Chrysoberyl from the Sabatini Volcanic Complex (Latium, Italy): chemical and petrological peculiarities

This paper describes mineralogical, chemical, and petrological features of an iron-rich chrysoberyl, containing up to 0.31 Fe atoms per formula unit (up to 17.24 wt % as Fe2O3), found in some rare volcanic ejecta of the Sabatini Volcanic Complex in central Italy. These rocks, locally named “sanidinites”, are ultrapotassic and undersaturated in silica, and they are well known for their abundance of HFSE-LILE-REE-rich well-crystallized minerals (HFSE: high-field-strength element; LILE: large-ion lithophile element; REE: rare earth element). Chrysoberyl occurs as yellow to brown crystals up to 2 mm in size, typically twinned, sometimes with penetration trilling of pseudohexagonal habit. Refined unit-cell parameters are a D 9:4507(5) Å, b D 5:4975(3) Å, c D 4:4364(2) Å, and V D 230:49(2) Å3. Its crystal structure has been refined the in Pnma space group down to R1 D 0:0397 for 420 unique reflections with F > 4σ (F ) and 41 refined parameters. The study of this occurrence provides insight into the partitioning of Fe into the Al(2) site and the role of Fe-to-Al substitution in the anisotropic chemical expansion of the structure. The differences between this occurrence and those reported in the literature are discussed, proposing a new metasomatic genesis for this anomalous chrysoberyl occurrence to explain the observed textural, chemical, and mineralogical evidence.