STONE COAL IN SOUTHERN SHAANXI PROVINCE, CHINA: MINERALOGY OF ENVIRONMENTALLY HAZARDOUS ELEMENTS

Organic-carbon rich black shale, locally known as stone coal (shíméi) from southern Shaanxi Province, People’s Republic of China has been examined to address environmental problems associated with its use and weathering in place. These organic-carbon rich rocks (typically > 15% TOC) are black shale that formed in anoxic to euxinic marine environments and are used for domestic combustion and in small industry. We have focused on the mineralogy and mode of occurrence of elements hazardous to human health. Mineral phases observed in the stone coals were identified and quantified using scanning electron microscopy, energy-dispersive analysis and electron microprobe wavelength-dispersive spectroscopy. We identified tetrahedrite-tennanite [Cu₁₀(Fe,Zn)₂(Sb,As)₄S₁₃], millerite [NiS], pentlandite [(Fe,Ni)₉S₈], sphalerite [ZnS], clausthalite [PbSe], galena [PbS], greenockite (or hawleyite) [CdS], pyrite [FeS₂], brannerite [(U,Ca,Y,Ce)(Ti,Fe)₂O₆] and uranium oxide (hydrated?) as hosts for environmentally hazardous elements; the host for fluorine also is being investigated.Reserves of stone coals are very large (over 1 billion tons) and play a significant role in the local energy budget of rural communities. Adverse health conditions resulting from stone coal combustion, disposal of ash, residual soil from weathering in place, and groundwater contamination have seriously affected some populations. Although, the distribution of hazardous elements in the stone coals can be highly variable, they present three main environmental problems; (1) the residual soils are enriched in the hazardous elements contained in the stone coals and, depending on their element speciation and bioavailability, so are the crops grown on these soils; (2) the weathering of the stone coal contaminates the local ground water and/or surface waters with heavy metals; and (3) local villagers and farmers use stone coal as a source of fuel, which presents a number of additional problems, especially from indoor use. Disposal of the ash on agricultural lands or nearby water supplies can contaminate both. Our studies may be used by local and Provincial public health officials to help assess the hazards of stone-coal use and to develop plans for mitigation.