98 Day and Allen — Isomorphism and Thermal 



AVe therefore planned an apparatus* which should be as 

 sensitive as possible to heat changes over a long range of tem- 

 peratures, and then prepared to examine the thermal behavior 

 of simple minerals of natural or artificial composition when 

 gradually heated or cooled. Changes of crystalline form 

 (Umwandhmgen) or of state (melting and solidifying) must 

 involve a more or less sharply marked absorption or release of 

 heat and be recorded as breaks in a smooth curve in the same 

 way as in the determination of metal melting points or the 

 singularities of any of the well-known chemical compounds at 

 lower temperatures. 



First Group of Minerals Investigated. — The particular 

 group of minerals chosen for the first investigation was the 

 soda-lime feldspar series, and orthoclase (microcline). The 

 reasons for this choice will be fairly obvious. Aside from its 

 being altogether the most important group of rock-forming 

 minerals, unusual interest has been attracted to it through 

 Tschermak's theory that these feldspars bear a very simple 

 relation to one another, that they are (orthoclase excepted ol 

 course) in fact merely isomorphous mixtures of albite and 

 anorthite. This hypothesis has given occasion for serious and 

 extended study both from the optical and thermal sides. 



A complete review of the literature of the feldspars will 

 not be attempted here. Although opinion is still somewhat 

 divided, f it is probably fair to say that the optical researches 

 have failed definitely to establish or disestablish the isomor- 

 phism of the albite-anorthite group, and that it is somewhat 

 uncertain whether conclusive evidence will be obtained by 

 optical means alone. Investigation from the thermal point of 

 view has been even less satisfactory by reason of the subjective 

 methods employed, to which reference has already been made, 

 though the recorded results indicate with reasonable unanimity 

 that the melting point of anorthite is above that of albite and 

 that the intermediate feldspars will probably fall between the 

 two4 Beyond this conclusion, the great body of evidence is 

 more or less contradictory and sometimes controversial in 

 character. 



Orthoclase (preliminary). — Somewhat unluckily, our meas- 

 urements began with natural orthoclase (microcline) from 



* For a detailed description of this apparatus see Day and Allen, Phys. 

 Kev., xix, p. 177, 1904. 



f Fouque et Levy, Synthese des Mineraux et des Roches, p. 145, 1882. 

 C. Viola, Tschermak's Min. u. Petr. Mitth., xx, p. 199, 1901. Lane, Journ. 

 Geol. XII, ii, p. 83, 1904. J. H. L. Vogt — Die Silikatschmelzlosungen. 

 Christiania, 1903. 



% J. H. L. Vogt, loc. cit., p. 154, expresses the opinion that the soda-lime 

 feldspars will be found to fall under Type III of Roozeboom's types of iso- 

 morphous series, with a minimum between anorthite and albite. (See p. 134, 

 seq.) 



