Resumo

Ferrochiavennite is a new beryllium silicate zeolite with chemical composition close to Ca_1–2FeSi₅Be₂O₁₃(OH)₂·2H₂O. It is described from two syenite pegmatite localities in Norway: Blåfjell, Langangen, Telemark, and the AS Granit larvikite quarry, Tvedalen, Vestfold. The mineral is monoclinic, P2₁/c, with a 8.759(5), b 4.864(2), c 31.258(6) Å, β 90.31(6)°, V 1331.7(6) Å3, and Z = 4. The crystal structure was refined to R₁ = 0.048 for 3651 observed reflections. The zeolite structure is isostructural with chiavennite, consisting of intersecting channels of nine-, six-, five-, and four-fold rings. The strongest eight reflections of the X-ray powder-diffraction pattern [d(obs.) in Å (I) (hkl)] are: 15.555 (100) (002), 4.104 (29) (1̅12, 112), 3.938 (36) (1̅13, 113), 3.909 (60) (008), 3.820 (30) (2̅04, 204), 3.251 (66) (017, 210, 2̅11), 3.186 (27) (2̅12, 212), 2.884 (64) (2̅15, 215). The mineral is biaxial (+) with refractive indices α 1.583(1), β 1.589(1), γ 1.602(1), measured at 590 nm. 2V (meas.) = 62(4)° from extinction curves 2V = 76(5)°; 2V (calc.) = 69°. The optical orientation is X ~ a, Y ~ c, and Z ~ b. The Mohs hardness is ~ 3; D(meas.) = 2.67(2) and D(calc.) = 2.709 g/cm3.