COAL MICROLITHOTYPES AND THEIR USAGE IN INTERPRETING DEPOSITION ENVIRONMENT

COAL MICROLITHOTYPES AND
THEIR USAGE IN INTERPRETING
DEPOSITIONAL ENVIRONMENT
BY
OLATINPO, OLUSEGUN AYOBAMI
14/68ET003

OUTLINE
Introduction
Coal Microlithotypes
Microlithotype Analysis
Significance of Coal Microlithotypes for
Interpreting Depositional Environment
Summary and Conclusion
7/11/2015 OLUSEGUN OLATINPO (2015) 2

Introduction
• Coal is described as an organic sedimentary rock
usually found in layers with other sedimentary
rocks such as shale, limestone and sandstone
• It is a combustible rock
• The major chemical elements in coal are: carbon,
hydrogen, oxygen, nitrogen, and sulphur

CONTD
The process of coal formation from organic
compounds includes two distinct phases:
1. Biochemical phase
plant material – peat – lignite
2. Geochemical phase
lignite – bituminous coal – anthracite

CONTD
Over time, the chemical and physical properties of the plant remains
were changed by geological action to create a solid material.

COAL MICROLITHOTYPES
 Microlithotype was developed to permit the designation of rock
types within coal that are at a microscopic scale and their definition
is based on maceral percentages.
 Microlithotypes are natural associations of macerals in coals at
microscopic level.
 In addition to the maceral content, 20–60% (vol.) of silicate or
carbonate minerals or 5–20% (vol.) sulfide minerals redefines
themicrolithotype as a carbominerite.
 All macerals have a suffix "-inite" and microlithotypes have the
suffix "-ite".

CONTD
Macerals are the basic and relatively homogenous
organo-petrographic entities of coal which by their
chemical composition and physical characteristics
determine its properties and utilization.
They are normally classified into 3 groups:
Liptinite-Hydrogen rich and show lowest reflectance
Vitrinite-Oxygen rich and show medium reflectance
Inertinite-Carbon rich and show highest reflectance
Macerals are the coalified remains of various plant
tissues or plant-derived substances existing at the time
of peat formation.

TABLE 1: ILLUSTRATION OF MACERAL GROUPS AND SOME MACERALS

CONTD
 The formation of macerals from plant remains during the early
stages of peat accumulation depends on th:
 type of plant community,
 climatic and
 ecological controls, and
 conditions of the depositional environment
 In polished sections under the microscope using incident light,
macerals are identified on the basis of their optical properties.
 Thus, layers of a particular maceral group greater than 0.05 mm
in thickness and consisting of greater than 95% of a particular
maceral are termed after the maceral group e.g vitrinite maceral
group forms vitrite microlithotype.

CONTD
Microlithtypes are classified into three groups based on
number macerals they contain :
Monomaceral- type consist of one maceral group
examples are vitrite (over 95% vitrinite) and inertite
(over 95% inertinite)
Bimaceral- consist of 2 maceral groups examples are
clarite, durite and vitrinertite.
Trimaceral-consist of 3 maceral groups examples are
duroclarite, clarodurite and vitrinertoliptite.

Microlithotypes of Hard Coals
(Data from Taylor et al., 1998)

CONTD
Fig. 1: Diagrammatic representation of Microlithotype Classification ( Bustin et al 1983)

CONTD
 The chemical properties of microlithotypes are very similar to those of the
predominating macerals
 Their physical properties, however, are related not only to those of the
macerals but also to the combined effect of the association.
 The microhardness of bi- and trimaceralic microlithotypes is always
higher than that of monomaceralic associations.
 The density of the microlithotypes varies with rank, maceral composition,
and size, as well as the form and quantity of associated minerals.
 The degree of heterogeneity in a microlithotype is also important in its
technological behavior, particularly in carbonization, combustion, and
gasification

CONTD
G
The maceral, mineral, and microlithotype
composition of a coal seam may change over
short distances both vertically and laterally, in
response to the conditions existing during the
formation of the original peat swamps. These
changes can be quantified by petrographic
assessment of the microlithotypes in relevant
coal samples.

MICROLITHOTYPE ANALYSIS
 It is used to determine the relative proportions of the various microlithotypes and
coal-mineral associations
 Although microlithotype analysis is carried out in a similar manner to maceral
analysis, a suitable 20-point reticule must be placed in one of the oculars of the
microscope as a substitute for the micrometer or cross-hairs.
 Two conventions (ICCP, 1963) must be observed:
 The minimum bandwidth of the association to be measured must be 50microns,
and
 Macerals present in the association in amounts smaller than 5% should be
disregarded (the 5% rule).
 Each observation on a 20-intersection reticule is regarded as one point in the
analysis, and each intersection on the reticule represents 5% of the total number of
intersections (20), providing guidance in use of the 5% rule.
 For a complete microlithotype analysis, at least 500 points should be measured, and
the results should be expressed as volume percentages. Microlithotype analysis is
less accurate than maceral analysis.

Significance of Coal Microlithotypes for
Interpreting Depositional Environment
 Maceral data needs to be considered in the light of different swamp
types and resulting coal microlithotypes in Teichmuller (1962), the coal
facies diagrams of Diessel (1986).
 The swamp and moor types described by Teichmuller (1962) indicate
that characteristic plants occur in each swamp type and plant remains
can be identified petrographically by maceral and microlithotype
analysis.
 Three swamp types illustrated on this basis include
 Forest swamp
 Reed swamp
 Open water swamp

CONTD
 The forest swamp is characterised by large contents of
structured vitrinite associated with spores, cuticles,
resinite, and frequent fusinite. Vitrite and clarite rich in
vitrinite are believed to form in this environment with a
high groundwater level
 The reed swamps occur at the transition into open water
swamp and this characterized by a decrease of structured
vitrinite, decomposed vitrinite and spores. More of
inertinite, cuticuloclarite, and duroclarite are found here.
 The open water swamps are dominated by algae and
subaquatic plant communities (especially floating plants).
Alginite, clarodurite, durite and carbargillite, and
associated washed-in cuticles and spores may be found

h
Fig. 2:Facies diagram of the seams with reference to lithotypes A, B, C, and D (Mastalerz, 1992)

CONTD
Fig 3: Coal depositional enviroments based on the composition of Microlithotypes (free of
mineral matter) Singh and Shukla (2004)
KEY
A: Lacustrine,
B: Fluvial,
C: Brackish water,
D: Upper deltaic, and
E:Lower deltaic environments.

SUMMARY AND CONCLUSION
 Vitrite is usually derived from stems, branches, and liquified tree roots where
trees were protected from oxidation by high water level.
 The association vitrite and clarite, particularly liptinite poor clarite, suggests
a strongly decomposed forest litter of wood and bark which can degrad to
humic detritus.
 Liptinite rich clarites are derived from reed peat and reed lignites
intermingled with subaquatic deposits.
 Clarites often display microlayers and suggest formation under wet conditions
normally associated with vitrinitic carbargillites and syngenetic pyrite.
 Durites are derived from a subaquatic ooze of oxidized peat characterized by
a low spore content and low water table.
 Vitrinertites are characteristic of frequent swamp dessication.

SUMMARY AND CONCLUSION
For geological research on coal basins and for an
evaluation of coal seam quality it is important to
know the quantitative composition of a coal in
terms of the macerals (and minerals in some
cases) or maceral groups and microlithotype.
This is because differences in maceral
composition may indicate differences in chemical
composition and consequently differences in the
technological properties of a coal like
liquefaction, carbonization, and combustion.

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COAL MICROLITHOTYPES AND THEIR USAGE IN INTERPRETING DEPOSITION ENVIRONMENT

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COAL MICROLITHOTYPES AND THEIR USAGE IN INTERPRETING DEPOSITION ENVIRONMENT