Melilite

The outer 1.5 mm is a mantle of polycrystalline melilite whose Mg content increases radially inwards, as does the abundance of included spinel. The interior of the inclusion is predominantly coarsely crystalline Ti-Al-pyroxene, melilite, and anorthite, all containing euhedral crystals of spinel. The entire inclusion is bounded by a fine-grained rim of complex mineralogy. The inclusion is 1.5 cm in diameter. 

Data from spinel (A) and melilite (A) define a good linear correlation between excess 2eMg and 27Alf24Mg ratios with slope roughly one-half that of the Bl isochron. Data from hibonite ( ], however, plot well above the isochron, suggesting either early formation of hibonite or heterogeneity in 26All27Al. 

The data from 3529-45 can also be interpreted as reflecting an initially heterogeneous distribution of 26A1. Melilite and hibonite may have formed from different reservoirs with enequal 26A1 contents in which case the different (26A1/27A1)Q ratios for... 

The Type A melilite data also reinforce earlier arguments about the absence of any connection between mineralogical alteration and Mg isotopic behavior. Melilites in 3529-45 are extensively altered, mainly to grossular and Na-rich plagioclase, yet show much less evidence for a disturbed Mg isotopic composition than relatively pristine melilites from B1 inclusions. B1 melilites exhibit much larger deviations from the standard isochron than anorthites (see also [1]). 

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Figure 3-30 An example of obtaining diffusivity (a) from mass exchange data with spheres using Equation 3-125 (Gas/melilite oxygen isotope exchange data of Hayashi and Muehlenbachs (1986)) and (b) using mass loss data from a single thin wafer using Equation 3-126 (garnet dehydration data of Wang et al. (1996)).

Hayashi T. and Muehlenbachs K. (1986) Rapid oxygen diffusion in melilite and its relevance to meteorites. Geochim. Cosmochim. Acta 50, 585-591. 

Morioka M. and Nagasawa H. (1991) Diffusion in single crystals of melilite, 11 cations. Geochim. Cosmochim. Acta 55, 751-759. 

Fort Union lignite is low in S (<0.5 wt% S03) and it forms Class-C fly ash that contains Ca-and/or Mg-bearing phases such as lime, anhydrite, C3A, periclase, melilite and merwinite. As Ca and Mg concentrations in the coal increase, so too does the amount of Ca- and Mg-bearing minerals in the fly ash. At the lower range of CaO concentrations (15 wt%), only a small percentage of the total CaO is... 

Fig. 9. Si/Ca ratio of the chemical bulk composition of BA in comparison to the amount of melilite measured by XRD.

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