AMMINEITE, CuCl2(NH3)2, A NEW SPECIES CONTAINING AN AMMINE COMPLEX: MINERAL DATA AND CRYSTAL STRUCTURE

Abstract

Ammineite, CuCl 2 (NH 3 ) 2 , the first mineral containing an ammine complex, was discovered at Calleta Pabellon de Pica, Tarapaca region, Chile. The mineral is named for the first described ammine complex as naturally occurring crystals. Ammineite occurs as hypidiomorphic mm-sized blue crystals and also as powdery masses, associated with halite, atacamite, salammoniac and darapskite. The Mohs hardness is 1, the streak is light blue, the luster is vitreous, and the calculated density is 2.38 g/cm 3 at 100 K and 2.31 g/cm 3 at 293 K. The mineral is biaxial positive, with α 1.676(2), β 1.715(2), γ 1.785(2), 2 V calc 76° (for λ 589 nm). The pleochroism for β and γ is dark blue, and for α, light blue; the optical orientation is X = c , Y = b and Z = a . The crystal structure has been solved by direct methods and refined to an R 1 value of 2.4% on the basis of 231 unique observed reflections at 100 K. Ammineite is orthorhombic, space group Cmcm , a 7.688(1), b 10.645(2), c 5.736(1) A, V 469.4(2) A 3 from single-crystal XRD data at 100 K, and a 7.790(1), b 10.643(1), c 5.838(1) A, V 484.1(1) A 3 , from powder XRD data at 293 K, Z = 4. The strongest six X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 6.285(69)(110), 4.278(55)(111), 3.898(56)(200), 2.920(100) (002), 2.763(36)(221) and 2.660(90)(040,112). The chemical composition of ammineite, Cu 37.60, Cl 41.67, N 16.54, H 3.32 wt.%, was determined from both the results of electron microprobe (for Cu and Cl) and elemental analyzer (for N and H). The empirical formula gives Cu 1.00 Cl 1.99 N 1.99 H 5.57 or, ideally, CuCl 2 (NH 3 ) 2 . The basic structural unit, the trans-diammine-dichloridocopper( II) complex, is layered parallel to (001) and connected to the parallel layered complexes up and down nearly perpendicular by two long Cu–Cl bonds. The resulting polyhedron is a [4 + 2]-distorted Cu 2+ N 2 Cl 4 octahedron. The distorted octahedra are connected by shared edges to form zigzag chains running along [001]. The chains are linked by hydrogen bonding between the equatorial NH 3 groups and the equatorial Cl atoms of the adjacent chains. Heating experiments with synthetic CuCl 2 (NH 3 ) 2 in air show that trans-diammine-dichlorido-copper(II) is stable up to 120°C. After 75 hours and heating to 200°C, the sample in air consists of copper chloride hydroxide hydrate and the synthetic analogue of eriochalcite, CuCl 2 ·2H 2 O, whereas the sample of CuCl 2 (NH 3 ) 2 sealed in a glass tube was unchanged after 75 hours at 200°C.