Abstract

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 forumla. 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 forumla 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 forumla as the alternative mechanism to twin gliding on forumla. 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 forumla 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 forumla, parallel to the edge forumla (2) translation gliding on forumla parallel to the edge forumla, effective at all temperatures; (3) translation gliding on forumla, parallel to the edge forumla, effective at low temperatures. Translation gliding on forumla 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.

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