Knoop indentation hardness numbers may be obtained on polished mineral grains as small as 10 microns in diameter. This paper gives average Knoop numbers for 127 opaque minerals and quartz. The limits of tests and the average hardness number are plotted on a logarithmic scale on which the lines representing the hardness range for a given mineral are reasonably comparable throughout the range from 1 to 1500.

Bismuth (0.7) is the softest mineral tested, berthierite (1140) the hardest. Other representative hardnesses (Talmage-scale equivalents in parentheses) are: argentite, 26 (A); galena, 75 (B); chalcopyrite, 175 (C: tetrahedrite, 299 (D); niccolite, 460 (E); magnetite, 528 (F); ilmenite, 965 (G). Quartz, 467 (7 on the Mohs scale), is approximately equivalent on the Knoop scale to niccolite, 460 (5–5½ on the Mohs scale), and goethite, 476 (5–5½ on the Mohs scale), and softer than magnetite, 536 (5½–6½ on the Mohs scale).

Some minerals exhibit striking differences in hardness in different crystallographic orientations. Molybdenite has a hardness of 12 parallel to {0001} cleavage and 60 at right angles to the cleavage; hucbnerite is 313 parallel to {010} cleavage and 463 at right angles to cleavage; enargite is 154 parallel to {110} cleavage and 378 at right angles to cleavage.

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