Tuesday, 1 April 2014

Micrograph of the Month: Foram with a Calcite Hypocoating

This month we have some micrographs from floor deposits in the refectory at Margat Castle, Syria. Here we have a fragment of lime, within which is embedded a lovely little foraminifera. Forams are tiny little marine creatures which have calcium carbonate shells, and when they die they form part of the fine sediment which is deposited on the sea floor. This in turn becomes sedimentary rock, and that's the origin of this tiny fragment of limestone.
In XPL we can see that the void space inside the foram has a coating of highly birefringent (sparkly) material. These are tiny little calcite crystals. Calcite hypocoatings can result from several processes, but in this case it is likely the result of carbonate rich water percolating through pores in the sediment. Similar coatings can be seen in other voids in this layer. When the water evaporates the carbonate precipitates on the walls of the voids. A bit like the layers of lime-scale that form on taps and kettles if you live somewhere with 'hard' (i.e. calcium rich) water.

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