Coal also contains minerals, which mostly occur as inorganic crystalline and noncrystalline particles or masses. A coal seam may consist of as much as 50 percent minerals. At more than 50 percent mineral matter, the rock would be termed a carbonaceous shale rather than coal. Most mined coals are less than 20 percent mineral matter, and many coal contracts require less than 10 percent ash yield, which is approximately (but generally less than) the actual mineral matter content of a coal. More than 120 minerals and inorganic compounds have been documented in coals worldwide (Finkelman, 1981; Vassilev and Vassileva, 1996; Ward, 2002; Schweinfurth and Finkelman, 2003).
Minerals in coal can be divided into two broad categories: (1) minerals that originated in the original coal-forming peat, termed syngenetic or primary minerals, and (2) diagenetic or secondary minerals, which were introduced into the coal, remobilized in the coal, or transformed from other minerals in the coal at any time after the original coal-forming peat was buried. Some minerals can also form in coal by oxidation of sulfides and other minerals when the coal is exposed at the surface. These would be termed tertiary minerals. Most tertiary minerals are transformations of minerals that are already in the coal (Mackowsky, 1982; Renton, 1982; Vassilev and Vassileva, 1996; Ward, 2002).
Syngenetic minerals tend to be tiny (microns) particles embedded in the coal matrix. Biogenic sources of syngenetic minerals include plants and organisms in the original coal-forming peats. Many plants incorporate minerals into their structures. An example of this is the common horsetail, or scouring rush, which incorporates silica in its tissues to provide structural strength. Some organism that live in, or are transported into peats, or whose remains are transported into peats also contain minerals in their body structures (see for example, Andrejko and others, 1983).
Detrital, syngenetic minerals include clays and other sediments transported into the coal-forming peats during floods. Detrital minerals also include dust and silt carried into the peat by wind. Thick, concentrated detrital sediments form rock partings in coal. Finer, less-concentrated sediment input is disseminated into the coal-forming peat without a visible rock laminae or bed.
A wide array of chemical precipitates may also occur in peat through various processes involved in peatification, including the bacterial reduction of sulfates in peat pore waters.
Once a peat is buried, mineral introduction and transformation is said to be diagenetic. Diagenetic minerals tend to be more coarsely crystalline than syngenetic minerals. After coal has reached the lignite stage, diagenetic minerals tend to occur as thicker concentrations in cleat and fracture fills, which are visible to the naked eye. This distinction is significant, because cleat-filling minerals are easier to remove from coal than from syngenetic, authigenic minerals (Rao and Gluskoter, 1973; Mackowsky, 1982; Renton, 1982; Ward, 1989; van Krevelen, 1993).
The amount, particle size, and type of mineral matter in coal influences a wide variety of uses, including combustion for steam for electrical energy production and the production of metallurgical coke to make steel. Understanding mineral components is not only important for the practical processes and maintenance of combustion equipment, but also because of the chemical reactions that occur during combustion, and the resultant composition of emissions and solid residues, especially in countries where the chemical composition of the emissions or residues is regulated.
- Go to the KGS Minerals page for help in identifying common minerals that can be found in in coal, including:
- calcite http://www.uky.edu/KGS/rocksmn/carbonates.htm#calcite
- clay minerals http://www.uky.edu/KGS/rocksmn/silicates.htm#montmorillonite
- marcasite and pyrite http://www.uky.edu/KGS/rocksmn/sulfides.htm#pyrite
- quartz http://www.uky.edu/KGS/rocksmn/silicates.htm#quartz
- siderite http://www.uky.edu/KGS/rocksmn/siderite.htm
- sphalerite http://www.uky.edu/KGS/rocksmn/sulfides.htm#sphalerite