DIFFERENTIATION OF IGNEOUS ROCKS 411 



mixtures of Or and Ab at about 50° below the melting point of 

 Ab. On account of the extreme viscosity of the melts, it, how- 

 ever, was impossible to determine the exact lowering of the melting 

 point. 



It is to be noted that Or and Ab form a mix-cry stal-eutectic 

 whose melting point lies near 1050° (or some degrees above 1050°). 

 We further notice that orthoclase crystallized in eruptive rocks 

 (predominantly composed of an average mixture of Or and Ab 

 which crystallized at a temperature only a few degrees below the 

 melting curve between Or and Ab,) can take up by the temperature 

 of the formation at least 55, possibly even 58 per cent or a little 

 more Ab. On the other hand plagioclase crystalHzed under similar 

 circumstances can absorb at least 25-30, and probably even a Httle 

 over 30, per cent Or. In the crystallization of igneous rocks 

 consisting of predominant Or+Ab with a quite subordinate admix- 

 ture of ferromagnesian silicates, etc., there wiU therefore only be 

 a very Uttle jump chemically between orthoclase very rich in 

 Na AlSijOg (Broggers soda-orthoclase or aoda-microcHne) and albite 

 or other plagioclase very rich in KAlSijOs' (Rosenbusch's anortho- 

 clase). When the two feldspars are formed at lower temperatures, 

 the solubihty in the soHd phase of Ab in orthoclase (or microcHne) 



' I find it unnecessary in this very short resume, which is based on Th. Vogt's 

 lecture (April, 1920), and which I myself have completed with some further studies, 

 to quote the numerous previous treatises on the very important problem here treated. 

 I shall only mention a treatise by E. Makinen, Helsingfors, "Uber Alkaiifeltspate " 

 {GeoL Foren. Fork., 191 7), and by R. Herzenberg, " Beitrag zur Kentniss der Kalinatron- 

 feltspate" Diss. Kiel, 1911. H. A. AUmg (see this Journal for 1921, No. 3) believes 

 that "both the potash and the soda feldspars are dimorphous, each existing in two 

 isomeric forms: each component crystallizing either in monoclinic or triclinic modifica- 

 tion, depending on the temperature and the viscosity of the magma, that orthoclase 

 and albite are high-temperature modifications and that microcline and possibly ( ?) 

 barbierite are relatively low-temperaturq forms." This supposition cannot, however, 

 be correct. The explanation advanced by Michel-Levy, and later adopted and proved 

 by Mallard, Rosenbusch, Groth, and Brogger, namely that orthoclase only is a crypto- 

 lamellic microcline, has now been given final proof, in that orthoclase and microcline 

 show an identical Laue-diagram (Hadding, " Rontgenographische Untersuchung von 

 Feldspat," Lunds Universitets Aarsskrift, Vol. XVII, 1921). And the mineral barbie- 

 rite, a monoclinic soda feldspar consisting of NaAlSisOg with only small admix- 

 tures of Or and An, is no doubt based on a mistake. 



