Synthesis and crystal chemistry of mukhinite, V-analogue of clinozoisite on the join Ca2Al3Si3O12(OH)–Ca2Al2VSi3O12(OH)

Abstract

This is the first report of the crystal structure of mukhinite, V-analogue clinozoisite, on the join Ca2Al3Si3O12(OH)–Ca2Al2V3+Si3O12(OH) synthesized at 1.5 GPa and 800 °C. The study was performed to clarify the distribution of V3+ among structurally independent octahedral M1, M2, and M3 sites, and the effect of V3+ on the crystal structure. Mukhinite and V3+-bearing clinozoisite in all run products are associated with zoisite, and also coexist with V-bearing phases such as vanadomalayaite, goldmanite, V-oxides, and unidentified Ca–Al-bearing vanadates. Mukhinite and V3+-bearing clinozoisite crystallized in the Run 20 product show a compositional gap between 0.33 and 0.74 V atoms per formula unit (apfu), and the V content attains 1.14 apfu. The coexistence of low V3+- and high V3+-clinozoisites indicates the presence of a miscibility gap at 1.5 GPa and 800 °C. Two mukhinite crystals with 0.75 and 0.83 V3+ apfu were used for X-ray single-crystal structure analysis. The unit-cell parameters are a = 8.8995(2), b = 5.6299(1), c = 10.1532(2) A, β = 115.327(1)°, and V = 459.81(2) A3 for the former, and a = 8.8999(1), b = 5.6357(1), c = 10.1499(1) A, β = 115.306(1)°, and V = 460.24(2) A3 for the latter. The resulting V3+ occupancies among the octahedral sites are M1(Al0.894(6)V0.106)M2(Al0.976(6)V0.024)M3(V0.621(6)Al0.379) for the former and M1(Al0.868(4)V0.132)M2(Al0.957(4)V0.043)M3(V0.652(2)Al0.348) for the latter. Site preference of V3+ at the octahedral sites is M3 > M1 > M2 as that of Fe3+ and Mn3+. The intracrystalline partition coefficient of V3+ and Al3+ between the M1 and M3 sites, KD = (V3+/Al)M1/(V3+/Al)M3, is 0.07–0.08, which is greater than those of Fe3+ and Al3+ (0.03–0.05) and of Mn3+ and Al3+ (0.04–0.06). Variations of the unit-cell parameters are strongly related to the variations of the M3−Oi and M1−Oi distances.