Petrologic Significance of the Chemical Composition of "Magnetite"

Abstract

In his previous paper (this Journal, Vol., LV, No., 646, 1949) the writer concluded that the amount of "magnetite" separated from a magma has an influence on the chemical composition of mafic minerals crystallized from the same magma, but the probable variability in the chemical composition of "magnetite" was not taken into account., In this paper this subject is discussed, based on chemical data of "magnetite" in volcanic rocks (Tables 3, 4 and 5) with the following conclusions : 1) FeO / (Fe 2O 3+TiO 2) in mol proportion of magnetites is comparatively high (up to 1., 4) for basic rocks, whereas almost 1., 0 for acid ones., Fig., 1 shows the variation of Feo, Fe 2O 3 and TiO 2 in the analysed magnetites against SiO 2 percent of the rocks in which they occur., 2) MnO shows no regular variation, and the data for NiO is insufficient in number for general consideration., The amounts of V 2O 3 in the magnetites of basic rocks except for alkaline basalt is higher than those of acid rocks., The M/R ratio (=amount of a certain oxide in magnetite / amount of the same oxide in the mother rock) varies with SiO 2 percent of the rocks, as shown amount of the same oxide in the mother rock) varies with SiO 2 percent of the rocks, as shown in Fig., 2., In acid rocks it varies widely and attains an extremely high value., 3) In basic rocks, the ratio 100/modal "magnetite" is higher than the M/R ratio for TiO 2, MnO and V 2O 3, while in acid rocks the former is smaller than the latter., This fact may show that the amount of modal "magnetite" is overestimated in acid rocks., Notwithstanding this fact, the conclusion in the writer's previous paper needs no revision.,