MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 639 
This is most clearly shown in the thin sections (see Figs. 38 and 
39), where the olivine individuals against the pyrrhotite show 
more or less well-developed idiomorphism; however, such that the 
edges are somewhat rounded. 
We especially remark the quite good idiomorphism of the small 
olivine individuals against the pyrrhotite. 
The rather common segregations of pyrrhotite-hypersthenite 
in hypersthene-rich norites 
carry from 10-20 to 50-60 
per cent of nickel-pyrrhotite. 
Besides the sulphides, they 
may carry hypersthene alone 
or hypersthene accompanied 
by more or less plagioclase, 
etc. The hypersthene here 
appears as porphyritic crys- Fic. 37-—Pyrrhotite-olivinite from Bruvand, 
fin the pyrrhotite; the fete Noms, Litto! niseal~ 
boundary planes of the crys- _ plende, black=pyrrhotite. (Natural size.) 
tals, however, usually are 
somewhat rounded, with small bends, etc. (see Figs. 40-43). 
A corresponding structure may also be observed in a pyrrhotite- 
norite from Dyrhaug in Verdalen, chiefly consisting of labradorite 
(Ab,An,, occasionally with zonal structure from about Ab,sAn,, in 
the kernel to about Ab, Ans in the exterior zone) and nickel- 
pyrrhotite (see Figs. 44, 45). 
The structure here described can only be explained to show that 
the olivine (Figs. 37-39), the hypersthene (Figs. 40-43), and the 
labradorite (Figs. 44-45) have crystallized at an earlier stage than the 
nickel-pyrrhotite. 
And this is in best conformity with the intervals of the melting- 
points, calculated for the different minerals at atmospheric pres- 
sure, and only quite unessentially changed at higher pressure 
(see a following paragraph). 
The melting-point of pyrrhotite is 1183° (or 1187°), and for a 
sulphide mixture, consisting of predominant pyrrhotite, 3-6 per 
cent pyrite, some chalcopyrites, and some pentlandite, we may 
fix the crystallization interval from about 1190° (or about 1200°), 
for the commencement of crystallization of the pyrite, down to, 
