THK FELSPARS. 



of symmetry parallel to the tlirco pairs of faces. But supposing' that 

 in preparing our hlock one pair of sides hail, while still at ri^lit angles 

 to the secoiul pair, been inclined at some other angle to the third pair, 

 a little consideration will show that placing the block on the glass 

 again we shall have the reflection as a continuation of the object in 

 only two of the six possible positions — that, therefore, there is only ohc 

 plane of symmetry, and with the block worked to a proper angle this 

 would be a model of our Felspar crystal. 



If now wo cut slices thin enough to see through, parallel to the two 

 cleavage planes, we shall lind some differences between them as to 

 their relation to polarised light. In both cases we find double refrac- 

 tion — that is, if the polarising and analysing prisms are so placed that 

 the field of the microscope is dark (if the prisms are crosxed, as it is 

 called), the film of crystal will, in both cases, enable light to pass 

 through the second prism. But, now, keeping the prisms crossed, 

 rotate the specimens on the stage. Four positions will be found in 

 which they no longer do this, but become dark like the rest of the field 

 of view. If now these four '• extinction " positions are accurately com- 

 pared with the positions of the Nicols prisms, it will be found that the 

 edge formed by the two cleavages is parallel to the principal planes, as 

 they are called, of these (that is, to the shorter diagonal of the face of 

 the prism) in one case, and inclined to them at an angle in the other. 

 The first of these will be found to coiTespond with the most perfect 

 cleavage, the other with that in the plane of symmetry. Some of the 

 larger crystals, when examined on the best (or basal) cleavage, will be 

 found to be divided into two parts, shown by the fact that the cleavage 

 on the one side makes a considerable angle with that on the other; and 

 where detached specimens can be observed it will be seen that the 

 appearance is that of two thin individuals, one of them turned round 

 half way with respect to the other, and partly penetrating each other. 

 This is called twinning, and this particular form is the most usual in 

 Orthoclase, and is termei the Carlsbad twin, from a locality where 

 good examples are found. It will be seen that the rotation is not 

 round an axis perpendicular to the faces in contact, but round one 

 lying in it. The laws governing the twinning of crystals show that 

 that plane, being the plane of symmetry, could not be the twinning 

 plane, as it is called, in contradistinction to the plane of composition. 

 It will be seen at once that merely tiirning these two rough models 

 round on that plane produces no difference in shape, and neither can 

 there be any in physical properties, seeing that these also are the 

 same on both sides of the plane in question. 



In thin slices this twinning shows very strikingly by the different 

 appearance of the two halves in polarised light, produced by the 

 different distribution of the optical properties in them. If the section 

 is accurately perpendicular to the plane of composition, although there 

 will be differences of colour in most positions, still both halves will 

 become dark at the same time. This, however, will happen but 

 rarely, but it is important, in view of what we shall find afterwards, 



