Polymorphs of a mineral are different forms that have the same chemical composition but different atomic structures. These different atomic arrangements reflect the pressures and temperatures, and ultimately the geological environments in which the different polymorphs form. The mineralogic community has worked diligently to understand the polymorphs of olivine, a mineral made of iron, magnesium, and silica, which constitutes a majority of Earth’s interior and also is a major component of many meteorites. The polymorphs of olivine found in the Earth’s deep interior inform geoscientists of conditions such as pressure, temperature, and density. In addition, the high-pressure polymorphs of olivine are of interest to the planetary science community because their presence in shocked meteorites can record the transient high-pressure conditions generated by impact events.
Although there are many olivine polymorphs predicted to form at high pressures, such as ringwoodite and wadsleyite, only a few have been observed in nature because samples from the deep interior of Earth are very difficult to obtain. However, an international team led by Naotaka Tomioka at the Japan Agency for Marine-Earth Science and Technology was able to study one of these polymorphs when it was found in three shocked chondritic meteorites. Chondritic meteorites are made of primitive materials that can become shocked when their parent bodies experience impact events. This particular polymorph of olivine, called poirierite, was predicted to exist by its namesake, Jean-Paul Poirier, approximately 40 years ago. Tomioka and colleagues used a variety of techniques, such as high-resolution transmission electron microscopy and single-crystal X-ray diffraction, to study poirierite in shocked chondritic meteorites on an atomic scale and better understand the processes that lead to its formation.
The study found that the atomic structure of poirierite is slightly deformed compared with the structures of the high-pressure polymorphs ringwoodite and wadsleyite and suggests that it forms as a transitional phase during decompression and cooling. Furthermore, the atoms in poirierite are arranged in a way that suggests that it forms due to shearing, which occurs when unaligned forces push in opposite directions, similar to spreading out a deck of cards. Shearing typically occurs in low-temperature, high-pressure environments such as subduction zones on Earth and/or in high-stress environments such as shocked meteorites. Not only did this study confirm the natural occurrence of a mineral predicted 40 years ago, but it also demonstrates that understanding the formation of minerals can lead to a better understanding of deep-Earth processes and the effects of shock metamorphism on planetary materials. READ MORE