8o ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



posite of divergent bundles shorter than the radius, which have been 

 added to one another as though new plumes had started from the ends 

 of earlier ones. 



In most cases the middle portion of the feldspar bundles consists of 

 stouter crystals than the outer parts. It also appears that the middle 

 portion is more prismatic, in certain cases somewhat cuboidal, the 

 outer parts becoming delicately tabular. This, with the divergence in 

 position, explains the spread of the outer part of the sphere. There is 

 a great increase in the number of individual crystals in the outer 

 portion of the spherulite, and in some cases the crystals also increase 

 in size in the outer part. 



The shapes of the crystals are due to the flattening of the crystal 

 parallel to the second pinacoid (oio), and its elongation parallel to 

 the crystal axis a. The outlines of the plates appear to conform to 

 traces of several pinacoids in the zone of the b axis, (ooi), (201), 

 (101), (201), (304), (203), not all of these occurring together. It is 

 quite probable that pinacoids in the zone of the c axis also may be 

 developed, but they were not recognized. 



Bladed forms in some cases prove to be aggregates of thin plates 

 not strictly parallel to one another in the plane of flattening, so that 

 the blade is curved and not straight in the direction of its longest axis. 



In some spherulites the component crystals are prisms throughout, 

 with no tabular flattening. The number of crystal prisms increases 

 from the center of the spherulite outward by the development of new 

 prisms at slightly divergent angles, in arborescent arrangement. 



The most complex arrangements are produced by twinning and 

 divergence combined, resulting in feather-like aggregates. Long, 

 narrow, tapering blades in albite twins form a shaft, elongated parallel 

 to the crystal axis a, on two sides of which diverge at a slight angle a 

 double set of thin blades, like barbs. These consist of branched 

 smaller blades or prisms, like barbules, the branch prisms having 

 approximately the direction of the crystal axis c. The two sets in 

 each "barb" are apparently related to one another as the halves of a 

 manebach twin. The small prisms are composed of many subpar- 

 allel plates flattened in the plane of the second pinacoid (010). These 

 correspond to barbicels in a feather. 



With respect to the size of the crystals it is extremely significant 

 that pure anorthite (An) develops in comparatively large plates, 5 mm. 

 thick and 20 to 30 mm. long, in a few hours, whereas the more sodic 

 the feldspar the smaller the individual crystals formed under almost 

 the same conditions of cooling. Thus with oligoclase (AhtAni) the 

 individual crystals composing a bundle of blades are considerably 



