[S-CG52] Petrology, Mineralogy & Resource Geology

[SCG52-P02] Crystal chemistry of synthetic mukhinite, V-analogue of clinozoisite, on the join Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂VSi₃O₁₂(OH)

*Mariko Nagashima¹, Daisuke Nishio-Hamane², Nobuhiko Nakano³, Toshisuke Kawasaki⁴ (1.Graduate school of Sciences and Technology for Innovation, Yamaguchi University, 2.The institute for Solid State Physics, the University of Tokyo, 3.Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 4.Faculty of Science, Ehime University)

Keywords:mukhinite, clinozoisite, synthesis, crystal chemistry, Raman spectroscopy, TEM

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 ºC (Run 20). 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. Although epidote-supergroup minerals occur widely in low- to medium-grade metamorphic mafic rocks, and hydrothermally altered rocks, clinozoisite- and allanite-subgroup minerals rich in V³⁺ are not common, and have been reported from restricted localities. Mukhinite from the Tashelginskoye iron ore deposit, southwestern Siberia, having the composition of Ca_2.0(Al_2.1V_0.8Fe_0.1)Si_3.1O₁₂(OH) has been regarded as a V³⁺-analogue of clinozoisite (Shepel and Karpenko 1969). However, its crystal structure and V³⁺ distribution among octahedra have not been investigated. Several syntheses and/or phase-equilibrium studies on clinozoisite-subgroup minerals have been performed. To the best of our knowledge, there has been no investigation of synthetic V-bearing epidote-supergroup minerals.
Syntheses were performed under the conditions of 1.2-1.5 GPa and 600-800 ºC, and run duration 89-261 hours. Mukhinite and V³⁺-bearing clinozoisite (Czo) in all run products are associated with zoisite (Zo), and also coexist with V-bearing phases such as vanadomalayaite, goldmanite, V-oxides and unidentified Ca-Al-bearing vanadates. Czo and Zo were distinguished by using Raman spectra. As the result of submicroscopic texture observation by TEM, Zo in Run 19 product often contain Czo lamellae with several nm thickness, and they have epitaxial relationship as Zo[010]//Cz[010]. Czo and Zo in Run 20 product are generally separated, but in a few cases, they contact each other at boundaries. However, the phases in Run 20 do not have an epitaxial relationship, and Zo intergrowth with Czo is not observed. Mukhinite and V³⁺-bearing clinozoisite crystallized in the Run 20 product show a compositional gap between 0.33-0.74 V atoms per formula unit (apfu), and the V content attains 1.14 apfu. The co-existence of low V³⁺- and high V³⁺-clinozoisites indicates the presence of a miscibility gap at 1.5 GPa and 800 ºC. Two mukhinite crystals with 0.75 and 0.83 V³⁺ apfu were used for X-ray single-crystal structure analysis (hereafter, labelled as 20d and 20k). Structure refinements of the 20d and 20k crystals converged to R₁ values of 4.20% and 3.33%, respectively. The unit-cell parameters are a = 8.8995(2), b = 5.6299(1), c = 10.1532(2) Å, beta = 115.327(1)º, and V = 459.81(2) ų for 20d, and a = 8.8999(1), b = 5.6357(1), c = 10.1499(1) Å, beta = 115.306(1)º, and V = 460.24(2) ų for 20k. The resulting V³⁺ occupancies among the octahedral sites of 20d and 20k are ^M1(Al_0.894(6)V_0.106)^M2(Al_0.976(6)V_0.024)^M3(V_0.621(6)Al_0.379) and ^M1(Al_0.868(4)V_0.132)^M2(Al_0.957(4)V_0.043)^M3(V_0.652(2)Al_0.348), respectively. 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, K_D = (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. The unit-cell volumes of mukhinites are similar to those of Ca₂(Al,Fe³⁺)₃Si₃O₁₂(OH)-epidote. However, it is noticed that the a-dimension is longer, and the c-dimensions shorter. The b angles of the mukhinites are also different from those of epidotes, but similar to those of Cr³⁺-clinozoisite. Although the behavior of V³⁺ and its effect on the epidote-type structure are expected to be similar to those in the case of Fe³⁺ due to their similar ionic radii, some structural variations cannot be explained only in terms of the similarity of ionic radii between V³⁺ and Fe³⁺.