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

Abstract

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. NH 4 and K micas and berthierine form small subparallel packets of a few layers separated by lowangle boundaries. One- and two-layer ordered polytypes, with some spot enlargement typical of minor disorder, occur in the NH 4 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 K- and NH 4 -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. NH 4 in tobelites was determined by analysis of NH 3 using Nessler’s reagent, basal spacing, and 1-(K + Na). The resulting values indicate NH 4 contents ranging from 30 to 59% of the interlayer site occupancy. At very low temperatures of metamorphism (e.g., North Sea), the intergrowths of NH 4 and K in micas is on the nanometer-scale. With an increase in temperature (e.g., Pennsylvania), NH 4 - and K-micas are present as different minerals. The Douro- Beira samples represent a still higher-temperature case and, therefore, NH 4 and K can be present in the same interlayer sheet but with one cation being dominant. NH 4 - and K-dominated micas have segregated into well-separated packets with scarce intergrowths and almost no mixed-layers. Hence they show a solvus relationship.