Detailed observations of the alteration products of sulfide oxidation in black shale at Ottawa, Canada, are presented using photographs taken at magnifications from times 1 to times 7000. X-ray diffraction analyses of the unaltered rock constituents and the alteration products, supplemented by X-ray fluorescence analyses are also presented.Growth of gypsum crystals imbedded in the shale appears to be responsible for the heaving reported for two buildings in Ottawa. Jarosite and natrojarosite alteration products, probably produced by bacterial action, fill open cavities and cracks in the rock. Jarosite commonly coats gypsum crystals imbedded in the shale and although probably critical to the overall alteration process may be of secondary importance in generation of heave pressures.Marcasite mineralization (probably hydrothermal) in a fault zone, plus the abundance of oxygen in the fractured rock, may play vital roles in the alteration.

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