Goethite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Iron oxide hydroxide
Chemical Formula: α-Fe3+O(OH)
Method(s) of Verification: given the number of written accounts of goethite, surprisingly few records have been fully verified. Taffs Well - XRD (National Museum of Wales, NMW X-1103); Rhandirmwyn - XRD (National Museum of Wales, NMW X-1104).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Hydrothermal: limestone hosted hematite deposits
Radiating goethite from Mwyndy Mine, Llantrisant, Mid Glamorgan. Vertical field of view 12 mm. National Museum of Wales Collection (NMW 85.131) donated to Cardiff Museum in 1885 by S. Vivian. Photo M.P. Cooper, © National Museum of Wales.
Goethite needles up to 4 mm in length from the Deep Adit Level of the Chidlaw Lode at Gwynfynydd Mine, in the Dolgellau Gold-belt. National Museum of Wales Collection (NMW 85.70G.M.23), ex R.W. Barstow Collection. Photo M.P. Cooper, © NMW.
Goethite pseudomorphs after cubic pyrite crystals. Blaengwynlais Quarry, Mid Glamorgan. I.E. Jones Collection. Photo M.P. Cooper, © National Museum of Wales.
Rusty orange goethite pseudomorphs after pyrite cubes, associated with calcite. Blaengwynlais Quarry, nr. Cardiff. National Museum of Wales Collection (NMW 87.73G.M.60) ex A. Dean Collection. © National Museum of Wales.
Introduction: goethite is a common weathering product of iron-bearing minerals in oxygenated environments and can be an important component of iron ore derived from weathered iron mineralization. Goethite is also a primary precipitate in hydrothermal, marine, and bog environments upon oxidation of reduced iron-bearing waters.
Occurrence in Wales: goethite is a common mineral in Wales, found almost everywhere that iron-bearing minerals have been weathered. As such only the more notable occurrences are described. The richest occurrences are associated with the southern outcrop of Carboniferous Limestone south of the South Wales Coalfield where hematite and goethite occur in large quantities both as replacement deposits in the limestone and along the boundary between limestone and overlying millstone grit and Triassic conglomerate (North, 1962). The main area of iron mineralization, in the vicinity of Llantrisant hosted a number of large iron mines with mineralization extending eastward to Taffs Well where the large Garth Mine is situated. Goethite was the dominant iron ore typically occurring massive, as botryoidal masses, or as stalactites. Goethite is also well-known as pseudomorphs after pyrite crystals, which have been collected from Llantrisant eastwards along the outcrop of Carboniferous Limestone through to Blaengwynlais and Machen. The most attractive specimens are those from Blaengwynlais Quarry. Goethite is widespread in the Central Wales Orefield, but is inconspicuous, typically replacing altered chalcopyrite, but always massive. At Llechweddhelyg Mine an unusually deep weathering zone has resulted in the survival of an unusally (for Wales – widespread glaciation removed much of the earlier surface cover) rich gossan, dominated by goethite richly invested with malachite. In North Wales, noteworthy goethite occurrences are few. At Dyserth large aggregates of bladed marcasite and cubic pyrite are totally replaced by goethite and porous goethite/limonite gossan forms the matrix of many classic anglesite specimens recovered from Parys Mountain on Anglesey.

Key Localities:

  • Blaengwynlais Quarry, near Cardiff, South Wales: well-known as a source of goethite pseudomorphs after pyrite cubes (Bevins, 1994; Bevins & Mason, 2000). Both aggregates and individual cubes occur on calcite and limestone. Some display an attractive orange ‘limonite’ coating. Individual cubes can reach 5 cm, but are typically 1-3 cm across.
  • Central Wales Orefield: goethite is widespread, particularly as an alteration product of chalcopyrite, but crystals are entirely absent. The richest specimens have been collected at Aberdaunant Mine near Llanidloes in Powys where goethite forms dark brown to black botryoidal crusts on galena cubes (Bevins, 1994). At Llechweddhelyg Mine in northern Ceredigion goethite is common within malachite-rich gossan encountered during sinking of an engine-shaft. This occurrence is thought to represent an unusually deep weathering zone that has survived glaciation.
  • Cwmleyshon Quarry, Draethen, South Wales: globular goethite occurs encrusting barite in veins cutting dolomitic limestone.
  • Dyserth, Clwyd: large aggregates of goethite pseudomorphing bladed marcasite crystals to 10 mm have been collected from Dyserth Quarry (National Museum of Wales specimen NMW 94.70G.M.10. While, a mass of cubic pyrite crystals replaced by goethite was found near the top of a mine shaft at Dyserth (National Museum of Wales specimen).
  • Garth Iron Mine, Taff’s Well, Mid Glamorgan: goethite occurs associated with hematite (Rankin & Criddle, 1985).
  • Gwynfynydd Mine, Ganllwyd, Dolgellau, Gwynedd: attractive copper-brown acicular sprays are associated with 'cog-wheel' marcasite collected from the Childlaw Lode during the early 1980s. Examples include National Museum of Wales specimen NMW 85.70G.M.23 from the R.W. Barstow Collection.
  • Hendy Quarry, Miskin, South Wales: thin plates of massive goethite pseudomorphing cubic pyrite – itself forming casts after an unknown – are well known from Hendy. See for example National Museum of Wales specimens.
  • Llanharry Iron Mine, Llantrisant, South Wales: goethite is extremely abundant at Llanharry making up the greater percentage of the iron ore mined. Goethite occurs in a number of forms including massive, stalactitic, botryoidal, acicular and pseudomorphous replacing aggregates of pyrite cubes. The National Museum of Wales holds a extensive suite of Llanharry specimens, many of which were collected during working operations at the mine. Stalactitic and botryoidal specimens from Llanharry rate amongst the best of these forms found in Wales.
  • Machen Quarry, Caerphilly, South Wales: pseudomorphs of goethite after pyrite are well known from Machen (Plant & Jones, 1995). Examples in the National Museum of Wales Mineral Collection include small cubic crystals overgrowing barite and a thin plate of cubo-octahedral pyrite totally replaced by goethite. A recent discovery of goethite pseudomorphs after aggregates of euhedral bladed marcasite crystals was made by a local collector.
  • Mwyndy Mine, Llantrisant, South Wales: goethite was an important ore at Mwyndy during the later half of the 19th Century. An old specimen from the Vivian collection now housed in the National Museum of Wales (NMW 85.131) displays attractive banded fibrous brown to golden goethite.
  • Parys Mountain, Anglesey: goethite was present near surface, forming the gossan host for the many anglesite specimens collected during the 18th and 19th centuries.
  • Sully Island, South Wales: small (1-2 mm) dark brown cubic pseudomorphs after pyrite occur in association with fluorite and barite from the south-eastern corner of the island.
  • Taff's Well Quarry, Pentyrch, South Wales: iron mineralization is often encountered within the quarry workings. Massive hematite-goethite similar to iron ore mined at Llanharry has been observed along with black stalactitic goethite and coatings of black velvety acicular goethite (all represented within the National Museum of Wales Mineral Collection).
  • Ton Mawr Quarry, Pentyrch, South Wales: recent (1998-2000) discoveries include attractive black velvety goethite coatings within cavities in dolomitic limestone (National Museum of Wales specimens, NMW 2002.52G.M.2 & 2002.55G.M.20.) and pseudomorphs after cubo-octahedral pyrite aggregates (E. Bravais Collection).
  • Tŷ Coch, near Porthcawl, South Wales: goethite occasionally occurs as banded botyoidal aggregates (National Museum of Wales specimen NMW 87.73G.M.240) and as compound crystallites interlayered with romanèchite in manganese ore (Criddle & Symes, 1977).
  • Tŷ Mawr, Rumney, South Wales: veins of dark brown radial goethite are held in the National Museum of Wales Mineral Collection.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  3. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  4. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  5. Criddle, A.J., & Symes, R.F., 1977. Mineralization at Tŷ Coch, Glamorgan (Mid Glamorgan), Wales: the second occurrence of pyrobelonite. Mineralogical Magazine, 41, 85-90.
  6. Plant, S.P. & Jones, I.E., 1995. Minerals of Machen Quarry, Mid Glamorgan, Wales. Journal of the Russell Society, 6(1), 31-36.
  7. Rankin, A.H. & Criddle, A.J., 1985. Mineralizing fluids and metastable low-temperature inclusion brines at Llanharry iron deposit, South Wales. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 94, B126-B132.