Ferriakasakaite-(La) and ferriandrosite-(La): new epidote-supergroup minerals from Ise, Mie Prefecture, Japan

摘要

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 Ca-A1 (A2) La (M1) Fe3+M2 (AlMn2+)-Mn-M3 (SiO4)(Si2O7) O(OH) (Z = 2, space group P2(1)/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) angstrom, beta = 113.845(2)degrees and V = 469.73(2) angstrom(3). According to the structural refinement (R-1 = 3.13%), the determined structural formula is (A1) (Ca0.54Mn0.462+) (A2) [(La0.48Ce0.20Pr0.07Nd0.18Gd0.02)(Sigma 0.95)Ca-0.05] (M1) (Fe0.423+V0.343+Al0.18Ti0.064+) (M2) (Al0.96Fe0.043+) (M3) (Mn-0. 50(2+) Fe0. 4 32+Mg0. 0 7) (SiO4) (Si2O7)O(OH). Ferriandrosite - (La), ideally (A1) Mn2+A2 La-M1 Fe3+M2 Al-M3 Mn2+(SiO4)(Si2O7) O(OH) (Z = 2, space group P2(1)/m), is the (M1) Fe3+ analogue of androsite. The unit-cell parameters are a = 8.8779(1), b = 5.73995(1), c = 10.0875(2) angstrom, beta = 113.899(1)degrees and V = 469.97(2) angstrom(3), and the structural formula is (A1) (Mn0.562+Ca0.44) (A2) [(La0.49Ce0.20Pr0.08Nd0.19Gd0.02)(Sigma 0.97)Ca-0.03](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) (R-1 = 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.