Silicates pyroxenes

Cameron M. and Papike 1 1 (1982). Crystal chemistry of silicate pyroxenes. In Reviews in Mineralogy, vol. 7, 2d ed., P. H. Ribbe (series ed.), Mineralogical Society of America. 

Because Li isotopes may be used as a tracer to identify the existence of recycled material in the mantle, systematic studies of arc lavas have been undertaken (Morignti and Nakamura 1998 Tomascak et al. 2000 Leeman et al. 2004 and others). However, most arc lavas have 5 Li values that are indistinguishable from those of MORB. Thus Li seems to be decoupled from other fluid mobile elements, becanse Li can partition into Mg-silicates (pyroxene, olivine) in the mantle (Tomascak et al. 2002). 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Spectra of chain silicates Pyroxene group 5.5.1 Background... 

Examples of both types are shown in Figs. 13, 14, and 15. Clearly the correlation of MnO with FeO, found in the lunar samples by Laul et al. 100 is due to the fact that Mn++ (R = 0.80 A) can easily replace Fe++ (R = 0.74 A) in the two most abundant Mg, Fe silicates, pyroxene and olivine. The correlation of the LIL elements (large-ion lithophile elements) first observed in KREEP is of the second type. 

I) that these are called the ordinary chondrites these constitute 35%, 38% and 8%, respectively, i.e. >80% of all falls Obviously, chondrite compositions with elements apportioned by chemical form are not continuous but, rather, quantized. Elsewhere (/) I list major element ratios diagnostic of specific chondritic groups. The total iron in some enstatite (E) chondrites - i.e. those in which the ferromagnesian silicate pyroxene (and the other low-Ca one, olivine) is Fe- and Ca-free MgSiOs - exceeds that in the H group of ordinary chondrites, denoting them as EH chondrites the EL chondrite designation is self-evident. 

A polymeric anion formed by sharing two oxygen atoms of the silicate anion results in a chain structure - chain silicates. Pyroxene minerals like diopside, enstatite, jadeite are examples. 

C Crystallization of amorphous silica to cristobalite, formation of corundum, spinels, mullite, Ca silicates, pyroxenes,... 

The resulting Mg(II) ion (shown as Mg SiO ) and Si(OH) are relatively soluble in water and can be washed away. The iron oxide Fe Oj colors the clay brown. Other silicates, pyroxene and others, are less soluble and more resistant to erosion, but will be weathered slowly. The only exception is quartz it may be pulverized mechanically (to become sand), but is resistant to chemical weathering. 

Brantley, S. L. and Chen, Y. (1995). Chemical weathering rates of pyroxenes and amphiboles. In Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America Washington, EXT, Reviews in Mineralogy 31,119-172. 

The chondrules contained in the chondrites contain olivine, pyroxene, plagiok-lase, troilite and nickel-iron they can make up 40-90% of the chondrites. Chondrules are silicate spheroids, fused drops from the primeval solar nebula. Because of their differing constitution, chondrites are further subdivided one group in particular is important for the question of the origin of life, and has thus been intensively studied—that of the carbonaceous chondrites. 

In studies of amphiboles (44), isolated strips of triplechain silicates were discovered embedded in the double-chain parent structure. It was later realized that new types of silicate structures, composed of recurrent triple chains, existed in nature. The part that HREM played in the identification of this new family of triple-chain silicates, which constitute a further step in the progression pyroxene, amphibole,. .. mica, was crucial. 

Figure 4.4 Infinite chain silicates (single, double, and sheet) (a) infinite single chain silicate with two corners shared per tetrahedron (pyroxene structure) (b) infinite double chain, with alternate two and three corners shared (am-phibole structure) (c) infinite sheet structure, with each tetrahedron sharing three corners (sheet silicates). (From Putnis, 1992 Figure 6.3, by permission of Cambridge University Press.)...

Figure 4.5 Structure of pyroxene minerals (a) demonstration of the end view of the single silicate chain (b) end view of the stacking arrangement of single chains, showing the position of the metal cations. There are two different cationic environments, Ml and M2. (After Putnis, 1992 Figure 6.11, by permission of Cambridge University Press.)...

Similarly to Mn(IV)- and Fe(III)-oxides, some primary minerals were shown to promote polymerization of hydroquinone (19). Olivines, pyroxenes, and amphiboles accelerated the polymerization reaction to a greater extent than micas and feldspars. Microcline and quartz were ineffective- The effect was greatest for tephroite, a manganese-bearing silicate with the ideal chemical formula M SiO. Fayalite, the corresponding Fe(II) analog (Fe2Si0 ), was effective, but to a lesser extent. 

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