Abstract
During the revision of beraunite like minerals from old localities represented by historical samples in the mineralogical collection of the National Museum in Prague, the second occurrence of tvrdýite was confirmed in the iron ores from abandoned iron deposit Krušná hora in the Central Bohemia (Czech Republic). Krušná hora is situated about 12 km NW of Beroun (30 km WSW of Prague) in an area of the Ordovician sedimentary rocks of the Teplá-Barrandian unit. Tvrdýite forms brown-green, yellow-green to light-green radial aggregates up to 3 mm in size. Tvrdýite is monoclinic, space group C2/c with following unit-cell parameters refined from the X-ray powder diffraction data: a 20.529(9), b 5.105(2), c 18.869(8) Å, β 92.8(4)° and V 1975.1(8) Å3; its empirical formula is (Na+0.13Fe2+0.86Mg2+0.01)Σ1.00(Al3+2.39Fe3+0.57)Σ2.96(Fe3+1.93Al3+0.03)Σ1.96[(PO4)3.99(VO4)0.01]Σ4.00(OH)4.61(OH2)4.00·2H2O. The mineral was found in association with smaller bluish radial aggregates of unidentified Fe-Al phosphate (probably Al-rich dufrénite or different generation of tvrdýite).
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