2 Morey & Bowen — Melting Potash Feldspar. 



a constant temperature (varying less than one degree) 

 in our furnace day and night, for a week or more. 



Preliminary Results. 



When we heated our artificial orthoclase for a week at 

 the temperature noted above (1200°) we obtained a 

 product with the appearance of a glass when examined 

 megascopically, and with the low refractive index (about 

 1.485) and the isotropic character of orthoclase glass 

 when examined under the microscope. 



However, under a high power (600 diameters) and with 

 the cone of light cut down to a small angle, this "glass'' 

 is found to show a sort of structure, visible only when it 

 is immersed in a liquid that matches it very closely in 

 refractive index. This structure can be described only 

 as a sort of fine cross-lining, usually rectangular and 

 giving therefore a grating effect. Day and Allen have 

 suggested that. orthoclase, on melting, loses the ordered 

 arrangement characteristic of the crystalline form only 

 very slowly. 3 Thinking, therefore, that this observed 

 structure was inherited from the crystalline material by 

 the extremely viscous liquid, we held it at a somewhat 

 higher temperature (1225°), where the liquid would be 

 more mobile, with the expectation that the structure would 

 disappear. Instead of this we found that it became more 

 distinct. At a still higher temperature (1250°) it became 

 apparent that the material was not a homogeneous glass 

 but was made up of two phases, the one occurring as 

 skeleton crystals in widely extended, branching forms of 

 rectangular pattern, and the other, of somewhat lower 

 refractive index, acting as a matrix for these. This 

 latter was subsequently shown to be glass, but under 

 crossed nicols the whole mass appears to be doubly 

 refracting with a grating structure recalling that of 

 microcline. At a still higher temperature (1300°), where 

 the crystals are present in smaller amount, they grow 

 as more discrete grains, though retaining skeletal ten- 

 dencies and appearing usually as rectangular crosses. 

 Finally, when formed at a temperature of 1400° or higher 

 the crystals assume definite rounded to polyhedral (icosi- 

 tetrahedral) forms, with indistinct patchy birefringence, 



3 Op. cit., p. 54. 



