Quartz Quartz is hexagonal and commonly occurs as crystals ranging in size form microscopic to crystals weighing several tons. Where it crystallizes unhindered by other crystals, such as in cavities in rock or in a liquid containing few other crystals, it shows well-developed hexagonal prisms and sometimes showing apparent hexagonal pyramids or dipyramid. When it crystallizes in an environment where growth is inhibited by the surroundings, it rarely show crystal faces. It is also found as microcrystalline masses, such as in the rock chert, and as fibrous masses, such as in chalcedony. Vitreous lusture. As visible crystals, Quartz is one of the more common rock forming minerals. It occurs in siliceous igneous rocks such as volcanic rhyolite and plutonic granitic rocks. It is common in metamorphic rocks at all grades of metamorphism, and is the chief constituent of sand. Because it is highly resistant to chemical weathering, it is found in a wide variety of sedimentary r
COAL ORIGIN AND ITS TYPES
FIGURE 1.1 ( COAL )
- Stratified, compact, mass of plant debris modified chemically and physically by natural agencies, Plant debris mainly but not exclusively from terrestrial plants
- Natural agencies causing physical and chemical changes: bacteria & fungi, oxidation, reduction, hydrolysis, condensation; effect of heat and pressure in presence of water
- Physical changes: reduction in strength of woody cell structure, development of colloidal properties, increase in depth and brightness of color, increase in hardness and brittleness
- Chemical properties of a coal depends upon properties of the different constituents of the vegetable matter, the nature and extent of the changes
- Inorganic matter proportion in coal is small, oil shales and carbonaceous shales which differ from coal only in having inorganic matter as their major component, are not classed as coal
- Cannel coals, boghead ( a cannel coal in which algal remains predominate and which is valuable as a source of paraffin oils and gas.) and similar products are classed as coal which in addition to plant products contain microscopic animal life such as zooplanktons of lakes and pools
- Peat, although of no industrial use as fuel, is classed as coal. But it is of scientific importance as its constituents and mode of formation provides information on the nature of coals as well as physicochemical changes occurring in the course of its development, or change of rank
In-situ Theory :
As important coal deposits have been formed almost entirely from terrestrial plants, it appears that in-situ material grew and accumulated under conditions similar to those existing in modern peat deposits
Accumulation of debris stops as conditions become unfavourable for growth or accumulation of vegetable matter for one or more of the following causes
a) Flooding and covering with a sedimentary
cover
b) Change of climate to extreme dryness or cold
c) Sudden storm of great intensity causing destruction of
living matter and erosion or disruption of plant debris
q For major in-situ coal deposits, changes of the surface level of the land are necessary, successively repeating the favourable conditions for accumulation
q Major in-situ coalfields are formed in brackish or fresh water, from massive plants growing in swamps or in swamps interspersed with shallow lakes
q A modern equivalent of coal formation under forming conditions is in the Ganges delta where plant debris is accumulating as layers of peat, alternating with sediments deposited during flooding
q Suitable conditions for growth and accumulation of plant matter in this manner have existed throughout most of the geological time during which life on land existed
q From Devonian on-wards in-situ coal deposit have been formed whenever geological conditions and time have allowed the full cycle required for their formation
q While large vegetable masses are widespread all over world, conditions for their preservation are limited. A peat deposit may remain uncovered for a long time and may be sooner or later denudated .
q Burial under a cover of sediments within a reasonable time is essential for effective preservation and this normally means submergence below water
q Formation of large in-situ coalfields required extensive accumulation of vegetable matter which are subjected to widespread submergence by sedimentary deposits
q The area where coal deposit is to be formed must be near water level
q When it is submerged by subsidence or through rise in water level, the entire area must be covered by sediments in a reasonable span of time
q If the land surface is to be reformed for growth and accumulation of another peat deposit, the entire area must become silted with sediments
q Therefore, extensive sedimentation and subsidence must characterize the region
q During large-scale in-situ coal forming periods, accumulation of vegetable matter and associated mineral matter, generally clay and sand, is balanced by the subsidence of the area
q If the area is near the sea such as delta or coastal swamp, and the rate of subsidence exceeds the rate of accumulation, the area sinks below the sea level by the invading sea covering the deposit with sand and calcareous mud
q If the area is inland with excessive subsidence, the area is flooded by fresh water and debris is covered with soil or clay
q When the rate of subsidence and deposition are about equal plant debris continues to accumulate and deposit is substantially free from extraneous mineral matter
When no subsidence occurs, the area rises gradually due to continued growth of plant life, but accumulation of plant debris ceases due to rapid decay by microbial agencies and erosion by wind and rain
Examples of in-situ deposits include most of the coalfields of N.W. Europe and North America.
Drift Theory :
q Drift material forming coal consists mainly of land plants transported by fresh or sea water
q Those transported by fresh water would not generally travel far but would be transported by floods during heavy rains from nearby forests and deposited in basins or lakes when flood velocity is reduced
q Such lakes and basins may contain much plant debris growing in-situ which will form coal deposits of mixed origin or the drift material may be deposited in places where vegetation had not previously accumulated forming a purely drift deposit
q Drift material carried by sea may travel far. Here also some drift material may be deposited upon material growing and accumulating as a peat deposit resulting in a mixed deposit (British Coal Measures, Rhur Coalfield of Germany)
q ‘Autochthonous’ used for deposits or portions of deposits of in-situ origin and ‘Allochthonous’ for portion of deposits or complete deposits of drift material
q Allocthonous portions of a deposit differs from autochthonous not only in the nature of organic components but also in the nature and proportion of inorganic content
q Invariably finely divided portions of organic material form large proportions of drift material often associated with fine clay and silt. This drift material form bands or pockets of high ash materials in the coal deposits and have widely different properties form that of the associated material, which is formed form more massive portions of the plant debris and are free from extraneous inorganic matter.
Indian
coals are almost entirely of drift origin. Two main coal forming periods in
Indian Peninsula: a) Gondwana period (Permo-Carboniferous) in which deposits
are of fresh water origin accumulating in four great basins or lakes and b)
Lower Eocene (Laki Stage) having Punjab, Salt Range, Rajputana and Baluchistan
coals of gulf (marine origin
Rank of
coal :-
FIGURE :1.3( RANK OF COAL ACCORDING TO ITS CALORIFIC VALUE )
n On the basis of physical and chemical properties of coal, it can be divided in to several characteristic classes
n In a classification based on the proportions of carbon present, the greater the carbon proportion, higher the rank
n Term Rank is thus an indication of chemical development of coal
n ‘Increase in rank’ denotes the natural processes due to which carbon content of a coal is increased at the expanse of hydrogen and oxygen contents
n It may be noted that differences in rank of various coals do not necessarily signify differences in geological age and apparent differences in rank (i.e., C/H ratios) may be due to presence of different proportions and types of chemical compounds in the coals as a result of variations in the original plant debris
n Principal changes related to increase in rank:
1. Progressive and uniform increase
in carbon
2. Decrease in hydrogen, at first gradually,
until carbon content reaches 89% and
then more rapidly
3. Decrease in proportion of volatile matter
4. Increase in calorific value until hydrogen
decreases to below 4.5%
5. Decrease in moisture content until anthracitous
rank is reached
6. Increase in absolute density
7. Decrease in solubility in alkaline solution
8. Increase in depth of colour, lusture, and reflectivity
9. Decrease in reactivity towards oxidizing or hydrogenating
agents
n It is generally agreed that all coal has been derived from peat and that it has assumed its present state as a result of various geological processes
n Peat: an accumulation of vegetable matter which has suffered varying degree of disintegration and decomposition, contains high percentage of water (80-90%) and oxygen (33%), physical character varies from a distinctly fibrous and woody, light brown material to dark brown jelly brown substance, seldom sufficiently compact to make a good fuel without compressing
n Lignite and Brown Coal: Amorphous or woody, brown colour (for more detail : https://destro53a.blogspot.com/2022/04/lignite.html )
n Sub-bituminous coal: intermediate stage between lignite and bituminous coal, parts along a surface nearly parallel to bedding plane and breaks into thin slabs irregularly and does not disintegrates into cubes like bituminous coal
n Bituminous Coal: burns with a long yellow flame and gives off a suffocating smell, more or less laminated, lusture of different layers varies greatly and may be resinous, silky, pitchy, or dull and earthy, soils the hands, colour from pitch black to dark grey, fracture irregular and splintery but almost roughly cubical, it is conchoidal in cannel coal, includes varieties like caking or coking coal, non-caking and non-coking coal, cannel coal, and boghead coal
n Anthracite: iron-black colour, dull to brilliant and even submetallic lusture, does not soil finger like bituminous coal, burns with a short pale blue flame with little smell, breaks with conchoidal fracture, hardest coal.
Comments
Post a Comment