The majority of solid chemicals have multiple meta-stable phases. This means that there are multiple different crystal structures that are kinetically stable. For example, aragonite and calcite are both stable at room temperature even though calcite is the only thermodynamically stable phase.

Ostwald's rule refers to the tendency for the less stable phase to crystalize first, i.e. it has a lower activation energy. Given enough time the more stable forms appear and eventually become dominant.

However, this "rule" could largely be observation bias: A polymorph that is both metastable and has a high kinetic barrier to formation would generally never form in the first place. However, if it could be created, that same kinetic barrier is a good thing: It means the phase will persist longer before converting back to a more stable phase.

Diamond vs graphite at low pressure is such an example. Diamond has a high kinetic barrier and so does not form when i.e. organic material is carbonized. Diamond only naturally forms under extreme pressure in which case it is more stable than graphite (heat helps overcome the kinetic barrier).

This makes me wonder if most ordinary substances have novel and unknown crystal structures that are reasonably stable at ambient conditions. In the case of diamonds, chemical vapour deposition turns the high kinetic barrier "bug" into a "feature": Reactive gases etch away any graphite formed but leave the diamond alone. How likely is it that common minerals such as ice, quartz, corundum, and iron oxide have undiscovered crystal structures that can exist at low pressure?


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Certainly, it would be shocking to think we've discovered the forms of all minerals, whether metastable or durable at STP. New morphs are being found for compounds as common as water ($\ce{H2O}$) and quartz ($\ce{SiO2}$).

Some metastable minerals formed by shock metamorphism have been discovered fairly recently.

  • Coesite is a polymorph of $\ce{SiO2}$ that is formed at 2–3 gigapascals and 700 °C. It was first synthesized in 1953, and first found in 1960 in the Barringer Crater, AZ, USA. One of the discoverers of coesite, Eugene Shoemaker, was interested in the "geology" of other bodies of the solar system, where other such minerals might be found.
  • Stishovite, first synthesized in 1961 by Sergey M. Stishov, is another polymorph of $\ce{SiO2}$. It is formed at 10 GPa and 1,200°C. It can be separated from surrounding quartz by dissolving the matrix in hydrofluoric acid!
  • Poirierite, a polymorph of magnesium iron silicate, was predicted in the 1980s by Jean-Paul Poirier and was found in shocked chondritic meteorites in 2021.
  • Reidite is a polymorph of $\ce{ZrSiO4}$, first synthesized in 1969 by Alan F. Reid and found in a few meteorite impact sites. It is formed above 9 GPa.

See some more such metastable metamorphs in the article by Tomioka and Miyahara. As Tom Lehrer put it, "And there may be many others, but they haven't been discavered [sic]."

In medicine, a prion is a protein with different folding, i.e., "crystal" form. Some forms have particularly bad habits, e.g., those involved in severe diseases. Unfortunately, there are likely many more prions, many disease-causing, to be found.

Quasicrystals were hypothesized, based on Penrose tilings, and discovered in aluminum by Dan Shechtman in 1982, for which he won a Nobel Prize.

New crystal forms have been the basis for some science fiction stories.


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