Abstract

Tvrdýite, Fe2+forumlaAl3(PO4)4(OH)5(OH2)4·2H2O, is a new phosphate mineral from the abandoned Huber open pit, in the Krásno ore district near Horní Slavkov, western Bohemia, Czech Republic. It was found along with Al-rich beraunite, fluorapatite and pharmacosiderite in a cavity of quartz gangue. Tvrdýite forms acicular to fibrous crystals with diameters in the range 0.5–5 μm and lengths 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 is very brittle with an 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 α = 1.650(2), β = 1.671(1) and γ = 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(4), b = 5.101(1), c = 18.883(4) Å, β = 93.68(3)° and V = 1976.7(7) Å3, Z = 4, a:b:c = 4.031:1:3.702. The strongest eight lines in the powder X-ray diffraction (XRD) pattern [d in Å (I)(hkl)] are 10.227 (100) (200), 9.400 (6) (002), 7.156 (14) (20forumla), 5.120 (7) (400), 3.416 (11) (600), 3.278 (6) (60forumla), 2.562 (5) (800) and 2.0511 (3) (forumla,0,0). 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 Zn0.52forumlaFforumlaAl2.75(PO4)3.86(AsO4)0.19OH4.60F0.23(OH2)4·2H2O. The ideal formula, Fe2+FforumlaAl3(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 > 2σ(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.

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