Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium calcium aluminium silicate hydroxide hydrate
Chemical Formula: (Na,Ca)Al4(Si,Al)8O20(OH)4.2H2O
Method(s) of Verification: X-ray diffraction (Gill et al., 1977; Merriman & Roberts, 1986; Jiang et al., 1990).

Chemical Group:

  • Silicates
Introduction: an interstratified clay mineral found in a variety of sedimentary environments. Rectorite also occurs in some low-temperature hydrothermal argillic alteration zones, along veins and pervasively replacing potassic feldspar. In altered bentonites; may develop from muscovite during diagenesis of shales. Rectorite, previously known as allevardite, is a clay mineral with a structure consisting of a regular interleaving of dioctahedral mica and dioctahedral smectite layers. It is a mineral which forms under low-temperature hydrothermal conditions, where it typically replaces potassic feldspar, or during by the diagenetic alteration of muscovite in altered bentonite (fine-grained clay horizons derived from volcanic dust).
Occurrence in Wales: Gill et al. (1977) noted the presence of allevardite associated with pyrophyllite in anchizone pelites from the anthracite area of the South Wales Coalfield; the mineral name allevardite has been superseded by rectorite. Elsewhere in Wales Merriman & Roberts (1985) recorded the presence of rectorite in a number of Arenig and Llanvirn (Ordovician) age pelites from Snowdonia and Llŷn, on the basis of X-ray diffraction analysis. It is restricted, however, to pelites which have undergone contact metamorphism prior to regional metamorphism. Microprobe analyses presented by Merriman & Roberts (1985) show that in the samples of Arenig age the expansible component of the rectorite is Ca-smectite, while in the samples of Llanvirn age it is a Na-smectite. A more recent X-ray diffraction study has recorded rectorite in a pelite sample in the valley of the Afon Seiont, 2 km southeast of Caernarfon Castle (Jiang et al., 1990).

Key Localities:

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