Hibschite topotype: crystal chemical characterization
Rinaldi, Romano; Passaglia, Elio
European Journal of Mineralogy Volume 1 Number 5 (1989), p. 639 - 644
published: Nov 16, 1989
manuscript accepted: Apr 20, 1989
manuscript received: Oct 27, 1988
ArtNo. ESP147050105004, Price: 29.00 €
Abstract : A topotype hibschite-bearing sample from Marienberg, Bohemia, Czechoslovakia shows the occurrence of very small garnet grains (approx. size 60 μm) consisting of two phases: a colourless octahedral outer shell with the composition: Ca3( Al1.61Fe3+0.22Mg0.10)(Si1.50S0.17)O6.64(OH)5.36 grown over a dark rhombic-dodecahedral core having the composition: (Ca2.87Mg0.03Fe2+0.10)(Fe3+1.12Al0.33Ti0.31Fe2+0.25)(Si2.60Al0.06)O10.65(OH)1.35. The outer shell can be identified as the mineral species hibschite, corresponding to a hydrogarnet with approximate composition grs83adr17 and a hydration degree of 44.7 %, i.e. considerably higher than that previously inferred. The X-ray cell dimension refinement (a = 12.22(1) Å) is in good agreement with its hydration degree. The dark inner core corresponds to a much less hydrous phase (hydration degree 11.3 %) of approximate composition adr84grs16 which, owing to its Ti content, may be classified as a melanite.