Pumpellyite-(Al), a new mineral from Bertrix, Belgian Ardennes

Abstract

Pumpellyite-(Al), ideally Ca 2 (Al,Fe 2+ ,Mg)Al 2 (SiO 4 )(Si 2 O 7 )(OH,O) 2 ·H 2 O, is a newly approved mineral species from Bertrix, Ardennes mountains, Belgium. It occurs as radiating fibrous aggregates reaching 5 mm in diameter, constituted by acicular crystals associated with calcite, K-feldspar and chlorite. Pumpellyite-(Al) is transparent to translucent and exhibits an emerald-green to white colour, sometimes with bluish tinges. The lustre is vitreous and the streak is colourless. The mineral is non-fluorescent, brittle, and shows a perfect {100} cleavage. The estimated Mohs hardness is 5½, and the calculated density is 3.24 g/cm 3 . Pumpellyite-(Al) is biaxial positive, α = 1.678(2), β = 1.680(2), γ = 1.691(1) (λ = 590 nm), colourless in thin section, 2 V = 46°, Y = b , no dispersion. Electron-microprobe analyses gave SiO 2 37.52, Al 2 O 3 25.63, MgO 1.99, FeO 4.97, MnO 0.11, CaO 23.21, BaO 0.01, Na 2 O 0.03, K 2 O 0.02, H 2 O calc . 6.71, total 100.20 wt. %. The resulting empirical formula, calculated on the basis of 8 cations, is (Ca 1.99 Na 0.01 ) ∑2.00 (Al 0.42 Fe 2+ 0.33 Mg 0.24 Mn 0.01 ) ∑1.00 Al 2.00 (SiO 4 )(Si 2 O 7 )(OH) 2.42 0.58H 2 O. The simplified formula is Ca 2 AlAl 2 (SiO 4 )(Si 2 O 7 )(OH) 3 , which requires SiO 2 38.16, Al 2 O 3 32.38, CaO 23.74, H 2 O 5.72, Total 100.00 wt. %. The unit-cell parameters, refined from X-ray powder diffraction data, are: a = 8.818(2), b = 5.898(2), c = 19.126(6) A, β = 97.26(3)°, V = 986.7(4) A 3 , space group A 2/ m. The eight strongest lines in the powder pattern [ d- values(in A)( I )( hkl )] are: 4.371(65)(200), 3.787(80)(202), 3.040(70)(204), 2.912(95)(300), 2.895(100)(302), 2.731(40)(206), 2.630(35)(311), 2.191(45)(402). Pumpellyite-(Al) belongs to the pumpellyite group, and corresponds to the Al-rich compositions where the M1 and M2 sites contain Al as predominant cation. The crystal structure of pumpellyite-(Al) has been refined by the Rietveld method, based on an X-ray powder diffraction pattern, to R Bragg = 7.09 %. The infrared spectrum is similar to those of minerals of the pumpellyite group. The mineral species and name were approved by the Commission on New Minerals and Mineral Names, IMA (no. 2005-016).