O. Andersen — Aventurine Feldspar. 393 



Microcline Perthite from Mineral Hills, Middletown, Delaware 

 County, Pennsylvania* 



The feldspar. — The aventurine feldspar from Mineral Hills 

 was described by Des Cloizeaux first as orthoclasef and later 

 as microcline perthite. :{: 



The albite inclusions are regularly extended in the direction 

 of the c-axis, but have very irregular cross sections, often as 

 much as l ram broad. On (001) we therefore see very irregular 

 patches or bands of albite in the microcline, on (010) we see 

 regular alternate stripes of microcline and albite approximately 

 parallel to the c-axis. The microcline is greenish-gray with 

 red spots ; under the microscope it shows the ordinary coarse 

 cross hatching. The albite is colorless and shows the usual 

 polysynthetic twinning after the albite law. 



The hematite lamellce. — Aventurization was only observed 

 in the red spots of the microcline ; the other parts of the 

 microcline and the albite contained no hematite lamellae. 



The lamellae are distorted six-sided, rhomb-shaped or form 

 elongated strips or irregular patches. The elongated strips 

 measure at most 0*2 X O'l 01111 , the more irregular lamellae about 

 0-1 X 0-l mm . 



Orientation of the lamellae. — The goniometric measurements 

 are contained in Table 10. The lamella? were oriented after 

 the faces : (001), (010), (112), (112), (150), (150). Projections 

 on (001) of elongated edges of lamellae after (112) and (112) 

 form often 20° with the a-axis. For other lamellae the cor- 

 responding angle is 76°. The angle between the projections 

 of the elongated edges of lamellae after (150) and (150) on 

 (010) and the a-axis is often about —80°. 



Microcline Perthite from Naskilen, Arendal, Norway. § 



The feldspar. — The cleavage pieces were rather fresh, of a 

 brownish-gray color. The cleavage faces were often curved. 



The feldspar was a microperthite with very fine, rod-shaped 

 albite inclusions of elliptical cross sections and with the 

 elongated direction approximately parallel to the c-axis. The 

 microcline showed a very fine cross. hatching. 



The hematite lamellae. — The aventurization was distinct, 

 produced by numerous small lamellae which never measured 

 more than - 2 mm in diameter. The lamellae often formed linear 

 strips, but were also sometimes approximately hexagonal or 

 rhomb-shaped. 



* Specimens from U. S. National Museum ; No. 78700. 



+ (Orthose aventurine) Nouv. Eecb., 1867, pp. 153 and 206. 



\ Ann. Chim. Phys. (5), ix, pp. 534, 460, 463, 1876. 



§ Specimens from the Mineralogical Museum of Kristiania University. 



