A colour atlas of carbonate sediments and rocks under the by A E Adams; W S MacKenzie

By A E Adams; W S MacKenzie

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43, 44 Stained thin section, Lower Cretaceous, Istrian Peninsula, × 34, 43 PPL, 44 XPL 43 44 34 Bioclasts The bimineralic nature of some Quaternary bivalve shells is shown in 46. 8), which is used to distinguish aragonite from calcite. Aragonite takes on a rather ‘spotty’ grey or black colour and calcite remains unchanged. The two curved shells in this photograph are apparently two-layered, with a thick, inner layer of aragonite and a thinner, outer layer of calcite. Unfortunately, the stain masks the structure of the aragonite layer, although, in fact, the structure of both layers is homogeneous.

57 is a view of a section that has been impregnated with a dye such that porosity shows blue. The rounded fragment, probably of an abraded bivalve, has been completely dissolved, although there is a little fine spar cement on the lower side. The clear areas in the sediment are detrital quartz grains. 55, 56 UnstainedBarbados, thin 55, 56 Unstained thin section, Quaternary, section, Quaternary, × 20, 55 PPL, 56 XPL with sensitive tint plate. Barbados, × 25, 55 PPL, 56 XPL with sensitive tint plate.

The variation in thickness of layers is characteristic, with the inner layer thickening towards the umbo. The boundary between the two layers would meet the inner wall of the shell in the pallial line. The wavy fragment in the upper right of the photo- graph is a gently ribbed bivalve preserved as a cast. 52 is part of a thick-shelled oyster showing the foliated calcite structure. Most brachiopods also show a foliated calcite structure, but it is much more regular than that of the oyster shown here.

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