Paragonite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Widespread
Chemical Composition: Sodium aluminium silicate hydroxide
Chemical Formula: NaAl2(Si3Al)O10(OH)2
Method(s) of Verification: XRD - Merriman & Roberts (1985); Jiang & Peacor (1993).

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
  • Metamorphic: medium-grade
Introduction: paragonite is a mica group mineral, similar to muscovite, except that the potassium in muscovite is replaced by sodium in paragonite. It has one polymorph brammallite. Paragonite is found in low and medium grade metamorphic rocks (blueschist, greenschist and amphibolite facies), typically occurring as fine-grained aggregates in association with other aluminium-rich minerals.
Occurrence in Wales: paragonite has been described from metasediments and metabasites (metamorphosed basic rocks) in North Wales which have undergone low-grade metamorphism. As with many other aluminium silicate minerals its distribution is likely to be far more widespread as a rock-forming mineral than is recorded in the literature. Merriman & Roberts (1985) provide a detailed account of the white mica content of Precambrian to Silurian sediments from localities across North Wales from Llŷn to Snowdonia. The presence of paragonite is revealed in many of these samples by X-ray diffraction of powders of less than 2 μm size. It is particularly common in the Llanberis Slate Formation, occurring with muscovite. Jiang & Peacor (1993) record paragonite in metabasites of Caradocian age from North Wales. Hydrothermal alteration of feldspar in the rock produced a metastable white mica which was subsequently transformed into paragonite and muscovite. The paragonite occurs in irregularly distributed crystal aggregates.

Key Localities:

There are no key localities for this specimen.

References:

  1. Jiang, W-T. and Peacor, D.R., 1993. Formation and modification of metastable intermediate sodium potassium mica, paragonite and muscovite in hydrothermally altered metabasites from northern Wales. American Mineralogist, 78, 782-793.
  2. Merriman, R.J. & Roberts, B., 1985. A survey of white mica crystallinity and polytypes in pelitic rocks of Snowdonia and Llŷn, North Wales. Mineralogical Magazine, 49, 305-319