Nioboixiolite-(Mn²⁺),<b> </b><span class="inline-formula"><span class="math">\(\left( {{\mathbf{N}}{{{\mathbf{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}}}}{\mathbf{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. } 3}}}^{{{\mathbf{2}} + }}} \right){{{\mathbf{O}}}_{{\mathbf{2}}}},\)</span></span> a New Ixiolite-Group Mineral from the Malkhan Pegmatite Field, Transbaikal Region, Russia

Abstract

The new ixiolte-group mineral nioboixiolite-(Mn2+), ideally \( \left( {{\text{N}}{{{\text{b}}}_{{{2 \mathord{\left/ {\vphantom {2 3}} \right. } 3}}}}{\text{Mn}}_{{{1 \mathord{\left/ {\vphantom {1 3}} \right. } 3}}}^{{2 + }}} \right){{{\text{O}}}_{{\text{2}}}}{\text{,}} \) the niobian analogue of ixiolite-(Mn2+), was discovered in the Sosedka granitic pegmatite vein, Malkhan pegmatite field, Zabaikalsky Krai (Transbaikal Region), Siberia, Russia. The associated minerals are albite, quartz, microcline, elbaite, beryl, bismuthinite, euxenite-(Y), zircon, rutile, cassiterite, and cannonite. Nioboixiolite-(Mn2+) occurs as clusters of dark brown to brown-black prismatic crystals up to 0.8 × 1.5 × 5 mm embedded in albite. The lustre is submetallic to adamantine, and the streak is brown. Cleavage is not observed. The Mohs’ hardness is 4.5–5. Density calculated using the empirical formula is equal to 5.803 g cm–3. The IR spectrum and reflectance spectra in visible range are given. The chemical composition of nioboixiolite-(Mn2+) is (electron microprobe, wt%): MnO 14.94, Sc2O3 1.80, Fe2O3 0.20, Y2O3 1.34, TiO2 7.66, ZrO2 1.74, SnO2 1.01, ThO2 0.26, UO2 1.44, Nb2O5 42.80, Ta2O5 26.77, total 99.96. The empirical formula is (Nb1.59\({\text{Mn}}_{{1.04}}^{{2 + }}\)Ta0.59Ti0.47Sc0.13Zr0.07Y0.06Sn0.03U0.03\({\text{Fe}}_{{0.01}}^{{3 + }}\))Σ4.02O8 (Z = 1). The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.0474. The new mineral is isostructural to other ixiolite-group members. Nioboixiolite-(Mn2+) is orthorhombic, space group: Pbcn, a = 4.762(2) Å, b = 5.739(1) Å, c = 5.149(1) Å, V=140.7(1) Å3. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.662 (29) (110), 2.984 (100) (111), 2.505 (21) (021), 1.775 (21) (130), 1.748 (28) (202), 1.726 (35) (221), 1.553 (20) (113), 1.473 (19) (023), 1.463 (30) (311, 132).