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:  
\[(Fe^{3{+}}_{2.70}Mg_{0.64}Al_{0.42}Mn_{0.06}Ti_{0.02}Sc_{0.02})(Be_{6.88}Si_{11.60})O_{36}*2H_{2}O(Na_{0.94}K_{0.02}Cs_{0.02})\]
compatible with the simplified formula:  
\[(Fe^{3{+}})_{3}(Mg,Fe^{2{+}})Na(Be_{6}Si_{12}O_{36})*2H_{2}O,\ or\ ideally\ Fe^{3{+}}_{4}Be_{6}Si_{12}O_{36}.\]

A rational classification of beryl group minerals based on the octahedral occupancy is proposed.

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