Crimsonite, PbFe3+2(PO4)2(OH)2, the phosphate analogue of carminite from the Silver Coin mine, Valmy, Nevada, USA

Abstract

AbstractCrimsonite (IMA2014-095), PbFe3+2(PO4)2(OH)2, the phosphate analogue of carminite, is a new mineral from the Silver Coin mine, Valmy, Iron Point district, Humboldt County, Nevada, USA, where it occurs as a low-temperature secondary mineral in association with fluorwavellite, goethite, hematite, hentschelite, plumbogummite and variscite on quartz. Crimsonite occurs in subparallel aggregates of deep red blades or plates flattened on {100} and up to 0.1 mm in maximum dimension. The streak is light purplish orange. Crystals are transparent and have adamantine lustre. The Mohs hardness is ∼3½, the tenacity is brittle, the fracture is irregular to splintery and an imperfect cleavage is likely on {101}. The calculated density is 5.180 g/cm3. Crimsonite is optically biaxial (+), with 2V = 85.5(5)° and γ – α = 0.011. Using the Gladstone-Dale relationship, the calculated indices of refraction are α = 2.021, β = 2.026 and γ = 2.032. The optical orientation isX=b;Y=a;Z=cand the pleochroism isXlight orange,Ylight yellow,Zred brown;Y<X< Z. Electron microprobe analyses provided PbO 40.69, CaO 0.60, ZnO 0.72, CuO 0.13, Fe2O323.36, Al2O30.34, V2O50.70, As2O512.05, P2O516.03, SO30.33 and H2O 3.64 (structure), total 98.59 wt.%. The empirical formula (based on 10 O apfu) is (Pb1.06Ca0.06)∑1.12(Fe1.71Zn0.05Al0.04Cu0.01)∑1.81(P1.32As0.61V0.05S0.02)∑2.00O8[(OH)1.64(H2O)0.36]∑2.00. Crimsonite is orthorhombic,Cccm,a= 16.2535(13),b= 7.4724(4),c= 12.1533(9) Å,V= 1476.04(17) Å3and Z = 8. The eight strongest lines in the powder X-ray diffraction pattern are [dobsin Å(I)(hkl)]: 5.86(42)(111); 4.53(45)(112); 3.485(64)(113); 3.190(100) (022); 3.026(40)(004); 2.902(54)(511); 2.502(77)(422) and 2.268(54)(224). The structure of crimsonite (R1= 3.57% for 740Fo> 4σF) contains FeO6octahedra that share edges to form dimers, which are then linked to other dimers by corner sharing to form chains along [010]. These chains are linked by PO4tetrahedra yielding sheets parallel to {001}. The sheets are linked to one another via bonds to 8-coordinated Pb2+atoms with non-stereoactive 6s2lone-electron pairs.