Morphology of Natural and Synthetic Ametrine. As<br>seen in most natural amethyst, natural ametrine<br>crystals typically have a prismatic habit, elongated<br>parallel to the optic axis (c-axis), and are terminated<br>by the rhombohedral faces r {101–<br>1} and z {011–<br>1} (figure 8). In some crystals, both of these rhombohedra<br>are equally developed, so that the terminations<br>resemble hexagonal pyramids; a small, rough c<br>(0001) face is occasionally present (Vasconcelos et<br>al., 1994). Prism faces m {101–<br>0} are commonly<br>developed, and are striated at right angles to the caxis. Rarely, other faces are developed, such as the<br>trigonal prism x {112–<br>0} and trigonal dipyramid s<br>{112–<br>1}; if present, they are very small.Unlike natural ametrine, synthetic ametrine<br>crystals are usually tabular, because a seed plate is<br>used to start the growth. As noted earlier, the crystal morphology varies depending on the characteristics of the seed plate and the growth conditions.<br>The crystals we examined for this study, all grown<br>on ZX-cut seeds, were bounded mainly by m and r<br>faces (figure 9); z faces were smaller, and the +x and<br>−x faces were seldom present. A short steel wire<br>used to suspend the seed plate in the autoclave<br>was usually present at one end of the crystals.
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