- Email: [email protected]
Computer-aided understanding of deformation microstructures
Computer-aided Understanding of Deformation Microstructures Carol Simpson Department of Earth Sciences, Boston University, 675 Commonwealth Ave., Boston MA 02215, U.S.A. [email protected]
Declan G. De Paor Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge MA 02138, U.S.A. [email protected]
A b s t r a c t - Using the Kodak PhotoCD T M file format, which can be viewed on either IBM PC or Macintosh computers, along with a set of stand-alone Macintosh appplications, we have developed an aid for teaching microstructural analysis in introductory through advanced structural geology courses. The PhotoCD stores digital images of 100 photomicrographs to serve as a reference atlas for research workers in the field. Each image is accompanied by a brief description of the illustrated microstructure. The collection is aimed at students and professional researchers who wish to evaluate their own microscopic samples of kinematic indicators, deformation fabrics, etc., by comparing and contrasting features. The images also serve as a useful resource for instructors who do not have access to a comprehensive teaching collection.
Introduction Teachers who wish to incorporate the study of microscopic deformation structures in their structural geology courses face a number of hurdles. Because of the decrease in class time devoted to optical mineralogy and microscopy in recent years, students today are generally ill-prepared for such analytical work. They may waste a lot of lab time because they have difficulty operating their microscopes; indeed, at introductory level, there may not be enough microscopes to go around in many departments and those that exist may be poorly maintained. Students may have difficulty in locating good examples of structures in their thin sections or in relocating such structures after the slide has been moved, and they may be confused by birefringence changes accompanying stage rotation. Professors whose research interests are on a regional or global tectonic scale may not have extensive collections of thin sections or photomicrographs suitable for 97
Structural Geology and Personal Computers
Fig. 1. Images from the Microstructures PhotoCD ROM. a) Strain induced removal of twin planes in a bent plagioclase porphyroclast deformed at upper greenschist facies. Borrego Springs mylonite zone. b) G-type porphyroclast in S-C mylonite showing sigma grain with recrystallized feldspar tails on upper left and lower right, consistent with left lateral shear. teaching large classes. All teaching collections are continuously eroded by careless students who drop and smash glass-mounted thin sections and even by careful, dedicated students who get excited by the discovery of a particularly good specimen of a microstructure and ram it with their high-power objective. In addition to the above difficulties associated with the viewing of natural examples of deformation microstructures, students tend to have difficulty visualizing submicroscopic scale processes such as dislocation glide, sub-grain formation, or the generation of crystal defects at a Frank Read source. Static illustrations in text books and research papers do little to reveal the nature of these dynamic processes. In order to help teachers overcome the above barriers to learning and to incorporate deformation microstructures both in their undergraduate and graduate level courses, we have assembled a group of computer-based teaching aids. These arebased upon our own teaching collections and lecture notes but they draw 98
Understanding Deformation Microstructures upon many excellent published works of theory and field observation (e.g., Frost & Ashby1982, H a n m e r & Passchier 1991, Hull & Bacon 1984, Nicolas & Poirier 1976, Passchier & Trouw1996, Wenk 1985)
The Microstructures CD ROM The Microstructures CD ROM evolved out of trivial origins. Many years ago, one of us (C.S.) made poster-sized photographic enlargements of her best photomicrographs to h a n g as decorations in her structure laboratory. They a t t r a c t e d a lot of attention over the years and, with the advent of color computer scanners, we decided to m a k e a collection of a half dozen similar photomicrographs to use as a slide set with theAfter Dark T M screen saver. These images were so eye-catching and attracted so much attention from colleagues and students alike t h a t we decided to assemble a larger collection to serve as a teaching aid and research reference set, or "electronic atlas". We chose the Kodak PhotoCD format (Fig. 1) because it was easy (relatively speaking) and cheap (US$120) to produce an initial disc with the help of our local Kodak processing agent and because the format could be accessed using either IBM or Macintosh platforms. In order to m a n u f a c t u r e replicas of the m a s t e r CD ROM, we contacted a disc m a n u f a c t u r e r licensed by Kodak (we were surprised at how few license holders there are in this rapidly growing market). Readers interested in replicating their own CDs should note t h a t a m i n i m u m of 1000 discs had to be m a n u f a c t u r e d for a much smaller potential usership so t h a t a sizable r e m a i n d e r had to be calculated into the project budget. Nowadays, many universities have CD authoring systems but these are not generally licensed to replicate Kodak format discs. Kodak also distributes more powerful multimedia authoring software called Kodak Portfolio T M which permits users to create buttons and text fields linked to images. Our CD is strictly a slide set!
V i e w i n g Images Initially, users had to purchase an application such as KodakAccess T M to read the Kodak format but now such capability is built into almost all personal computers and graphics editing programs and it is rumored t h a t it may be incorporated on the PhotoCD medium itself in the future. An uncropped standard size image displayed in millions of colors requires 1 MB of memory. The memory required can be reduced by setting the monitor to display fewer colors but images will not appear n a t u r a l in fewer t h a n 256 colors. We strongly recommend t h a t users view the microstructures imagery with Adobe Photoshop T M sol, ware. This application permits several levels of zoom analogous to changing lens on a traditional microscope. When a s t u d e n t zooms in, she or he sees fine details, such as tiny rutile needles in quartz grains, t h a t were not visible at the lower magnification. Instructors can retouch and annotate copies of individual images and incorporate them in their own m u l t i m e d i a presentations, lecture notes, or overhead projection transparencies. 99
Structural Geology and Personal Computers The CD images can also serve as reference collections of n a t u r a l structures; for example, if students think they see a particular structure or t e x t u r e in their research materials, they can compare the samples with type specimens on the PhotoCD. Figure 1 shows representative (though greatly reduced) samples of the images.
S i m u l a t i o n s of D e f o r m a t i o n M e c h a n i s m s One of us (D.P.) has developed a set of microstructure simulations for the Macintosh computer. They work on any model from Mac Plus t h r o u g h PowerMac and PowerWave clones and require between 1 and 2 MB of free RAM d e p e n d i n g on screen size and monitor color depth. In order to minimize the learning curve, each deformation mechanism is illustrated in a separate stand-alone application with a m i n i m u m n u m b e r of menu options and dialog boxes. Thus s t u d e n t s can concentrate on the process they are supposed to be studying r a t h e r t h a n the details of the computer's operation.
Glide The first application in the group, called Glide version 7.0, d e m o n s t r a t e s the processes of dislocation glide and climb and the migration of screw dislocations (Fig. 2). It is impossible to convey the full operation of this program with still illustrations. For example, all of the atoms in Fig. 2 undergo rapid r a n d o m oscillations about their mean positions as they would in a crystal structure at a finite t e m p e r a t u r e (to get an idea of w h a t this looks like, hold Fig. 2 close to your face and shake it violently). When a sinistral simple shear strain is applied to the crystal lattice, the lower atoms move to the right, stretching crystal bonds. At a critical point, the stretched bonds break sequentially and new, shorter bonds form, effectively causing a half plane to migrate through the crystal lattice. Optionally, students may display a 'T' symbol representing the edge of the half plane and a Burger's vector (Fig. 2a). Students have particular difficulty visualizing the migration of a screw dislocation. This is simulated in Fig. 2b using a three dimensional view of the crystal structure. The process of dislocation climb also causes students a lot of trouble. It is illustrated upon selection of a menu option. An impurity atom is added to the crystal structure and the bonds between it and ordinary atoms are highlighted. If these bonds are stronger t h a n normal, then the passage of a dislocation through the crystal may be impeded (Fig. 2c). Students m u s t wait and watch for a vacancy which migrates along a random path through the structure. When the vacancy reaches the tip of the half plane (Fig. 2d), the dislocation is said to climb (it migrates down the screen in this example, reinforcing the point t h a t "climb" can occur in any direction, not just upwards). After the climb event, the dislocation continues to migrate.
Porphyroblast A second application, called Porphyroblast illustrates the concepts of pre-, syn-, and post-tectonic crystallization (Fig. 3). The program displays a rock fabric in its 100
Understanding Deformation Microstructures
Fig. 2. Screen shot of the Glide program, a) When running, the location of the half plane sweeps through the atomic array as bonds break in front of it and are reformed in its wake. The T-bar and Burger's vector are optional features, b) A three dimensional simulation demonstrates the glide of a screw dislocation, c, d) Demonstration of dislocation climb. Dark atom with thick bonds represents impurity, c) Dislocation glide halted by strong bonds, d) Vacancy migration reduces height of half plane, releasing dislocation.
initial state and lets the s t u d e n t select the crystallization history desired. Upon pressing the Go button, the student watches as crystals grow and as the fabric becomes deformed. Figure 3 illustrates the case of pre-tectonic crystallization. The fabric is enclosed in the growing crystal and the enclosed portion is protected from f u r t h e r deformation. For simplicity, the imposed deformation in this case is a horizontal pure shear, as illustrated by the straight horizontal fibers t h a t grow progressively from the ends of the crystals. The program works with the aid of two off-screen bitmaps. One holds the initial image and the second is a scratchpad for assembling images before transfer onto the screen. To minimize memory requirements, a small image of the initial fabric is tiled onto the screen. If you look closely, you will detect a wallpaper-like repetition of the pattern. The growth of crystals is controlled by mathematical functions and the appropriate sizes and locations of circles are m a r k e d on the scratchpad at each increment. The fabric inside each crystal is copied from the appropriate part of the initial image without any distortion whereas the fabric outside the crystals is stretched using a function t h a t maps a bitmap from a source 101
Structural Geology and Personal Computers
Fig. 3. Screen shot from the application Porphyroblast for the case of pre-tectonic crystallization. rectangle to a destination rectangle. The assembled scratchpad image is transferred to the screen very quickly and no intermediate steps are visible.
Other Applications Glide and Porphyroblast are two examples of a set of teaching mini-applications t h a t we use in microstructure laboratory classes. Others include simulations of the generation of dislocations at a F r a n k Read source, the formation of sigma and delta type porphyoclast systems, grain boundary migration recrystallization, grain boundary sliding, basal and prism slip systems, syntaxial vs. antitaxial fiber growth, the formation of tiltwalls, the domino effect in fractured feldspars, vacancy migration, and stylolite formation
Discussion A PhotoCD is like a carefully indexed set of photographic negatives (when did you last file your slides or negatives?). Unlike negatives or transparencies, individual images on CD won't get lost or spoiled. Unlike glass microscope slides, CDs don't deteriorate with age and they don't easily break. 102
Understanding Deformation Microstructures The Microstructures CD ROM is intended to make available to students on remote or under-financed campuses the benefits of a thin section and photomicrograph collection that might not otherwise be accessible to them. We believe that this technology has great potential for democratization of educational access. Without large financial expenditure, many readers could create their own archives of teaching and research materials on Kodak PhotoCD ROM. The accompanying applications simulating deformation microstructures seem to aid visualization, especially among students with weak backgrounds in optical or sub-optical mineralogy and atomic physics.
References Frost, H. J. & Ashby, M. F. 1982. Deformation-mechanism Maps. The Plasticity and Creep of Metals and Ceramics. Pergamon Press, Oxford. Hanmer, S. & Passchier, C. W. 1991. Shear sense indicators: A review. Geol. Surv. Canada Pap. 90: 1-17. Hull, D. & Bacon, D. J. 1984. Introduction to Dislocations (3rd. ed.) Pergamon Press, Oxford. Nicolas, A. & Poirier, J. P. 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. J. Wiley & Sons, New York. Passchier, C. W. & Trouw, R. A. J. 1996. Microtectonics. Springer- Verlag, New York. Wenk, H-R. (ed.) 1985. Preferred Orientation in Deformed Metals and Rocks: An Introduction to Modern Texture Analysis. Academic Press, New York.
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Declan G. De Paor Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge MA 02138, U.S.A. [email protected]
A b s t r a c t - Using the Kodak PhotoCD T M file format, which can be viewed on either IBM PC or Macintosh computers, along with a set of stand-alone Macintosh appplications, we have developed an aid for teaching microstructural analysis in introductory through advanced structural geology courses. The PhotoCD stores digital images of 100 photomicrographs to serve as a reference atlas for research workers in the field. Each image is accompanied by a brief description of the illustrated microstructure. The collection is aimed at students and professional researchers who wish to evaluate their own microscopic samples of kinematic indicators, deformation fabrics, etc., by comparing and contrasting features. The images also serve as a useful resource for instructors who do not have access to a comprehensive teaching collection.
Introduction Teachers who wish to incorporate the study of microscopic deformation structures in their structural geology courses face a number of hurdles. Because of the decrease in class time devoted to optical mineralogy and microscopy in recent years, students today are generally ill-prepared for such analytical work. They may waste a lot of lab time because they have difficulty operating their microscopes; indeed, at introductory level, there may not be enough microscopes to go around in many departments and those that exist may be poorly maintained. Students may have difficulty in locating good examples of structures in their thin sections or in relocating such structures after the slide has been moved, and they may be confused by birefringence changes accompanying stage rotation. Professors whose research interests are on a regional or global tectonic scale may not have extensive collections of thin sections or photomicrographs suitable for 97
Structural Geology and Personal Computers
Fig. 1. Images from the Microstructures PhotoCD ROM. a) Strain induced removal of twin planes in a bent plagioclase porphyroclast deformed at upper greenschist facies. Borrego Springs mylonite zone. b) G-type porphyroclast in S-C mylonite showing sigma grain with recrystallized feldspar tails on upper left and lower right, consistent with left lateral shear. teaching large classes. All teaching collections are continuously eroded by careless students who drop and smash glass-mounted thin sections and even by careful, dedicated students who get excited by the discovery of a particularly good specimen of a microstructure and ram it with their high-power objective. In addition to the above difficulties associated with the viewing of natural examples of deformation microstructures, students tend to have difficulty visualizing submicroscopic scale processes such as dislocation glide, sub-grain formation, or the generation of crystal defects at a Frank Read source. Static illustrations in text books and research papers do little to reveal the nature of these dynamic processes. In order to help teachers overcome the above barriers to learning and to incorporate deformation microstructures both in their undergraduate and graduate level courses, we have assembled a group of computer-based teaching aids. These arebased upon our own teaching collections and lecture notes but they draw 98
Understanding Deformation Microstructures upon many excellent published works of theory and field observation (e.g., Frost & Ashby1982, H a n m e r & Passchier 1991, Hull & Bacon 1984, Nicolas & Poirier 1976, Passchier & Trouw1996, Wenk 1985)
The Microstructures CD ROM The Microstructures CD ROM evolved out of trivial origins. Many years ago, one of us (C.S.) made poster-sized photographic enlargements of her best photomicrographs to h a n g as decorations in her structure laboratory. They a t t r a c t e d a lot of attention over the years and, with the advent of color computer scanners, we decided to m a k e a collection of a half dozen similar photomicrographs to use as a slide set with theAfter Dark T M screen saver. These images were so eye-catching and attracted so much attention from colleagues and students alike t h a t we decided to assemble a larger collection to serve as a teaching aid and research reference set, or "electronic atlas". We chose the Kodak PhotoCD format (Fig. 1) because it was easy (relatively speaking) and cheap (US$120) to produce an initial disc with the help of our local Kodak processing agent and because the format could be accessed using either IBM or Macintosh platforms. In order to m a n u f a c t u r e replicas of the m a s t e r CD ROM, we contacted a disc m a n u f a c t u r e r licensed by Kodak (we were surprised at how few license holders there are in this rapidly growing market). Readers interested in replicating their own CDs should note t h a t a m i n i m u m of 1000 discs had to be m a n u f a c t u r e d for a much smaller potential usership so t h a t a sizable r e m a i n d e r had to be calculated into the project budget. Nowadays, many universities have CD authoring systems but these are not generally licensed to replicate Kodak format discs. Kodak also distributes more powerful multimedia authoring software called Kodak Portfolio T M which permits users to create buttons and text fields linked to images. Our CD is strictly a slide set!
V i e w i n g Images Initially, users had to purchase an application such as KodakAccess T M to read the Kodak format but now such capability is built into almost all personal computers and graphics editing programs and it is rumored t h a t it may be incorporated on the PhotoCD medium itself in the future. An uncropped standard size image displayed in millions of colors requires 1 MB of memory. The memory required can be reduced by setting the monitor to display fewer colors but images will not appear n a t u r a l in fewer t h a n 256 colors. We strongly recommend t h a t users view the microstructures imagery with Adobe Photoshop T M sol, ware. This application permits several levels of zoom analogous to changing lens on a traditional microscope. When a s t u d e n t zooms in, she or he sees fine details, such as tiny rutile needles in quartz grains, t h a t were not visible at the lower magnification. Instructors can retouch and annotate copies of individual images and incorporate them in their own m u l t i m e d i a presentations, lecture notes, or overhead projection transparencies. 99
Structural Geology and Personal Computers The CD images can also serve as reference collections of n a t u r a l structures; for example, if students think they see a particular structure or t e x t u r e in their research materials, they can compare the samples with type specimens on the PhotoCD. Figure 1 shows representative (though greatly reduced) samples of the images.
S i m u l a t i o n s of D e f o r m a t i o n M e c h a n i s m s One of us (D.P.) has developed a set of microstructure simulations for the Macintosh computer. They work on any model from Mac Plus t h r o u g h PowerMac and PowerWave clones and require between 1 and 2 MB of free RAM d e p e n d i n g on screen size and monitor color depth. In order to minimize the learning curve, each deformation mechanism is illustrated in a separate stand-alone application with a m i n i m u m n u m b e r of menu options and dialog boxes. Thus s t u d e n t s can concentrate on the process they are supposed to be studying r a t h e r t h a n the details of the computer's operation.
Glide The first application in the group, called Glide version 7.0, d e m o n s t r a t e s the processes of dislocation glide and climb and the migration of screw dislocations (Fig. 2). It is impossible to convey the full operation of this program with still illustrations. For example, all of the atoms in Fig. 2 undergo rapid r a n d o m oscillations about their mean positions as they would in a crystal structure at a finite t e m p e r a t u r e (to get an idea of w h a t this looks like, hold Fig. 2 close to your face and shake it violently). When a sinistral simple shear strain is applied to the crystal lattice, the lower atoms move to the right, stretching crystal bonds. At a critical point, the stretched bonds break sequentially and new, shorter bonds form, effectively causing a half plane to migrate through the crystal lattice. Optionally, students may display a 'T' symbol representing the edge of the half plane and a Burger's vector (Fig. 2a). Students have particular difficulty visualizing the migration of a screw dislocation. This is simulated in Fig. 2b using a three dimensional view of the crystal structure. The process of dislocation climb also causes students a lot of trouble. It is illustrated upon selection of a menu option. An impurity atom is added to the crystal structure and the bonds between it and ordinary atoms are highlighted. If these bonds are stronger t h a n normal, then the passage of a dislocation through the crystal may be impeded (Fig. 2c). Students m u s t wait and watch for a vacancy which migrates along a random path through the structure. When the vacancy reaches the tip of the half plane (Fig. 2d), the dislocation is said to climb (it migrates down the screen in this example, reinforcing the point t h a t "climb" can occur in any direction, not just upwards). After the climb event, the dislocation continues to migrate.
Porphyroblast A second application, called Porphyroblast illustrates the concepts of pre-, syn-, and post-tectonic crystallization (Fig. 3). The program displays a rock fabric in its 100
Understanding Deformation Microstructures
Fig. 2. Screen shot of the Glide program, a) When running, the location of the half plane sweeps through the atomic array as bonds break in front of it and are reformed in its wake. The T-bar and Burger's vector are optional features, b) A three dimensional simulation demonstrates the glide of a screw dislocation, c, d) Demonstration of dislocation climb. Dark atom with thick bonds represents impurity, c) Dislocation glide halted by strong bonds, d) Vacancy migration reduces height of half plane, releasing dislocation.
initial state and lets the s t u d e n t select the crystallization history desired. Upon pressing the Go button, the student watches as crystals grow and as the fabric becomes deformed. Figure 3 illustrates the case of pre-tectonic crystallization. The fabric is enclosed in the growing crystal and the enclosed portion is protected from f u r t h e r deformation. For simplicity, the imposed deformation in this case is a horizontal pure shear, as illustrated by the straight horizontal fibers t h a t grow progressively from the ends of the crystals. The program works with the aid of two off-screen bitmaps. One holds the initial image and the second is a scratchpad for assembling images before transfer onto the screen. To minimize memory requirements, a small image of the initial fabric is tiled onto the screen. If you look closely, you will detect a wallpaper-like repetition of the pattern. The growth of crystals is controlled by mathematical functions and the appropriate sizes and locations of circles are m a r k e d on the scratchpad at each increment. The fabric inside each crystal is copied from the appropriate part of the initial image without any distortion whereas the fabric outside the crystals is stretched using a function t h a t maps a bitmap from a source 101
Structural Geology and Personal Computers
Fig. 3. Screen shot from the application Porphyroblast for the case of pre-tectonic crystallization. rectangle to a destination rectangle. The assembled scratchpad image is transferred to the screen very quickly and no intermediate steps are visible.
Other Applications Glide and Porphyroblast are two examples of a set of teaching mini-applications t h a t we use in microstructure laboratory classes. Others include simulations of the generation of dislocations at a F r a n k Read source, the formation of sigma and delta type porphyoclast systems, grain boundary migration recrystallization, grain boundary sliding, basal and prism slip systems, syntaxial vs. antitaxial fiber growth, the formation of tiltwalls, the domino effect in fractured feldspars, vacancy migration, and stylolite formation
Discussion A PhotoCD is like a carefully indexed set of photographic negatives (when did you last file your slides or negatives?). Unlike negatives or transparencies, individual images on CD won't get lost or spoiled. Unlike glass microscope slides, CDs don't deteriorate with age and they don't easily break. 102
Understanding Deformation Microstructures The Microstructures CD ROM is intended to make available to students on remote or under-financed campuses the benefits of a thin section and photomicrograph collection that might not otherwise be accessible to them. We believe that this technology has great potential for democratization of educational access. Without large financial expenditure, many readers could create their own archives of teaching and research materials on Kodak PhotoCD ROM. The accompanying applications simulating deformation microstructures seem to aid visualization, especially among students with weak backgrounds in optical or sub-optical mineralogy and atomic physics.
References Frost, H. J. & Ashby, M. F. 1982. Deformation-mechanism Maps. The Plasticity and Creep of Metals and Ceramics. Pergamon Press, Oxford. Hanmer, S. & Passchier, C. W. 1991. Shear sense indicators: A review. Geol. Surv. Canada Pap. 90: 1-17. Hull, D. & Bacon, D. J. 1984. Introduction to Dislocations (3rd. ed.) Pergamon Press, Oxford. Nicolas, A. & Poirier, J. P. 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. J. Wiley & Sons, New York. Passchier, C. W. & Trouw, R. A. J. 1996. Microtectonics. Springer- Verlag, New York. Wenk, H-R. (ed.) 1985. Preferred Orientation in Deformed Metals and Rocks: An Introduction to Modern Texture Analysis. Academic Press, New York.
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