The new REE-rich, monoclinic, epidote-supergroup minerals ferriakasakaite-(La) and ferriandrosite-(La), found in tephroite ± calcite veinlets cutting the stratiform ferromanganese deposit from the Shobu area, Ise City, Mie Prefecture, Japan, were studied using electron microprobe analysis and single-crystal X-ray diffraction methods. Ferriakasakaite-(La), ideally A1CaA2LaM1Fe3+M2AlM3Mn2+ (SiO4)(Si2O7)O(OH) (Z = 2, space group P21/m), has a new combination of dominant cations at A1(Ca) and M3(Mn2+), which are the key sites to determine a root name for epidote-supergroup minerals. The unit-cell parameters are a = 8.8733(2), b = 5.7415(1), c = 10.0805(3) Å, β = 113.845(2)° and V = 469.73(2) Å3. According to the structural refinement (R1 = 3.13%), the determined structural formula is A1(Ca0.54Mn0.462+)A2[(La0.48Ce0.20Pr0.07Nd0.18Gd0.02)Σ0.95Ca0.05]M1(Fe0.423+V0.343+Al0.18Ti0.064+)M2(Al0.96Fe0.043+)M3(Mn0.502+Fe0.432+Mg0.07)(SiO4)(Si2O7) O (OH). Ferriandrosite-(La), ideally A1Mn2+A2LaM1Fe3+M2AlM3Mn2+(SiO4)(Si2O7)O(OH) (Z = 2, space group P21/m), is the M1Fe3+ analogue of androsite. The unit-cell parameters are a = 8.8779(1), b = 5.73995(1), c = 10.0875(2) Å, β = 113.899(1)° and V = 469.97(2) Å3, and the structural formula is A1(Mn0.562+Ca0.44)A2[(La0.49Ce0.20Pr0.08Nd0.19Gd0.02)Σ0.97Ca0.03]M1(Fe0.403+V0.283+Al0.20Fe0.052+Ti0.074+)M2(Al0.97Fe0.033+)M3(Mn0.502+Fe0.402+Mg0.10)(SiO4)(Si2O7)O(OH) (R1 = 2.93%). The two new minerals, which are compositionally very similar overall, are distinguished by occupancy of A1, Ca vs. Mn2+. The structural properties of these minerals depend not only on the REE content at A2, but also on the Mn content at A1.

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