Maceral
Encyclopedia
A maceral is a component of coal
or oil shale
. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral
' in reference to igneous or metamorphic
rocks. Examples of macerals are inertinite
, vitrinite
and liptinite
.
Macerals of liptinite are sporinite
, cutinite
, resinite, alginite
(telalginite
and lamalginite
), liptodetrinite, fluorinite, and bituminite.
spores, fossilised leaves, remnant cellular structure and similar. In rare cases, maceral and fossilised pollen can be found in terrestrial sedimentary rocks.
Maceral maturity can be estimated by vitrinite reflectance. This gives information on the carbon, hydrogen and nitrogen composition of the coal, and determines the type of coal: lignite
, brown coal, bituminous coal
, anthracite or graphite
.
Macerals found in kerogen
source rocks are often observed under the microscope to determine the kerogen maturity of the sedimentary formations. This is a vital component of oil and gas exploration.
Macerals are observed under the petrographic microscope
under reflected light. Coal fragments must be extremely highly polished down to less than half a micrometre before they can be observed under the microscope.
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
or oil shale
Oil shale
Oil shale, an organic-rich fine-grained sedimentary rock, contains significant amounts of kerogen from which liquid hydrocarbons called shale oil can be produced...
. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
' in reference to igneous or metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
rocks. Examples of macerals are inertinite
Inertinite
Inertinite is oxidized organic material or fossilized charcoal. It is found as tiny flakes within sedimentary rocks. The presence of inertinite is significant in the geological record, as it signifies that wildfires occurred at the time that the host sediment was deposited. It is also an indication...
, vitrinite
Vitrinite
Vitrinite is one of the primary components of coals and most sedimentary kerogens. Vitrinite is a type of maceral, where "macerals" are organic components of coal analogous to the "minerals" of rocks. Vitrinite has a shiny appearance resembling glass . It is derived from the cell-wall material or...
and liptinite
Exinite
Liptinite is an umbrella term used in coal geology, referring to the finely-ground and macerated remains found in coal deposits. It replaced the term Exinite as one of the four categories of kerogen. Liptinites were originally formed by spores, pollen, dinoflagellate cysts, leaf cuticles, and plant...
.
Inertinite
Inertinite is considered to be the equivalent of charcoal and degraded plant material. It is highly oxidised in nature and may be said to be burnt. A large portion of South Africa's coal reserves consist of inertinite.Vitrinite
Vitrinite is shiny, glass-like material that is considered to be composed of cellular plant material such as roots, bark, plant stems and tree trunks. Vitrinite macerals when observed under the microscope show a boxlike, cellular structure, often with oblong voids and cavities which are likely the remains of plant stems. This has a high calorific value (24 - 28 MJ/kg) and a large proportion of volatile matter (24 - 30 %). It often occurs interbanded or interlaminated with inertinite and can be recognised as bright bands.Liptinite
Liptinite macerals are considered to be produced from decayed leaf matter, spores, pollen and algal matter. Resins and plant waxes can also be part of liptinite macerals. Liptinite macerals tend to retain their original plant form, i.e., they resemble plant fossils. These are hydrogen rich and have the highest calorific values of all coal macerals.Macerals of liptinite are sporinite
Sporinite
Sporinite is a kind of exinite maceral found in coal formed from spores and pollen. It is a Type II kerogen....
, cutinite
Cutinite
Cutinite is a liptinite maceral formed from terrestrial plant cuticles, and often found in coal deposits. It is classified as a Type II kerogen....
, resinite, alginite
Alginite
Alginite is a component of some types of kerogen alongside amorphous organic matter. Alginite consists of organic-walled marine microfossils, distinct from inorganic -walled microfossils that comprise diatomaceous earth. At least two forms of alignite are distinguishable, "alginite A" and...
(telalginite
Telalginite
Telalginite is a structured organic matter in sapropel, composed of large discretely occurring colonial or thick-walled unicellular algae such as Botryococcus, Tasmanites and Gloeocapsomorpha prisca. Telalginite is present in large algal bodies. It fluoresce brightly in shades of yellow under...
and lamalginite
Lamalginite
Lamalginite is a structured organic matter in sapropel, composed of thin-walled colonial or unicellular algae that occur as distinct laminae, cryptically interbedded with mineral matter. It displays few or no recognisable biologic structures. Lamalginite fluoresce brightly in shades of yellow...
), liptodetrinite, fluorinite, and bituminite.
Nature of macerals
Macerals are considered to be dehydrogenated plant fragments. Evidence for this includes remnant pollenPollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
spores, fossilised leaves, remnant cellular structure and similar. In rare cases, maceral and fossilised pollen can be found in terrestrial sedimentary rocks.
Maceral maturity can be estimated by vitrinite reflectance. This gives information on the carbon, hydrogen and nitrogen composition of the coal, and determines the type of coal: lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
, brown coal, bituminous coal
Bituminous coal
Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...
, anthracite or graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
.
Macerals found in kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
source rocks are often observed under the microscope to determine the kerogen maturity of the sedimentary formations. This is a vital component of oil and gas exploration.
Macerals are observed under the petrographic microscope
Petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...
under reflected light. Coal fragments must be extremely highly polished down to less than half a micrometre before they can be observed under the microscope.