Materials Advent 2018 3 (and 4 and 5): Pearl and its components

A black pearl rests on an iridescent shell in a black background
A black pearl on the shell of a black-lipped pearl oyster. From Wikipedia: https://en.wikipedia.org/wiki/Pearl

Pearls are quite beautiful and also (kind of) rare, which makes us appreciate them even more. But it turns out they are made of very common components – a variant of chalk (calcium carbonate) bound in strings of the polymer that makes up insect, shrimp, and crab shells (called chitin, as well as a few other polymers). The calcium carbonate variant (or “polymorph“) is called aragonite, which technically has a different crystal structure from the form we actually use as chalk, which is called calcite. To make pearls even more interesting, aragonite isn’t actually the stable form of calcium carbonate at typical temperatures on the surface of Earth, but something about the way clams and other molluscs grow shells assembles that structure instead of calcite (and researchers still aren’t entirely sure why). 

Image result for crystal structure of aragonite

Despite essentially being a combination of chalk and insect shells, pearls and the related materials in mollusc shells are incredibly strong. We often attribute this to the strength of curves at our macro level, but it also turns out the way these parts are combined at the micro level is really important. The main solid part is essentially laid out slabs made of smaller bricks of aragonite, not unlike a brick house. And they are separated by small spaces due to rough bits on each brick, which is also where the entangling bits of polymers go. It’s really hard for cracks to get far in this arrangement – to travel through a slab it has to keep going through the alternating order of bricks and through the more flexible polymers separating them, and to go through multiple slabs a crack has to get through even more polymer and also change preferred directions along the crystals because the bricks change arrangement across slabs. As a result, pearls and clam shells are about 10 times stronger than individual hunks of aragonite or the polymers. This makes pearls an incredibly interesting composite that engineers want to mimic. In fact,a professor I work with at UVA has published multiple papers about the strength of mother-of-pearl (often called nacre in technical contexts) and tried to mimic this structure with graphene

original
An electron microscope image of flakes of aragonite in a mother-of-pearl sample

Industrially, humans get a lot of chitin from industrial processing of shellfish and use it as fertilizer as a food additive to help improve texture. 

Image result for chitin

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