0. Andersen — Aventurine Feldspar. 351 



Art. XXVIII. — On Aventurine Feldspar ; by Olaf Ander- 

 sen. With Plates I-III. 



CONTENTS. 



INTRODUCTION 351 



PEEVIOUS WORK. 352 



I. METHODS OF EXAMINATION AND GENERAL RESULTS.... 354 



Determination of the orientation of the lamella 354 



Planes of orientation 355 



Measurem ents with the microscope 355 



Remarks on the optics of aventurine feldspars ._ 356 



Goniometric measurements 361 



Results of measurements 363 



. Orientation of the edges of the lamellce 367 



The properties of the reflecting lamella 369 



Size ; crystal outlines 369 



Interference colors ; thickness 369 



Absorption colors _. 370 



Double refraction 374 



Chemical tests ._ 374 



Some observations on hematite 375 



Summary of the properties of the lamellae- 375 



Thermal data 376 



Origin of the hematite lamella 379 



II. DESCRIPTION OF THE SPECIMENS 380 



— - Albite from, Fisher Hill Mine. 381 



Albite from near Media 382 



— Oligoclase from Statesville 384 



— - Oligoclase from Kragero 384 



Oligoclase from Tvedestrand 386 



Oligoclase from Aamland 389 



Labradorite from Labrador . . 390 



Microcline perthite from Perth 391 



Microcline perthite from Mineral Hills 393 



Microcline perthite from Naskilen 393 



Microcline perthite from Stene 395 



-~ Miscellaneous occurrences _. 396 



SUMMARY 397 



INTRODUCTION. 



Different varieties of feldspar show a more or less distinct 

 metallic schiller, aventurization (sunstone schiller), when light 

 rays fall in certain directions on cleavage faces or artificially 

 polished faces. This schiller is caused by oriented lamellar 

 inclusions which reflect the light with great intensity. The 

 "fire" of the schiller is due to the brilliant interference colors 

 produced by the thin film action of the reflecting lamellae. 



Aventurization may be defined as a play of light and colors 

 caused by strong reflections from thin oriented lamellae of 

 visible size included in the feldspar. 



The terms aventurine feldspar and sunstone have been used 

 interchangeably by previous authors. It seems advisable to 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 238. — October, 1915. 

 23 



