Mineral Database (Saesneg yn unig)

Mineral Database (Saesneg yn unig)
Mineralogy of Wales

Aegirine

Crystal System: Monoclinic
Formula: NaFe3+Si2O6
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium iron silicate
Method(s) of Verification: Mynydd Mawr - optical identification by Nockolds (1938).
Chemical Group:
  • Silicates
Geological Context:
  • Igneous
Introduction: aegirine is a sodic member of the pyroxene group. It is found in alkali igneous rocks (including carbonatites), pegmatites, and is also found in amphibolite, granulite and blueschist facies metamorphic rocks of suitable composition. It is typically associated with other alkali minerals such as sodic amphiboles, potassium feldspar, nepheline, astrophyllite, eudialyte, serandite, and apophyllite.
Occurrence in Wales: aegirine, also known as acmite, is only known from the Mynydd Mawr microgranite, North Wales. Although the crystals were noted by Harker (1888) and Bonney (1888) neither could resolve their composition optically because of the small size of the crystals. This was subsequently undertaken by Nockolds (1938).
Key Localities:
  • Mynydd Mawr, Gwynedd: aegirine is distributed throughout the Mynydd Mawr microgranite pluton and is associated with riebeckite-arfvedsonite (sodic amphibole). The pyroxene crystals form needle-like crystals up to 0.30 mm long and 0.06 mm wide.
References:
  • Bonney, T.G., 1888 On a peculiar variety of hornblende from Mynydd Mawr, Carnarvonshire.  Mineralogical Magazine, 8,103-107.
  • Harker, A., 1888a Notes on the geology of Mynydd Mawr and the Nantlle Valley.  Geological Magazine, New Series, Decade 3, 5, 221-226.
  • Nockolds, S.R., 1938 On the occurrence of acmite in the riebeckite-microgranite of Mynydd Mawr, Carnarvonshire.  Mineralogical Magazine, 25, 35-37.