Taniajacoite and strontioruizite, two new minerals isostructural with ruizite from the N’chwaning III mine, kalahari manganese field, South Africa

Two new mineral species, taniajacoite and strontioruizite, ideally SrCaMn^3þ₂Si₄O₁₁(OH)₄2H₂O and Sr₂Mn^3þ₂ Si₄O₁₁(OH)₄2H₂O, 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/cm³, respectively, for taniajacoite and 3.20(2) and 3.16 g/cm³ for strontioruizite. Optically, both taniajacoite and strontioruizite are biaxial (–), with a = 1.686(2), b = 1.729(2), c = 1.746(2) (white light), 2V (meas.) = 63.7(5)8, 2V (calc.) = 62.58 for the former and a = 1.692(2), b = 1.734(2), c = 1.747(2) (white light), 2V (meas.) = 59.1(5)8, 2V (calc.) = 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(Ca_0.81Sr_0.19)_R1.00(Mn^3þ_1.90Fe^3þ_0.15 Al_0.01)_R2.06Si_3.96O₁₁(OH)₄2H₂O for taniajacoite and (Sr_1.61Ca_0.42)_R2.03(Mn^3þ_1.95Fe^3þ_0.05)_R2.00Si_3.98O₁₁(OH)₄2H₂O for strontioruizite. Taniajacoite and strontioruizite are isostructural with ruizite. Strontioruizite, like ruizite, is monoclinic with space group C2 and unit-cell parameters a = 9.1575(4), b = 6.2857(4), c = 12.0431(6) Å, b = 91.744(4)8, and V = 692.90(6) ų, whereas taniajacoite is triclinic, with space group C1 and a = 9.1386(5), b = 6.2566(3), c = 12.0043(6) Å, a = 90.019(4), b = 91.643(4), c = 89.900(4)8, and V = 686.08(6) ų. 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 Sr^2þ and Ca^2þ occupy the seven-coordinated interstices. Unlike monoclinic ruizite and strontioruizite, taniajacoite with Sr:Ca’ 1:1 is triclinic, owing to the ordering of Sr^2þ and Ca²þ into two crystallographically distinct sites, indicating an incomplete solid solution between Ca and Sr endmembers. The unit-cell volumes for ruizite, taniajacoite, and strontioruizite appear to vary linearly with the Sr/(Ca þ Sr) ratio.