Fayalite-forsterite

Mossbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported. 

Figure 5.5 Relative distributions of minor components Mn2Si04, Ni2Si04, and Ca2Si04 in fayalite-forsterite mixtures (weight %).

Figure 3. Temperature dependence of the amorphization dose for (A) pyrochlore, zircon, monazite and perovskite and for (B) fayalite, forsterite, zirconolite, and hnttonite. The lines represent a least-squares fit to Equation (15).

A2Si04-B2Si04 Olivine Mg2Si04-Fe2Si04, end members forsterite and fayalite, sobd solution is olivine... 

Olivine is assumed to be the only mineral precipitating and the fa and fo subscripts refer to pure fayalite and forsterite. Upon crystallization of the incremental amount dM i of cumulate, the incremental amount dMfa of fayalite is precipitated, and, since the iron content of fayalite is constant... 

CfaFe° and CfoMg0 being the contents of FeO in fayalite and MgO in forsterite, the... 

Olivine is the principal mineral (in terms of mass) of the earth s upper mantle where it is present as a combination of the two main components forsterite (85-95 mol.%) and fayalite (5-15 mol.%). In the earth mantle the increase in seismic velocity with depth is believed to be due to the presence of denser modifications of common minerals. 

Fayalite (melting point 1205°C) and forsterite (melting point 1890°C) form continuous solid solutions (the melting diagram of their binary mixtures correspond to the type exemplified in Fig. 2.28). Phase equilibria as a function of pressure, show, however, transition into the spinel structure. (cF56-MgAl204, spinel structural type). 

Olivine Name Forsterite Chrysolite Hyalosiderite Hortonolite F err o-hortonolite Fayalite... 

COMPONENT - Each phase in a system is in turn composed of chemical components. The choice of the various components is, under certain provisos, arbitrary. For instance, the olivine solid mixture (Mg, Fe)2Si04 may be considered to be composed of fayalite (Fe2Si04) and forsterite (Mg2Si04) end-members, of oxides... 

Table 4.2 Defect energies in forsterite and fayalite based on lattice energy calcnlations. 7 = ionization potential E = electron affinity = dissociation energy for O2 A77 = enthalpy of defect process (adapted from Ottonello et af, 1990).

Table 5.4 Olivine major element compositions (in weight %). Samples occur in different types of rocks (1) = forsterite from a metamorphosed limestone (2) = hortonolite from an olivine gabbro (3) = fayalite from a pantelleritic obsidian (4) = fayalite from an Fe-gabbro (5) = forsterite from a cumulitic peridotite (6) = forsterite from a tectonitic peridotite. Samples (1) to (4) from Deer et al. (1983) sample (5) from Ottonello et al. (1979) sample (6) from Piccardo and Ottonello (1978). ...

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... 

Akamatsu T, Kumazawa M., Aikawa N., and Takei H. (1993). Pressure effect on the divalent cation distribution in nonideal solid solution of forsterite and fayalite. Phys. Chem. Minerals, 19 431-444. 

Wood B. J. and Kleppa O. J. (1981). Thermochemistry of forsterite-fayalite olivine solutions. Geochim. Cosmochim. Acta, 45 529-534. 

Example 4.1. Suppose olivine and garnet are in contact and olivine is on the left-hand side (x<0). Ignore the anisotropic diffusion effect in olivine. Suppose Fe-Mg interdiffusion between the two minerals may be treated as one dimensional. Assume olivine is a binary solid solution between fayalite and forsterite, and garnet is a binary solid solution between almandine and pyrope. Hence, Cpe + CMg= 1 for both phases, where C is mole fraction. Let initial Fe/(Fe- -Mg) = 0.12 in olivine and 0.2 in garnet. Let Xq = (Fe/Mg)gt/ (Fe/Mg)oi = 3, >Fe-Mg,oi = 10 ° mm+s, and Dpe-Mg,gt =... 

Forstente. Pure forsterite is rare in nature. Most natural magnesium orthosilicates form solid solutions of fayalite, Fe2Si04, and forsterite. Forsterite refractories are usually made by calcining magnesium silicate rock such as dunite, serpentine, or olivine with sufficient magnesia added to convert all excess silica to forsterite and all sesquioxides to magnesia spinels. 

Olivine - solid solution of Mg2Si04 (forsterite) and Fe2Si04 (fayalite). 

This oxidation of iron is of importance for carbonation of magnesium oxide-based silicates for CCS, using for example olivine which a mixture of Mg2Si04 (forsterite) and Fe2Si04 (fayalite). In other similar situations, the oxidation of Fe or Mn ions can limit the reaction with (dissolved) C02. 

Compositional variations of the olivine spectra across the forsterite-fayalite series (fig. 5.10) indicate ranges of A0 and CFSE of Fe2+ ions to be... 

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