On twinning and microstructures in calcite and dolomite

Larsson, Ann-Kristin and Christy, Andrew G. (2008) On twinning and microstructures in calcite and dolomite. American Mineralogist, 93 1: 103-113. doi:10.2138/am.2008.2520


Author Larsson, Ann-Kristin
Christy, Andrew G.
Title On twinning and microstructures in calcite and dolomite
Journal name American Mineralogist   Check publisher's open access policy
ISSN 0003-004X
1945-3027
Publication date 2008-01-01
Year available 2007
Sub-type Article (original research)
DOI 10.2138/am.2008.2520
Open Access Status Not yet assessed
Volume 93
Issue 1
Start page 103
End page 113
Total pages 11
Place of publication Chantilly, VA, United States
Publisher Mineralogical Society of America
Language eng
Abstract Apparent superlattice reflections obtained in electron diffraction patterns of modulated calcite and dolomite have been ascribed to local domains of various different metastable Ca-Mg ordering schemes that are not known as macroscopic phases. We show that the type "c" reflections in diffraction patterns of supposed superstructures can in fact be produced by superposition of diffraction from the host crystal and that from domains that are in an orientation related to the host by twinning on (104). From details of the additional reflections present, we deduce that the carbonate anions are orientationally disordered in the twin nanodomains, which have the R3̄m space group of high-temperature disordered calcite. This twinning can explain the diffraction ascribed to type "γ"/"μ" /"ν" superstructures, and resolves controversies over the occurrence of these purportedly different superstructures. The relationships between composition, orientational order of the carbonates, molar volume and known macroscopic structures, and the possibility of interfacial strain reduction by static disorder in the twin domain, are discussed. We stress the importance of checking for presence of twinned nanodomains using microdiffraction before attributing an apparent superstructure modulation to local cation ordering. High-resolution imaging may not be diagnostic, since overlap of small domains and host matrix can give Moiré patterns that resemble superlattice fringes.
Keyword Calcite
Dolomite
Electron diffraction
Superstructures
Twinning
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Earth and Environmental Sciences
 
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