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The development of slaty cleavage in part of the french alps—discussion
Tectonophysics, 47 (1978) 185-191 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
185
THE DEVELOPMENT OF SLATY CLEAVAGE IN PART OF THE FRENCH ALPS - DISCUSSION
G. OERTEL Department of Earth and Space Sciences, Catif. 90024 (U.S.A.) (Received August 8,1977;
University of California, Los Angeles,
accepted for publication October 28,1977)
In an informative paper on the transition with increasing deformation from marl to slate between Barcelonnette and Savines in the French Alps, Siddans (1977, fig. 19) published pole figures of the basal planes of chlorite and illite from four samples. There are large differences in each of the samples between the measured preferred orientations of the two minerals. Both the maxima of pole concentration in multiples of average concentration and the shapes of the contour lines disagree sharply in each case. Understandably, Siddans found that such evidence does not allow easy assessment of “the quantitative significance of the features of the textures” (p. 549). He drew the general conclusion (p. 555), repeated from an earlier paper (Siddans, 1976), that the use of pyllosilicate textures for a determination of strain by the method of March (1932) is impossible. Granted that there may be difficulties with the theoretical foundations for the application of March’s simple geometrical model to real rocks (e.g., Oertel, 1970, pp. 1183-1184; Oertel and Phakey, 1972, pp. 5-B), the surprising fact remains that strain calculated according to March from the preferred orientation of phyllosilicates is in many cases nearly the same as strain measured by completely independent methods. This holds for compacted sediments (Oertel and Curtis, 1972) and for slates (Oertel and Wood, 1974; Tullis and Wood, 1975; Wood et al., 1976). The preferred orientations for different phyllosilicates in rocks of low metamorphic grade have been found to be virtually identical in the same sample whenever two of them could be measured. The difference between Siddans’ negative and other workers’ more encouraging results may be due to differences in the data they used, caused in turn by differences in measuring technique. Siddans (1976, p. 47) used the X-ray goniometer in a reflection mode, scanning on a single ground surface for the most significant portion of the pole figure and augmenting this by a transmission scan exclusively for’ those directions that could not be obtained in reflection. My technique involves two or three scans in the transmission mode, with orthogonal section planes so chosen that the most significant portion of the pole figure is covered by two of the scans (Wood et al., 1976, pp. 30-32).
186
The two modes of X-ray goniometry produce different kinds of sampling. Only a thin layer at the surface of the rock slab contributes the overwhelming majority of diffracted X-rays counted in the reflection mode. In contrast, in transmission a relatively large volume is analyzed throughout the specimen. For details about the geometry of the two modes, see Siddans (1976, figs. 1 and 2, pp. 45-46). As a consequence of sampling to a small depth only, the reflection mode depends for accurate results on the quality of the surface. It must be smooth, free from even minor undulations, and grains near the surface must not be distorted by grinding or polishing. For the sampling geometry used by Siddans, the smooth surface must be produced at a small angle to the plane of slaty cleavage. It is therefore unavoidable that portions of rock above a mica flake that ideally should remain completely inside the sample will frequently be plucked out during grinding, thus exposing disproportionately large surfaces of mica, preferentially with a basal cleavage surface uppermost. Higher proportions of mica (or illite) relative to chlorite will thus be exposed in rocks containing both, suppressing preferentially chlorite grains with basal planes near-parallel with the slaty cleavage. This is because chlorite is more brittle (Deer et al., 1962, p. 152) and cleaves less easily than mica (Ramdohr, 1948, p. 601). Anisotropy of the fabric will lead to anisotropy of this surface effect, different for each mineral species. In the transmission mode the analysis is not significantly affected by details of the surface. The great drawback of this mode is the strong absorption of both the primary beam and the diffracted X-rays by the sample material. To c’ompensate for this requires long scans, eleven hours instead of the customary forty minutes for reflection, and extensive shielding of the useful X-ray path from scattered X-rays. The measures taken to reduce background radiation due to scattering and due to X-ray fluorescence have been described by Oertel et al. (1973, pp. 178-179) and by Lipshie et al. (1976, pp. 92-93). There does not seem to be any way of speeding up an X-ray scan in transmission as long as commercially available X-ray tubes cannot exceed the present flux density at the target. Slow scans, however, do yield reproducible results. To dispel doubts (Siddans, 1976, p. 48), contours on pole figures published by myself and my collaborators required a small degree of smoothing only occasionally in the lowest and least significant contour. For varying degrees of observed smoothness, compare Oertel (1974, fig. 3, p. 446) with, say, Wood et al. (1976, fig. 3, p. 31, observations by Oertel). The differences in smoothness are due to differences in abundance, grain size, and degree of preferred orientation of the phyllosilicate mineral that diffracts the X-rays. I am keen to learn how the preferred orientation of illite and chlorite in the marls and slates of Barcelonnette is correlated with the independently measured distortion of ammonites on bedding planes. I therefore invite Siddans to send me small portions of his samples so that I may measure them in the transmission mode and so that we may jointly study the suitability of my technique for the estimation of strain.
187
ACKNOWLEDGEMENTS
J.M. Christie, W.A. Dollase, W.G. Ernst, S.R. Lipshie, and J.L. Rosenfeld have helped to improve this paper. REFERENCES Deer, W.A., Howie, R.A. and Zussman, J., 1962. Rock-Forming Minerals. Longmans, London, 3: 270 pp. Lipshie, S.R., Oertel, G. and Christie, J.M., 1976. Measurement of preferred orientation of phyllosilicates in schists. Tectonophysics, 34: 91-99. March, A., 1932. Mathematische Theorie der Regelung nach der Korngestalt bei affiner Deformation. Z. Kristallogr., 81: 285-297. Oertel, G., 1970. Deformation of a slaty, lapillar tuff in the Lake District, England. Geol. Sot. Am. Bull., 81: 1173-1188. Oertel, G., 1974. Unfolding of an antiform by the reversal of observed strains. Geol. Sot. Am. Bull., 85: 445-450. Oertel, G. and Curtis, C.D., 1972. Clay-ironstone concretion preserving fabrics due to progressive compaction. Geol. Sot. Am. Bull., 83: 2597-2606. Oertel, G. and Phakey, P.P., 1972. The texture of a slate from Nantlle, Caernarvon, North Wales. Texture, 1: 1-8. Oertel, G. and Wood, D.S., 1974. Finite strain measurement: a comparison of methods. Trans. Am. Geophys. Union, 55: 695 (abstr.). Oertel, G., Curtis, C.D. and Phakey, P.P., 1973. A transmission electron microscope and X-ray diffraction study of muscovite and chlorite. Miner. Mag., 39: 176-188. Ramdohr, P., 1948. Klockmann’s Lehrbuch der Mineralogie. Enke, Stuttgart, 13th ed., 674 pp. Siddans, A.W.B., 1976. Deformed rocks and their textures. Philos. Trans. R. Sot. Lond., ser. A, 283, 43-54. Siddans, A.W.B., 1977. The development of slaty cleavage in a part of the French Alps. Tectonophysics, 39: 533-557. Tullis, T.E. and Wood, D.S., 1975. Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales. Geol. Sot. Am. Bull., 86: 632638. Wood, D.S., Oertel, G., Singh, J. and Bennett, H.F., 1976. Strain and anisotropy in rocks. Philos. Trans. R. Sot. London, Ser. A, 283: 27-42.
THE DEVELOPMENT OF SLATY CLEAVAGE FRENCH ALPS - REPLY
IN A PART OF THE
A.W.B. SIDDANS Department
of Earth Sciences,
University of Leeds, Leeds LS2 9JT (Great Britain)
(Received October 21, 1977; accepted for publication
October 28, 1977)
In discussion of my paper on the development of slaty cleavage in a part of the French Alps (Siddans, 1977), Oertel draws attention to differences
188
between pole-figures for phyllosilicates in low-grade metamorphic rocks obtained in our two laboratories; and hence to our different views on the use of the model of deformation proposed by March (1932) as a method of finite strain measurement. One of the differing aspects of our pole-figures is the difference I find between the textures of chlorite (002) and illite (002) in the same samples (Siddans, 1976, figs. 6, 7, 8; 1977, fig. 19), whereas Oertel states in his discussion that “the preferred orientations for different phyllosilicates in rocks of low metamorphism have been found to be virtually identical in the same sample whenever two of them could be measured”. I find this claim difficult to assess for in all the references quoted by him (Oertel and Curtis, 1972; Oertel and Wood, 1974; Tullis and Wood, 1975; Wood et al., 1976) there is only one rock for which pole-figures from two minerals are presented. These are for kaolinite (001) and illite (002) in the Peninstone Shale (Oertel and Curtis, 1972, fig. 3e, f) and there is in fact an 11% difference between the maximum pole-densities for these two. The range of differences between maximum pole-densities for chlorite (002) and illite (002) in “terres noires” samples is from 4 to 154% (Siddans, 1976, fig. 6). Another difference between our pole-figures is in their symmetry. I find that most shales and slates have phyllosilicate textures that are triclinic in detail (e.g. Siddans, 1976, fig. 5; 1977, fig. 19c, d), though some do approach orthorhombic symmetry (e.g. Siddans, 1977, fig. 19a, b), whereas Oertel and Curtis (1972, fig. 3), Tullis and Wood (1975, fig. 3) and Wood et al. (1976, fig. 3) present pole-figures that all strongly approximate to orthorhombic symmetry.. It is interesting to compare other published data on phyllosilicate textures in low-grade metamorphic rocks with these two sets of pole-figures. Holeywell and Tullis (1975), in a detailed study of mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania, note that there is some difference in orientation between the chlorite and mica maxima in some samples. They also present pole-figures for these two minerals that show different maximum pole-densities and contour patterns in the same samples (Holeywell and Tullis, 1975, fig. 10). Etheridge and Lee (1975, fig. 5) present a graph of the orientation of mica (001) relative to bedding and cleavage traces, in sections normal to both bedding and cleavage, in a slate from Lady Loretta, Queensland. This graph shows a maximum parallel to cleavage but is strongly skewed towards the bedding trace. A graph such as this for the sample illustrated in Siddans (1977, fig. 19d) would look very similar, with the maximum parallel to bedding. The work of Etheridge and Lee (1975) was an optical study and it was found possible to relate grain size to orientation, larger micas having a bimodal orientation distribution parallel to bedding and cleavage. An earlier draft of my manuscript (Siddans, 1977) contained an account of S.E.M. studies on the “terres noires” samples, including a photomicrograph of the specimen whose pole-figures are shown in fig. 19c, d. This demonstrated that the phyllosilicate textures are indeed skewed as the pole-figures suggest. Similar skewed rose-diagrams are pre-
189
sented by Williams (1972, figs. 11, 12, 14) for optically determined mica textures in low-grade greywackes from Bermagui, Australia. More recently Weber (1976, figs. 16-30) has published many pairs of pole-figures for mica (001) and chlorite (002) for samples from the northeast Rheinisches Schiefergebirge. These include pole-figures that range from almost orthorhombic to triclinic in their symmetry, samples in which the mica and chlorite textures are almost identical, samples in which they are different, samples with unimodal textures and samples with bimodal textures, Weber (1976, pl. 2,4-8) also demonstrates the reality of these textures in a series of S.E.M. photomicrographs. I thus find my pole-figures for the “terres noires” samples eonsistent with other published work on slate textures. Oertel attributes the differences in our pole-figures to our different measurement techniques. His laboratory uses the combined transmission mode scans method (Wood et al., 1976, pp. 30-32), ours the combined reflection/ transmission mode scans method (Siddans, 1976, pp. 45-47). The advantages and disadvantages of these procedures were discussed by Siddans (1976) and the reasons for preferring the combined reflection/transmission mode scans method were stated. Oertel is, of course, quite correct in stating that in reflection mode only a thin layer at the surface of the specimen contributes to the overwhelming majority of diffracted X-rays detected at the counter. The thickness of this layer decreases with decrease in diffracting angle, 0, and increase in specimen tilt, p (Siddans, 1976, fig. la). In slaty rocks a typical value for the linear absorption coefficient has been found to be 0.013. Using this value in Siddans (1976, eq. 3) and relationships describing the incident beam geometry in reflection mode (Frost and Siddans, 1978), the depth below the surface of the specimen at which the flux of the incident X-ray beam has decreased to 0.001 of that in profile section immediately prior to entering the specimen can be calculated. These values are tabulated below, in ,um, for a range of diffracting angles (0) and a range of tilt angles (p) that are used in texture analysis of slates: 3
p = 0”
p = 30”
p= 50°
/3=70°
5.14” 7.34” 12.15” 21.32” 29.44’
4.24 8.69 23.49 70.32 128.24
3.67 7.52 20.34 60.90 111.05
2.72 5.59 15.10 45.22 82.45
1.45 2.97 8.03 24.05 43.85
illite (002) Co& chlorite (002) CoK, quartz (100) CoK, quartz (110) CoK, quartz (112) CoK,
As Oertel notes care must be taken in preparing samples for reflection mode work, especially at low diffracting angles. Our confidence in the combined reflection/transmission mode scans meth-rd is based on two facts: (a) pole-figures for the same d-spacing taken from ‘cifferently oriented slabs through the same sample are the same after the necessary rotations are made and (b) the textures implied by the pole-figures have sometimes been seen optically (see above).
190
The time comparisons made by Oertel for scans in the reflection and transmission modes do not, of course, apply to our step-scanning instrument. We aim to accumulate a minimum of 1000 counts at each data point on the pole-figure. This reduces the probable fractional error of the raw data to 0.02. In the reflection mode, accumulating counts for a pre-set time, usually necessitates counting times of 20-40 sec. Using an angular increment of 5” between and around small circles, scanning through tilts of O-60”, thus requires 8;-13; hour duration scans. These considerations are discussed more fully by Frost and Siddans (1978). I am pleased to find that Oertel also finds “difficulties with the theoretical foundations for the application of March’s simple geometrical model to real rocks”. The reasons for my scepticism of its use have already been stated (Siddans, 1976, pp. 52-53). Perhaps the most convincing demonstration of its inapplicability comes from the H.V.E.M. studies by Knipe and White (1975), who describe in detail the bimodel phyllosilicate textures of Ordovician slates from Rhosneigr, Anglesey (see also the bimodal pole-figures determined by Whalley, 1973, for these phyllosilicates, obtained using our combined reflection/transmission mode scans method). They also describe the microstructure of a Cambrian slate from Penrhyn, North Wales, as “containing pods of phyllosilicates with their (001) planes at high angles to the cleavage” and identify the main cleavage forming process as one of crystallisation at sites of high lattice bending within and around these pods. Finally, I am sorry to see that Oertel regards my results as “negative” in comparison with “other workers’ more encouraging results”, different from some other workers’ results would be a more reasonable description. REFERENCES Etheridge, M.A. and Lee, M.F., 1975. Microstructure of slate from Lady Loretta, Queensland, Australia. Geol. Sot. Am. Bull., 86: 13-22. Frost, M.T. and Siddans, A.W.B., 1978. A new and fully automated texture goniometer for analysis of textures in rocks. J. Appl. Crystallogr., in press. Holeywell, R.C. and Tullis, T.E., 1975. Mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania. Geol. Sot. Am. Bull., 86: 12961304. Knipe, R.J. and White, S.H., 1975. Microstructural development of slaty cleavage. In: J.A. Venables (Editor), Developments in E.M. and Analysis. Proc. EMAG, 75: 521524. March, A., 1932. Mathematische Theorie der Regelung nach der Korngestalt bei affiner Deformation. Z. Kristallogr., 81: 285-297. Oertel, G. and Curtis, C.D., 1972. Clay-ironstone concretion preserving fabrics due to progressive compaction. Geol. Sot. Am. Bull., 83: 2597-2606. Oertel, G. and Wood, D.S., 1974. Finite strain measurement: a comparison of methods. Trans. Am. Geophys. Union, 55: 695 (abstr.). Siddans, A.W.B., 1976. Deformed rocks and their textures. Philos. Trans. R. Sot. London, Ser. A, 283: 43-54. Siddans, A.W.B., 1977. The development of slaty cleavage in a part of the French Alps. Tectonophysics, 39: 533-557.
191 Tullis, T.E. and Wood, D.S., 1975. Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales. Geol. Sot. Am. Bull., 86: 632638. Weber, K., 1976. Gefiigeuntersuchungen an transversalgeschieferten Gesteinen aus dem ostlichen Rheinischen Schiefergebirge. Geol. Jahrb., 15: 3-98. Whalley, J.S., 1973. Finite strain and texture variation associated with folds in a greywacke sequence at Rhosneigr., Anglesey. M.&z. Thesis, University of London, 98 pp. Williams, P.F., 1972. Development of metamorphic layering and cleavage in low grade metamorphic rocks at Bermagui, Australia. Am. J. Sci., 272: l-47. Wood, D.S., Oertel, G., Singh, J. and Bennet, H.F., 1976. Strain and anisotropy in rocks. Philos. Trans. R. Sot. London, Ser. A, 283: 27-42.
185
THE DEVELOPMENT OF SLATY CLEAVAGE IN PART OF THE FRENCH ALPS - DISCUSSION
G. OERTEL Department of Earth and Space Sciences, Catif. 90024 (U.S.A.) (Received August 8,1977;
University of California, Los Angeles,
accepted for publication October 28,1977)
In an informative paper on the transition with increasing deformation from marl to slate between Barcelonnette and Savines in the French Alps, Siddans (1977, fig. 19) published pole figures of the basal planes of chlorite and illite from four samples. There are large differences in each of the samples between the measured preferred orientations of the two minerals. Both the maxima of pole concentration in multiples of average concentration and the shapes of the contour lines disagree sharply in each case. Understandably, Siddans found that such evidence does not allow easy assessment of “the quantitative significance of the features of the textures” (p. 549). He drew the general conclusion (p. 555), repeated from an earlier paper (Siddans, 1976), that the use of pyllosilicate textures for a determination of strain by the method of March (1932) is impossible. Granted that there may be difficulties with the theoretical foundations for the application of March’s simple geometrical model to real rocks (e.g., Oertel, 1970, pp. 1183-1184; Oertel and Phakey, 1972, pp. 5-B), the surprising fact remains that strain calculated according to March from the preferred orientation of phyllosilicates is in many cases nearly the same as strain measured by completely independent methods. This holds for compacted sediments (Oertel and Curtis, 1972) and for slates (Oertel and Wood, 1974; Tullis and Wood, 1975; Wood et al., 1976). The preferred orientations for different phyllosilicates in rocks of low metamorphic grade have been found to be virtually identical in the same sample whenever two of them could be measured. The difference between Siddans’ negative and other workers’ more encouraging results may be due to differences in the data they used, caused in turn by differences in measuring technique. Siddans (1976, p. 47) used the X-ray goniometer in a reflection mode, scanning on a single ground surface for the most significant portion of the pole figure and augmenting this by a transmission scan exclusively for’ those directions that could not be obtained in reflection. My technique involves two or three scans in the transmission mode, with orthogonal section planes so chosen that the most significant portion of the pole figure is covered by two of the scans (Wood et al., 1976, pp. 30-32).
186
The two modes of X-ray goniometry produce different kinds of sampling. Only a thin layer at the surface of the rock slab contributes the overwhelming majority of diffracted X-rays counted in the reflection mode. In contrast, in transmission a relatively large volume is analyzed throughout the specimen. For details about the geometry of the two modes, see Siddans (1976, figs. 1 and 2, pp. 45-46). As a consequence of sampling to a small depth only, the reflection mode depends for accurate results on the quality of the surface. It must be smooth, free from even minor undulations, and grains near the surface must not be distorted by grinding or polishing. For the sampling geometry used by Siddans, the smooth surface must be produced at a small angle to the plane of slaty cleavage. It is therefore unavoidable that portions of rock above a mica flake that ideally should remain completely inside the sample will frequently be plucked out during grinding, thus exposing disproportionately large surfaces of mica, preferentially with a basal cleavage surface uppermost. Higher proportions of mica (or illite) relative to chlorite will thus be exposed in rocks containing both, suppressing preferentially chlorite grains with basal planes near-parallel with the slaty cleavage. This is because chlorite is more brittle (Deer et al., 1962, p. 152) and cleaves less easily than mica (Ramdohr, 1948, p. 601). Anisotropy of the fabric will lead to anisotropy of this surface effect, different for each mineral species. In the transmission mode the analysis is not significantly affected by details of the surface. The great drawback of this mode is the strong absorption of both the primary beam and the diffracted X-rays by the sample material. To c’ompensate for this requires long scans, eleven hours instead of the customary forty minutes for reflection, and extensive shielding of the useful X-ray path from scattered X-rays. The measures taken to reduce background radiation due to scattering and due to X-ray fluorescence have been described by Oertel et al. (1973, pp. 178-179) and by Lipshie et al. (1976, pp. 92-93). There does not seem to be any way of speeding up an X-ray scan in transmission as long as commercially available X-ray tubes cannot exceed the present flux density at the target. Slow scans, however, do yield reproducible results. To dispel doubts (Siddans, 1976, p. 48), contours on pole figures published by myself and my collaborators required a small degree of smoothing only occasionally in the lowest and least significant contour. For varying degrees of observed smoothness, compare Oertel (1974, fig. 3, p. 446) with, say, Wood et al. (1976, fig. 3, p. 31, observations by Oertel). The differences in smoothness are due to differences in abundance, grain size, and degree of preferred orientation of the phyllosilicate mineral that diffracts the X-rays. I am keen to learn how the preferred orientation of illite and chlorite in the marls and slates of Barcelonnette is correlated with the independently measured distortion of ammonites on bedding planes. I therefore invite Siddans to send me small portions of his samples so that I may measure them in the transmission mode and so that we may jointly study the suitability of my technique for the estimation of strain.
187
ACKNOWLEDGEMENTS
J.M. Christie, W.A. Dollase, W.G. Ernst, S.R. Lipshie, and J.L. Rosenfeld have helped to improve this paper. REFERENCES Deer, W.A., Howie, R.A. and Zussman, J., 1962. Rock-Forming Minerals. Longmans, London, 3: 270 pp. Lipshie, S.R., Oertel, G. and Christie, J.M., 1976. Measurement of preferred orientation of phyllosilicates in schists. Tectonophysics, 34: 91-99. March, A., 1932. Mathematische Theorie der Regelung nach der Korngestalt bei affiner Deformation. Z. Kristallogr., 81: 285-297. Oertel, G., 1970. Deformation of a slaty, lapillar tuff in the Lake District, England. Geol. Sot. Am. Bull., 81: 1173-1188. Oertel, G., 1974. Unfolding of an antiform by the reversal of observed strains. Geol. Sot. Am. Bull., 85: 445-450. Oertel, G. and Curtis, C.D., 1972. Clay-ironstone concretion preserving fabrics due to progressive compaction. Geol. Sot. Am. Bull., 83: 2597-2606. Oertel, G. and Phakey, P.P., 1972. The texture of a slate from Nantlle, Caernarvon, North Wales. Texture, 1: 1-8. Oertel, G. and Wood, D.S., 1974. Finite strain measurement: a comparison of methods. Trans. Am. Geophys. Union, 55: 695 (abstr.). Oertel, G., Curtis, C.D. and Phakey, P.P., 1973. A transmission electron microscope and X-ray diffraction study of muscovite and chlorite. Miner. Mag., 39: 176-188. Ramdohr, P., 1948. Klockmann’s Lehrbuch der Mineralogie. Enke, Stuttgart, 13th ed., 674 pp. Siddans, A.W.B., 1976. Deformed rocks and their textures. Philos. Trans. R. Sot. Lond., ser. A, 283, 43-54. Siddans, A.W.B., 1977. The development of slaty cleavage in a part of the French Alps. Tectonophysics, 39: 533-557. Tullis, T.E. and Wood, D.S., 1975. Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales. Geol. Sot. Am. Bull., 86: 632638. Wood, D.S., Oertel, G., Singh, J. and Bennett, H.F., 1976. Strain and anisotropy in rocks. Philos. Trans. R. Sot. London, Ser. A, 283: 27-42.
THE DEVELOPMENT OF SLATY CLEAVAGE FRENCH ALPS - REPLY
IN A PART OF THE
A.W.B. SIDDANS Department
of Earth Sciences,
University of Leeds, Leeds LS2 9JT (Great Britain)
(Received October 21, 1977; accepted for publication
October 28, 1977)
In discussion of my paper on the development of slaty cleavage in a part of the French Alps (Siddans, 1977), Oertel draws attention to differences
188
between pole-figures for phyllosilicates in low-grade metamorphic rocks obtained in our two laboratories; and hence to our different views on the use of the model of deformation proposed by March (1932) as a method of finite strain measurement. One of the differing aspects of our pole-figures is the difference I find between the textures of chlorite (002) and illite (002) in the same samples (Siddans, 1976, figs. 6, 7, 8; 1977, fig. 19), whereas Oertel states in his discussion that “the preferred orientations for different phyllosilicates in rocks of low metamorphism have been found to be virtually identical in the same sample whenever two of them could be measured”. I find this claim difficult to assess for in all the references quoted by him (Oertel and Curtis, 1972; Oertel and Wood, 1974; Tullis and Wood, 1975; Wood et al., 1976) there is only one rock for which pole-figures from two minerals are presented. These are for kaolinite (001) and illite (002) in the Peninstone Shale (Oertel and Curtis, 1972, fig. 3e, f) and there is in fact an 11% difference between the maximum pole-densities for these two. The range of differences between maximum pole-densities for chlorite (002) and illite (002) in “terres noires” samples is from 4 to 154% (Siddans, 1976, fig. 6). Another difference between our pole-figures is in their symmetry. I find that most shales and slates have phyllosilicate textures that are triclinic in detail (e.g. Siddans, 1976, fig. 5; 1977, fig. 19c, d), though some do approach orthorhombic symmetry (e.g. Siddans, 1977, fig. 19a, b), whereas Oertel and Curtis (1972, fig. 3), Tullis and Wood (1975, fig. 3) and Wood et al. (1976, fig. 3) present pole-figures that all strongly approximate to orthorhombic symmetry.. It is interesting to compare other published data on phyllosilicate textures in low-grade metamorphic rocks with these two sets of pole-figures. Holeywell and Tullis (1975), in a detailed study of mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania, note that there is some difference in orientation between the chlorite and mica maxima in some samples. They also present pole-figures for these two minerals that show different maximum pole-densities and contour patterns in the same samples (Holeywell and Tullis, 1975, fig. 10). Etheridge and Lee (1975, fig. 5) present a graph of the orientation of mica (001) relative to bedding and cleavage traces, in sections normal to both bedding and cleavage, in a slate from Lady Loretta, Queensland. This graph shows a maximum parallel to cleavage but is strongly skewed towards the bedding trace. A graph such as this for the sample illustrated in Siddans (1977, fig. 19d) would look very similar, with the maximum parallel to bedding. The work of Etheridge and Lee (1975) was an optical study and it was found possible to relate grain size to orientation, larger micas having a bimodal orientation distribution parallel to bedding and cleavage. An earlier draft of my manuscript (Siddans, 1977) contained an account of S.E.M. studies on the “terres noires” samples, including a photomicrograph of the specimen whose pole-figures are shown in fig. 19c, d. This demonstrated that the phyllosilicate textures are indeed skewed as the pole-figures suggest. Similar skewed rose-diagrams are pre-
189
sented by Williams (1972, figs. 11, 12, 14) for optically determined mica textures in low-grade greywackes from Bermagui, Australia. More recently Weber (1976, figs. 16-30) has published many pairs of pole-figures for mica (001) and chlorite (002) for samples from the northeast Rheinisches Schiefergebirge. These include pole-figures that range from almost orthorhombic to triclinic in their symmetry, samples in which the mica and chlorite textures are almost identical, samples in which they are different, samples with unimodal textures and samples with bimodal textures, Weber (1976, pl. 2,4-8) also demonstrates the reality of these textures in a series of S.E.M. photomicrographs. I thus find my pole-figures for the “terres noires” samples eonsistent with other published work on slate textures. Oertel attributes the differences in our pole-figures to our different measurement techniques. His laboratory uses the combined transmission mode scans method (Wood et al., 1976, pp. 30-32), ours the combined reflection/ transmission mode scans method (Siddans, 1976, pp. 45-47). The advantages and disadvantages of these procedures were discussed by Siddans (1976) and the reasons for preferring the combined reflection/transmission mode scans method were stated. Oertel is, of course, quite correct in stating that in reflection mode only a thin layer at the surface of the specimen contributes to the overwhelming majority of diffracted X-rays detected at the counter. The thickness of this layer decreases with decrease in diffracting angle, 0, and increase in specimen tilt, p (Siddans, 1976, fig. la). In slaty rocks a typical value for the linear absorption coefficient has been found to be 0.013. Using this value in Siddans (1976, eq. 3) and relationships describing the incident beam geometry in reflection mode (Frost and Siddans, 1978), the depth below the surface of the specimen at which the flux of the incident X-ray beam has decreased to 0.001 of that in profile section immediately prior to entering the specimen can be calculated. These values are tabulated below, in ,um, for a range of diffracting angles (0) and a range of tilt angles (p) that are used in texture analysis of slates: 3
p = 0”
p = 30”
p= 50°
/3=70°
5.14” 7.34” 12.15” 21.32” 29.44’
4.24 8.69 23.49 70.32 128.24
3.67 7.52 20.34 60.90 111.05
2.72 5.59 15.10 45.22 82.45
1.45 2.97 8.03 24.05 43.85
illite (002) Co& chlorite (002) CoK, quartz (100) CoK, quartz (110) CoK, quartz (112) CoK,
As Oertel notes care must be taken in preparing samples for reflection mode work, especially at low diffracting angles. Our confidence in the combined reflection/transmission mode scans meth-rd is based on two facts: (a) pole-figures for the same d-spacing taken from ‘cifferently oriented slabs through the same sample are the same after the necessary rotations are made and (b) the textures implied by the pole-figures have sometimes been seen optically (see above).
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The time comparisons made by Oertel for scans in the reflection and transmission modes do not, of course, apply to our step-scanning instrument. We aim to accumulate a minimum of 1000 counts at each data point on the pole-figure. This reduces the probable fractional error of the raw data to 0.02. In the reflection mode, accumulating counts for a pre-set time, usually necessitates counting times of 20-40 sec. Using an angular increment of 5” between and around small circles, scanning through tilts of O-60”, thus requires 8;-13; hour duration scans. These considerations are discussed more fully by Frost and Siddans (1978). I am pleased to find that Oertel also finds “difficulties with the theoretical foundations for the application of March’s simple geometrical model to real rocks”. The reasons for my scepticism of its use have already been stated (Siddans, 1976, pp. 52-53). Perhaps the most convincing demonstration of its inapplicability comes from the H.V.E.M. studies by Knipe and White (1975), who describe in detail the bimodel phyllosilicate textures of Ordovician slates from Rhosneigr, Anglesey (see also the bimodal pole-figures determined by Whalley, 1973, for these phyllosilicates, obtained using our combined reflection/transmission mode scans method). They also describe the microstructure of a Cambrian slate from Penrhyn, North Wales, as “containing pods of phyllosilicates with their (001) planes at high angles to the cleavage” and identify the main cleavage forming process as one of crystallisation at sites of high lattice bending within and around these pods. Finally, I am sorry to see that Oertel regards my results as “negative” in comparison with “other workers’ more encouraging results”, different from some other workers’ results would be a more reasonable description. REFERENCES Etheridge, M.A. and Lee, M.F., 1975. Microstructure of slate from Lady Loretta, Queensland, Australia. Geol. Sot. Am. Bull., 86: 13-22. Frost, M.T. and Siddans, A.W.B., 1978. A new and fully automated texture goniometer for analysis of textures in rocks. J. Appl. Crystallogr., in press. Holeywell, R.C. and Tullis, T.E., 1975. Mineral reorientation and slaty cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania. Geol. Sot. Am. Bull., 86: 12961304. Knipe, R.J. and White, S.H., 1975. Microstructural development of slaty cleavage. In: J.A. Venables (Editor), Developments in E.M. and Analysis. Proc. EMAG, 75: 521524. March, A., 1932. Mathematische Theorie der Regelung nach der Korngestalt bei affiner Deformation. Z. Kristallogr., 81: 285-297. Oertel, G. and Curtis, C.D., 1972. Clay-ironstone concretion preserving fabrics due to progressive compaction. Geol. Sot. Am. Bull., 83: 2597-2606. Oertel, G. and Wood, D.S., 1974. Finite strain measurement: a comparison of methods. Trans. Am. Geophys. Union, 55: 695 (abstr.). Siddans, A.W.B., 1976. Deformed rocks and their textures. Philos. Trans. R. Sot. London, Ser. A, 283: 43-54. Siddans, A.W.B., 1977. The development of slaty cleavage in a part of the French Alps. Tectonophysics, 39: 533-557.
191 Tullis, T.E. and Wood, D.S., 1975. Correlation of finite strain from both reduction bodies and preferred orientation of mica in slate from Wales. Geol. Sot. Am. Bull., 86: 632638. Weber, K., 1976. Gefiigeuntersuchungen an transversalgeschieferten Gesteinen aus dem ostlichen Rheinischen Schiefergebirge. Geol. Jahrb., 15: 3-98. Whalley, J.S., 1973. Finite strain and texture variation associated with folds in a greywacke sequence at Rhosneigr., Anglesey. M.&z. Thesis, University of London, 98 pp. Williams, P.F., 1972. Development of metamorphic layering and cleavage in low grade metamorphic rocks at Bermagui, Australia. Am. J. Sci., 272: l-47. Wood, D.S., Oertel, G., Singh, J. and Bennet, H.F., 1976. Strain and anisotropy in rocks. Philos. Trans. R. Sot. London, Ser. A, 283: 27-42.