S degrees 298 , Delta H degrees (sub f,298) , and Delta G degrees (sub f.298) of ferrosilite are 93.05+ or -0.3J/mol . K, -1194.872+ or -1.0kJ/mol, -1117.124+ or -1.0 kJ/mol; those of grunerite 721.8+ or -10.0 J/mol . K, -9648.304+ or - kJ/mol (minimum), and -8988.759+ or -20 kJ/mol (minimum). Phase relations for iron formations classified into three types: (I) olivine-free, (II) olivine- and quartz-bearing, and (III) olivine-bearing and quartz-free relations. Each type may or may not include grunerite; its presence is strongly influenced by the magnitude of fH 2 O.The formation of retrograde grunerite appears to be dependent upon fH 2 O rather than on the lowering of T.--Modified journal abstract.

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