M⁺M³⁺₂As(HAsO₄)₆and α- and β-M⁺M³⁺(HAsO₄)₂(M⁺M³⁺= RbAl or CsFe): six new compounds crystallizing in three closely related structure types

Abstract

The crystal structures of hydrothermally synthesized (T = 493 K, 7-9 d) rubidium aluminium bis[hydrogen arsenate(V)], RbAl(HAsO), caesium iron bis[hydrogen arsenate(V)], CsFe(HAsO), rubidium dialuminium arsenic(V) hexakis[hydrogen arsenate(V)], RbAlAs(HAsO), and caesium diiron arsenic(V) hexakis[hydrogen arsenate(V)], CsFeAs(HAsO), were solved by single-crystal X-ray diffraction. The four compounds with the general formula MM(HAsO) adopt the RbFe(HPO) structure type (R-3c) and a closely related new structure type, which is characterized by a different stacking order of the building units, leading to noncentrosymmetric space-group symmetry R32. The second new structure type, with the general formula MMAs(HAsO) (R-3c), is also a modification of the RbFe(HPO) structure type, in which one third of the MO octahedra are replaced by AsO octahedra, and two thirds of the voids in the structure, which are usually filled by M cations, remain empty to achieve charge balance.

Naalasite, NaAl(AsO3OH)2⋅H2O, the Al analogue of nafeasite from the Torrecillas mine, Iquique Province, Chile

“…(2014) were parascorodite, Fe(H 2 O) 2 AsO 4 ; the isostructural minerals scorodite, Fe(H 2 O) 2 AsO 4 , mansfieldite, Al(H 2 O) 2 AsO 4 , and yanomamite, In(H 2 O) 2 AsO 4 ; and berzeliite, (NaCa 2 )Mg 2 (AsO 4 ) 3 , and manganberzeliite, (NaCa 2 )Mn 2 (AsO 4 ) 3 , which have the garnet structure. None of these structures is very similar to those of naalasite and nafeasite; however, the naalasite and nafeasite frameworks are topologically equivalent to those of synthetic RbAl(AsO 3 OH) 2 and CsFe 3+ (AsO 3 OH) 2 , which, like naalasite, crystallise in space group R 32 (Schwendtner and Kolitsch, 2018).…”