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An etching technique for revealing dislocation structures in deformed quartz grains
Tectonophysics, 37 (1977) T9-T14 o Elsevier Scientific Publishing Company,
Amsterdam
- Printed
in The Netherlands
Letter Section
-_
An etching technique for revealing dislocation structures iy deformed quartz gEiklS
A. BALL’ and S. WHITE’ ‘Department of Metallurgy and Materials Science, University of Cape Town, Cape Town (South Africa) ‘Departments of Geology and Metallurgy, Imperial College of Science and Technology, London (Great 3r~ta~n~ (Submitted
August
13, 1976; accepted
October
25, 1976)
ABSTRACT Ball, A. and White, S., 1977. An etching technique for revealing deformed quartz grains. Tectonophysics, 37: T9-T14.
dislocation
structures
in
A simple, inexpensive etching technique for revealing the dislocation structures in deformed quartz grains is outlined. Sub-grain sizes and dislocation densities determined on etched surfaces are similar to those measured in micrographs, from the same specimens, obtained with a high-voltage transmission electron microscope.
INTRODUCTION
The development of convenient techniques for the determination of dislocation structures in quartz is important since recent studies (White, 1975,1976a) have indicated that the dislocation structures may preserve a memory of past deformation conditions. White has proposed that the sub-grains decrease in size as the stress or strain rate increases and has used sub-grain sizes to estimate stresses and strain rates for crustal deformations. A correlation between the increase in dislocation density and increase in stress for exper~ent~ly deformed olivine has been applied by Kolstedt et al. (1976) to the determination of paleo-stresses in the mantle from the dislocation densities in naturally deformed olivine. Therefore it is important that structural geologists be able to determine sub-gram size and dislocation densities in deformed minerals. The dislocation structures in quartz have only been studied with ease in a high-voltage electron microscope (EVER) since beam damage occurs rapidly at lower voltages, for example at 100 kV (White, 1973,1976b). The use of a HVEM also enables the investigator to examine thicker sections of his sample. As a consequence of this, larger areas of specimens can be photographed in
T10
50pm Fig.1. Typical sub-grain structures in quartz grains in a specimen of the Witwatersrand quartzite observed by transmission electron microscopy. (a) using an accelerating voltage of 200 kV, and (b) using an accelerating voltage of 1000 kV. The area enclosed in the rectangle is the same as in (a).
order to determine sub-grain dimensions and to study the distribution of dislocations. The increase in area transparent at 1000 kV compared to that at 200 kV can be seen in Fig.1. It is clear, that for this specimen it is difficult to measure sub-grains in conventional 100 or 200 kV transmission electron microscopes. The dislocations within the sub-grains are shown in Fig.2. Few geologists have access to a HVEM and there is a need to develop simple and inexpensive, but nonetheless effective, techniques for revealing dislocations in quartz. It should be noted that some sub-grain structures in naturally deformed quartz can be detected in a petrological microscope. However, this optical technique is limited because the frequent presence of smaller, less misorientated sub-grains within the larger sub-grains cannot be detected (White, 1973) and individual dislocations are invisible. Attempts to ‘decorate’ dislocations with impurity ions have been made (White, 1970) but the observed structures are not reliable since the necessary elevated temperature treatment required for decoration promotes recovery and alters the original structure. Much effort has been given to the development of simple and rapid techniques
Tll
Fig.2. An electron micrograph (1000 kV) showing the dislocations inside a sub-grain.
of etch pitting dislocation sites in quartz (Pate1 et al., 1965; Vol’skaya, 1969). However, prior to the work reported in this paper no successful procedure has been devised that is capable of revealing, in polished sections, the sub-grain structures and densities of dislocations (10” to 1014 m-*) known to exist in naturally deformed quartzite. THE ETCH PITTING TECHNIQUE
Crystal dislocations possess an environ of strained lattice or an atmosphere of impurity ions. Etch pits may be developed on a crystal surface plane at dislocation sites when the specimen is immersed in a specific solution if these local differences in elastic or chemical energy promote a differential in dissolution rates. In order to resolve these sites by optical or scanning electron microscopy the etching solution or conditions must be adjusted in order to optimize this differential. In the case of quartz, it is well known that a solution containing 48% hydrofluoric acid in water causes a general faceted dissolution of crystal planes (Fig.3) and also creates large pits along scratches and at impurity particles. We found that the selectivity of the etching could be increased by diluting the 48% hydrofluoric acid in concentrated nitric acid and observed that a solution of 3 parts of 48% hydrofluoric acid in 97 parts of con-
T12
Fig.3. A scanning electron micrograph by 48% hydrofluoric acid.
showing
the rapid general etching
which
is produced
centrated nitric acid produced an etch pit array in certain grains, namely those with their surface cut parallel or nearly parallel to the [OOOl] zone axis, of a polished section of Witwatersrand quartzite. The etch structures correlate well with the dislocation structures observed by transmission electron microscopy. It was found necessary to carefully prepare the cut sections and give them a final polish with 0.05 micron alumina on a silk or rayon lap. Typical etching times are between 24 and 30 hours. The etch pits were revealed by using a reflected light optical microscope fitted with a Nomarski interference attachment (Fig.4) or by scanning electron microscopy (Figs.5 a and b). The pits are crystallographic in nature and their detailed shape corresponds with that defined by Frondel (1962) for prism planes. The etch patterns correlate well with the dislocation structures observed in foils from the same specimens with the HVEM (Fig.1). The etching clearly reveals the linear arrays which form the sub-grain structures within the grains. The etching technique has the obvious advantage of displaying all the subgrains in a grain and also of showing the sub-grain structures in several grains after a single etch. The individual dislocations in some walls and within the interiors of the sub-grains are best revealed in a scanning electron microscope.
T13
iion. The sible .
Fig.5. Typical etch pit structures and distributions revealed by scanning electron microscopy at (a) low magnification, and (b) higher magnifications.
We found that many of the small pits clearly evident in scanning electron micrographs (Fig.5b) were not well resolved in optical micrographs. Thus the dislocation densities measured from optical micrographs were consistently an order of magnitude less than those measured from scanning or transmission electron micrographs. Thus in the Witwatersrand quartzite studied, the optical micrographs gave a density of between 10” and 101’ rnw2whereas in both
T14
types of electron micrographs, the average densities are between lo1 ’ and 10’ 2 rne2. This indicates that accurate determinations of dislocation densities from etch pits can only be done in a scanning electron microscope. However, the sub-grain sizes were similar in optical, scanning and transmission electron micrographs and an average diameter of 46 I_tmwas obtained from both optical and scanning electron micrographs of etched surfaces compared with 43 ym obtained from high voltage electron micrographs from the same specimens. We found that it was most convenient to measure sub-grains from optical micrographs taken at low magnification. CONCLUSION
We wish to emphasize that the simple and inexpensive technique of etch pitting described can provide much qualitative and quantitative information of the type mentioned which is of great value to structural geologists who are concerned with dislocation structures in quartz and with deformation conditions in the earth’s crust. Previously this information could only be obtained with a high-voltage electron microscope. ACKNOWLEDGEMENTS
The Royal Society are thanked for the support of S. White through a Mr. and Mrs. John Jaffe Donation Research Fellowship. The Chamber of Mines of South Africa are thanked for financial support given to this work and for supplying specimens.
REFERENCES Frondel, C., 1962. Dana’s System of Mineralogy. Vol. III, Silica Minerals. Wiley, New York, 334 pp. Kolstedt, D.L., Goetze, C., Durham, W.B. and Van der Sande, J., 1976. New technique for decorating dislocations in olivine. Science, 191: 1045-1046. Patel, A.R., Bahl, O.P. and Vagh, A.S., 1965. Etching of rhombohedral cleavages of quartz. Acta Cryst., 19: 757-758. Vol’skaya, O.B., 1969. Selective etchants for quartz. Sov. Phys. Cryst., 13: 620-622. White, S., 1970. Defect and Diffusion Studies of Quartz. Unpubl. thesis, Univ. Melbourne. White, S., 1973. The dislocation structures responsible for the optical effects in some naturally deformed quartzites. J. Mater. Sci., 9: 490-499. White, S., 1975. Estimation of strain rates from microstructural features. Q. J. Geol. Sot. London, 131: 577-583. White, S., 1976a. Estimation of deformation parameters from dislocation sub-structures in naturally deformed quartz. In: J.A. Venables (editor), Developments in Electron Microscopy and Analysis. Academic Press, London, pp.505-505. White, S., 1976b. Effects of strain on the microstructures, fabrics and deformation mechanisms. Philos. Trans. R. Sot. London, Ser. A (in press).
Amsterdam
- Printed
in The Netherlands
Letter Section
-_
An etching technique for revealing dislocation structures iy deformed quartz gEiklS
A. BALL’ and S. WHITE’ ‘Department of Metallurgy and Materials Science, University of Cape Town, Cape Town (South Africa) ‘Departments of Geology and Metallurgy, Imperial College of Science and Technology, London (Great 3r~ta~n~ (Submitted
August
13, 1976; accepted
October
25, 1976)
ABSTRACT Ball, A. and White, S., 1977. An etching technique for revealing deformed quartz grains. Tectonophysics, 37: T9-T14.
dislocation
structures
in
A simple, inexpensive etching technique for revealing the dislocation structures in deformed quartz grains is outlined. Sub-grain sizes and dislocation densities determined on etched surfaces are similar to those measured in micrographs, from the same specimens, obtained with a high-voltage transmission electron microscope.
INTRODUCTION
The development of convenient techniques for the determination of dislocation structures in quartz is important since recent studies (White, 1975,1976a) have indicated that the dislocation structures may preserve a memory of past deformation conditions. White has proposed that the sub-grains decrease in size as the stress or strain rate increases and has used sub-grain sizes to estimate stresses and strain rates for crustal deformations. A correlation between the increase in dislocation density and increase in stress for exper~ent~ly deformed olivine has been applied by Kolstedt et al. (1976) to the determination of paleo-stresses in the mantle from the dislocation densities in naturally deformed olivine. Therefore it is important that structural geologists be able to determine sub-gram size and dislocation densities in deformed minerals. The dislocation structures in quartz have only been studied with ease in a high-voltage electron microscope (EVER) since beam damage occurs rapidly at lower voltages, for example at 100 kV (White, 1973,1976b). The use of a HVEM also enables the investigator to examine thicker sections of his sample. As a consequence of this, larger areas of specimens can be photographed in
T10
50pm Fig.1. Typical sub-grain structures in quartz grains in a specimen of the Witwatersrand quartzite observed by transmission electron microscopy. (a) using an accelerating voltage of 200 kV, and (b) using an accelerating voltage of 1000 kV. The area enclosed in the rectangle is the same as in (a).
order to determine sub-grain dimensions and to study the distribution of dislocations. The increase in area transparent at 1000 kV compared to that at 200 kV can be seen in Fig.1. It is clear, that for this specimen it is difficult to measure sub-grains in conventional 100 or 200 kV transmission electron microscopes. The dislocations within the sub-grains are shown in Fig.2. Few geologists have access to a HVEM and there is a need to develop simple and inexpensive, but nonetheless effective, techniques for revealing dislocations in quartz. It should be noted that some sub-grain structures in naturally deformed quartz can be detected in a petrological microscope. However, this optical technique is limited because the frequent presence of smaller, less misorientated sub-grains within the larger sub-grains cannot be detected (White, 1973) and individual dislocations are invisible. Attempts to ‘decorate’ dislocations with impurity ions have been made (White, 1970) but the observed structures are not reliable since the necessary elevated temperature treatment required for decoration promotes recovery and alters the original structure. Much effort has been given to the development of simple and rapid techniques
Tll
Fig.2. An electron micrograph (1000 kV) showing the dislocations inside a sub-grain.
of etch pitting dislocation sites in quartz (Pate1 et al., 1965; Vol’skaya, 1969). However, prior to the work reported in this paper no successful procedure has been devised that is capable of revealing, in polished sections, the sub-grain structures and densities of dislocations (10” to 1014 m-*) known to exist in naturally deformed quartzite. THE ETCH PITTING TECHNIQUE
Crystal dislocations possess an environ of strained lattice or an atmosphere of impurity ions. Etch pits may be developed on a crystal surface plane at dislocation sites when the specimen is immersed in a specific solution if these local differences in elastic or chemical energy promote a differential in dissolution rates. In order to resolve these sites by optical or scanning electron microscopy the etching solution or conditions must be adjusted in order to optimize this differential. In the case of quartz, it is well known that a solution containing 48% hydrofluoric acid in water causes a general faceted dissolution of crystal planes (Fig.3) and also creates large pits along scratches and at impurity particles. We found that the selectivity of the etching could be increased by diluting the 48% hydrofluoric acid in concentrated nitric acid and observed that a solution of 3 parts of 48% hydrofluoric acid in 97 parts of con-
T12
Fig.3. A scanning electron micrograph by 48% hydrofluoric acid.
showing
the rapid general etching
which
is produced
centrated nitric acid produced an etch pit array in certain grains, namely those with their surface cut parallel or nearly parallel to the [OOOl] zone axis, of a polished section of Witwatersrand quartzite. The etch structures correlate well with the dislocation structures observed by transmission electron microscopy. It was found necessary to carefully prepare the cut sections and give them a final polish with 0.05 micron alumina on a silk or rayon lap. Typical etching times are between 24 and 30 hours. The etch pits were revealed by using a reflected light optical microscope fitted with a Nomarski interference attachment (Fig.4) or by scanning electron microscopy (Figs.5 a and b). The pits are crystallographic in nature and their detailed shape corresponds with that defined by Frondel (1962) for prism planes. The etch patterns correlate well with the dislocation structures observed in foils from the same specimens with the HVEM (Fig.1). The etching clearly reveals the linear arrays which form the sub-grain structures within the grains. The etching technique has the obvious advantage of displaying all the subgrains in a grain and also of showing the sub-grain structures in several grains after a single etch. The individual dislocations in some walls and within the interiors of the sub-grains are best revealed in a scanning electron microscope.
T13
iion. The sible .
Fig.5. Typical etch pit structures and distributions revealed by scanning electron microscopy at (a) low magnification, and (b) higher magnifications.
We found that many of the small pits clearly evident in scanning electron micrographs (Fig.5b) were not well resolved in optical micrographs. Thus the dislocation densities measured from optical micrographs were consistently an order of magnitude less than those measured from scanning or transmission electron micrographs. Thus in the Witwatersrand quartzite studied, the optical micrographs gave a density of between 10” and 101’ rnw2whereas in both
T14
types of electron micrographs, the average densities are between lo1 ’ and 10’ 2 rne2. This indicates that accurate determinations of dislocation densities from etch pits can only be done in a scanning electron microscope. However, the sub-grain sizes were similar in optical, scanning and transmission electron micrographs and an average diameter of 46 I_tmwas obtained from both optical and scanning electron micrographs of etched surfaces compared with 43 ym obtained from high voltage electron micrographs from the same specimens. We found that it was most convenient to measure sub-grains from optical micrographs taken at low magnification. CONCLUSION
We wish to emphasize that the simple and inexpensive technique of etch pitting described can provide much qualitative and quantitative information of the type mentioned which is of great value to structural geologists who are concerned with dislocation structures in quartz and with deformation conditions in the earth’s crust. Previously this information could only be obtained with a high-voltage electron microscope. ACKNOWLEDGEMENTS
The Royal Society are thanked for the support of S. White through a Mr. and Mrs. John Jaffe Donation Research Fellowship. The Chamber of Mines of South Africa are thanked for financial support given to this work and for supplying specimens.
REFERENCES Frondel, C., 1962. Dana’s System of Mineralogy. Vol. III, Silica Minerals. Wiley, New York, 334 pp. Kolstedt, D.L., Goetze, C., Durham, W.B. and Van der Sande, J., 1976. New technique for decorating dislocations in olivine. Science, 191: 1045-1046. Patel, A.R., Bahl, O.P. and Vagh, A.S., 1965. Etching of rhombohedral cleavages of quartz. Acta Cryst., 19: 757-758. Vol’skaya, O.B., 1969. Selective etchants for quartz. Sov. Phys. Cryst., 13: 620-622. White, S., 1970. Defect and Diffusion Studies of Quartz. Unpubl. thesis, Univ. Melbourne. White, S., 1973. The dislocation structures responsible for the optical effects in some naturally deformed quartzites. J. Mater. Sci., 9: 490-499. White, S., 1975. Estimation of strain rates from microstructural features. Q. J. Geol. Sot. London, 131: 577-583. White, S., 1976a. Estimation of deformation parameters from dislocation sub-structures in naturally deformed quartz. In: J.A. Venables (editor), Developments in Electron Microscopy and Analysis. Academic Press, London, pp.505-505. White, S., 1976b. Effects of strain on the microstructures, fabrics and deformation mechanisms. Philos. Trans. R. Sot. London, Ser. A (in press).