Bulletin Mineralogie Petrologie

Vrtiška Luboš, Sejkora Jiří, Malíková Radana, Kadlec Tomáš

Volume 25, issue 2 (2017), pages 277-305

fluorwavellite, variscite, kidwellite, natrodufrénite, strengite, phosphate occurence, uranium minerals, unit-cell parameters, chemical composition, Raman spectra, Na Zoubku, Černovice, Tábor, Czech Republic

An interesting supergene phosphate and uranium mineral association was found on the historical wavellite occur- rence Černovice near Tábor (Vysočina region, Czech Republic). Phosphates are bound to cracks and cavities in graphitics gneisses and quartzites. Beraunite forms radially fibrous greenish, greenish brown to reddish brown aggregates up to 1.5 mm in size; its empirical formula is (Fe²⁺_0.58Zn²⁺_0.08)_Σ0.66(Fe³⁺_4.44Al³⁺_0.56)_Σ5.00[(PO₄)_3.86(SiO₄)_0.13(VO₄)_0.01]_Σ4.00

(OH)_4.45·6H₂O and refined unit-cell parameters are a 20.647(4), b 5.1332(15), c 19.214(4) Å, β 93.6(3)° and V 2032.3 ų. Carnotite was found as yellow powdery aggregates up to 1 mm accross in association with metatorbernite, phosphosiderite and metavariscite; its empirical formula is  K_1.88Ba_0.10Ca_0.01(UO₂)_1.99(VO₄)_2.00·3H₂O. Historically known wavellite has been redefined as fluorwavellite. It is the most abundant phosphate in the studied mineral association, it usually occurs as spherical aggregates up to 5 mm in size. Its colour is significantly zonal, from blue in the centre of aggregates to yellow in the marginal part. Its empirical formula (Al_2.74V_0.12Cr_0.01Fe_0.01K_0.01)_Σ2.89[(PO₄)_1.98(SiO₄)_0.02]_Σ2.00(F_0.72(OH)_0.28) (OH)_1.57·5H₂O and refined unit-cell parameters are a 9.6285(13), b 17.374(3), c 6.9953(8) Å, V 1170.2(3) ų. Phosphosiderite-metavariscite forms light green crusts on quartz crystals thickeness up to 0.1 mm; its empirical formula is (Fe_0.57Al_0.40)_Σ0.97[(PO₄)_0.99(VO₄)_0.01]_Σ1.00F_0.03·2H₂O and refined unit-cell parameters are a 5.324(14), b 9.83(2), c 8.722(19) Å, β 90.6(3)° and V 456(1) ų.  Cacoxenite occurs as yellow crusts an radial aggregates up to 0.2 m in size with empirical formula K_0.07Ca_0.06Fe³⁺_20.21Al_3.89O_6.00[(PO₄)_16.66(SiO₄)_0.28(VO₄)_0.06]_Σ17.00(OH)_9.75·75H₂O and unit-cell parameters: a 27.556(9), c 10.5570(3) Å, and V 6942.4(8) ų.  Kidwellite occurs as green spherical aggregates up to 0.1 mm in size. Its empirical formula is (Na_0.56K_0.03□_0.41)_Σ1.00(Al_1.16Ca_0.26Mg_0.06Fe³⁺_8.15)_Σ9.63[(PO₄)_5.91(VO₄)_0.06(SiO₄)_0.03]_Σ6.00(OH)_11.20·3H₂O and refined unit-cell parameters are a 20.12(12), b 5.187(18), c 13.974(9) Å, β  107.1(6)° and V 1395(1) ų. Leucophosphite was found only as a very rare yellow to yellowish green tabular crystals and their aggregates up to 0.01 mm in size on hemispherical beraunite; it was verified only by Raman spectroscopy and qualitative EDS analysis. Metatorbernite forms green tabular crystals up to 3 mm in size; its empirical formula is (Cu_1.13Ca_0.01)_Σ1.14(UO₂)_1.97[(PO₄)_1.99(VO₄)_0.01]_Σ2.00

·8H₂O and refined unit-cell parameters are a 6.969(4), c 17.3316(3) Å, and V 841.9(5) ų. Natrodufrénite was found as dark green to bluish green aggregates with light green or yellowish green zones forming radial aggregates up to  1.5 mm in size; its empirical formula is (Na_0.65Ca_0.13K_0.01□_0.21)_Σ1.00(Fe²⁺_0.96Zn_0.04)_Σ1.00(Fe³⁺_4.44Al_0.34)_Σ4.78[(PO₄)_3.93 (SiO₄)_0.07]_Σ4.00(OH)_5.33·2H₂O and refined unit-cell parameters are a 25.872(12), b 5.149(3), c 13.785(8) Å, β 111.5(5)° and V 1708(1) ų. Strengite was observed as spherical aggregates of white, light gray, beige or yellow to orange color up to 4 mm in size in four different mineral associations; empirical formulas of three most common strengites are: Fe_0.77Al_0.21(PO₄)_1.00·2H₂O (strengite I; association with natrodufrénite); Fe_0.92Al_0.06(PO₄)_1.00·2H₂O (strengite II; association with turquoise); Fe_0.84Al_0.10((PO₄)_0.98(VO₄)_0.02)_Σ1.00·2H₂O (strengite III; association with kidwellite); refined unit-cell parameters for all studied types are comparable a 8.720(4), b 9.877(5), c 10.115(5) Å and V 871.1(7) ų. Turquoise forms apple green zonal crystals up to 0.1 mm in size; empirical formula (Cu_0.59□_0.35K_0.03Zn_0.02Ca_0.01)_Σ1.00(Al_5.03Fe³⁺_0.81)_Σ5.84

[(PO₄)_3.64(PO₃OH)_0.35(SiO₄)_0.01]_Σ4.00(OH)_6.40F_0.37.4H₂O (central parts of aggregates) and (Cu_0.70□_0.25K_0.01Zn_0.03Ca_0.01)_Σ1.00

(Al_4.28Fe³⁺_1.52)_Σ5.80[(PO₄)_3.73(PO₃OH)_0.25(SiO₄)_0.01]_Σ3.99(OH)_6.76F_0.13·4H₂O (marginal parts) and unit-cell parameters are a 7.425(8), b 7.628(7), c 9.921(8) Å, α 68.59(7)°, β 69.67(8)°, γ 65.08(7)° and V 461.8(5) ų. Two varieties of variscite was observed; white variscite forms crusts on the surface of spherical aggregates of the strengite II; its empirical formula is Al_0.87Fe_0.14(PO₄)_1.00F_0.03·2H₂O; and green variscite with Al_0.97Fe_0.05((PO₄)_0.98(SiO₄)_0.01(VO₄)_0.01)_Σ1.00F_0.05·2H₂O  was found as light green crusts of thickness up to 1 mm in size. Unit-cell parameters of green variscite are a 9.909(4), b 9.664(3), c 17.184(6) Å, V 1645.5(6) ų.

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