ATOLL HYDROGARNETS AND MECHANISM OF THE FORMATION OF ACHTARANDITE PSEUDOMORPHS

The atoll hydrogarnets have been observed in achtarandite pseudomorphs from the Wiluy deposit in Yakutia, Russia. Atoll structure in hydroga rnets is described for the first time. Core, intermediate zone and rin g (atoll) are the structural components of the atoll hydrogarnets. Cor e and internal zone of the ring are hydrogrossulars with compositions of Grs(80)Adr(20) and Grs(74)Adr(24)Pyr(2), respectively. External zon e of the ring is hydroandradite (Adr(58)Grs(37)Pyr(5)). Atoll hydrogar nets are involved in pole-like formations and curved plates, which are elements of the hydrogarnet skeleton of achtarandite pseudomorphs. Th ey formed as a result of the epitaxial replacement of protoachtarandit e (wadalite) by hydrogarnet. Model of the atoll hydrogarnet growth is proposed, which explains a mechanism of the formation of achtarandite pseudomorphs.