406 



MINERALOGY 



commonly occurring twins in orthoclase. The name is derived 

 from Carlsbad in Bohemia, where they are found in great perfection. 

 2. Baveno law, twinning axis perpendicular to n (021), with n 

 as the composition plane. Since the angle n A n' = 89 13' or 

 nearly 90, the outward form of the square section perpendicular 

 to the clinoaxis is not changed. As the composition plane passes 

 diagonally across the prism face M, making unequal angles with 

 the two faces, upon a revolution of 180 around the twinning axis 

 there will appear in the twin crystal a reentrant angle at one 

 extremity much larger than at the other. This habit of twinning 

 may be repeated with the other diagonal of the square section as 

 the twinning plane, when the crystal will be composed of four 

 individuals. The name is derived from Baveno in Piedmont, where 



the pink granite quarries furnish 

 many fine crystals twinned after 

 this law. 



3. Manebach law, twinning axis 

 is perpendicular to the base and the 

 composition face is the base. Since 

 the base and clinopinacoid are at 

 right angles, the square section of 

 the crystal is not changed, but the 

 form which terminates the crystal 

 will show a reentrant angle at one 

 extremity only. When developed 

 polysynthetically, the twinning 

 planes will appear on the clinopin- 

 acoidal cleavage faces and not on 

 the basal cleavage. 



In sections, orthoclase when fresh 

 is colorless and transparent, but 

 often milky or cloudy through kao- 

 linization. In many cases the crys- 

 tal sections will show numerous fine inclusions of hematite, which 

 may impart a slight reddish tinge to the mineral. Orthoclase 

 seems to possess the peculiar habit of collecting these oxide 

 of iron inclusions from the magma, and when porphyritic the 

 individual crystals may be pink to red in color from the numerous 

 inclusions. 



In most rocks orthoclase is one of the last minerals to crystallize 

 and therefore may include such minerals as magnetite, zircons, 



FIG. 456. Baveno Twins of Ortho- 

 clase, from Baveno. 



