Twin Gliding Research Articles

TRANSFORMATION OF INDICES IN TWIN GLIDING

For computation the most convenient form to express transformation of indices attending twin gliding is that of a matrix comparable to matrices used in connection with changes in the axes of reference. The matrix can be obtained by the usual equations, first derived by Mugge (1889a, p. 286–294), or directly from the changes of indices of a suitable set of three faces. These can usually be chosen by inspection when the elements of twin gliding are established. The reciprocal lattice and the gnomonic projection are used to represent the several types of twin gliding. Transformation of crystal forms, of the lattice, and of the crystal structure are illustrated by stereographic projections and diagrams. Geometric criteria can sometimes be used not only to determine the possibility of twin gliding but also to decide the sense of the glide. Simple shear describes twin gliding megascopically, but this concept requires modification hi describing known cases of atom movements. Structures at the moving twin boundary are related to polymorphs of the crystal undergoing gliding in certain cases.

EXPERIMENTAL DEFORMATION OF CALCITE CRYSTALS

This paper reports the results of experimental plastic deformation of cylinders cut from single crystals of clear calcite. A wide range of crystallographic orientation in relation to compression or extension of cylinders is involved. Most experiments were conducted at 20°C and 5000 or 10,000 atmospheres confining pressure, or at 300°C and 5000 atmospheres. Temperatures of 150°C and 400°C were employed in a few additional cases. Shortening or extension of the whole cylinder ranges from 2 to 20 per cent; but in some extension experiments necking of the cylinder has locally increased the strain by a factor of 3 or 4. Stress-strain curves for typical experiments are given. Where the orientation permits, the dominant mechanism of deformation at all temperatures is twin gliding on ![Graphic][1] . Cylinders so oriented that twin gliding cannot occur deform plastically by some alternative mechanism. At 20°C calcite is many times stronger when oriented unfavorably for ![Graphic][2] twin gliding than when favorably oriented; but with rising temperature this difference in strength rapidly diminishes. Analysis of stress-strain data for variously oriented crystals at 300°C points to translation gliding on ![Graphic][3] as the alternative mechanism to twin gliding on ![Graphic][4] . However, no satisfactory correlation of stress-strain data for 20°C could be established on the basis of this or any other simple glide system. An independent approach to the problem is based on analyses of rotational effects observed microscopically in thin sections of the deformed material. Deformed sectors ( e.g ., kink bands) in the cylinder are found to be externally rotated about an axis parallel to the glide plane and normal to the glide line of the active system. At the same time, early-formed lamellae (such as ![Graphic][5] twin lamellae) become internally rotated within the deformed crystal, the axis of rotation being the intersection of the glide plane and the rotated lamella. The senses of internal and external rotation in a given sector of the crystal are mutually opposed, and for a given glide plane each can be deduced for a given stress system. Analysis of directions and amounts of internal and external rotation in many instances leads to unique identification of the active glide system. The glide systems so identified include (1) twin gliding on ![Graphic][6] , parallel to the edge ![Graphic][7] (2) translation gliding on ![Graphic][8] parallel to the edge ![Graphic][9] , effective at all temperatures; (3) translation gliding on ![Graphic][10] , parallel to the edge ![Graphic][11] , effective at low temperatures. Translation gliding on ![Graphic][12] in the sense opposite to that of twin gliding is discarded as a possible mechanism of deformation; there is likewise no evidence of gliding on {0001}. Visible effects of deformation (lamellae, partings, deformation bands, kink bands, etc.) for individual experiments embracing the complete range of orientation are described in detail and illustrated by photographs, line drawings, and projections. The criteria by which various kinds of internally rotated lamellae may be recognized are summarized (Table 6), and the possible applications of our conclusions in interpreting the fabric of an experimentally deformed multicrystalline aggregate—Yule marble—are discussed. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif [7]: /embed/inline-graphic-7.gif [8]: /embed/inline-graphic-8.gif [9]: /embed/inline-graphic-9.gif [10]: /embed/inline-graphic-10.gif [11]: /embed/inline-graphic-11.gif [12]: /embed/inline-graphic-12.gif