546 



Palache and Warren — Chemical Composition and 



only and thus lacks the grating structure characteristic of 

 microcline in general. In basal sections the twinning is seen 

 to be very finely polysynthctic. The individual lamellas appear 

 as short strips slightly elongated parallel to 010. Their bound- 

 aries are as a rule not sharp. The two sets of lamellae extin- 

 guished symmetrically on either side of the trace of the 

 twinning plane at an angle of 16 degrees (average of 12 meas- 



Fig. 4. 



Fig. 4. Micro-photograph of a basal section of microcline cut from a 

 freely developed crystal of the pocket lining, Fallon quarry pegmatite. 

 Shows twinning after the albite law only. One set of twin lamellae is extin- 

 guished at an angle of 16° with the trace of 010. 



Crossed nicols. Magnification about 300 diameters. 



urements). The clearer growths of later age are in parallel 

 position to the older crystal and in them the twinning lamellae 

 are often longer and more sharply defined. In the small 

 microclines throughout the finer-grained portions of the pipe 

 the twinning is usually more sharply defined. The extinction 

 of 010 sections was found from the average of ten measure- 

 ments to be 5°. Figure 1 is a micro-photograph, using 

 polarized light, of a basal section of a small microcline 

 crystal. 



