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Francolite in a concretion from argillaceous sediments in the Westphalian of Yorkshire

Published online by Cambridge University Press:  05 July 2018

Mike J. Pearson
Mike J. Pearson*
Affiliation:
Department of Geology and Mineralogy, The University, Marischal College, Aberdeen AB9 1AS
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Synopsis

A phosphate-rich carbonate nodule from carboniferous mudstones has been studied by wet chemistry, X-ray diffraction, and electron microprobe. Partial heavy-liquid separation has enabled characterization of the phosphate phase as francolite (carbonate fluorapatite) with a composition Ca10(PO4)5·59(CO3)1·00((OH)1·49F2·10). Diffractometer data for this mineral is presented and cell dimensions are calculated as a = 9·349 Å, c = 6·887 Å. with c/a = 0·737. The problem of the structural positioning of carbon in francolite is discussed.

The francolite is seen in thin section as a structureless groundmass enclosing siderite spheru-lites. Electron microprobe X-ray distribution photographs for P, Al, Si, Mg, and Fe show the relationship of these two phases to quartz, pyrite, and at least two clay minerals. One of the latter is intimately mixed with francolite.

Possible origins of phosphatic nodules are discussed. Textural evidence in the present case strongly suggests that the francolite is concretionary and formed during early diagenesis possibly by replacement of earlier calcite. Phosphate was probably derived from protein degradation, which also maintained the alkaline pH necessary for francolite formation.

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Synopsis
Information
Mineralogical Magazine , Volume 41 , Issue 318 , June 1977 , pp. 287
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1977

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