88 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



tinctly branching, curved, radiating aggregates, in some cases exhibit- 

 ing albite twinning. In places these prismatic, rod-like forms broaden 

 out to rectangular cuboidal shapes, which extend in short prismatic 

 branches almost at right angles to the longitudinal direction of the 

 long prisms. These cuboidal crystals show to a slight extent a cross 

 twinning which is in the position for pericline twinning. The angle 

 between the traces of the cuboidal faces is nearly 90, which indicates 

 that the crystals have been cut parallel to the basal plane and the 

 sections are bounded by the first and second pinacoids (ioo) (oio). 

 The sections parallel to the flat side of the blades show an intricate 

 structure composed of parallel and subparallel thin plates with crystal 

 outlines at several angles, the most frequent being nearly qo. These 

 aggregates of plates form bands that branch in feather-like structures 

 ( Plate X) . There are occasionally small rectangular but quite irregu- 

 larly outlined sections, whose shape is that of cuboidal crystals, with 

 zonal markings about the center, which have developed small pris- 

 matic projections parallel to one axis (?a), the prisms being located 

 at the four corners of the rectangular section. This corresponds to 

 the microlitic crystals of feldspar found in volcanic glasses, where the 

 projecting prisms are delicate fibers. The feldspar crystals in this 

 preparation appear to be homogeneous. 



( 22 a-b-c) The preparation was first cooled rapidly from a melted 

 condition, then heated again to 1250 at a maximum. This has had 

 a very interesting result, namely, two periods of crystallization, the 

 first rapid, the second slower. The main mass of the preparation 

 consists of bundles of feldspar fibers and delicate network of crossed 

 fibers. The bundles of fibers are about 0.015 mm. wide and 0.165 mm. 

 long, and occur singly or cross one another at various angles, several 

 intersecting in the middle. Single bundles have two strong tapering 

 fibers on the outside spreading slightly. These bundles of fibers 

 extinguish light parallel, or at a small angle (7 ), to the length of the 

 fiber. The fastest ray vibrates nearly parallel to the fibers, which 

 appear to be elongated in the direction of the axis a. There is a small 

 amount of isotropic glass. 



Within this mass are spherulites about 10 mm. in diameter, the 

 outer shell, 1 millimeter thick, having a somewhat different appear- 

 ance from the central portion. The central part consists of radiating, 

 branching prisms or blades, not in straight rays but in plumose aggre- 

 gations. The outer marginal zone is a blend of the inner spherulite 

 and surrounding matrix of small bundles of fibers (Plate XI), and it 

 is evident that the spherulitic feldspar crystallization had advanced 

 in the already crystalline matrix by a process of recrystallization, the 



