Generalized Titanomagnetite in Hawaiian Volcanic Rocks 1 
Takashi Katsura 2 
ABSTRACT: A ferromagnetic oxide mineral with spinel structure was separated 
from Hawaiian volcanic rocks ranging from basalt to trachyte. The chemical compo- 
sitions 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 titanomagnemite. The composition of Hawaiian 
titanomagnetites is compared with that of titanomagnetites 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 rhom- 
bohedral. 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 tem- 
peratures in order to interpret the stability rela- 
tionship between temperature and oxygen partial 
pressure at a total pressure of 1 atmosphere. The 
studies showed that the extremely oxidized 
titanomagnetite which is usually called titano- 
maghemite, or y-phase, is not stable at tempera- 
tures above 1000° C, and seems to be unstable 
even below 1000° because the range of the solid 
solution field of magnetite in the system Fe- 
Ti-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 num- 
ber of known occurrences of titanomaghemite 
in igneous rocks, sometimes with titanomagne- 
tite and sometimes with neither titanomagnetite 
nor any other phase of the solid solution Fe 2 Oa- 
FeTiOa. 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 Tech- 
nology, 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 vol- 
canic rocks are either basalt free from olivine 
(tholeiite basalt) or olivine-bearing tholeiite. 
At most 5 % consist of alkali-rich types — alkali- 
olivine basalt, nepheline-bearing basalt, mugear- 
ite, hawaiite (as defined by Macdonald, I960), 
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 inter- 
pretation 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 alkali- 
olivine basalts. 
By generalized titanomagnetite is meant the 
ferromagnetic oxide mineral with spinel struc- 
ture, essentially composed of FeO, Fe 2 03 , and 
TiOv, disregarding vacancies in its unit cell. If 
there are a large number of vacancies, as in the 
case of y-hematite, we call the mineral titano- 
maghemite. The term titanomagnetite will be 
used in this paper for the mineral of stoichio- 
metric composition, or near it. 
Acknowledgments. The writer wishes to ex- 
press 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 
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