The melting mechanism of Na2SiO3, a crystal with pyroxene structure, includes three distinct reactions. All are driven by heating with each reaction commencing at a different temperature. The first two reactions proceed within the crystal at temperatures well below the melting point and are expressed by distinctive crystallographic, calorimetric, and Raman spectroscopic changes to the crystal. With the reactions identified and explained for Na2SiO3(c) and the melting mechanism elucidated, the Na2SiO3 system becomes the “Rosetta Stone” by which to decipher the melting mechanisms of all pyroxenes and other silicate minerals.
The last two reactions proceed by nucleophilic substitution where Si centers are attacked to form fivefold-coordinated activated complexes. Si-O− acts as nucleophile in the second reaction (producing Q3 species), and O2− acts as nucleophile in the third reaction (producing Q1). Taken in reverse, these mechanisms describe the formation of nuclei in crystallizing melts and in addition provide insight into the elusive changes that occur at the glass transition. Elucidation of the melting mechanism could thus provides a unified framework within which melting, crystallization, and the glass transition can be understood.