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