Magnesio-ferri-fluoro-hornblende from Portoscuso, Sardinia, Italy: description of a newly approved member of the amphibole supergroup

Magnesio-ferri-fluoro-hornblende has the ideal formula A□B Ca2C(Mg4Fe3+)T(Si7Al)O22WF2(Hawthorne et al., 2012). The holotype sample described in this work occurs as prismatic crystals in vugs of volcanic rocks (Seruci ignimbrites), found along the coast road ∼5.5 km northeast of Portoscuso, Cagliari, Sardinia; associated minerals are tridymite, todorokite, magnetite, and hematite. The name and the mineral were approved by the IMA CNMNC (2014-091). Holotype magnesio-ferri-fluoro-hornblende is monoclinic, space group C2/m, a = 9.839(5), b = 18.078(9), c = 5.319(3) Å, β = 104.99(3)°, V = 913.9(9) Å3, Z = 2. The density calculated from the empirical formula is 3.315 g cm–3. In plane-polarized light, magnesio-ferri-fluoro-hornblende is pleochroic, X = pale grey (least), Y = dark grey (most), Z = pale brownish grey (intermediate); X^a= 47.6° (β obtuse), Y // b, Z^c= 33.4° (β acute). It is biaxial negative, α = 1.669, β = 1.676, γ = 1.678, all ±0.002; 2Vobs= 74(1)°, 2Vcalc= 56°. The strongest eight lines in the powder X-ray diffraction pattern are [d in Å (I)(hkl)]: 2.711 (100)(151), 8.412 (89)(110), 3.121 (64)(310), 2.553 (61)(2̄02), 3.389 (55)(131), 2.599 (45)(061), 2.164 (36)(261), and 2.738 (34)(3̄31). Electron-microprobe analysis of the refined crystal gave SiO245.34, Al2O36.18, TiO21.22, FeO 15.24, Fe2O36.27, MgO 9.71, MnO 0.78, ZnO 0.06, CaO 10.18, Na2O 1.35, K2O 1.15, F 3.22, Cl 0.30, H2Ocalc 0.37, sum 99.95 wt.%. The empirical formula unit, calculated on the basis of 24 (O, OH, F, Cl) apfu with (OH + F + Cl) = 2 apfu is: (Na0.15K0.22)∑0.37(Na0.25Ca1.66Mn0.09)∑2.00(Mg2.20Fe2+1.94Mn0.01Zn0.01Fe3+0.72Ti0.13)∑5.01(Al1.11Si6.89)∑8.00O22[F1.55(OH)0.37Cl0.08)∑2.00.