364 0. Andersen — Aventurine Feldspar. 



It was found that the lamellae, in all the different varieties 

 examined, were oriented parallel to definite, crystallographic 

 faces of the feldspar, and all these faces had rather simple 

 indices. The lamellae causing the aventurization on the cleav- 

 age faces were oriented along the same crystal faces in all the 

 varieties. 



Altogether it was found that the planes of orientation of the 

 lamellae were parallel to faces of_the following forms : (112), 

 (112), (113), (150), (150), (110), (110, (021), (010), (001). 



Of these (001), (010), (110) and (110) are known as actual 

 faces in practically all feldspar crystals ; (021) is also a fairly 

 ■common form, whereas (112), (112), (113), (150) and (150) 

 belong to the very rare forms that have been observed only 

 occasionally and with insignificant faces. 



Table 1 gives the angles p P , p M , <f> P and <£ M of the forms men- 

 tioned, for albite, anorthite and orthoclase calculated from 

 the known axial ratios of these minerals.* 



For the sake of completeness the angles of the forms (113) 

 and (021) are also given. All the angles are given with posi- 

 tive and acute values. The direction in which the angle should 

 be counted and whether the positive or ,the negative a-axis 

 should be used as base is always easily inferred. 



For comparison with the measurements the calculated angles 

 of the aventurine plagioclases were simply computed by inter- 

 polation between the angles of albite and anorthite on the basis 

 -of the known compositions of the plagioclases (see Tables 2-8). 

 This method may not be strictly correct, but it is accurate 

 enough for our purpose and, in fact, probably the most accu- 

 rate method available,! as the axial ratios of the different 

 plagioclases are but imperfectly known. 



The angles measured on microclines (microcline perthites) 

 .are compared with the calculated angles of orthoclase for the 

 reason that the microcline, as usual, was always so finely twin- 

 ned that the orientation of the reflecting lamellae could not be 

 measured in relation to single twinning lamellae (as in the case 



*Albite: a: b: c = 0-6367 : 1 : 05593 



a = 94° 15' ; H - U6° 37' ; y = 87° 41'. 

 {X = 86° 24' ; u = 63° 28' ; v = 90° 28'.) 

 C. Dreyer und V. Goldschmidt, Meddelelser om Gronland, xxxiv, 1907. 

 .(In the table of elements, p. 43, is given 2. = 86° 42' instead of 86° 24'.) 



Anorthite : a : b: c = 06347 : 1 : 0-5501 



a = 93° 13' ; P = 115° 56' ; y - 91° 12'. 



(A = 85-° 50' ; \i = 63° 56' ; v = 87° 6'). 

 V. Goldschmidt, Winkeltabellen, (1897, p. 141). 



Orthoclase: a.- b: c: =0-6585 : 1 : 05554 



/?= 116° 3'. 

 V. Goldschmidt, Winkeltabellen (1897, p. 143). 

 t See V. Goldschmidt, Winkeltabellen (1897, p. 404). 



