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American Mineralogist; April 2008; v. 93; no. 4; p. 520-527; DOI: 10.2138/am.2008.2662
© 2008 Mineralogical Society of America
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White mica domain formation: A model for paragonite, margarite, and muscovite formation during prograde metamorphism

Kenneth J.T. Livi1,*, George E. Christidis2, Péter Árkai3 and David R. Veblen1

1 The Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
2 Technical University of Crete, Department of Mineral Resources Engineering, 73100, Chania, Greece
3 Institute for Geochemical Research, Hungarian Academy of Sciences, Budaoersi ut 45, 1112 Budapest, Hungary


Figure 1
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  		FIGURE 1. STEM EDS images of white mica flakes from Gravera, Switzerland (MF-925). The top half of the image includes a bright-field image (BF) and X-ray maps. The Na, Ca, and K images were combined to form a false-color RGB map (Na-Ca-K) of Pg (red), Mrg (green), and Ms (blue). The spectra summed from areas A and B are given in the lower right. The boundaries between Mrg and Ms (A and B) are sharp, indicating crystallographic control of lamellar orientation.

 

Figure 2
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  		FIGURE 2. STEM EDS images of white mica from Gravera, Switzerland (MF-925). The interface between Pg and Ms are more irregular in this flake.

 

Figure 3
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  		FIGURE 3. STEM EDS images of white mica flakes from the Ravdoucha beds, Crete (GR-5). This is the clearest and sharpest image of Pg and Ms segregation within the basal plane of the micas. Summed spectra from areas A and B show that not only are the interlayer cations well segregated, but that the Al/Si ratio also changes during segregation.

 

Figure 4
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  		FIGURE 4. STEM EDS images of Cretan micas showing both more diffuse and sharp Pg/Ms boundaries.

 

Figure 5
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  		FIGURE 5. XRD patterns of the Liassic (from Livi et al. 1997) and Ravdoucha samples. Numbers above pattern are d-spacings in nanometers.

 

Figure 6
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  		FIGURE 6. Diagram of the domain model. (a) Initial state of clay minerals dominated by fixed (purple) and expandable (white) layers, while the cations within the layers are thought to be disordered and contain immiscible cations in close proximity. Schematic XRD patterns are given on the right. (b) Growth of fixed layers at the expense of expandable. (c) Initiation of segregation of immiscible cations creates regions within the basal plane of relatively pure compositions. There is still disorder along the 00l direction. (d) Coarsening of domains creates strain energy due to mismatch of cell parameters across basal plane. This is minimized by alignment of like domains along 00l. (e, f) Minimization of grain boundaries (surface area reduction) leads to discrete grains.

 




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