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Hardness - harder substance will scratch or cut into a...

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Figure 1.11 Augite. A dark green to black, rock-forming, pyroxene mineral. It has two directions of cleavage at 87° and 93°. (Photo by E. J. Tarbuck) Figure 1.13 Plagioclase feldspar, variety labradorite. It has two directions of cleavage with striations commonly visible on one cleavage surface. (Photo by E. J. Tarbuck) most minerals with a metallic luster are opaque, while vitreous minerals are either translucent or transparent. Figure 1.12 Potassium feldspar, variety microcline. The potassium feldspars are common rock-forming minerals with various colors (commonly pink, cream, or white) and two directions of cleavage at nearly 90°. (Photo by E. J. Tarbuck) Hardness Hardness, one of the most useful diagnostic properties of a mineral, is a measure of the resistance of a mineral to abrasion or scratching. It is a relative property in that a
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Unformatted text preview: harder substance will scratch, or cut into, a softer one. In order to establish a common system for determining hardness, Friedrich Mohs (1773-1839), a German mineralogist, developed a reference scale of mineral hardness. The Mohs scale of hardness (Figure 1.14), widely used today by geologists and engineers, utilizes ten index minerals as a reference set to determine the hardness of other minerals. The hardness value of 1 is assigned to the softest mineral in the set, talc, and 10 is assigned to the hardest mineral, diamond. Higher-numbered minerals will scratch lower-numbered minerals. For example, quartz, with a hardness of 7, will scratch calcite, which has a hardness of 3. It should be remembered that Mohs scale is a relative ranking and does not imply that mineral number 2, gypsum, is twice as hard as mineral 1, talc....
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