Grandidierite Inclusions in Sapphires
Recently the authors independently encountered an inclusion that has not been previously reported in sapphire. Both stones contained colorless crystals that were identified by Raman analysis as the mineral grandidierite. The sapphire examined by author MH in Carlsbad, California, contained colorless lath-like inclusions (figure 1). The sapphire examined by author EBH in Bangkok contained a colorless crystal that reached the surface of the sapphire host and displayed a duller luster in reflected light, and birefringent interference colors when examined using cross-polarized light (figure 2). Both observations are consistent with what one would expect for grandidierite.
Grandidierite, named after French naturalist Alfred Grandidier (1836–1921), is an extremely rare orthorhombic Mg-Fe aluminous borosilicate with the formula (Mg,Fe)Al3(BO3)(SiO4)O2. The mineral is described as bluish green to greenish blue; the blue color increases with Fe content. It is transparent to translucent with a pale yellow to colorless, greenish blue, and blue trichroism. Since its discovery, grandidierite has been found as a rare accessory mineral in aluminous boron-rich pegmatite; in aplite, gneiss, and crystalline rock associated with charnockite; and in rock subjected to local high-temperature, low-pressure metamorphism (contact aureoles and xenoliths). To the authors’ knowledge, these are the first observations of grandidierite as an inclusion in sapphire.