Search for content and authors
 

Synthesis of high-pressure MgCO3 - FeCO3 carbonate solid solutions: in situ Raman spectroscopy study

Anna V. Spivak 1Natalia A. Solopova 1,2Elena Bykova 2Egor S. Zakharchenko 1Yuriy A. Litvin 1Leonid S. Dubrovinsky 2

1. Institute of Experimental Mineralogy RAS (IEMRAS), Acad. Osipiana St, Chernogolovka 142432, Russian Federation
2. Bayerisches Geoinstitut Universität Bayreuth (BGI), Bayreuth D-95440, Germany

Abstract

There is growing mineralogical and experimental evidence that Ca-, (Ca, Mg, Fe)- and Na-Ca - carbonates are stable at the conditions of the Earth deep interior. Data on carbonate   high-pressure high-temperature transformations would reveal their behavior at PT-conditions of the Earth’s transition zone and lower mantle.

Fe2+ and Mg2+ substitution is common in mineral systems. Particularly, Fe2+ and Mg2+ form a complete solid solution in the FeCO3-MgCO3 system. Behavior of components of Mg1-xFexCO3 solid solution (x=0.05, 0.09, 0.2, 0.4) is characterized in situ by Raman spectroscopy at high pressure and ambient temperature.

Samples of Mg-Fe carbonates of variable composition were synthesized using multianvil apparatus at 18 GPa and 2000oC. High-pressure experiments were carried out in a diamond anvil cell (DAC).  The LabRam system (Horiba Sceintific Inc.) with a He-Nd-laser (excitation wavelength 632 nm) was used for in situ registration of the Raman spectra.

Magnesite-stuctured carbonates with rhombohedral symmetry have six characteristic vibrational modes in Raman spectra. Four internal modes are connected with vibrations within the (CO3)2- unit, and two modes - with external vibrations of the crystal lattice. For external modes T and L, (1) position of Raman band for end members siderite and magnesite are, correspondingly, 174/213 cm-1 and 273/330 cm-1; (2) pressure – induced shiftfor solid-solution variable within Mg0.6Fe0.4CO3-Mg0.95Fe0.05CO3 composition are in the 197-210 cm-1 and 301-328 cm-1 range, correspondingly. The end member positions and solid solution pressure – induced shifts are characterized by the internal modes ν4  (720/737 cm-1 and 730-737 cm-1 range), ν1 (1074/1092 cm-1 and 1086-1092 cm-1 range), ν3 (1425/1442 cm-1 and 1427-1442 cm-1 range), 2ν2 (1711/1759 cm-1 and 1733-1758 cm-1 range). Raman shifts calibrated as a function of Mg or Fe carbonate content may be used in turn to evaluate the chemical composition of natural carbonate samples. Apart Raman bands characteristic for magnesite-type materials, it was observed the additional Raman peak in range 868-874 cm-1 for solid solutions of composition Mg0.6Fe0.4CO3-Mg0.95Fe0.05CO3.  Similar peak was determined at 870 cm-1 for siderite and described as additional one not predicted by factor group analyses by Boulard et al. (2011). This peak may be explained to an infrared mode becoming Raman active due to a local loss of inversion symmetry as a result of Fe ordering [Langille and Oshea, 1977].

At high pressure in situ Raman study, it was analyzed the behavior of four modes of carbonates with following compositions Mg0.6Fe0.4CO3, Mg0.8Fe0.2CO3, Mg0.91Fe0.09CO3 and Mg0.95Fe0.05CO3. The high pressure Raman spectra show that the frequency of ν1 of all studied carbonates increases monotonically up to 40-42 GPa, where addition peak appeared at 1220 cm-1. Position of the peak of internal mode ν4 is increases monotonically in all cases. Frequency of external modes T and L of Mg0.91Fe0.9CO3 increase linearly up to 46 GPa, and above this pressure change it slope. For all carbonates with intermediate compositions additional peak in range 868-874 cm-1 at 0 GPa is observed. This peak not detectable in the DAC at low pressures, but become visible above 30 GPa with frequency increasing monotonically.

This work was funded by grant of the President RF MK-1386.2013.5, The Ministry of education and science of Russian Federation, project 8317, 16.740.11.0621 and grants RFBR 13-05-00835, 12-05-33044, 11-05-000401.

 

Legal notice
  • Legal notice:
 

Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 6, by Anna V. Spivak
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-04-01 14:27
Revised:   2013-04-03 11:30