Centennialite, CaCu₃(OH)₆Cl₂.nH₂O,n≈ 0.7, a new kapellasite-like species, and a reassessment of calumetite

Abstract

The new mineral centennialite (IMA 2013-110), CaCu3(OH)6Cl2·nH2O, was identified from three cotype specimens originating from the Centennial Mine, Houghton County, Michigan, USA, where it occurs as a secondary product, after acid water action upon supergene Cu mineralization in association with, and essentially indivisible from, other copper-containing minerals such as calumetite and atacamite family minerals. It forms as pale to azure blue encrustations, often taking a botryoidal form. Centennialite is trigonal, , a = 6.6606(9) Å, c = 5.8004(8) Å, V= 222.85(6) Å3, Z= 1. The strongest powder X-ray diffraction lines are dobs/Å [I%] (hkl), 5.799 [100] (001), 2.583 [75] (201), 2.886 [51] (111), 1.665 [20] (220), 1.605 [17] (023), 1.600 [15] (221), 1.444 [11] (222). The X-ray refined structure forms a kagome net of planar coordinated CuO4 units with Jahn-Teller coordinated Cl apices to form octahedra that edge-share to in-plane adjacent and flattened CaO6 octahedra, which are centred about the lattice origin. All oxygen sites are protonated and shared between one Ca-octahedron and one CuO4 planar unit. Three protonated sites are linked, by hydrogen-bonding to Cl sites, which sit on the triad axis. Each lattice has one Cl above and one below the Ca-Cu polyhedral plane. Consequently, the layers are stacked, along ⟨001⟩, with two Cl sites between layers. In addition to this kapellasite-like topology, an extra c/2 site is identified as being variably water-hosting and extends the coordination of the Ca-site to 8-fold, akin to the body-diagonal Pb-Cu sheet in murdochite. Centennialite conforms to the description of the ‘Unidentified Cu-Ca-Cl Mineral’ noted in Heinrich's Mineralogy of Michigan and is almost certainly identical to the supposed hexagonal basic calcium-copper hydroxychloride monohydrate of Erdös et al. (1981). We comment upon relationships between calumetite and centennialite and propose a substructure model for a synthetic calumetite-like phase that is related directly to centennialite.

Synthesis and structure of calumetite-like SrCu4(OH)8Cl2⋅3.5H2O

“…However, and as discussed in Crichton and Müller (2017), there are a significant number of additional reflections that indicate hkl-dependent peak shape, specifically (hk0) reflections of a notionally doubled pseudotetragonal (a = √2a 1 ) lattice, with significant peak asymmetry, that can typically arise from stacking defects.…”

“…The recovered sample was placed into a 0.9 mm glass capillary and mounted on a sample spinner at ID31, ESRF, where ADX data were collected at λ = 0.3999(1) Å (Fitch, 2004). Given the evident similarity of 1 with calumetite and Erdös et al 's phase, the model was refined with initial coordinates given by Crichton and Müller (2017), in Cmcm . The resulting refinement, in Jana2006 (Petříček et al , 2014), given in Tables 3,4 and Fig.…”

“…Another phase is described as rehydrating in a hexagonal lattice with composition 3Cu(OH) 2 ⋅CaCl 2 ⋅H 2 O, with a = 6.66 Å and c = 5.81 Å. This latter phase has been isolated synthetically and described, from natural samples, as centennialite, a relative of the kapellasite family (Crichton and Müller, 2017). The synthesis of single crystals has been further elaborated (Sun et al , 2016), which has also afforded more detailed structural analysis at 213 K, to a noncentrosymmetric P 3 m 1 with disordered Ca positions (Yoshida et al , 2017).…”

“…a = 6.2733(4) Å and c = 5.7472(5) Å, haydeeite; Malcherek and Schlüter, 2007); Mn, a = 6.42533(17) Å and c = 5.71048(11) Å (cf. a = 6.4168(4) Å and c = 5.7099(4) Å, misakiite; Nishio-Hamane et al, 2017), centennialite, calumetite (and both together), Crichton and Müller (2017); clinoatacamite-like: Cd, with a = 6.3889(10) Å, b = 7.05329(11) Å, c = 9.3378(11) Å and β = 99.034(15)° (e.g. McQueen et al, 2011); Ni, with a = 6.1746(6) Å, b = 6.8821(5) Å, c = 9.0308(7) Å and β = 99.46(8)° and rhombohedral forms: Co, with a = 6.8476(4) Å and c = 14.1382( 22) Å (cf.…”

The atacamite family of minerals – a testbed for quantum spin liquids

“…Misakiite, Cu 3 Mn(OH) 6 Cl 2 , is isostructural with kapellasite, while centennialite, Cu 3 Ca(OH) 6 Cl 2 ÁnH 2 O (Sun et al, 2016;Crichton & Mü ller, 2017), contains about 0.7 apfu molecular water in the interlayer space. The different radii of Ca 2+ (1.0 Å ) and Cu 2+ (0.77 Å ) (Shannon, 1976) support the assumption of little mixing of those cations on the same site, hence a kagome lattice fully occupied by paramagnetic Cu 2+ cations is expected in centennialite.…”