Taniajacoite and Strontioruizite, Two New Minerals Isostructural with Ruizite from the N'Chwaning III Mine, Kalahari Manganese Field, South Africa

Abstract

Two new mineral species, taniajacoite and strontioruizite, ideally SrCaMn2Si4O11(OH)4 2H2O and Sr2Mn2 Si4O11(OH)4 2H2O, respectively, have been identified from the N’Chwaning III mine, Kalahari manganese field, South Africa. Both minerals occur as brown radiating groups or aggregates of acicular or prismatic crystals, with individual crystals up to 0.15 3 0.04 3 0.02 mm for taniajacoite and 1.3 3 0.2 3 0.2 mm for strontioruizite. Minerals associated with taniajacoite include sugilite, aegirine, pectolite, richterite, potassic-ferri-leakeite, and lipuite, whereas those associated with strontioruizite include sugilite, potassic-magnesio-arfvedsonite, and lipuite. Both taniajacoite and strontioruizite are brown in transmitted light, transparent with very light brown streak and vitreous luster. They are brittle and have a Mohs hardness of 5–5.5; cleavage is good on {010} and no parting or twinning is observed macroscopically. The measured and calculated densities are 3.05(2) and 3.09 g/cm, respectively, for taniajacoite and 3.20(2) and 3.16 g/cm for strontioruizite. Optically, both taniajacoite and strontioruizite are biaxial (–), with a1⁄4 1.686(2), b1⁄4 1.729(2), c1⁄4 1.746(2) (white light), 2V (meas.)1⁄4 63.7(5)8, 2V (calc.)1⁄4 62.58 for the former and a1⁄4 1.692(2), b1⁄4 1.734(2), c1⁄4 1.747(2) (white light), 2V (meas.)1⁄4 59.1(5)8, 2V (calc.)1⁄4 56.68 for the latter. The calculated compatibility index based on the empirical formula is 0.008 for taniajacoite and 0.015 for strontioruizite. An electron microprobe analysis yielded an empirical formula (based on 17 O apfu) of Sr(Ca0.81Sr0.19)R1.00(Mn 3þ 1.90Fe 3þ 0.15 Al0.01)R2.06Si3.96O11(OH)4 2H2O for taniajacoite and (Sr1.61Ca0.42)R2.03(Mn1.95Fe0.05)R2.00Si3.98O11(OH)4 2H2O for strontioruizite. Taniajacoite and strontioruizite are isostructural with ruizite. Strontioruizite, like ruizite, is monoclinic with space group C2 and unit-cell parameters a 1⁄4 9.1575(4), b 1⁄4 6.2857(4), c 1⁄4 12.0431(6) Å, b 1⁄4 91.744(4)8, and V 1⁄4 692.90(6) Å, whereas taniajacoite is triclinic, with space group C1 and a1⁄49.1386(5), b1⁄46.2566(3), c1⁄412.0043(6) Å, a1⁄490.019(4), b1⁄491.643(4), c 1⁄4 89.900(4)8, and V 1⁄4 686.08(6) Å. Their structures are characterized by chains of edge-sharing MnO6 octahedra extended along [010], which are linked together by corner-shared SiO4 tetrahedra in four-membered [Si4O11(OH)2] linear clusters, giving rise to a so-called ‘‘hetero-polyhedral framework’’. The large cations Sr2þ and Ca2þ occupy the seven-coordinated interstices. Unlike monoclinic ruizite and strontioruizite, taniajacoite with Sr:Ca ’ 1:1 is triclinic, owing to the ordering of Sr2þ and Ca2þ into two crystallographically distinct sites, indicating an incomplete solid solution between Ca and Sr endmembers. The unitcell volumes for ruizite, taniajacoite, and strontioruizite appear to vary linearly with the Sr/(Ca þ Sr) ratio.