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

Details Francolite geochemistry--compositional controls during formation, diagenesis, metamorphism and weathering Francolite geochemistry--compositional controls during formation, diagenesis, metamorphism and weathering

Jan 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985gecoa..49...23m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 49, Issue 1, pp.23-35

Computer Science

5

Scientific paper

Francolite in unaltered Tertiary phosphorites is highly substituted with about 1.2% Na, 0.25% Sr, 0.36% Mg and 2.7% SO 4 within the structure in substitution for Ca and PO 4 . 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 SO 4 at sea water concentrations. The structura-CO 2 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% CO 2 . For the samples considered here F/P 2 O 5 ratios vary with X-ray CO 2 contents showing that PO 4 3- is replaced by (CO 3 2- + F-) and not by CO 3 2- alone. In Phosphoria Formation francolite the Na/P 2 O 5 ratios and X-ray CO 2 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.

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