92 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



was somewhat more uniform from all sides, at the beginning, but 

 became more rapid, developing very thin plates or prisms outside 

 the nucleus. 



These are illustrated in fig. 25. In one (a) several minute grains 

 form a nucleus of a tabular feldspar microlite, whose form indicates 

 that it is tabular parallel to the pinacoid (010) and is bounded by 

 (101) and (201). The central part of this plate is thicker than the 

 margin, and thins out in irregularly placed rays, like spurs and gulches 

 leading from a mesa to a plain. The microlite is double, consisting of 

 two thin plates in parallel position. A cross-section of such a double- 

 plated microlite is shown in (b). 



In (c) the central part is a flattened prism or blade, passing at the 

 extremities into fibers or needle-like prisms and then into a thin plate 

 completely surrounding the nucleus. In (d) the thin plates at both 

 ends of the blade do not unite. The shape of the plate shows only a 

 center of symmetry; diagonally opposite corners are sharp angles a 

 little less than 90 (101) and (201). The other corners are rounded or 

 formed by two obtuse angles. The outline corresponds to the traces 

 of the basal pinacoid (001) and the two pinacoids of the second kind 

 (201) and (101) on the second pinacoid (010). The crystals are 

 elongated in the direction of the crystal axis, a, and flattened parallel 

 to the second pinacoid (010). In (e) a small rectangular prismatic 

 crystal is enlarged to a more elongated form with fibrous termina- 

 tion. In (/) a squarish form with rectangular projections is extended 

 in opposite directions as straight fibers. There are narrow prismatic 

 forms with needle-like extensions at the ends. These are isolated or 

 more often grouped in subparallel bundles and in almost parallel 

 lines, as in (g). These microscopic crystals vary in size from a length 

 of 0.64 mm. to less. They are so small that their optical characters 

 can not be used to determine their chemical composition. They 

 appear to be alike and homogeneous, and may be assumed to have 

 the composition of the preparation Ab3An,. 



(28). This preparation is colorless glass, with feldspar crystals in 

 rods or prisms about 0.08 mm. long and 0.005 mm. wide. A few of 

 these are isolated ; most of them are crossed groups of two or more, or 

 form radiating aggregates. Relatively few crystals have the shape of 

 blades or plates. The prisms are frequently double, joined in the 

 middle like an H. The central connecting part sometimes occupies 

 a small portion of the length of the double prism, sometimes a large 

 portion. The prisms are grouped in subparallel bundles as in prepara- 

 tion (32). The extinction angle is almost zero in nearly all bundles 

 of prisms. The feldspars appear to be homogeneous, and must have 



