376 0. Andersen — Aventurine Feldspar. 



(3) Chemical tests show that the lamellae contain iron oxide.* 



(4) Thermal experiments (to be described below) show that 

 the lamellae do not undergo any essential change even by a 

 prolonged heating of the aventurine feldspar at temperatures 

 around 1050°. If the lamellae consisted of goethite or other 

 hydrated iron oxides we should expect a considerable change 

 due to the decomposition of such hydrates by heating. 



All these data lead to the conclusion that the reflecting 

 lamellae of the aventurine feldspars consist of hematite in 

 tabular crystals after the base, as first suggested by Scheerer. 



Thermal Data. 



In order to obtain, if possible, some information on the 

 stability relations between the feldspar and the hematite in- 

 clusions, a number of heating experiments were carried out.f 

 Fresh, transparent cleavage pieces containing hematite lamellae 

 of various size and thickness were selected for the experi- 

 ments. The outlines of the cleavage pieces and their included 

 lamellae were drawn with camera lucida and the colors of the 

 different lamellae in transmitted light were noted. After 

 appropriate heating (in the electric resistance furnace) the cleav- 

 age pieces were removed to the air, examined under the micro- 

 scope and compared with the drawings. 



A brief record of the experiments is given below. 



(1) Cleavage piece with numerous transparent hematite 

 lamellae heated for one hour at 1260° : The feldspar remained 

 birefringent throughout with outlines sharp as before the 

 •heating; colorless; transparent, but somewhat dim. All 

 hematite inclusions disappeared. 



(2) Cleavage piece with few light-colored hematite lamellae 

 of sharp outlines heated for one hour at 1150° : No visible 

 change. 



(3) Same piece heated for one-half hour at 1200° : No 

 change. 



(4) Same piece heated for one-half hour at 1230° : Feldspar 

 unchanged. Some hematite lamellae disappeared ; others be- 

 came lighter and were corroded at the edges. 



(5) Piece from exp. (1) heated for twenty-four hours at about 

 1050° : Feldspar milk white, dull, full of very fine black dust. 



(6) Cleavage piece with numerous hematite lamellae heated 

 for one-half hour at 1235° : Feldspar birefringent ; colorless ; 

 somewhat dim, but still transparent. All hematite disappeared. 



*In this connection it should be noticed that lamellae of similar qualities 

 forming inclusions in carnallite have been analyzed separately and found to 

 consist of Fe 2 O a without H 2 (O. Ruff, "Kali", i, 81, 1907). 



f Specimens from Aamland, Sondeled, Norway, yielded the most favorable 

 material for these experiments on account of the freshness of the feldspar 

 and the large size of the hematite lamellae. 



