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Discussion |
Geography Department, Birmingham University
A. J. Gerrard writes: In his paper, the author provides valuable new information on the hardness of minerals and establishes, quite convincingly, that the grades on Mohs' scale of hardness are spaced in a generally systematic manner. This is useful evidence, largely substantiating the work carried out in the Department of Mining Geology at the Royal School of Mines, London in the 1960s and reported by Duncan (1969); however, this work seems to have been overlooked. The ten minerals on Mohs' scale were investigated using both Vickers and Knoop diamond indenters. A virtually linear relationship was obtained when the logarithm of Mohs' hardness number was plotted against the Vickers' microhardness value. This allowed a recomputation of Mohs' numbers which established that the numbers are far from arbitrary and are spaced in a reasonably systematic manner, except possibly for the value for diamond. In comparing these results with those presented by West, the only difference is that the Vickers' microhardness value places orthoclase feldspar closer to apatite and further away from quartz. Apart from this discrepancy the results are very similar. It is not possible to compare the results further because no detailed information is provided concerning the way in which the experiments were conducted. It is well known that mineral anisotropy affects hardness, different values being obtained on different crystal faces as well as quite considerable variation on individual faces. Hardness variations are also affected by the relations between cleavage plane orientations and the direction of indentation. Presumably this accounts
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