297 



The crystals may be either tabular in consequence of the conspicuous 

 development of M (this is the most common form in British rocks) or 

 columnar in consequence of the equal development of P and M, and the 

 elongation of the crystal in the direction of the clino-diagonal axis. In the 

 columnar crystals the most common terminal forms are I and y. The most 

 perfect cleavage is parallel to P, but that parallel to M is often almost as 

 perfect. There is also an imperfect cleavage parallel to I (110) and in 

 sanidine a tendency to break along planes roughly parallel to the ortho-pina- 

 coid (100). The cleavages are often not recognizable in thin sections. Zonal 

 structures are frequently seen in the porphyritic crystals of the granitic and 

 trachytic rocks; they are, however, as we should naturally expect, much 

 more marked in the latter than in the former. The porphyritic crystals of 

 the trachytic rocks are often completely honeycombed by inclusions of the 

 ground-mass in the manner already described in speaking of the plagioclase- 

 (Tvstals of the trachytic rocks of basic and intermediate composition. 



Twinning on the Carlsbad plan is very common. In this case the 

 vertical axis is the twinning axis. Two adjacent individuals interpenetrate 

 or else the face of composition is the clino-pinacoid. The basal cleavages 

 of the two halves do not coincide and this produces the well-known 

 macroscopic appearance indicative of twinning on this plan. Thin sections 

 of a Carlsbad twin out of the zone 100: 001, extinguish simultaneously in 

 both halves when the trace of the clino-pinacoid (usually the face of 

 composition) lies parallel with the principal section of one of the nicols. 

 This, as already stated, is one of the most useful tests in distinguishing 

 between orthoclase and plagioclase under the microscope. In normal 

 orthoclase the optic axial plane is at right angles to the plane of 

 symmetry. Cleavage flakes parallel to M are at right angles to the obtuse 

 bisectrix which is positive (b=7). Sections parallel to the ortho-pinacoid 

 (100) give straight extinction, and show in convergent light the negative 

 bisectrix. The bisectrices for different colours are dispersed in a plane 

 parallel to M,but this dispersion cannot be recognized in the ordinary thin 

 sections. The optic axial angle varies greatly in different varieties of 

 orthoclase. In orthoclase proper it is large (in air 112-125); in some 

 varieties of sanidine it is very small indeed, the crystal appearing almost 

 uniaxial. DES CLOISEAUX has shown that by heating a section of ortho- 

 clase the optic axes may be made to approach and finally to expand in the 

 plane of symmetry. If the temperature be raised above 500 C. the original 

 positions of the axes are not recovered on cooling the ellipsoid of 

 elasticity is permanently deformed. The sanidine crystals in liparite tuff's 

 are often found to have their optic axial plane in this abnormal position. 

 It seems probable, therefore, that the variability in the optical characters of 

 different varieties of orthoclase is largely due to physical causes operating 

 after the development of the crystals. In twinning on the Carlsbad plan 

 the optic axial plane in the one half lies parallel with that in the other ; 

 in twinning on the Baveno plan the optic axial plane in one half is at right 

 angles to that in the other. Intergrowths of plagioclase and orthoclase may 

 frequently be observed. They are much more common in the granitic than 



