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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