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Journal of the Geological Society; 1980; v. 137; issue.6; p. 661-667;
DOI: 10.1144/gsjgs.137.6.0661
© 1980 Geological Society of London

Article

Microstructural contrasts between some sedimentary francolites

R. A. Hewitt

Phosphatic nodules contain early diagenetic francolite cements and carbonate replacements. Lower Cambrian phosphatized carbonates are composed of granular and mammillated fabrics. They show large (112) X-ray diffraction peaks, and probably consist of fluorapatite. Granular and prismatic francolites are found in concentrically banded nodules from the Lower Lias (Lower Jurassic) of Blockley, England. They are interpreted as originally phosphatic burrows. The francolite microstructures of Lingula, Linnarssonia, Micromitra, Hyolithellus, Torellella, Serpuloides, Trachyteuthis and Mesozoic teuthids, contain variably recrystallized granules and needles.