The basic principal is, that it characterises the scratch resistance of a softer mineral being scratched a harder mineral.
Developed in 1822 by Friedrich Mohs a German Geologist and mineralogist, it is one of several methods used to describe the hardness of minerals and materials in science, however it is the one most commonly used by geologists. Although it has been 200 years since the first implementation of the Mohs hardness scale, the method of comparing one mineral to another to ascertain its hardness is much older. The technique was mentioned in 300BCE by Theophrastus in his treatise On Stones, and then again in 77CE by Pliny the Elder in his Naturalis Historia.
The scale each uses a different mineral, and is ordinal in nature. The hardest known substance to man at the time of the scales creation was diamond, and as such diamond is at the top of the scale. The complete scale is as follows;
TALC- Mohs hardness 1- Absolute hardness 1
GYPSUM- Mohs hardness 2- Absolute hardness 3
CALCITE- Mohs hardness 3- Absolute hardness 9
FLOURITE- Mohs hardness 4- Absolute hardness 21
APATITE- Mohs hardness 5- Absolute hardness 48
ORTHOCLASE FELDSPAR- Mohs hardness 6- Absolute hardness 72
QUARTZ- Mohs hardness 7- Absolute hardness 100
TOPAZ- Mohs hardness 8- Absolute hardness 200
CORRUNDUM- Mohs hardness 9- Absolute hardness 400
DIAMOND- Mohs hardness 10- Absolute hardness 1600
Whilst it is possible to buy a set of “Mohs mineral pencils” a set of pencils each mounted on the tip with a small piece of each of the minerals above, these for geologists are often more show pieces, and in the field it is important to know the hardness of ordinary materials to make mineral identification easier. A steel knife (very good for many applications in the field) has a hardness of about 6-6.5 whist a fingernail has a hardness of between 2 and 2.5. Just these two tools can make mineral identification so much easier!
0 Commentaires