Graftonite-(Mn), ideally M1MnM2,M3Fe2(PO4)2, and graftonite-(Ca), ideally M1CaM2,M3Fe2(PO4)2, two new minerals of the graftonite group from Poland

• Published in: Mineralogical magazine Vol. 82; no. 6; pp. 1307 - 1322
• Main Authors: Pieczka, Adam, Hawthorne, Frank C, Ball, Neil A, Abdu, Yassir A, Golebiowska, Bozena, Wlodek, Adam, Zukrowski, Jan
• Format: Journal Article
• Language: English
• Published: London Mineralogical Society 01.12.2018; Cambridge University Press
• Subjects: beryl; Central Europe; Classification; columbite; crystal structure; Crystals; Dolnoslaskie Poland; Europe; formula; Geology; graftonite; granites; Grants; High temperature; igneous rocks; lattice parameters; lithiophilite; Lower Silesia; Lutomia Poland; melts; Metamorphism; Michalkowa Poland; Mineralogy; Minerals; Museums; new minerals; niobates; nonsilicates; optical properties; oxides; pegmatite; Phosphates; plutonic rocks; Poland; Quartz; ring silicates; saline composition; sarcopside; silicate melts; silicates; Sowie Mountains; space groups; Sudeten Mountains; temperature; triphylite; unit cell; X-ray diffraction; Poland; Canada
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/minmag.2017.081.109

Two new minerals of the graftonite group, graftonite-(Mn), ideally M(1)MnM(2),M(3)Fe2(PO4)2, and graftonite-(Ca), ideally M(1)CaM(2),M(3)Fe2(PO4)2, were discovered in phosphate nodules of two beryl-columbite-phosphate pegmatites at Lutomia and Michalkowa, respectively, in the Gory 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 the M(1) site, the formulae of the holotype crystals are M(1)(Mn0.70 Ca0.30)M(2),M(3) (Fe1.34 Mn0.60 Mg0.06 Zn0.01)Σ3(PO4)2 for graftonite-(Mn) and M(1)(Ca0.98 Mn0.02)M(2),M(3) (Fe1.38 Mn0.56 Mg0.05)Σ3(PO4)2 for graftonite-(Ca). Both crystal chemistry and crystal-structure refinement (R1=2.34 and 1.63%, respectively) indicate that the M(1) site is occupied dominantly by Mn in graftonite-(Mn) and by Ca in graftonite-(Ca), and the M(2) and M(3) sites are occupied by Fe2+ and Mn2+, with Fe2+ dominant over Mn2+ at the aggregate M(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 symmetry P21/c), with the unit-cell parameters a=8.811(2) Å, b=11.494(2) Å, c=6.138(1) Å, β=99.23(3)° and V=613.5(4) Å3, and a=8.792(2) Å, b=11.743(2) Å, c=6.169(1) Å, β=99.35(3)° and V=628.5(1) Å3, respectively. The densities calculated on the basis of molar weights and unit-cell volumes are 3.793 g/cm3 for graftonite-(Mn) and 3.592 g/cm3 for 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, (311); 2.952, 55, (131); 2.916, 53, (112); 2.899, 44, (300); 3.016, 35, (102); and 3.654, 100, (130); 2.979, 85, (221); 3.014, 77, (230); 3.042, 76, (040+112); 2.834, 68, (311); 3.097, 57, (131); 3.133, 56, (102); 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.