Behavior of Quartz during Indentation

The ductility of single crystals of quartz has been studied using identation techniques. Indentations made by a Knoop diamond pyramid were studied by interferometry, X-ray, and electron microscopy. Large permanent deformation essentially free of gross fracturing characterizes identations less than 1/10 mm. in size. Within such identations, where effective confining pressure exerted on the quartz is of the order of 20 kb., closely spaced planar features occur which are parallel with {11̄011} and {0001}. These features are offset in shear and, at a surface, closely resemble slip lines in metals and NaCl. They are parallel with the sides of the pressure figures produced by Shubnikov and Zinserling with a spherical indenter. Strength of quartz as calculated from hardness is close to that measured in conventional compression tests. Hardness decreases with impurity content and with temperature. Most synthetic quartz is softer than the natural quartz studied. Impurity concentrations of the order of a few 100 p.p.m. measurably influence hardness. From analysis of various microscopic and macroscopic observations it is concluded that the apparent ductility observed is more likely due to systematic microfracturing on a scale too small to be observed, rather than to slip.