Abstract

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) Å3. The cation distributions determined at A1, A2 and M3 are Ca0.61Mn0.39, (La0.46Ce0.14Pr0.07Nd0.18)Σ0.85Ca0.15 and Fe0.562+Mn0.302+Mg0.06V0.053+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(Al0.92Fe0.083+), and (2) M1(V0.583+Fe0.423+)M2(Al0.92Ti0.084+). 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.

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