Aragonitic nacreous layers

Figure 14.2. Structures forming the shells ofbivalves [6]. Parts (a)-(d) indicate how the nacreous layer of pearl is formed. A =platy aragonite crystals 0 = organic sheet ...

Isoenzymes III and VII have a more specialized distribution. Carbonic anhydrase III is abundant in adipocytes which use bicarbonate in fatty acid synthesis.7 Isoenzyme V is present in the mitochondrial matrix and is also abundant in both adipocytes and liver.7 8 Isoenzyme IV is a larger membrane-associated form, while VI is secreted into the saliva.10 Carbonic anhydrase has also been identified in E. coli.,11 in a methanobacterium,12 and in green plants.13 133 A 60-kDa carbonic anhydrase called nacrein is found in the organic matrix of the nacreous layer of the pearl oyster, the layer that forms aragonite (orthorhombic calcium carbonate) in the shell and in pearls.14... 

Some shells have a porcellaneous inner layer, while others produce a nacreous layer. The nacreous effect is called mother-of-pearl and is caused by tiny, overlapping platelets of calcium carbonate in the form of aragonite, which disperse the light to give a play-of-colour. The platelets are polygonal and lie horizontal to the surface. In gastropods and nautilus they are stacked like coins, but they can also be alternated, like a brick wall, as is the case in most bivalves. There can be more than one nacreous layer, as in, for example, freshwater mussels. 

Fig. 4.10. Schematic representation of the structural relationships between protein sheets, aragonite crystals, and chitin fibres in the nacreous layer of N. repertus ...

According to the optical model of a pearl (illuminant model) based on a computer-assisted visual simulation, the aragonite layer in the nacreous layer of a pearl should be highly transparent and also have point light source transmitting rays in... 

Figure 19.7 Molecular correspondence of the inorganic-organic interface in the nacreous shell layer of Nautilus repertus. (a) Structural relationships between protein sheets, aragonite crystals and chitin fibres, (b) Possible complementarity of Ca binding. (From Mann et al., 1989. Reproduced with permission from John Wiley Sons., Inc.)...

Nacreous structure layer a layer consisting of stacked platy aragonite crystals in brick-like layers and an interstitial protein layer. 

It is this ability to secrete the organic and inorganic compounds that make up the shell, which gives the mollusc the ability to secrete the same substances to produce pearls. All pearls consist of approximately 90 per cent calcium carbonate, plus water, and an organic matrix. As with the nacreous inner layer of the shell, the calcium carbonate in pearls fbnns as aragonite platelets and gives the pearl its nacreous lustre (sometimes called orient ). 

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