Chondrules textures

Bremervorde (4) and Barwell (5 (Figs. 6c and 6d). Bremervorde is a breccia of type 3.8 and type 4 (14) our section is entirely type 4 material. Chondrule mesostases luminesce brightly, often highlighting chondrule textures. The non-chondrule silicate portions of the meteorites appear to consist of fragmental material similar to the chondrules in its CL colors and intensities clearly any interstitial... 

Figure 18). Chondrule textures are consistent with condensation of melts. 

Chondrules exhibit a bewildering variety of compositions and textures (F ig. 6.1 a,b). Most are composed primarily of olivine and/or pyroxene, commonly with some glass. (For a crash course in mineral names and compositions, see Box 6.1.) If melt solidifies so quickly that its atoms cannot organize into crystalline minerals, it quenches into glass. Iron-nickel metal and iron sulfide occur in many chondrules, often clustered near the peripheries. The textures of... 

Chondrule-matrix integration No chondrules Sharp chondrule boundaries Some chondrules can be discerned, fewer sharp edges Chondrules poorly delineated Primary textures destroyed... 

Primitive achondrites exhibit metamorphic textures, as appropriate for the solid residues from which melts were extracted. In effect, these meteorites represent an extension of the highly metamorphosed type 6 chondrites, which show no eutectic melting of metal and sulfide. In a few cases, some primitive achondrites have recognizable chondritic textures, but often they are so thoroughly recrystallized that chondrules are not identifiable. 

The acapulcoites (Fig. 6.7a) have recrystallized textures, but a few relict chondrules have been reported (McCoy et al., 1997a). The relative proportions of the minerals (olivine, orthopyroxene, diopside, plagioclase, metal) in acapulcoites are similar to those in ordinary chondrites, and the abundances of the major elements are nearly chondritic. However,... 

Figure 8.5 Different textural types of chondrules and a Type-B CAI shown in backscatter electron microscope images samples from A through D suffered increasing levels of heating (A) agglomeratic chondrules (scalebar 100 pm) (B) Type-IA (scalebar 100 pm) (C) Type-IIA (scalebar 100 pm) (D) compound barred chondrule set (scalebar 100 pm) (E) layered chondrules with fine grains (scalebar 500 pm) (F) a Type-B CAI (scalebar 1mm).

Chondrules display a wide variety of compositions and textural types (see Fig. 8.5). The volume fraction occupied by chondrules in chondritic meteorites ranges from 85% (ordinary chondrites) down to 0% (Cl chondrites). Without question the most abundant type of chondrule is dominated by ferromagnesian (Fe, Mg-rich) silicates (Figs. 8.4 and 8.5 Lauretta et al. 2006). Ferromagnesian chondrules are primarily composed of olivine, pyroxene (minor Ca-rich pyroxene), glass, spinels, Fe, Ni-rich metal, FeS, and other minor phases (< 1 vol%). These are subdivided into FeO-poor and FeO-rich, or Type-I and Type-II chondrules (Brearley Jones 1998 Jones et al. 2000, and references therein Lauretta et al. 2006, and references therein). The division is delineated by the Mg-number [or Mg defined as 100 x Mg/(Mg + Fe)] of the olivine and low-Ca pyroxene > 90 is FeO-poor, and < 90 is FeO-rich, which roughly translates in a bulk difference of 10 wt% FeO < 10 wt% = FeO-poor and >10 wt% = FeO-rich. 

Chondrules. Chondrules are major constituents of chondrites, which are millimeter- to submillimeter-sized spherules consisting of silicate phenocrysts (relatively large crystals), glassy mesostasis, and a small fraction of opaque phases (Lauretta et al. 2006 and references therein Chapter 8). Their textural, mineralog-ical, and chemical features suggest that chondrules formed from dust aggregates that were melted by localized transient high-temperature events and were cooled relatively rapidly. 

The most commonly used classification scheme divides chondrules into two broad categories Type I and Type II, based on bulk FeO contents. Type-I chondrules are characterized by the presence of FeO-poor olivine and pyroxene (Fo and En > 90). Type-II chondrules contain FeO-rich olivine and pyroxene (Fo and En < 90). The textural characteristics of both the Type-I and Type-II series are gradational. These two main categories are further subdivided into Subtypes A and B based on their abundances of olivine and pyroxene. Type-IA and IIA chondrules contain abundant olivine (> 80 vol%), Type-IB and IIB chondrules contain abundant pyroxene (> 80 vol%), and chondrules with intermediate abundances of olivine and pyroxene are classified as Type IAB or IIAB. 

Perhaps the most well-known, unsolved problem in cosmochemistry is the question of the mechanism whereby dust grain aggregates were thermally processed to form chondrules and some rounded refractory inclusions. Chondrule compositions and textures require rapid heating and somewhat slower cooling for their explanation a globally hot nebula is inconsistent with these requirements... 

Figure 9 Combined elemental maps of (a) the C03.1 carbonaceous chondrite Kainsaz and (b) ungrouped CO/CM-like carbonaceous chondrite Acfer 094. Kainsaz contains abundant small chondrules, CAIs, and AOAs. Acfer 094 is texturally and mineralogically similar to CO chondrites, but contains higher abundance of matrix. AOA = amoeboid ohvine aggregate BO = barred olivine chondrule PO(P)i n = type I (II) porphyritic olivine (pyroxene) chondrule.

CV (Vigarano-like) and ungrouped CV-like chondrites. The CV chondrites (Figure 12) are characterized by (i) millimeter-sized chondrules with mostly porhyritic textures, most of which are magnesium rich and —50% of which are surrounded by coarse-grained igneous rims ... 

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