Stoppaniite, (Fe,Al,Mg)(4)(Be6Si12O36)*(H2O)(2)(Na,square) a new mineral of the beryl group from Latium (Italy)

This paper relates the finding of stoppaniite, a new mineral of the beryl g roup, at Capranica, Vice volcanic complex, Latium, Italy. Stoppaniite occur s inside the miarolitic cavities of a volcanic ejectum, as very rare, trans parent, light-blue, hexagonal-prismatic euhedral crystals, up to 0.5 mm in length. It is optically uniaxial (-), and shows a weak pleochroism from col ourless to very light blue. The refraction indices are: epsilon = 1.619(3) and omega = 1.625(3); the density is: D-obs = 2.79(3) g/cm(3) D-calc = 2.81 1 g/cm(3). Stoppaniite is hexagonal, s.g. P6/mcc, with a(Angstrom) = 9.397( 1), c(Angstrom) = 9.202(2). Strongest X-ray powder diffraction lines (Gando lfi camera) are (d in Angstrom, intensities visually estimated): 3.278 (vS) , 8.12 (S), 2.903 (S), 4.00 (m), 2.553 (mw), 1.752 (mw). The empirical crystal-chemical formula is: (Fe2.703+Mg0.64Al0.42Mn0.06Ti0.02Sc0.02)(Be6.88Si11.60)O-36*2H(2)O(Na0.94K0 .02Cs0.02) compatible with the simplified formula: (Fe3+)(3)(Mg,Fe2+)Na(Be6Si12O36)*2H(2)O, or ideally Fe43+Be6Si12O36. A rational classification of beryl group minerals based on the octahedral o ccupancy is proposed.