Waipouaite, Ca3(V 4.5 4 + V 0.5 5 +)O9[(Si2O5(OH)2][Si3O7.5(OH)1.5]·11H2O, a new polyoxovanadate mineral from the Aranga Quarry, New Zealand.

Waipouaite, Ca3 (V 4.5 4 + V 0.5 5 + ) O9[(Si2O5(OH)2][Si3O7.5(OH)1.5]·11H2O, is a new mineral from the Aranga Quarry, Northland Region, New Zealand. It occurs in basalt as overgrowths on thompsonite-Ca and chabazite-Ca and as inclusions within calcite and okenite. It forms dark olive green to almost black prismatic crystals to 0.3 mm in length. Crystals are transparent to translucent with a vitreous luster. The Mohs hardness is ~2, and the measured density is 2.24(2) g/cm3. The new mineral is biaxial (+), with α = 1.620(5), β = 1.622(5), γ = 1.628(5) (white light). The calculated 2V is 60.2°. Dispersion could not be observed. The optical orientation is Z = b. Pleochroism is X blue-green, Y olive green, Z olive; X > Y >> Z. Electron microprobe analyses gave the empirical formula (based on 36 O apfu) (Ca2.90Na0.05K0.04Sr0.01)Σ3.00 (V 4.60 4 + V 0.44 5 + ) Σ 5.04 (Si4.97Al0.02)Σ4.99O21.45OH3.55·H2O11.00. Waipouaite is monoclinic, P21/c, a = 12.843(3), b = 23.589(5), c = 11.560(2) Å, β = 115.54(3)°, V = 3160.0(13) Å3, and Z = 4. The eight strongest reflections in the X-ray powder diffraction pattern are [dobs in Å (I) (hkl)]: 11.78 (100) (020, 100), 9.54 (16) (011), 7.85 (19) (021), 6.29 (32) (031), 5.92 (31) (040), 5.22 (21) (1 22), 3.140 (18) (3 33), 2.850 (17) (180, 242). The crystal structure was refined using synchrotron single-crystal X-ray data to R1 = 6.85% for 6594 reflections with I>2σI. Waipouaite is the first natural polyoxovanadosilicate and has a novel structure based on [(V4+,V5+)5OI7] polyoxovanadate units, which are unique in natural and synthetic phases. Synthesis of polyoxovanadosilicates has proved to be a great challenge, and the discovery of waipouaite demonstrates that these compounds can form under natural conditions. [ABSTRACT FROM AUTHOR]