Francis J. Turner

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

Preferred orientation in experimentally deformed limestone

Over sixty syntectonic deformation experiments in uniaxial compression have been done on fine-grained limestones in the stability fields of calcite I, calcite II and aragonite. X-ray techniques and spherical harmonic analysis of the data were used to determine preferred orientation quantitatively, and inverse pole-figures were derived for these axially symmetric specimens. They display in most cases strong preferred orientation which varies as a function of the experimental conditions, mainly temperature and pressure. At temperatures below 350° C recrystallization is lacking and flattened grains indicate that translation, twin gliding and kinking have been the dominant deformation mechanisms. The inverse pole-figure shows a maximum at c with a shoulder towards or a second maximum at e. This is in agreement with preferred orientation observed in experimentally deformed Yule marble and can be explained as the product of dominant twin gliding on e and translation gliding on r (Turner et al., 1956). At high temperatures (900–1000° C) strong grain growth (from 4 to 50 microns) indicates that the fabric recrystallized. Grains are equidimensional and clear with a marble-like texture. The inverse pole-figure shows a single maximum at r, and c-axes are oriented in a small circle around the axis of compression, σ1. Such a pattern of preferred orientation would be expected on thermodynamic grounds assuming that recrystallized grains will be oriented in such a way that the strain energy is a maximum (e.g. MacDonald, 1960). Decrease in confining pressure caused a decrease of the maximum at c and the formation of a secondary maximum at highangle positive rhombs in the inverse pole-figure. This can be interpreted as r translation dominating over e twinning. In all deformation experiments an equilibrium in preferred orientation was reached after 20 percent shortening. The strength of preferred orientation decreased with increasing temperature. Aragonite was produced within its hydrostatic stability field at temperatures above 500° C. Close to the phase boundary, coarse-grained textures showed preferred orientation with poles to (010) parallel to σ1. At higher pressures the fabric is fine-grained and [001] is aligned parallel to σ1. Evidence is given that the phase change from calcite to aragonite in these deformation experiments is a diffusive and not a martensitic transformation.

DEFORMATION OF YULE MARBLE. PART VII: DEVELOPMENT OF ORIENTED FABRICS AT 300°C–500°C

Yule marble has been deformed at 400° and 500° C and a confining pressure of 5000 atmospheres. Its strength, relative to that at room temperature, is 47 per cent at 400° C and 41 per cent at 500° C, measured in all cases at 3 per cent strain. Textures of the deformed rock are similar to those of marble comparably deformed at 300° C. Sanders method of “axial-distribution analysis” has been used to test homogeneity of fabric in the original marble and in a specimen shortened 19 per cent at 300° C. In both cases aggregates of a few tens of grains show local very strong preferred orientation of the lattice. In this respect there is no effect that can be correlated with deformation. Petrofabric analysis of specimens shortened 40 per cent or elongated 90–120 per cent demonstrates strong preferred orientation of the c axis at 10°–30° to the axis of compression or at 60°–80° to the axis of extension. The symmetry of the deformed fabrics corresponds with the symmetry of strain, which is determined by the orientation of the axis of extension or compression in relation to the preferred orientation pattern of the original fabric. Behavior of grains is predicted on the basis of a hypothesis of approximately homogeneous deformation, involving twin gliding on ![Graphic][1] and translation gliding on ![Graphic][2] . Behavior of individual grains is also reconstructed from evidence of observed internal rotation of early e lamellae, interpreted in the light of previous observations on single crystals of calcite. The same data are used to compute shortening or elongation of individual grains parallel to the axis of applied force. The behavior and strain so deduced and the details of observed preferred-orientation patterns agree with prediction. It should prove possible to compute strain in geologically deformed rocks on the basis of petrographically measured effects of internal rotation. However, at present this can be attempted only for those few minerals, such as calcite and dolomite, whose glide systems have been ascertained experimentally. Possible analogy between a natural marble fabric and a fabric resulting from experimental compression is noted. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

DEFORMATION OF YULE MARBLE: PART III—OBSERVED FABRIC CHANGES DUE TO DEFORMATION AT 10,000 ATMOSPHERES CONFINING PRESSURE, ROOM TEMPERATURE, DRY

Preferred orientation patterns are recorded for c axis, a 1, a 2, a 3 axes, ![Graphic][1] lamellae, edges [ e:e ] between prominent ![Graphic][2] lamellae, and ![Graphic][3] lamellae in Yule marble deformed to various degrees in the laboratory at 10,000 atmospheres confining pressure, room temperature, dry. The changes of fabric observed correspond strikingly with those predicted by Handin and Griggs (Part II) on their hypothesis of homogeneous deformation. Visible ![Graphic][4] lamellae can be correlated with gliding (probably twinning) in the direction and sense appropriate for twinning. Translation on ![Graphic][5] in the opposite sense, though effective in many cases, leaves no microscopically visible trace. Gliding in the twinning sense on ![Graphic][6] could account for development of relatively insignificant lamellae. Comments are added regarding interpretation of petrofabric data in general, in the light of the results here recorded. [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

Reappraisal of the metamorphic facies concept

AbstractEskolas concept of metamorphic facies, now 50 years old, is reappraised in the light of current knowledge and usage among petrologists. Facies should be defined solely in terms of observable geologic criteria. LikeEskola, we continue to view each facies as a set of mineral assemblages that approximate equilibrium within a definite range of temperature; but this is inference and must be excluded from the definition of facies.Mutual boundaries between facies are transitional. Division into subfacies has proved unacceptable to many writers, and has led to confusion in the physical interpretation of metamorphic parageneses. We propose henceforth not to recognize subfacies.Eleven facies are recognized in this paper, and their terminology has been adapted as nearly as possible to current general usage: A.Low-pressure facies commonly but not exclusively of contact metamorphism. In order of increasing temperature:(1)Albite-epidote-hornfels.(2)Hornblende-hornfels.(3)Pyroxene-hornfels.(4)Sanidinite.B.High-pressure low-temperature facies of regional metamorphism. In order of increasing pressure:(5)Zeolitic.(6)Greenschist.(7)Glaucophane-lawsonite-schist.C.High-pressure medium- to high-temperature facies of regional metamorphism. In order of increasing temperature:(8)Albite-epidote-amphibolite.(9)Amphibolite.(10)Granulite.D.Facies of extreme pressure and wide temperature range:(11)Eclogite. The pressure-temperature regime of a metamorphic terrane can be discussed in terms of the facies there represented, with detailed gradients inferred from sequences of mineral assemblages in basic, pelitic and calcareous rocks. The general simplicity of metamorphic mineral assemblages and their tendency to recur in space and time suggests a simple relation between such theoretically independent pressure variables as load pressurePl fluid pressurePj partial pressure of water,

Crystal bending in metamorphic calcite, and its relations to associated twinning

P_{{\text{H}}_{\text{2}} {\text{O}}}

DEFORMATION OF YULE MARBLE: PART VI— IDENTITY AND SIGNIFICANCE OF DEFORMATION LAMELLAE AND PARTINGS IN CALCITE GRAINS

. As a first approximation we accept for non-carbonate, hydrous assemblages a model in which