Phlogopite-chlorite reaction mechanisms and physical conditions during retrograde reactions in the Marble Formation, Franklin, New Jersey

Abstract

A retrograde assemblage in a specimen from the Franklin Marble Formation containing an unusual occurrence of three micas has been studied. Microprobe analyses of the margarite, muscovite and phlogopite reveal significant sodium and fluorine but otherwise show little mutual solid solution. The conditions that prevailed during the retrogression estimated from phase equilibria are T=370–450° C and $$X_{{\text{CO}}_{\text{2}} }$$ =0.03–0.3 at an assumed pressure of 2 kb. Examination of the phlogopite by TEM revealed replacement of phlogopite by chlorite. The chlorite occurred as small packets of layers interlayered with phlogopite. AEM analyses revealed that the chlorite composition approximates that of clinochlore. Transformations from a single phlogopite layer to one chlorite layer (a 1∶1 reaction) and also from two phlogopite layers to one chlorite layer (a 2∶1 reaction) have been observed. Both reactions result in a large volume change causing local strain at transition fronts. An apparently strain-free, volume-preserving transformation of 14 phlogopite layers terminating against 10 chlorite layers was observed which indicates that pervasive replacement may occur through a combination of both reactions. Topological and site occupancy changes during the transformation suggest that the reaction involves not only gain and loss of layer units, but also considerable local rupturing and reformation of bonds, concomitant with ion diffusion and interchange. Dislocations (layer terminations) at the transition front imply the existence of pathways for the fluid flow and ion transport required for mass balance.