Phlogopite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Potassium magnesium aluminosilicate hydroxide
Chemical Formula: KMg3Si3AlO10(F,OH)2
Method(s) of Verification: Arenig Station Quarry - XRD at the Natural History Museum, London (X-ray numbers x10844, x10845 and x10846); Esgair Hir Mine - XRD (Rust and Mason, 1988).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic
Introduction: a member of the mica group of minerals. It forms a continuous series with biotite, phlogopite having Mg:Fe > 2:1. Phlogopite occurs in metamorphosed dolostones and magnesium-rich limestones and in ultramafic rocks.
Occurrence in Wales: Williams (1927) considered strongly pleochroic pale-brown mica, associated with chlorite in rutile-bearing tuffs of Ordovician age exposed on Lliwedd, Gwynedd to be most probably phlogopite. Unpublished X-ray diffraction studies of Natural History Museum specimens (numbers B.M. 1931, 483, B.M. 1931, 485 and B.M. 1931, 486) from Arenig Station Quarry, Llanycil, Gwynedd identified phlogopite in association with calcite. Ti-rich phlogopite occurs in biotite-pyroxenite xenoliths from the Great House agglomeratic diatreme near Usk, Gwent (D.J. Moffat, unpublished data). An unusual occurrence (in association with hydrocerussite) was reported from Esgair Hir Mine, Ceredigion by Rust and Mason (1988).

Key Localities:

  • Arenig Station Quarry, Llanycil, Gwynedd: phlogopite occurs in association with calcite on specimens B.M. 1931, 483, B.M. 1931, 485 and B.M. 1931, 486 in the Natural History Museum Collection.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: Rust and Mason (1988) describe under the heading phlogopite, 'a mica group mineral originally confirmed in association with hydrocerussite'. They go on to add that a similar white mica has been found replacing some small fragments of brecciated mudstone. The description and geological setting do not suggest phlogopite. It is not known where the original XRD analysis was performed.
  • Great House, Usk, Monmouthshire: pleochroic pale yellow to dark reddish-brown, Ti-rich phlogopite occurs in biotite-pyroxenite xenoliths from the agglomeratic diatreme exposed near Great House (D.J. Moffat, unpublished data).
  • Lliwedd, Central Snowdonia, Gwynedd: Williams (1927) considered strongly pleochroic pale-brown mica, associated with chlorite in rutile-bearing tuffs of Ordovician age exposed on Lliwedd to be phlogopite.

References:

  1. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.
  2. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.