GROATITE, Na Ca Mn2+2 (PO4) [PO3(OH)]2, A NEW MINERAL SPECIES OF THE ALLUAUDITE GROUP FROM THE TANCO PEGMATITE, BERNIC LAKE, MANITOBA, CANADA: DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Groatite, ideally Na Ca Mn 2+ 2 (PO 4 ) [PO 3 (OH)] 2 , is a new mineral species from the Tanco pegmatite at Bernic Lake, Manitoba, Canada. It was found in a phosphate–carbonate mass in a spodumene-rich boulder. Groatite occurs sparingly as slightly divergent sprays of acicular crystals, embedded stellate sprays and as a tabular mass of densely intergrown acicular crystals. Groatite is closely associated with whitlockite, crandallite, an unidentified Na–Al phosphate and quartz; fairfieldite and manganese-rich overite are among the less closely associated phosphates; this assemblage may be secondary, following apparent dissolution of lithiophosphate and possibly of primary lithiophilite. Groatite is translucent, colorless to pale yellow and pale orange with a white streak and vitreous luster; it does not fluoresce under ultraviolet light. The Mohs hardness is 3. Groatite is brittle with an uneven fracture, and has a calculated density of 3.213 g/cm 3 . It is biaxial positive with α 1.622, β 1.634, γ 1.663 (all ±0.001), and is nonpleochroic; 2 V (obs.) = 67(1), 2 V (calc.) = 66.5°. It is monoclinic, space group C 2/ c , a 12.5435(9), b 12.4324(9), c 6.7121(4) A, β 115.332(2)°, V 946.07(19) A 3 , Z = 4. The six strongest lines in the X-ray powder-diffraction pattern [ d in A(I)( hkl )] are as follows: 3.187(100)(112), 2.726(90)(402, 240), 6.204(80)(020), 2.788(80)(330), 5.653(70)(200), and 2.580(70)(132, 420). Chemical analysis by electron microprobe gave P 2 O 5 46.66, FeO 0.49, MnO 29.31, CaO 12.51, Na 2 O 6.87, H 2 O (calc.) 3.93, sum 99.77 wt.%, where the H 2 O content was determined by crystal-structure refinement as OH = 2.0 apfu . The resulting empirical formula, on the basis of 10 O atoms and 2 (OH) groups pfu , is Na 1.02 Ca 1.02 (Mn 1.90 Fe 2+ 0.03 ) ∑ 1.93 P 3.02 O 10 (OH) 2 . The crystal structure of groatite was refined to an R index of 2.7% based on 831 unique reflections collected on a four-circle diffractometer with Mo K α X-radiation and a 4K CCD detector. There are two tetrahedrally coordinated T -sites occupied by P. There are two octahedra, M (1) and M (2), and an eight-coordinated A (2) site which are occupied by Ca, Mn and Na, respectively. The observed site-scattering, mean bond-lengths and bond-valence sums at these sites are in agreement with complete chemical order of Ca at M (1), Mn 2+ at M (2) and Na at A (2). A (PO 4 ) group is centered at the T (1) site and a PO 3 (OH) group at T (2). The H atom is located near the O(4) anion and provides a strong hydrogen bond to the O(2) anion. Groatite is isostructural with the acid arsenate mineral o’danielite and is a member of the alluaudite group; it is the first alluaudite-group mineral with an acid phosphate group. The new mineral is named in honor of Lee A. Groat, Professor of Mineralogy at the University of British Columbia in Vancouver. The species and the name groatite were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2008–054).