Aragonite brick

Fig. 4.19. Schematic of a cross-sectional view of the correlated inter-platelet arrangement of aragonite bricks and interlamellar macromoleculax organic mortar material in the architecture of nacre.

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

Fig. 2. Nacre of red abalone shell Halitotis refescens), imaged here by transmission electron microscopy (left), has a bricks-and-mortar structure. The bricks are CaCOs (aragonite) platelets, and the mortar is a composite of macromolecules, including structural proteins and polysaccharides, that form a thin film around the platelets. The three-dimensional structure is depicted on the right [23]...

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. 

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