Olivine thermal expansion

With increasing T and constant P, olivines expand their octahedral sites Ml and M2, whereas the tetrahedral group [8104] remains virtually unaffected. Table 5.9 lists linear thermal expansion coefficients for Ml-O and M2-0 polyhedra in some olivine compounds, according to Lager and Meagher (1978). These values are generally higher than those obtained by the method of Hazen and Prewitt (1977) (see eq. 1.93 and section 1.14.2). In particular, thermal expansion is anom-... 

Table 5.10 Isobaric thermal expansion of olivine componnds. Regressions for Mg2Si04 and Fe2Si04 from Fei and Saxena (1986).

Incidentally, it must be noted that, although most olivine compounds are stable (at P = 1 bar) up to the melting temperature, in some compounds polymorphic transitions are observed at temperatures below the melting point (for instance, Ca2Si04 has a transition to a monoclinic polymorph at 1120 K). Forsterite at 1 bar melts at 7) = 2163 K, whereas fayalite melts at a much lower temperature Tf = 1490 K). The significance of this difference, as shown by Hazen (1977), must be ascribed to different polyhedral thermal expansions for polyhedra containing Mg and Fe cations. Hazen (1977) pointed out that, if the curve of volumes of the members of the (Mg,Fe)2Si04 mixture is extrapolated at various... 

Combining thermal expansion and compressibility data for compounds in the (Mg,Fe)2Si04 mixture, Hazen (1977) proposed the following equation of state for olivines of the series Mg2Si04-Fe2Si04 ... 

---