Letter. High-pressure I2/c-P21/c phase transformation in SrAl2Si2O8 feldspar

• Published in: The American mineralogist Vol. 96; no. 7; pp. 1182 - 1185
• Main Authors: Pandolfo, Francesco, Ballaran, Tiziana Boffa, Nestola, Fabrizio, Müller, Monika Koch, Mrosko, Maria, Bruno, Emiliano
• Format: Journal Article
• Language: English
• Published: De Gruyter 01.07.2011
• Subjects: high pressure; phase transition; single crystal; SrAl2Si2O8 feldspar
• DOI: 10.2138/am.2011.3797
• ISSN: 0003-004X

In situ, high-pressure single-crystal X‑ray diffraction experiments were performed on a feldspar with composition SrAl Si (space group I2/c). The measurements were performed at 19 different pressures up to 7.9 GPa. The unit-cell parameters and the unit-cell volume show a strong discontinuity at 6.6 GPa indicating a first-order phase transition. The systematic absences clearly show that the transition occurs from I2/c to P2 /c space group symmetry, as for the transition recently reported for a feldspar with composition Sr Ca Al Si Our results suggest that the P2 /c phase is softer than the I2/c one. Moreover, within the I2/c symmetry two pressure-volume trends are evident due to a volume softening above 4.2 GPa. The softening did not allow fitting the data to a single equation of state, and only the data up to 3.67 were fitted with a third-order Birch-Murnaghan equation of state. Based on our and previous experimental results we propose a possible composition-pressure stability field for the Sr-rich side along the CaAl Si -SrAl Si join.