Twinning is very common and helps to distinguish orthoclase from the plagioclase feldspars. In orthoclase the twins are simple, that is, only two crystals growing together, and are united on one of the faces, as if one of them had been revolved 180° with the other; or, while related to each other as in the preceding case, they may seem to grow through each other. On [plate 34] are three orthoclase crystals showing this simple type of twinning. The first (A) is a simple crystal; the second (B) shows the simplest type of twinning where the left-hand crystal has revolved 180° on the p face, and the end is composed, half of the upper end of one crystal, and half of the lower end of the adjacent crystal. The presence of reëntrant angles calls attention to the twinning. The third figure (C) is a case of intergrowing crystals.
In the plagioclase feldspars twinning is multiple, a large number of crystals, each thin, sometimes as thin as paper, growing side by side, the first one in normal position, the next at 180° with it, the third revolved 180° to the second and thus parallel to the first, and so on. The result is first of all a striated appearance, and second that, as plagioclase crystals have their prism faces intersecting at 86°, there is a series of low roofs and valleys, which are best seen by holding the piece of feldspar so the light reflects from a cleavage face, when it will appear striated; then by tilting it about 8 degrees a second set of reflections, also appearing striated, will appear. The light was first reflected from one side of the roofs, and in the second case from the other side. Figure D, [Pl. 34], is a diagram showing the relation of the individual crystals in a multiple twinned piece of plagioclase, in which the crystals are represented as rather large. [Plate 35], under labradorite, shows a photograph of a cleavage piece, on which is readily seen the striation which is characteristic of the plagioclase feldspars.
Mixtures of albite and anorthite occur in bewildering numbers, one or the other predominating, and each mixture being uniform throughout the crystal and in the whole mass; so each combination is a mineral, each with its special properties; but the different plagioclase feldspars are so similar in appearance, that by the naked eye it is impossible to separate the closely related ones. This can be done under the microscope by studying the angles at which light is cut off, and also by chemical analyses. For our purposes six types will suffice to illustrate the group, and their composition may be indicated as follows.
Albite is albite with up to 15% of anorthite mixed with it.
Oligoclase is albite with from 15-25% of anorthite mixed with it.
Andesite is albite with from 25-50% of anorthite mixed with it.
Labradorite is anorthite with from 25-50% of albite mixed with it.
Bytownite is anorthite with from 15-25% of albite mixed with it.
Anorthite is anorthite with up to 15% of albite mixed with it.
The best method for distinguishing these feldspars of the plagioclase group is to measure the angle between the two perfect cleavage faces, and even this requires careful measurement. The angles between these faces are as follows: