EPITAXY OF ACHTARANDITE ON GROSSULAR - THE KEY TO THE PROBLEM OF ACHTARANDITE

Achtarandite, a pseudomorph after an unknown mineral of the point grou p of ($) over bar 43 m, was discovered in the epitaxial growing with z onal grossular crystals (a = 11.881-11.905 Angstrom, grs(65)adr(35)-gr s(77)adr(23)) in the hydrogrossular serpentinite from Yakutia, which w as formed after skarned clay-carbonatic rock. Hydrogrossular (a = 11.9 62-12.061 Angstrom, grs(71)adr(16)pyr(13)-grs(94)adr(6)) is transforme d by epitaxy to achtarandite forming we-like crystals. It is assumed, that protoachtarandite was formed during a high temperature contact, c omparable with a process in cement kilns. Protoachtarandite is propose d to be a mineral close to mayenite (12CaO . 7Al(2)O(3), I ($) over ba r 43d, a = 11.98 Angstrom).