0. Andersen — Aventurine Feldspar. 389 



Oligoclase from Aamland, Sondeled, Norway* 



The occurrence is much like the one at TVedestrand (accord- 

 ing to Weiby's description of the latter). At the place where 

 the specimens were found the prevailing gneiss included a great 

 number of irregular pegmatite veins varying in thickness from 

 0*5 m. down to a few mm. The main minerals of the veins 

 were oligoclase (sunstone), quartz and cordierite ; accessory 

 minerals, hornblende, biotite, apatite and magnetite. 



The feldspar. — The oligoclase is generally very fresh, in 

 places strong red from the hematite inclusion, sometimes gray- 

 ish or colorless. 



Extinction angle on (001) = +1° — 2° 

 " " " (010) = +5° 



/3 = 1*545 

 Refractive index of glass, n Na = 1*507 ± 0*001 

 Composition : Ab 78 An 22 , oligoclase, practically the same 

 as the sunstone from Tvedestrand. 



The crystals are often twinned with coarse lamellae, after 

 the albite and pericline laws, thus showing striation on (001) 

 and (010). Yery often, also, large pieces without twinning 

 are observed. 



The aventurization was of a variable intensity and seldom 

 uniform over large pieces ; many specimens contained no reflect- 

 ing lamellae ; others contained many and showed a beautiful 

 aventurization, not inferior to that of the best Tvedestrand 

 specimens. 



The hematite lamellaz. — The lamellae were large as in the 

 Tvedestrand sunstone, up to 2 X 3 mm or sometimes more. Most 

 of them were not six-sided, but showed distinct, unsymmetrical 

 eight or ten-sided outlines. Often they were quite irregular. 

 Fig. 2, PI. II, and fig. 1, PI. Ill, show the characteristic 

 shape of lamellae in sections after (001). 



Orientation of the lamellae. — The angles of the goniometric 

 measurements are given in Table 7. The planes of orientation 

 were : (001), (010), ( 112), (112), (150), (150), (110), (110). Of 

 these (112) contained the larger number of lamellae; (112) 

 only a few; (150) and (150) contained a considerable number, 

 the other forms very few. 



The determinations of the planes (110) and (110) were based 

 chiefly on the microscopic measurements ; for (110) : <$> P = 

 57° 10' ; M = 63 9 30' ; for (-110) ; <£ P = 57° 35' ; <£ M = 63° 58'. 



In one case direct goniometric measurements could be made 

 on exposed lamellae after (110) : /a M = 61° 5' ; <£ M = 63° 55'. 



* This locality was recently discovered by Mr. Torje Torjesen, Eisor. The 

 specimens for the present description were collected by the author at the 

 locality. 



