Dickite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - Type Locality In Wales
Distribution: Locally Abundant
Chemical Composition: Aluminium silicate hydroxide
Chemical Formula: Al2Si2O5(OH)4
Method(s) of Verification: Aberfan – XRD (Natural History Museum X-ray no. 1901F).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
  • Sedimentary
Dickite, from the type locality at Trwyn-Bychan, near Amlwch, Anglesey. Specimen measures 11 cm x 8.75 cm. National Museum of Wales Collection (NMW 78.85G.M.9). Photo M.P. Cooper, © National Museum of Wales.
Dickite coating millerite, Fforchaman Colliery, Mid Glamorgan. National Museum of Wales Collection (NMW 39.232.GR.4). © National Museum of Wales.
Introduction: dickite is a clay mineral of the kaolinite-serpentinite group with three polymorphs, halloysite, kaolinite and nacrite. Prior to dickite being established as a new mineral species by Ross and Kerr (1930) there was considerable confusion in the literature with kaolinite, nacrite and dickite all described under the name kaolinite. Dickite is named after Allan B. Dick, who provided a detailed description of this mineral from Trwyn-Bychan in northern Anglesey (Dick 1888, 1908). The type locality for dickite is Pant-y-Gaseg Mine, Trwyn-Bychan, Anglesey. This mineral is most commonly found in hyrothermal vein deposits and as an authigenic mineral (a mineral which has grown after the sediment was deposited) in sedimentary rocks.
Occurrence in Wales: dickite localities are scattered across Wales forming occurrences in vein assemblages and as a rock forming mineral. Brown & Smithson (1957), Smithson (1954) and Smithson & Brown (1957) describe dickite as an authogenic mineral forming in pore spaces within sandstones of Carboniferous age from various localities in North Wales.

Key Localities:

  • Aberfan, South Wales: dickite in association with nacrite has been confirmed from the Aberfan borehole by X-ray data.
  • Pant-y-Gaseg Mine, Anglesey: at the type locality dickite forms colourless to white hexagonal plates or stacks of plates up to 0.1 mm across, on quartz and dolomite.
  • South Wales Coalfield: in South Wales dickite is present in clay ironstone nodules found within the Coal Measures (Firth, 1971). Specimens from the following localities are present in the collections of the National Museum of Wales; Avon Colliery; Bryn Henllys Colliery; Coed Ely Colliery; Ferndale; Gelli Colliery; Markham Colliery; Nant Helen Opencast; Parc Slip West Opencast, Margam Opencast; St John’s Colliery; Wyndham Deep Colliery.

References:

  1. Brown, G. & Smithson, F., 1953. Distribution of dickite in some British sandstones. Nature, 72, 317.
  2. Dick, A., 1888. On kaolinite. Mineralogical Magazine, 8, 15-27.
  3. Dick, A., 1908. Supplimentary note on the mineral kaolinite. Mineralogical Magazine, 15, 124-127
  4. Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
  5. Ross, C.S. & Kerr, P.F., 1930. Dickite, a kaolin mineral. American Mineralogist, 15, 34-39
  6. Smithson, F., 1954. The petrography of dickite sandstones in North Wales and Northern England. Geological Magazine, 91, 177-188.
  7. Smithson, F. & Brown, G., 1957. Dickite from sandstones in Northern England and North Wales. Mineralogical Magazine, 31, 381-391.