Waipouaite, Ca3(⁠V4.54+V0.55+⁠)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(⁠V4.604+V0.445+⁠)Σ5.04(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) (122), 3.140 (18) (333), 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+)5O17] polyoxovana-date 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.

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