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Optical properties of carbonized vitrinites — a reply
Letter to the Editor Optical properties of carbonized vitrinites - a reply R. J. Marshall and D. G. Murchison Department of Geology, Organic Geochemistry St. Thomas’Street. Newcastle upon Tyne, NE1 (Received 6 March 1972)
Unit, University 7RU, UK
In a Letter to the Editor Chandra’, commenting on a recent article by Marshall and Murchison’, has suggested that these authors made certain statements which created misunderstandings about the contents of an earlier paper by Bond, Chandra and Dryden in which the optical properties of three vitrinites (in two low-rank coals and one highrank coal) carbonized at different temperatures were discussed. These comments deserve replies, since the misunderstandings seem to arise because of some confusion on Chandra’s part about the precise meaning and relevance of optical data obtained in such carbonization experiments. (1) Marshall and Murchison stated that the two low-rank coals carbonized by Bond, Chandra and Dryden were isotropic and remained so during carbonization. Chandra would have preferred a more elaborate description of the carbonization behaviour of these coals and he points out’ that they ‘ . . . remained almost isotropic at each carbontiing temperature. . . . But the differences in the maximum and minimum reflectances in oil are so small that for most practical purposes the carbonized coals may be considered almost isotropic.’ Despite the fact that Chandra’ refers to another paper4, which he says makes it clear that these very same coals are anisotropic and that this anisotropy increases with temperature, we felt we had made a not unreasonable, albeit abbreviated, interpretation of the intention of the statement by Bond, Chandra and Dryden. In particular, we were concerned to emphasize that vitrinites of a similar level of rank to those carbonized by Bond, Chandra and Dryden could display anisotropy on heating and even a substantial amount of anisotropy above the resolidification temperature. (2) A comment by Chandra which has more serious implications is that the method used by Marshall and Murchison to present their results tends to ‘emphasize the anisotropy’. This statement suggests a lack of appreciation by Chandra of the precise relevance of reflectances measured on a number of randomly oriented particles from a single sample of what is at present widely accepted to be an optically negative uniaxial material, namely vitrinite or its carbonized products_ Measurements along the principal directions of (say) 10 such particles will yield 10 true maximum reflectances within experimental error and 10 lower reflectances from the other principal direction of the particles. The values of these lower reflectances could range widely, depending on the orientation of the vitrinite particles, from that of the maximum reflectance (in the isotropic section) to the true minimum value (in the optical axial plane). Since only the difference between true maximum and true minimum reflectance yields a bireflectance which estimates the true anisotropy of the vitrinite, a poor assessment of the anisotropy of the vitrinite must be obtained if the course suggested by Chandra is adopted, namely the averaging of the 10 measured lower reflectances
252
FUEL, 1972, Vol 51, July
of Newcastle,
Porter Building,
and the subtraction of this average value from the average maximum reflectance to give the bireflectance. Our concern was to obtain as realistic an estimate as possible for the anisotropy of the carbonized vitrinites. We would not claim that averaging the lowest 20% of lower reflectances from a set of measurements and subtracting this value from the average maximum reflectance would necessarily be the most satisfactory way to achieve this aim (there are other possibilities to be investigated), but the bireflectance estimate thus obtained will certainly always lie much closer to the true value than any estimate obtained by Chandra’s procedure. (3) Chandra states that the refractive-index data obtained by us for an anthracitic vitrinite do not differ significantly from the refractive indices determined by Bond, Chandra and Dryden on a vitrinite of slightly lower rank for which no decrease of refractive index at carbonizing temperatures above approximately 600°C was observed. It is unfortunate that in making this claim Chandra has chosen first to refer to only a limited portion of our data and secondly to quote it out of the context in which our interpretation was made. Reference to figure 5 (Marshall and Murchison2) will show a fall of refractive index for the carbonized anthracitic vitrinite between 600 and 750°C over the greater part of the visible spectrum. The fall is very pronounced in the red region of the spectrum and it diminishes towards the blue region. Only at wavelengths below approximately 500 nm does comparatively little decrease occur. Our conclusion was based on a comparison of the behaviour of dispersion curves between 403 and 709 nm for the carbonized vitrinites at different temperatures. Not unreasonably, because of the few data points we had at individual wavelengths, we did not feel that we could make reliable judgements from the type of plot’ Chandra has used. (4) Finally, Chandra indicates that based on Seyler’s coal classification we were wrong to refer to the high-rank coal carbonized by Bond, Chandra and Dryden as an anthracite. The point is taken, but, depending on the classification employed, the coal can be described as an anthracite, although much more frequently as a semi-anthracite. To suggest, as Chandra does, however, that we should perpetuate the scientific tautology, ‘carbonaceous coal’, to describe the sample hardly helps the development of a nomenclature that is explicit to the general reader.
REFERENCES 1 2
3 4
Chandra, D. Fuel, Land. 1972,51, 88 Marshall, R. J. and Murchison, D. G. Fuel, Land. 1971,50, 4 Bond, R. L., Chandra, D. and Dryden, I. G. C. Rev. ind. minbizle (Numb-o spkcial, 15.7.58) 1958, p 171 Chandra, D. Fuel, Lond. 1963,42,69
Unit, University 7RU, UK
In a Letter to the Editor Chandra’, commenting on a recent article by Marshall and Murchison’, has suggested that these authors made certain statements which created misunderstandings about the contents of an earlier paper by Bond, Chandra and Dryden in which the optical properties of three vitrinites (in two low-rank coals and one highrank coal) carbonized at different temperatures were discussed. These comments deserve replies, since the misunderstandings seem to arise because of some confusion on Chandra’s part about the precise meaning and relevance of optical data obtained in such carbonization experiments. (1) Marshall and Murchison stated that the two low-rank coals carbonized by Bond, Chandra and Dryden were isotropic and remained so during carbonization. Chandra would have preferred a more elaborate description of the carbonization behaviour of these coals and he points out’ that they ‘ . . . remained almost isotropic at each carbontiing temperature. . . . But the differences in the maximum and minimum reflectances in oil are so small that for most practical purposes the carbonized coals may be considered almost isotropic.’ Despite the fact that Chandra’ refers to another paper4, which he says makes it clear that these very same coals are anisotropic and that this anisotropy increases with temperature, we felt we had made a not unreasonable, albeit abbreviated, interpretation of the intention of the statement by Bond, Chandra and Dryden. In particular, we were concerned to emphasize that vitrinites of a similar level of rank to those carbonized by Bond, Chandra and Dryden could display anisotropy on heating and even a substantial amount of anisotropy above the resolidification temperature. (2) A comment by Chandra which has more serious implications is that the method used by Marshall and Murchison to present their results tends to ‘emphasize the anisotropy’. This statement suggests a lack of appreciation by Chandra of the precise relevance of reflectances measured on a number of randomly oriented particles from a single sample of what is at present widely accepted to be an optically negative uniaxial material, namely vitrinite or its carbonized products_ Measurements along the principal directions of (say) 10 such particles will yield 10 true maximum reflectances within experimental error and 10 lower reflectances from the other principal direction of the particles. The values of these lower reflectances could range widely, depending on the orientation of the vitrinite particles, from that of the maximum reflectance (in the isotropic section) to the true minimum value (in the optical axial plane). Since only the difference between true maximum and true minimum reflectance yields a bireflectance which estimates the true anisotropy of the vitrinite, a poor assessment of the anisotropy of the vitrinite must be obtained if the course suggested by Chandra is adopted, namely the averaging of the 10 measured lower reflectances
252
FUEL, 1972, Vol 51, July
of Newcastle,
Porter Building,
and the subtraction of this average value from the average maximum reflectance to give the bireflectance. Our concern was to obtain as realistic an estimate as possible for the anisotropy of the carbonized vitrinites. We would not claim that averaging the lowest 20% of lower reflectances from a set of measurements and subtracting this value from the average maximum reflectance would necessarily be the most satisfactory way to achieve this aim (there are other possibilities to be investigated), but the bireflectance estimate thus obtained will certainly always lie much closer to the true value than any estimate obtained by Chandra’s procedure. (3) Chandra states that the refractive-index data obtained by us for an anthracitic vitrinite do not differ significantly from the refractive indices determined by Bond, Chandra and Dryden on a vitrinite of slightly lower rank for which no decrease of refractive index at carbonizing temperatures above approximately 600°C was observed. It is unfortunate that in making this claim Chandra has chosen first to refer to only a limited portion of our data and secondly to quote it out of the context in which our interpretation was made. Reference to figure 5 (Marshall and Murchison2) will show a fall of refractive index for the carbonized anthracitic vitrinite between 600 and 750°C over the greater part of the visible spectrum. The fall is very pronounced in the red region of the spectrum and it diminishes towards the blue region. Only at wavelengths below approximately 500 nm does comparatively little decrease occur. Our conclusion was based on a comparison of the behaviour of dispersion curves between 403 and 709 nm for the carbonized vitrinites at different temperatures. Not unreasonably, because of the few data points we had at individual wavelengths, we did not feel that we could make reliable judgements from the type of plot’ Chandra has used. (4) Finally, Chandra indicates that based on Seyler’s coal classification we were wrong to refer to the high-rank coal carbonized by Bond, Chandra and Dryden as an anthracite. The point is taken, but, depending on the classification employed, the coal can be described as an anthracite, although much more frequently as a semi-anthracite. To suggest, as Chandra does, however, that we should perpetuate the scientific tautology, ‘carbonaceous coal’, to describe the sample hardly helps the development of a nomenclature that is explicit to the general reader.
REFERENCES 1 2
3 4
Chandra, D. Fuel, Land. 1972,51, 88 Marshall, R. J. and Murchison, D. G. Fuel, Land. 1971,50, 4 Bond, R. L., Chandra, D. and Dryden, I. G. C. Rev. ind. minbizle (Numb-o spkcial, 15.7.58) 1958, p 171 Chandra, D. Fuel, Lond. 1963,42,69