twinning striations on the brachypinacoid. 



391 



ing to the so-called pericline law, that is the twinning plane is 

 a plane at right angles to the macro-axis, &. The two individu- 

 als in twin position are not united by the basal plane, when 

 they would not quite match along the composition face, but by 

 a plane deviating a little from the base, passing through the & 

 axis, and so inclined that the four plane angles which its inter- 

 section makes with the prisms If and m and pinacoid b are all 

 equal, this is the so-called rhombic section {der rhombische Schnitt 

 of vom Eath). Owing to the variation in the axial angles of the 

 triclinic feldspars, especially the angle between the a and & axes, 

 which in albite = 87° 51£ v , in anorthite 91° 12', 

 the direction of the rhombic section changes 

 very greatly. This will be better understood 

 from the accompanying figure and the following 

 table copied from Tschermak.* If the direction 

 above the line, parallel to the edge c : &, be called 

 positive -f- and below negative — , the direction 

 of the rhombic section in the different feldspars 

 in the albite-anorthite series should be about as 

 follows : 



Albite Na 2 Al 2 Si 6 16 Ab, 



Oligoclase AbgAnj 



Andesine AbjAnj, 



Labradorite Ab 1 An 3 , 



Anorthite Ca a Al 4 Si 4 16 An 



If these striations can be found, therefore, on i-i, they should 

 give a ready means of determining the position of the feldspar 

 in the albite-anorthite series. The following analyses will 

 serve to show the relation between the direction of the stria- 

 tions and the chemical composition of the feldspars. 



Oligoclase and Albite. — In Professor Brush's collection, leav- 

 ing out of consideration the crystallized specimens of albite 

 and pericline, there are cleavage specimens from six different 

 localities which show distinct striations upon i-i. There are 

 also a few others in which the striations are not very distinct, 

 in all, nearly one-half of the specimens show these striations. 

 On the basal plane the striae are usually a little finer than on 

 i-i, and a rectangular marking is sometimes seen due to the 

 meeting of the two sets of striations according to the albite 

 and pericline laws. There is also a tendency for the feldspars 

 to separate, or cleave, parallel to the rhombic section and a 

 combination parting of this with the basal cleavage causes the 

 feldspar to break out into wedge-shaped blocks. The follow- 

 ing six analyses by Mr. Sperry are arranged according to the 





#Na 2 



$CaO 



+ 22 



11-8 







+ 4 



8-1 



5-2 



— 2 



5-7 



10-4 



— 9 



2-8 



15-3 



— 18 



•o 



20-1 



* Lehrbuch der Mineralogie, 1884, 458. 



