Characterization of coexisting NH4- and K-micas in very low-grade metapelites

Organic-rich Carboniferous shales with associated coal seams, from the Bacia Carbonífera do Douro-Beira (north Portugal), have been studied by TEM as well as by a variety of other methods. NH4 and K micas and berthierine form small subparallel packets of a few layers separated by lowangle boundaries. Oneand two-layer ordered polytypes, with some spot enlargement typical of minor disorder, occur in the NH4 micas (i.e., metamorphism tobelites). They exhibit all the characteristics commonly described for sub-greenschist-facies, including a lack of textural and chemical equilibrium. The compositions of the 2:1 layers vary considerably for both Kand NH4-micas, and except for Ti, exhibit similar compositional ranges. The most significant compositional variations are explained by phengitic substitution (Si from 3 to 3.25, Fe + Mg from 0.1 to 0.3), but no evidence of an illitic substitution has been found. NH4 in tobelites was determined by analysis of NH3 using Nessler’s reagent, basal spacing, and 1–(K + Na). The resulting values indicate NH4 contents ranging from 30 to 59% of the interlayer site occupancy. At very low temperatures of metamorphism (e.g., North Sea), the intergrowths of NH4 and K in micas is on the nanometer-scale. With an increase in temperature (e.g., Pennsylvania), NH4and K-micas are present as different minerals. The DouroBeira samples represent a still higher-temperature case and, therefore, NH4 and K can be present in the same interlayer sheet but with one cation being dominant. NH4and K-dominated micas have segregated into well-separated packets with scarce intergrowths and almost no mixed-layers. Hence they show a solvus relationship. NIETO: NH4 AND K MICAS OF VERY LOW-GRADE METAPETITES 206 ciated with the semi-anthracite and anthracite coal measures of Pennsylvania and interpreted NH4-illites as forming from the replacement of K in illite by NH3 expelled from coal seams during a very low-grade metamorphic coalification process. Likewise, Williams et al. (1989, 1992) concluded that NH4 released from organic matter migrated with oil to become incorporated into phyllosilicates, with a maximum fixation coincident with the “oil window” (Williams and Ferrell 1991; Schroeder and McLain 1998). Daniels and Altaner (1990) interpreted the origin of the Pennsylvania tobelites in the same way as Juster et al. (1987), but considered kaolinite instead of K-illite as the precursor. The same type of origin was described by Liu et al. (1996) for NH4-illites in coal-bearing strata in North China. Sucha et al. (1994) found NH4-rich illite, together with Kand Na-illite, in shales associated with meta-anthracite coalfields in the Western Carpathians. The estimated formation temperature interval was 200–270 ∞C, as determined from vitrinite reflectance. Ward and Christie (1994) found NH4illite in the semi-anthracite material of Coal Seams in the Bowen Basin (Australia). In contrast, the mixed-layer, smectite-NH4illite described by Bobos and Ghergary (1999) in the fossil hydrothermal system of Harghita Bǎi (Romania) are not associated with coal or organic matter. Wilson et al. (1992) described tobelites and NH4-illites in veins of black shales from the Oquirrh Mountains, Utah and analyzed them by EMP methods. They reported a complete set of compositions, including O and N, with low to high NH4 contents.