Carbonate-Bearing, F-Overcompensated Fluorapatite in Magnesian Exoskarns from Valea Rea, Budureasa, Romania

Abstract

A carbonate-bearing, fluorine-overcompensated fluorapatite (F = 4.42 wt.% as compared with 3.77 wt.% F in the Ca5(PO4)3F end-member), was identified in forsterite-bearing skarns from Valea Rea (N 46°39′48″, E 22°36′43″), located near the contact of the granodiorite laccolith from Budureasa, of Upper Cretaceous Age, with Anisian dolostones. The chemical structural formula (with carbonate not included) is: (Ca4.989Mn0.001Fe2+0.003Mg0.003Ce0.001La0.001)(P2.992Si0.008)(O11.894F1.202Cl0.001). No major structural distortions due to (CO3F)3--for-(PO4)3- replacement were identified by single crystal X-ray diffraction, Raman or FTIR. The mineral crystallizes in space group P63/m, having as cell parameters a = 9.3818(1) Å and c = 6.8872(1) Å. The indices of refraction are: ω = 1.634(2) and ε = 1.631(1). The calculated density is Dx = 3.199 g/cm3 and the measured density is Dm = 3.201(3) g/cm3. Calculation of the Gladstone–Dale compatibility indices gave in all cases values indicative of superior agreement between physical and chemical data. In the infrared spectra, the multiplicity of the bands assumed to phosphate modes (1ν1 + 2ν2 + 3ν3 + 3ν4) agrees with the reduction of the symmetry of PO43− ion from Td to C6. Chemical peculiarities and textural relations agree with a hydrothermal origin of the mineral, crystallized from F-rich fluids originating from the granodiorite intrusion.