Heulandite-Ba, a new zeolite species from Norway

Abstract

Heulandite-Ba, ideally (Ba,Ca,Sr,K,Na) 5 Al 9 Si 27 O 72 ·22H 2 O, is a new zeolite species in the heulandite series, occurring as an accessory mineral in hydrothermal veins of the Kongsberg silver deposit type at the Northern RavnAs prospect, southern Vinoren, 14 km NNW of Kongsberg town, Kongsberg ore district, Flesberg community, Buskerud county, Norway. The mineral has also been found at the Bratteskjerpet mine, Saggrenda near Kongsberg, and in hydrothermal veins in quartzite at Sjoa in Sel community, Oppland county. Heulandite-Ba occurs as well developed, thick tabular, trapezoidal crystals up to 4 mm across, showing the forms {100}, {010}, {001}, 111} and {201}. The mineral is colourless to white, rarely very pale yellowish white or pale beige, with a white streak; transparent to translucent, with a vitreous lustre, pearly on {010}. The mineral has a perfect {010} cleavage; subconchoidal to uneven fracture. It is non-fluorescent in long- or short-wave ultraviolet light. The Mohs9 hardness is 3½; D meas = 2.35(1) and D calc = 2.350 g/cm 3 . Heulandite-Ba is biaxial positive with n α = 1.5056(5), n β = 1.5064(5) and n γ = 1.5150(5); Δ = 0.0094, n (mean) = 1.5090. 2V γ (calc) = 34.1°, 2V γ (meas) = 38(1)°; distinct dispersion, r > v; α Λ c varying from ≅ 39° to ≅ 51° in obtuse angle β, γ = b. An average of 14 electron microprobe analyses on heulandite-Ba from the Northern RavnAs prospect, Kongsberg, gave SiO 2 54.26, Al 2 O 3 15.27, MgO 2 O 0.34, K 2 O 0.58, H 2 O 13.1 (from TGA), total 99.99, corresponding to (Ba 2.49 Ca 1.41 Sr 0.30 K 0.37 Na 0.33 ) Σ4.90 Al 8.96 Si 27.00 O 72.00 ·21.75H 2 Oon the basis of 72 framework oxygen atoms. Chemical zoning is frequent, with transitions to heulandite-Ca and heulandite-Sr. Heulandite-Ba is monoclinic, C 2/ m , with a = 17.738(3), b = 17.856(2), c = 7.419(1) A, β = 116.55(2)°, V = 2102.0(7) A 3 , Z = 1. The strongest five X-ray diffraction lines of the powder pattern [ d in A( I )( hkl )] are: 2.973(100)(151), 3.978(97)(131), 7.941(66)(200), 4.650(66)(-131), 2.807(65)(-621). The crystal structure refinements (R = 3.5%) of heulandite-Ba were done in space groups C 2/ m, Cm, C 2, and C 1, but refinements in space groups with lower symmetry than C 2/ m did not improve the structural model.