Achalaite, Fe2+TiNb2O8, a New Member of the Wodginite Group from the La Calandria Granitic Pegmatite, CÓrdoba, Argentina

Abstract

Achalaite is a new mineral of the wodginite group from the La Calandria granitic pegmatite, Cañada del Puerto, Córdoba province, Argentina (31°25′S, 64°55′W). The mineral occurs as polygranular aggregates up to 1.5 cm across, included in albite ± quartz with exsolutions of Ta-rich rutile and intergrown with microlite-group minerals. Achalaite has a black color and streak, metallic luster, H = 5½, Dcalc = 6.285 g/cm3, and appears to lack cleavage. In reflected light, it is light grey with a slightly violet tint in air, and light grey with an olive tint in oil. Its bireflectance in both air and oil is weak but noticeable. Pleochroism is not observed. With crossed polarizers anisotropy in air is weak, particularly noticeable along grain boundaries; in oil, it is medium with light to intermediate olive grey tint. Internal reflections have not been observed either in air or oil. Data on minimum and maximum reflectances for the COM wavelengths are [λ (nm): Rair, Roil (%)]: 470: 16.89, 17.72 and 5.26, 5.76; 546: 16.49, 17.26 and 4.98, 5.32; 589: 16.34, 17.04 and 4.97, 5.26; 650: 16.23, 16.94 and 4.86, 5.19. Achalaite, indexed by analogy with wodginite, is monoclinic, space group C2/c, Z = 4. Refined unit-cell dimensions are a 9.422(4) Å, b 11.427(3) Å, c 5.120(1) Å, β 90.12° (4), V 551.2(3) Å3. The strongest five lines in the X-ray powder diffraction pattern are as follows (d [Å], I%, hkl): 3.630, 40, 220; 2.964, 100, forumla, 221; 2.493, 40, 041; 1.735, 40, forumla; 1.711, 50, forumla, 441. Eight electron microprobe analyses (WDS) gave the following mean composition: WO3 2.41, Nb2O5 30.18, Ta2O5 37.56, TiO2 6.90, ZrO2 0.88, SnO2 3.82, Fe2O3 4.35, FeO 7.54, MnO 5.14, CaO 0.02, UO2 0.38, total 99.18 wt.%. The average empirical formula is: (Fe2+2.211Mn1.524Fe3+0.226U0.030Ca0.009)Σ4.000(Ti1.819Fe3+0.920Ta0.579Sn0.533Zr0.150)Σ4.000(Nb4.781Ta3.000W0.219)Σ8.000O32, ideally Fe2+TiNb2O8. Achalaite is the first member of the wodginite group with Nb dominant over Ta; it is genetically considered a post-magmatic phase. The name of achalaite (CNMNC – IMA 2013-103) derives from the Achala granite batholith, located in Córdoba, Argentina.