ARMENITE: A REALLY RARE MINERAL?

Armenite is a quite uncommon double-ring Ba-Al-Ca silicate hydrate belonging to the milarite-osumilite group and with the general formula BaCa2Al6Si9O30·2H2O. It generally forms pseudo-hexagonal whitish-pinkish crystals. However, in its structure, Si, Al ordering and H2O positions produce the deviation from hexagonal symmetry, explaining the belonging to the Pnna or Pnc2 space groups. In thin section, armenite is quite elusive. In fact, it appears colorless, with low relief and low first-order interference color. More complication arises from the tartan-like twinning patterns (resembling that of microcline), patchy-like and/or undulose extinction as well as the monoaxial to strongly biaxial (2V up to 65°) behavior. Its affinity to hexagonal or orthorhombic space groups as well as the reasons for its anomalous optical features have formerly been an object of debate. Up to now, armenite has only been found in a dozen of places worldwide, among which Armen mine (Norway), Quebec (Canada), New South Wales (Australia), Scotland, Switzerland, and Sardinia (Italy). It typically forms veins within the host rocks in different geological environments. These include metasomatic basic to intermediate igneous rocks, mineralized skarn and hornfels, and gneisses indicating that the interaction between fluid phases and a primary Ba source is required for its formation. Here we report the third occurrence of armenite in Sardinia, from the Rosas mine area (Mitza Sermentus mineworks, south-west Sardinia). Armenite-bearing samples were collected along the contact between a sulfide-mineralized skarn vein and a black phyllite host-rock. The black phyllite matrix consists of muscovite, chamosite and quartz with feldspars, clinozoisite, titanite, and calcite as accessory phases. The skarn is made up of clinopyroxene, amphibole, epidote, chlorite and wollastonite, and calcite; accessory minerals are titanite, apatite, prehnite, and baryte. The ore minerals mainly consist of galena, sphalerite, chalcopyrite, and pyrite. Armenite is usually concentrated in mm-wide white veinlets along the contact between the sulfide mineralization and the host rock or more rarely dispersed in the phyllite matrix. At first, interpreted as an altered feldspar, it was identified by SEM-EDS analyses. Despite being semi-quantitative, the analyses provided compositions very close to stoichiometric armenite, with SiO2 ~ 48 wt.%, Al2O3 ~ 28 wt.%, BaO ~ 13 wt.% and CaO ~ 10 wt.%. This finding was further confirmed by XRPD analyses on armenite-rich polymineralic samples in which more than 20 peaks were assigned to this phase leading to a good match with an armenite in the PDF database (Ref. code 00-037-0432). Beyond its supposed rarity and its peculiar crystal structure, three reasons make armenite deserving of attention: (i) understanding its genesis could better constrain the P-T-fluid conditions of rocks in which armenite is found and that are often mineralized; (ii) given its difficult recognition by base techniques, it is likely that armenite is more common than previously thought and is usually overlooked; (iii) since its formation requires a primary Ba source, armenite could be used as an indicator of the proximity of Ba-rich deposits.