ALBITE. 45 



AbjAnj. (Plate XVII). 



With Ab 4 Ani a third proof of the identity of composition of the first 

 crystals to separate and the vitreous residue was obtained. The 

 optical identification of this feldspar is absolute. If we could obtain 

 crystals at all in a melt of this chemical composition, therefore, it 

 would offer a crucial test of the relation of the solid and liquid phases 

 in a part of the curve where no melting point or specific gravity deter- 

 mination upon crystals was possible. After some days of nearly con- 

 tinuous heating at a temperature somewhat below its assumed melting 

 point, a number of crystals of Ab 4 Ani were obtained and identified. 



Albite. 



From the experiments upon natural albite and orthoclase, which 

 have been described, and after observing the effect of the increasing 

 viscosity as we approached the albite end of the artificial plagioclase 

 series, we had no expectation of finding a melting point for either in 

 the ordinary sense. Nor did we in fact succeed in locating a point of any 

 real significance in this connection. The various trials which were made 

 were simply calculated to throw all the light possible upon the char- 

 acter of the change from (crystalline) solid to liquid in such extremely 

 viscous substances. The return change or recrystallization of such 

 substances from the melt (solidifying point) without the introduction 

 of modifying conditions has never been accomplished. The time 

 required to do it is certainly very great, probably much greater than 

 the demonstration is worth at the present stage of experimentation in 

 this field. 



Crystalline albite has been produced under exceptional conditions 

 several times by Hautefeuille,* by heating a very alkaline alumino- 

 silicate with sodium tungstate for 30 days at 900 to iooo ; by Friedel 

 and Sarasinf, using an atmosphere of water-vapor under very high 

 pressure and a moderately high temperature (an aqueo-igneous fusion) ; 

 by J. Lenarcict, at ordinary pressure and high temperature by crys- 

 tallization out of a mixture of melted albite and magnetite (1 part 

 magnetite, 2 parts albite by weight), and by others. It may be noted 

 in passing that, entirely apart from the solution relations, the last- 

 mentioned process reduces the viscosity to an entirely different order 

 of magnitude from that of pure albite ; magnetite melts to form a thin 

 liquid almost of the consistency of water and even in 1 : 10 solution 

 with albite forms a fairly mobile liquid. We endeavored to repeat 



* Hautefeuille, Annales de l'Ecole Normale Superieure, 2d sen, 9, p. 363, 1880. 

 t Friedel & Sarasin, Bull. Min., p. 158, 1879; p. 71, 1881. 

 I J. Lenarcic, Centralblatt f. Min., 23, p. 705, 1903. 



