4 o 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



scopic examination of slides cut from the different portions showed 

 the two to be optically identical. 



We have here another instance of the tendency of the iron to con- 

 centrate in the crystals which first form, a tendency which was often 

 noticed throughout our work.* It also appeared to matter little 

 whether the first crystals formed at the surface or at the bottom of the 

 charge. This phenomenon may have significance in ore deposition, 

 but we have not thus far been able to give it adequate attention. 



Ab,An! (Plates XII, XIII). 



With this member of the feldspar group a difficulty in effecting 

 crystallization in the molten mass becomes noticeable. Undercooling 

 will continue until the vitreous melt becomes rigid, unless the cooling 

 is slow or some special effort in the way of mechanical disturbance or 

 the introduction of nuclei is applied. Furthermore, when once pre- 

 cipitated, crystal formation goes on slowly, even when the charge is 

 finelv powdered, and the crystals are always small. Of the feldspars 

 at least it is possible to say that the size of individual crystals varied 

 chiefly with the viscosity; the thinner, calcic feldspars always gave 

 large individuals, while AbiAni, Ab 2 Ani, AbaAiii and Ab 4 Ani crystal- 

 lized in closely interwoven, increasingly smaller fibers, which gave 

 much difficulty in microscopic study. In comparison with this appar- 

 ent effect of the viscosity, the rate of cooling was altogether insignifi- 

 cant in determining the size of individual crystals. 



Several days were required to complete the crystallization of ioo 

 grams of AbiAm under the most favorable conditions which we were 

 able to bring to bear upon it. The melting temperature of the crys- 

 talline feldspar was still fairly well marked, however, and crystalliza- 

 tion began in the powdered vitreous material as low as 700 . 



The melting point of this feldspar is: 



AbjAnj. 



Melting temperature, i4ig c 



* See also J. P Iddings, Bull. Phil. Soc. Wash., Vol. XI, p. 97, 1888-1891 



