The crystal structure of alstonite, BaCa(CO 3 ) 2 : an extraordinary example of ‘hidden’ complex twinning in large single crystals

Abstract

Alstonite, BaCa(CO 3 ) 2 , is a mineral described almost two centuries ago. It is widespread in Nature and forms magnificent cm-sized crystals. Notwithstanding, its crystal structure was still unknown. Here, we report the crystal-structure determination of the mineral and discuss it in relationship to other polymorphs of BaCa(CO 3 ) 2 . Alstonite is trigonal, space group P 31 m , with unit-cell parameters a = 17.4360(6), c = 6.1295(2) Å, V = 1613.80(9) Å 3 and Z = 12. The crystal structure was solved and refined to R 1 = 0.0727 on the basis of 4515 reflections with F o > 4σ( F o ) and 195 refined parameters. Alstonite is formed by the alternation, along c , of Ba-dominant and Ca-dominant layers, separated by CO 3 groups parallel to {0001}. The main take-home message is to show that not all structure determinations of minerals/compounds can be solved routinely. Some crystals, even large ones displaying excellent diffraction quality, can be twinned in complex ways, thus making their study a crystallographic challenge.