Tag Archives: apatite

Fox spine.

Bones have a fractal-like structure making them super strong and flexible

Zoom in close enough and bones betray an incredible structural sophistication.

Fox spine.

Fragment of fox spine.
Image via Pixabay.

Researchers from the University of York and the Imperial College London have produced a 3D, nanoscale reconstruction of bone’s mineral structure. Their work reveals a surprising ‘hierarchical organisation’ which underpins the material’s mechanical versatility.

Bred in the bones

Bone is a surprisingly versatile material. Different varieties of bone can be both strong and flexible, maintaining the lithe form of cheetas, the impressive bulk of elephants, or the lightweight frames of birds alike.

These enviable properties are owed to a sophisticated internal structure. However, the exact nature of this structure and of the interactions between the main components of bone — collagen protein strands and the mineral hydroxyapatite — has so far been unknown. According to new research, however, the ‘hierarchical organization’ of bone is based on small elements coming together to form larger and larger structures.

Their results have shown that individual mineral crystals inside bone tissue come together into larger, more complex structures — ones that come together into even more complex levels of organization, the team reports.

For the findings, the team used advanced 3D nanoscale imaging of the mineral component of human bone. They used a combination of electron microscopy-based techniques to reveal its main mineral building blocks. These nanometer-sized crystals of apatite take on a curved, needle-like shape and merge together into larger, twisted platelets that resemble the shape of propeller blades.

These blades, in turn, merge together and split apart throughout the protein phase of the bone. This overarching weaving pattern of mineral and protein is what provides the material’s strength and flexibility.

“Bone is an intriguing composite of essentially two materials, the flexible protein collagen and the hard mineral called apatite,” Lead author, Associate Professor Roland Kröger, says Associate Professor the University of York’s Department of Physics, lead author of the paper.

“The combination of the two materials in a hierarchical manner provides bone with mechanical properties that are superior to those of its individual components alone and we find that there are 12 levels of hierarchy in bone.”

The paper describes the structure as “fractal-like”, containing 12 different levels of complexity. The needle-like crystals merge into the propeller-like platelets in a roughly parallel arrangement with gaps of roughly 2 nanometers between them. These stacks of platelets, along with some single platelets and acicular crystals, come together into larger “polycrystalline aggregates”. These latter ones are larger laterally than the collagen fibers, and can even span several adjacent fibers — providing a continuous, cross-fiber mineral structure that lends resilience to the bone.

Bone structure.

The model of crystal organization in bone proposed by the team.
Patterns specified by the model at the top alongside the mineral organization in different directions (bottom).
Image credits N. Reznikov et al., 2018, Science.

These nanostructures woven into the bone also show a slight curvature, twisting the overall geometry, the team further reports. For example, the individual crystals are curved, the protein (collagen) strands are braided together, mineralized collagen fibrils twist, and the bone themselves have a twist (such as a curvature of a rib).

The team concludes that this fractal-like structure they discovered embedded in our bones is one of the cornerstones of their remarkable physical properties.

The paper “Fractal-like hierarchical organization of bone begins at the nanoscale” has been published in the journal Science.

apatite_moon_sample

New evidence suggests the moon never was abundant in water

apatite_moon_sample

(c) NASA

An eminent team of US researchers found that it is highly unlikely that the moon ever once harbored important quantities of water, after studying a mineral called apatite. Generally speaking, scientists have always though the moon was water barren, a theory confirmed by the initial rock samples brought back by the Apollo missions, however in the past decades or so new modern mineral analysis suggested that Earth’s satellite is much wetter than some believe.

Now, a new set of findings reported by a team of researchers from the US, some of whom supported the wet moon theory in the past, suggests that the high hydrogen content from volcanic glasses and apatite in lunar rocks results from a more complex chemical process, one that doesn’t fits a wet moon scenario. Conveniently enough, the name apatite comes from the Greek word ‘apat’, meaning ‘deceit’.

Lead author Dr. Jeremy Boyce, a Nasa Early Career Fellow at the University of California, Los Angeles, along with his distinguished colleagues, simulated the formation of apatite minerals containing different amounts of volatile elements – hydrogen, chlorine and fluorine.

The simulation demonstrated that you could start off with any kind of water composition in the magma, yet still end up with the same apatite configuration simply by varying the chlorine content and degree of crystallization. The findings suggest that the mineral’s structure formation is not as simple as saying the more hydrogen in the apatite, the more there was in the magma. Instead, it all boils down to competition between elements .

Apatite wants to pair up with fluorine, since its electron configuration is the most stable to join with. Then comes chlorine, and right at the end when there isn’t much left, hydrogen.

“So all the apatites are taking all the fluorine and hiding it from the melt. Then the melt forgets that it had all that fluorine and the apatites get more chlorine-rich and more hydrogen-rich,” the researchers write.

Back where we started: a dry moon

It’s because of this that the original indication that claimed the moon may once have been abundant in water could be false. Moreover, a lot of water on the moon contradicts the current prevailing theory concerning its formation. The theory goes that during our solar system’s infancy, a giant planetary body called Theia collided with Earth, blasting rock into Earth orbit. These debris coalesced to eventually form the moon, but in the process volatile materials must have been boiled off, leaving little water on the moon relative to Earth. So a less watery Moon ties in better with this theory.

Not only this, there are potentially other sources of hydrogen on the moon like the solar wind.

“There is some hydrogen that’s coming in from the solar wind and getting stuck on the surface. But there is some amount – definitely seems to be less than on Earth – that the Moon started with,” said Dr Francis McCubbin, senior research scientist at the University of New Mexico.

If anything, scientists may be back to drawing board as these latest findings create new uncertainty as to how much water there used to be on the moon.

“Clearly, we did the best we could at the time. But that’s the progress of science – there are course corrections,” Dr Boyce explained.

“Definitely, there is still water on the Moon. Those rocks are not completely anhydrous. There’s a really interesting record of heavy chlorine and hydrogen isotopes. But the abundances, we’ve demonstrated, are difficult to interpret.”

Dr McCubbin commented: “Forty years ago, the Apollo astronauts built a building and the elevator was on a floor where [the water abundance] was one part per billion (ppb).

“We took it up to where we were near terrestrial abundances, and then we realised we were on the wrong floor. We’ve taken it back down, but not all the way down to where we were 40 years ago.”

The findings were reported in the journal Science.