Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets

Abstract

Andradite, Ca 3 Fe 2 (SiO 4 ) 3 , single crystals up to 0.4 mm were synthesized in a piston cylinder device from a glass at 1473 K and 20 kbar. The samples were characterized optically, by single-crystal infrared and by 57 Fe Mossbauer spectroscopy. Precise single-crystal X-ray diffraction data between 100 and 500 K were collected from a synthetic crystal and a natural nearly end-member andradite with a low OH content. Both samples yielded similar oxygen coordinates (within 5 esd's) and anisotropic displacement parameters (within 5 esd's). In addition, a 500 K single-crystal X-ray data set on synthetic pyrope, Mg 3 Al 2 (SiO 4 ) 3 , was measured for crystal chemical comparisons. A slight temperature dependent out-of-centre displacement was observed for Ca in andradite, in spite of the Ca overbonding on the dodecahedral X-site. This X-site disorder is less than the stronger'rattling effects' of Mg and Fe 2+ in pyrope and almandine, respectively, but still measurable. Anisotropic displacement parameters of oxygen were successfully modelled by rigid tetrahedral librations and translations. Structural strain in silicate garnets is discussed on the basis of the bond valence model. There is no difference in strain between Ca garnets refined in space group Ia3d and I1. It is suggested that strain relaxation in Ca-rich garnets may be accomplished by a small pyralspite or hydrogarnet substitution