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The crystal structure of alstonite, BaCa(CO (sub 3) ) (sub 2) ; an extraordinary example of 'hidden' complex twinning in large single crystals

Luca Bindi, Andrew C. Roberts and Cristian Biagioni
The crystal structure of alstonite, BaCa(CO (sub 3) ) (sub 2) ; an extraordinary example of 'hidden' complex twinning in large single crystals
Mineralogical Magazine (October 2020) 84 (5): 699-704

Abstract

Alstonite, 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 group P31m, with unit-cell parameters a = 17.4360(6), c = 6.1295(2) Aa, V = 1613.80(9) Aa3 and Z = 12. The crystal structure was solved and refined to R1 = 0.0727 on the basis of 4515 reflections with Fo > 4sigma (Fo) and 195 refined parameters. Alstonite is formed by the alternation, along c, of Ba-dominant and Ca-dominant layers, separated by CO3 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.


ISSN: 0026-461X
EISSN: 1471-8022
Coden: MNLMBB
Serial Title: Mineralogical Magazine
Serial Volume: 84
Serial Issue: 5
Title: The crystal structure of alstonite, BaCa(CO (sub 3) ) (sub 2) ; an extraordinary example of 'hidden' complex twinning in large single crystals
Affiliation: Universita di Firenze, Dipartimento di Scienze della Terra, Florence, Italy
Pages: 699-704
Published: 202010
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 33
Accession Number: 2021-002112
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Geological Survey of Canada, CAN, CanadaUniversita di Pisa, ITA, Italy
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, Copyright, Mineralogical Society of Great Britain and Ireland. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2021
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