Rattling iron concretions from the Waikato Coal Measures

Abstract

Abstract Two iron-rich concretions from the Waikato Coal Measures are described. Each has a hard iron oxide shell with a weathered outer surface. In one case there is a solid siderite-rich interior; in the other the shell encloses a void which contains a clay rattle. Concentrations of Fe, AI, Ca, K, Si, P, S, Mn, Ti, Mo, Mg, and Cu, in various parts of the concretions have been measured and the dominant minerals in these parts have been determined. The concretions are considered to represent different stages in a process which begins when iron mobilised by decomposing organic matter is pre cipitated under reducing conditions as siderite in sediments. Subsequent oxidising conditions oroduce a series of “boxes” each with a dense ferric oxide shell. The shell thickens as the enclosed ferrous ions are oxidised and eventually a void is created within the concretion. The kaolin rattle represents the weathered remnant of the host sediment.