HIGH-PRESSURE BEHAVIOR OF CA0.2SR0.8AL2SI2O8 FELDSPAR

The feldspar structure can accommodate in the cavities of its tetrahedral framework a wide range of cations that determine its behavior as a function of pressure and temperature. In anorthite, for example, a P-1 to I-1 phase transition occurs both as a function of temperature and pressure. The high-pressure low-temperature and the high-temperature low-pressure phase boundaries have different Clapeyron slopes (Hackwell and Angel, Am. Min. 1995) possibly indicating different mechanisms for the two phase transitions. A further phase transformation from I-1 to a yet undefined structure has been observed by Raman spectroscopy (Daniel et al., J. Geophys. Res. 1997) at about 10 GPa. In Sr-feldspar, an analogue of anorthite, instead, a monoclinic to triclinic phase transformation has been observed (McGuinn and Redfern, Min. Mag. 1994). The range of different symmetries stable at room conditions due to substitution of Ca by Sr into the M site of the anorthite P-1 structure gives an excellent opportunity to investigate the possible reason of such different high-pressure behavior of the two end-member feldspars.

A single-crystal of Ca_0.2Sr_0.8Al₂Si₂O₈ having I-1 symmetry at room conditions has been loaded into a diamond anvil cell together with a quartz single-crystal as internal pressure standard and a 4:1 methanol:ethanol mixture as pressure medium. Unit-cell parameters were measured as a function of pressure up to 7.7 GPa. At pressure above 4 GPa a first-order transition from triclinic to monoclinic symmetry is observed, indicated by the change in a and g unit-cell angles. Careful cycling above and below the transition has shown the presence of hysteresis between 4.0 and 4.3 GPa. This transition is likely to be the I-1 to I2/c phase transformation observed for the same sample with increasing temperature, suggesting that also for the triclinic CaSr-feldspars, as in the case of anorthite, the high-pressure low-temperature and the high-temperature low-pressure phase boundaries have different Clapeyron slopes. EoS parameters for the triclinic phase calculated up to 4.1 GPa using a Birch-Murnaghan equation of state are V₀=1393.97(5) Å K₀=88.7(7) GPa and K'=1.5(2). Evidence of a further first-order phase transformation from monoclinic to monoclinic symmetry have been found at pressure above 7 GPa with a volume change of ~ 1.5% and is at the moment under investigation.