Khomyakovite and manganokhomyakovite, two new members of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada

Khomyakovite, ideally Na12Sr3Ca6Fe3Zr3W(Si25O73)(O,OH,H2O)(3)(OH,Cl)(2) and manganokhomyakovite, ideally Na12Sr3Ca6Mn3Zr3W(Si25O73)(O,OH,H2O)(3)(OH,Cl )(2) are two mew members of the eudialyte group from Mont Saint-Hilaire, Qu ebec. They occur as orange to orange-red, pseudo-octahedral crystals rangin g in size from 0.5 mm (khomyakovite) to 5 mm (manganokhomyakovite). Associa ted minerals include, for khomyakovite: analcime, annite, calcite, natrolit e, pyrite, and titanite, and for manganokhomyakovite: aegirine, albite, ana lcime, annite, cerussite, galena, kupletskite, microcline, molybdenite, nat rolite, pyrite, pyrrhotite, sodalite, sphalerite, titanite, wohlerite and z ircon. Both minerals are transparent to translucent, with a vitreous luster and white streak. They are brittle, with a hardness of 5-6 (Mohs scale). T hey have no cleavage, no parting and an uneven fracture. They are uniaxial negative, for khomyakovite: omega = 1.6279(5) and epsilon = 1.6254(5), and for manganokhomyakovite: omega = 1.629(1) and epsilon = 1.626(2). They are trigonal, space group R3m. For khomyakovite: a 14.2959(8), c 30.084(3) Angs trom, V 5324.6(7) Angstrom(3), and for manganokhomyakovite: a 14.282(3), c 30.12(1) A, V5320(3) Angstrom(3), Z = 3. The strongest X-ray powder-diffrac tion lines, measured for manganokhomyakovite [d in Angstrom (I)(hkl)], are 11.500(90)(101), 9.535(70)(012), 6.452(50)(104), 6.072(50)(021), 5.735(50)( 202), 3.406(50)(131), 3.213(50)(208), 3.167(50)(217), 2.980(100)(315) and 2 .856(80)(404). The infrared spectrum of manganokhomyakovite is given. An el ectron-microprobe analysis of the khomyakovite crystal used for single-crys tal work gave Na2O 11.35, K2O 0.52, MgO 0.04, CaO 10.42, MnO 1.63, FeO 4.33 , SrO 8.45, Al2O3 0.09, La2O3 0.21, Ce2O3 0.18, SiO2 43.70, TiO2 0.11, ZrO2 10.62, HfO2 0.18, Nb2O5 1.33, Ta2O5 0.02, WO3 3.80, Cl 0.67, H2O 0.87, O e quivalent to Cl 0.15, total 98.37 wt.%. The proportion of H2O was calculate d by stoichiometry from the structure analysis. The empirical formula of kh omyakovite, based on 78.70 anions as determined in the structure analysis, is (Na12.26Ca0.33K0.38Sr0.13REE0.08)(Sigma 13.05) (Sr2.78Na0.22)(Sigma 3) C as (Fe2.05Mn0.78Mg0.03)(Sigma 2.86) (Zr2.94Ti0.05Hf0.03)(Sigma 3.02) (W0.56 Nb0.34)(Sigma 0.90) (Si24.78Al0.06)(Sigma 24.84) O-73 (O,OH,H2O)(Sigma 3.70 ) (OH1.36Cl0.64)(Sigma 2) For manganokhomyakovite, an average of four elect ron-microprobe analyses gave Na2O 10.37 (10.05-10.59), K2O 0.41 (0.37-0.45) , CaO 10.20 (10.08-10.35), MnO 4.21 (3.91-4.38), FeO 2.58 (2.40-2.65); SrO 9.17 (8.63-9.63), Al2O3 0.02 (0.00-0.05), La2O3 0.12 (0.00-0.27), Ce2O3 0.2 0 (0.00-0.30), SiO2 42.98 (42.51-43.39), TiO2 0.02 (0.00-0.07), ZrO2 10.43 (10.40-10.50), HfO2 0.17 (0.16-0.19), Nb2O5 1.58 (1.35-2.05), Ta2O5 0.03 (0 .00-0.05), WO3 4.48 (3.66-4.83), Cl 0.84 (0. 78-0.89), H2O 0.83; O equivalent to Cl 0.19, total 98.45 wt. %. The proport ion of H2O was calculated by stoichiometry from the crystal-structure relat ionship with khomyakovite, confirmed by TGA and LR data. The empirical form ula of manganokhomyakovite, based on 78.70 anions as found in the crystal s tructure of khomyakovite, is (Na11.15K0.30Ca0.25Sr0.04REE0.07)(Sigma 12.17) Sr-3 Cas (Mn2.04Fe1.23)(Sigma 3.27) (Zr2.91Hf0.03Ti0.01)(Sigma 2.95) (W0.6 6Nb0.41Ta0.01)(Sigma 1.08) (Si24.60Al0.01)(Sigma 24.61) O-73 (O,OH,H2O)3.70 (OH1.19Cl0.81)(Sigma 2) For khomyakovite, D-calc. = 3.14 g/cm(3), and for manganokhomyakovite, D-calc. = 3.17 g/cm(3), D-meas = 3.13(3) g/cm(3). Both minerals possess the eudialyte structure. Single-crystal data for khomyako vite show that W dominates over Nb in M(3), and Sr over Na in Na(4). M(2) i s dominated by Fe in khomyakovite, and by Mn in manganokhomyakovite.