Fig. 4.  (a, b) Molar compositions of individual analyses of maskelynite in ALH 84001. (c) Plot of percent of deviations from plagioclase stoichiometry for Al vs. Si in ALH 84001 maskelynite. Large positive deviations in Si from stoichiometry suggest that some maskelynite is a mixture of feldspar and quartz/tridymite. The length of the arrow represents the effect of 25% Na loss on a stoichiometric plagioclase with initial An-Ab-Or composition equal to that of the average maskelynite.