Abstract
Atoll Garnets in "Achtarandite" Serpentinites: Morphology, Composition and Mode of OriginAtoll garnets in aposkarn serpentinite from the Wiluy River, Republic of Sakha-Yakutia, Russia, have the classic form comprising a garnet core, an intermediate zone filled with chlorite-group minerals and an outer garnet atoll. The core of an illustrated example is complexly zoned from schorlomite to grossular-andradite. Morphologically, the core is a rhombic dodecahedral crystal. The atoll crystallized as a tetragon-trisoctahedron with minor rhombic dodecahedron faces and is composed of hibschite and "hydroandradite". The atoll garnet formed as the result of selective dissolution and substitution by chlorite of an internal hibschite zone with columnar structure that became unstable under new conditions of crystallization. The pattern of dissolution traces defects in the garnet crystal. The growth of the atoll garnets reflects the main stages in the evolution of the Wiluy deposit itself and is associated with the development of the Siberian traps.
Highlights
Beginning twelve years ago, during work on “achtarandite” represented by hibschite pseudomorphs of wadalite (Galuskin et al 1995; Galuskina et al 1998), we encountered minerals of the hydrogarnet group for the first time
Searching the literature on hydrogarnet, we discovered the valuable early work on crystal chemistry of hydrogarnets published by Professor Witoldabiñski in 1965 ( ̄abiñski 1965a, b)
Our friendship with Professorabiñski began which ongoing common scientific interests connected with research on the minerals of the garnet and vesu
Summary
Beginning twelve years ago, during work on “achtarandite” represented by hibschite pseudomorphs of wadalite (Galuskin et al 1995; Galuskina et al 1998), we encountered minerals of the hydrogarnet group for the first time. Atoll structures are characteristic of minerals of both metamorphic and metasomatic origin. They are three-fold structures comprising an internal core, an intermediate zone and an external ring – the atoll. Atoll structures characterizing metamorphic garnets (Rast 1965; Atherton, Edmunds 1966; Cooper 1972; Smellie 1974; Ushakova, Usova 1990; Homam 2003; Cheng et al 2007), hydrogarnets of metasomatic genesis (Galuskina et al 1998), magnetite in skarns (Grigoriev, Zhabin 1975) and chromite-magnetite spinel from serpentinite (Galuskina et al 2007a) have been described over the years. Electron Backscattered Diffraction (EBSD) and orientation contrast (OC) imaging contrast have been used with success in recent atoll garnet investigations (Homam 2003; Cheng et al 2007)
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