Transmission Electron Microscopic Study of the Kaolinitization of Muscovite

Abstract

Abstract--Hydrothermally kaolinitized muscovite from the Otago schist of Brighton, New Zealand, has been studied by transmission and analytical electron microscopy (TEM and AEM) to determine the mechanism of alteration and to compare reactant-product relations for di-and trioctahedral micas. The muscovite is a primary metamorphic phase having a phengitic composition. It occurs as well-ordered, two-and three-layer polytypes, in grains as thick as 30 tim. Kaolinite occurs as packets of layers, each about 100-600/k thick, which alternate with packets of muscovite or smectite-like layers. Most of the kaolinite is highly disordered in stacking sequence, although a one-layer polytype is also present, occurring as relatively thick sequences of layers. Phase boundaries between kaolinite and muscovite are invariably parallel to the 001 lattice fringes with no strain contrast; i.e., no transitions exist along layers. Parallelism of 001 and 11! reflection rows of both kaolinite and muscovite implies a topotaxial intergrowth. A smectitelike phase is also present, occurring as packets of wavy layers, which locally have periodic contrast that may reflect R 1 ordering of illite/smectite. This material appears to be a direct, "along-layer" alteration product of muscovite. Electron diffraction data and lattice-fringe images imply that kaolinite alternates with micaceous phase(s) with some regularity; i.e., micaceous layers are separated by approximately equal numbers of kaolinite layers. Similar long-range periodicity occurs in contrast variations within packets of kaolinite layers.The data collectively suggest that the alteration interface was self-perpetuating and that alteration proceeded rapidly along layers once it initiated in 2:1 layers at crystal edges or strained areas, with no observable component normal to the layers. They also suggest that smectite may have formed as an intermediate phase during the hydrothermal kaolinitization of muscovite. In the previous study of alteration of biotite in the same sample, "along-layer" transition boundaries were commonly observed, and a second, intermediate product phase was not detected, implying a relation between the alteration mechanisms and the chemical differences between reactants and products.

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

“…This Fe-Mg chlorite is inferred to be a primary hydrothermal, pre-metamorphic product that originated during ore-forming processes, or a metamorphic phase that evolved from possible hydrothermal precursors such as smectite and mixed-layer chlorite/smecdte. The presence of hybrid phases usually is suggested to result from retrograde metamorphic or hydrothermal reactions under non-equilibrium conditions (e.g., Banfield et al, 1989;Jiang and Peacor, 1991). The complex, heterogeneous mixed-layer chlorite/berthierine, berthierine, and Fe-rich chlorite in sample KC-2505 have properties that are consistent with non-equilibrium states and, therefore, are likely alteration products of homogeneous Fe-Mg chlorite formed earlier.…”

“…Specimen preparation and instrumental settings were detailed in Jiang and Peacor (1991) and Slack et al (1992). A selected area aperture 10 #m in diameter was utilized to produce the electron diffraction patterns.…”

“…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.…”

Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies

“…Kaolinite, like berthierine, has d(001) 7 /~, and these minerals can therefore be distinguished from each other only with difficulty. Kaolinite is especially difficult to identify by TEM methods since it is rapidly damaged by interaction with the electron beam and becomes amorphous within a few seconds (Jiang and Peacor 1991). Curtis et al (1984,1985) concluded that the presence of alkalis in their analyses implied contamination by admixed illite or residual smectite.…”

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…Preparation of TEM specimens was detailed in Jiang and Peacor (1991). Carbon-coated, ion-milled TEM specimens were first studied by optical microscopy and scanning electron microscopy (SEM) to identify largescale textural relations and to locate areas of interest for TEM observations.…”