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By A E Adams; W S MacKenzie

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The structure of the wall is not really evident in the photograph, but is, at least in part, prismatic. 53 Unstained thin section, Lower Cretaceous, Middle East, PPL, × 18. 53 54 Stained thin section, Lower Cretaceous, Istrian Peninsula, PPL, × 17. 54 39 Carbonate Sediments and Rocks Under the Microscope In pre-Quaternary limestones, most originally aragonitic bivalves are preserved as moulds or casts. 55 and 56 show two curved bivalve shells that have suffered from solution and were therefore likely to have been originally composed of aragonite.

In small fragments the sweeping extinction pattern is not always visible. 89, 90 Stained Unstained thin thin section, Quaternary, Barbados, × 50, 89 PPL, 90 XPL. 89 90 55 Carbonate Sediments and Rocks Under the Microscope Older scleractinians are believed to have had the same structure, but being aragonite, they are usually preserved as moulds and casts and the original structure is lost. 91 is a photograph of a section of a colony of Jurassic corals. All the hard parts of the coral have been replaced by mauve-stained ferroan calcite and none of the original microstructure is visible.

The most important wall structures are foliated, prismatic, crossed-lamellar and homogeneous. Majewske (1969) includes an invaluable table summarising the shell mineralogy and structure of the major bivalve families. Bioclasts 42 is a photograph of a Jurassic limestone and shows four different bivalve shell structures. Dominating the left centre field of view is a transverse section of a shell with prismatic structure. Individual prisms with their polygonal cross-section can readily be seen. Along the top of the picture is a shell fragment with a foliated structure.

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