0. Andersen — Aventurine Feldspar. 395 



Projections on (001) of the long edges of lamellae after (112) 

 and (ll2) formed frequently 15° with the a-axis ; others were 

 approximately parallel to the o-axis. Projections on (010) of 

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

 often —83° with the <z-axis. 



Microcline Perthite from IStene, Sannokedal, Norway.* 



The occurrence is an ordinary granite pegmatite dike (feld- 

 spar quarry) containing microcline perthite, plagioclase (oligo- 

 clase), quartz and biotite as main minerals and a number of 

 other minerals in smaller quantities. Different varieties of 

 graphic granite are abundant. 



The feldspar. — The perthite structure was in part coarse 

 with visible lamellae of albite (after 110), in part a very fine 

 microperthite structure, with extremely thin rod-shaped in- 

 clusions of albite oriented approximately parallel to the c-axis. 

 There were all transitions between these two structures, both 

 often being found in the same little cleavage piece. The 

 coarse perthite often formed isolated patches in the micro- 

 perthite. 



In the coarse perthite the microcline was developed with the 

 ordinary cross hatching. In the microperthite the microcline 

 structure was very fine and there was no regular cross hatch- 

 The hematite lamella). — The aventurization was restricted 

 to the microperthitic parts of the feldspar and appeared with 

 medium intensity about equally distinct on either of the two 

 cleavage faces. 



The lamellae measured at most 0*l mm in diameter and were 

 very variable as to shape. Some showed very regular hexa- 

 gonal outlines, others were rhomb-shaped or elongated and 

 still others perfectly irregular. 



The lamellae parallel to (001) were often collected in groups 

 with regular hexagonal or rhomb-shaped outlines. The single 

 lamellae were small, mostly irregular, but sometimes hexagonal 

 or rhomb-shaped and with the edges parallel to the outlines of 

 the groups. The groups often measured 1 — 2 nim in diameter 

 and their outlines were definitely oriented. 



Orientation of the lamella). — The goniometric measurements 

 are given in Table 12. The planes of orientation _were the 

 following faces: (001), (010), (112), (112), (150), (150), (110), 

 (110). _Most of the lamellae were parallel to (112), (112), (150) 

 and (150) with about equally many after each ; a considerable 

 number were also oriented after (001), (010), (110) and (110). 



*This locality was discovered some years ago by Mr. Peder P. Tangen, 

 Ki'agero. The specimens examined were partly obtained from the Minera- 

 logical Museum of Kristiania University, partly collected by the author at 

 the locality. 



