378 0. Andersen — Aventurine Feldspar. 



the change from darker to lighter brown red or to yellowish 

 brown is very slow and in the lightest lamellae a change is 

 hardly detectible, even after a long heating. The opaque 

 lamellae are seldom homogeneous, but generally contain a num- 

 ber of irregular spots of transparent substance which shows 

 the same colors as the entirely transparent hematite lamellae. 

 By a short heating these transparent parts undergo practically 

 no change while the opaque parts become transparent and soon 

 acquire the same color as the transparent parts so that the 

 lamellae become homogeneous. It seems as if the changes 

 take place with retarded velocity and gradually cease when a 

 certain stage is reached. 



These changes might be explained as being due to a direct 

 solution of the hematite into the feldspar (in the solid state) 

 whereby the hematite lamellae grew thinner and lighter. In 

 that case we should expect the most conspicuous change in the 

 thinnest lamellae, some of which ought to disappear if the heat- 

 ing were continued for a sufficiently long time. As the ex- 

 periments show, however, the thinnest lamellae were evidently 

 the most persistent ones, none of them disappearing and most 

 of them undergoing no visible change even after 22 days' 

 heating. 



The change may be explained by assuming that there is a 

 transition from a darker to a lighter modification of the 

 hematite. The darker form is perhaps a secondary product 

 and the original, lighter form is restored by heating. An ex- 

 planation of this nature seems to account for the actual 

 behavior of the different hematite lamellae on heating. As 

 the experimental data are few and only suggestive we have no 

 basis for a detailed discussion of these problems. 



The sudden disappearance of the hematite lamellae at 1235° 

 is most reasonably interpreted as due to a simultaneous melting 

 of hematite together with a portion of the feldspar surround- 

 ing it ; perhaps a eutectic melting or possibly a melting with a 

 reaction between the feldspar and the hematite. Owing to the 

 extreme thinness of the lamellae the amount of feldspar neces- 

 sary for such a melting must in any case be small. The liquid 

 (glass) resulting from the melting will therefore occupy only 

 very thin films in the place of the lamellae and will escape 

 detection under the microscope. It will look as if the lamellae 

 disappeared without leaving any trace, while the surrounding 

 feldspar was unchanged and showed no sign of melting. The 

 fact that the opaque lamellae reappear in the same places by 

 heating at a lower temperature proves that their substance can 

 not have travelled far. The substance of these reappearing 

 lamellae is evidently different from that of the original lamellae, 

 and the iron oxide must therefore have undergone some change 



