Dickite

dickite

chalcedony

nacrite

millerite

Images

Formula: Al2Si2O5(OH)4
Phyllosilicate (sheet silicate), kaolinite - serpentine group, paramorph of halloysite-7Å, kaolinite and nacrite
Specific gravity: 2.60
Hardness: 2 to 2½
Streak: White
Colour: White, or tinted by impurities
Common impurities: Ti,Fe,Mg,Ca,Na,K
Environments

Sedimentary environments
Metamorphic environments
Hydrothermal environments

Dickite is a secondary mineral, usually of hydrothermal origin, derived in part from the alteration of aluminosilicate minerals; it also occurs as an authigenic (formed in place) sedimentary phase. Chalcedony is a common associate (Mindat, MW, HOM).

Localities

At the Francon quarry, Montréal, Québec, Canada dickite occurs as a silky white coating on quartz (Min Rec 37-1.30).

At the Imiter Mine, Tinghir Cercle, Tinghir Province, Drâa-Tafilalet Region, Morocco, dickite occurs as 0.2 mm white scales with cerussite in altered galena (Min Rec 422.123).

At the type locality, the Pant-y-Gaseg mine, Amlwch, Isle of Anglesey, Wales, UK, dickite forms colourless to white hexagonal plates up to 0.1 mm across, on quartz and dolomite (MW).

At Aberfan, Merthyr Tydfil, Mid Glamorgan, Wales, UK, dickite in association with nacrite has been confirmed from the Aberfan borehole by X-ray data (MW).

At the Fforchaman Colliery, Rhondda Cynon Taf, Wales, UK, dickite has been found coating millerite (MW).

At the South Wales Coalfield, Wales, UK, dickite is present in clay ironstone nodules found within the Coal Measures. 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 and Wyndham Deep Colliery (MW).

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