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The idea discussed in the paper you reference is about transferability of Mohs ordinal scale to true hardness. A mineral's ability to scratch is related to it's hardness, but only an indentation test measures absolute hardness (e.g. the Vickers scale). Mineral orientation relative to planar structure can change the result in a scratch test, and two minerals ...


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It may be difficult to ascertain the hardness of a mineral relative to an other mineral in a Mohs streak test if one (or both) are not homogeneous. Already a small local grain of an impurity may put you off; you might remember the occasions of the sudden scratching sound of chalk on the green board. Which is why pen-shaped hardness picks are advantageous: ...


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