Orthopyroxene-olivine ratios

In pro-grade metamorphic reactions, on the other hand, effects of intersite cation ordering at elevated temperatures could influence Fe/Mg ratios of orthopyroxene-olivine assemblages in granulite facies rocks. For the olivine-orthopyroxene exchange reaction... 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

The angrites are basaltic in composition, and critically silica undersaturated—all have normative olivine, Ca2Si04, and nepheline and lack orthopyroxene (Mittlefehldt et al., 2002). Angrites are also characterized by extreme depletions in moderately volatile elements (Figure 1). They have very low abundances of all the alkali elements, and have Ga/Al ratios lower than for any other achondrite, lunar sample, or martian meteorite... 

Pyroxene pallasites are represented by only two members Vermillion and Yamato 8451. The two meteorites share the common feature of containing pyroxene, but differ substantially from one another. Yamato 8451 consists of —60% ohvine, 35% metal, 2% pyroxene, and 1% troihte (Hiroi et al., 1993 Yanai and Kojima, 1995). In contrast, Vermillion contains around 14% olivine, and less than 1% each of orthopyroxene, chromite, and phosphates (Boesenberg et al., 2000). The iron concentration in olivine (Fogg 9o) is similar to main group pallasites, although pyroxene pallasites have lower Fe/Mn ratios (Mittlefehldt et al., 1998). The two pyroxene pallasites do not share a common metal composition with each other or, in detail, with other pallasites (Wasson et al., 1998). 

Normal mantle melting, such as that which leads to the formation of oceanic crust and oceanic islands, produces magma of basaltic to picritic composition and leaves a residue consisting of olivine, orthopyroxene, chnopyroxene and an aluminous phase. The compositions of minerals in this residue, and their relative proportions, are unlike those in old subcontinental hthosphere Mg-Fe ratios of the ferromagnesian minerals are too low, and the amount of chnopyroxene and spinel or garnet is too high. 

The oxidation of natural olivines (Mg,Fe)2Si04 has been studied as a function of the Mg Fe ratio.308 The results indicate that the reaction occurs in two parts the initial breakdown of the fayalite component, Fe2Si04, is followed by reaction of the Si02 so formed with the forsterite component Mg2Si04, to give magnetite and orthopyroxene [reactions (59)—(61)]. A compositional control on the oxidation process was also apparent thus,... 

In natural systems, similar effects may be observed. For example many of the olivine-rich lavas from Nuanetsi have almost identical CaO AI2O3 Na20 ratios, and therefore lie close to a plane in the ternary join with MgO and Si02. Melting experiments on these rocks (Cox and Jamieson, 197 ) have shown that increases in K2O content from approximately % to 3 cause an extension of the crystallization range of olivine by up to 130 0 before the entry of orthopyroxene. This corresponds to an increased primary phase volume of olivine relative to orthopyroxene with increasing K2O content (cf. Fig. 2). 

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