Epitaxial lateral overgrowth (ELO); the mechanism of formation of scepter, skeletal, cathedral and related quartz morphologies

Scepter quartz consists of pseudo-hexagonal prismatic trunk and a larger but shorter, pseudo-hexagonal bipyramidal prismatic cap. The so-called skeletal, cathedral, and other related morphologies of quartz crystals belong to the same category. Modes of occurrence of scepter quartz in a druse, morphological and surface microtopographic differences, as well as differences in crystal clarity, distribution of inclusions and perfection are compared between trunk and cap portions of scepter quartz of hydrothermal and pegmatitic origins. It has been clarified that the surface of trunk portion was masked by precipitation of foreign minerals like clay and the growth of trunk portion was interrupted. Then cap portion grew from a newly supplied purer hydrothermal solution. Epitaxial lateral overgrowth (ELO) is concluded as the most reasonable mechanism for the formation of these morphologies of quartz crystals. This conclusion was verified by a series of hydrothermal experiments to reproduce scepter quartz artificially, using masked and unmasked seed crystals.