ABSTRACT

Extensive diagenetic changes occurring in peat during burial to approximately 1.5 km may result in the formation of lignite (or medium brown coal). Examination of available literature indicates very little research dealing specifically with the behavior of inorganic (non-carbon-based) compounds during this early postburial diagenesis. Because these inorganic constituents determine both the quantity and quality of ash in coals and may act as potential diagenetic agents in associated rocks, their behavior throughout the peat to lignite transformation is potentially important in several areas of study. Laboratory simulations of the early postburial diagenesis of peat offer a new approach for the study of such processes.

In the present research, laboratory simulations have been used to study diagenesis of several well-characterized peats. During the experimental runs, approximately 80 g of peat were compressed and heated in a PTFE-lined reaction cell from atmospheric conditions to 2100 psi and 60°C, thus approximately simulating conditions necessary for the formation of lignite. Pressure and temperature were increased on a stepwise pattern, and prior to each increase, expelled solution from the previous interval was collected. Following completion of each experimental run, the residual solid plug was extracted from the cell intact.

Extensive reorganization of organic compounds in the produced residual solids during laboratory simulations was documented by Rollins et al. (1991). In addition, extensive petrographic changes can be observed, including increases in gelatinous material, darkening of colors, and increases in reflectance.

Concentrations of major inorganic constituents (Na, K, Ca, and Mg) released in solution are typically constant, or decrease, with increasing pressure and temperature for high-ash peats; whereas, minor inorganic concentrations (Fe, Cu, Zn, Mn, and Al) increase, occasionally reaching a maximum and then decreasing. With low-ash peats, concentrations generally are much lower than high-ash peats, and both major and minor inorganic concentrations typically increase throughout the experimental runs. For all peats, the ratio of major to minor inorganics decreases. The general behavior of inorganics during the diagenesis of these peats is strongly related to the composition of the original peat and to the stage of alteration. In addition, organic acids, including acetic and others, are present within the solutions. These organic acids may act as organic complexors and thus contribute to the mobilization of inorganic constituents.