Redefinition of beraunite, Fe³⁺₆(PO₄)₄O(OH)₄  ⋅  6H₂O, and discreditation of the name eleonorite: a re-investigation of type material from the Hrbek Mine (Czech Republic)

Abstract

Re-investigation of the type material of beraunite from the Hrbek Mine, Svatá Dobrotivá, Czech Republic, proved the identity of beraunite and eleonorite. Based on this study, the mineral eleonorite was discredited, and the ideal formula of beraunite was redefined from Fe2+Fe53+(PO4)4(OH)5 ⋅ 6H2O to Fe63+(PO4)4O(OH)4 ⋅ 6H2O. Beraunite from Hrbek Mine usually forms prismatic crystals flattened on {100}; elongated along b axis, striated on {100} || b, up to 0.2 × 0.5 × 3 mm, often in radial clusters up to 6 mm in size in association with black goethite, olive-green to dark green dufrénite and yellow to orange cacoxenite. Beraunite is hyacinth red, dark red, or red-brown and has a light orange-red streak; crystals are translucent, with a vitreous luster. The symmetry of the structure was found to be lower (Cc) than reported in the past (C2/c); this has been proven by the crystal structure refinement, Robs= 2.7 % for Cc and ∼ 8 % for C2/m. Refined unit-cell parameters obtained from powder X-ray diffraction data of the original material of Friedrich August Breithaupt are a= 20.653(2), b= 5.1433(6), c= 19.241(2) Å, β=93.560(9)∘, and V= 2039.9(2) Å3. Calculated density is 2.961 g cm−3. Beraunite is optically biaxial (+), α= 1.768(2), β= 1.781(3), γ>1.805, 2Vmeas= 69(4)∘, and 2Vcalc is not possible to calculate. Dispersion of optical axes is strong, r>v. The orientation is Y=b, X≈a, and Z≈c. Pleochroism is strong; Z (brown-red) ≫Y>X (both brownish yellow). The empirical formula of the original beraunite sample calculated on the basis of P = 4 apfu is (Fe5.763+Al0.15Zn0.01)5.92(PO4)4.00O0.99(OH)3.77 ⋅ 6H2O. The 57Fe Mössbauer spectrum of beraunite neotype (National Museum, Prague) from type locality implies that all iron atoms in beraunite structure are exclusively trivalent, located at the M site with different next-nearest-neighbor configurations. There is no spectral evidence for Fe2+ or iron in some impurities (e.g., limonite).