Studying sea squirts has revealed the crystal structure of the mineral vaterite — a mystery which had spanned almost a century.
Vaterite is a variant form of calcium carbonate. Little was known about its structure as large sample crystals of vaterite are rare in nature and attempting to synthesise the mineral generally yields poor results.
However, vaterite also occurs as a biomineral in freshwater pearls, some types of fish ear bone and, importantly, in sea squirts. Sea squirts use vaterite to create spicules — tiny support structures, which act as a kind of skeleton for the creatures.“This organism makes the best crystal,” said Pupa Gilbert, an expert on biocrystallisation at the University of Wisconsin. Lead author, Boaz Pokroy, adds, “The Herdmania momus spicules were known to be made of vaterite since 1975 when a paper by the late Heinz
Lowenstam of CalTech was published in Science. It was
clear that this would be the best source of biogenic vaterite and
geologic vaterite we could ever find.”By using aberration-corrected high-resolution transmission
electron microscopy the research team discovered that vaterite was
actually composed of at least two different crystallographic
structures which coexist in the one crystal.
But vaterite is proving to be the Russian doll of
biomineralisation mysteries — in answering one question, the team
have raised another. The major structure is now known to take a
hexagonally symmetrical form, but the minor structure which is
tucked away in spaces within the major matrix is unknown.
Edited by Liat Clark