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

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. Huoro-edenite is transparent, intense yellow, non-fluorescent, has vitreous to resinous luster, and gives a yellow streak parallel to the c axis; Mobs' 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, alpha = 1.6058(5), beta = 1.6170(5), gamma = 1.6245(5), 2V(calc) = 78.09 degrees, Y = beta perpendicular to (010), and gamma :Z = 26 degrees. No pleochroism is observed. Fluoro-edenite is monoclinic, space group C2/m, a = 9.847(2) Angstrom, b = 18.017(3) Angstrom, c = 5.268(2) Angstrom, beta = 104.84(2)degrees, V = 903.45 Angstrom (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,(5) over bar 10), 2.938(2,221), 1.649(2, 461), 3.376(2,131), 1.438(2,(6) over bar 61). 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)(Na-0.56 K-0.15) (B)(Na-0.30 Ca-1.62 Mg-0.03 Mn-0.05) (C)(Mg-4.68 Fe-0.19(2+) Fe-0.10(3+) Ti-0.03(4+) (T)(Si-7.42 Al-0.58) O-22 (O3)(F-1.98 Cl-0.02)(2).