380 0. Andersen — Aventurine Feldspar. 



either as hematite or as a constituent of a ferric compound. 

 By a change in the exterior conditions prevailing at the time 

 of formation, e. g. change in temperature, the equilibrium of 

 the solid solution may be disturbed in such a way that Fe 2 3 

 can no longer be held in solution, but must separate in 

 individual crystals. The hematite molecules will then move 

 towards the centers of crystallization (that is the locations of the 

 hematite lamella?) and feldspar molecules must move in the oppo- 

 site directions. From the extreme thinness of the lamellae we 

 conclude that practically all these movements have taken place 

 in the planes of orientation of the lamellae. These planes, there- 

 fore, seem to represent definite structural planes in the feldspar, 

 perhaps translation planes,* along which the molecules can move 

 relatively easily. In such planes there will again be certain direc- 

 tions, translation directions,* of maximum mobility of the mole- 

 cules, and these may account for the distortions of the hema- 

 tite lamellae. It should be noticed that some of the planes 

 of orientation actually are important structure planes of the 

 feldspar. Thus (001) and (010) are both cleavage planes and 

 twinning planes. Of (110) and (110) one or both are cleavage 

 planes and (021), is a twinning plane. It is, therefore, reason- 

 able to consider the other planes of orientation, especially 

 (112), (112), (150) and (150), which are observed in all aven- 

 turine feldspars, as definite structural planes, planes of transla- 

 tion, as suggested. 



We may summarize the conclusions as to the origin of the 

 hematite lamellae as follows : The aventurine feldspars have 

 been formed by unmixing of an originally homogeneous solid 

 solution of the feldspar and hematite (or a ferric compound) in 

 such a manner that thin hematite lamellae have separated along 

 structural planes (translation planes) of the feldspar. 



II. DESCRIPTION OF THE SPECIMENS. 



The present section contains brief descriptions, including 

 tabulations of measurements, of all the specimens examined. 



The optical properties of the feldspars were only determined 

 to the extent necessary for an identification of the species, f 

 Extinction angles on (001) and (010) were determined on thin 

 sections or cleavage pieces. Refractive indices were deter- 

 mined on powdered material by the immersion method. As a 

 rule only approximate determinations of the mean refractive 

 index j3 were made in white light. Exceptionally the refractive 



*See A Johnsen, Fortschritte der Mineralogie, vol. iii, p. 93, 1913. 

 ■|-The graphical plob devised by F. E. Wright, this Journal (4), xxxvi, 540, 

 1913, was used in the determinations of the plagioclase. 



