Fluoralforsite, Ba5(PO4)3F – a new apatite-group mineral from the Hatrurim Basin, Negev Desert, Israel

Abstract

AbstractFluoralforsite, ideally Ba5(PO4)3F, (space group P63/m (#176), Z = 2, a = 10.0031(2) Å, c= 7.5382(2) Å and V = 653.23(3) Å3), is a new mineral species of the apatite group – a Ba-analogue of fluorapatite and a F-analogue of alforsite. It was discovered in rankinite paralava filling cracks in pyrometamorphic gehlenite hornfels near the tributary of wadi Zohar and Gurim Anticline, Hatrurim Basin, Negev Desert, Israel. Fluoralforsite occurs in small intergranular spaces between large gehlenite and garnet crystals and in enclaves inside large rankinite crystals with other Ba minerals such as walstromite, zadovite, bennesherite, gurimite, mazorite, barioferrite and baryte. It forms tiny transparent, colourless crystals up to 50 μm with a white streak and a vitreous lustre. The cleavage was not observed. It exhibits a brittle tenacity and a conchoidal fracture. The estimated Mohs hardness is 4–4½, and its calculated density is 4.57 g/cm–3. Fluoralforsite is uniaxial (–) with refractive indices (589 nm) nω = 1.689(3) and nɛ = 1.687(3). The empirical crystal-chemical formula for the holotype calculated on the basis of 8 cations is: (Ba3.81Ca0.97Na0.07K0.05Sr0.05Fe0.05)Σ5(P5+2.32V5+0.29S6+0.22Si0.17)Σ3O12(F0.85Cl0.13)Σ0.98. The crystal structure was refined from single-crystal X-ray diffraction data with R1 = 0.0192. The structural investigation indicated an ordered arrangement of Ba/Ca at the M1 site within individual columns running along the c-axis, but a disordered distribution among adjacent columns throughout the structure, which enables the maintenance of the P63/m space group. Fluoralforsite was formed at the final stage of crystallisation as a result of a reaction between the primary mineral assemblages and residual melt.