Metamorphism micas

Discussions with M. Nespolo (University of Nancy) greatly influenced this chapter. Useful suggestions came from S. V. Soboleva (IGEM, Moscow). Constructive comments have been provided by the referees, S. Guggenheim and M. Rieder. Research was financially supported by MURST ( Layer silicates Crystal chemical, structural and petrologic aspects project) and CNR ( Igneous and metamorphic micas project). 

N.S.L.S., Brookhaven National Laboratory Jeremy Delaney, Tony Lanzirotti and Steve Sutton. Financial supports for our experimental work and for its evaluation and interpretation were granted by M.U.R.S.T. (Project COFIN 1999 Phyllosilicates crystal-chemical, structural and petrologic aspects ), C.N.R. (Project 99.00688.CTOS Igneous and metamorphic micas ), and I.N.F.N. (Project DAONE-Lighf ) in Italy, and by N.S.F. 

Guidotti CV, Sassi FP (1998b) Miscellaneous isomorphous substitutions in Na-K white micas A review with special emphasis to metamorphic micas. Rend Fis Acc Lincei 9 57-78 Guidotti CV, Sassi FP, Blencoe JG (1989) Compositional controls on the a and b cell dimensions of 2M, muscovite. EurJ Mineral 1 71-84... 

Vance D, Ayres M, Kelley SP, Harris N (1998) The thermal response of a metamorphic belt to extension Constraints from laser Ar data on metamorphic micas. Earth Planet Sci Lett 162 153-164 Verchovsky AB, Shukolyukov YuA (1976a) Isotopic composition of neon in the crustal rocks and the origin of Ne rad in natural gases. Geochem Inti 13 95-98 Verchovsky AB, Shukolyukov YuA (1976b) Neon isotopes in minerals with excess of helium and argon (in Russian). Geokhimiya 3 315-322. 

Brewer MS (1969) Excess Radiogenic Argon in Metamorphic Micas from the Eastern Alps, Austria. Earth Planet Sci Lett 6 321-331... 

Micas are primary phases in several types of igneous, metamorphic, and sedimentary rocks. Table 5.54 furnishes a rough scheme of main mica occurrences. 

In metamorphic rocks, muscovite occurs in low-grade terrains of the regional metamorphism (albite-chlorite-sericite schists). It must be noted here that the term sericite identifies fine-grained white micas (muscovite, paragonite). 

Table 5.54 Occurrences of main compositional terms of mica in igneous, metamorphic and sedimentary rocks.

Table 5.55 Chemical analyses of natural micas (from Deer et al., 1983). Note that ionic fractions are retrieved on a 24-anion basis—i.e. double the canonical formula. (1) Muscovite from a low-grade metamorphic prasinite schist (2) glauconite from a sandstone (3) phlogopite from a marble (4) biotite from a quartz-bearing latite (5) lepidolite from a pegmatite. ...

Microlithofacial classification of the sandstones is based on Dott s classification modified by Pettijohn et al. (1972). They are mostly arenites and subarkose and quartz wackes (rare sublithic, sporadically lithic and arkosic). Quartz is the main component of the sandstones (about 60-70 vol. percent). Feldspars (6 vol. percent) are mostly represented by potassium feldspars with plagioclases in lesser amounts. Some micas (muscovite and biotite) and chlorites are observed. Mica content of arenites reaches 3 vol. %, but is higher in the wackes. Heavy minerals present include zircon, sphene, rutile and apatite. Magmatic rocks (volcanic more than Plutonic) are predominant among lithoclasts (about 2 vol. %), but some metamorphic and sedimentary clasts being present too. 

Figure 13.5. White mica in a regional metamorphic rock using the decoration method [5]. 

S. Tomura, M. Kitamura, and I. Sunagawa, Surface microtopography of metamorphic white micas, Phys. Chem. Min., 5,1979, 65-81... 

Figure 11 indicates the necessary change in composition which a muscovite would need to become stable under conditions in a sedimentary rock where chlorite is present (x to y). The solid solution for mica-illites is delimited by the shaded area which represents a much larger variation than is possible under metamorphic or igneous conditions. The detrital muscovite (composition x) is in itself stable if the bulk composition of the sediment as projected into the coordinates is found at x. 

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

Schist A strongly foliated metamorphic rock with visible micas or other platy or linear minerals (compare with slate and phyllite). 

On an AFM diagram (Fig. 16) all the minerals from iron-formations proper fall into a narrow and clearly defined field, while the rock-forming minerals of the associated iron-rich shales from BIF of Superior type (chamosite, ripidolite and other chlorites, garnets and some biotites) form another broader and less clear-cut field. These two fields do not overlap, which indicates the isochemical character of metamorphism and the limited mobility of the main rock-forming minerals, at least within individual layers and bands of the iron-formations and shales. The chemical composition of stilpnomelane occupies an intermediate position both in iron content and in alumina content. However, a high content of potassium (up to 2.2% K2O), an element not typical of most BIF, is necessary for the formation of this mica. 

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