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

Abstract

Manitobaite, ideally Na 16 Mn 2+ 25 Al 8 (PO 4 ) 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 ( −3 , respectively. Manitobaite is biaxial negative with α 1.682, β 1.691, γ 1.697 (all ±0.001), with X ∧ a = 31.7° (in β obtuse), Y || b , Z ∧ c = 20.2° (in β acute); 2 V obs = 78.1(6)°, 2 V calc = 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) A, β 101.579(10)°, V 4397.1(6) A 3 , Z = 2, a:b:c = 1.07482:1:2.13027. The strongest seven lines in the X-ray powder-diffraction pattern [ d in A( 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 P 2 O 5 44.19, Al 2 O 3 6.91, FeO 7.79, MnO 27.57, ZnO 0.54, MgO 0.73, CaO 1.71, Na 2 O 9.97, for a total of 99.58 wt%. The resulting empirical formula, using the valence states of Fe determined by Mossbauer spectroscopy, is Na 15.55 Ca 1.47 Mg 0.88 Fe 2+ 4.19 Mn 2+ 18.78 Zn 0.32 Al 6.54 Fe 3+ 1.05 P 30.08 O 120 based on 120 O 2− anions pfu . The general formula is ( Na ,Ca,□) 16 ( Mn 2+ ,Fe 2+ ,Ca,Al,Fe 3+ ,Mg,Zn) 25 ( Al ,Fe 3+ ,Mg,Mn 2+ , Fe 2+ ) 8 (PO 4 ) 30 , and the end-member formula is Na 16 Mn 2+ 25 Al 8 (PO 4 ) 30 . The crystal structure of manitobaite is an ordered superstructure of the alluaudite arrangement with a cell volume five times that of alluaudite.