Numanoite, Ca₄CuB₄O₆(OH)₆ (CO₃)₂, a new mineral species, the Cu analogue of borcarite from the Fuka mine, Okayama Prefecture, Japan

Numanoite, the Cu analogue of borcarite, is found in an irregular patch in crystalline limestone near gehlenite-spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Numanoite (up to 1 mm across) is observed as a core or zones in horcarite crystals up to 5 mm long. The mineral is also found as veinlets up to 0.4 mm wide in aggregates of borcarite crystals. The associated minerals are nifontovite, bultfonteinite, calcite and an unidentified magnesium silicate mineral. In hand specimen, the mineral is blue-green to colorless and transparent with a vitreous luster. The streak is white to pale blue-green. Numanoite is monoclinic, space group C2/m, a 17.794(2), b 8.381(1), c 4.4494(7) Å, β 102.42(2)° and Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are 7.57(100)(110), 2.671(84)(4̄21), 2.727(68)(221), 1.887(52)(041,440), 2.272(48)(3̄31), 2.899(44)(600) and 1.698(34)(640). Electron-microprobe and thermogravimetric analyses gave B₂O₃ 24.09, CaO 38.11, CuO 10.32, MgO 1.02, ZnO 0.51, CO₂ 15.80, H₂O 9.75, sum 99.60 wt.%. The empirical formula, calculated on the basis of O = 18, is Ca_3.898(Cu_0.744Mg_0.145 Zn_0.036)_∑0.925B_3.969 O_5.615(OH)_6.208(CO₃)_2.059, ideally Ca₄Cu B₄O₆(OH)₆ (CO₃)₂. The mineral is optically biaxial negative, α 1.618(2), β 1.658(2), γ 1.672(2), and 2V_calc = 60°. The mineral has perfect cleavages in two directions. The density is 2.96(2) g/cm³ (meas.) and 2.93 g/cm³ (calc.). The Vickers microhardness is 376 (290-464) kg/mm² (25 g load), and the Mohs hardness number is 41/2. The differential thermal analysis curve shows two endothermic peaks at 489° and 692°C. It is likely that numanoite from the Fuka mine formed by precipitation from late Cu- and Mg-bearing hydrothermal solutions.