Introduction: arfvedsonite belongs to the sodic amphibole group and forms series with riebeckite [(â–¡,Na)₂(Fe²⁺,Fe³⁺,Mg)₅Si₈O₂₂(OH,F)₂] and eckermannite [(Na,â–¡)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 Electon microprobe analyses (J.M. Horák, unpublished data) shows that a range of composition in the riebeckite-arfvedsonite series are present.

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