Fluorite on Muscovite ( #7)

Pale green octahedral fluorites to 2 5/8" with pink corners on silvery foliated books of muscovite, ex Heliodor / Brad van Scriver.
Asia Nagar near Allahabad, Hunza Valley, Pakistan 5 ½" x 4 ½" x 4 ½"

 

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In 1812, Friedrich Mohs, a German mineralogist, created a scale of relative hardness by comparing the resistance to scratching of many minerals. Although there are other ways to measure hardness, the Mohs scale is the most easily applied and useful. The scale is not exactly linear (that is, the difference in Mohs hardness between numbers 2 and 3 is not the same as between numbers 4 and 5, or numbers 7 and B, for example). The minerals used by Friedrich Mohs and their hardnesses are:

1) Tacl 6) Orthoclase
2) Gypsum 7) Quartz
3) Calcite 8) Topaz
4) Fluorite 9) Corundum
5) Apatite 10) Diamond

The Mohs hardness, or "scratch" hardness, of a mineral is determined by attempting to scratch a specimen with something of known hardness (another mineral, a knife blade, a fingernail, etc.). If the specimen is scratched, it is softer than the tool used to scratch it. If not, it is harder. A fingernail has hardness about 2.5, a copper penny 3, a steel knife blade about 5.5, glass between 6 and 7, depending on quality. The fluorite in this specimen has hardness 4, by definition. Muscovite, one of the mica family of minerals, is considerably softer at hardness about 2.5. So fluorite would scratch muscovite, but not vice versa. Scratching a mineral involves breaking the weakest, or sometimes the fewest, atomic bonds. Thus the weakest bonds in fluorite are stronger than the weakest bonds in muscovite, although the strongest bonds in muscovite are stronger than any in fluorite. It's the weak bonds, along with their systematic arrangement within the crystal structure, that determine hardness.