The Earth Story

Vaterite … from fish ears to crystal lattices

Deep within the ear of a fish you will find a little bone, an otolith. This bone acts as part of the sensory system of the ear, part accelerometer, part gravity sensor, part sound sensor. Otoliths are formed from calcium carbonate minerals, and different species of fish exploit different types of CaCO3 mineral. These CaCO3 "polymorphs" all have the same chemistry, but the arrangements of atoms within the crystal lattice of each are different, just as diamond and graphite are two polymorphs of carbon. Usually, a fish otolith grows as aragonite, sometimes as calcite, a different polymorph of calcium carbonate, and sometime as the third CaCO3 polymorph, vaterite.

Vaterite is rather rare in nature and almost always occurs as a biomineral. If you try and grow it in the laboratory it will tend to change rather quickly over a few days into the more stable polymorph, calcite. But it seems to be something of a biological sticking plaster. Very often carbonate-forming organisms such as fish and molluscs grow vaterite instead of calcite or aragonite when they are "stressed" … when the environmental conditions are non-ideal, for example in polluted waters, or if they are suffering disease. So, fish otoliths can show vaterite infilling as a sign of poor living conditions. In fact, otoliths act as a chemical record of a fish's life story, with growth rings and variations that reflect the fish's health and the water chemistry of its home as it grows. Since otoliths are preserved in ocean sediments, they can also be used to chart palaeoceanographic conditions into the distant geologic past.

Despite its widespread use as a biomineral marker, the detailed atomic crystal structure of vaterite has remained a mystery for more than a century. Until this week, that is. A recent report in the journal "Science" describes the crystal structure of a crystal of vaterite taken from a sea squirt spilicule. The extraordinary thing about the structure is that, unlike the structures of the calcite and aragonite forms of CaCO3, vaterite seems to be composed of two interspersed lattices of CaCO3. But this does not seem to be the last word on the subject …. the authors, Pokroy and Gilbert, show that one of these two lattices is the previously-known hexagonal crystal structure, and the other remains yet unknown.

~SATR

Image: Silver sea trout (Cynoscion nothus) otolith, Florida Fish and Wildlife Conservation Commission's Fish and Wildlife Research Institute (Creative Commons License)

http://www.sciencemag.org/content/340/6131/454.abstract

http://hal.archives-ouvertes.fr/docs/00/69/10/53/PDF/Morat_et_al._EFF_2008.pdf