Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Iron carbonate
Chemical Formula: FeCO3
Method(s) of Verification: mainly visual, limited XRD data available
- Hydrothermal: mesothermal polymetallic veins
Lustrous, only slightly weathered rhombic siderite crystals (up to 5 mm across) from Llantrisant, South Wales. National Museum of Wales Collection (NMW 48.264.GR.141), ex Cymmer Welfare Library and Institute. © National Museum of Wales.
Unusual stellate twinned siderite crystals associated with sphalerite (brown, centre right) and millerite (tarnished needles), from Wyndham Colliery. Specimen B. Taylor collection (National Museum of Wales), © National Museum of Wales.
Introduction: siderite is widely developed in a range of geological settings, in particular some hydrothermal veins (where it may be the chief gangue mineral and may also be mined if pure enough), and in sedimentary rocks, where it is often an important constituent of ironstones. Siderite, like other iron-bearing carbonates, is white to creamy-yellow when unweathered: however it does weather rapidly, turning first a lustrous golden-brown and then a dull mid-to dark-brown as the process progresses. This is a property it shares with ankerite, and the two minerals are not always readily distinguished on a purely visual basis.
Occurrence in Wales: there are two main occurrences of siderite in Wales, both related to sedimentary ironstones. Siderite is a significant component of the Ordovician bedded ironstones exposed across North Wales from Cadair Idris to Anglesey (Pulfrey, 1933). In South Wales, it is the principal carbonate mineral in the clay-ironstone nodule beds of Upper Carboniferous age which occur within the South Wales Coalfield (North, 1916; Firth, 1971). This area, unlike North Wales, produces some attractive siderite crystals, in association with quartz, millerite and other minerals.
- Central Wales Orefield: Siderite was noted as a gangue mineral at several Central Wales mines, including Ystrad Einion, Geufron and Siglenlas, by Jones & Moreton (1977). However, all analyses done to date on the heavy, brown-weathering carbonates in the Central Wales veins have shown them to fall within the dolomite-ankerite series.
- North Wales: North Wales ironstones (Ordovician): Noted as a fine-grained intergrowth with chamosite, magnetite and other minerals in these oolitic ironstones, although not generally as specimens of interest to anybody but the sedimentary petrologist. Iron-mining sites where it occurs include: Tremadog; Maes Y Gaer (near Aber); Llandegai (Anglesey) and the Garreg and Ystrad-Fawr mines at Betws Garmon.
- South Wales Coalfield: South Wales ironstones (Carboniferous): Widespread in the coal-measures strata, occurring in often large nodules of clay-ironstone, many of which contain septarian cavities lined with pearly-white, brown-weathering siderite crystals. These form the matrix for fine specimens of millerite and other sulphides. Due to ongoing coal-tip reclamation, worthwhile specimens only tend to be recovered during the actual reclamation works themselves. Specimens are also found in short-lived but sometimes productive opencast coalmines. Of particular note are the rare stellate twins, typically 0.5-1 mm in diamether, collected in the 1980s when the tips at Wyndham Colliery were reclaimed.
- Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
- Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
- North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.
- Pulfrey, W., 1933. The iron-ore oolites and pisolites of North Wales. Quarterly Journal of the Geological Society of London, 89, 401-430.