0. Andersen — Aventurine feldspar. 377 



(7) Cleavage piece with some transparent and some opaque 

 lamellae heated for one hour at 1230° : Feldspar practically 

 unchanged. Nearly all hematite lamellae disappeared. 



(8) Piece from exp. (7) heated for eighteen hours at about 

 1050° : Feldspar white, dull, only little transparent. The 

 opaque lamellae reappeared in the same positions and with the 

 same outlines as before the heating (of exp. 7). They did not 

 reflect the light as before, however, and were evidently made 

 up of a fine aggregate of an opaque or very dark brown sub- 

 stance. The originally transparent lamellae did not reappear, 

 but at the places they had occupied a dense crowding of black 

 dust was seen. 



(9) Piece from exp. (8) heated for one hour at 1235° : The 

 opaque lamellae again disappeared. 



(10) Same piece heated for 40 hours at about 1050° : 

 Opaque lamellae reappeared as in exp. (8). The feldspar 

 opaque, full of black dust. 



(11) Cleavage piece with opaque and transparent lamellae 

 heated for one hour at 1235° : Feldspar practically unchanged, 

 only a little dim. Both the opaque and the transparent hema- 

 tite lamellae disappeared. 



(12) Same piece heated for 45 hours at about 1050° : Feld- 

 spar dim and full of black dust. Opaque lamellae reappeared 

 in the same positions as before. The substance of the lamellae 

 now a dark aggregate as in exp. (8). 



(13) Cleavage piece with opaque and transparent hematite 

 heated for 22 days at about 1050°. The piece was examined 

 and replaced after 1, 2, 7 and 14 days of heating. After 1 day 

 some of the opaque lamellae had become transparent with a 

 deep brown color ; others remained opaque. The originally 

 transparent lamellae were apparently unchanged. After 2 days 

 no further change visible. After 7 and 14 days all lamellae 

 had become visibly lighter, some of the originally opaque 

 lamellae now being reddish brown, others being deep red. 

 After 22 days the same relations persisted. The feldspar 

 remained perfectly clear throughout (without formation of 

 dark dust as in the cases where the heating had been first 

 carried up to 1230°- 1260°). The reflections from the lamellae 

 were just as intense as before the heating. 



The result of these experiments may be summarized as 

 follows : 



By heating fresh cleavage pieces of aventurine feldspar at 

 temperatures below 1230° (around 1050°) the hematite lamellae 

 undergo a slow change. The opaque lamellae become more or 

 less transparent and the transparent ones generally grow a 

 little lighter. The change from opaque to transparent with 

 brown red color seems to take place rather quickly, whereas 



