Mohs hardness scale
The Mohs hardness scale is a tool developed in 1822 by Austrian mineralogist Friedrich Mohs to measure the hardness of minerals based on their resistance to scratching. The scale consists of ten common minerals ranked from 1 (the softest, talc) to 10 (the hardest, diamond), allowing for a relative comparison of hardness. This scale is particularly useful in the field of mineral identification, as the ability of one mineral to scratch another indicates its hardness. For example, a mineral that can scratch another is considered harder than the latter.
Testing involves sliding a pointed corner of one mineral across the surface of another, with clear indications of hardness differences emerging through scratches or smears left behind. In practice, common objects, such as hardness pencils with mineral tips, assist jewelers and gemologists in determining the hardness of gemstones. While the original scale remains widely recognized, modern adaptations have introduced new index minerals for industrial purposes, elevating minerals like quartz and garnet to higher rankings and incorporating synthetic materials such as silicon carbide and boron carbide. Understanding the Mohs hardness scale provides valuable insights into the physical properties of minerals and their applications in various fields.
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Mohs hardness scale
The Mohs hardness scale, proposed in 1822, provides a method of ranking minerals according to their relative hardness and thus is a way to help identify them.
Definition
The resistance of minerals to abrasion or scratches is a valuable diagnostic physical property used in mineral identification. In 1822, Friedrich Mohs, an Austrian mineralogist, developed a relative scale of mineral hardness. This scale consisted of ten common minerals that were ranked from 1 (softest) to 10 (hardest). The values assigned to each member of the scale indicate the relative hardness of the minerals. Intervals between minerals in the scale are approximately equal, except between nine and ten.
Overview
The resistance of a mineral to scratch is tested by sliding a pointed corner of one mineral across the smooth surface of another mineral. If the mineral with the point is harder, it will cut or scratch the other mineral. The scratch should be as short as possible, not more than five or six millimeters. If the pointed mineral is softer, a smear or powdered residue is left on the flat surface of the other mineral. This residue could be mistaken for a scratch; however, the smear can be easily rubbed off. A mineral from the high end of the scale will usually produce a significant “bite” on the softer mineral. Two minerals that have the same hardness will scratch each other equally well. Common objects are sometimes used as aids in hardness determination.
Brass rods set with conical-shaped fragments of test minerals on the ends are sometimes used to determine the hardness of small specimens and gemstones; these rods are known as hardness pencils. Most gems, with the exception of pearls, have a hardness of 6 or above. In testing rough and uncut gems, some jewelers use these pencils to determine the specific hardness of the stones. Other minerals, such as chrysoberyl, epidote, olivine, and zircon, are included with the set of instruments. Six test pencils are sometimes conveniently arranged in a hardness wheel.
With the advent of extremely hard manufactured abrasives in the second half of the twentieth century, a new sequence of index minerals was proposed for the upper part of the Mohs scale. This modified Mohs scale has found some use in industry. In this scale, quartz was elevated to 8, garnet was introduced as 10, and corundum was elevated to 12. Diamond, the hardest naturally occurring substance derived from the Earth, topped the scale at 15. The artificial abrasives silicon carbide and boron carbide were designated as 13 and 14, respectively. Silicon carbide is produced by heating a mixture of carbon and sand in a specially designed electric furnace. Boron carbide, the hardest known substance next to diamond, is manufactured in an electric furnace from coke and dehydrated boric acid.
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"Mohs Hardness Scale." National Park Service, 12 Apr. 2023, www.nps.gov/articles/mohs-hardness-scale.htm. Accessed 23 Dec. 2024.