Olivine weathering

Another useful concept is that of congruent and incongruent reactions. These terms describe reactions involving the dissolution of minerals. If all the products of a dissolution reaction are soluble, the reaction is called congruent, as in the case of the quartz dissolution reaction (1.6) described above. Because, as written, the olivine weathering reaction leads to quartz precipitation it is an incongruent reaction. 

The sharing of tetrahedral oxygens in other silicates varies from the maximum sharing in quartz and feldspar to the complete independence of the tetrahedra in olivine. Olivine weathers very rapidly when exposed to water and air. Pyroxenes... 

The end members of the olivine group, the structure of which consists of individual Si04 tetrahedra linked by divalent ions in sixfold coordination, are forsterite (Mg2Si04) and fayalite (Fe2Si04). Those commonly found in soils are usually intermediate in composition and partially altered because of the ease with which olivine weathers. The structure is orthohombic. 

Silicates also lead to uptake of CO2. Weathering of a non-aluminum silicate like Mg-olivine may be written... 

The collection of spectra obtained at Spirit s landing site reveals various miner-alogical signs of weathering. Spectra obtained on the basaltic rocks and soil on the plains show mainly olivine and pyroxene and small amounts of nonstoichio-metric magnetite, and only comparably small amounts of weathering. An example... 

The spectra of the rocks in the plains are very similar to the spectra obtained on the soil (see above). The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater. 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

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. 

Schott, J., and R. A. Berner (1985), "Dissolution Mechanism of Pyroxenes and Olivines During Weathering", in J.l. Drever, Ed, The Chemistry of Weathering, NATO ASI SERIES C 149, 35-53. 

Molar chemical diagram showing different weathering trends for terrestrial basalts (small black circles) and Martian basalts (gray circles), resulting from differences in acidity of fluids. Experiments (large black circles) demonstrate that olivine is leached under acid conditions. Modified from Hurowitz ef al. (2006). 

The Hostrock and Backfill Material. Most crystalline igneous rocks, including granite and gneiss, are composed of a comparatively small number of rock forming silicate minerals like quartz, feldspars (albite, microcline, anorthite etc.) micas (biotite, muscovite) and sometimes pyroxenes, amphiboles, olivine and others. Besides, there is a rather limited number of common accessory minerals like magnetite, hematite, pyrite, fluorite, apatite, cal cite and others. Moreover, the weathering and alteration products (clay minerals etc.) from these major constituents of the rock would be present, especially on water exposed surfaces in cracks and fissures. 

Schott, J., and Berner, R. A. (1985). Dissolution mechanisms of pyroxenes and olivines during weathering. In The Chemistry of Weathering (J. I. Drever, ed.), pp. 35-53. Reidel Publ., Dordrecht, The Netherlands. 

Brindley (1951) hard greenish-gray clay derived from alteration of olivine-basalt by sub-aerial weathering, Ayrshire coal fields, area III, England analyst B.E. Dixon. 

Mars 1.5 78.3 6,796 280 0.01 Mafic silicates in daik regions, pyroxenes detected locally, olivine suspected Fe3t phases in bright regions probably include nanophase hematite and sulphate phases these and clay silicates in global dust are oxidative weathering products HjO (frost) condensates ice at north pole dry ice (C02) at south pole. 

In Table 3, susceptibility to weathering increases down the list as fewer silicon-oxygen bonds need to be broken to release silicate. Consequently, quartz and feldspars especially, but also mica in temperate soils, are common inherited minerals in the coarse particle size fractions of soil (the silt and sand fractions, 0.002-2 mm). The amphiboles, pyroxenes, and olivine are much more easily weathered. Thus, soils derived from parent material with rock containing a predominance of framework silicates e.g. granite, sandstone) tend to be more sandy, while those derived from rocks containing the more easily weathered minerals tend to be more clayey. 

Fig. 15.3. Crustose lichen Lecidea lactea growing on dunite, a rock composed of the silicate olivine ((Mg, Fe)2Si04). Note dark stain (arrowed) below lichen representing amorphous silica coated in iron produced by lichen weathering (Purvis, 2000).

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). 

Iron(II) silicates are widely distributed in nature. Most of the iron in the primary magmatic rocks is in the form of Fe(II) silicates (olivines, pyroxenes, amphiboles, andbiotites). When exposed on the Earth s surface, they undergo oxidative hydrolytic weathering to give iron(III) hydroxyoxides such as goethite, the overall stoichiometry being described by equation (1). This form of iron is very insoluble, but subsequent microbial reduction to iron(II) makes it available to the biosphere. 

Venera 13 -7.55 303.69 0.8 0.3 Navka Planitia at E. end of Phoebe Regio rise, landscape similar to Venera 10 site, mafic aUcahne rocks such as weathered olivine leucitite, nephelinite, volume density = 1.4-1.5 g cm from impact loading XRF, redox expt., TV imaging, impact load... 

The Mg is also another measure of depletion, but is more susceptible to modification by secondary metasomatic processes in the mantle, or to alteration by serpentinization and/or marine weathering. Olivine Mg is sometimes used to avoid problems of grain-boundary... 

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