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Calcite, silica, pyrite...
Shells of Inarticulata are generally made of calcium phosphate and chitin, those of Articulata of proteines and a stable variety of calcium carbonate named calcite. The mineral-bearing parts can thus fossilize without significant change in their crystalline structure. However, it happens that the existing minerals are gradually replaced, molecule by molecule, by another mineral, without touching with the fossil's morphology. This phenomenon is called epigeny. Silica (in the form of quartz, opal...), pyrite, dolomite being the most common components. Limonite also can be formed by oxidation of other iron minerals.
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Fluorite epigeny is rather rare
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Oxfordian shells infilled with minerals :
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small geodized shell with quartz crystals
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two generations of calcite crystals in a Terebratula shell
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This fossil has been replaced with grey-bluish chalcedony, a cryptocristalline form of silica.
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Internal moulds turned into
Green, green, green...
Larger parts of the shell of this Moutonithyris, from the lower Barremian deposits, are covered with tiny green Glauconite crystals. Sometimes, the complete shell shows olive green, black green to bluish green colour.
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Glauconite is an iron potassium silicate mineral (mica group). It is considered diagnostic of marine depositional environments (depth 50-500m).
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