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Vitrinite anisotropy under differential stress and high confining pressure and temperature: preliminary observations
International Journal of Coal Geology, 6 (1986) 343-351 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
343
Vitrinite anisotropy under differential stress and high confining pressure and temperature: p r e l i m i n a r y observations R.M. B U S T I N , J.V. ROSS and IAN MOFFAT
Department of Geological Sciences, The University of British Columbia, Vancouver, B.C. V6T 2B4, Canada ( Received August 28, 1985; revised and accepted March 26, 1986)
ABSTRACT Bustin, R.M., Ross, J.V. and Moffat, I., 1986. Vitrinite anistropy under differential stress and high confining pressure and temperature: preliminary observations. Int. J. Coal Geol., 6: 343-351. The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500 ° C, confining pressures of 500 and 800 MPa and a strain rate of 10 ~ s 1. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500 °C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70 ° from close to parallel to a~ in the undeformed state to perpendicular to a~ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.
INTRODUCTION
The optical reflectance of the coal maceral vitrinite has been extensively used as a measure of the degree of organic maturation (Teichmfiller and Teichmfiller, 1982). Although the mean reflectance of randomly oriented vitrinite particles is considered to be mainly the product of time-temperature history, there is considerable field evidence to suggest that the anisotropy of the reflectance indicatrix (optical indicating surface) in polarized light is a product of stress or strain conditions during organic metamorphism. In a number of field studies the orientation of the optical indicating surface of vitrinite has been utilized to infer the orientation and magnitude of differential stress during coalification (Stone and Cook, 1979; Hower and Davis, 1981a,b; Levine and Davis, 1984; Ting, 1984). In most studies vitrinite has been found to be uniax-
0166-5162/86/$03.50
© 1986 Elsevier Science Publishers B.V.
344
ial negative with the maximum reflectance contained in the plane of bedding (Dahme and Mackowsky, 1951; Williams, 1953) thereby suggesting that the orientation of the optical indicating surface was fixed during burial with the averaged maximum stress direction vertically orientated. In some areas of deformed strata however vitrinite has been demonstrated to have biaxial optical properties (Williams, 1953; Hevia and Virgos, 1977; Levine and Davis, 1984). The biaxial nature of vitrinite has been attributed to a triaxial stress field during at least part of the coalification history which promoted preferential orientation of aromatic-graphite like lammellae (Levine and Davis, 1984). Thus, because the aromatic structure (micro-structure) is anisotropic (Rouzaud and Oberlin, 1983), vitrinite reflectance is anisotropic as well (Cook et al., 1972; Stone and Cook, 1979; Forrest et al., 1984; Levine and Davis, 1984). Reorientation of the optical indicating surface of vitrinite in laboratory experiments has not been previously described, although at exceedingly high temperatures ( 2800-3400 °C ) reorientation of pyrolytic graphite with the "c" axis (minimum reflectance) parallel to the maximum compressive stress has been documented (Moore, 1973). A series of deformation experiments at high confining pressure and temperature were designed in an effort to produce reorientation of the optical indicating surface of vitrinite in a deviatoric stress field. In this paper we describe some of our preliminary observations and present some conclusions that may have a bearing on the interpretation of reflectance anisotropy and coal rank determinations based on reflectance. EXPERIMENTAL METHODS
In the initial experiments, reported here, homogeneous vitrain of anthracite rank from Pennsylvania was selected for analysis from the sample collection at the University of British Columbia. Anthracite was chosen both because it has a higher vitrinite bireflectance than lower rank coals and because the chemical structure of anthracite has been better documented ( Teichmfiller and Teichmfiller, 1982; Rouzaud and Oberlin, 1983). Prior to design of the experiments, it was determined by methods described later that the undeformed vitrinite had biaxial optical properties with the maximum and intermediate reflectance values (maximum and intermediate axes of the biaxial indicating surface ) located in the plane of bedding, and the minimum reflectance (minor axis of the indicating surface) normal to bedding. Thus the samples were deformed parallel to bedding inasmuch as it was reasoned that deforming the specimens normal to bedding would not reorient an indicating surface already normal to the direction of planned shortening (Fig. 1 ). It was necessary to core the sample slightly oblique (18 ° ) to the previously determined maximum reflectance orientation because of the shape of the sample. All cored samples were placed in talc assemblies and deformed in a solid
345 -A
-B-
Cores cut parallel
Cores deformed
to bedding
paraltel to bedding
-C
Analysed surfaces
-D-
Calculation of the orientation and magnitude of maximum and intermediate reflectance values
°'
Fig. 1. Diagrammatic summary of experimental methods utilized. A. All samples were cored parallel to bedding and each other from a single block of almost pure vitrain. B. The samples were deformed in a solid medium apparatus parallel to bedding. Because of the shape of the original block, the cores were cut with pre-experiment orientation of Romax in bedding oblique (18 ° ) to the core axis. C. At least three non-coplanar surfaces normal to bedding were cut and polished from each core and the magnitude and direction of Ro .... and Romi . determined for each face. D. The orientation and magnitude of the Romax values (vectors) determined on each face (dashed lines) were used to define an ellipse centered about an arbitrary origin. These vectors were used to calculate the orientation and magnitude of"true" Romaxand Roint~rm (vectors) in the bedding plane (solid lines). m e d i u m d e f o r m a t i o n a p p a r a t u s ( F i g . 1B; G r e e n e t al., 1 9 7 0 ) . T h e e x p e r i m e n t a l c o n d i t i o n s u t i l i z e d f o r e a c h s a m p l e a r e s u m m a r i z e d i n T a b l e 1. T h e d e g r e e o f c o n f i n e m e n t o f t h e v o l a t i l e s d u r i n g t h e e x p e r i m e n t s is u n k n o w n . T h e a s s e m bly was sealed by a loose fitting gas sampling bag during deformation but gas chromatography revealed no gas that could be attributed to devolatilization of the coal. Following the experiments the samples together with surrounding assembly TABLE 1 Summary of experimental conditions
GY-0 GY-190 GY-191 GY-192 GY-193 ]
Temperature (°C)
Confining pressure (MPa)
undeformed 350 350 500 500
500 500 500 800-500
-
Strain rate per s -
10 ~' 10 ~ 10 ~' 10 ~'
]The confining pressure for sample GY-193 during the onset of the experiment was 800 MPa and was subsequently lowered to 500 MPa.
346
were impregnated with epoxy and cut and polished along at least three noncoplanar orientations normal to bedding (Fig. 1C). The m a x i m u m (Ro ..... ) and minimum (Romin) reflectance values in oil and their orientation were determined from each of the polished faces utilizing a computerized Leitz MPV II and standard techniques {I.C.C.P., 1971; Bustin et al., 1983). For comparative and statistical purposes we also measured the orientation of Roma,, and Romm in sections cut as parallel to bedding as possible. On each analyzed face of each sample the measured maximum vitrinite reflectance was found to be parallel to bedding and the minimum vitrinite reflectance normal to bedding, within the precision with which reflectance orientation could be measured {2-3 ~ ). The orientation and magnitude of the measured maximum vitrinite reflectance values measured on each face are apparent maximum values that can range in value from the true maximum to the intermediate reflectance value depending on the orientation of the analyzed face. The apparent measured values are vectors that can be used to define an ellipse centered about an arbitrary origin in the plane of bedding (Fig. 1D; Stone and Cook, 1979). The ellipse is the calculated bedding plane section of the indicating surface ( CBPSIS of Stone and Cook, 1979) with major and minor axes that correspond in magnitude and direction to the "true" maximum and intermediate vitrinite reflectance values of the indicating surface. Such calculations assume that the major and intermediate axes (vitrinite reflectance values) of the indicating surface are parallel to bedding which was supported by measurements on all our samples. If the major and intermediate axes are not parallel to bedding the indicating ellipsoid would have to be calculated which is numerically possible although difficult to rigorously evaluate statistically. RESULTS
The reflectance data of the undeformed and deformed specimens are summarized in Table 2. The calculated bedding plane sections of the indicating surface (CBPSIS) are shown diagrammatically together with the deviatoric stress/strain curves for the deformed specimens in Fig. 2. The undeformed specimen GY-0 has biaxial negative optical properties with the maximum and intermediate reflectance values located in (or close to) the plane of bedding. The deformed samples are also biaxial negative and, although the indicating ellipsoid was not calculated, all the measurements indicate the maximum and intermediate reflectance values are in the plane of bedding and the minimum reflectance is perpendicular to bedding. In the undeformed specimen, which is only slightly anisotropic in the plane of bedding {bireflectance = 0.19%, Table 2), the calculated Ro ..... value makes an angle of about 18 ° to the core axis. In the deformed specimens the degree of anisotropy (bireflectance) and the magnitude of calculated Ro .... is greater and Romax occurs at larger angles to the core axis. The magnitude and degree of bireflectance of the deformed samples
4.90+0.12 4.75_+0.18 4.67+0.15 4.65_+0.30 5.11 _+0.12
0°
4.88+0.15 4.42+0.20 4.40_+0.17 5.40_+0.30 4.47_+0.13
45 °
Ro .... a z i m u t h f r o m core axis
4.75+0.11 4.55_+0.17 4.76_+0.12 6.33_+0.45 4.82+0.14
90 ° 2.66+0.40 2.38_+0.25 2.39+0.21 2.45_+0.44 2.41 + 0 . 1 6
Romin
4.92 4.94 5.11 6.34 5.70
Ro . . . .
4.73 4.40 4.40 4.64 4.60
Romter m
0.19 0.54 0.71 1.70 1.10
Birefl.
C a l c u l a t e d v a l u e s in b e d d i n g p l a n e s e c t i o n 2
18 ° + 3 ° 56 ° _+ 10 ° 48 ° _+ 5 ° 87 ° + 9 : 51 ° _+22 °
f r o m core a x i s
azimuth and StD
Roma x v e c t o r
~Ro..... a n d Ro,,,m m e a s u r e d v a l u e s were d e t e r m i n e d f r o m t h r e e s e c t i o n s o u t f r o m o r i e n t e d cores p e r p e n d i c u l a r to b e d d i n g at 0 , 4 5 ° a n d 90 ° to t h e core axis (Fig. 1 ). T h e v a l u e s s h o w n are t h e m e a n o f 50 m e a s u r e m e n t s a n d t h e s t a n d a r d d e v i a t i o n ( 9 5 % ) . ~Calculated values in b e d d i n g p l a n e s e c t i o n ( C B P S I S ) were d e t e r m i n e d as s u m m a r i z e d in Fig. 1. T h e s t a n d a r d d e v i a t i o n ( 9 5 % ) of t h e o r i e n t a t i o n of t h e calculated m a x i m u m r e f l e c t a n c e v e c t o r is b a s e d on 50 m e a s u r e m e n t s o n t h e b e d d i n g p l a n e s u r f a c e u s i n g Von M i s e s d i s t r i b u t i o n .
GY-0 GY-190 GY 191 GY-192 GY-193
Sample no.
Measured values 1
Measured and calculated vitrinite reflectance values
TABLE 2
348 O R I E N T A T I O N S OF C A L C U L A T E D
MAXIMUM
AND INTERMEDIATE
R E F L E C T A N C E V A L U E S IN B E D D I N G P L A N E S E C T I O N
3000
z w
2000,
u.
LU a:
GY
0
GY - 190
GY - 191
G Y 192
GY
193
(undeformed) 1000
-
~~:
G Y - 190
~
/
~__2Y
- 192
GY
191
193
STRAIN
(%)
Fig. 2. Stress/strain curves for the deformedspecimens and diagrammaticrepresentation of"the orientation and magnitudeof the Ro.....and Roi.t~rmvalues calculatedon the beddingplane surface (CBPSIS). varies according to the experimental conditions. Samples GY-190 and 191 deformed at 350 ° C have lower calculated Romax and lower bireflectance values in bedding t h a n samples GY-192 and 193 deformed at 500 ° C. Of the samples deformed at 350 and 500 oC, the samples with the highest strains have greater bireflectance and calculated Romax values (Table 2). In sample GY-192 deformed at 500 °C (23% axial strain) measured and calculated Romax values are nearly perpendicular to the core axis and thus also nearly perpendicular to a] during deformation. In the other three deformed specimens Ro .... occurs at angles of 48 ° , 51 o and 56 ° to the core axis. Thus in all the deformed specimens Romax appears to have rotated away from a~ when compared to the undeformed specimen. It is noteworthy t h a t in sample GY-191 (deformed at 350 ° C ) which was subjected to the greatest strain (33% axial strain) of the deformed samples, the indicating surface has rotated less t h a n in sample GY-192 with 23% axial strain but deformed at 500 ° C. The orientation Ro .... in bedding plane section of sample GY-193 was found to vary considerably within domains of the sample; in some areas Ro .... was essentially normal to the core axis whereas elsewhere Romax occurred at a variety of angles. Thus the orientation of the mean vector is poorly defined as indicated by the large standard deviation (Table 2 ).
349 DISCUSSION AND CONCLUSIONS The results of the experiments reported above indicate that reorientation of the optical indicatrix of vitrinite in anthracite coal is possible utilizing the experimental conditions of this study. Furthermore the results suggest that temperature, and stress or strain are important parameters: from comparison of the deformed specimens it is evident that higher temperature and larger strain facilitates reorientation of the indicating surface. Both the deformed and undeformed specimens have biaxial optical properties with the maximum and intermediate reflectance values close to or within the bedding plane revealing that the optical indicating surface has rotated within in the plane of bedding. Such results are consistent with the experimental conditions where ~ = a:~. In a review of the possible mechanisms by which aromatic lamellae in vitrinite may reorient, Levine and Davis (1984) suggest that preferred crystal growth is the most likely mechanism for naturally coalified material. They suggest that new carbon atoms may be added to the aromatic structure in an existing stress field without greatly involving pre-existing structure such that the resulting reflectance fabric would represent the cumulative effect of stress and temperature. Levine and Davis (1984) argue that, analogous to graphite, the stable orientation would be with the "c" axis and thus minimum reflectance aligned in the direction of maximum compressive stress. Alternative mechanisms for reorientation suggested by Levine and Davis are intracrystalline slip and twinning and rigid rotation of inequant grains. Transmission electron microscopy studies by Rouzaud and Oberlin (1983) led them to suggest that the "statistical molecular orientation" observed in anthracite results from flattening of pores under shear stress. The mechanism(s) of which reorientation of the optical indicating surface in the present study occurred is unknown. In the experiments in this study an increase in Romaxw a s observed suggesting that an increased level of metamorphism, as measured by Ro .... accompanied deformation. Whether or not the increase in reflectance was a product of reorientation of the aromatic lamellae, flattening of pores, or a result of devolatilization and increased aromaticity and size of aromatic clusters, is unknown. The presence of epoxy, with which the samples were impregnated after deformation, resulted in unreliable chemical analysis. Orientations of the optical indicating surface in the deformed specimens suggest that under some conditions the indicating surface may be a composite product of the original indicatrix orientation and an orientation produced during the deformation as postulated by Levine and Davis (1984). In samples GY190, 191 and 193 the optical indicating surface is at a high angle to the deviatoric stress direction used in the experiments and thus cannot be in a stable orientation. It is thus possible that the orientation of the maximum reflectance rather than indicating a discrete orientation of molecular groups is a composite
350
of reoriented and non-reoriented molecular groups. The wide variation in maximum reflectance orientations in sample GY-193 may be the result of the presence of completely reoriented domains in the sample and domains in which only partial reorientation has occurred. Resolving the molecular orientation in these samples may be facilitated by transmission electron microscopy studies such as those of Rouzaud and Oberlin (1983). In contrast to samples GY-190, 191 and 193, the orientation of the indicating surface of sample GY-192 is such that the maximum reflectance value is nearly perpendicular to the core (parallel to plane of flattening during deformation) indicating the indicating surface has rotated through at least 70 ° to conform with the normal to the principal stress direction during deformation. In this sample the original orientation of the indicatrix has been completely overprinted by the deformation. In summary, the results of the experiments discussed above indicate that the reorientation of the indicating surface of vitrinite in anthracite coal is possible utilizing our experimental conditions. Clearly, additional studies are required prior to drawing conclusions with regard to the geological applicability of the indicating surface as a stress and/or strain marker. In particular the importance of magnitude of differential stress, strain rate, confining pressure, time (kinetics) and temperature must be investigated. Also it is unknown if any of our results are applicable to less mature (lower rank) organic matter which has a less ordered structure than the anthracite investigated. The mechanism(s) involved in reorientation of the indicating surface are unknown and the results of this study suggest that the orientation of the indicating surface may be a composite product of the deformation/temperature history as proposed by Levine and Davis (1984). It is also premature to suggest that the mechanisms of reorientation of the optical indicating surface using our experimental conditions with high temperature and large strains are the same as in nature. Additional deformation studies together with transmission electron microscopy presently underway may answer some of these questions. ACKNOWLEDGMENTS
Financial support for this study was received from the Natural Sciences and Engineering Research Council of Canada. We thank Dr. T.H. Brown of The University of British Columbia for his help with mathematical treatment of the data. We are particularly grateful to two anonymous reviewers whose unusually careful reading of an earlier draft of the manuscript has increased the quality of this paper greatly.
REFERENCES Bustin, R.M., Cameron, A.C., Greive, D. and Kalkreuth, W.D., 1983. Coal petrology: its principles, methods and applications. Geol. Assoc. Can., Short Course Notes 3,273 pp.
351 Cook, A.C., Murchison, D.G. and Scott, E., 1972. Optically biaxial vitrinites. Fuel, 51: 180-184. Dahme, A. and Mackowsky, M.-Th., 1951. Mikroskopische, chemische, und rontgenographische Untersuchungen an Anthraziten. Brennst.-Chem., 32: 175-186. Forrest, R.A., Marsh, H. and Cornford, C., 1984. Optical properties of anisotropic carbon. In: P. Thrower (Editor), Chemistry and Physics of Carbon, Vol. 19. Marcel Dekker, New York, N.Y., pp. 211-330. Green, H.W., Griggs, D.T. and Christie, J.M., 1970. Syntectonic and annealing recrystallization of fine-grained quartz aggregates. In: P. Paulitsch (Editor), Experimental and Natural Rock Deformation. Springer, New York, N.Y., pp. 272-335. Hevia, V. and Virgos, J.M., 1977. The rank and anisotropy of anthracites: the indicating surface of reflectivity in uniaxial and biaxial substances. J. Microsc., 100: 23-28. Hower, J.C. and Davis, A., 1981a. Application of vitrinite reflectance anisotropy in the evaluation of coal metamorphism. Geol. Soc. Am. Bull., 92: 350-366. Hower, J.C. and Davis, A., 1981b. Vitrinite reflectance anisotropy as a tectonic fabric element. Geology, 9: 165-168. International Committee for Coal Petrography (I.C.C.P.), 1971. International Handbook of Coal Petrology. Centre National de la Recherche Scientifique, Paris, 2nd ed., 1st suppl. Levine, J.R. and Davis, A., 1984. Optical anisotropy of coals as an indicator of tectonic deformation, Broad Top Coal Field, Pennsylvania. Geol. Soc. Am. Bull., 95: 100-108. Moore, A.W., 1973. Highly oriented pyrolytic graphite. In: P.L. Walker and P.A. Thrower (Editors), Chemistry and Physics of Carbon, Vol. 11. Marcel Dekker, New York, N.Y., pp. 69-187. Rouzaud, Z.N. and Oberlin, A., 1983. Contribution of high resolution electron microscopy {TEM ) to organic materials characterization and interpretation of their reflectance. In: B. Durand (Editor), Their Phenomena in Sedimentary Basins. Int. Colloquium, Bordeaux, June 7 10, 1983, Editions Technip, Paris, pp. 127-132. Stone, I.J. and Cook, A.C., 1979. The influence of some tectonic structures upon vitrinite reflectance. J. Geol., 87: 497-508. Teichmiiller, M. and Teichmiiller, R., 1982. The geological basis of coal formation. In: E. Stach, M.-Th. Mackowsky, M. Teichmiiller, G.H. Taylor, D. Chandra and R. Teichmiiller (Editors), Stachs Text Book of Coal Petrology. Borntraeger, Berlin, pp. 5 86. Ting, F.C.T., 1984. Uniaxial and biaxial vitrinite reflectance models and their relationship to paleotectonics. In: J. Brooks (Editor), Organic Maturation Studies and Fossil Fuel Exploration. Academic Press, New York, N.Y., pp. 379-391. Williams, E., 1953. Anisotropy of vitrain of south Wales coals. Fuel, 32: 88-99.
343
Vitrinite anisotropy under differential stress and high confining pressure and temperature: p r e l i m i n a r y observations R.M. B U S T I N , J.V. ROSS and IAN MOFFAT
Department of Geological Sciences, The University of British Columbia, Vancouver, B.C. V6T 2B4, Canada ( Received August 28, 1985; revised and accepted March 26, 1986)
ABSTRACT Bustin, R.M., Ross, J.V. and Moffat, I., 1986. Vitrinite anistropy under differential stress and high confining pressure and temperature: preliminary observations. Int. J. Coal Geol., 6: 343-351. The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500 ° C, confining pressures of 500 and 800 MPa and a strain rate of 10 ~ s 1. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500 °C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70 ° from close to parallel to a~ in the undeformed state to perpendicular to a~ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.
INTRODUCTION
The optical reflectance of the coal maceral vitrinite has been extensively used as a measure of the degree of organic maturation (Teichmfiller and Teichmfiller, 1982). Although the mean reflectance of randomly oriented vitrinite particles is considered to be mainly the product of time-temperature history, there is considerable field evidence to suggest that the anisotropy of the reflectance indicatrix (optical indicating surface) in polarized light is a product of stress or strain conditions during organic metamorphism. In a number of field studies the orientation of the optical indicating surface of vitrinite has been utilized to infer the orientation and magnitude of differential stress during coalification (Stone and Cook, 1979; Hower and Davis, 1981a,b; Levine and Davis, 1984; Ting, 1984). In most studies vitrinite has been found to be uniax-
0166-5162/86/$03.50
© 1986 Elsevier Science Publishers B.V.
344
ial negative with the maximum reflectance contained in the plane of bedding (Dahme and Mackowsky, 1951; Williams, 1953) thereby suggesting that the orientation of the optical indicating surface was fixed during burial with the averaged maximum stress direction vertically orientated. In some areas of deformed strata however vitrinite has been demonstrated to have biaxial optical properties (Williams, 1953; Hevia and Virgos, 1977; Levine and Davis, 1984). The biaxial nature of vitrinite has been attributed to a triaxial stress field during at least part of the coalification history which promoted preferential orientation of aromatic-graphite like lammellae (Levine and Davis, 1984). Thus, because the aromatic structure (micro-structure) is anisotropic (Rouzaud and Oberlin, 1983), vitrinite reflectance is anisotropic as well (Cook et al., 1972; Stone and Cook, 1979; Forrest et al., 1984; Levine and Davis, 1984). Reorientation of the optical indicating surface of vitrinite in laboratory experiments has not been previously described, although at exceedingly high temperatures ( 2800-3400 °C ) reorientation of pyrolytic graphite with the "c" axis (minimum reflectance) parallel to the maximum compressive stress has been documented (Moore, 1973). A series of deformation experiments at high confining pressure and temperature were designed in an effort to produce reorientation of the optical indicating surface of vitrinite in a deviatoric stress field. In this paper we describe some of our preliminary observations and present some conclusions that may have a bearing on the interpretation of reflectance anisotropy and coal rank determinations based on reflectance. EXPERIMENTAL METHODS
In the initial experiments, reported here, homogeneous vitrain of anthracite rank from Pennsylvania was selected for analysis from the sample collection at the University of British Columbia. Anthracite was chosen both because it has a higher vitrinite bireflectance than lower rank coals and because the chemical structure of anthracite has been better documented ( Teichmfiller and Teichmfiller, 1982; Rouzaud and Oberlin, 1983). Prior to design of the experiments, it was determined by methods described later that the undeformed vitrinite had biaxial optical properties with the maximum and intermediate reflectance values (maximum and intermediate axes of the biaxial indicating surface ) located in the plane of bedding, and the minimum reflectance (minor axis of the indicating surface) normal to bedding. Thus the samples were deformed parallel to bedding inasmuch as it was reasoned that deforming the specimens normal to bedding would not reorient an indicating surface already normal to the direction of planned shortening (Fig. 1 ). It was necessary to core the sample slightly oblique (18 ° ) to the previously determined maximum reflectance orientation because of the shape of the sample. All cored samples were placed in talc assemblies and deformed in a solid
345 -A
-B-
Cores cut parallel
Cores deformed
to bedding
paraltel to bedding
-C
Analysed surfaces
-D-
Calculation of the orientation and magnitude of maximum and intermediate reflectance values
°'
Fig. 1. Diagrammatic summary of experimental methods utilized. A. All samples were cored parallel to bedding and each other from a single block of almost pure vitrain. B. The samples were deformed in a solid medium apparatus parallel to bedding. Because of the shape of the original block, the cores were cut with pre-experiment orientation of Romax in bedding oblique (18 ° ) to the core axis. C. At least three non-coplanar surfaces normal to bedding were cut and polished from each core and the magnitude and direction of Ro .... and Romi . determined for each face. D. The orientation and magnitude of the Romax values (vectors) determined on each face (dashed lines) were used to define an ellipse centered about an arbitrary origin. These vectors were used to calculate the orientation and magnitude of"true" Romaxand Roint~rm (vectors) in the bedding plane (solid lines). m e d i u m d e f o r m a t i o n a p p a r a t u s ( F i g . 1B; G r e e n e t al., 1 9 7 0 ) . T h e e x p e r i m e n t a l c o n d i t i o n s u t i l i z e d f o r e a c h s a m p l e a r e s u m m a r i z e d i n T a b l e 1. T h e d e g r e e o f c o n f i n e m e n t o f t h e v o l a t i l e s d u r i n g t h e e x p e r i m e n t s is u n k n o w n . T h e a s s e m bly was sealed by a loose fitting gas sampling bag during deformation but gas chromatography revealed no gas that could be attributed to devolatilization of the coal. Following the experiments the samples together with surrounding assembly TABLE 1 Summary of experimental conditions
GY-0 GY-190 GY-191 GY-192 GY-193 ]
Temperature (°C)
Confining pressure (MPa)
undeformed 350 350 500 500
500 500 500 800-500
-
Strain rate per s -
10 ~' 10 ~ 10 ~' 10 ~'
]The confining pressure for sample GY-193 during the onset of the experiment was 800 MPa and was subsequently lowered to 500 MPa.
346
were impregnated with epoxy and cut and polished along at least three noncoplanar orientations normal to bedding (Fig. 1C). The m a x i m u m (Ro ..... ) and minimum (Romin) reflectance values in oil and their orientation were determined from each of the polished faces utilizing a computerized Leitz MPV II and standard techniques {I.C.C.P., 1971; Bustin et al., 1983). For comparative and statistical purposes we also measured the orientation of Roma,, and Romm in sections cut as parallel to bedding as possible. On each analyzed face of each sample the measured maximum vitrinite reflectance was found to be parallel to bedding and the minimum vitrinite reflectance normal to bedding, within the precision with which reflectance orientation could be measured {2-3 ~ ). The orientation and magnitude of the measured maximum vitrinite reflectance values measured on each face are apparent maximum values that can range in value from the true maximum to the intermediate reflectance value depending on the orientation of the analyzed face. The apparent measured values are vectors that can be used to define an ellipse centered about an arbitrary origin in the plane of bedding (Fig. 1D; Stone and Cook, 1979). The ellipse is the calculated bedding plane section of the indicating surface ( CBPSIS of Stone and Cook, 1979) with major and minor axes that correspond in magnitude and direction to the "true" maximum and intermediate vitrinite reflectance values of the indicating surface. Such calculations assume that the major and intermediate axes (vitrinite reflectance values) of the indicating surface are parallel to bedding which was supported by measurements on all our samples. If the major and intermediate axes are not parallel to bedding the indicating ellipsoid would have to be calculated which is numerically possible although difficult to rigorously evaluate statistically. RESULTS
The reflectance data of the undeformed and deformed specimens are summarized in Table 2. The calculated bedding plane sections of the indicating surface (CBPSIS) are shown diagrammatically together with the deviatoric stress/strain curves for the deformed specimens in Fig. 2. The undeformed specimen GY-0 has biaxial negative optical properties with the maximum and intermediate reflectance values located in (or close to) the plane of bedding. The deformed samples are also biaxial negative and, although the indicating ellipsoid was not calculated, all the measurements indicate the maximum and intermediate reflectance values are in the plane of bedding and the minimum reflectance is perpendicular to bedding. In the undeformed specimen, which is only slightly anisotropic in the plane of bedding {bireflectance = 0.19%, Table 2), the calculated Ro ..... value makes an angle of about 18 ° to the core axis. In the deformed specimens the degree of anisotropy (bireflectance) and the magnitude of calculated Ro .... is greater and Romax occurs at larger angles to the core axis. The magnitude and degree of bireflectance of the deformed samples
4.90+0.12 4.75_+0.18 4.67+0.15 4.65_+0.30 5.11 _+0.12
0°
4.88+0.15 4.42+0.20 4.40_+0.17 5.40_+0.30 4.47_+0.13
45 °
Ro .... a z i m u t h f r o m core axis
4.75+0.11 4.55_+0.17 4.76_+0.12 6.33_+0.45 4.82+0.14
90 ° 2.66+0.40 2.38_+0.25 2.39+0.21 2.45_+0.44 2.41 + 0 . 1 6
Romin
4.92 4.94 5.11 6.34 5.70
Ro . . . .
4.73 4.40 4.40 4.64 4.60
Romter m
0.19 0.54 0.71 1.70 1.10
Birefl.
C a l c u l a t e d v a l u e s in b e d d i n g p l a n e s e c t i o n 2
18 ° + 3 ° 56 ° _+ 10 ° 48 ° _+ 5 ° 87 ° + 9 : 51 ° _+22 °
f r o m core a x i s
azimuth and StD
Roma x v e c t o r
~Ro..... a n d Ro,,,m m e a s u r e d v a l u e s were d e t e r m i n e d f r o m t h r e e s e c t i o n s o u t f r o m o r i e n t e d cores p e r p e n d i c u l a r to b e d d i n g at 0 , 4 5 ° a n d 90 ° to t h e core axis (Fig. 1 ). T h e v a l u e s s h o w n are t h e m e a n o f 50 m e a s u r e m e n t s a n d t h e s t a n d a r d d e v i a t i o n ( 9 5 % ) . ~Calculated values in b e d d i n g p l a n e s e c t i o n ( C B P S I S ) were d e t e r m i n e d as s u m m a r i z e d in Fig. 1. T h e s t a n d a r d d e v i a t i o n ( 9 5 % ) of t h e o r i e n t a t i o n of t h e calculated m a x i m u m r e f l e c t a n c e v e c t o r is b a s e d on 50 m e a s u r e m e n t s o n t h e b e d d i n g p l a n e s u r f a c e u s i n g Von M i s e s d i s t r i b u t i o n .
GY-0 GY-190 GY 191 GY-192 GY-193
Sample no.
Measured values 1
Measured and calculated vitrinite reflectance values
TABLE 2
348 O R I E N T A T I O N S OF C A L C U L A T E D
MAXIMUM
AND INTERMEDIATE
R E F L E C T A N C E V A L U E S IN B E D D I N G P L A N E S E C T I O N
3000
z w
2000,
u.
LU a:
GY
0
GY - 190
GY - 191
G Y 192
GY
193
(undeformed) 1000
-
~~:
G Y - 190
~
/
~__2Y
- 192
GY
191
193
STRAIN
(%)
Fig. 2. Stress/strain curves for the deformedspecimens and diagrammaticrepresentation of"the orientation and magnitudeof the Ro.....and Roi.t~rmvalues calculatedon the beddingplane surface (CBPSIS). varies according to the experimental conditions. Samples GY-190 and 191 deformed at 350 ° C have lower calculated Romax and lower bireflectance values in bedding t h a n samples GY-192 and 193 deformed at 500 ° C. Of the samples deformed at 350 and 500 oC, the samples with the highest strains have greater bireflectance and calculated Romax values (Table 2). In sample GY-192 deformed at 500 °C (23% axial strain) measured and calculated Romax values are nearly perpendicular to the core axis and thus also nearly perpendicular to a] during deformation. In the other three deformed specimens Ro .... occurs at angles of 48 ° , 51 o and 56 ° to the core axis. Thus in all the deformed specimens Romax appears to have rotated away from a~ when compared to the undeformed specimen. It is noteworthy t h a t in sample GY-191 (deformed at 350 ° C ) which was subjected to the greatest strain (33% axial strain) of the deformed samples, the indicating surface has rotated less t h a n in sample GY-192 with 23% axial strain but deformed at 500 ° C. The orientation Ro .... in bedding plane section of sample GY-193 was found to vary considerably within domains of the sample; in some areas Ro .... was essentially normal to the core axis whereas elsewhere Romax occurred at a variety of angles. Thus the orientation of the mean vector is poorly defined as indicated by the large standard deviation (Table 2 ).
349 DISCUSSION AND CONCLUSIONS The results of the experiments reported above indicate that reorientation of the optical indicatrix of vitrinite in anthracite coal is possible utilizing the experimental conditions of this study. Furthermore the results suggest that temperature, and stress or strain are important parameters: from comparison of the deformed specimens it is evident that higher temperature and larger strain facilitates reorientation of the indicating surface. Both the deformed and undeformed specimens have biaxial optical properties with the maximum and intermediate reflectance values close to or within the bedding plane revealing that the optical indicating surface has rotated within in the plane of bedding. Such results are consistent with the experimental conditions where ~ = a:~. In a review of the possible mechanisms by which aromatic lamellae in vitrinite may reorient, Levine and Davis (1984) suggest that preferred crystal growth is the most likely mechanism for naturally coalified material. They suggest that new carbon atoms may be added to the aromatic structure in an existing stress field without greatly involving pre-existing structure such that the resulting reflectance fabric would represent the cumulative effect of stress and temperature. Levine and Davis (1984) argue that, analogous to graphite, the stable orientation would be with the "c" axis and thus minimum reflectance aligned in the direction of maximum compressive stress. Alternative mechanisms for reorientation suggested by Levine and Davis are intracrystalline slip and twinning and rigid rotation of inequant grains. Transmission electron microscopy studies by Rouzaud and Oberlin (1983) led them to suggest that the "statistical molecular orientation" observed in anthracite results from flattening of pores under shear stress. The mechanism(s) of which reorientation of the optical indicating surface in the present study occurred is unknown. In the experiments in this study an increase in Romaxw a s observed suggesting that an increased level of metamorphism, as measured by Ro .... accompanied deformation. Whether or not the increase in reflectance was a product of reorientation of the aromatic lamellae, flattening of pores, or a result of devolatilization and increased aromaticity and size of aromatic clusters, is unknown. The presence of epoxy, with which the samples were impregnated after deformation, resulted in unreliable chemical analysis. Orientations of the optical indicating surface in the deformed specimens suggest that under some conditions the indicating surface may be a composite product of the original indicatrix orientation and an orientation produced during the deformation as postulated by Levine and Davis (1984). In samples GY190, 191 and 193 the optical indicating surface is at a high angle to the deviatoric stress direction used in the experiments and thus cannot be in a stable orientation. It is thus possible that the orientation of the maximum reflectance rather than indicating a discrete orientation of molecular groups is a composite
350
of reoriented and non-reoriented molecular groups. The wide variation in maximum reflectance orientations in sample GY-193 may be the result of the presence of completely reoriented domains in the sample and domains in which only partial reorientation has occurred. Resolving the molecular orientation in these samples may be facilitated by transmission electron microscopy studies such as those of Rouzaud and Oberlin (1983). In contrast to samples GY-190, 191 and 193, the orientation of the indicating surface of sample GY-192 is such that the maximum reflectance value is nearly perpendicular to the core (parallel to plane of flattening during deformation) indicating the indicating surface has rotated through at least 70 ° to conform with the normal to the principal stress direction during deformation. In this sample the original orientation of the indicatrix has been completely overprinted by the deformation. In summary, the results of the experiments discussed above indicate that the reorientation of the indicating surface of vitrinite in anthracite coal is possible utilizing our experimental conditions. Clearly, additional studies are required prior to drawing conclusions with regard to the geological applicability of the indicating surface as a stress and/or strain marker. In particular the importance of magnitude of differential stress, strain rate, confining pressure, time (kinetics) and temperature must be investigated. Also it is unknown if any of our results are applicable to less mature (lower rank) organic matter which has a less ordered structure than the anthracite investigated. The mechanism(s) involved in reorientation of the indicating surface are unknown and the results of this study suggest that the orientation of the indicating surface may be a composite product of the deformation/temperature history as proposed by Levine and Davis (1984). It is also premature to suggest that the mechanisms of reorientation of the optical indicating surface using our experimental conditions with high temperature and large strains are the same as in nature. Additional deformation studies together with transmission electron microscopy presently underway may answer some of these questions. ACKNOWLEDGMENTS
Financial support for this study was received from the Natural Sciences and Engineering Research Council of Canada. We thank Dr. T.H. Brown of The University of British Columbia for his help with mathematical treatment of the data. We are particularly grateful to two anonymous reviewers whose unusually careful reading of an earlier draft of the manuscript has increased the quality of this paper greatly.
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