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

Abstract

Fluoro-edenite, ideally NaCa₂Mg₅(Si₇Al)O₂₂F₂, 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/cm³, 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 ų, 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 SiO₂ 52.92, TiO₂ 0.29, Al₂O₃ 3.53, FeO_t 2.50, MnO 0.46, MgO 22.65, CaO 10.83, Na₂O 3.20, K₂O 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 Fe²⁺_0.19 Fe³⁺_0.10 Ti⁴⁺_0.03) ^T(Si_7.42 Al_0.58) O₂₂^O3(F_1.98 Cl_0.02)₂.