The role of medium range order on phase transitions in chain silicates upon compression

Raman spectroscopic measurements of the tetrahedrally coordinated crystal MnSiO₃ (rhodonite) in an argon pressure medium show that it becomes amorphous above 33 GPa. This observation consolidates our findings and explanation for the global structural trends exhibited by the extended chain silicate family {\mathrm {AA'BO_{3}}} ({\mathrm {A,A'}} : Mg, Ca, Mn, Fe; B: Si) upon compression. In particular, crystals of this family are made of two types of building blocks coined P and C. Those crystals comprised solely of P blocks transform to dense higher coordinated crystalline phases; those comprised of P and C blocks, such as MnSiO₃ rhodonite, become amorphous; whereas those comprised solely of C blocks show both crystalline and amorphous regions upon compression. The reason that this medium range order length scale (building block scale) classification is correlated with the type of transitions taking place upon compression is due to the instability of C blocks and C-P interfaces with respect to P blocks and P-P interfaces at high pressures.