Stability of genthelvite, Zn₄(BeSiO₄)₃S: an exercise in chalcophilicity using exchange operators

Despite differences in ionic radii and chemical affinities, Be and Zn have somewhat similar crystal chemical tendencies, although they rarely occur together in minerals. The only silicate in which they do so is genthelvite, a rare accessory in alkaline to peralkaline granites, syenites, and related pegmatites and greisens. The formula of genthelvite, Zn₄(BeSiO₄)₃S, can be expressed as a mixture of phenakite, willemite, and sphalerite. This fact, plus the fact that Zn is much more chalcophile than Be, allows the prediction of a series of seven sulfide-silicate reactions that involve SO_-1 (the "acid anhydrite' of H₂S) and the seven phases sphalerite, bromellite, zincite, phenakite, willemite, genthelvite, and quartz. Addition of Al₂O₃ and F₂O_-1 to the model system gives further information on the stability of genthelvite. -from Author