Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Sodium iron silicate
Chemical Formula: NaFe3+Si2O6
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.


  1. Bonney, T.G., 1888. On a peculiar variety of hornblende from Mynydd Mawr, Carnarvonshire. Mineralogical Magazine, 8,103-107.
  2. Harker, A., 1888a. Notes on the geology of Mynydd Mawr and the Nantlle Valley. Geological Magazine, New Series, Decade 3, 5, 221-226.
  3. Nockolds, S.R., 1938. On the occurrence of acmite in the riebeckite-microgranite of Mynydd Mawr, Carnarvonshire. Mineralogical Magazine, 25, 35-37.