Characterization of oriented inclusions in cat's-eye, star and other chrysoberyls

Milky-appearing alexandrite samples from Tanzania (Lake Manyara) and India (Kerala) were examined, as were chatoyant and asteriated chrysoberyl/ alexandrite from India (Orissa), Brazil, Madagascar and Sri Lanka, and also phenomenal synthetic alexandrite from Kyocera in Japan. Sixteen oriented thin sections were studied by a combination of optical microscopy, micro-Raman spectroscopy, and electron microprobe techniques employing backscattered electron (BSE) imaging, qualitative point analysis and Ti compositional mapping. Rutile was identified as needle-like inclusions and V-shaped platelets in planes perpendicular to the a-axis, elongated parallel to the c-axis, and parallel to symmetry-equivalent <011> directions. Additional structures consisting of rutile needles and/or channels in various samples were oriented parallel to the a-axis. Also observed were single or multiple-intergrown rutile platelets elongated parallel to the a-axis, and rutile platelets showing rectangular, L-shaped or zigzag crosssections parallel to <012> directions. In a four-rayed star chrysoberyl, elongated ilmenite particles were oriented along the c-axis. Various optical effects such as a whitish appearance, asterism or chatoyancy are caused by, and depend upon, the presence, size and concentration of the combined different types of needlelike or platy inclusions.