Vanadoallanite-(La): a new epidote-supergroup mineral from Ise, Mie Prefecture, Japan Available to Purchase

The new mineral, vanadoallanite-(La), found in the stratiform ferromanganese deposit from the Shobu area, Ise City, Mie Prefecture, Japan, was studied using electron microprobe analysis and single-crystal X-ray diffraction methods. Vanadoallanite-(La) is a rare-earth element-rich monoclinic epidote-supergroup mineral with simplified formula CaLaV³⁺AlFe²⁺(SiO₄)(Si₂O₇)O(OH) (Z = 2, space group P2₁/m) characterized by predominantly V³⁺ at one of three octahedral sites, M1. The crystal studied shows large V (~8.4 V₂O₃ wt.%), Fe(~13.8 Fe₂O₃ wt.%; Fe²⁺/total Fe = 0.58) and Mn (~8.8 MnO wt.%) contents. A small amount of Ti is also present (~1.3 TiO₂ wt.%). Structural refinement converged to R₁ = 2.96%. The unit-cell parameters are a = 8.8985(2), b = 5.7650(1), c = 10.1185(2) Å_, β = 114.120(1)° and V = 473.76(2) ų. The cation distributions determined at A1, A2 and M3 are Ca_0.61Mn_0.39, (La_0.46Ce_0.14Pr_0.07Nd_0.18)_Σ0.85Ca_0.15 and $Fe0.562+Mn0.302+$Mg_0.06$V0.053+Fe0.033+$⁠, respectively. On the other hand, depending on Ti assignment, two different schemes of the cation distribution at M1 and M2 can be considered: (1) ^M1(⁠$V0.583+Fe0.343+Ti0.084+$⁠)^M2(Al_0.92$Fe0.083+$⁠), and (2) ^M1(⁠$V0.583+Fe0.423+$⁠)^M2(Al_0.92$Ti0.084+$⁠). In both cases, the dominant cations at A1, A2, M1, M2 and M3 are Ca, La, V³⁺, Al and Fe²⁺, respectively. According to ionic radius, Ti⁴⁺ possibly prefers M2 rather than Fe³⁺. A large Mn²⁺ content at A1 also characterizes our vanadoallanite-(La). The structural change of Mn²⁺-rich allanite-group minerals is considered to be controlled by two main factors. One is the large Mn²⁺ content at A1 in vanadoallanite-(La), which modifies the topology of the A1O₉ polyhedron. The other is the expansion of M3O₆ and M1O₆ octahedra caused by large octahedral cations, such as Fe²⁺ and Mn²⁺, at M3 and the trivalent transition elements, V³⁺ and Fe³⁺, at M1.