Parting, Translation-lamellae and Undulose Extinction in Olivine.
Olivine has a distinct cleavage parallel to (010) and a less perfect cleavage parallel to (100). In the olivine of peridotites this second plane of mechanical weakness (perpendicular to γ) may be rendered more obvious by the presence of minute rod-like opaque or semi-opaque schiller inclusions which tend to be sharply oriented in (100) of the host crystal (cf. Harker 1919, p. 84). Among the rocks discussed in this paper this feature is particularly well shown in some of the fresh black peridotites of Rum (e.g. 7497*) described by Harker (1908), and a partially serpentinised dunite (5308) from the vicinity of Bluff (Service 1937, p. 212, no. 208). The black peridotite of Abhuinn Fiadh-innis, Rum, has been described by Phillips (1938, p. 134) as showing a remarkable development of lamellae parallel to (100), but in the writer's sections of this rock it is difficult to estimate the relative importance of fine schiller inclusions and of actual planes of parting in determining this sharp,
[Footnote] * Unless otherwise indicated, numbers refer to specimens and sections in the collections of the Geology Department, University of Otago.
finely lamellar appearance, as seen under high-power magnification using a universal stage without the analyser. Iddings (1906, p. 370) mentions inclusions of ilmenite arranged parallel to (100) in the olivine of some basalts, so that in general the presence of schiller inclusions of this type in olivine is not necessarily connected either with plutonic conditions of crystallisation or with deformation.
Certainly of deformational origin, however, are closely-spaced, discontinuous but sharply-defined lamellae parallel to (100) that may be observed in sections of many New Zealand dunites when viewed between crossed nicols. In general appearance these recall the translation-lamellae of deformed quartz figured in a recent paper by H. W. Fairbairn (1941, pl. 2, fig. 1), and are so interpreted by the present writer (cf. Ernst, 1935, p. 153). The olivine lamellae are very closely spaced, usually are restricted to parts of any particular grain, and not infrequently terminate before reaching the grain boundary. In a random section as seen between crossed nicols with an ordinary microscope the lamellae fall into two alternating series with a difference of 5° to 10° in extinction position, thus simulating polysynthetic twinning (cf. Harker, 1919, p. 84). Careful measurements on a universal stage show, however, that the crystallographic orientations of adjacent lamellae are always only slightly different; indeed, the lamellar structure often disappears when the plane of the lamellae is brought parallel to the axis of the microscope-tube. Even where the difference in optic orientation is a maximum, stereo-graphic plotting of α β and γ for each set of lamellae shows that the only twin relationships that could be responsible for the observed phenomena would be irrational types, viz. either (a) normal twinning with the twin axis inclined at about 5° to γ (= crystallographic a), or (b) parallel twinning with the twin plane inclined at 85° to γ and the twin axis at 3° to 5° to α or β. Twinning must therefore be eliminated as a possible explanation, and translation-gliding remains the most likely mode of origin for lamellar structure sub-parallel to (100) in olivine. Buerger (1930, p. 13) has noted that in general translation-gliding is commonly accompanied by slight changes in optic orientation along the translation planes.*
Undulose extinction, so commonly observed in the olivine of peridotites in general, is particularly prominent in the dunites and other peridotites of the western portion of the South Island of New Zealand. Within any large undulose superindividual, the boundaries of the subindividuals or extinction-bands are invariably subparallel to (100) and not infrequently take the form of sharply defined surfaces of rupture. If the lamellar structure referred to in the preceding paragraph is also present, it maintains constant orientation within any one subindividual, but there may be slight differences of orientation between lamellae of adjacent subindividuals.
[Footnote] * Similar sharply defined lamellae parallel to (100) are developed in crystals of enstatite associated with lamellar olivine in South Island peridotites. Though this is highly suggestive of origin by translation-gliding in such cases (cf. N. F. M. Henry, Min. Mag., vol. xxvi, pp. 179–189, 1942), universal application of this explanation to cover all cases of lamellar hypersthenes in norites, dolerites, etc., is in the writer's opinion still unwarranted.
Considerable attention has recently been given to the phenomenon of undulose extinction, especially as it occurs in deformed grains of quartz (e.g. Sander 1930, pp. 173–180; Knopf 1938, pp. 170, 171; Hietanen 1938, pp. 33–35; Fairbairn 1941, pp. 1273–1276). It is generally agreed that the undulatory bands result from a permanent distortion of the space-lattice, which not infrequently leads at last to rupture and slight translation parallel to the boundary surfaces of the subindividuals. According to Hietanen (loc. cit.) the primary mechanism by which undulose extinction develops in bands parallel to the vertical axis in quartz is translation and flexure on the basal plane; Fairbairn on the other hand favours a hypothesis of translation governed by an invariable glide line (the edge m:r) and variable irrational glide-planes inclined at low angles to the basal plane. In both cases the causal translation movement is pictured as operating on planes perpendicular to the trend of the undulose bands.
If a like mechanism is assumed for the origin of undulose extinction in olivine, certain conditions follow immediately with regard to the relative parts played by glide-directions and glide-planes in the deformation. The possibility of deformation governed by a definite glide-plane without any predominating glide-direction must be discarded for olivine. For in such a case the boundaries between the extinction bands of any crystal could be parallel to any plane perpendicular to the glide-plane; actually they are restricted to a single crystallographic plane, (100). All that is necessary to produce the effects observed in olivine is the presence of a controlling invariable glide-direction parallel to γ (a crystal axis); translation could occur on any plane containing γ, i.e. on any plane in the zone , or it could be restricted to an invariable plane in that zone. It is interesting to note at this point that in olivine the direction of maximum atomic density, and hence the most likely potential glide-direction, is the a crystal axis (cf. Bragg 1937, p. 147); furthermore on the similar grounds the most likely glide-plane is (010).
The crystallographic planes and directions of structural weakness with which petrofabric data should be compared are as follows:—(010); ⊥ α; most distinct cleavage; on theoretical grounds the most probable glide-plane.
(100); ⊥ γ; indistinct cleavage; most pronounced plane of schillerization and accompanying parting; plane of microscopically observable translation-lamellae, and undulose banding.
; = γ; on theoretical grounds the most likely glide-direction; glide-direction assumed for development of undulose extinction by flexure-gliding in a plane or planes perpendicular to the resultant banding.
; = β; on theoretical grounds a probable glide-direction.