Thermal stability, low- and high-temperature behavior of bergslagite, a berylloarsenate member of the gadolinite supergroup

Abstract

Bergslagite, Ca₂Be₂As₂O₈(OH)₂, is one of the only three known berylloarsenate minerals and is a member of the gadolinite supergroup. To date, very little is known about the thermal behavior of beryllium compounds and not much more about arsenates, while the thermal behavior of berylloarsenates (both natural and synthetic) has not been previously studied at all. In this work, the low and high-temperature behavior and thermal stability of bergslagite were studied in situ using single-crystal X-ray diffraction. Besides, its Raman spectrum was obtained and compared to the calculated one. Bergslagite does not undergo a phase transition in the temperature range − 173 to 700 °C, whereas it amorphizes at higher temperatures. The TO₄-based (T = Be, As) framework remains stable, while the CaO₆(OH)₂ polyhedra are slightly expanding. The volume thermal expansion coefficient (32 × 10^− 6 °C^− 1) is comparable with borosilicate / beryllophosphate analogues (30–35 × 10^− 6 °C^− 1). The low thermal stability of bergslagite can be associated with the vacant octahedral position, which is occupied by divalent cations in more thermally stable beryllosilicate analogues.