BARBERIITE, NH4BF4, A NEW MINERAL FROM VULCANO, AEOLIAN ISLANDS, ITALY

Barberiite, ammonium tetrafluoroborate, occurs as a fumarolic encrusta tion at Fossa crater on Vulcano, Aeolian Islands, Italy. Associated mi nerals are sulfur, malladrite, realgar, salammoniac, cannizzarite, gal enobismutite, and bismuthinite. The mineral is orthorhombic and occurs as minute crystals tabular to platy on {001} and sometimes elongated [010] or [100], forming globular aggregates (average diameter 1-2 mm). It is admixed with sulfur and sometimes with malladrite, salammoniac, and realgar. The very thin crystals (about 1 mum in thickness) range from about 50 to 300 mum in length and are very unstable in air. The m ineral is colorless, transparent to translucent, nonfluorescent, with a vitreous luster and a white streak. VHN25 = 14.2 kg/mm2 (range: 13.0 -15.4), cleavage {100} perfect, {010} and {001} good, fracture not obs erved. D(calc) = 1.90, D(meas) = 1.89(3) g/cm3. Barberiite is biaxial, 2V(meas) = 90 +/- 2-degrees; the mean n, calculated from the Gladston e-Dale relationship, is 1.308. It has a = 9.0615(7), b = 5.6727(6), c = 7.2672(6) angstrom, V = 373.5(t) angstrom3, a:h:c = 1.5974:1:1.2811, Z = 4, space group Pnma. The strongest six lines in the X-ray powder diffraction pattern are [d in angstroms (I/I0) hkl]:3.183(100)211, 3.5 40(90)210, 2.8982(80)112, 4.472(75)011, 2.1631(70)113, 2.5362(65)121. On the basis of chemical analysis, IR spectroscopy, and X-ray data, ba rberiite corresponds to the synthetic compound NH4BF4. The mineral is named after Franco Barberi, professor of volcanology at Pisa Universit y.