'Super rainbow garnet' from Tenkawa, Nara Prefecture, Japan

Iridescent andradites are extremely rare and have been reported from only a few localities in the world. Recently, an enormous volume of iridescent andradites were found from near Kose mine, Tenkawa, Nara Prefecture, Japan. In the present study, textures of this newly-found 'Super Rainbow Garnet' are first examined with optical microscopy, SEM-EDX and TEM.<Br> Iridescent andradite used in the present study occurs as brown translucent euhedral crystals with well-developed {110} faces about 1 cm across. Greenish schiller is noticeable. A crystal of the garnet, whose average chemical composition is And_96.0Grs_3.3Sps_0.7, was cut perpendicular to the [001] axis. In the (001) thin section, several {110} sectors with banding textures parallel to {110} faces were observed. Strong iridescence was observed in four sectors – (011), (-101), (0-11) and (101) sectors, whereas iridescence was not observed in the other four – (110), (1-10), (-1-10) and (-110) sectors. In the latter four sectors, {110} layers are perpendicular to the thin section, and therefore parallel to the transmitted light. This fact suggests that the interference of light with {110} layers originate the iridescence.<br> The (011) sector in this thin section was thinned by ion milling and observed with TEM. Fine lamellar texture can be observed. The layer texture was found to be composed of alternating major and minor lamellae with very sharp interfaces between the adjacent lamellae. The major lamellae are 100 to 300 nm in thickness and the minor lamellae are about 20 nm. The origin of the fine lamellar texture is probably exsolution as proposed by Hirai and Nakazawa (1986) or oscillatory zoning by Akizuki et al. (1984). In either case, it can be concluded that iridescence of the present Tenkawa garnet should be due to these fine {110} layer texture.