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29 Si NMR and Raman spectroscopic studies demonstrate that fusion of crystalline orthosilicates and metasilicates produces melts more polymerized than their precursor crystals. Forsterite, for example, consists of 100% Q 0  species, whereas its melt consists of ~50 mol% of Q 1  species (Q = a Si tetrahedron and the superscript indicates the number of bridging oxygen atoms in the tetrahedron). Polymerization during melting can be rationalized from an energetics perspective. Si-NBO-M moieties of Q species are more susceptible to librational, rotational, and vibrational modes than are Si-BO-Si moieties (NBO = non-bridging oxygen; BO = bridging oxygen; M = counter cation). Thermal agitation activates these additional modes, thus increasing the  C  P  and free energy of melts. The reaction of Q n  to Q n+1  species during melting eliminates Si-NBO-M moieties and produces Si-O-Si moieties that are less susceptible to the additional modes, thereby minimizing the  C  P  of melts. By decreasing the abundances of Q 0 , Q 1 , and Q 2  species in favor of Q 3  and Q 4  species, melts become more stable. In the absence of polymerization, melting temperatures of minerals would be appreciably greater than observed.

american mineralogist

Polymerization during melting of ortho- and meta-silicates: Effects on Q species stability, heats of fusion, and redox state of mid-ocean range basalts (MORBs)