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Fluoro-edenite, ideally NaCa 2 Mg 5 (Si 7 Al)O 22 F 2 , was found both as prismatic or acicular crystals of millimetric size and as fibers in the rock cavities in gray-red altered benmoreitic lavas occurring at Biancavilla (Etnean Volcanic Complex, Catania, Italy). It is associated with feldspars, quartz, clino- and orthopyroxene, fluoro-apatite, ilmenite, and hematite, and probably crystallized from late-stage hydrothermal fluids. Fluoro-edenite is transparent, intense yellow, non-fluorescent, has vitreous to resinous luster, and gives a yellow streak parallel to the c axis; Mohs’ hardness 5-6, D calc  = 3.09 g/cm 3 , perfect cleavage on {110}, and conchoidal fracture. In plane-polarized light, fluoro-edenite is birefringent (1 st  order), biaxial negative, α = 1.6058(5), β = 1.6170(5), γ = 1.6245(5), 2V calc  = 78.09°, Y ≡ β ⊥ (010), and γ:Z = 26°. No pleochroism is observed. Fluoro-edenite is monoclinic, space group C2/m, a = 9.847(2) Å, b = 18.017(3) Å, c = 5.268(2) Å, β = 104.84(2)°, V = 903.45 Å 3 , Z = 2; the ten strongest X-ray diffraction lines in the powder pattern are [d(I, hkl)]: 3.125(10, 310), 8.403(6,110), 3.271(5,240), 2.807(4,330), 2.703(3,151), 1.894(2,5̄10), 2.938(2,221), 1.649(2, 461), 3.376(2,131), 1.438(2,6̄61). IR analysis showed absorption bands at 1066, 991, 791, 738, 667, 517, 475 cm -1 , and no bands in the OH-stretching region. Structure refinement allowed determination of cation site-preference and ordering. Microprobe analysis of the refined crystal gave SiO 2  52.92, TiO 2  0.29, Al 2 O 3  3.53, FeO t  2.50, MnO 0.46, MgO 22.65, CaO 10.83, Na 2 O 3.20, K 2 O 0.84, F 4.35, Cl 0.07 wt%, and the crystal-chemical formula obtained by combining all the available data is:  A (Na 0.56  K 0.15 )  B (Na 0.30  Ca 1.62  Mg 0.03  Mn 0.05 )  C (Mg 4.68  Fe 2+  0.19  Fe 3+  0.10  Ti 4+  0.03 )  T (Si 7.42  Al 0.58 ) O 22  O3 (F 1.98  Cl 0.02 ) 2 .

Fluoro-edenite from Biancavilla (Catania, Sicily, Italy): Crystal chemistry of a new amphibole end-member