Density Measurement of Iron-Bearing Sodium Disilicate by High-Pressure X-ray Absorption Method

Density is one of the important physical properties of silicate melts to understand the magmatic process in the Earth inteiror. Recently, we have applied x-ray absorption technique to measure density of silicate glasses and melts at high-pressure, using highly brilliant synchrotron x-ray and a multi-anvil apparatus. Very low absorption of silciates for x-ray reduces absorption contrast between silicate samples and surrounding materials to make density measurement difficult. We have overcome this difficulty by use of a single crystal diamond ring as a sample container, which is transparent for x-ray and less reactive with silicate melt. High-pressure and temperature x-ray absorption experiments were conducted using the cubic-type press SMAP-1 at BL22XU of SPring-8, Japan. Starting sample is the Fe-bearing Na disilicate glass with the composition of Na₂FeSi₂O₆. Energy of monochromatic x-ray is tune to be 25 keV and intensities of incident and transmitted x-rays are measured by ion chambers. Density is evaluated from x-ray absorption profile along a radial direction of a cylindrical sample using equation of I = I₀exp(-\mu\rho t). We reported the results of the density measurements of Na₂FeSi₂O₆ glass up to 5 GPa and 600 K at AGU Fall meeting in 2003. Now we succeed the density measurement of that melt up to 3 GPa and 1400 K. Althogh these pressure and temperature conditions are out of the stability field of diamond, diamond ring survives after several hours heating. Measured density of Na₂FeSi₂O₆ changes discontinuously with phase transitions from glass to crystal and from crystal to melt. Apparent density increase of Na₂FeSi₂O₆ melt with increasing pressure is also observed, indicating that this method has a potential to determine equation of state of silicate melts.