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  • 1
    Publication Date: 2015-11-21
    Description: Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na 2 Mg(CO 3 ) 2 –K 2 Mg(CO 3 ) 2 join only the Na-end-member eitelite ( R space group), was reported in nature. The K-end-member ( R m ) readily hydrates even at low temperatures, therefore, only baylissite, K 2 Mg(CO 3 ) 2 ·4H 2 O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 . Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V 0 = 396.2(4), 381.2(5), and 347.1(3) Å 3 and K 0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 , respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high- P phase is 1% denser than the low- P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25–600 °C and ambient pressure and are α 0 = 14.31(5) x 10 –5 K –1 and 16.73(11) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a - and c -axis: α 0 ( a ) = 2.84(6) x 10 –5 and 4.78(5) x 10 –5 K –1 and α 0 ( c ) = 10.47(11) x 10 –5 and 8.72(5) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 2
    Publication Date: 2012-10-01
    Description: Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered ( R ), but transforms with increasing temperature into a disordered higher symmetry structure ( R c ). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300–1350 K and 2.6–4.2 GPa. The long-range order parameter s , quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20–30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe . Complete disorder is attained in dolomite at ~1240 K, 100–220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction.
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 3
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    Mineralogical Society of America
    Publication Date: 2013-05-11
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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