Equilibrium form of negative crystals in igneous quartz

Morphology and internal textures of negative crystals in igneous quartz were studied by combining microscopic observation and annealing experiments of natural specimens. Judged from its symmetry and the annealing experiments, the equilibrium form of the negative crystals is estimated to be rounded bipyramidal. The rounded bipyramidal shape suggests that only rhombohedral faces are singular under the growth conditions of quartz crystals in felsic magmas. Thermal history of these crystals after the eruption of magmas was estimated by the morphology and the internal texture of the negative crystals.