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COAL
1. Origin
Factors favouring the development of a coal forming peat (and its
subsequent preservation) are:(a) climate --- warm, high rainfall (tropical) --- to encourage growth of
vegetation.
(b) plants --- abundant land plants --- to increase chance of coal
forming
(c) rate of subsidence --- must not be too slow --- so plant material will
not oxidise.
A CYCLOTHEM = a sedimentary sequence which is repeated.
Question: Complete this diagram of a cyclothem.
Rock type
Environment of deposition.
New forest grows
New land develops, new swamp
soil forms
Sandstone
Sandstone & shale
Shale With “mussel” bands
Coal
Seat earth
Deltas spread
o
n
e
c
y
c
l
o
t
h
e
m
Sea-level rises & there is rapid
subsidence, forest covered by
marine deposits
Swamp forest
Swamp soil
2. Coal Types

The nature of the original plant material influences the type of coal formed.

The plant material is further modified by:(a) Biochemical changes --- resulting from fungal and bacterial decay.
This attacks the cellulose, lignin and proteins from the plant, which leaves the
peat richer in the waxy constituents.
(b) Later physical and chemical changes --- linked to the length of time the
material is under the higher pressures and temperatures inside the crust.
These changes include diagenetic changes and metamorphism.

A number of different coals are formed.

The coal series ranging from peat to anthracite shows these changes:Chemical changes
1.
2.
3.
4.
Loss of water
Loss of methane
Loss of CO2
Increase in C content.
Physical changes
1.
2.
3.
4.
Loss of plant structures
Increase in compaction
Increase in hardness
Increase in density
TABLE TO SHOW CONTENT OF DIFFERENT TYPES OF COAL
RANK
PEAT
LIGNITE
BITUMINOUS
COAL
ANTHRACITE
% FIXED
CARBON
5
25
55
% VOLATILES
% MOISTURE
15
30
35
80
45
10
83
10
7
Show this information on a composite graph.
SUMMARY OF CHANGES LEADING TO THE FORMATION
OF A HARD BRITTLE COAL FROM PEAT
% Carbon
60%
CHANGES
PEAT
1
Increased compaction
LIGNITE
BITUMINOUS COAL
95%
ANTHRACITE
Increase in
Rank
2
Loss of moisture
3
Loss of plant structure
4
Increased hardness
5
Increased carbon content &
changed colour (brown to
black)
6
Increased density
DESCRIPTIONS OF DIFFERENT TYPES OF COAL
PEAT = almost unaltered
vegetable matter.
Porous, spongy,
brown colour.
Formed today on badly drained
moorland
LIGNITE = vegetable matter decomposed
further than in peat
More compact than peat. Brown
colour, plant structures difficult to
recognise. Breaks into fragments
when dry. Only mined where no
higher ranks of coal or where can
be mined cheaply.
ANTHRACITE = Black hard
coal which is unbanded Light,
& no trace of organic structure.
Brilliant lustre, conchoidal
fracture, H>2
Only found in deepest part of
coalfield or areas + high T/P
BITUMINOUS COAL = most
common type. Rarely
shows plant structures. Black,
hard, brittle. Breaks into
rough cubes.
Bedding planes- covered with
Soft powdery material called
FUSAIN. Main part of coal
made up of other
materials giving dull & shiny
layers