Pyroxene group, silicates

Spectra of chain silicates Pyroxene group 5.5.1 Background... 

The pyroxene group and the amphibole group, respectively, are representatives of silicate minerals having single-chain and double-chain tetrahedral networks. Pyroxenes are believed to be significant components of Earth s mantle, whereas amphiboles are dark-colored minerals commonly found in continental rocks. 

The ionsilicates (chain silicates) are comprised of either single or double chains of linked silica tetrahedrons. The single chains (pyroxene group) have the formula and consist of a chain of tetrahedrons that are linked together by two of their corner oxygens. The double chains (amphibole group) have the formula... 

Iron group Silicates group Pyroxene group Augite Green earth Clark et al. (1969) Rutley (1988)... 

Pyroxene group (Chaiti silicates) Aegirine Augite Diopside Enstatite (Wollastonite). 

Pyroxene group of important rock-forming silicate minerals found in many igneous and metamorphic rocks presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass. 

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. 

Present M sbauer Studies of Natural Pyroxenes and Olivines. Table IX gives the major element chemical compositions of the silicate minerals examined in this study. Table X compares the Mossbauer parameters of these minerals, while Figures 9-13 show representative Mossbauer spectra. Fayalite (Figure 9) is the only olivine in this group. The two lines are, however, somewhat broadened (0.35 and 0.39 mm./ sec.) compared with the width of natural iron foil lines observed with our source (0.24 mm./sec.) and suggest the near coincidence of two quadrupole-split doublets resulting from Mi and M2 sites. Analysis of this "two-line spectrum into a four-line spectrum in the manner described by Evans et al (11) could possibly yield parameters for the two iron sites, but this was not undertaken since both lines appear symmetric. The "two-line quadrupole splitting of 2.78 mm./sec. is somewhat smaller... 

Pyroxenes are poly silicates ( inosilicates in the mineralogical classification), crystallizing, respectively, in the monoclinic clinopyroxenes spatial groups C2lc, Pljlc, P2ln) and orthorhombic systems orthopyroxenes spatial groups... 

The pyroxenes are chemically complex but common rock-forming minerals. They resemble the amphiboles in many ways, but are actually single-chain silicates. The tetrahedral basic unit of the pyroxenes, [(Al,Si)20g] , was schematically depicted in Fig. 2.1C. The general formula for the group is Ai (B, C)i+ TjOfi, where A = Ca" Fe Li+, Mg "", Na-" B = Mg ", Fe Mn" Sc+ C = Fe A Cr" Ti " and T = Si Al+l Within the group are several mineral series and several species that often occur in acicular or fibrous forms. One species that occurs in fibrous form is jadeite, [Na(Al,Fe )Si206], a relatively familiar name because of the popularity of this material with Oriental sculptors. 

In this case, it is not necessary to overcome an energy barrier for nucleation to occur. Decomposition into two phases occurs without involving the process of nucleation. Separation into two phases in this type of system is called spinodal decomposition, and is seen among silicate minerals such as the pyroxene and feldspar groups. Generally, the boundaries between the two phases are wavy rather than flat, but the two phases are in a specific crystallographic relation. 

Stony irons are nonchondritic meteorites that contain roughly equal proportions of silicate minerals and metal. Two types of stony irons - pallasites and mesosiderites - are distinguished. Pallasites consist of approximately equal amounts of metal and olivine (one small group contains pyroxene as well). Mesosiderates also have approximately equal proportions of metal and silicate, but the silicate fraction is basalt. 

Mesosiderites are a highly enigmatic group of differentiated meteorites. They are breccias composed of iron-nickel metal and silicate in roughly equal proportions. The metal represents molten material from the deep interior of an asteroid, whereas the silicate fraction consists of basalts and pyroxene cumulates similar to HED meteorites that formed near the surface. It is difficult to construct models that allow mixing of such diverse materials, but these disparate materials are generally thought to have been violently mixed by impact. 

Pyroxenes e.g., Mg2[Si2Ofi] = 2MgSi03, enstatite) and amphiboles (double chain silicates containing OH groups) form chains of Si04 tetrahedra. By interlinking the chains we arrive at layer silicates. 

Silicates. This mineral classification encompasses the largest group of mineral species and includes most of the important rock-forming minerals, such as the feldspars, feldspathoids, pyroxenes, amphiboles. 

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