The egg capsule of a sea snail resists cracks due to substituting calcium with magnesium.

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The sea snail Odontocymbiola magellanica has evolved to substitute magnesium for calcium in the calcite of its eggshells in order to make them more durable and resistant to cracks. The precise chemical mechanism is unknown but crack propagation seems to be reduced with the addition of the magnesium content. The eggshells of O. magellanica contain ~21% of their Ca substituted for Mg.

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"A surprising discovery is what we believe is the first record of high-magnesium calcite in molluscs. This mineral is a calcium carbonate polymorph in which Ca2+ is randomly substituted to some extent by Mg2+ in the calcite lattice; Mg substitutions over 4% (values found in O. [Odontocymbiola] magellanica capsules were 13.4– 21.1%) are considered to be high (Ries & Blaustein, 2003)...It is thought that magnesium substitution may impart a greater crack resistance to the calcite (Magdans & Gies, 2004)." (Bigatti et al. 2010:284, 286)

"The calcareous layer is made of high-magnesium calcite, a calcium carbonate polymorph in which Ca is partially substituted by Mg in the calcite lattice. Mg substitution is thought to confer a greater crack resistance to the mineral; it is found in many invertebrates, but apparently has not been reported before in molluscs. Odontocymbiola magellanica is a long-lived species, investing heavily in its egg capsules." (Bigatti et al. 2010: 279)

"The X-ray powder diffraction pattern obtained from recently laid egg capsules showed that the calcareous material was magnesium calcite, a calcium carbonate polymorph. Atomic absorption analysis indicated that 21.1% of the Ca was substituted by Mg in the calcite lattice." (Bigatti et al. 2010: 282)

"Magnesium calcite occurs in either shells or skeletal structures of a variety of nonmolluscan groups... It is thought that magnesium substitution may impart a greater crack resistance to the calcite." (Bigatti et al. 2010: 286)

"Biological systems have evolved strategies for high magnesium incorporation and stabilization, yielding favourable material properties. In sea urchin spines, for example, the inhomogeneous distribution of Mg2+ impedes crack propagation, thus enhancing fracture resistance." (Gayathri et al.  2007:3263)

Journal article
The calcareous egg capsule of the Patagonian neogastropod Odontocymbiola magellanica: morphology, secretion and mineralogyJournal of Molluscan StudiesApril 17, 2010
G. Bigatti, M. Giraud-Billoud, I. A. Vega, P. E. Penchaszadeh, A. Castro-Vazquez

Journal article
In vitro study of magnesium-calcite biomineralization in the skeletal materials of the seastar Pisaster giganteus

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