ABSTRACT

     Manganokukisvumite, ideally Na₆MnTi₄Si₈O₂₈·4H₂O, is a new mineral species from the Poudrette Quarry, Mont Saint-Hilaire, Rouville County, Quebec (IMA 2002-029). It occurs in small cavities in an albite-rich syenite breccia associated with aegirine, microcline, albite, annite, chalcopyrite, pyrite, pyrrhotite, natrolite, labuntsovite-Mn, titanite, calcite, a chlorite-group mineral, magnetite, fluorapatite, elpidite and sodalite.

     Manganokukisvumite crystals are colorless and transparent with a vitreous luster and a white streak. It is non-fluorescent in short- and long-wave ultraviolet light. The crystals are extremely thin, flattened and sword-shaped to 0.5 mm in length, commonly forming rosette-like radiating groups. Forms observed are pinacoids {100} and {010} and a prism that we could not measure. No twinning was observed. The hardness is 5½ to 6. It is sectile, slightly flexible, with no apparent cleavage but with a splintery fracture.

     Manganokukisvumite is biaxial negative, α(calc.) 1.657, β 1.744 ± 0.003 and γ 1.792 ± 0.003 (for λ = 589 nm); 2V_meas 70° (Kamb's method). There is no dispersion or pleochroism; the optical orientation is X = a, Y = b and Z = c. It is orthorhombic, space group Pccn, with unit-cell parameters refined from powder data: a 29.05(2), b 8.612(6), c 5.220(4) Å, V 1305.9(3) A³ and Z = 2.

     The strongest X-ray powder-diffraction lines [d in Å(I)(hkl)] are:

     An electron-microprobe analysis supported by infrared-absorption spectrometry gave:

     The empirical formula based on O = 32, is:


(Na_5.81K_0.05Ca_0.02Ce_0.01)_Σ5.89 (Mn_0.89Fe_0.09Mg_0.07Al_0.04)_Σ1.09


(Ti_3.88Nb_0.06)_Σ3.94Si_8.01O₂₈ · 4H₂O or, ideally, Na₆MnTi₄Si₈O₂₈ · 4H₂O.

     The calculated density (from the empirical formula) is 2.88 g/cm³, and the measured density is 2.86(1) g/cm³. A Gladstone-Dale calculation gives a compatibility index of 0.038 (excellent). Manganokukisvumite is the manganese-dominant analogue of kukisvumite and is closely related to lintisite.

Key words: manganokukisvumite, new mineral species, Mont Saint-Hilaire, Quebec.