Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Sodium iron silicate hydroxide
Chemical Formula: (Na,□)3(Fe2+,Fe3+,Mg)5Si8O22(OH)2
Method(s) of Verification: Mynydd Mawr - electron microprobe (J.M. Horák, unpublished data, National Museum of Wales).
Introduction: arfvedsonite belongs to the sodic amphibole group and forms series with riebeckite [(□,Na)2(Fe2+,Fe3+,Mg)5Si8O22(OH,F)2] and eckermannite [(Na,□)3(Mg,Fe2+,Al)5Si8O22(OH,F)2]. It is found in intermediate and acid alkaline igneous rocks, commonly associated with aegirine (sodic pyroxene) where it forms either by crystallizing directly from the magma or by later growth as a subsolidus mineral, once the magma has solidified. Reducing conditions are required to form arfvedsonite. Many so called ‘riebeckite granites’ are more likely to contain amphiboles in the arfvedsonite-riebeckite series. This series, found in igneous rocks, contrasts with the glaucophane-ferroglaucophane series which is only found under high pressure metamorphic conditions.
Occurrence in Wales: in a description of the Mynydd Mawr microgranite, North Wales, Harker (1888a) referred to the presence of both hornblende and tourmaline, although Bonney (1888) considered both minerals to be varieties of hornblende, possibly arfvedsonite. Harker (1888b) subsequently confirmed that the crystals he had observed were riebeckite, and this was the first description of this mineral in the British Isles. Electon microprobe analyses (J.M. Horák, unpublished data) shows that a range of composition in the riebeckite-arfvedsonite series are present.
There are no key localities for this specimen.
- 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.
- Harker, A., 1888b. Additional note on blue hornblende of Mynydd Mawr. Geological Magazine, New Series, Decade 3, 5, 221-226.