Hard mode infrared spectroscopy of plagioclase feldspars
Atkinson, Alison J.; Carpenter, Michael A.; Salje, Ekhard K. H.
published: Feb 11, 1999
manuscript accepted: Sep 8, 1998
manuscript received: Apr 20, 1998
ArtNo. ESP147051101008, Price: 29.00 €
Abstract Local states of order and the character of phase transitions in a suite of natural and heat-treated plagioclase feldspars have been examined by Hard Mode Infrared Spectroscopy at room temperature. Autocorrelation analysis has been used to determine variations of average linewidths of groups of peaks in the primary IR spectra. Changes in frequency of selected peaks and an autocorrelation parameter from the frequency range 500-650 cm-1 scale with the square of the macroscopic order parameter for the C1̄ ↔ I1̄ transition. Combining the autocorrelation data, which provide information relating to the local (unit-cell) scale behaviour, with lattice strain, which indicates the average macroscopic behaviour, leads to a qualitative differentiation between the two incommensurate phases, ei (An-rich) and e2 (Ab-rich). Within individual domains of the ei structure in crystals of composition ~An70, the local homogeneity and Al/Si ordering conformation appear to be indistinguishable from I1̄ structural states. The degree of I1̄ order diminishes with increasing Ab-content and would extrapolate to zero somewhere between ~An40 and ~An50. The e2 structures appear to relate to the Al/Si ordering of albite butjetain a degree of heterogeneity on a phonon length scale which is comparable to that found in disordered C1̄ crystals. On the length scale of the IR experiments, there is an abrupt cut off for the formation_of PTdomains between_~An86 and ~An90 as Ab-component is added to anorthite. Transitions of the type C1̄ ↔ I1̄, I1̄ ↔ P1̄ and I1̄ ↔ e1 appear to be continuous as a function of composition at a local structural scale. Deviations from ideal mixing in the heat-treated series arise from Al/Si ordering at An-rich compositions.