New rubidium pentaborate Rb[B5O7(OH)2] · 0.5H2O with a 5: [4Δ + 1T] anionic block and its relation to larderellite (NH4)[B5O7(OH)2] · H2O on the basis of the OD theory

Abstract

A new rubidium pentaborate is synthesized under hydrothermal conditions. Its crystal structure is studied by the heavy-atom method without any a priori knowledge of chemical formula. The chemical formula is Rb[B5O6(OH)4] · 0.5H2O, sp. gr. \(\bar P1\), lattice parameters a = 7.679(4) A, b = 9.253(6) A, c = 12.053(9) A, α = 98.55(5)°, β = 106.80(5)°, γ = 91.71°, R = 0.0573, Rw = 0.0638, S = 1.07. The anionic part of the structure consists of a chain of fundamental building blocks 5:[4Δ + 1T] built by four B triangles bound to one B tetrahedron, which are common to Na, K, Rb, and Cs pentaborates. This new pentaborate is closely related to the mineral larderellite (NH4)[B5O6(OH)4] · H2O but possesses an original structure, which manifests itself in the different morphology of the new pentaborate and the absence of perfect cleavage. The Dornberger-Schiff OD theory allows one to describe in detail the structural relationships, predict possible hypothetical structures, and write the OD groupoid.