Letter | Published:

Origin of sedimentary francolite from its sulphur and carbon isotope composition

Naturevolume 302pages516518 (1983) | Download Citation



Francolite, the phosphorus-bearing mineral of nearly all sedimentary phosphorites, has a variable chemical composition which can be represented by (Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2–3. It is now well established that this mineral can form authigenically in organic-rich muds1 or by replacement of precursor carbonate2 but the origin of some phosphorite deposits is still in dispute or unknown. Recently the isotopic composition of structural PO4–O, CO3–C+O, and SO4–S has been considered in attempts to resolve this problem3–9. Structural CO3–C has proved particularly useful in discriminating between phosphatized carbonates and authigenic phosphorites, the former preserving the δ13C of its precursor and the latter containing a substantial proportion of lighter carbon generated by anaerobic bacterial degradation of organic matter4,5. From new δ34S data we present here a more refined interpretation of the formational environment. δ34S values heavier than that of sea-water indicate that genesis has occurred within the bacterial sulphate reduction zone whilst δ34S values lighter than that of seawater delimit formation to a zone between sulphate reducing and overlying oxic conditions where isotopically light H2S is re-oxidized.

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  1. Isotope Geology Unit, Institute of Geological Sciences, 64 Gray's Inn Road, London, WC1X 8NG, UK

    • Richard A. Benmore
    •  & Max L. Coleman
  2. Department of Geology, University College London, Gower Street, London, WC1E 6BT, UK

    • Richard A. Benmore
    •  & John M. McArthur


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