LABRADORITE (aBjAN,,). 87 



the extinction angle, measured from the cleavage plane (001), is 

 about 33 . The crystals appear to be alike and homogeneous, hav- 

 ing the composition AbiAns. 



Labradorite (Ab,An s ). 



(6o a_b ). The preparation consists of radiating plates or blades, 

 about 0.05 mm. to less in width and as much as 0.7 mm. long. Two to 

 five blades intersect at various angles, wedging out at the point of 

 intersection (Plate VI). Bach plate consists of two or more twinned 

 lamellae. Between the thicker plates there are more delicate crystals 

 composed of subparallel plates and skeleton growths of extremely 

 thin blades with crystal outline, probably the traces of (001) and 

 (201) on the second pinacoid (010). The more solid plates or blades 

 feather out at the ends to somewhat divergent plumes. There are 

 in some cases branching, feather-like forms in crystallographic posi- 

 tions suggesting the extension of a single crystal in directions parallel 

 to the a and c, and possibly the b, axes, the angles of branching being 

 about 64, and in some cases about 90 . Albite twins yield maximum 

 symmetrical extinction angles of 37 . The crystals appear to be 

 homogeneous. 



(6i a ~ b ). The preparation is glass, with an index of refraction 

 higher than that of balsam, and feldspar spherulites about 10 mm. 

 in diameter. The spherulites are very beautiful aggregations of 

 somewhat divergent, plumose bundles of prismatic crystals (Plate VII) 

 that appear as distinct crystals at the surface of the spherulite, from 

 which they project at slightly different lengths into the surrounding 

 glass, each prism, 0.003 or 0.004 mm. in diameter, being terminated 

 by crystal faces nearly equally inclined to the long axis of the prism. 

 The component short bundles show similar plagiohedral terminations 

 to the individual prisms composing them. These in some cases are 

 flattened and blade-like, and are in subparallel aggregations, the 

 plates having nearly rectangular outline. In one part of section 

 (6i a ) there are groups of albite twinned feather-like aggregates similar 

 to those in (58 b ). The groups are shorter and less parallel, and are 

 more curved. There are longitudinal sections of radial elements of 

 the spherulites (Plate VIII) with the same feather-like structure 

 observed in (58 a ). The feldspars of the spherulites appear to be 

 homogeneous optically, and are probably so chemically. 



(23 a_b ). An aggregation of radiating blades and possibly prisms 

 somewhat spherulitic, the radii being 5 mm. long in some cases (Plate 

 IX) . The apparently prismatic forms may be cross-sections of blades 

 which are recognizable as such in other positions. They form dis- 



