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Coordination of Sr and Mg in calcite and aragonite

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Coordination of Sr and Mg in calcite and aragonite. / Finch, Adrian Anthony; Allison, Nicola.

In: Mineralogical Magazine, Vol. 71, 10.2007, p. 539-552.

Research output: Contribution to journalArticle

Harvard

Finch, AA & Allison, N 2007, 'Coordination of Sr and Mg in calcite and aragonite' Mineralogical Magazine, vol. 71, pp. 539-552. https://doi.org/10.1180/minmag.2007.071.5.539

APA

Finch, A. A., & Allison, N. (2007). Coordination of Sr and Mg in calcite and aragonite. Mineralogical Magazine, 71, 539-552. https://doi.org/10.1180/minmag.2007.071.5.539

Vancouver

Finch AA, Allison N. Coordination of Sr and Mg in calcite and aragonite. Mineralogical Magazine. 2007 Oct;71:539-552. https://doi.org/10.1180/minmag.2007.071.5.539

Author

Finch, Adrian Anthony ; Allison, Nicola. / Coordination of Sr and Mg in calcite and aragonite. In: Mineralogical Magazine. 2007 ; Vol. 71. pp. 539-552.

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@article{ee39091c07964d10963c2f8f3e20d5cc,
title = "Coordination of Sr and Mg in calcite and aragonite",
abstract = "Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2{\%}) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5{\%} dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg-O bond distances of similar to 2.12 angstrom, which are much smaller than the Ca-O bond distance of 2.35 angstrom but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain natrodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.'08 angstrom) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.",
keywords = "EXAFS, XANES, impurities, palaeo-environmental proxies, calcite, aragonite, NEAR-EDGE STRUCTURE, X-RAY, CRYSTAL-STRUCTURE, K-EDGE, MAGNESIUM, SUPERSTRUCTURES, PRECIPITATION, STRONTIANITE, TEMPERATURE, RELAXATION",
author = "Finch, {Adrian Anthony} and Nicola Allison",
year = "2007",
month = "10",
doi = "10.1180/minmag.2007.071.5.539",
language = "English",
volume = "71",
pages = "539--552",
journal = "Mineralogical Magazine",
issn = "0026-461X",
publisher = "Mineralogical Society",

}

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TY - JOUR

T1 - Coordination of Sr and Mg in calcite and aragonite

AU - Finch, Adrian Anthony

AU - Allison, Nicola

PY - 2007/10

Y1 - 2007/10

N2 - Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg-O bond distances of similar to 2.12 angstrom, which are much smaller than the Ca-O bond distance of 2.35 angstrom but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain natrodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.'08 angstrom) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.

AB - Strontium and Mg in calcite and aragonite are widely used as proxies of temperature in palaeo-environmental reconstructions. We use X-ray absorption fine structure (XAFS) to examine Sr and Mg substitution in calcite and aragonite. We have measured the K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Mg and Sr-bearing calcite and aragonite, plus the carbonates: strontianite, hydromagnesite, magnesite, dolomite and a suite of calcites with differing amounts of Mg. The Sr substitutes ideally for Ca in aragonite but causes a small (2%) dilation of the site. Strontium substitutes for octahedral Ca in calcite but with a 6.5% dilation and distortion. Magnesium in the calcites studied provides a variable XANES indicating that the Mg structural state in calcite is variable. Refinement of EXAFS gives Mg-O bond distances of similar to 2.12 angstrom, which are much smaller than the Ca-O bond distance of 2.35 angstrom but consistent with published amounts of relaxation of the calcite structure. The XANES and EXAFS are consistent with a model whereby some calcites contain natrodomains, e.g. of dolomite and/or huntite structures. The variability in the XANES can be explained by domains of different types and/or sizes. Substitution of Mg into aragonite has 9-fold coordination but relatively short bond distances (2.'08 angstrom) demonstrating either: (1) substantial distortion of the site; or (2) that Mg is accommodated in nanodomains of an unknown phase. Variability in the Mg structural state in calcite may be linked to the variety of temperature dependences observed, e.g. in foraminiferal calcite.

KW - EXAFS

KW - XANES

KW - impurities

KW - palaeo-environmental proxies

KW - calcite

KW - aragonite

KW - NEAR-EDGE STRUCTURE

KW - X-RAY

KW - CRYSTAL-STRUCTURE

KW - K-EDGE

KW - MAGNESIUM

KW - SUPERSTRUCTURES

KW - PRECIPITATION

KW - STRONTIANITE

KW - TEMPERATURE

KW - RELAXATION

UR - http://www.scopus.com/inward/record.url?scp=42949109781&partnerID=8YFLogxK

U2 - 10.1180/minmag.2007.071.5.539

DO - 10.1180/minmag.2007.071.5.539

M3 - Article

VL - 71

SP - 539

EP - 552

JO - Mineralogical Magazine

T2 - Mineralogical Magazine

JF - Mineralogical Magazine

SN - 0026-461X

ER -

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ID: 453597