0. Andersen — Aventurine Feldspar. 353 



Very little was known about the exact localities or the 

 mineral associations of the snnstones. The first description of 

 an occurrence was given by K. G. Fiedler,* who discovered a 

 locality near Werchne ITdinsk, Siberia. Fiedler does not 

 describe the feldspar and the explanation of the aventuriza- 

 tion is disposed of in the following remarks : '' Ueber diesen 

 Feldspath ist noch zu bemerken, dasz er seinen G-oldschimmer 

 der Vulkanitat verdankt in welcher er entstand." 



The well known occurrence at Tvedestrand, Norway, was 

 discovered by Weiby in 1844. This locality has furnished a 

 large quantity of good specimens, the first of which were care- 

 fully examined by Th. Scheerer,f who analyzed the feldspar 

 (oligoclase) and described and gave drawings of the reflecting 

 inclusions, which he determined as hematite. He found these 

 inclusions to be oriented parallel to (001), (010), (221) and a 

 vertical prism and summarized his explanation of the origin of 

 the sunstone as follows :\ " Man musz also annehmen, dasz Oli- 

 goklas und Eisenglanz die Producte eines gleichzeitigen Krys- 

 tallizationsactes sind, und dasz beide, in ihrer regelmassigen 

 "Werwachsung ein dem Schriftgranite ahnliches Gemenge dar- 

 stellen." 



A. Kenngott§ discussed the qualities of the reflecting lamel- 

 lae, and concluded that they were goethite (" pyrrhosiderit"). 



E. Reuschf discussed, in connection with a careful study of 

 moonstones, the problem of reflections and refractions in bodies 

 like sunstones and moonstones and made a few observations on 

 sunstone from Tvedestrand, correcting and supplementing some 

 of Scheerer's measurements. 



Isaac Lea^f made microscopic examinations of sunstones from 

 different (mostly American) localities and described the reflect- 

 ing inclusions, which were considered goethite, but did not 

 attempt to determine their orientation, thinking that " they 

 usually lie parallel with the principal cleavage of the feldspar."** 



A. Schraufff studied aventurization on labradoi'ites and also 

 examined the inclusions in sunstone from Tvedestrand. It 

 was shown that the inclusions from the sunstone were identi- 

 cal with those found in carnallite, the latter determined to be 

 hematite. The lamellae causing aventurization on (010) of cer- 

 tain labradorites (which also showed labradorization) were 

 found to be oriented approximately parallel to (180) and (170). 



J. W. Judd,^ in discussions on the schiller of minerals, 



*Pogg. Ann., xlvi, 189, 1839. + Pogg. Ann., lxiv, 153, 1845. 



X Loc. cit., x>. 161. § Sitz.-Ber. Akad. Wien, x, 179, 1853. 



I Pogg. Ann., cxvi, 396, 1862. 



TfProc. Acad. Nat. Sc. Philad., 1866, 110. 



** Loc. cit., p. 111. 



•H Sitz.-Ber. math, naturw. CI. Ak. Wien, lx, I, 1024, 1869. 



H Quart. Jouin. Geol. Soc, xli, 374, 1885. Min. Mag., vii, No. 33, 81, 1886. 



