510 T. L. Watson — Kragerite, a Rutilebearing Rock 



of its content of rutile. In theoretic connection, the speaker 

 considered this rock, which consists almost exclusively of 

 albite ami rutile, a differentiation product of a gabbro magma, 

 analogous to the appearance of lestiwarite, an aplitic differen- 

 tiation product of a nepheline-syenite magma, etc. In addition, 

 mention was made by analogy of routivarite, anorthosite, and 

 oligoclasite. The content of titantic acid in the kragerite was 

 attributed to pneumatolytic processes during formation*" 



Pelroc/raphic Character. 



General description. — The rutile-bearing rock (kragerite) 

 from Krageroe, Norway, submitted to analysis is medium- 

 grained and of light color, with a pronounced granitic habit. 

 Its most prominent megascopic constituents are light gray and 

 pinkish feldspars and nearly black rutile, with a little quartz. 

 Several small areas of a green ferromagnesian mineral, prob- 

 ably pyroxene partly altered and associated with rutile, were 

 noted. The feldspar and quartz grains measure 1 to 2 miD in 

 cross-section ; the rutile grains rarely exceed l mni and many are 

 less than - 5 mm . The quantity of nearly black rutile and the 

 pink color of a part of the feldspar increase the depth of color 

 of the rock. Most of the feldspar shows line albitic twinning 

 on cleavage surfaces under a pocket lens. The rutile is in 

 small grains, partly disseminated through the rock, but mostly 

 segregated along roughly parallel lines which give it a streaked 

 or banded appearance, and which Professor Brogger refers to 

 as schlieren of local enrichments of the mineral. 



Microscopic description. — A thin section of the less rich 

 rutile portion of the rock was examined under the microscope 

 and found to consist essentially of feldspar, much rutile, some 

 quartz, and a little ilmenite. No ferromagnesian silicate 

 minerals were observed. 



Feldspar is much the most abundant constituent and is com- 

 posed chiefly of a sodic plagioclase (albite-oligoclase) together 

 with some microcline and orthoclase, the latter intergrown with 

 albite as microperthite. Some of the feldspar anhedra show 

 partial micropoikilitic structure developed from inclosures of 

 other feldspar, quartz, and rutile. The feldspars are almost 

 wholly fresh, but occasional small spots are altered to colorless 

 mica. Partial peripheral granulation of a part of the feldspar 

 and quartz was indicated, accompanied by slight bending of 

 the lamellae of several plagioclase individuals. 



Rutile, the second mineral in quantity, ranges from small 

 microscopic idiomorphic crystals to irregular massive grains 

 about l mm in diameter. Some of the larger grains show partial 

 crystal outline. It is deep red-brown, usually pleochroic, and 

 frequently shows cleavage and twinning. Some ilmenite, 



