Muscovite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Potassium aluminium silicate hydroxide
Chemical Formula: KAl2(Si,Al)4O10(OH,F)2
Method(s) of Verification: Central Anglesey - EMPA (Horák, 1993).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Igneous
  • Metamorphic: medium-grade
  • Sedimentary: allogenic (detrital)
Plate of secondary muscovite in Coedana Granite, central Angelsey. Specimen National Museum of Wales (93.12G0, © National Museum of Wales.
Introduction: muscovite is a member of the mica familiy, and is commonly referred to as a white mica (i.e. not a dark mica which contains iron and magnesium, such as the biotite micas). It is a rock-forming mineral in some granites and in pegmatites; is also common in low and medium grade metamorphosed pelitic rocks; and occurs as a detrital mineral in sedimentary rocks.
Occurrence in Wales: muscovite is very widespread in Wales, being a component in sedimentary, igneous and metamorphic rocks. In many rocks the muscovite forms as fine-grained crystal aggregates, forming a secondary low-temperature alteration product, typically after magmatic feldspar. Analyses of such muscovite are provided by Bevins & Rowbotham (1983).

Key Localities:

  • Central Anglesey: the Coedana Granite on Anglesey, consists of several different granite types, of which one of the leucocratic units (quartz and feldspar-rich) contains crystals of muscovite up to 4 mm in length. Whereas fine grained muscovite associated with biotite and fine-grained muscovite in feldspar are secondary alteration products and not igneous in origin, the large flakes of muscovite are thought to be magmatic (Horák, 1993), and reflect the aluminium-rich composition of the granite.

References:

  1. Bevins, R.E. & Rowbotham, G., 1983. Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
  2. Horák, J.M., 1993. The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.