Fig. 18.—Sanidine crystal f, showing Carlsbad twin (which, as it consists of two parts only, may be called simple), and quartz q in rhyolite. Crossed nicols.
Structure:
(a) Twinning, generally noticed by the parts of the twin not extinguishing at the same time. It may also be observed, without crossed nicols, just as in the case of macroscopic minerals.
Twinning may be described as: simple, Fig. 18; polysynthetic, due to repeated twinning after the same law, Fig. 12; and crossed or “gridiron,” due to repeated twinning after two laws, Fig. 19.
Fig. 19.—Microcline, showing crossed or “gridiron” twinning. Crossed nicols.
(b) Zonal structure, often only made visible by the zones extinguishing at different times. It may, however, be noticed by the zones being of slightly different color, or by the zonal distribution of inclusions. In the case of the feldspars the zonal structure may be caused either by the crystal being formed of zones of different chemical composition (the successive zones in the plagioclases growing more acid towards the exterior), or by ultra-microscopic twinning,[67] Fig. 20.
Fig. 20.—Zonal feldspar (Carlsbad twin) in trachyte. Crossed nicols.
(c) Aggregate structure, being a confused mass of separate little crystals, scales or grains all extinguishing at different times, Fig. 21.
Fig. 21.—Sphærulites in felsite. Ground mass shows aggregate structure. Crossed nicols.
(d) Sphærulitic structure, produced by the aggregation, in a radiate form, of crystals or crystallites. It is generally easily perceived by the dark cross, resulting from the extinguishing of the light in those crystals whose directions of vibration are parallel to the planes of vibration of the nicols. When the stage is revolved the arms of the cross do not rotate, Fig. 21.
Fig. 22.—Olivine decomposed to serpentine. The pseudomorphism has been almost complete, only small portions of the original olivine remaining. The outline of the parent crystal can be quite distinctly seen. Crossed nicols.
(e) Pseudomorphic structure, which may be partial or complete and is noticed by the changed portions producing different optical effects from those of the original mineral. Sometimes, although the pseudomorphism has been almost complete, the form of the original mineral or crystal may still be seen, Fig. 22.
Characters Observed by Convergent Polarized Light.
Convergent light is obtained by passing the rays of polarized light through a strong condensing lens, which generally fits like a cap over the top of the polarizer. By means of a suitable adjustment the condensing lens can be brought very close to the lower surface of the section on the stage. The lens thus sends a cone of light through the section, and used in connection with crossed nicols a series of optical phenomena, called interference figures,[68] are produced.
Each direction in which rays are sent is traversed by a minute bundle of parallel rays and these rays extinguish and produce interference colors as already described for parallel light. Hence each direction yields a spot or picture in the field of view and from all these spots combined there results an “interference figure” or picture, depending upon the structure of the section for all the directions traversed by the rays.
A very high power objective[69] must be used, and when the eye-piece is removed, a small image of the interference figure will be seen. In some microscopes an arrangement is made for getting a magnified image of the interference figure, by retaining the eye-piece and using an additional Bertrand lens.
In order to get good results care must be taken to have strong illumination and the condensing lens close up under the section. The tests are best made with monochromatic light, but with white light the effects are substantially the same, the only difference being that the rings and curves are variously colored instead of being simply light and dark.
Isotropic substances show no interference figures.
Uniaxial Interference Figures.
(a) Sections perpendicular to the optic or vertical axis ć show a dark cross, with or without colored rings, Figs. 23 and 24. The figure is symmetrical to the center, as the optical behavior of uniaxial crystals is symmetrical to the optic axis.
Fig. 23.
Fig. 24.
The arms of the cross are parallel to the planes of vibration of the nicols, and the figure does not move when the stage carrying the section is rotated.[70]
(b) Sections oblique to the optic axis show a portion of a dark cross, with or without colored rings, Fig. 25. The centre of the cross is not in the axis of rotation, and as the stage bearing the section is revolved, the centre of the cross describes a circle, the arms always maintaining parallel positions.
If the section is still more oblique to the optic axis the centre of the interference cross may be outside the field of view, and only portions of the dark arms will be seen.
Sections parallel to the optic axis show a vague dark cross, which, on rotating the stage, dissolves into hyperbolic curves (suggesting biaxial figure), which very rapidly disappear in the direction of the optic axis. When the interference figure shows colors, these colors are lower in order in the quadrants containing the optic axis. Knowing thus the position of the optic axis the optical character can be obtained by the method for parallel light, p. 43.
Fig. 25.
Sections which are thick and have strong double refraction will show the cross and rings clearly and sharply defined, there being quite a number of rings crowded close together. Sections which are very thin and have weak double refraction show only a broad dark cross and no rings. The interference figures will vary between these extremes, depending on the thickness of the section and the strength of the double refraction.
To obtain the most characteristic figures, observations must be made on sections about perpendicular to the optic axis, that is sections which remain dark or nearly dark during complete rotation between crossed nicols in parallel light.
Optical Character, Positive or Negative. After having obtained an uniaxial interference figure, test it by means of a ¼ undulation mica plate. This plate must be introduced between the objective[71] and the analyzer in such a way that its vibration direction c, marked on the plate, makes an angle of 45° with the planes of vibration of the nicols.
When this is done the interference figure changes, or may more or less disappear, two dark spots or blotches being brought prominently into view. If rings are still seen it will be noticed that they have expanded in the quadrants occupied by the dark spots, and have contracted in the remaining quadrants. This movement of the rings may make it possible to determine the optical character of a section, which is so oblique to the optic axis that the dark spots are not seen after the introduction of the mica plate.