Transmission electron microscope study of bubbles and dislocations in amethyst and citrine quartz

Abstract

Crystals of amethyst and citrine that have been turned milky (or turbid) by annealing at temperatures above 600°C for a few hours have been examined by transmission electron microscopy. It has been found that the milkiness is due to light scattering from nearly spherical bubbles whose diameters vary from about 200 to 1000 A. On annealing for longer periods the bubbles became crystallographic in shape ("negative crystals"). An expression governing the equilibrium shape has been derived and used, together with the electron micrographs, to estimate the ratio of the surface energies of the faces of the negative crystals. There is evidence that the bubbles contain H20 molecules. Spherical bubbles are not in equilibrium and they are frequently linked by dislocations. The origin of these dislocations is discussed. For comparison, crystals of natural milky quartz have also been examined.