The glauconite-phosphate association in hardgrounds: examples from the Cenomanian of Devon, southwest England

Glauconite and phosphate from the Cenomanian of Devon, southwest England, may be classified into different groups on the basis of petrography. Chemical analysis of these different fabrics reveals a strong association between glauconite and phosphate occurring on a sub-microscopic scale. Despite this intimate association, certain deductions may be made about the mineralogy of the glauconite (notably the K concentration) and its relationship to its morphology. Glauconite and phosphate formation took place in the transitional zone between oxic and anoxic environments (suboxic zone). Iron oxyhydroxides were a potential source of phosphorus and iron, while 'sulphide fluxing' maintained pH levels appropriate for carbonate dissolution and subsequent mineralisation.