Abstract
AbstractAlstonite, BaCa(CO3)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(CO3)2. Alstonite is trigonal, space groupP31m, with unit-cell parametersa= 17.4360(6),c= 6.1295(2) Å,V= 1613.80(9) Å3andZ= 12. The crystal structure was solved and refined toR1= 0.0727 on the basis of 4515 reflections withFo> 4σ(Fo) and 195 refined parameters. Alstonite is formed by the alternation, alongc, of Ba-dominant and Ca-dominant layers, separated by CO3groups 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.
Highlights
Alstonite, BaCa(CO3)2, is polymorphous with barytocalcite, paralstonite and a recently published synthetic monoclinic phase (Spahr et al, 2019)
It is typically found in lowtemperature hydrothermal veins associated with other bariumand calcium-rich phases
We report only the results from the first locality because the tested crystal is of excellent diffraction quality with respect to the others
Summary
BaCa(CO3)2, is polymorphous with barytocalcite, paralstonite and a recently published synthetic monoclinic phase (Spahr et al, 2019). The other three polymorphs of BaCa(CO3)2 have all had their crystal structures determined: barytocalcite (Dickens and Bowen, 1971), paralstonite (Effenberger, 1980) and the recently described synthetic monoclinic form (Spahr et al, 2019).
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