384 0. Andersen — Aventurine Feldspar. 



Oligoclasefrom near Statesville, Iredell County, North Carolina* 



The feldspar. — The cleavage pieces had a grayish-brown 

 color, and were not quite fresh, being clear in spots only. 



Extinction angle on (001) = + 1/2° 

 " " « (010) = + 8° 



j8 = T54 

 Composition : Ab 63 An ls , oligoclase.f 



The cleavage pieces form polysynthetic twins after the 

 albite law, the lamellae of the one individual being compara- 

 tively broad, those of the other extremely narrow. 



The hematite lamellae. — In the clear spots of the feldspar 

 there were numerous lamellae which produced a stroug aven- 

 turization. The lamellae formed sometimes long narrow strips 

 and sometimes distorted six-sided or rhomb-shaped plates. The 

 largest dimensions of the strips were 0*8 X 0"03 mm ; the plates 

 were seldom more than 03 X - l mm . 



Orientation of the lamellae. — The goniometric measurements 

 are given in Table 4. The_ planes of orientation were: (001), 

 (010), (112), (112),^ (113), (021), (150), (150). 



The most_prominent set of lamellae were parallel to (112), 

 (150) and (150). After (112) there were fewer, and after the 

 other planes, (113), (021), (001) and (010), very few. 



The projections of the long edges of lamellae after (112) on 

 (001) often formed about +74° with the a-axis. These lamellae 

 were elongated, strip-shaped, and traversed a great number of 

 twinning lamellae. Six-sided lamellae after (112) were oriented 

 with the projections of one of the edges on (001) forming —80° 

 with the a-axis. The lamellae after (112) were often elongated 

 with the projections of the long edges on (001) approximately 

 parallel to the a-axis. Lamellae after (150) and (150) had dis- 

 torted hexagonal or rhombic outlines. The projections of 

 their elongated edges on (010) sometimes formed +11° with 

 the a-axis. 



Oligoclase from Kragerd, Norway. \ 



The feldspar. — The cleavage pieces have a strong, red color, 

 and are usually perfectly fresh and transparent. 



Extinction angle on (001) = +1° 

 " " " (010) = +7° 



/3 = 1-543 

 Composition : Ab 80 An go , oligoclase. 



* Specimens from U. S. National Museum, No. 80324. 



t Q. F. Kunz has described orthoclase sunstone from Statesville, N. C. 

 (History of the Gems fouud in North Carolina, p. 27). This may be the 

 same sunstone as the one described here. Owing to the very fine twinning 

 striation the feldspar might have been mistaken for orthoclase, by a macro- 

 scopic examination. 



\ Specimens from U. S. National Museum, No. 44776. 



