguish genuine gems from less valuable minerals. Then, as the science of mineralogy was developed, mineralogists looked for ways to tell one mineral from another, and one of these ways was by the relative hardness of the minerals (see MINERALS AND MINERALOGY). In 1812 German mineralogist Friedrich Mohs (1773–1839) established a scale of hardness to help in identification.

The hardest natural mineral is diamond, crystalline carbon, and one of the softest is talc, a magnesium clay silicate (see CARBON). Mohs gave a value of 10 to the hardness of diamond and a value of 1 to talc. Investigating how various minerals were scratched by harder specimens, he made a scale of 1 to 10, with a different mineral at each point on the scale (see the table below). The surface of any mineral on this scale will be scratched by all those ol higher

number, and the mineral tested will scratch the surface of all those of lower number. That this scale shows Mohs’ skill in choosing the minerals.

The points on the scale are not equal; in fact, diamond is more than four times harder than corundum, and the first six are all very close in real terms. A glance at this list of minerals will show that the harder ones are gemstones. A number of everyday objects can be added to the scale. A fingernail is 2.5, a copper coin 5, and a steel knife blade 7. One of the main advantages of the minerals that Mohs chose is that they are fairly common.

Many attempts have been made to provide a scale of equal intervals, but it has always proved impractical. The Knoop test gives a measure of real hardness. This test is carried out with a special instrument and indicates the depth to which a diamond point, at a controlled pressure, will mark the mineral surface.


Testing hardness

In order to find the hardness of a given mineral on the Mohs’ scale, it is sensible to scratch its surface with minerals from the scale in a definite sequence. Zircons can be finished to look very similar to diamonds, and they are often used to make less expensive jewelry. How can Mohs’ scale identify them? First the stone is scratched with a diamond. A clear mark is left on its surface. Corundum will also mark the stone, as will topaz. Quartz will not mark it—in fact, the zircon leaves a mark on the quartz surface. Zircon is therefore softer than topaz but harder than quartz. It is given a hardness of 7. 5.

Suppose a mineral is suspected of being the sulfide of lead, galena. Working down the scale, the test mineral is consistently marked, until gypsum is used. If the mineral is harder than gypsum (2) but softer than calcite (3), the specimen is galena, with a hardness of 2.5. It is important to apply this methodical approach to hardness testing.

The hardness of a mineral is a result of the strength of the bonds between the constituent atoms, and it may vary on different surfaces of the mineral and in different directions on the same surface (see CHEMICAL BONDS). For example, the aluminum silicate mineral kyanite has hardness 5 parallel to the crystal length and 7 across the crystal.

C. PELLANT/H. PELLANT

See also: CARBON; CRYSTALS AND CRYSTALLOGRAPHY; GEOCHEMISTRY; MINERALS AND MINERALOGY; OPTICAL MINERALOGY; SOI IDS.

Further reading:

Perkins, Dexter. 2002. Mineralogy. Upper Saddle River, N.J.: Prentice Hall.

Perseroff, Joyce. 1999. The Hardness Scale. Pittsburgh: Carnegie Mellon University Press.

-974-

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