Abstract:
Problem of formation of achtarandite — the pseudomorph after an unknown mineral from Vilyuiskoye deposit (Vilyui River, Yakutia) is well known for a long time. Various minerals were named as primary ones for achtarandite, which is characterized by various forms of its facing; there arc among them tritetrahedron {112}, combining forms of positive and negative trigontritetrahedra (112) + (112) and their twins of intergrowth. All these forms have the visible summetry 43m. However, there is a suggestion that the protoachtarandite could crystallized in the same symmetry type as a garnet — т3т (Kuznetsova, Shafranovsky. 1966). The authors have specified various intermediate forms of transiuons from trigontriterahedral achtarandite to tetragontrioctahedral one; and they connected the hypomorphy of achtarandite with development of skeletal forms of a garnet-like mineral or the mere garnet. In 1995 skeletal-zonal garnet crystals with trigontriterahedral habit have been discovered at the deposit. Zoning from the center to outer part, they are represented by: the Ti grandite (110) core — hydrogarnet {110} dendrites of microcrystals — grossular with combination of forms {211}({112}) + {110}. Grossular replaces crust of skeletal hydrogarnet-achtarandite crystals and forms the case. In general, the reported study of their morphology has approved the concept of V. Kuznetsova and I.I. Shafranovsky: formation of achtarandite crystals should be connected with skeletal growth of hydrogarnet.