Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Sodium iron silicate hydroxide
Chemical Formula: (□,Na)2(Fe 2+,Fe 3+,Mg)5Si8O22(OH,F)2
Method(s) of Verification: Mynydd Mawr - EMPA (J.M. Horák, unpublished data).
Introduction: riebeckite is a sodium iron silicate hydroxide belonging to the amphibole group which forms a series with arfvedsonite [(Na,□)3(Fe2+,Fe3+,Mg)5Si8O22(OH)2] and magnesioriebeckite [(□,Na)2(Mg,Fe2+,Fe3+)5Si8O22(OH)2]. It is found in acid igneous rocks (e.g. riebeckite granite) but it may also be found in high grade-metamorphic schists and metamorphosed ironstones. In the latter the fibrous form of the mineral was previously referred to as crocidolite.
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. Electron microprobe analyses (J.M. Horák, unpublished data) show that a range of composition in the riebeckite-arfvedsonite series are present.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.