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Knoop hardness numbers for 127 opaque minerals

Forbes Smith Robertson
Knoop hardness numbers for 127 opaque minerals
Geological Society of America Bulletin (1961) 72 (4): 621-637


Knoop identation 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 1,500. Bismuth (0.7) is the softest mineral tested, berthierite (1,140) 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 (5-5 1/2 on the Mohs scale), and goethite, 476 (5-5 1/2 on the Mohs scale), and softer than magnetite, 536 (5 1/2-6 1/2 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; huebnerite 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.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 72
Serial Issue: 4
Title: Knoop hardness numbers for 127 opaque minerals
Pages: 621-637
Published: 1961
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Accession Number: 1961-006913
Categories: Mineralogy
Document Type: Serial
Bibliographic Level: Analytic
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
Update Code: 1961
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