LIME-SODA FELDSPARS CRYSTALLIZED IN OPEN 

 CRUCIBLES FROM FUSED CONSTITUENTS. 



INTRODUCTION. 



The results of these synthetical experiments agree closely in some 

 respects while differing in others. They agree in general in the habit 

 and arrangement of the crystals of the different feldspars produced, 

 while differing in the size of the crystals of the various feldspars 

 according to their composition. These results have an important 

 bearing on the problem of texture and granularity in igneous rocks. 



First, as to the habit of the feldspar crystals produced from solu- 

 tion of the feldspar constituents without admixture of other material. 

 So far as can be determined by microscopical study of the sections, the 

 crystals are in most cases blade-like in form ; that is, they are elongated 

 plates. They vary, however, from one extreme to another, being in 

 some cases equidimensional plates of extreme thinness, in other cases 

 prisms, elongated in one direction with the other two dimensions 

 equal. The development of these forms takes place in feldspars of 

 various compositions, and appears to be chiefly a function of the rate 

 of crystallization and not of the chemical composition of the feld- 

 spar, except as this modifies the viscosity of the solution. It is not 

 possible to recognize any fixed relation between the habit of the 

 crystals and the composition of the feldspar. This is, of course, in 

 accord with the well-known isomorphism of the feldspar group. 



The common mode of crystallization in these preparations is that 

 of spherulitic aggregations, more or less completely developed in 

 spherical forms. 



The elements of the spherulites are bundle- or sheaf-like aggrega- 

 tions of long, thin blades, which blades lie nearly parallel to one 

 another in the middle or narrower part of the bundle, and diverge at 

 the ends into fan-like or plumose forms. Several of these bundles or 

 blades cross one another at the middle, and when there are a sufficient 

 number of bundles, or when they diverge sufficiently, a completely 

 spherulitic aggregation results. 



In some cases a spherulite consists of bundles or prisms that extend 

 uninterruptedly from the center to the outer margin, the rays of the 

 spherulite being nearly straight. In other cases the spherulite is a com- 



79 



