Correlation between crystallization pressure and structural parameters of phengites

Linear correlations of lnP vs the thickness t of the TOT mica module and vs the ditrigonal distortion α are established by analyzing a population of 43 phengites for which an estimate of the crystallization pressure P independent of the Si content is available. The equations for the fitting regression lines are: lnP = -0.35(3)α + 5.0(3) (R 0.90, 26 observations) and lnP = -29(2)t + 293(22) (R 0.90, 43 observations). These correlations evidence the strict links involving genetic conditions, chemical composition and structural parameters c (represented by t) and α. In their turn, for the structural parameters the following correlation holds: t = 0.0110(9)α + 9.859(7) (R 0.92, 26 observations). The linear correlations of lnP, t and α vs the Si content have equations: lnP = 4.2(4)Si - 12(1) (R 0.86, 43 observations), t = -0.13(1)Si + 10.37(4) (R 0.87, 43 observations), and α = -12.7(9)Si + 50(3) (R 0.94, 26 observations). The observed dispersion of the data is mainly due to the uncertainty in the pressures estimated from mineral assemblages. The possibility of calculating P from a measure of c only does not overcome this type of uncertainty, but opens the possibility of estimating the crystallization pressure of phengites relics at nanoscale (e.g., by transmission electron diffraction).