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, Dcalc = 3.09 g/cm3, perfect cleavage on {110}, and conchoidal fracture. In plane-polarized light, fluoro-edenite is birefringent (1st order), biaxial negative, α = 1.6058(5), β = 1.6170(5), γ = 1.6245(5), 2Vcalc = 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 Å3, 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(2,5̅10), 2.938(2,221), 1.649(2, 461), 3.376(2,131), 1.438(2, 6̅61). 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, FeOt 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(Na0.56 K0.15) B(Na0.30 Ca1.62 Mg0.03 Mn0.05) C(Mg4.68 Fe2+0.19 Fe3+0.10 Ti4+0.03) T(Si7.42 Al0.58) O22O3(F1.98 Cl0.02)2.

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