Abstract

Petrographie examination of 96 thin sections of least oxidized, silicate-rich samples of the thin-bedded facies of the Trommald iron-formation in the Cuyuna district, Minnesota, showed the following minerals in order of decreasing abundance: Mn-Mg-siderite, stilpno-melane, minnesotaite, magnetite, quartz, two unidentified iron silicates, goethite, hematite, pyrite, amphibole and arsenopyrite. Much coarse nematoblastic minnesotaite is oriented along secondary rock cleavage traces. Reconstitution and recrystallization of minerals along certain beds is attested by the wavy or scalloped structures that have destroyed primary layering.

Three samples of stilpnomelane and one of minnesotaite were concentrated from frac-ture-fillings in iron-formation and subjected to chemical, optical, spectrographic, x-ray diffraction and differential thermal analyses. Stilpnomelane samples gave a distinct ammoniacal odor when heated at 70° C, and chemical analysis showed as much as 0.22 per cent ammonia. It is suggested that ammonia, from volcanic activity that produced lava flow's and tuffs which overlie the iron-formation, entered the stilpnomelane structure as ammonium ions.

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