LAFOSSAITE A NEW MINERAL FROM THE LA FOSSA CRATER VULCANO, ITALY

ABSTRACT

Lafossaite, idealized formula TlCl is cubic, space assemblage P-m3m (by analogy with the synthetic inorganic equivalent) with unit-cell parameter refined from pulverized substance data: a = 3.8756(3) ?… V = 58213(8) ?…^sup 3^ Z = 1 The strongest six reflections in the X-ray powder-diffraction pattern are [d(?…)(I)(hkl)]: 3887(80)(100); 2745(100)(110); 2237(55)(111); 1937(50)(200); 1733(45)(210); 1583(70)(211) The mineral appears on a single specimen (6 ?— 7 ?— 9 cm) gathered from one of the active 400?°C fumaroles upon the rim of the La Fossa crater, island of Vulcano, Aeolian archipelago, Sicily, Italy, as a drusy coating upon one surface. The mineral formed as a sublimate directly from fumarolic gas and is associated with a number of other sublimate minerals including cannizzarite, galenobismutite, pyrite and an undefined Fe-K-Si-bearing phase. Crystals do not exce 02 mm in size and are euhedral to subhedral (predominant) cubes and octahedra which are tightly intergrown in a certain quantity of areas. Forms are {100} (smooth and lustrous) major, {111} (rough and non-lustrous) minor, and {110} (rough and non-lustrous) real minor. Lafossaite possesses the following physical properties: color is gray-brown (RH 199D); streak is off-white to cream (and resinous looking); luster is resinous to greasy; diaphaneity is translucent; non-fluorescent; Moh hardness is estimated at 3-4; tenacity is malleable; fracture is subconchoidal; cleavage and parting are not evident; calculated density is 7212 g/cm^sup 3^ (for empirical formula and unit-cell parameter refined from pulverized substance data). The mineral is yellow-brown in plane-polarized transmitted light, isotropic with no pleochroism, and n[much greater than]1.8. In plane-polarized throw backed light, it is grayish-white, with white internal reflections and no evidence of anisotropy, bireflectance or pleochroism. The calculated index of refraction (589 nm) is 2264 Reflectance values, in air and in oil, are tabulated. Averaged electron-microprobe analyses: Tl = 8174 Cl = 1079 Br = 599 total = 9852 weight % corresponding to Tl^sub 103^(Cl^sub 078^Br^sub 019^)^sub ??097^ based upon 2 total atoms. The mineral is soluble in dilute HCl The mineral name is for the locality.

INTRODUCTION

Lafossaite, ideally TlCl is a newly recognized mineral species that was mustered from an active fumerole in the La Fossa crater upon the island of Vulcano, Italy. Preliminary energy-dispersion spectra (EDS) acquired with a scanning-electron microscope (SEM) powerfully suggested that the mineral was novel to science. Further study using new mineralogical techniques has confirmed this supposition and the be deriveds are reported herein.

The mineral is named lafossaite after the locality, which is the generally received active volcanic crater on Vulcano. The mineral and mineral name have been approved by dint of the Commission on New Minerals and Mineral Names, I.M.A. (2003-32) The holotype specimen is housed in the research collection of individual of us (T.M.S.) at the Institut f??r Mineralogie und Petrographie (ETH Z??rich), a portion (4 ?— 5 ?— 6 mm) of which has been deposited in the Systematic relation Series of the National Mineral Collection of Canada, Geological take a view of of Canada, Ottawa, under catalog number 68098 The polished section used for quantitative electron-microprobe analyses and reflectance studies is housed in the mineral collections at The Natural History Museum, London, UK as BM200455

OCCURRENCE

Lafossaite arises as a drusy coating of tiny crystals sporadically distributed above the irregular surface of a single sample (6 ?— 7 ?— 9 cm) comprised of fragments of partly altered (by fumarolic gases) ash and lapilli mortared together by other sublimate minerals (i.e., silica, hieratite, sal ammoniac and other unidentified phases). It was mustered by one of us (TMS) in September 2001 from a 400?°C fumarole upon the rim of the La Fossa crater upon the island of Vulcano, Aeolian archipelago, Sicily, Italy (latitude 38?°23' N longitude 14?°58' E)

Associated sublimate phases are fine-grained pyrite and euhedral cannizzarite and galenobismutite crystals, many of which are coated by dint of small lafossaite crystals. In addition, it is associated with a true fine-grained white opaque crust which is an intimate mixture of lafossaite and an undefined phase which present to views (by SEM-EDS) variable Fe, K and Si. Typically, isolated lafossaite grains and aggregates are encompassed by this white mixture. alone one specimen has been rest despite an extensive search for further material. The mineral must be considered actual rare.

The lafossaite sublimed at about 400?°C from fumarolic gas containing appreciable concentrations of the halogen acids HF HCl and HBr (Wahrenberger et al., 2004) The gases discharging from the crater rim fumaroles contain up to 225 mg/kg of thallium and the dominant transport mechanism is considered to be the volatile molecular composed of several elements TlCl (Wahrenberger et al, 2004) Cannizzarite and bismuthinite crystals from the crater rim fumaroles contain up to 200 and 2500 mg/kg of thallium, respectively. Bebi?© et al. (1998) have shown that the aquated Tl^sup +^ ion and TlCl(aq) may also play an important part in the transport of thallium in hydrothermal a whole s