Fe-Cr garnet

Table 5.25 Parameters of the Kieflfer model for calculation of Cy and harmonic entropy for the 12 garnet end-members in the system (Mg, Fe, Ca, Mn)3(Al, Fe, Cr)2Si30i2. is the lower cutoff...

Carswell D. A., Harvey M. A., and Al-Samman A. (1983) The petrogenesis of contrasting Fe-Ti and Mg-Cr garnet peridotite types in the high grade gneiss complex of western Norway. Bull. Mineral. 106, 727-750. 

The CV, CO, and CR chondrites are mostly anhydrous and were considered in Chapter 11. However, the CV3oxB chondrites experienced significant aqueous alteration. The matrices of these meteorites are heavily altered and contain phyllosilicates, fayalite, Fe,Ni sulfides and carbides, Ca,Fe pyroxene, and andradite garnet. The CVoxA chondrites were apparently also aqueously altered, but were subsequently dehydrated by thermal metamorphism. The matrices of CR2 chondrites contain alteration minerals that resemble those in Cl chondrites, including phyllosilicates, magnetite (Fig. 12.15d), carbonates and sulfides, although the alteration is not as extensive. Chondrule mesostasis was affected in some CR chondrites. Minor phyllosilicates occur in the matrix of chondrites, but these meteorites contain no carbonates or sulfates. 

Cr-poor variety widespread, locally abundant (e.g.. Monastery). Garnets, clino- and orthopyroxenes, phlogopite and ihnenite most common, zircon and olivine rarer. Debatable whether phlogopite and olivine are members of Cr-poor suite. Wide range in chemistry but Cr-poor, Fe-Ti-rich relative to type I (low-Z) peridotite minerals. Mineral chemistry and estimated equilibration P/Ts overlap those of type V (high-Z) Iherzolites. Some Slave craton Cr-poor megacrysts show mineral chemistry links to type II megacrystalline pyroxenite xenoliths. See review of Schulze (1987). 

The AI2O3 content of clinopyroxene varies between 1 wt.% and 7 wt.% and shows a positive correlation with both T of equilibration and the bulk composition. Up to 3 wt.% Cr203 is observed in xenolith clinopyroxenes. The molar Cr/Al in clinopyroxene is nearly identical to that of the bulk rock and correlates positively with the level of depletion. Up to 35% of all iron in clinopyroxene is Fe and the partition of this cation between clinopyroxene and garnet may vary with T or/o (Luth and Canil, 1993 Canil and O Neill, 1996). 

Figure 29 X-ray maps of Mn, Ca, Fe/(Fe + Mg) ( FM ), Cr, and Sc in garnets from Fall Mountain, New Hampshire. White box indicates area of major element maps. Different generations of growth can be linked to prograde reaction history, to better understand mineralogical changes attending metamorphism and anatexis, and overall P-T evolution. Black line shows location of composition traverse plotted in Figure 30...

Garnet-structure ferrites. Garnet, ideally A3 B2[T04]3, is isometrie (Ia3d) with a straeture built of TO4 tetrahedra and BOe oetahedra linked by shared vertiees that form AOg dodeeahedra. The A-sites aeeommodate divalent (Ca, Mn, Fe) and trivalent (Y, REE) elements. The B-site may contain various di-, tri- (Fe, Al, Ga, Cr, Mn, In, Sc, Co), tetra- (Zr, Ti, Sn, Ru) and even pentavalent elements (Nb, Ta, Sb). The T-sites are filled with tetravalent (Si, Ge, Sn) but may be also occupied by trivalent (Al, Ga, Fe) and pentavalent (P, V, As) elements. Synthetic garnets form multicomponent solid solutions that provide opportunities for incorporating of HEW elements into the structure [149]. 

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