In situ observation of the pyroxene-majorite transition in Na₂MgSi₅O₁₂ using synchrotron radiation and Raman spectroscopy of Na-majorite

In situ X-ray diffraction study of the pyroxene to majorite transition in Na₂MgSi₅O₁₂ was carried out in Kawai-type high-pressure apparatus coupled with synchrotron radiation. The phase boundary between Na-pyroxene and Na-majorite was determined over the temperature interval of 1073-1973 K and was described by a linear equation P (GPa) = 12.39 + 0.0018×T (K). The Clapeyron slope (dP/ dT) determined in this study is similar to the one predicted by computer simulations (Vinograd et al. 2011) but smoother than the one obtained by quenched experiments (Dymshits et al. 2010). The presence of sodium in the system lowers the pressure of pyroxene-to-majorite transformation. For the first time Na-majorite was characterized using Raman spectroscopy. Raman peaks of Na-majorite are broader than pyrope due to the substitution of Mg²⁺ for Na⁺ at the X site. Both Si-O symmetric stretching (A_1g-n₁) and O-Si-O symmetric bending (A_1g-n₂) modes of Na-majorite are significantly shifted to higher frequencies relative to corresponding bands of pyrope. In contrast the A_1g-R (SiO₄) mode of Na-majorite (342 cm^-1) displays a lower frequency than that of pyrope (365 cm^-1). Our results enable further understanding of the mechanisms responsible for phase transformations in the Earth's transition zone and lower mantle.