352 0. Andersen — Aventurine Feldspar. 



make some distinction between them by using aventurine 

 feldspar as the general term, embracing all feldspars which 

 show aventurization, without regard to the intensity of the 

 phenomenon. Sunstone should then be the special term for 

 those varieties which have intense aventurization and there- 

 fore sometimes find use as gem stones. 



Aventurine feldspars (snnstones) should be sharply dis- 

 tinguished from the other group of color-playing feldspars 

 known as moonstones, murchisonites and labradorites. These 

 feldspars are characterized by a rather subdued, generally blu- 

 ish or greenish play of colors (" glaukisiren "*) which is 

 not caused by any visible lamellae but perhaps by submicro- 

 scopic inclusions. The colors are probaWy due to scattering 

 of light by particles smaller than the wave length of light, f 

 and cau not be explained as ordinary interference colors of 

 thin films. 



A survey of the literature shows that the conceptions of the 

 problems connected with aventurine feldspars have been 

 rather diverse. A general treatment of the subject based on 

 thorough examinations of different aventurine feldspars has 

 never been attempted. It, therefore, seemed of considerable 

 interest to subject these problems to a somewhat closer study. 



1 had at my disposal good material from a number of occur- 

 rences. Specimens from Norwegian localities were obtained 

 from the Mineralogical Museum of Kristiania University 

 through the kindness of Professor Dr. W. C. Brogger and Mr. 

 Jacob Schetelig. During visits to some of these localities I 

 have also had the opportunity to collect specimens and to study 

 their occurrences. From American localities I obtained good 

 specimens from U. S. National Museum through the courtesy 

 of Dr. Gr. P. Merrill and Dr. E. T. Wherry, who placed at my 

 disposal (among other specimens) the entire feldspar collection 

 of Isaac Lea, containing the type specimens for the paper 

 referred to below. 



PEEVIOUS WORK4 



Aventurine feldspars or sunstones are mentioned in some 

 of the earliest systematic works on mineralogy,§ in which they 

 are described as varieties of feldspars without explanation of 

 the aventurization. 



*C. Viola, Zs. Kryst., xxxiv, 171, 1901. 



+ Cf. explanation of the blue of the sky, C. Viola, loc. cit., p. 188. 



| The following review is not intended to be complete, the most important 

 work only being mentioned. Some additional references will be found in 

 different parts of the present paper. 



§ e. g. Delametherie, Th6orie de la Terre, vol. II, p. 201, 1797, where the 

 term heliolithe is used. 



R. Jameson, A System of Mineralogy, 1820, vol. II, p. 17, where aven- 

 turine feldspars from The White Sea and. Archangel are mentioned. 



