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

Fluoralforsite, ideally Ba₅(PO₄)₃F, (space group P6_3/m(#176), Z = 2, a = 10.0031(2) A, c= 7.5382(2) A, V = 653.23(3) A³), is a new mineral species of the apatite group-a Ba-Analogue of fluorapatite and an 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 4-41/2, and its calculated density is 4.57 g/cm^-3. Fluoralforsite is uniaxial (-) with refractive indices (589 nm) nω = 1.689(3), nϵ = 1.687(3). The empirical crystal-chemical formula for the holotype calculated based on 8 cations is as follows: (Ba_3.81Ca_0.97Na_0.07K_0.05Sr_0.05Fe_0.05)_Σ5(P_5+2.32V_5+0.29S_6+0.22Si_0.17)_Σ3O₁₂(F_0.85Cl_0.13)_Σ0.98. The crystal structure was refined from single-crystal X-ray diffraction data with R = 0.0192. The structural investigation indicated an ordered arrangement of Ba/Ca at 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 P6^3/mspace group. Fluoralforsite was formed at the final stage of crystallisation as a result of a reaction between the primary mineral assemblages and residual melt.