Francolite geochemistry—compositional controls during formation,diagenesis, metamorphism and weathering

Francolite in unaltered Tertiary phosphorites is highly substituted with about 1.2% Na, 0.25% Sr, 0.36% Mg and 2.7% SO₄ within the structure in substitution for Ca and PO₄. These substitutions reflect the composition of the solution from which francolite forms and are not influenced by the mechanism, rate or redox conditions of formation. The remarkably constant composition for a wide range of samples reflects the relative constant composition of sea water, during Tertiary times. Francolite in sulphate-reducing sediments forms in an uppermost zone where bioturbation maintains the concentrations of Na, Sr, Mg and SO₄ at sea water concentrations. The structura-CO₂ content of unaltered francolites vary from 1.4% to 6.3%, in response to variations in pH. The maximum level of carbonate substitution is limited by the disruption of the francolite structure at levels above 6.3% CO₂. For the samples considered here F/P₂O₅ ratios vary with X-ray CO₂ contents showing that PO₄^3? is replaced by (CO₃^2? + F?) and not by CO₃^2? alone. In Phosphoria Formation francolite the Na/P₂O₅ ratios and X-ray CO₂ contents show areal trends related to burial depth and thermal metamorphism. These two processes together with weathering and fresh water diageneses explain all observed variation in francolite composition.