A new method for studying cleavage in optically uniaxial materials by means of the universal stage was applied to quartz. Statistical analysis of the results indicates that low quartz, when crushed, cleaves most readily parallel to r {1011} or z {0111} and next most readily parallel to ξ {1122}. Less pronounced cleavages exist parallel to c{0001},s{1121}, x {5161}, m {1010} and, perhaps, j{3032} and a {1120}. The direction d {1012}, though reported in the literature, was not observed as a direction for cleavage. The conchoidal fractures of quartz may represent submicroscopic combinations of cleavage planes ξ,z, r, c, s, x, m and a.

The number of Si—O bonds cut per unit area was determined for a set of rational planes in the (0001): (1120) zone and a second set in the (0001): (1010)zone. Of the first set of planes, ξ cut the fewest bonds; of the second set, r cut the fewest bonds, cutting even fewer than ξ.

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