Tvrdyite, Fe2+Fe23+ Al-3(PO4)(4)(OH)(5)(OH2)(4)center dot 2H(2)O, a new phosphate mineral from Krasno near Horni Slavkov, Czech Republic

摘要

Tvrdyite, Fe2+Fe23+Al3(PO4)(4) (OH)(5) (OH2)(4) center dot 2H(2)O, is a new phosphate mineral from the abandoned Huber open pit, in the Krasno ore district near Horni Slavkov, western Bohemia, Czech Republic. It was found along with Al-rich beraunite, fluorapatite and pharmacosiderite in a cavity of quartz gangue. Tvrdyite forms acicular to fibrous crystals with diameters in the range 0.5-5 mu m and lengths up to 300 mu m, partly grouped in radiating aggregates up to 3 mm in size. It has a silvery to olive, greyish green colour with pearly lustre, greyish-white streak and is very brittle with an uneven fracture; individual fibres are somewhat flexible. Cleavage on {100} is good; the Mohs hardness is similar to 3-4. The calculated density is 2.834 g cm(3). Tvrdyite is optically biaxial (-), with alpha = 1.650(2), beta = 1.671(1) and gamma = 1.677(1) (white light); 2V = 56(1)degrees; dispersion: r > nu, strong; optical orientation: Z = b, X approximate to a, Y approximate to c; pleochroism: X = greenish blue, Y = yellowish orange, Z = yellowish orange (X Y > Z). Tvrdyite is monoclinic, space group C2/c, a = 20.564(4), b = 5.101(1), c = 18.883(4) angstrom, beta = 93.68(3)degrees and V = 1976.7(7)angstrom(3), Z = 4, a:b:c = 4.031:1:3.702. The strongest eight lines in the powder X-ray diffraction (XRD) pattern [d in angstrom (I)(hkl)] are 10.227 (100) (200), 9.400 (6) (002), 7.156 (14) (20 (2) over bar), 5.120 (7) (400), 3.416 (11) (600), 3.278 (6) (60 (2) over bar), 2.562 (5) (800) and 2.0511 (3) ((10) over bar0,0). Chemical analyses by electron microprobe yielded MnO 0.01, ZnO 5.08, FeO 4.31, Fe2O3 21.16, Al2O3 16.71, P2O5 32.64, As2O5 2.56, F 0.53, H2O (calc.) 17.84, O = F -0.22, total 100.62 wt.%. The resulting empirical formula, calculated on the base of 27 anions, obtained from the crystal structure, is Zn0.52Fe0.502+ Fe-2.21(3+) Al-2.75(PO4)(3.86)(AsO4)(0.19)OH4.60F0.23(OH2)(4) center dot 2H(2)O. The ideal formula, Fe2+Fe23+Al3 (PO4)(4) (OH)(5) (OH2)(4) center dot 2H(2)O, requires FeO 8.75, Fe2O3 19.44, Al2O3 18.62, P2O5 34.56, H2O 18.64, total 100.00 wt.%. The crystal structure of tvrdyite was solved from single-crystal data (synchrotron beamline) and refined to R-1 = 0.038 for 2276 reflections with I > 2 sigma(I). Tvrdyite is isostructural with beraunite, but contains dominant Al in two of the four independent M sites, which are all occupied by Fe in beraunite.