Graftonite-(Mn), ideallyM1MnM2,M3Fe2(PO4)2, and graftonite-(Ca), ideallyM1CaM2,M3Fe2(PO4)2, two new minerals of the graftonite group from Poland

Abstract

AbstractTwo new minerals of the graftonite group, graftonite-(Mn), ideallyM(1)MnM(2),M(3)Fe2(PO4)2, and graftonite-(Ca), ideallyM(1)CaM(2),M(3)Fe2(PO4)2, were discovered in phosphate nodules of two beryl–columbite–phosphate pegmatites at Lutomia and Michałkowa, respectively, in the Góry Sowie Block, Lower Silesia, southwest Poland. Graftonite-(Mn) is pinkish brown, whereas graftonite-(Ca) shows more brownish colouration. Both minerals have a vitreous lustre, a good cleavage observed along (010) and irregular fracture; both are transparent and neither of them is fluorescent. They are brittle and have a Mohs hardness of ~5. The minerals are non-pleochroic, colourless in all orientations, biaxial (+), with mean refractive indices α = 1.710(2) and 1.690(2), β = 1.713(2) and 1.692(2), and γ = 1.725(2) and 1.710(5), respectively. With complete order of Ca at theM(1) site, the formulae of the holotype crystals areM(1)(Mn0.70Ca0.30)M(2),M(3)(Fe1.34Mn0.60Mg0.06Zn0.01)Σ3(PO4)2for graftonite-(Mn) andM(1)(Ca0.98Mn0.02)M(2),M(3)(Fe1.38Mn0.56Mg0.05)Σ3(PO4)2for graftonite-(Ca). Both crystal chemistry and crystal-structure refinement (R1= 2.34 and 1.63%, respectively) indicate that theM(1) site is occupied dominantly by Mn in graftonite-(Mn) and by Ca in graftonite-(Ca), and theM(2) andM(3) sites are occupied by Fe2+and Mn2+, with Fe2+dominant over Mn2+at the aggregateM(2) +M(3) sites. Graftonite-(Mn) and graftonite-(Ca) are isostructural with graftonite,M(1)FeM(2),M(3)Fe2(PO4)2(monoclinic system; space-group symmetryP21/c), with the unit-cell parametersa= 8.811(2) Å,b= 11.494(2) Å,c= 6.138(1) Å, β = 99.23(3)° and V = 613.5(4) Å3, anda= 8.792(2) Å,b= 11.743(2) Å,c= 6.169(1) Å, β = 99.35(3)° andV= 628.5(1) Å3, respectively. The densities calculated on the basis of molar weights and unit-cell volumes are 3.793 g/cm3for graftonite-(Mn) and 3.592 g/cm3for graftonite-(Ca). The eight strongest lines in powder X-ray diffraction patterns on the basis of single-crystal data are, respectively [d, Å,I(hkl)]: 2.874, 100, (230 + 040); 2.858, 79, (221); 3.506, 73, (130); 2.717, 79, ($\bar{3}$11); 2.952, 55, (131); 2.916, 53, ($\bar{1}$12); 2.899, 44, (300); 3.016, 35, ($\bar{1}$02); and 3.654, 100, (130); 2.979, 85, (221); 3.014, 77, (230); 3.042, 76, (040 +$\bar{1}$12); 2.834, 68, ($\bar{3}$11); 3.097, 57, (131); 3.133, 56, ($\bar{1}$02); 2.542, 30, (311). Both minerals are common primary phosphates in phosphate nodules, occurring as lamellar intergrowths with sarcopside ± triphylite/lithiophilite, products of exsolution from a (Li,Ca)-rich graftonite-like parent phase crystallized at high temperature from P-bearing hydrosaline melts.