Optical anomaly in near-end-member grossular garnet from the Jeffrey mine, Asbestos, Quebec, Canada

Abstract

The optical anomalies, surface and lamellar textures of a birefringent grossular garnet crystal from the Jeffrey mine Canada have been investigated by optical polarizing microscope, electron-probe micro-analyzer (EPMA) and infrared spectrometer from the stand point of crystal growth. This grossular shows one-to-one correlation between surface features and its internal textures. The average chemical composition measured by EPMA is Grs97.4Alm1.2Sps1.2Pyr0.1 as a near-end-member grossular. The surface features correspond to the internal textures observed under crossed polarizers. Two growth hillocks with regular growth steps elongated parallel to [001] vertical direction of the (110) face, produce sectoral twins. Such growth steps indicate the orthorhombic symmetry of the crystal correlated with the growth direction. Some irregular or curved growth steps appearing as lamellae in (110) thin section are parallel to the sides of (110) face but inclined to the [001] growth direction; this suggests the monoclinic symmetry. The crystal is optically biaxial (+) and its 2Vx angle being close to 90° could not be measured. Infrared spectroscopy data revealed the presence of [(OH)4] group substitution at [SiO4], the tetrahedral site. Optical birefringence of the grossular is about 0.002. Back-scattered electron imaging could not detect any zonation and compositional differences of the lamellar texture.

Keywords

• grossular
• lamellar texture
• cation order
• optical anomaly
• garnet
• crystal growth.