X-ray single-crystal and infra-red spectral studies of vanadium-bearing garnets from the Menipa mountain mass, central part of the Sør Rondane Mountains, East Antarctica were carried to determine crystal structures and examine existence of OH-group. Three modes of occurrences of V-bearing garnet are recognized: 1) large porphyroblast (type-1); 2) fine crystal (type-2) in kelyphite rim around type-1 garnet; 3) fine-grained porphyroblast (type-3) in matrix. The structure refinements of type-1 and type-3 garnets with the space group of
Ia-3
d were reduced with
R1 of 0.0278 and 0.0689, respectively. The site occupancies determined using both X-ray diffraction data and electron microprobe analysis data are:
VIII[Ca0.976Mg0.017M0.007] and
VI[Al0.930V0.040Cr0.01Ti0.015 Fe0.005] for type-1 garnet, and
VIII[Ca0.885Mg0.027M0.057Fe0.030] and
VI[Al0.315V0.520Cr0.10Ti0.015 Fe0.05] for type-3 garnet. Mean interatomic distances are Type-1 <
X-O> = 2.407(1) Å, <
Y-O> = 1.931(1) Å, <
T-O> = 1.651(1) Å, andType-3 <
X-O> = 2.426(2) Å, <
Y-O> = 1.989(2) Å, <
T-O> = 1.655(2) Å. The increase of <Y-O> distances is caused by the substitution of V
3+ for Al
3+. By the expansion of the octahedral site with this substitution, the unit-cell parameters increase from
a = 11.8686(8) Å of type-1 garnet to
a = 11.9694(6) Å of type-3 garnet. The mean <
T-O> distances of 1.651-1.655 Å are longer than ordinary Si-O distances in silicates. It has been known that the substitution of OH in garnet structure results in an expansion of the tetrahedral and a corresponding increase in the unit-cell parameter (Cohen-Addad et al. 1967). In fact, OH absorptions in the wavenumber regeion 3500-3700 cm
-1 were confirmed in the infrared spectra of type-1, -2 and -3 garnets.
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