644 J Hb. VOGE 
show idiomorphic contours against the pyrrhotite. This phenome- 
non is due to the fact that the melted FeS even at so low a 
temperature as about 1200° is very thin fluid.” 
FeS, is so little soluble in the silicate magma that the limit of 
the solubility, when only about o.1-0.2 per cent FeS, is present, 
is usually reached by the cooling of the magma to somewhat above | 
the upper boundary of the crystallization interval of the silicates. 
The melting-point of FeS, lies (at high pressure) at a still higher 
temperature. FeS, consequently, in the common igneous rocks, 
separates in the solid phase. 
The solubility of FeS in silicate magmas at any temperature 
depends, as explained in my treatise “‘Die Sulfid-Silikatschmelz- 
l6sungen,”’ on the chemical composition of the magma; the solubil- 
ity under otherwise similar conditions increasing with the basicity. 
And as usual with dissolved substances, the solubility of FeS 
also increases with the temperature. In melts of gabbroidic 
composition, the solubility at 1350-1400° and at the pressure of one 
atmosphere is about 0. 25—-0.4 per cent FeS but decreases rapidly at 
a little lower temperature, as about 1300°. When the limit of 
solubility is reached by the cooling of the silicate magma, FeS 
accordingly separates in the /zquid phase. 
On account of the great difference existing at high pressure 
between the melting-points of FeS, and FeS, an essential difference 
arises with regard to the physics of the segregation: FeS, crystal- 
lizes while FeS, on the other hand, separates in liquid phase and 
remains in this condition till near the termination of the solidifica- 
tion of the rock. 
The inirusive deposits of pyrite, which often are characterized 
by nearly exclusively pyrite, and which usually do not contain 
any pyrrhotite, prove also that FeS, at especially high pressure may 
exist in the liquid phase. 
The formation of these deposits is probably due to the fact 
that locally in the igneous magmas so much Fes, has been present, 
that the latter at especially iigh temperature has been secreted at 
tSee ‘Die Sulfid-Silikatschmelzlésungen”’ (1917), where I have discussed the 
importance of the thinness of the pyrrhotite magma for the interpretation of the 
morphology of these deposits. 
a 
