Phengite

Garnet and cordierite blastesis postdating S1/S2 fabric contain inciusion traiis of quartz, biotite and phengitic micas and indicate the influence of a iate thermai overprint due to intrusion of the Pabineau Granite. 

Mineral Quartz Phengite Omphacite Garnet Rutile... 

Fe content in octahedral sites of natural micas are compared (Figure 8). The phengites, micas produced by high pressure and moderate temperature... 

Figure 7. Phases present along the compositional join muscovite (Mu)— MgFe +. celadonite (Ce) at 2Kb pressure. MU = muscovite-phengite Mica = dioctahedral mica of unidentified composition CE = celadonite mica ...

Figure 8. Compositions of natural potassic micas as a function of their AlVI—Fe + contents (octahedrally coordinated ions). Crosses represent phengite micas and dots celadonite-glauconite micas (K + Na + Ca/2 =...

VELDE (B.), 1965b. Phengite micas synthesis, stability and natural occurrence. Amer. Journ. Sci. 262, 886-913. 

Complete unmixing of the mixed-layered minerals requires temperatures on the order of 400°C or a relatively high degree of metamorphism (phengite -> muscovite + chlorite). Velde (1964), Weaver (1965), Raman and Jackson (1966), and Weaver and Beck (1971a) have presented data to indicate chloritic layers are present in most, if not all, 10A illites. Based on chemical data, Raman and Jackson concluded that the illites they had examined contained 20—29% chlorite layers. Weaver and Beck believe that most of the chloritic layers are dioctahedral. 

The type phengite suggested by Foster (1956) has a composition almost identical to that of the Belt illite (No.8). When the H20 content is appreciably higher and the K20 content lower than muscovite, the minerals have been called hydromicas or hydromuscovites. The excess H20 in some instances is present as interlayer water, particularly in the trioctahedral hydrobiotites. Table XII contains a selection of sericite and hydromuscovite analyses and Table XIII the structural formulas. The H20 and K20 values of these minerals are similar to those reported for the illite minerals however, the MgO content of the sericites and hydromuscovites is lower and the NazO contents higher than for the illites (Table XIV). 

Interestingly, there is a strong mineralogical control on the observed values of 6. Watson and Lupulescu (1993) report 6 > 60° in clinopyrox-enite-H20 system at 1.5 GPa, 900-950 °C. Ono et al. (2002) studied pyrope-H20 at 4 -13 GPa and 900-1,200 °C. They found that 0 increased with pressure up to 9 GPa. At 9 GPa and above, 6 was >60°. They suggested that in the metabasaltic portion of a subducting slab, lawsonite or phengite dehydration at > 9 GPa would produce an H20-rich fluid, which would be trapped in the... 

Domanik K. J. and Holloway J. R. (1996) The stability and composition of phengitic muscovite and associated phases from 5.5 to 11 GPa implications for deeply subducted sediments. Geochim. Cosmochim. Acta 60, 4133-4150. 

Catlos E. J. and Sorensen S. S. (2003) Phengite-based chronology of K- and Ba-rich fluid flow in two paelosubduc-tion zones. Science 299, 92-95. 

Excess Ar in phengitic mica of HP-UHP rocks has been documented frequently and is the cause of aberrant ages (Li et al., 1994, 2000 Giorgis et al., 2000 Jahn et al., 2001a). Incorrect... 

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