Hollandite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Locally Abundant
Chemical Composition: Barium manganese oxide
Chemical Formula: Ba(Mn4+,Mn2+)8O16
Method(s) of Verification: Mochowgryn - chemical analysis, XRD & oscillation photographs (Lynas, 1973); Nant yr Helfa - XRD & EPMA (National Museum of Wales, NMW X-1177); Mynydd Nodol - XRD & EPMA (NMW X-1397).

Chemical Group:

  • Oxides & Hydroxides

Geological Context:

  • Metamorphic: low-grade
  • Supergene: in situ natural oxidation & weathering deposits
Acicular hollandite from Mynydd Nodol, Arenig District, Gwynedd. Field of view 8 mm wide. National Museum of Wales specimen. Photo T.F. Cotterell, © National Museum of Wales.
Botryoidal hollandite from Mynydd Nodol, Arenig District, Gwynedd. National Museum of Wales Collection (NMW 27.111.GR.27) ex G.J. Williams Collection. © National Museum of Wales.
Introduction: hollandite is a member of the cryptomelane group of manganese oxides. Hollandite forms both as a primary mineral in contact metamorphic manganese ores, and as a secondary weathering product of earlier manganese-bearing minerals.
Occurrence in Wales: Lynas (1973) provided the first account of hollandite from the British Isles. Psilomelane and pyrolusite (of Dewey & Bromehead, 1915) associated with minor N-S vertical faults on Mochowgryn, in the Migneint area of Gwynedd, was shown to be a hollandite group mineral, based on chemical analysis, X-ray powder photography and oscillation photographs. Lynas (1973) suggested that a systematic study of other so-called pyrolusites and psilomelanes would reveal other occurrences of hollandite group minerals. This has proved to be exactly the case with hollandite now confirmed from trials on Nant yr Helfa and at Mynydd Nodol both in the Arenig District.

Key Localities:

  • Arenig Station Mn Mine (Nant yr Helfa), Arenig District, Llanycil, Gwynedd: specimens of botryoidal manganese oxides in the National Museum of Wales Mineral Collection are composed largely of hollandite.
  • Mochowgryn, Arenig District, Llanycil, Gwynedd: manganese mineralization (dominated by a hollandite group mineral) occurs, associated with minor N-S vertical faults, hosted by yellowish tuff of the Llyn Conwy Formation (Lynas, 1973).
  • Mynydd Nodol Mine, Arenig District, Llanycil, Gwynedd: highly altered yellowed tuffs of Caradoc age are mineralized, along numerous joints and thin veins, by manganese oxides (including hollandite) forming botryoidal crusts (Bevins & Mason, 1998). Velvety crusts of acicular, metallic dark grey crystals lining cavities within botryoidal manganese ore have also been shown to be hollandite (T.F. Cotterell, unpublished data).

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. Dewey, H. & Bromehead, C.E.N., 1915. Tungsten and manganese ores. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 1.
  4. Lynas, B.D.T., 1973. The Cambrian and Ordovician rocks of the Migneint, North Wales. Journal of the Geological Society, London. 129, 481-503.