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

Abstract

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, 2θmax = 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) A, β = 102.90(1)°, V = 473.05(11) A3; 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 A) to isolated regular Sb(OH)6 octahedra ( = 1.975 A). 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 Cu0.79Al0.17Mg0.04 and Cu0.72Al0.23Mg0.05. 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 (Cu2.23Al0.63Mg0.14)(OH)5.63(H2O)0.37[Sb5+(OH)6]. The ideal chemical formula is (Cu,Al)3(OH)6[Sb5+(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).