The crystal structure of camerolaite and structural variation in the cyanotrichite family of merotypes

Abstract

We present Raman data for camerolaite, cyanotrichite and carbonatecyanotrichite, and using synchrotron single-crystal X-ray diffraction have solved the structure of camerolaite from the Tistoulet Mine, Padern, Aude Department, France. Camerolaite crystallizes in space group P1 with the unit-cell parameters: a = 6.3310(13) Å, b = 2.9130(6) Å, c = 10.727(2) Å, α = 93.77(3)°, β = 96.34(3)°, γ =79.03(3)º, V = 192.82(7) Å3 and Z = ⅓, with respect to the ideal formula from the refinement, Cu6Al3(OH)18(H2O)2[Sb(OH)6](SO4). The crystal structure was solved to R1 = 0.0890 for all 1875 observed reflections [Fo > 4σFo] and 0.0946 for all 2019 unique reflections. The P cell has been transformed into a C-centred cell that aids comparison with that of the structurally related khaidarkanite by aC = 2aP – bP, giving parameters a = 12.441(3), b = 2.9130(6), c = 10.727(2) Å, α = 93.77(3), β = 95.57(3), γ = 92.32(3)º and Z = ⅔ in C1. Edge-sharing octahedral ribbons Cu2Al(O,OH,H2O)8 form hydrogen-bonded layers || (001), as in khaidarkanite. The partially occupied interlayer Sb and S sites of the average structure are in octahedral and tetrahedral coordination by oxygen, respectively. They cannot be occupied simultaneously, which leads to regular alternation of [Sb(OH)6]– and SO42– groups in rods || y, resulting in local tripling of the periodicity along y for the Sb(OH)6–SO4 rods. Thus, camerolaite has a ‘host–guest’ structure in which an invariant host module (layers of Cu–Al ribbons) has embedded rod-like guest modules with a longer periodicity. Coupling between the phases of these rods is only short-range, resulting in diffuse X-ray scattering rather than sharp superstructure reflections. Similar disorder is known for parnauite, and is deduced for other members of the cyanotrichite group (cyanotrichite, carbonatecyanotrichite and khaidarkanite). Group members all share the Cu–Al ribbon module but have interlayer rods of different compositions and topologies; thus, they form a merotypic family. The low symmetry of the camerolaite average structure suggests other possibilities for structure variation in the group, which are discussed.

The crystal structure of cyanotrichite

“…2). This is consistent with the IR spectra of the cyanotrichite sample M50793 of Mills et al (2014), which showed the same features, particularly the H 2 O bend at 1648 and SO 4 stretching bands at 1093 and 1035 cm À1 . The lack of bands in the 1100À1500 cm À1 region is noteworthy, both confirming the lack of CO 3 2À within the sample, and supporting the inference that such bands, when observed, do arise from the presence of carbonate (Mills et al, 2014).…”

“…Note that there is probably a strong preference for regular alternation of upper and lower sulfates, as deviation from that pattern would require rearrangement of the associated water molecules, omission of some H 2 O to avoid impossibly close contacts and loss of some H bonding. The location of the H 2 O at O7 makes impossible any local ordering pattern with 2b periodicity, as was proposed by Mills et al (2014). ordered P2 1 /a superstructure with such an arrangement would be possible, in which adjacent rods along the a direction are related by the glide plane.…”

“…Hence, the two S sites (and their attendant O4ÀO6 sites) cannot both be occupied simultaneously (see discussion below). As was the case for the structure of camerolaite (Mills et al, 2014), the two oxygen atoms (O4) of the sulfate tetrahedron bridge two adjacent Cu(OH,O,H 2 O) 6 octahedra along the edge of the CuÀAl ribbon. If the sulfate SÀO distance is close to the grand average value 1.459 Å of Hawthorne et al (2000), then the O_O distance along the tetrahedral edge must be close to H(˚Ù¯)61.459 = 2.382 Å , significantly less than the b parameter (~2.9 Å ).…”

“…Mills et al (2014) predicted that cyanotrichite has a structure in which SO 4 2À + 2H 2 O repeat along b within the interlayer and that the up/down sense of the sulfate tetrahedron is not necessarily ordered. This results in the minimum periodicity along any given sulfate chain being 2b.…”

Multidimensional structural variation in the cyanotrichite family of merotypes: camerolaite-3b-F\bar 1

“…The crystal structures of minerals in the cyanotrichite group share a [Cu 2 Al(OH) 6 ] + ribbon of edge-sharing Cu and Al polyhedra || b of a (pseudo)monoclinic subcell. The ribbons lie en echelon in layers || (001) (Mills et al, 2014).…”