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Phosphatization of calcium carbonate in phosphorites: microstructure and importance
Author(s) -
LAMBOY MICHEL
Publication year - 1993
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1993.tb01090.x
Subject(s) - carbonate , apatite , geology , phosphorite , calcium carbonate , mineralogy , dissolution , precipitation , pseudomorph , geochemistry , materials science , chemical engineering , phosphorus , metallurgy , paleontology , composite material , physics , quartz , meteorology , engineering
Fabrics of phosphatized calcium carbonate particles in various phosphorites have been studied using scanning electron microscopy coupled with X‐ray dispersive microanalysis. Replacement of calcium carbonate by apatite has been observed in bivalve shell fragments and in foraminiferal tests; replacement proceeds at constant volume with excellent preservation of the original microtextures. In some deposits, replacement of carbonate by apatite is the main phosphogenic process. However, in general, the process seems to be far less important than might be believed purely on the basis of thin section observations. In many phosphorites, internal or external apatite moulds of bioclasts are common, including very small particles such as coccoliths in phosphatized chalks. Apatite precipitation was typically followed by carbonate dissolution. Later apatite precipitation within the dissolution voids may produce partial or total phosphate pseudomorphs of the original carbonate grain. In these examples direct replacement of carbonate by phosphate cannot be demonstrated.