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 nmtual solid solution. The conditions that prevailed during the retrogression estimated from phase equilibria are T= 370-450 ~ C and Xco 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.

Transmission Electron Microscopic Study of the Kaolinitization of Muscovite

“…Such major changes demand at least partial dissolution of octahedral and tetrahedral sheets of the mica structure and the crystallization of kaolinite. Dissolutioncrystallization mechanisms for transitions between phyllosilicates under low-temperature conditions have also been suggested in other studies; e.g., Yau et al (1984) for the phlogopite-to-chlorite transition, Ahn and Peacor (1986) and Yau et al (1987) for the smectite-to-illite reaction, Ahn and Peacor (1987) for the biotite-to-kaolinite transition, and Banfield and Eggleton (1988) for biotite-to-vermiculite and/or -kaolinite alteration. Dissolution-crystallization through a fluid medium is required simply because solid-state mechanisms are too sluggish at low temperatures, as in the samples studied here.…”

“…Veblen and Ferry (1983) and Olives and Amouric (1984) described the transitions of two biotite layers to one chlorite layer and one biotite layer to one chlorite layer, respectively, resulting in a large volume loss and gain, respectively. In a study of the phlogopite-to-chlorite transition, Yau et al (1984) showed that both volume loss and gain can occur locally and suggested that they may compensate for each other in volume change on a larger scale, as was also suggested by Veblen and Ferry (1983). Nearly "equal-volume" reactions are rare but do exist, e.g., Yau et aL (1984) described a single example of a transition of 14 phlogopite layers to 10 chlorite layers.…”

Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

“…The great variety of interstratificadons and the styles of mixed layering are uncharacteristic ofberthierine or mixed-layer chlorite/berthierine observed in prograde diagenetic/metamorphic rocks (Lee and Peacor, 1983;Ahn and Peacor, 1985;Amouric et al, 1988;Jahren and Aagaard, 1989). Low-temperature, hydrothermal or retrograde alteration of one phyllosilicate to another commonly results in a wide range of intergrowth relations reflecting heterogeneous local environments (e.g., Veblen and Ferry, 1983;Yau et al, 1984;Ahn and Peacor, 1987;Banfield and Eggleton, 1988;Jiang et al, 1990;Sharp et al, 1990;Jiang and Peacor, 1991). All of the phyllosilicate grains in sample KC-813 have high concentrations of strain features, such as kink bands and stacking faults, which apparently reflect the effect of tectonic stress during dynamic metamorphism.…”

An Electron Optical investigation of Aluminosilicate Cements in Silcretes

“…The discontinuity in the structure at the point of layer termination may serve as channel for transport of ions in solution. This mechanism of ion transport has been considered to be active in many mineral alteration processes (Veblen and Buseck, 1980;Yau et al, 1984;Singh and Gilkes, 1991) that involve gain or loss of ions at a weathering front progressing along a single layer. Thus, these very small and disordered kaolinite crystals may be the residues of the much larger and more highly ordered kaolinite that occurs in saprolite.…”