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Recycled mollusc shells 193

shell’s core structure or prismatic layer is made of calcite. The aragonite present is
linked to nacre forming the building blocks of the structure. The next crystalline
form, the vaterite, is a polymorph which is less stable and present due to the shell’s
formation and growing processes.
In the work of Siriprom et al. (2012), it can be seen a comparison of the X-ray
difraction (XRD) profile of one type of clam (Anadara granosa) with one of mussel
(Perna viridis). The composition of the clam shell was 100% aragonite, while that
of the mussel is 13% calcite and 78% aragonite.
As with all bivalves, mussels are composed of two calcareous shells joined by a
ligament. This is the protective system of the mollusc and this defence grows as the
mussel develops. These shells are formed by biomineralisation of CaCO 3 together
with a small amount of organic matrix which holds the structure together (Safi
et al., 2015).
Mussel shells, like all bivalves, can be divided in three parts: the outer layer,
periostracum, the middle layer called the prismatic layer and the inner layer referred
to as nacre (Fig. 8.2). The periostracum is secreted in the groove between the outer
and middle layers of the mantle. It is un-mineralised and composed mainly of a
protein with a thickness of around 40 μm; its function is to protect the prismatic
layer from abrasion and dissolution by acids (Martı ´nez-Garcı ´a et al., 2017). The
central and thickest layer (around 400 μm) (Martı ´nez-Garcı ´a et al., 2017) has a
prismatic structure with an array of parallel calcite prisms (Safi et al., 2015), which
are polygonal in cross section and grow from the periostracum at an angle of
around 30 degrees to the surface. The last layer, about 10 μm in width, is known as
nacre which could be classified as a biomineralised composite. It comprises laminar
aragonite, which is oriented parallel to the surface and held together with an organic
matrix composite of polysaccharides (chitin), proteins and glycoproteins (Safi et al., 2015).

Figure 8.2 Description and SEM details of mussel shell structure (Martı ´nez-Garcı ´a et al., 2017).