Synthetic ANaB(NaxLi1 ¡ xMg1)CMg5Si8O22(OH)2 (with x = 0.6, 0.2 and 0) P21/m amphiboles at high pressure: a synchrotron infrared study

The high-pressure behavior of three synthetic amphiboles crystallized with space group P2(1)/m at room conditions in the system Li(2)O-Na(2)O-MgO-SiO(2)-H(2)O has been studied by in situ synchrotron infrared absorption spectroscopy. The amphiboles have compositions (A)Na (B)(Na (x) Li(1 - x) Mg(1)) (C)Mg(5) Si(8) O(22)(OH)(2) with x = 0.6, 0.2 and 0.0, respectively. The high-P experiments up to 32 GPa were carried out on the U2A beamline at Brookhaven National Laboratory (NY, USA) using a diamond anvil cell under non-hydrostatic or quasi-hydrostatic conditions. The two most intense absorption bands in the OH-stretching infrared spectra can be assigned to two non-equivalent O-H dipoles in the P2(1)/m structure, bonded to the same local environment (M1M3)Mg(3)-OH-(A)Na, and pointing toward two differently kinked tetrahedral rings. In all samples these bands progressively merge to give a unique symmetrical absorption with increasing pressure, suggesting a change in symmetry from P2(1)/m to C2/m. The pressure at which the transition occurs appears to be linearly correlated to the aggregate B-site dimension. The infrared spectra collected for amphibole (B)(Na(0.2)Li(0.8)Mg(1)) in the frequency range 50 to 1,400 cm(-1) also show a series of changes with increasing pressure. The data reported here support the inference of Iezzi et al. (Am Miner 91:479-482, 2006a) regarding a new high-pressure amphibole polymorph.