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When Rock Classification is not hard anymore, thank Mohs Scale of Hardness

The views expressed are those of the author and are not necessarily those of Scientific American.


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Talc – Gypsum – Calcite – Fluorite – Apatite – Feldspar – Quartz – Topaz – Corundum – Diamond -  “Mohs Scale of Mineral Hardness ” should be familiar to rock-hounds and earth-science students alike, as it lists common minerals in the order of relative hardness (talc as the softest and diamond as the hardest mineral occurring in nature). Almost all  basic classification charts include this scale, as mineral hardness can be a quite useful criteria to identify unknown minerals and can be easily determinate also in the field (a steel blade corresponds to fluorite and a piece of glass to quartz).

Mohs scale  is appropriately named after the German mineralogist Carl Friedrich Christian Mohs (lithograph by Joseph Kriehuber, 1832), born January 29, 1773 in Gernrode (at the time located in the principality of Anhalt-Bernburg), son of a middle-class family.


After school he worked in his father’s business as merchant, but in 1796 he went to the University of Halle to study  mathematics, physics and chemistry. He continued his studies at the famous Royal Saxon Mining Academy of Freiberg, where he studied under the even more famous geologist Abraham Gottlob Werner. Werner had published in 1787 a “Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten” (Short classification and description of the various rock types), a classification guide that used – unusual at a time when most rocks were classified based on the complex rock-chemistry -easily recognizable features (like color)  to identify minerals and rocks.

Mohs was impressed by the approach of Werner and in 1804 published himself a “student-friendly” classification chart for minerals, based on his experience in the mining district of the Harz and as consultant for wealthy mineral-collectors.
In the work”"Über die oryktognostische Classification nebst Versuchen eines auf blossen äußeren Kennzeichen gegründeten Mineraliensystems” (The genetic-geological classification and an attempt to introduce a mineral-system based on superficial properties) Mohs combines various physical properties of minerals (like color, hardness an density) with 6 classes of crystal shapes  (in part in use even today) to identify 183 different minerals.

Fig.2. and 3. Specimens of Quartz (Mohs hardness 7) and Calcite (Mohs hardness 3), both minerals are common and can be very similar in shape and color, however they are easily recognizable by the different hardness, calcite can be scratched with a knife blade, quartz not.

After 1812, now as a professor in the Austrian city of Graz, he continued to improve his mineral classification scheme and to publish guidelines for mineral identification. In 1818 he succeeded Werner and became professor in Freiberg and between 1822-1824 Mohs finally published his famous hardness scale in the book  “Grund-Riß der Mineralogie” (Essentials of Mineralogy).

Bibliography:

HÖLDER, H. (1989): Kurze Geschichte der Geologie und Paläontologie – Ein Lesebuch. Springer Verlag, Heidlberg: 243
WAGENBRETH, O.(1999): Geschichte der Geologie Deutschland. Georg Thieme Verlag: 264

David Bressan About the Author: Freelance geologist dealing with quaternary outcrops interested in the history and the development of geological concepts through time. Follow on Twitter @David_Bressan.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Spironis 12:54 pm 01/29/2014

    A drop of vinegar would also separate quartz from calcite.

    Link to this

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