Magnetite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Iron oxide
Chemical Formula: Fe3O4
Method(s) of Verification: all cited occurrences identified by optical or magnetic properties.

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Hydrothermal
  • Igneous
  • Hydrothermal: mesothermal polymetallic veins
  • Metamorphic: skarn mineralization
  • Metamorphic
Coarse (to 7 mm across) octahedral magnetite crystals in metabasite. Rhoscolyn, Holy Island, Anglesey. National Museum of Wales Collection (NMW 2002.15G.M.12), ex Tony Ellis Collection. Photo T.F. Cotterell, © National Museum of Wales.
Photomicrograph, polished section of magnetite porphyroblasts (white) overprinting pisolith of chamosite-magnetite (concentric dark/pale rings). From recrystallized, Ordovician, oolitic bedded iron ore, Tyllau Mwyn, Gwynedd. © J.S. Mason
Introduction: magnetite occurs in a wide range of geological settings. It is common in both fresh and metamorphosed basic and ultrabasic igneous rocks. Sedimentary rocks such as sandstones, derived from the weathering and erosion of igneous rocks, may contain significant magnetite. It is present in sedimentary iron-ores, particularly those which have undergone contact or regional metamorphism and is often found in skarn-type deposits and some types of hydrothermal veins. Magnetite, like other members of the spinel group of minerals, to which it belongs, tends to crystallize as well-formed sharp octahedra, but the key to its identification is that it is strongly magnetic, hence the name.
Occurrence in Wales: records of magnetite in Wales are many and date back to the early 20th Century, for example in the case of an occurrence on the west coast of Anglesey (Greenly, 1919). Magnetite was in fact an important component of the Ordovician bedded iron ores mined in many areas of North Wales as noted by Pulfrey (1933) and Trythall (1988); other significant but unexploited occurrences include the relatively recently-discovered hydrothermal breccia at Shadow Gully on Snowdon (Colman & Appleby, 1991).

Key Localities:

  • Afon Mawddach, Ganllwyd, Gwynedd: magnetite occurs as scattered euhedral crystals, typically 0.5-1 mm in size, on joints in a contact-metamorphosed highly altered carbonate-rich intrusive sill exposed in the bed of Afon Mawddach, Gwynedd. Associated minerals are andradite, hematite, chalcopyrite and epidote. The locality is generally inaccessible as it is usually deep underwater (M.J. Liezers & J.S. Mason, unpublished data).
  • Cregennen, Dolgellau, Gwynedd: a bed of sedimentary ironstone of Ordovician age has been contact-metamorphosed in this area, where it has been explored by very limited trial workings. The metamorphosed ironstone consists of a hard, jaspery matrix studded and locally crowded with 2-3 mm magnetite octahedra (J.S. Mason, unpublished data).
  • Cwm Llan, Snowdon, Gwynedd: the bed of Shadow Gully, Cwm Llan, high on the slopes of Snowdon is floored by a magnetite-cemented breccia. The magnetite forms a granular groundmass of interlocking small (<2 mm) euhedral crystals. Hematite, pyrite, minor chalcopyrite, cassiterite and scheelite are also present. The breccia has been interpreted as forming during hydrothermal activity related to the evolution of the volcanic caldera and the tin-tungsten component to indicate a degree of magmatic input to the hydrothermal fluids. Magnetite-bearing sandstones additionally occur nearby (Colman & Appleby, 1991).
  • Cwm Tregalan, Snowdon, Gwynedd: magnetite occurs as 'swirling bands' associated with hematite in quartz veins and infilled vescicles in pillowed basalts outcropping near the head of the cwm (Colman & Appleby, 1991).
  • Holy Island, Anglesey: at Rhoscolyn, on Holy Island, spectacular octahedral crystals to 5 mm, associated with chromite, occur in chlorite schists of the New Harbour Group (part of the Monian Supergroup) in this area (Greenly, 1919; Maltman, 1977).
  • Llyn Cwellyn, Beddgelert, Gwynedd: an unusual (for the area) fluorite-dominated vein on the margin of the Mynydd Mawr microgranite carries magnetite as swirl-like bodies in chlorite (Colman & Appleby, 1991).
  • Rhiw, Llŷn, Gwynedd: magnetite is abundant in the basic to ultrabasic rocks of the Rhiw Intrusion (Hawkins, 1970; Cattermole, 1976).
  • Tan-yr-allt, Nant Peris, Gwynedd: a thin horizon of Ordovician spessartine-magnetite 'schist' was noted by Williams (1927, 1930), with individual crystals to 0.5 mm.
  • Tyllau Mwyn, Drws-y-nant, Gwynedd: at this remote site, in the Arans to the NE of Dolgellau, small (<1 mm) but lustrous magnetite octahedra form crowded aggregates in an oolitic carbonate-chamosite-magnetite matrix (Matthews & Scoon, 1964).

References:

  1. Cattermole, P.J., 1976. The crystallization and differentiation of a layered intrusion of hydrated alkali olivine-basalt parentage at Rhiw, North Wales. Geological Journal,11,45-70.
  2. Colman, T.B. & Appleby, A.-K., 1991. Volcanogenic quartz-magnetite-hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Hawkins, T.R.W., 1970. Hornblende gabbros and picrites at Rhiw, Caernarvonshire. Geological Journal, 7, 1-24.
  5. Maltman, A.J., 1977. Serpentinites and related rocks of Anglesey. Geological Journal, 12, 113-128.
  6. Matthews, D.W. & Scoon, J.H., 1964. Notes on a new occurrence of stilpnomelane from North Wales. Mineralogical Magazine, 33, 1032-1037.
  7. Pulfrey, W., 1933. The iron-ore oolites and pisolites of North Wales. Quarterly Journal of the Geological Society of London, 89, 401-430.
  8. Trythall, R.J.B., 1989. The mid-Ordovician oolitic ironstones of North Wales: a field guide. In: The mid-Ordovician oolitic ironstones of North Wales: a field guide, in Phanerozoic Ironstones (eds Young, T.P. and Taylor, W.E.G.) Geological Society of London Special Publication, 46, 213-220.
  9. Williams, H., 1930. The Snowdon District: Report of the Easter field meeting of the Geologists’ Association. Proceedings of the Geologists’ Association, 41,190-205.
  10. Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.