Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2-Na2Mg(CO3)2 join
Articolo
Data di Pubblicazione:
2015
Citazione:
Crystal structure, high-pressure, and high-temperature behavior of carbonates in the K2Mg(CO3)2-Na2Mg(CO3)2 join / A. Golubkova, M. Merlini, M.W. Schmidt. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - 100:11/12(2015), pp. 2458-2467. [10.2138/am-2015-5219]
Abstract:
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 Na2Mg(CO3)2-K2Mg(CO3)2 join only the Na-end-member eitelite (R3 space group), was reported in nature. The K-end-member (R3m) readily hydrates even at low temperatures, therefore, only baylissite, K2Mg(CO3)2·4H2O, has been observed. Because of the role of (K,Na)Mgdouble carbonates in mantle metasomatism, we performed high P-T experiments on K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)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 V0 = 396.2(4), 381.2(5), and 347.1(3) Å3 and K0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K2Mg(CO3)2, (K1.1Na0.9)2Mg(CO3)2, and Na2Mg(CO3)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 É0 = 14.31(5) × 10-5 K-1 and 16.73(11) × 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a- and c-axis: É0(a) = 2.84(6) × 10-5 and 4.78(5) × 10-5 K-1 and É0(c) = 10.47(11) × 10-5 and 8.72(5) × 10-5 K-1 for K2Mg(CO3)2 and Na2Mg(CO3)2, respectively.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
alkali-alkali earth double carbonates; high pressure; phase transition; synchrotron; geochemistry and petrology; geophysics
Elenco autori:
A. Golubkova, M. Merlini, M.W. Schmidt
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