AEGIRITE AND RIEBECKITE ROCKS FROM OKLAHOMA 285 
eight of these occur near the center of the largest mountain just 
northwest of Granite. ‘They vary in width from one to sixteen inches. 
The prevailing color is a greenish-gray inclined to reddish-gray in 
places. The rocks are fine-grained with equidimensional constituents 
averaging about 1™™in diameter. With a lens glassy grains of quartz, 
cleavable feldspars, black prismatic crystals (riebeckite), usually 
small but sometimes 3™™ in length, and minute greenish patches and 
needles (aegirite) can be recognized. 
In thin sections these rocks are seen to be fine-grained mosaics of 
quartz, microperthite, and accessories with the typical panautomorphic 
texture of aplites. Both the feldspar and quartz have a tendency to 
assume a crystal form. The microperthite is an intergrowth of cloudy 
orthoclase with clear albite which is recognized by extinction angles 
and index of refraction. The albite occurs in patches within the 
orthoclase, on its border and in a few cases, independently. The 
orthoclase often occurs in Carlsbad twins. The quartz is xeno- 
morphic to automorphic in various specimens. The best automor- 
phic quartzes were hexagonal in outline though only a few of these 
were basal sections. The riebeckite, one of the accessory minerals 
usually occurs in large patchy aggregates so characteristic for this 
mineral, but in one rock (No. 12) it was in the form of small grains. 
Its color varies from green through a bluish-green and greenish-blue 
to deep, almost opaque blue. The axial colors are @, very deep 
blue; b, deep blue; C, green. A>D>SC. The extinction is practi- 
cally parallel. In addition to cleavage parallel to (110), parting 
parallel to (oro) was noticed. The aegirite varies from minute 
needles to small prismatic crystals with the octagonal cross-section 
characteristic of the pyroxenes. The pleochroism is yellowish-green 
to emerald-green, and the maximum extinction 5 or 6°. Magnetite 
and microcline are found in small amount. 
The order of consolidation seems to have been first, the orthoclase, 
then the albite, next the aegirite and finally the quartz. ‘The aegirite 
occurs as inclusions in the quartz (but not in the orthoclase), but 
mostly between the quartz and orthoclase. In one rock (No. 44) 
aegirite needles occur with their long axes parallel to outlines 
of the quartz. In this case it looks as though the quartz had simply 
pushed aside the aegirite, as there is no constant orientation due to 
