want

A want is a natural hollow or depression that formed in a coal seam before overlying strata were deposited on top of it. Unlike a fracture or fault that breaks the continuity of rock, a want represents missing coal where the seam never developed or was eroded away before burial. The void was then filled by sandstone, mudstone, or other material that hardened in place, creating a permanent defect in the seam.

Wants occur most commonly in the upper portions of coal seams where the coal-forming environment was interrupted. This might happen when a river channel cut through a developing peat layer, or when coastal erosion removed part of an organic-rich deposit before subsidence and renewed sedimentation resumed. The overlying rock that fills a want is often coarser or of visibly different lithology than the surrounding strata, making the feature recognizable in mine sections and drill cores.

Detection and mining impact

Wants are identified through borehole logging, mine mapping, and seam sampling. Because they represent non-coal material interrupting the working seam, they directly reduce the volume of recoverable coal and create unpredictable changes in mining conditions. A thin want a few meters across may be worked around; a large one covering several hectares requires route planning to avoid excessive waste rock and processing burden.

The term reflects old mining terminology where such features were considered absent or "wanted" coal. In deep mines, wants can also affect roof stability and ventilation patterns if they contain clay or soft material that slakes or consolidates under load. Modern mining surveys always map known wants to inform extraction planning and to predict where equipment may encounter unexpected material changes.