Expansivity and compressibility of wadeite-type K2Si4O9 determined by in situ high T/P experiments, and their implication

Abstract

Wadeite-type K2Si4O9 was synthesized with a cubic press at 5.4 GPa and 900 °C for 3 h. Its unit-cell parameters were measured by in situ high-T powder X-ray diffraction up to 600 °C at ambient P. The T–V data were fitted with a polynomial expression for the volumetric thermal expansion coefficient (αT = a0 + a1T), yielding a0 = 2.47(21) × 10−5 K−1 and a1 = 1.45(36) × 10−8 K−2. Compression experiments at ambient T were conducted up to 10.40 GPa with a diamond-anvil cell combined with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the P–V data, yielding KT = 97(3) GPa and V0 = 360.55(9) A3. These newly determined thermal expansion data and compression data were used to thermodynamically calculate the P–T curves of the following reactions: 2 sanidine (KAlSi3O8) = wadeite (K2Si4O9) + kyanite (Al2SiO5) + coesite (SiO2) and wadeite (K2Si4O9) + kyanite (Al2SiO5) + coesite/stishovite (SiO2) = 2 hollandite (KAlSi3O8). The calculated phase boundaries are generally consistent with previous experimental determinations.