Pressure-induced (Al,Si)[4]-ordering in dioctahedral micas?

Pure, synthetic 2M₁-muscovites (Ms) and 2M₁-paragonites (Pg) prepared at 7, 13.5, and 18 kbar have been subjected to careful X-ray powder diffractometry and IR powder spectrometry. The results of X-ray studies show that the lattice constants of these micas are independent of the pressure employed in synthesizing them. Similarly, the IR powder spectra of Ms and Pg remain unaffected by the synthesis pressure. Neither the Al^4]-O-Al^4] vibrations decrease in relative intensity nor do the Si-O-Al^4] vibrations increase in intensity with increasing pressure of synthesis, as they should, if pressure-induced (Al, Si)^4] ordering had been operative. Finally, the v _OH bands in Ms and Pg are also found to retain their fine structural details, regardless of the pressures employed in synthesizing these phases. In particular, the half widths of v _OH in these (Al,Si)^4]-disordered micas remain constant at around 60 cm⁻¹, compared to ∼ 10 cm⁻¹ in (Al,Si)^4]-ordered mica margarite. In the light of this study, shortening of the b lattice parameter and progressive (Al,Si)^4] ordering with increasing pressure of synthesis of muscovites reported by Velde (1980) would appear to be anomalous. Consequently, any discussion of the effect of (Al,Si)^4] ordering on the stability of muscovite is a moot point at present.