David T. Griggs

Quartz: Anomalous Weakness of Synthetic Crystals.

The strength of a synthetic quartz crystal drops rapidly at 400�C, and at 600�C is a hundredfold lower than at 300�C. Large plastic deformations can be produced without fracture. The predominant mechanism of deformation is translation gliding. The preferred explanation for this anomalous weakness is that this synthetic quartz contains water which has hydrolyzed the silicon-oxygen bonds. The silanol groups so formed are presumed to be rendered sufficiently mobile by elevating the temperature to 400�C so that they align themselves in dislocation lines and move through the crystal with the dislocation under the small applied shear stress.

Microstructures and Preferred Orientations of Experimentally Deformed Quartzites

An experimental study of the plastic deformation of quartzite has produced microstructures and preferred orientations similar to those found in many natural rocks, and has identified the operative orienting mechanisms in most cases. The microstructures vary widely with conditions and presumably are related to the deformation mechanisms. Below 850°C at 10−5/sec (or 650°C at 10−7/sec), no recrystallization occurs; the deformation of the original grains is very inhomogeneous and deformation lamellae of many orientations are observed. At higher temperatures or slower strain rates, grain boundary recrystallization is present; the original grains are continuously flattened with increasing strain and only basal and prismatic deformation lamellae are observed. Above 800°C at 10−7/sec, recrystallization is complete after low strain. Below 800°C at 10−5/sec (or 600°C at 10−7/sec), a maximum of c axes develops parallel to the compression direction (σ1), while at higher temperatures and slower strain rates, a small-circle girdle of c axes develops about σ1. The opening half-angle of this girdle ranges from 20° to 45° and increases with increasing temperature and decreasing strain rate. Super-imposed on both of these c axis patterns is a tendency for the poles to positive trigonal forms, and the pole to the second order prism to be aligned parallel to σ1. The preferred orientations of the c axes and the prisms are consistent with external rotations produced by the observed intragranular glide. The difference in the preferred orientations of the positive and negative forms is due to mechanical Dauphine twinning. Strong evidence exists that these same orienting mechanisms have operated in many naturally deformed rocks.

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

Experimental Deformation and Recrystallization of Quartz

Plastic deformation of quartz, as evidenced by undulatory extinction, deformation bands, and deformation lamellae, was obtained in sand, quartzite, and single crystals deformed experimentally in several types of apparatus. Extensive syntectonic recrystallization of flint and quartzite was also produced in experiments at high temperatures. Undulatory extinction commonly occurs in zones subparallel to the c-axis and, rarely, in zones with other orientations. Deformation bands vary considerably in orientation, but most of the bands are oriented subparallel to the c-axis. The lamellae form in three more or less distinct orientations: the most common lamellae are inclined at small angles to the base; a weaker concentration is inclined at 20°-60° to the base; and the weakest concentration is subparallel to the c-axis. Deformation lamellae subparallel to the base and zones of undulatory extinction and deformation bands subparallel to the c-axis originate by slip on (0001), accompanied by bending or kinking of the slip planes. Lamellae, bands, and undulatory extinction inclined at moderate angles (20°-60°) to the base are present only in grains in which the shear stress on (0001) is low. They must be produced by another slip mechanism (or mechanisms) not yet identified. Lamellae oriented subparallel to the c-axis and bands and undulatory extinction subparallel to the base are considered to have formed by slip parallel to the c-axis. The poles of lamellae in polycrystalline aggregates with nearly random orientation form small-circle girdle patterns about the axis of greatest compressive stress. The deformation lamellae and bands originate in planes of high shear stress, and it is shown that the orientation of lamellae in rocks may be used to determine the orientation of the stress when the lamellae were formed. In short-term tests flint recrystallizes at 900° C. and above and quartzite at 1,000° C. and above. The products are similar, texturally, to quartzites recrystallized in nature, and the recrystallized grains show strong preferred orientations. The amount of

Syntectonic and Annealing Recrystallization of Fine-Grained Quartz Aggregates

H_{2}O

The Factor of Fatigue in Rock Exfoliation

present in the sample does not apparently influence the temperature at which recrystallization begins, but the recrystallized grains appear to be larger when more

DEFORMATION OF YULE MARBLE: PART II—PREDICTED FABRIC CHANGES

H_{2}O