Olivine mineral residues

Experimental studies of mineral weathering rates in the presence of oxalic acid demonstrate the importance of LPD. For example, in the presence of 1 mM oxalic acid, rates of silica elution from feldspar can increase up to 15-fold at circumneutral pH, while A1 elution rates can increase by two orders of magnitude (Barker et al, 1997). Similar results are reported for quartz and olivine (Grandstaff, 1986 Bennett et al, 1988), and indicate that oxalate leaching of aluminium, calcium, magnesium and other cations from primary silicate minerals can yield a silica-rich residue similar to that found in association with endolithic lichens (Johnston Vestal, 1993 Lee Parsons, 1999). 

Fig. 5.9. Electron density distributions in olivines (a) Experimental difference density map of part of the forsterite structure showing the residual peaks around Si. Contours are at intervals of 0.1 electrons A negative contours being broken and zero contours dotted. Numbers in decimal fractions of the a length indicate the heights of the atoms. The tetrahedron formed by oxygen atoms around Si is shown (after Fujino et al., 1981 reproduced with the publisher s permission), (b) A comparison of a theoretical difference density map (i) of the O-Si-O group in the monosilicic acid molecule [Si(OH)4] with an experimental map (ii) of the same group in the monosilicate mineral andalusite (AljSiO,). Contours are at intervals of 0.07 electrons A in (ii). The region around the nucleus of each atom in the theoretical map represents the core region, where the data are not expected to be accurate (after Gibbs, 1982 reproduced with the publisher s permission).

These studies highlight the importance of finding other ways to more systematically evaluate whole-rock peridotite Re-Os isotope model ages. Ideally, Re-Os dating studies on peridotites should be carried out on primary sulphides included in minerals such as olivine. However, as already indicated, most cratonic peridotites are residues of large degrees of melting. The removal... 

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

There is not much resistance to weathering in these minerals because of the relative lack of Si—O—Si bonding, especially in island silicates such as olivine. Layer silicate minerals rich in Mg (e.g., trioctahedral smectites, chlorite, serpentine) may form from the siliceous residue if leaching does not deplete in the weathering zone. 

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