Subsolidus Phase Relations in the System CaCO<sub>3</sub>-MgCO<sub>3</sub>

Goldsmith, Julian R.; Heard, H.C.

doi:10.1086/626715

Cited by 331 publications

(97 citation statements)

References 9 publications

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“…Thus, as it was already shown by Sluiter and Kawazoe, 35 the high-temperature enthalpy can be predicted from the excess enthalpies of just two single defects. The predicted type of asymmetry with the maximum shifted to magnesite-rich compositions is consistent with the shape of the phase diagram of the calcite-magnesite system, 36 which shows that the miscibility gap at magnesite-rich compositions is wider than the gap at calcite-rich compositions.…”

Section: Enthalpy In the High-temperature Limitsupporting

confidence: 75%

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

2009

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The thermodynamic mixing properties of a binary ͑A x B 1−x ͒R solid solution are evaluated from the enthalpies of supercell structures A m−2 B 2 R m and B m−2 A 2 R m , where m is the number of the exchangeable sites in the supercell. The excess enthalpies of these structures are converted into concentration-dependent pairwise effective cluster interactions J n , i.e., the enthalpies of the intracrystalline reactions AA+ BB 2AB acting at the n-neighbor distance within the supercell. The pairwise interactions calculated in this way for all possible distances within 3 ϫ 3 ϫ 1 supercells of R3c calcite and magnesite ͑m =54͒ are combined to form an effective Ising-type Hamiltonian from which temperature-dependent enthalpies, entropies, and free energies of mixing are evaluated with the Monte Carlo method. The calculated phase diagram with two miscibility gaps separated by a field of stability of the R3 dolomite phase is in good agreement with available experimental data, thereby showing that the existence of the intermediate ordered compound can be predicted from the analysis of the supercell structures whose compositions approach the diluted limits.

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Section: Enthalpy In the High-temperature Limitsupporting

confidence: 75%

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

2009

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“…The NanoSIMS ion probe provides quantitative information on Mg concentrations at a resolution lower than X-PEEM, but much higher than the X-ray microdiffraction used here. The spatial resolution is Ϸ100 nm and the probing depth of each scan is Ϸ10 nm (23). Fig.…”

Section: Resultsmentioning

confidence: 99%

“…NanoSIMS analyses were carried out with a primary beam of negatively-charged oxygen focused to a spot-size of Ϸ100 nm on the surface of the sample. The secondary ions 24 Mg ϩ and 44 Ca ϩ were detected in multicollector mode with electron multipliers at a mass resolving power of Ϸ5,000, which allows all potentially problematic mass-interferences to be resolved (23,24). Images of 256 by 256 pixels were obtained by rastering the primary beam across a presputtered surface with a pixel dwelling time of 10 ms.…”

Section: Nanosimsmentioning

confidence: 99%

“…A particularly powerful method is the combination of microbeam diffraction with scanning, that enables the production of 2-dimensional maps from the diffraction patterns (21,22). In our case, information on the magnesium distribution can be determined from the dependence of the diffraction angle of Bragg reflections on the Mg concentration (23). The nano-scale secondary ion mass spectrometer (NanoSIMS) represents the latest generation of ion microprobes, which links high-resolution microscopy with isotopic analysis.…”

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confidence: 99%

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The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

Aichmayer

Paris

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

162

157

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The sea urchin tooth is a remarkable grinding tool. Even though the tooth is composed almost entirely of calcite, it is used to grind holes into a rocky substrate itself often composed of calcite. Here, we use 3 complementary high-resolution tools to probe aspects of the structure of the grinding tip: X-ray photoelectron emission spectromicroscopy (X-PEEM), X-ray microdiffraction, and NanoSIMS. We confirm that the needles and plates are aligned and show here that even the high Mg polycrystalline matrix constituents are aligned with the other 2 structural elements when imaged at 20-nm resolution. Furthermore, we show that the entire tooth is composed of 2 cooriented polycrystalline blocks that differ in their orientations by only a few degrees. A unique feature of the grinding tip is that the structural elements from each coaligned block interdigitate. This interdigitation may influence the fracture process by creating a corrugated grinding surface. We also show that the overall Mg content of the tooth structural elements increases toward the grinding tip. This probably contributes to the increasing hardness of the tooth from the periphery to the tip. Clearly the formation of the tooth, and the tooth tip in particular, is amazingly well controlled. The improved understanding of these structural features could lead to the design of better mechanical grinding and cutting tools. echinoderms ͉ grinding tool ͉ self-sharpening ͉ spectromicroscopy ͉ X-ray microdiffraction

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“…The experimental study of Goldsmith and Heard (1961) identified the essential features of subsolidus phase relations: two asymmetric miscibility gaps separated by a narrow stability field for the dolomite-structure phase. Calorimetric studies by Navrotsky and Capobianco (1987), Chai et al (1995), Chai and Navrotsky (1996) and Navrotsky et al (1999) showed that the enthalpy of formation of ordered dolomite is negative, relative to a mechanical mixture of calcite and magnesite, but the formation enthalpy for a disordered solid solution of the same composition is positive.…”

Section: Introductionmentioning

confidence: 99%

Activity–composition relations in the system CaCO3–MgCO3 predicted from static structure energy calculations and Monte Carlo simulations

Vinograd

Burton

Gale

et al. 2007

Geochimica et Cosmochimica Acta

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Thermodynamic mixing properties and subsolidus phase relations of the rhombohedral carbonate system, (1-x)·CaCO 3 -x·MgCO 3 , were modelled with static structure energy calculations based on well constrained empirical interatomic potentials. Relaxed static structure energies of a large set of randomly selected structures in a 4x4x1 supercell of R3c calcite (a = 4.988 Å, c = 17.061Å) were calculated with the General Utitility Lattice Program (GULP). These energies were cluster expanded in a basis set of 12 pair-wise effective interactions. Temperature-dependent enthalpies of mixing were calculated by the Monte Carlo method. Free energies of mixing were obtained by thermodynamic integration of the Monte Carlo results. The calculated phase diagram is in good agreement with experimental phase boundaries.

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Subsolidus Phase Relations in the System CaCO₃-MgCO₃

Cited by 331 publications

References 9 publications

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

Activity–composition relations in the system CaCO3–MgCO3 predicted from static structure energy calculations and Monte Carlo simulations

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Subsolidus Phase Relations in the System CaCO3-MgCO3

Cited by 331 publications

References 9 publications

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution

Activity–composition relations in the system CaCO3–MgCO3 predicted from static structure energy calculations and Monte Carlo simulations

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Subsolidus Phase Relations in the System CaCO₃-MgCO₃