Rare sulfosalts from Vulcano, Aeolian Islands, Italy. I. Se-bearing kirkiite, Pb-10(Bi,As)(6)(S,Se)(19)

This note reports the first occurrence of a Se-bearing variety of kirkiite, a rare lead - bismuth - arsenic sulfosalt, from the deepest part of fumaro le F11, La Fossa crater, island of Vulcano, Italy. Small crystals of sphale rite and aggregates of cannizzarite occur sporadically on the kirkiite crys tals. Optical properties, X-ray powder-diffraction data and the results of electron-microprobe analyses are compared with those of kirkiite from type locality as well as of phase A? the synthetic analogue. An electron-microdi ffraction study revealed the presence of three structural varieties of kirk iite. The Se content ranges from 0.73 to 1.16 wt%. As and Bi concentrations vary from 6.49 to 7.98 wt% and from 14.32 to 16.84 wt%, respectively. The composition field of kirkiite from Vulcano can be described by the formula Pb-10(As6-xBix)(Sigma 6)(S19-ySey)(Sigma 19), where 2.3 less than or equal to x less than or equal to 2.8 and 0.3 less than or equal to y less than or equal to 0.5, which is close to the ideal formula Pb-10(As,Bi)(6)(S,Se)(19 ). Kirkiite and its associated minerals at Vulcano crystallized at a high f ugacity of sulfur and a high temperature (470 degrees C, direct measurement s). This temperature is in agreement with that known for the synthetic anal ogue, stable at 400 degrees C.