Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Calcium sulphate hydrate
Chemical Formula: CaSO4.2H2O
Method(s) of Verification: no fully quantitative analyses have been made of Welsh gypsum.
- Supergene: post-mining oxidation & weathering deposits
- Sedimentary: chemical deposit (evaporites & carbonates)
Selenite crystal spray from Agen Allwedd, Mynydd Llangatwg, Breconshire. 9.5 cm across at its widest point. National Museum of Wales Collection (NMW 58.239.GR.1.1). Photo T.F. Cotterell, © National Museum of Wales.
Introduction: gypsum is a common sulphate mineral typically formed as an evaporite mineral particularly by the evaporation of lakes in arid regions. Gypsum also forms as thin bladed crystals on the walls of caves and mines as the variety selenite.
Occurrence in Wales: in Wales gypsum occurs in a wide range of geological settings and in a variety of forms. In South Wales, massive gypsum known as alabaster forms nodules and beds of nodules in the undivided ‘red mudstones’ and the Blue Anchor Formation of the Mercia Mudstone Group of Triassic age exposed in the Penarth-Barry area (North, 1916; Waters & Lawrence, 1987; Bevins, 1994). Veins of fibrous ‘satin spar’ also occur, particularly within the ‘red mudstones’ as witnessed in coastal cliffs between Penarth and Lavernock. Crystallized gypsum is quite common in caves and within disused mine workings, where it precipitates as the variety selenite. Crystals are typically colourless to creamy orange, translucent to transparent and usually very brittle. Good examples have been found in a number of Carboniferous Limestone caves in South Wales and within acidic mine workings, including Britannia Mine on Snowdon, Cae Coch Mine, near Trefriw and Castell Carndochan Mine, near Bala, all in North Wales.
- Abereiddi, Pembrokeshire: thin films of selenite crystals are found on black shales of Ordovician age, probably as a result of the decay of pyrite within the shales (Bevins, 1994).
- Britannia Mine, Snowdonia, Gwynedd: finely acicular friable selenite encrusts fragments of rock from underground workings (National Museum of Wales specimen NMW 80.43G.M.2).
- Cae Coch Mine, Trefriw, Gwynedd: gypsum is common underground. Long acicular crystals and aggregates of smaller prismatic crystals coated with iron sulphates are present on National Museum of Wales specimens.
- Castell Carn Dochan Mine, Llanuwchllyn, Bala, Gwynedd: the variety selenite forms extensive crusts on gossan from No. 3 adit. See for example National Museum of Wales specimen NMW 92.15G.M.2.
- Dylife Mine, Penegoes, Powys: selenite is noted as a recent formation on mine heaps and as transparent colourless prisms associated with greenockite in vughs in sphalerite (Jones & Moreton, 1977).
- Goginan Mine, Goginan, Ceredigion: selenite is reported by Jones & Moreton (1977) from mine heaps at Goginan.
- Gwaithgoch Mine, Pontrhydygroes, Ceredigion: selenite is present as friable crystal aggregates on the walls of No. 3 adit.
- Henfwlch Mine, Ceulanymaesmawr, Ceredigion: small colourless crystals occur associated with serpierite in a post-mining assemblage developed within altered sulphide-bearing veinstone used as backfill underground (T.F. Cotterell, unpublished data).
- Llangattwg Limestone Caves, Gwent: selenite crystals occur on the walls of caves in the Carboniferous Limestone, including Ogof Agen Alwedd and Ogof Daren Cilau (Bevins, 1994). Crystals from Agen Alwedd are amongst the finest from Wales, forming clusters to 9.5 cm across, see for example National Museum of Wales specimen NMW 58.239.GR.1.1.
- North Plynlimon Mine, Plynlimon, Ceredigion: friable bladed selenite occurs in small clusters on the back of the adit in this short trial working (T.F. Cotterell, unpublished data).
- Ogof Ffynnon-ddu, Craig-y-nos, Powys: coarse selenite crystals encrust a thin calcite stalactite in the National Museum of Wales Mineral Collection (NMW 57.64.GR.1).
- Ogofau Mine, Pumpsaint, Dyfed: crusts of selenite occur on mud/siltstone from the ‘Roman’ stope (National Museum of Wales specimens).
- Penarth-Barry coast, South Wales: gypsum is abundant in the form of white and pink alabaster exposed in cliffs between Penarth and Lavernock. Fibrous layers of satin spar to several centimetres in thickness are also present. Colourless to white bladed selenite crystals to 24 mm long coat friable Rhaetic black shales from the beach just south of Penarth, while a large aggregate of cream-coloured crystals has been collected from Jackson’s Bay on Barry Island (D. Wellings specimen).
- Rheidol United Mines, Cwmrheidol, Ceredigion: large acicular crystals have been collected from underground workings on the south side of the Rheidol Valley (R. Starkey Collection).
- Ystrad Einion Mine, Furnace, Ceredigion: small colourless crystals coat rock fragments used as backfill underground (National Museum of Wales specimens).
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- 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.
- Waters, R.A. & Lawrence, D.J.D., 1987. Geology of the South Wales Coalfield, Part III, the Country around Cardiff. 3rd edition. Memoirs of the Geological Survey of England and Wales. Explanation of Sheet 263.