Open-pit mining
After mining finishes, the mine area may undergo land rehabilitation. Waste dumps are contoured to flatten them out, to further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidise the sulfides to produce sulfuric acid, a phenomenon known as acid mine drainage. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the environment can handle the load of acid and associated heavy metals. There are no long term studies on the success of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become "acid neutral" and stop leaching to the environment. The dumps are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with a fence, to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to deep groundwater levels. Instead of returning the land to its former natural state, it may also be reused, converting it into recreational parks or even residential/mixed communities.
Clay mineral X-ray diffraction
Clay minerals are one of the most diverse minerals but all have a commonalty of crystal or grain sizes below 2 µm. Chemically clays are defined by crystal structure and chemical composition. Sometimes fine grain sediments are mistakenly described as clays, this is actually a description of the “clay-size fraction” rather than the mineralogy of the sediment. There are three crystallographic clay groups: platy clays (phyllosilicates), fibrous clay minerals, and amorphous clay. Phyllosilicates are the most abundant clays and are categorized based on the layering of a tetrahedral and an octahedral layer. For most clays, the octahedral layer is centered with Al, Fe, or Mg(OH), but sometimes Zn, Li, and Cr can substitute as well. Si is normally the center of the tetrahedral layer but Al will often partially substitute and create a charge imbalance. Two-layer clays are composed of a tetrahedral layer and an octahedral layer (T-O) while three-layer clays contain an octahedral layer sandwiched by two tetrahedral layers (T-O-T). When substitution of Al for Si creates a charge imbalance, an interlayer cation will fill in between tetrahedral layers to balance the charge of the clay.