The crystal structure of cualstibite-1M (formerly cyanophyllite), its revised chemical formula and its relation to cualstibite-1T

The crystal structure of the rare secondary mineral cualstibite-1M (formerly cyanophyllite), originally reported to have the chemical formula 10CuO·2Al₂O₃·3Sb₂O₃·25H₂O and orthorhombic symmetry, was solved from single-crystal intensity data (Mo-Kα X-radiation, CCD area detector, 293 K, 2θ_max?=?80) collected on a twinned crystal containing very minor Mg. The mineral is monoclinic, P2₁/c (no. 14), with a?=?9.938(1), b?=?8.890(1), c?=?5.493(1) Å, β?=?102.90(1)°, V?=?473.05(11) ų; R1(F)?=?0.0326. All crystals investigated turned out to be non-merohedric twins. The atomic arrangement has a distinctly layered character. Brucite-like sheets composed of two 4?+?2]-coordinated (Cu,Al,Mg) sites are linked by weak hydrogen-bonding (O···O?~?2.80 Å) to isolated regular Sb(OH)₆ octahedra (<Sb-O>?=?1.975 Å). The layered, pseudotrigonal character explains the perfect cleavage and the proneness to twinning. The Sb site is fully occupied and the two (Cu,Al,Mg) sites have occupancies of Cu_0.79Al_0.17Mg_0.04 and Cu_0.72Al_0.23Mg_0.05. The Cu-richer site shows a slightly stronger Jahn-Teller-distortion. The resulting empirical formula, which necessitates a H₂O-for-OH substitution to obtain charge balance, is (Cu_2.23Al_0.63Mg_0.14)(OH)_5.63(H₂O)_0.37Sb⁵⁺(OH)₆]. The ideal chemical formula is (Cu,Al)₃(OH)₆Sb⁵⁺(OH)₆], with Cu:Al = 2:1. The structure is closely related to those of trigonal cualstibite-1T Cu₂AlSb(OH)₁₂, P-3, with ordered Cu-Al distribution in the brucite sheets], and its Zn analogue zincalstibite-1T Zn₂AlSb(OH)₁₂]. Cualstibite-1M and cualstibite-1T are polytypes and, together with zincalstibite-1T, zincalstibite-9R and omsite, belong to the cualstibite group within the hydrotalcite supergroup, which comprises all natural members of the large family of layered double hydroxides (LDH).