Reference:
▪ Sejkora, J., Grey, I.E., Kampf, A.R., Price, J.R., Cejka, J. (2016): Tvrdýite, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, a new phosphate mineral from Krásno near Horní Slavkov, Czech Republic. Mineralogical Magazine, 80, 1077-1088.
Abstract:
Tvrdýite, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, is a new phosphate mineral from the abandoned Huber open pit, the Krásno ore district near Horní lavkov, western Bohemia, Czech Republic. It was found along with Al-rich beraunite, fluorapatite and pharmacosiderite in cavity of quartz gangue. Tvrdýite forms acicular to fibrous crystals with diameters in the range 0.5 - 5 μm and length up to 300 μm, 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 it is very brittle with uneven fracture, individual fibres are somewhat flexible. Cleavage on {100} is good, the Mohs hardness is ~3–4. The calculated density is 2.834 g cm-3. Tvrdýite is optically biaxial (-), with alpha = 1.650(2), beta = 1.671(1) and gamma = 1.677(1) (white light); 2V = 56(1)°; dispersion: r > v, strong; optical orientation: Z = b, X ≈ a, Y ≈ c; pleochroism: X = greenish blue, Y = yellowish orange, Z = yellowish orange (X >> Y > Z). Tvrdýite is monoclinic, space group C2/c, a = 20.564, b = 5.101(1), c = 18.883(4) Å, beta = 93.68(3)° and V = 1976.7(7) Å3, with Z = 4, a:b:c = 4.031:1:3.702. The strongest eight lines in powder X-ray diffraction (XRD) pattern [d in Å (I)(hkl)] are: 10.227 (100) (200), 9.400 (6) (002), 7.156 (14) (20-2), 5.120 (7) (400), 3.416 (11) (600), 3.278 (6) (60-2), 2.562 (5) (800) and 2.0511 (3) (1000). The 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 Zn0.52Fe2+ 0.50Fe3+ 2.21Al2.75(PO4)3.86(AsO4)0.19OH4.60F0.23(OH2)4·2H2O. The ideal formula, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, requires FeO 8.75, Fe2O3 19.44, Al2O3 18.62, P2O5 34.56, H2O 18.64, total 100.00 wt.%. The crystal structure of tvrdýite was solved from single-crystal data (synchrotron beamline) and refined to R1 = 0.038 for 2276 reflections with [I > 2δ(I)]. Tvrdýite is isostructural with beraunite, but contains dominant Al in two of the four independent M sites, which are all occupied by Fe in beraunite.
▪ Sejkora, J., Grey, I.E., Kampf, A.R., Price, J.R., Cejka, J. (2016): Tvrdýite, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, a new phosphate mineral from Krásno near Horní Slavkov, Czech Republic. Mineralogical Magazine, 80, 1077-1088.
Abstract:
Tvrdýite, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, is a new phosphate mineral from the abandoned Huber open pit, the Krásno ore district near Horní lavkov, western Bohemia, Czech Republic. It was found along with Al-rich beraunite, fluorapatite and pharmacosiderite in cavity of quartz gangue. Tvrdýite forms acicular to fibrous crystals with diameters in the range 0.5 - 5 μm and length up to 300 μm, 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 it is very brittle with uneven fracture, individual fibres are somewhat flexible. Cleavage on {100} is good, the Mohs hardness is ~3–4. The calculated density is 2.834 g cm-3. Tvrdýite is optically biaxial (-), with alpha = 1.650(2), beta = 1.671(1) and gamma = 1.677(1) (white light); 2V = 56(1)°; dispersion: r > v, strong; optical orientation: Z = b, X ≈ a, Y ≈ c; pleochroism: X = greenish blue, Y = yellowish orange, Z = yellowish orange (X >> Y > Z). Tvrdýite is monoclinic, space group C2/c, a = 20.564, b = 5.101(1), c = 18.883(4) Å, beta = 93.68(3)° and V = 1976.7(7) Å3, with Z = 4, a:b:c = 4.031:1:3.702. The strongest eight lines in powder X-ray diffraction (XRD) pattern [d in Å (I)(hkl)] are: 10.227 (100) (200), 9.400 (6) (002), 7.156 (14) (20-2), 5.120 (7) (400), 3.416 (11) (600), 3.278 (6) (60-2), 2.562 (5) (800) and 2.0511 (3) (1000). The 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 Zn0.52Fe2+ 0.50Fe3+ 2.21Al2.75(PO4)3.86(AsO4)0.19OH4.60F0.23(OH2)4·2H2O. The ideal formula, Fe2+Fe3+2Al3(PO4)4(OH)5(OH2)4·2H2O, requires FeO 8.75, Fe2O3 19.44, Al2O3 18.62, P2O5 34.56, H2O 18.64, total 100.00 wt.%. The crystal structure of tvrdýite was solved from single-crystal data (synchrotron beamline) and refined to R1 = 0.038 for 2276 reflections with [I > 2δ(I)]. Tvrdýite is isostructural with beraunite, but contains dominant Al in two of the four independent M sites, which are all occupied by Fe in beraunite.