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At temperatures less than ~1500 K, previously published  C  P  data demonstrate that the heat capacities of orthoenstatite, proto-enstatite, diopside, and pseudowollastonite include primarily Debye type vibrational and anharmonic contributions, whereas the alkali chain, sheet, and ring silicates, Na 2 SiO 3 , Li 2 SiO 3 , K 2 SiO 3 , and Na 2 Si 2 O 5  include a third contribution. The third contribution to  C  P  arises from defect formation due to the mobility Na, K, Li, and O 2− . The contribution becomes apparent at temperatures above 700–800 K for Na and K silicates, and above 900–1000 K for Li metasilicate. With strong thermal agitation, alkali-non-bridging oxygen (NBO) bonds are ruptured with the cations exiting their structural sites to occupy interstitial sites, thereby producing intrinsic Frenkel defects, which contribute to the  C  P  of the alkali silicates. The magnitudes of the  C  P  defect contributions correlate inversely with cation-oxygen bond strengths, as measured by bond dissociation energies. K-O and Na-O bond strengths are weak (239 and 257 kJ/mol) and defect contributions are large for these alkali chain, ring, and sheet silicates. The greater bond strength of Li-O (341 kJ/mol) correlates with a weaker defect contribution to the  C  P  of Li 2 SiO 3 . Mg-O and Ca-O bonds are stronger still (394 and 464 kJ/mol) and no  C  P  defect contributions are observed for the pyroxenes and pseudowollastonite up to ~1500 K.

Defect contributions to the heat capacities and stabilities of some chain, ring, and sheet silicates, with implications for mantle minerals