Olivine composition

Homogeneity of olivine compositions > 5% mean deviations <5% Homogeneous ... 

Silicates with olivine composition (MgxFe(i x))2Si04 are common in chondrites, comets, IDPs, and in protoplanetary disks. The Mg-rich end-member of the olivine family is forsterite, also often termed as Foioo the Fe-rich end-member is fayalite (Foo). The interstellar medium contains a similar concentration of the FeO- and MgO-rich silicates (see Chapter 2). Correspondingly, amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks, in cometary dust, and in chondritic IDPs. In stark contrast, crystalline dust is almost always dominated by Mg-rich grains in protoplanetary disks (e.g. Malfait et al. 1998 Bouwman etal. 2008), comet tails (e.g. Crovisier el al. 1997 Wooden et al 2004 Harker et al. 2005 Lisse et al. 2006), in the most primitive and least processed chondritic matrices, and IDPs (for a review, see Wooden et al. 2007). 

It has long been known that suites of MORE samples recovered from a relatively limited spatial area often display regular variations in major element oxides as a function of MgO content (Figure 6) or Mg (100 X molecular [Mg/(Fe -F Mg)]). In addition, the Mg s of most MORBs (mean Mg = 55 7.6 (la) 1.3 X lO" ) are not in equilibrium with presumed mantle olivine compositions (Mg 90 Roeder and Emslie, 1970 Langmuir and Hanson, 1981). These findings, combined with the results of experimental... 

Fig. 10. Histograms of Trd ages for on- and off-craton peridotites from southern Africa co-plotted with histograms of olivine mg-numbers. Kaapvaal low-L peridotites xenohths, n = 96. Data from Nixon et al. (1983), Walker et al. (1989), Pearson et al. (1995a), Carlson et al. (1999), Menzies et al. (1999) and Irvine et al. (2002). Lrd ages for peridotite xenoliths from kimberlites from East Griqualand, 170 km from the Lesotho on-craton kimberlites (Pearson et al. 1998) and for peridotites from the Farm Louwrencia kimberlite. Southern Namibia (Pearson et al. 1994 Pearson 1999a). It should be noted that no Archaean Trd ages are observed for off-craton peridotites. Olivine compositional data from Boyd Nixon (1979), Nixon (1987) and Boyd (unpubl. data).

Next, the MgO content is constrained from the observation that molar Mg/(Mg - - Fe ) (i.e., bulk-rock Mg ) ought not to be affected by modal segregation, since mineral Mg s are nearly the same in olivine and in pyroxenes (as shown by experimental phase equilibrium studies and empirical studies of coexisting phases in peridotites). Hence, (Mg )oi = (Mg )buik, to a very good approximation, in spinel Iherzolites. Extraction of partial melt causes an increase in Mg, so that the most primitive samples should have the lowest Mg. Histograms of olivine compositions in spinel Iherzolites from a wide variety of environments show that the most fertile samples that have not been grossly disturbed by metasomatism have Mg s from 0.888 to —0.896. Since the uncertainty with which olivine Mg s are determined in individual samples is typically —0.001, we interpret this distribution as implying an Mg for the primitive depleted mantle of 0.890, which value we adopt. Hence,... 

R.Dominko, M. Bele, M. Gaberscek,, M. Remskar, D. Hanzel, J.M. Goupil, S. Pejovnik, and J. Jamnik, Porous olivine composites systhesized by sol-gel technique. Journal of Power Sources, 153, 274-280, (2006). 

The authors did not have an electron microprobe to determine the compositions of the minerals after the experiments. The compositions were determined by X-ray diffraction, comparing lattice parameter measurements with those Ifom standards of known composition. For the pyroxenes, this was not sufficiently precise, so compositions were determined graphically by projecting tie Unes from known coexisting olivine compositions through the bulk composition to the pyroxene boundary. This explanation may not be clear if you have no experience with phase diagrams suffice it to say that the whole process is difficult and time-consuming. 

Dominko, R. et al. Porous olivine composites qmthesized by sol-gel technique./. Power Sources, 153,274 [2006]. 

Dominko R, Bele M, Gaberscek M, Remskar M, Hanzel D, GoupU JM, Pejovruk S, Jamnik J (2006) Porous olivine composites synthesized by sol-gel technique. J Powtn Some 153 274-280... 

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