Fluorcarmoite-(BaNa), the first Mg-dominant mineral of the arrojadite group

Fluorcarmoite-(BaNa), ideally ^A1Ba^A2□^B1,2Na^Na1,2Na₂^Na3□^CaCa^MMg₁₃Al(PO₄)₁₁(PO₃OH)^WF₂, was found in a pebble of the riverbed of the upper Maremola Creek, close to the village of Isallo, in the Magliolo municipality (Savona, Liguria, Italy). The root-name is after Monte Carmo di Loano, the highest peak in the area, namesake of the tectonic unit where the mineral was found and the first locality where phosphate mineralization has been found in the region. The mineral is associated with quartz and almandine and has microscopic inclusions of fluorapatite and possible graftonite. It occurs as yellow–orange and translucent crystals in an anhedral centimetric nodule embedded in quartz. Fluorcarmoite-(BaNa) is brittle, and no cleavage or parting was observed. It has a yellow–orange streak, a vitreous lustre, does not fluoresce under shortwave or longwave ultraviolet light and is weakly pleochroic (light yellow). Fluorcarmoite-(BaNa) is optically biaxial positive, with α = 1.6240(5), β = 1.6255(5), γ = 1.6384(5) (589 nm), 2V_meas = 35(2)° and 2V_calc = 37.9°. Raman spectroscopy shows the presence of weak bands in the OH-stretching region. The average chemical composition is (wt%, wavelength-dispersive-mode electron microprobe): Na₂O 5.83, K₂O 0.36, CaO 2.64, SrO 0.46, BaO 7.12, MnO 2.01, FeO 17.68, MgO 15.12, Al₂O₃ 2.57, P₂O₅ 44.96, F 2.14, –O = F₂ 0.90, H₂O_calc 0.33, total 100.32. The empirical formula calculated on the basis of 50 O + F + (OH) atoms per formula unit (apfu), is: (Na_3.77Ca_0.94Ba_0.93K_0.15Sr_0.09□_0.12)_Σ=6.00(Mg_7.52$Fe4.932+Mn0.572+$⁠)_Σ=13.02Al_1.01(PO₄)₁₁(PO₃)(OH_0.74F_0.26)F₂. Strongest lines in the X-ray powder diffraction pattern are [d in Å (I_calc) hkl]: 4.959 (25) 020, 4.524 (20) 114, 3.188 (28) 206, 3.012 (100) 42$4¯$⁠, 2.735 (32) 60$2¯$⁠, 2.682 (39) 226, 2.526 (25) 424. The crystal structure has been refined using single-crystal X-ray diffractometer data (R_int = 4.1%) in space group Cc (no. 14) to R₁ = 0.0342 for 11 511 reflections with F_o > 4σ|F| and 0.0417 for all 13 232 data. Refined unit-cell parameters are: a = 16.4013(3) Å, b = 9.9487(1) Å, c = 24.4536(8) Å, β = 105.725(2)°, V = 3840.80(15) ų (Z = 4). Fluorcarmoite-(BaNa) is the first Mg-dominant mineral of the arrojadite group. Mg orders preferentially in the M1, M2b, M3a,b, M4a,b and M7a,b sites whereas the non-dominant Fe²⁺ and very minor Mn²⁺ show site preference for M2a, M5a,b and M6a,b. The A1 site is mostly populated by Ba, the A2 site is empty, and minor Fe²⁺ occurs at the B1b site. A significant, but not dominant occupancy of the Na3 site by Na is also observed. Only Ca and Al are present at the Ca and Al sites, respectively. The type material is deposited in the mineralogical collection of the Museo Regionale di Scienze Naturali di Torino, Sezione di Mineralogia, Petrografia e Geologia, Torino (Italy). The mineral and its name have been approved by the IMA-CNMNC (2015-062).