HIGHLY UNDERSATURATED ANIONS IN THE CRYSTAL STRUCTURE OF ANDYROBERTSITE - CALCIO-ANDYROBERTSITE, A DOUBLY ACID ARSENATE OF THE FORM K (Cd,Ca) [Cu2+5 (AsO4)4 ] (H2O)2

Abstract

The crystal structure of andyrobertsite, ideally K Cd [Cu 2+ 5 (AsO 4 ) 4 {As(OH) 2 O 2 }] (H 2 O) 2 , monoclinic, a 9.8102(9), b 10.0424(6), c 9.9788(7) A, β 101.686(7)°, V 962.6(1) A 3 , P 2 1 / m , Z = 2, has been solved by direct methods and refined to an R index of 2.3% for 2140 observed ( F o > 5σ F ) reflections measured with Mo K α X-radiation. There are three crystallographically distinct As sites, each occupied by As 5+ in tetrahedral coordination. One of the As sites is coordinated by two O atoms and two (OH) groups to form a doubly acid arsenate group, {As(OH) 2 O 2 }. There are four crystallographically distinct Cu sites, each occupied by Cu 2+ in square-pyramidal coordination. There is one crystallographically distinct M site occupied by Cd, Ca and Mn 2+ coordinated by four O atoms and two (H 2 O) groups in a trigonal-prismatic arrangement. Variation in the occupancy of this site gives rise to the two related species andyrobertsite (Cd dominant) and calcio-andyrobertsite (Ca dominant). There is one distinct K site occupied by K and coordinated by four O atoms, two (OH) groups and two (H 2 O) groups in a distorted-cube arrangement. Four (CuO 5 ) polyhedra link to a central anion to form a [Cu 4 O 13 ] group. The anions at the base of this group corner-share to four (AsO 4 ) tetrahedra that link to a fifth (CuO 5 ) square pyramid. A single (Asϕ 4 ) (ϕ: unspecified anion) tetrahedron links to the central anion of the [Cu 4 O 13 ] group to form a [Cu 5 (AsO 4 ) 4 (Asϕ 4 ) O 9 ] cluster. These clusters are arranged at the vertices of non-coplanar 4 4 nets parallel to (100) and corner-link to form a heteropolyhedral framework with interstitial M (= Cd, Ca, Mn 2+ ) and K (= K) sites between adjacent 4 4 nets. Two of the anions in the structure show anomalously low incident-bond-valence sums (1.60 and 1.69 vu : valence units).