| Abstrakt: |
29Si 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% Q0species, whereas its melt consists of ~50 mol% of Q1species (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 CPand free energy of melts. The reaction of Qnto Qn+1species during melting eliminates Si-NBO-M moieties and produces Si-O-Si moieties that are less susceptible to the additional modes, thereby minimizing the CPof melts. By decreasing the abundances of Q0, Q1, and Q2species in favor of Q3and Q4species, melts become more stable. In the absence of polymerization, melting temperatures of minerals would be appreciably greater than observed. |
