NH-Bearing Illite in Very Low-Grade Metamorphic Rocks Associated with Coal, Northeastern Pennsylvania

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NHo-bearing illite occurs in mudrocks (siltstones, shales, and mudstones) from the anthracite and semi-anthracite coalfields of northeastern Pennsylvania, where the coexisting mineral assemblage and coal rank indicate very low grade metamorphism (Z: 200-27 5'C). xno, infrared absorption, and decomposition experiments all suggest the presence of an illite (polytype 2M,) with approximately 0.2-0.55 NHf, ions per 12 oxygens. Analysis of the basal spacings of coexisting illite-NHo-bearing illite pairs srrggests the presence of an asymmetric rniscibility gap, which exists below 450"C (Shigorova et al., 1981). The anomalously large interlayer spacing of dioctahedral NHo-bearing rnicas cannot be attributed solely to the radius of the NHi ion; it is likely that the covalently bonded and tetrahedral nature of the NHo+ ion also affects the structure. The available data suggest that NHobearing illite formed by isomorphous alkali exchange with muscovite-illite, supported by the presence of a transitional mixed-layer phase that is concentrated in very fine size separates. Analysis of the system C-O-H-N at T : 250"C and P: 2000 bars suggests that, atlogfo, values of approximately the QFM buffer, speciation strongly favors N, over NH. and that NH, was not a dominant fluid component. Thus incorporation of significant NH. in minerals as NHo+ can occur only if the distribution coefficient between illite and fluid is very high (KD : 10-100). Although N is considered to be a minor component of metamorphic fluids, it may be considerably more important at very low grades prior to complete graphitization of carbonaceous material.

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American Mineralogist, v. 72, p. 555-565