Generalized Titanomagnetite in Hawaiian Volcanic Rocks'

A ferromagnetic oxide mineral with spinel structure was separated from Hawaiian volcanic rocks ranging from basalt to trachyte . The chemical compositions of all the specimens have been arranged on an oxygen reaction line, and can safely be interpreted as the result of a process of either oxidation or reduction of material with composition on or near this line. In the trachyte the mineral was found to be highly oxidized ritanomagnemite. The composition of Hawaiian titanomagnerites is compared with that of riranomagnetires found in Japanese volcanic rocks belonging to the calc-alkali rock series. IT IS WELL KNOWN that there are two kinds of ferromagnetic oxide minerals in igneous rocks, one with spinel structure and the other rhombohedral. Many investigators have studied the chemical composition, magnetism, and stability against heat of these two mineral series. In addition, phase equilibria of the system Fe-Ti-O related to the ferromagnetic minerals found in igneous rocks have been studied by Webster and Bright (1961), MacChesney and Muan (1959), and Taylor (1961) at fairly high temperatures in order to interpret the stability relationship between temperature and oxygen partial pressure at a total pressure of 1 atmosphere. The studies showed that the extremely oxidized tiranomagnerite which is usually called titanomaghernire, or y-phase, is not stable at temperatures above 1000 0 C, and seems to be unstable even below 1000 0 because the range of the solid solution field of magnetite in the system FeTi-O decreases with decreasing temperature, as shown by the experimental results of Darken and Gurry (1946), Webster and Bright (1961) , and Taylor (1961). In spite of this; there are a number of known occurrences of riranomaghemite in igneous rocks, sometimes with riranomagnetire and sometimes with neither riranomagnerite nor any other phase of the solid solution Fe20sFeTiOs. In Japan, this was found to be true 1 Contribution No. 62-18 from College of Mineral Industries, Pennsylvania State University, University Park, Pennsylvania. Manuscript received October 30, 1962 . 2 Department of Chemistry, Tokyo Institute of Technology, Tokyo, Japan. predominantly in alkali-rich basalts (Katsura and Kushiro, 1961). Recent work by Macdonald and Katsura has shown that a large proportion of Hawaiian volcanic rocks are either basalt free from olivine (tholeiite basalt) or olivine-bearing tholeiite. At most 5% consist of alkali-rich types-alkaliolivine basalt, nepheline-bearing basalt, mugearite, hawaiite (as defined by Macdonald, 1960), and trachyte. In almost all cases ferric iron was more abundant in such alkali-rich types than in tholeiite, though ferric iron is generally fixed as magnetite rather than hematite. The interpretation of petrological relationships between these two types of rocks is not yet settled, but it is interesting to study the spinel-type minerals in comparison with those found in Japanese volcanic rocks, especially in Japanese alkaliolivine basalts. By generalized tiranornagnerite is meant the ferromagnetic oxide mineral with spinel structure, essentially composed of FeO, Fe20s, and Ti02, disregarding vacancies in its unit cell. If there are a large number of vacancies, as in the case of y-hemarite, we call the mineral titanomaghemire, The term riranomagnetire will be used in this paper for the mineral of stoichiometric composition, or near it. Acknowledgments. The writer wishes to express his thanks to Dr. Gordon A. Macdonald, University of Hawaii, for encouragement when the writer was a staff member of the Hawaii Institute of Geophysics. The chemical analyses in Table 1 were made in the laboratories of the University of Hawaii under National Science