New epidote-supergroup minerals from the Mogurazawa mine, Gunma, Japan: akasakaite-(Ce), akasakaite-(La), vanadoakasakaite-(Ce), and vanadoakasakaite-(La)

Abstract

New allanite-group minerals, akasakaite-(Ce), akasakaite-(La), vanadoakasakaite-(Ce), and vanadoakasakaite-(La), associated with small quartz lenses within rhodonite-enriched rocks of the stratiform manganese deposits at the Mogurazawa mine, Gunma, Japan, were studied using electron microprobe analysis, Raman spectroscopy, and single-crystal X-ray diffraction methods. Akasakaite is defined by the ideal formula, ^A1Ca^A2REE^3+M1Me^3+M2Al^M3Mn²⁺(SiO₄)(Si₂O₇)O(OH) (Z = 2, space group P2₁/m). The four new minerals are distinguished by the dominant cation at the M1 site (Al or V³⁺) and the dominant REE³⁺ at the A2 site (Ce or La). The unit-cell parameters are a = 8.885-8.925, b = 5.694-5.744, c = 10.100-10.153 Å, β = 113.62-113.79°, and V = 468.0-476.4 ų. Structure refinements converged to R1 values of 2.20-2.86%. At the M1 site, Al content reaches 85% in akasakaite-(La), whereas V³⁺ reaches 82% in vanadoakasakaite-(La). The cation assignments for these specimens are as follows: ^A1Ca_0.54Mn²⁺_0.46, ^A2La_0.53Ce_0.18Nd_0.05Pr_0.02Sr_0.04Ca_0.18, ^M1Al_0.85V³⁺_0.13Ti_0.02, ^M2Al_1.00, and ^M3Mn²⁺_0.75Fe_0.01V³⁺_0.16Al_0.07 for akasakaite-(La) and ^A1Ca_0.60Mn²⁺_0.40, ^A2La_0.45Ce_0.17Nd_0.06Pr_0.03Sr_0.15Ca_0.14, ^M1V³⁺_0.82Al_0.18, ^M2Al_0.64V³⁺_0.33Ti⁴⁺_0.03, and ^M3Mn²⁺_0.71V³⁺_0.26Al_0.02 for vanadoakasakaite-(La). All studied specimens are enriched in Mn²⁺, which predominates at the M3 site and also occupies over 40% of the A1 site. The unit-cell dimensions of vanadoakasakaites are larger than those of akasakaites. This variation appears to be attributed mainly to the elongation of <M1-O> caused by the substitution of Al for V³⁺ at the M1 site. Moreover, the expansion of the M3 site by the substitution of divalent octahedral cations, such as Mn²⁺, elongates the M3-O8 distance, which shortens the A2-O8 distance despite no cation substitution at the A2 site. This suggests that the variation in the akasakaite structure is largely influenced by cation substitution at the M3 and M1 sites. The topological change of the A1 polyhedra due to Mn²⁺ substitution for Ca at the A1 site, as reported in previous studies, was also observed.