Generalized Titanomagnetite in Hawaii — KATSURA 
225 
Foundation grant G- 1.1 3 19 for geochemical in- 
vestigations of Hawaiian lavas. 
His thanks are also expressed to Dr. E. F. 
Osborn and Dr. A. Muan, of the Pennsylvania 
State University, for their kind permission to 
pursue the present work at Pennsylvania State 
University. Mr. C. O. Xngamells kindly gave 
permission to use the laboratory of chemical 
analysis at Pennsylvania State University. Dr. 
J. W. Greig, visiting professor at Pennsylvania 
State University, kindly prepared the polished 
sections of ferromagnetic mineral fractions sep- 
arated from the rocks. Dr. I. Kushiro, of the 
University of Tokyo, polished the rocks to 
identify the presence of hematite. The writer 
wishes to express his deep thanks to those per- 
sons who gave him many kindnesses. 
SAMPLES 
About 80 samples of Hawaiian volcanic rocks 
were examined under the microscope to de- 
termine the grain size of the opaque mineral. In 
general, the grains of opaque mineral in the 
tholeiites were so small that it was a really 
difficult problem to obtain a pure separation of 
it from the coexisting silicate minerals. An at- 
tempt was made to separate it as purely as 
possible by applying the same technique as that 
described in a previous paper (Akimoto and 
Katsura, 1959). The purity of the separated 
ferromagnetic fraction attained was only 70% 
by weight in the case of sample 12 (Tables 1, 
2 ) , which may represent the tholeiite type, but 
the coexisting silicates consisted of felsic min- 
erals. Therefore, when we concentrate our at- 
tention on the great difference in composition 
which generally exists between titanomagnetite 
and titanomaghemite, there may be no need to 
be concerned over any significant divergence 
from the true picture of the essential composi- 
tion of the ferromagnetic minerals based on the 
three major components, FeO, Fe 2 0 3 , and Ti0 2 „ 
In contrast, It was easy to separate the opaque 
mineral in alkali-rich rocks when no hematite 
was present. The purity of the ferromagnetic 
fraction separated was without exception greater 
than 90% (the sum of FeO, Fe 2 0' 3 , and Ti0 2 ). 
The X-ray diffraction method was used to 
check whether or not the separated fraction con- 
sisted of spinel. Microscopic examination of 
polished sections was made to identify the ex- 
solved rhombohedrai phase in titanomagnetite. 
In the present study only the ferromagnetic 
fraction that was shown by X-ray to have spinel 
structure was used. 
Brief remarks on the volcanic rocks from 
which the generalized titanomagnetite was sep- 
arated are given below, and their chemical com- 
positions are given In Table 1. 
No. 1. Alkali-rich type; gabbro vein cutting 
massive hawaiite valley-filling lava flow at Kaena 
Quarry, upper Waianae series, Oahu. The sum 
of the normative magnetite and ilmenite is 
about 21% (see Table 1), and the amount of 
ferromagnetic fraction actually separated was 
18.5% by weight. 
No. 2. The advanced stage of tholeiite dif- 
ferentiation, according to Tilley (1950); hy- 
persthene dolerite with quartz, Palolo Quarry, 
Oahu. The rocks in the Palolo Quarry are some- 
what complicated, and sometimes needle-like 
hematite is found in cavities. In the present 
sample a small amount of hematite was present. 
No. 3. Alkali-rich type; hawaiite containing 
numerous flakes of dark mica, in quarry 1 mile 
north of Paauilo, Laupahoehoe series, Mauna 
Kea, Hawaii. 
No. 4. Tholeiite type; reheated picrite-basalt 
with new hypersthene, block ejected during 
1924 explosions of Kilauea, Hawaii, the same 
as a sample studied by Tilley (1950). 
No. 5. Alkali-rich type; nepheline basalt, Ka- 
laheo road cut, Kauai. 
No. 6. Alkali-rich type; trachyte, Pun Koae 
dome, Honolua series, West Maui. 
No. 7. Alkali-rich type, trachyte, near the base 
of Ukumehame dome on west wall of Ukume- 
hame Canyon, Honolua series, West Maui. 
No. 8. Alkali-rich type; mugearite, in high- 
way cut at McGregor Point, Honolua series, 
West Maui. 
No. 9- Alkali-rich type; hawaiite containing 
abundant dark mica, in quarry at Keanokolu 
road, Laupahoehoe series, Mauna Kea, Hawaii. 
No. 10. Tholeiite type; massive pahoehoe, in 
shallow gulch running up the face of Nanakuli 
Valley, Waianae Range, Oahu. 
No. 11. Alkali-rich type; massive hawaiite, 
valley-filling flow at the same locality as No. 1. 
No. 12. Tholeiite type; olivine basalt, near 
highway 1.4 miles south of Volcano Observa- 
