TODOROKITE, GOETHITE, AND HEMATITE

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Abstract
We investigated alteration products of siderite in east Texas lignite overburden sediments using x-ray diffraction and optical and scanning electron microscopy. Siderite commonly occurs in these sediments as lenticular masses and nodules of various sizes. When exposed to oxidizing conditions, the siderite alters to either hematite or goethite. Alteration of siderite to hematite probably occurs before the siderite is exposed to surface weathering conditions. The moist weathering environment characteristic of strip-mined areas in east Texas favors transformation of the siderite to goethite through dissolution and reprecipitation reactions. Manganese that substitutes for Fe2+ in the siderite structure is also released upon weathering and precipitates as a very thin, dark coating of an Mn-oxide mineral (todorokite) on surfaces of partially weathered siderite concretions. The morphology of the todorokite varies from tabular or platy particles (< 5 μm long) consisting of twinned individual todorokite fibers to what appears to be a nonfibrous phase. Weathering of siderite may release potentially toxic levels of Mn and possibly other undesirable ions that may have drastic effects on reclamation of the strip-mined lands in east Texas.

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