THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION 
AND MAGMATIC DIFFERENTIATION 
OF IGNEOUS ROCKS 
J. H. L. VOGT 
Trondhjem, Norway 
IV 
MAGNETITE 
Magnetite and olivine (with fayalite)—As discussed in “‘Silikat- 
schmelzlésungen,” I (1903), the individualization boundary between 
magnetite and fayalite (reckoned by weight) lies at about: 
©. 2=O) 25 BOO 2© So. aS), 6 
Near the same boundary, probably with somewhat less magne- 
tite and somewhat more olivine, we find in the “‘titanomagnetite’’- 
olivinites, chiefly consisting of the so-called ‘titanomagnetite,” 
which is a mechanical mixture of magnetite and ilmenite, and of 
olivine, the latter with proportions intermediate between Fe,SiO, 
and Mg,siO, (partly stoechiometrically about 0.4 Fe,SiO,- 
0.6 Mg,SiO,, partly still more Fe,SiO, and less Mg,SiO,). As 
described several years ago by earlier investigators, among them 
also myself, the olivine in these rocks appears partly porphyric in 
the “titanomagnetite” when it is quite abundant. As an example 
we may take the ‘“‘titanomagnetite’’-olivinite from Cumberland in 
Rhode Island, according to C. H. Warren" with an average mineral- 
ogical composition of 46.1 per cent olivine (0.39 Fe,SiO, -0.61 
Mg,SiO,), 20.7 magnetite, 18.6 ilmenite, 9.2 labradorite (Ab,An,), 
3.6 spinel (and in addition a little Or and sulphide). 
The specimen at my disposal (Fig. 27) gives the same pro- 
portions between olivine and magnetite plus ilmenite, but less 
labradorite and less spinel. The olivine shows a more or less 
well-developed idiomorphic contour against the iron-ore minerals. 
t Amer. Jour. of Sci. (1908), p. 175. 
627 
