Coal is a fossil fuel found in sedimentary rocks and is currently one of the most dominant commercial energy sources in the world. Coal is quite different from other sedimentary rocks.
Unlike limestone and chert, which are rich in calcite or silica, coal is made primarily of organic matter. Close examination of a lump of charcoal under a microscope or magnifying glass often reveals plant structures such as leaves, bark, and wood that have been chemically altered but are still identifiable.
This supports the conclusion that coal is the end product of vast amounts of plant material buried over millions of years. Furthermore, it is an important source of rock for gas and, in some cases, for oil as well.
The Coal corresponds to sedimentary rocks that are brown to black in color, soft to hard, low in density and rich in organic carbon.
By definition, they must contain less than 33% inorganic material (impurities) but generally have less than 10% clay, silt and sand, as well as a large amount of minerals present in minor and trace amounts.
They are a very common (and economically important) type of sediment that is easily recognized in the field. Most of the coals are humic, formed by the in situ accumulation of plant material.
Sapropelic (or current) smuts, on the other hand, have formed from the dispersal and subsequent accumulation of plant remains, spores, pollen, and/or fragmented algae. Carbon rank refers to the degree of metamorphism (or coalification) of organic matter.
Humic coals can be divided based on their rank into several categories.
Sedimentary characteristics of coal
The Coal it is one of the well-known sedimentary rocks in formations of all ages since the late Devonian, shortly after land plants first evolved and proliferated.
They are easily recognized in the field, based on their black color, light weight, and associated rootlets, and provide important environmental information.
However, many of its specific characteristics are more easily defined by routine sampling and laboratory analysis.
Much of the sedimentological work on coals is carried out on cores recovered from shallow wells.
Deposit
Coals tend to be thinner (3 m) but more extensive laterally in deltaic settings, whereas they are thicker and more restricted in fluvial sequences.
Some of the thickest coal seams (up to several hundred meters) are found in fluvial and alluvial fan systems.
Even very thin layers (5 cm) can have an extensive root system.
Charcoal beds and thicker, well-established root systems may have leached the underlying sediment (sand or mud) giving a distinctive, usually pale-colored layer of bedrock.
structures
No dynamic sedimentary structures are observed, but stratification from millimeters (lamination) to centimeters of different microlithotypes is common.
These are known as bands.
A somewhat subjective and time-consuming but very useful method for the macroscopic description of coals involves visual recording of banding in coal seams, both in the field and in cores.
Each layer can develop a distinctive banding profile, which can facilitate identification and correlation of the layer across the coal basin.
The layers can be characterized as follows:
- C1 Bright carbon (90% bright bands).
- C2 Carbon with bright band (bright bands 90–60%).
- C3 Carbon bands (bright bands 60–40%).
- C4 Opaque carbon with bands (bright bands 40–10%).
- C5 Opaque carbon (bright bands 10%).
- C6 matte with a satin luster, friable, with up to 10% other lithotypes of carbon.
Vertical seams (known as wads in coal seams) occur in many coals, often coated with different minerals.
There may also be evidence of syn-sedimentary faulting and/or slip surfaces parallel to the layers.
Cleavages of other rock types (eg, carbonaceous shale, sandstone, volcanic ash) may be present within coal seams, further aiding identification and correlation.
Root horizons are common below coal seams and will even occur in the absence of a preserved coal seam.
Its presence in a sedimentary succession may indicate the occurrence of laterally adjacent coals.
Texture
Coal is a compact, fine-grained rock, with a grain size equivalent to mud rocks (mudstones).
Therefore, individual grains are not visible in the field, even with a hand lens (magnifying glass).
Thin sections polished in the laboratory show that some of the component macerals (see below) may be the size of silt or sand.
In peat, the coarsest fragments of undecomposed plant material can be seen.
Composition
The fundamental (microscopic) components of coals are known as macerals, which are derived from different parts of the original plant material (Table 2).
| Main components of coal and their origin. | |
| main macerals | primary origin |
| showcase group | Derived from the cell wall material (woody tissue) of plants composed of polymers, cellulose and lignin. |
| liptinite group | Derived from waxy and resinous parts of plants, such as pores, cuticles, and resins, and also algae. Sensitive to advanced carbonation and generally disappears from mid to low volatile range bituminous coals. |
| inertinite group | Derived from plant material that has been heavily disturbed and degraded during the peat stage of oxidative carbonization. It includes fossil carbon (known as fusinite). |
Table 2
These can vary significantly for different types and ages of coal.
Maceral assemblages characterize specific microlithotypes, which when visible on hand samples are known as charcoal lithotypes.
The most common lithotypes are:
- Vitrain: bright bands, glassy, brittle, conchoidal fracture.
- Clarain: shiny and dull bands, finely laminated, silky, with a smooth fracture.
- Durain: opaque, hard, dull bands.
- Fusain: similar to coal, soft, dusty, dirty fingers.
With increasing range, the macerals tend to lose their specific character and the coal becomes more homogeneous and progressively harder.
Macroscopic plant material (fossils) can only be clearly recognized in the lowest rank coals: peat and lignite.
A variety of inorganic components may also be present in coal.
These include detrital quartz, clay minerals, heavy minerals, sulfates, and phosphates, as well as diagenetic nodules of pyrite (very common), marcasite, siderite, ankerite, dolomite, and calcite.
Other field observations
Certain other features may be very important for coal studies. The type and nature of the underlying and overlying strata (sandstone, shale, etc.), as well as the proportion of coal and non-carbonized sediments, are important to mining engineers.
So is carbon geometry – fast thinning/thickening, layer wedging. The presence of igneous, intrusive, or extrusive rocks may have caused the coal to become affected by heat, to have matured too quickly, and thus to deteriorate in quality.
It is important to note any features that may aid correlation, such as the presence of separations, cinder layers, diagenetic nodules, and bands.
Syn-sedimentary faults, as well as observations on cleat directions and other structural features, will help with the overall interpretation of the coal basin.
Geological setting and occurrence of coal
Coal facies are primarily associated with both (deltaic and coastal) and lime successional (fluvial and lacustrine) environments, and do not occur within fully marine systems.
They are formed from the accumulation of mainly terrestrial plant material and its conservation in anoxic or low-oxygen conditions. This occurs more easily in humid climates.
The most abundant coals are of Carboniferous and Late Permian age in the Northern Hemisphere (especially in North America and Europe), mainly in deltaic and coastal successions. Permian-age coals in limnic successions are typical of many southern hemisphere occurrences formed on the ancient continent of Gondwanaland.
Other peaks of coal formation occur around the world, and especially in China, in the Jurassic, Cretaceous, and middle Tertiary periods. Tertiary coals are typically lower rank lignitic or brown coals.
Paleozoic and Precambrian rare, early coals are exclusively derived from algae. Northern Hemisphere Carboniferous coals are typically rich in vitrinite group macerals (60–90%), along with 5–15% liptinites and 5–40% inertinite.
Tertiary coals are typically lower burning lignitic or brown coals, except where deeply buried or heavily influenced by igneous/tectonic heating.
Channel (fluvial) and slough coals are dominated by liptinite macerals. Rare, early Paleozoic and Precambrian coals can also be found and are exclusively of algal derivation.