By using state-of-the-art high-resolution transmission electron microscopy to examine crystals found in and on the bodies of small marine organisms called sea squirts, a team of scientists led by the Technion-Israel Institute of Technology has resolved the nearly 100 year-old mystery about the crystal structure of the mineral called vaterite. The findings are published in the current issue (April 26, 2013) of SCIENCE.
Scanning electron microscope image of sea squirt vaterite spicules
Vaterite is a form of calcium carbonate – one of nature’s most abundant minerals. It can be found in gallstones and in certain geological structures. It is also a component of cement, where its quick transformation into other more stable forms of calcium carbonate when exposed to water, helps make cement hard and water resistant.
Scientists have long been studying the development and formation of vaterite in biomineralization, the process in which living organisms control the production of different minerals on the atomic level. For example, when a mollusk receives a blow that cracks its shell, it uses vaterite to repair the damage. As a biomineral, vaterite is also found in freshwater pearls and in the inner ear structures of some fish.
“Unlike most minerals, vaterite has defied every effort to resolve its crystal structure, perplexing scientists for nearly a century. The structure of a mineral crystal is a critically important feature and is determined by how atoms are arranged in the crystal. The arrangement of atoms and the resulting crystal structure, for instance, make the difference between graphite and diamond, both forms of pure carbon,” said Assistant Professor Boaz Pokroy of the Technion Department of Materials Science and Engineering, who led the project.
Prof. Pokroy and his graduate student Lee Kabalah-Amitai, and University of Wisconsin-Madison scientists, worked together to unveil the secrets of vaterite with the help of a needlelike spicule from a sea squirt found in the Mediterranean and Red Seas.
Ferreting out the structure of the mineral was challenging because of the difficulty of finding sufficiently large, pure single crystals of vaterite.
Using state-of-the-art transmission electron microscopy at the Technion to examine the large, single crystal that makes up the spicule of the solitary sea squirt Herdmania momus, Pokroy and his team were able to unmask the atomic structure of its constituent biomineral vaterite. They concluded that vaterite is composed of at least two different crystal structures that “coexist within a pseudo-single crystal.”
“We never envisaged this scenario,” explains Pokroy. “It was a total surprise, but at the same time it made so much sense knowing the years of conflicting results from different groups publishing on the structure of vaterite.”
“This small organism produces a bundle of crystals from vaterite that are very sharp and relatively large, making them easier to work with,” explains Kabalah-Amitai. “This is why we examined these crystals, and this is the first time they have ever been studied on the atomic level.
“We found that vaterites actually consisted of two different atomic arrangements that exist in harmony with one another. The second atomic arrangement was found in an area so tiny – about 40,000 times thinner than a human hair – which is why it eluded the eyes of scientists who believed this was a singular structure rather than a dual structure, explains Pokroy.”
The research team also included Boaz Mayzel, a marine biologist from Tel Aviv University, who collected the specimens; Dr. Yaron Kaufman, who aided in examining the crystals under the Technion’s “Titan” electronic microscope, Dr. Andy Fitch, a scientists at the synchrotron in Grenoble, France, Leonid Bloch from the Technion, and Prof. Pupa Gilbert of the University of Wisconsin-Madison Department of Physics.