Mohs hardness scale Moos scale anniversary
Posted by kabeeragate on November 9th, 2019
Moos scale anniversary
For more than two hundred years, gemologists all over the world have been using the scale of a rough delineation of the hardness of minerals. Compiled in 1811 by the German scientist Friedrich Moos (Carl Friedrich Christian Mohs, 1773-1839), it has not lost its relevance to this day. The principle of comparative determination of rock hardness was extremely convenient. Even far from the reference parameters of the human nail have found application in the table.
It should be recognized that the method for identifying the resistance of any mineral to scattering to establish indicators of its hardness is both simple and witty. Moos suggested taking ten very common rocks - from the softest to the hardest - and arrange them in a table so that with increasing numbers the degree of strength of the intermolecular bonds of the mineral increases, understood by science as hardness.
It is natural to expect that the softest mineral (talc) is not able to leave a scratch on any of the more durable materials. The hardest - diamond - cuts a mark on any of the stones of natural origin.
The stone diagnosed by the mineralogist can be tested by scratching with any of the reference samples - which allows you to determine the relative and suggest (albeit very approximately) the absolute hardness of the rock.
For example, a green transparent mineral, scratched by topaz, and leaving a mark on quartz, may well turn out to be beryl. The stone, similar to an emerald and found in the Urals, is not able to scratch rock crystal during testing, but it leaves a mark on the orthoclase. So this is chrysolite.
Despite the conventionality and imperfection of such a classification (many minerals are characterized by hardness of a rather wide range), the Mohs table has taken root as a convenient applied tool for diagnosing stone.
For gemologists, such characteristics of the processed materials turned out to be key: the hardness of a gemstone is an indicator of its resistance to abrasive wear, which determines the scope of the practical use of the crystal.
The list of minerals used by Moos to build the table of hardness standards remained unchanged, however, to our time was supplemented by analogues that help diagnose test samples:
Talc. Talc is the softest mineral; the absolute hardness (determined by instrumental methods in the laboratory) is equal to one. It can be scratched with a fingernail (hardness of about 2.5 units). Graphite has similar parameters.
Gypsum. Gypsum is more than twice harder than talc, but it is almost as easily scratched with a fingernail. Mica, salt crystals, and some other chlorites are characterized by a similar level of hardness.
Calcite. He is calcareous spar, almost three times superior to mica in hardness, and no longer lends itself to fingernail. But it easily scratches the steel blade of a penknife (hardness of about 5.5 units), as well as copper and its alloys. Noble metals - pure gold, pure silver and also layered biotite are solid to the same extent as calcite.
Fluorite. Or fluorspar, lends itself to steel cutter and a glass fragment, but more than doubles the hardness of the previous reference mineral. Dolomite and sphalerite are characterized by similar hardness (but less aesthetic expressiveness of semiprecious stones).
Apatite. Apatite, a gem demanded by the jewelry industry, is not scratched by all steel. It is more than two times harder than fluorite. Window glass leaves a subtle scratch on the surface of apatite. Strictly brilliant hematite and dazzling blue lapis lazuli are as hard as apatite.
Orthoclase. Orthoclase, aka feldspar, is about twice as hard as apatite. Orthoclase itself already scratches glass, and lends itself only to carbide steels. Opal and rutile often growing in crystals are characterized by the hardness of orthoclase.
Quartz. Quartz is one and a half times harder than orthoclase. Quartz can be processed with corundum and diamond. The colored silica species - garnet - and two-tone tourmaline are no less solid than rock crystal (i.e. crystalline quartz).
Topaz. Topaz is one of the very hard gems. It is twice harder than quartz, and is similar in hardness to spinel and aquamarine.
Corundum. Corundum is four times harder than topaz. Along with corundum are tungsten carbide (in recent years, jewelers have learned to make impressive men's rings from this material), sapphires, rubies.
Diamond. Diamond is the champion of hardness among minerals of natural origin. It is four times harder than corundum, and even theoretically approaches the limit of possible hardness for any material objects.
When choosing materials for the full-scale version of the Mohs scale, one should know: minerals mined in different deposits can vary significantly in hardness. Australian diamonds are harder than South African. Sri Lankan sapphires are harder than any ruby, and Kashmir sapphires are softer...
It is useful for a practicing mineralologist to have a set of Mohs standards mounted in (for convenience) metal tubular frames in the arsenal of tools. You can do without the most expensive element of the scale — diamond — by replacing it with an elbor, an artificial material that is close to natural diamond in hardness.
Material diagnostics should be carried out with neat, short movements of the reference samples on a flat (and preferably smooth) surface. Scratches must be considered at optical magnification: the naked eye is not always able to distinguish which of the test materials crumbled and which survived.
Since the bulk of minerals of natural origin has a hardness in the range from 2 to 6 units, it is useful to start the test by scratching the test sample with apatite or glass (hardness 5).
It must be borne in mind that in some cases different planes of crystals of the same mineral may have different hardness indices (such, in particular, kyanite) - which in itself is a characteristic diagnostic feature.
Certain materials, the structure of which is different from crystalline, can give false research results. Hematite, found in the aggregate state of red ocher, may seem less solid than it actually is.
The verification of jewelry inserts, carried out in order to determine the authenticity of the stone, should affect the areas of the stone hidden from the observer by the frame. Leaving a scratch on the visible edge of the gem can spoil an expensive product!
In fairness, it should be noted that attempts to create an even more detailed scale of the relative hardness of minerals were made after Mohs - however, the real success fell on the universal scale of relative hardness created by Karl Friedrich Mohs.
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