412 



/. n. L. VOGT 



and of Or in plagioclase (albit-oligoclase, oligoclase, etc.) is very 

 remarkably diminished, as may be illustrated by the following:^ 



Temperature of 

 Formation 



About 900° (or 850°) 

 About 700° (or 750°) 

 About 30c>-400° 



Percentage of 



Ab (or Ab+.4;() in 

 Orthoclase 



Or in Albite 



Oligoclase- Albite, 



Oligoclase 



Granite 



Granitic pegmatites 



Aplite veins 



Low temp 



About 33 

 About 30 (28) 

 Adularia about 15 

 About 10 



About 13 

 About 10 

 Albite about 5 

 About 3 (?) 



Most of the proper syenites — hence not including the nepheline- 

 nor quartz-syenites — ^contain Or and Ab+y4« in proportions 

 between 55 Or : 45 Kh+An and 30 Or : 70 Ab+^«. That is, 

 some have a relatively small surplus of Or, and others a similar 

 relatively small surplus of Ab+An above the eutectic boundary 

 between Or and Ah -\- An. In numerous syenites we find almost 

 precisely the eutectic proportion of Or and P^-\-An. 



The binary eutectic between Ab and Ca Mg SizOe, according to 

 the investigations of Bowen,^ lies at almost 100 per cent Ab and 

 very little Ca Mg SiaOe. The extreme viscosity renders a precise 

 determination impossible, but by extrapolation only a small (2 or 3) 

 per cent Ca Mg Si206 might be estimated. The eutectic between 

 Na, K-feldspar and aegirite — ^or a point on the eutectic boundary 

 curve between Ab+Or and Na Fe Si^Og with a Httle Ca Mg Si^Oe 

 etc. — consists of 



2.5 p.c. aegirite 

 97.5 — Na, K-feldspar 



(66 Ab with a trifle An and 34 Or). 



This agrees with petrographic experience, viz., that the ferro- 

 magnesian siHcates (augite, amphibole, biotite, etc.) when their 

 quantity in the syenites amounts at least to a small per cent (say 



>= See my treatise on the " Physikalisch-Chemische Gesetze der Krystallisationsfolge 

 in Eruptivgesteinen," Tscherm. Min. Petrogr. Mitt., XXIV (1905), PP- 528-42; further 

 C. N. Warren, "A Quantitative Study of Certain Pertitic Feldspars," Proc. Amer. 

 Acad, of Arts and Set., Vol. LI (1915), and Makinens' treatise of 1917. 



' Amer. Jour. Sci., XL (1915). 



