Transformation of siderite in the zone of hypergenesis

The structure of siderite aggregates and its weathering products from the Ruza deposit, Moscow Region, has been studied by X-ray phase analysis and electron microscopy. The initial siderite is a dense FeCO$_3$ aggregate of micron-sized grains. The nodules are covered with an ocher weathered crust, the color intensity of which varies from light yellow to brown-black. A successive replacement of siderite by FeOOH goethite and lepidocrocite was revealed. It is accompanied by a sharp decrease in volume ($\sim$30%) and the formation of a microporous structure.