Stoppaniite, (Fe,Al,Mg)4(Be6Si12O36)*(H2O)2(Na,□) a new mineral of the beryl group from Latium (Italy)
Ventura, Giancarlo Della; Rossi, Paolo; Parodi, Gian Carlo; Mottana, Annibale; Raudsepp, Mati; Prencipe, Mauro
European Journal of Mineralogy Volume 12 Number 1 (2000), p. 121 - 127
published: Feb 7, 2000
manuscript accepted: Sep 14, 1999
manuscript received: Jul 20, 1999
ArtNo. ESP147051201026, Price: 29.00 €
Abstract This paper relates the finding of stoppaniite, a new mineral of the beryl group, at Capranica, Vico volcanic complex, Latium, Italy. Stoppaniite occurs inside the miarolitic cavities of a volcanic ejectum, as very rare, transparent, light-blue, hexagonal-prismatic euhedral crystals, up to 0.5 mm in length. It is optically uniaxial (-), and shows a weak pleochroism from colourless to very light blue. The refraction indices are: ε = 1.619(3) and ω = 1.625(3); the density is: Dobs = 2.79(3) g/cm3, Dcalc = 2.811 g/cm3. Stoppaniite is hexagonal, s.g. P6/mcc, with a(Å) = 9.397(1), c(Å) = 9.202(2). Strongest X-ray powder diffraction lines (Gandolfi camera) are (d in Å, 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: (Fe3+2.70Mg0.64Al0.42Mn0.06Ti0.02Sc0.02)(Be6.88Si11.60)O36*2H2O(Na0.94K0.02Cs0.02) compatible with the simplified formula: (Fe3+)3(Mg,Fe2+)Na(Be6Si12O36)*2H2O, or ideally Fe3+ 4Be6Si12O36. A rational classification of beryl group minerals based on the octahedral occupancy is proposed.