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On the Elimination of Plagioclase Twinning ON THE ELIMINATION OF PLAGIOCLASE TWINNING BY E. A. V. PRASAD (Department of Geology, Sri Venkateswara University, Tirupati, d.P.) Reeeivetl April 27, 1966 (Communicated by Prof. C. S. Piehamuthu, F.A.se.) ABSTRACT A study of the cataclasticaUy deformed plagioclase from a fault zone reveals that the characteristic plagioclase twinning is destroyed in some way. It is presumed that twinning in plagioclase is an .irreversible or thermodynamically unstable imperfection. It is concluded that cataclasis and the consequent induced twinning are pre-'tectonie crystal- lisation features, while elimination of twinning is a post-tectonic deforma- tional feature. INTRODUCTION DURING the petrographic study of the rocks occurring in a fault zone near Ramapuram, Anantapur District of Andhra Pradesh, it was noticed that the fundamental and characteristic plagioclase twinning had been destroyed in some way. The rocks include cataclasites (quartzitic breccia) and mylonites which are essentially composed of extensively crushed and shattered feldspar grains in a microcrystalline mosaic of quartz in which irregular patches, streaks and single crystals of coarse quartz also occur. The quartz grains display strongly developed undulose extinction, deformation lameLlae and deformation bands, which occasionally tend to be replaced by "Boehrn lamellae" or relic lines of inclusions. Many authors have been interested in investigating the origin of twinning; they have looked for the cause of the twinning, but nobody has ever given thought to the elimination except Vance (1961). Ramberg (1961, p. 11) mentions albite crystals having a few narrow lamellae of albite twins confined to the middle part, while on either side are the untwinned parts. His explana- tion is that the feldspar started to grow as a twinned nucleus but continued to grow in an untwinned state, due probably to rapid growth for a relatively short-time just after nucleation, and a subsequent slower growth. In the 296 On the Elimination of Plagioclase Twinning 297 course of the present study, the writer has observed that the effacing of plagio- clase twinning has been brought about in more than one way: (1) In some plagioclase grains all the twin lamellae become bent and coalesce ultimately producing an untwinned grain. (2) In some grains, as was noticed by Vance, at one end there is the development of secondary glide lamellae and these bend gradually until they merge into one broad lamella, and the other part of the grain forms the second individual of the twinned grain. (3) In some grains, there is a tendency for the reduction in the number, size and continuity of twinning lamellae, until all the lamellae disappear, giving rise to an untwinned grain. It is well known that plagioclase feldspars are very prone to twinning which is mostly a mechanical feature and a reflection of the environmental history of the crystal. Twinning is initiated or developed by heating as shown by experimental investigations (Scholler, 1941; Tuttle and Bowen, 1950; Muir, 1965). Also, the stress-strain pattern of mechanical deforma- tion induces certain cystallographic elements of which twinning planes are the most common among feldspars. Such mechanical twinning has drawn the particular attention of many investigators, as is seen from a review of the Norsk Feldspar Volume (Norsk Geologisk Tidsskrift, Bd. 42, 2 halvbind, 1962), and also in the recent papers of Vogel (1964), Vogel and Seifert (1965), Laves (1965) and Tobi (1965). These papers deal with mechanical twinning, expanding it to cover all plagioclases at all temperatures. According to Suwa (1956), deformation of a fully grown crystal is considered to be least effective in forming plagioclase twins. However, the writer believes that the deformation caused by continually imposed mechanical stresses may be effective in effacing the twinning. That is, the rearrangement and reorgan- isation of the deformed grain, such as intragranular lattice displacement, grain rotation, and ionic migration by diffusion or solution which take place without destroying the cohesion of the grain, constitute the internal mecha- nism for the elimination of twinning; and thus, the final untwinned plagio- clase is believed to be the form attained in response to continually applied external forces. Because the microfabric which results from deformation of a certain kind will obviously be affected by recrystallisation of the consti- tuent minerals, it might be thought that such recrystallisation of post-deforma- tion date (post-tectonic crystallisation, pre-crystalline deformation) would obliterate most of the microfabric, and that the elimination of twinning is probably, a consequence. 298 E.A.V. PRASAD Further, twinning in plagioclase is presumed to be one of the several imperfections which have been referred to as irreversible or thermodyna- mically unstable. Pressure, temperature, composition and size alone are not the factors to contribute to this imperfection which is dependent also upon the history of the crystal. Therefore, twinning in ptagioclase which is thought to be an irreversible imperfection, tends to disappear in course of time when the crystal experiences appreciable thermal mobility inside, or when it is left to itself for sufficient period of time under mechanical stresses. Such induced mechanical twinning and its subsequent elimination will not be possible over short periods of time but will be possible during periods which are sufficiently long to permit appreciable thermal mobility resulting probably in the migration of elements, particularly of Si and A1. Such periods of time may vary in different members of the isomorphous mixtures of plagioclase series. The individual grains undergoing different degrees of deformation display different stages of elimination. In general, such grains exhibit ill-defined composition planes which are not properly aligned. The twin planes may be discontinuous or curved. All the deformed plagio- clase grains are highly turbid with intense sericitic alteration. On this question of elimination of plagioclase twinning, Barth (personal communication) remarks that some external energy--mechanical energy or stress is responsible for the elimination of twin structure, the thermal energy being too low to eliminate twinning immediately. He suggests an alternative view that twinning may be a minor kind of disorder, transitional between high temperature disorder and low temperature order. Ramberg (personal communication) remarks that the twinning in feldspar is eliminated by a process of recrystallisation and/or annealing of mechanically deformed feldspar grains, the slowness of metamorphic recrystallisation favouring the untwinned grain to form. Thus, in plagioclas,e feldspars, cataclasis and the consequent induced twinning may be pre-tectonic crystallisation features, while elimination of twinning is a post-tectonic deformational feature. The observations made in this study of cataclastically deformed plagioclase do not agree with the suggestion made by Emmons and Mann (1953) that the untwinned plagioclase is in reality an unzoned plagioclase for whatever reason and is incapable of twinning, because in the light of the present observations, an untwinned plagiaclase may possibly have been originally twinned and the twinning might have been effaced later. On the Elimination of Plagioclase Twinning 299 ACKNOWLEDGEMENTS The writer desires to express his gratitude to Prof. M. G. Chakrapani Naidu for his guidance and encouragement in this study. He is highly grate- ful to Prof. C. S. Pichamuthu for his critical reading of this manuscript. The writer is indebted to Prof. T. F. W. Barth, Hans Ramberg and J. J. W. Rogers for their critical comments and many helpful suggestions. Thanks are due to Dr. K. V. Subbarao for assisting in taking the photomicrographs. REFERENCES I. Emmons, R. C. and Mann, "A twin zone relationship in plagioclase feldspar," Geol. V. Soc. Amer., Mere., 1953, 52, 41-54. 2. Laves, F. .. "Mechanical twinning in acid plagioclases," Amer. Min., 1965, 50, 511-14. 3. Muir, I.D. .. "Transitional optics of some andesines and labradorites," Min. Mag., 1955, 30, 545-68. 4. Ramberg, H. • • "A study of veins in Caledonian rocks around Trondheim Fjord, Norway," Norsk Geologisk Tidsskrifl, 1961, 41, Part I, p. 11. 5. Scholler, H. "Versuche zur Temperaturabhiingigkeit der plagioklasoptik," Tscherm. Mineralog. Petrogr. Mitt., 1941, 53, 180-221. 6. Suwa, K. "Plagioclase twinning in Ryoke Metamorphic rocks from the Mitsue-mura area, Kii peninsula, Central Japan," dour. Earth Sci., Nagoya University, 1956, 4(2), 91-122. 7. Tobi, A. C. "On the cause of internal optical scatter in plagioclase and the occurrence of lamellar albite-Ala B twinning," Am. dour. Sei., 1965, 263, 712-18. 8. Tutfle, O. F. and Bowen, "High temperature albite and contiguous feldspars," dour. N.L. Geol., 1950, 58, 572-83. 9. Vance, J.A. .. "Polysynthetic twinning in plagioclase," Amer. Min., 1961, 46, 1097-1119. 10. Vogel, T.A. .. "Optical crystallographic scatter in plagioclase," Ibid., 1964, 49, 614-33. 11. ~ and Seifert, K. E... "Deformation twinning in ordered plagioclase," Ibid., 1965, 59, 514-18. EXPLANATION OF PLATE XlII Fins. 1~3 Cataclastically deformed plagioclase. All photographs taken under crossed nicols, x 60. FiG. 1. Plagioclase with numerous closely spaced narrow twinning lamellae, and intensely tra- versed by network of cracks filled with iron oxide material. 300 E. A. V. PvoLs,~ Fxo. 2. Plagioclase with twin larnellae bent, broken and off-set, and infiltra~M by secondary quartz granules. The grain is also frayed along twin planes. FIo. 3. Patterns of plagioclase twinning. The large grain is partly twinned and partly untwinned and highly turbid. In certain parts of the grain the twin lame|lae are seen to be merged. Reduction in the number, size and continuity of the twin lameUae is also observable~ E A. V. Prasad Proc. Ind. Acad. Sci., B, Vol. LXIV, Pl. XIII FIGS. 1-3. .
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