Mineral Database (Saesneg yn unig)
Muscovite
Crystal System: Monoclinic
Formula: KAl2(Si,Al)4O10(OH,F)2
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Potassium aluminium silicate hydroxide
Method(s) of Verification: Central Anglesey - EMPA (Horák, 1993).
Chemical Group:
- Silicates
Geological Context:
- Metamorphic : low-grade
- Igneous
- Metamorphic : medium-grade
- Sedimentary : allogenic (detrital)
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:
- Low-grade metamorphism within the Welsh sector of the paratectonic Caledonides. Geological Journal, 18, 141-167
- The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.
