410 



MINERALOGY 



Microcline is always twinned with striations differing from those 

 of orthoclase. It has been advanced as a theory that orthoclase is 

 a triclinic feldspar and that its monoclinic symmetry is a pseudo 

 one, and caused by thin laminae, so thin as to be submicroscopic, 

 and when considered from this standpoint it would not differ from 

 microcline. 



In addition to the twins found in orthoclase there are two other 

 important twinning laws represented in the twins of microcline : 



1. The albite twins, in which the twinning axis is the normal to 

 the pinacoid 010, and the composition plane is parallel to 010; 

 when repeated polysynthetically, this produces striations on the 

 base and bands between crossed nicols on the basal cleavage parallel 

 to the intersection of the basal and brachypinacoids. 



2. Pericline twins, in which the twinning axis is the crys- 

 tallographical axis b and the composition plane is parallel to the 

 axis b and inclined to the base, at an angle depending upon the 

 composition of the feldspar. These twins are also repeated, and 

 produce striations and bands on both the brachypinacoidal 

 cleavage and on the basal cleavage. Those on the base are 

 at right angles to the striations produced by the Carlsbad and 

 albite twins and the two sets of bands between crossed nicols from 



FIG. 460. Section of Microline, showing the Gridiron Structure and inclosing 

 a Crystal of Plagioclase. 



