MANITOBAITE, Na16 Mn2+ 25 Al8 (PO4)30, A NEW PHOSPHATE MINERAL SPECIES FROM CROSS LAKE, MANITOBA, CANADA

T. SCOTT ERCIT, KIMBERLY T. TAIT, MARK A. COOPER, YASIR ABDU, NEIL A. BALL, ALAN J. ANDERSON, PETR CERNÝ, FRANK C. HAWTHORNE, MIGUEL A. GALLISKI

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Año: 2010 vol. 48 p. 1455 - 1463

0008-4476

Manitobaite, ideally Na16 Mn2+25 Al8 (PO4)30, is a new mineral species from Cross Lake, Manitoba, Canada. It occurs as large crystals or cleavage masses intergrown with other phosphate minerals in a phosphate pod in the intermediate and core zones of pegmatite #22 on the southeastern shoreline of a small unnamed island in Cross Lake, Manitoba, about 5 km north–northwest of the Cross Lake settlement, longitude 54° 41’ N, latitude 97° 49’ W. Associated minerals are fluorapatite, chlorapatite, bobfergusonite, eosphorite, dickinsonite, fillowite, triploidite, goyazite, perloffite, beusite, triplite, as well as quartz, K-feldspar,muscovite, schorl, beryl, spessartine, gahnite and (Nb,Ta,Sn) oxides. Manitobaite is opaque in large crystals (up to 4 cm), and transparent to translucent in small (<1 mm) grains. Color varies from green to brown, with a colorless to very pale green or very pale greenish brown streak and a vitreous to resinous luster; manitobaite does not fluoresce under ultraviolet light. Thecleavage is perfect on {010}, there is no parting, the tenacity is brittle, and the fracture is hackly. The following properties were measured on the green variety of manitobaite. The measured and calculated densities are 3.621(6) and 3.628 g cm–3, respectively.Manitobaite is biaxial negative with a 1.682, b 1.691, g 1.697 (all ±0.001), with X- a = 31.7° (in b obtuse), Y || b, Z and c = 20.2° (in b acute); 2Vobs = 78.1(6)°, 2Vcalc = 77.9°. It is pleochroic X = orange brown, Y = green, Z = greenish brown, with absorption Y ≥ Z > X and dispersion r > v, medium. Manitobaite is monoclinic, Pc, a 13.4516(15), b 12.5153(16), c 26.661(3) Å, b 101.579(10)°, V 4397.1(6) Å3, Z = 2, a:b:c = 1.07482:1:2.13027. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.715(100)(242), 2.730(50)(404), 3.494(47)(313), 3.078(27)(317), 2.518(22)(515), 2.881(21)(119),and 6.260(20)(020). A chemical analysis with an electron microprobe gave P2O5 44.19, Al2O3 6.91, FeO 7.79, MnO 27.57, ZnO 0.54, MgO 0.73, CaO 1.71, Na2O 9.97, for a total of 99.58 wt%. The resulting empirical formula, using the valence states of Fe determined by Mössbauer spectroscopy, is Na15.55 Ca1.47 Mg0.88 Fe2+4.19 Mn2+18.78 Zn0.32 Al6.54 Fe3+1.05 P30.08 O120 based on 120 O2– anions pfu. The general formula is (Na,Ca,□)16 (Mn2+,Fe2+,Ca,Al,Fe3+,Mg,Zn)25 (Al,Fe3+,Mg,Mn2+, Fe2+)8 (PO4)30,and the end-member formula is Na16 Mn2+25 Al8 (PO4)30. The crystal structure of manitobaite is an ordered superstructure of the alluaudite arrangement with a cell volume five times that of alluaudite.Keywords: manitobaite, new mineral species, phosphate, alluaudite superstructure, granitic pegmatite, electron-microprobe analysis, Mössbauer spectroscopy, optical properties, X-ray powder-diffraction pattern, Cross Lake, Manitoba, Canada.