Metal contamination of soils is widespread at many current and former industrial and military sites. Lead, As, Cr, and Cd are of particular concern because of their potential toxicity. In situ immobilization is one strategy for reducing bioavailability. Mineralogy is a key factor in the bioavailability of the contaminant metals. We sampled a Histosol in a peat bog that receives runoff and seepage water from the site of a former lead smelter. The water table is at or above the surface for most of the year and the site is covered with common reed (Phragmites australis) and cattail (Typha sp.). There is a redox gradient with depth that probably varies as the water table fluctuates. Along with major elements such as Fe and S, the soil contains 1,700 mg kg^-1 Pb, 2,200 mg kg^-1 As, 930 mg kg^-1 Cr, 210 mg kg^-1 Cd. We used a binocular microscope to select aggregates ~100 to 200 mm in diameter that had distinctive morphologies and appeared to be inorganic. We used the synchrotron x-ray microprobe on beamline X26A at the National Synchrotron Light Source at Brookhaven National Laboratory to obtain micro x-ray diffraction and x-ray fluorescence patterns of each aggregate. Preliminary results indicate that Pb is associated with akaganeite in the surface 0-10 cm. Gypsum (CaSO₄·2H₂O) is abundant, particularly for samples collected during drier periods, and goethite (FeOOH) is a common precipitate around roots. Several sulfide minerals have been identified in reduced horizons, including realgar (AsS), greigite (Fe²⁺Fe³⁺₂S₄), galena (PbS), and possibly sphalerite (Zn,Fe²⁺)S. Most of these minerals occur as almost pure phases in the submillimeter aggregates and appear to be secondary phases that have precipitated from solution. The close physical association of minerals that are stable under different redox conditions makes this soil a challenging material for in situ remediation.