Field of Science

Carl Friedrich Christian Mohs's Mineralogical Legacy

Carl Friedrich Christian Mohs, lithography by Joseph Kriehuber (1832).

Talc – Gypsum – Calcite – Fluorite – Apatite – Feldspar – Quartz – Topaz – Corundum – Diamond - the Mohs Scale of Mineral Hardness is familiar to rock-hounds and earth-science students alike. The ten-point hardness scales lists common minerals in the order of the relative hardness, with talc being the softest and diamond the hardest mineral found in nature.

The Mohs scale is named after German mineralogist Carl Friedrich Christian Mohs, born January 29, 1773, in the town of Genrode, at the time part of the principality of Anhalt-Bernburgs. After attending school, he worked in his father's business as a merchant, but in 1796 he went to the University of Halle to study there mathematics, physics and chemistry. He continued his studies at the famous Royal Saxon Mining Academy of Freiberg, where he studied under the renowned geognost Abraham Gottlob Werner. Werner published in 1787 a »Kurze Klassifikation und Beschreibung der verschiedenen Gesteinsarten« - Short classification and description of the various rock types - as a guide for identifying and classifying rocks and minerals. Unlike other mineralogists at the time, mostly using chemical analysis, Werner uses easily recognizable features, like color or crystal shape, to classify minerals and rocks. Mohs is impressed by Werner's approach. In 1804, he publishes himself a “student-friendly” classification chart for minerals, based on his experience in the mining district of the Harz mountain and as a consultant for wealthy mineral-collectors. In his book »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 outer properties - Mohs combines various physical properties of minerals, like color, hardness and density, with six classes of crystal shapes, to identify 183 different minerals.

Mohs scale of hardness sets from the 19th century, Mohs's geological hammer, and a letter to his wife.

Mohs continues to travel, collect material and improve his mineral classification system. He visits Štiavnica in Slovakia, famous for the local Mining Academy, and the mining district of Bleiberg in Carinthia. He visits and studies mines in Hungary, Transylvania and Scotland, and quarries in Germany and Austria.

In 1812, now a professor in the Austrian city of Graz, he creates a preliminary hardness scale and continues to publish guidelines for mineral identification. In 1818 he returns to Freiberg and between 1822-1824 Mohs publishes his final version of the hardness scale in the book »Grund-Riß der Mineralogie« - Essentials of Mineralogy.

The Mohs scale of mineral hardness is based on the ability of one natural sample to scratch another sample visibly. The samples of matter used by Mohs are readily available to a student or miner. Minerals with a hardness of 1 or 2 can be scratched with a fingernail. A coin will scratch minerals with a hardness of 3, the blade of a pocket knife scratches minerals of the hardness 5 and 6. Glass will scratch minerals with a hardness of 7, and harder minerals scratch each other.

Calcite crystals, example of a common mineral with hardness 3.