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□BCa2C(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 C 2/ m , a = 9.839(5), b = 18.078(9), c = 5.319(3) A, β = 104.99(3)°, V = 913.9(9) A3, 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 A ( I )( hkl )]: 2.711 (100)(151), 8.412 (89)(110), 3.121 (64)(310), 2.553 (61)(![Graphic][1] 02), 3.389 (55)(131), 2.599 (45)(061), 2.164 (36)(261), and 2.738 (34)(![Graphic][2] 31). Electron-microprobe analysis of the refined crystal gave SiO2 45.34, Al2O3 6.18, TiO2 1.22, FeO 15.24, Fe2O3 6.27, 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)Σ0.37(Na0.25Ca1.66Mn0.09)Σ2.00(Mg2.20![Graphic][3] Mn0.01Zn0.01![Graphic][4] Ti0.13)Σ5.01(Al1.11Si6.89)Σ8.00O22[F1.55(OH)0.37Cl0.08)Σ2.00. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif