NH ₄ -bearing illite in very low grade metamorphic rocks associated with coal, northeastern Pennsylvania

NH₄-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 (T = 200-275°C). xrd, infrared absorption, and decomposition experiments all suggest the presence of an illite (polytype 2M₁) with approximately 0.2-0.55 ions per 12 oxygens. Analysis of the basal spacings of coexisting illite-NH₄-bearing illite pairs suggests the presence of an asymmetric miscibility gap, which exists below 450°C (Shigorova et al., 1981). The anomalously large interlayer spacing of dioctahedral NH₄-bearing micas cannot be attributed solely to the radius of the ion; it is likely that the covalently bonded and tetrahedral nature of the ion also affects the structure. The available data suggest that NH₄-bearing 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, at log f_o2 values of approximately the QFM buffer, speciation strongly favors N₂ over NH₃ and that NH₃ was not a dominant fluid component. Thus incorporation of significant NH₃ in minerals as can occur only if the distribution coefficient between illite and fluid is very high (K_D = 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.