On the relative fracture surface energies of the major and minor rhombohedra in low-quartz

The habits of both natural and synthetic crystals of low-quartz reveal that, under conditions of hydrothermal grówth, the major rhombohedronr {1 0 ¯1 1} is more developed than the minor rhombohedronz {01 ¯1 1}. The present work demonstrates that this preference ofr overz is reversed during fracture. It is also shown that neither the Hartman and Perdok 1] method nor that of Herring 2] can predict any difference in surface energy forr andz but that, by reducing the problem to the fracture or growth of a single SiO₄ tetrahedron, it is possible to account for both observations.