Olivine forsterite

LL3.0 Semarkona Smectite, fayalitic olivine, forsterite, enstatite, calcite, magnetite 10... 

In deeper systems dominated by calcium-rich saline fluids, it has been shown that both solubility constraints and silicate reactions act to further remove bicarbonate ions as precipitated calcium and magnesium carbonates, often adjusting pH to levels greater than 9 (Barnes and O Neil, 1971 Fritz et al., 1987a Clauer et al., 1989). For example, during closed-system dissolution of magnesium olivine (forsterite), a major component of many ultramafic rocks, as the silicate water reaction proceeds water breaks down, H" " ions are consumed, carbonates precipitate, and hydroxyl ions force the pH to rise (Barnes and O Neil, 1971 Drever, 1988). 

Acid hydrolysis of a simple silicate, for example the magnesium-rich olivine, forsterite, is summarized by ... 

Figure 7.1 Goldich s sequence of increasing weatherability of common minerals (cf. Loughnan 1969 Faure 1991). In parentheses are the lifetimes in years from Table 7.1, assuming olivine = forsterite, augite = diopside, hornblende = tremolite, Ca-plagioclase = anorthite, Na-plagioclase = albite, K-feldspar = microcline, and the stability of muscovite is comparable to that of the related clay, illite.

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... 

Grandstaff, D. E. (1986), The Dissolution Rate of Forsteritic Olivine from Hawaiian Beach Sand", in S. M. Colman and D. P. Dethier, Eds., Rates of Chemical Weathering of Rocks and Minerals, Academic Press, pp. 41 -59. 

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. 

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. 

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

Figure 1,17 Absorption spectrum of a forsteritic olivine under polarized light. Ordinate axis represents optical density (relative absorption intensity, ///q). From R. G. Burns (1970), Mineralogical Applications of Crystal Field Theory. Reprinted with the permission of Cambridge University Press.

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 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... 

Reaction 6. This reaction was investigated in detail by Finnerty and Boyd (1978) and was more recently recahbrated by Kohler and Brey (1990). Because the Ca content in olivine is R-dependent, due essentially to a large solvus between monticellite and forsterite that expands with pressure (cf. section 5.2.5), this reaction should act as a sensitive barometric function. However, the enthalpy of reaction is quite high and the effect of T on the equilibrium is also marked. The calibrated P-T slope has an inflection, the origin of which is not clear at first glance. 

Grandstalf D. E. (1980). The dissolution rate of forsterite olivine from Hawaiian beach sand. In Third International Symposium on Water-Rock Interaction Proceedings, Alberta Research Council, Edmonton. 

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

Other general cases in binary systems are referred to as interdiffusion or binary diffusion. For example, Fe-Mg diffusion between two olivine crystals of different Xpo (mole fraction of forsterite Mg2Si04) is called Fe-Mg interdiffusion. Inter-diffusivity often varies across the profile because there are major concentration changes, and diffusivity usually depends on composition. 

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