Crystal System: Monoclinic
Status of Occurrence: Unconfirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Potassium sodium iron aluminium magnesium silicate hydroxide.
Chemical Formula: (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2
Method(s) of Verification: Visual and optical microscopy.
Introduction: glauconite is a member of the mica group and can be considered as an Fe-rich illite. It is a green mineral, typically found as grains or pellets in marine sedimentary rocks such as sandstones and impure limestones and is abundant in ‘greensands’ such as the Lower Cretaceous of Southern Britain. Glauconite forms from the alteration of detrital biotite or volcanic glass in shallow water and under slow rates of sedimentation.
Occurrence in Wales: North (1916) reported the presence of glauconite in Sutton Stone from the Bridgend area. Sutton Stone is a cream coloured limestone formed as the shallow water near shore equivalent to the lower part of the Liassic limestone succession (and known as the ‘marginal facies’). Although the sedimentary conditions in the Sutton Stone could have supported glauconite formation, subsequent studies on these rocks (e.g. Willey, @@@@) have not confirmed North’s identification and therefore the presence of glauconite remains dubious until it can be positively confirmed.
There are no key localities for this specimen.
- North, F.J., 1916. The minerals of Glamorgan. Transactions of the Cardiff Naturalists' Society, 49, 16-51.