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MINING GEOLOGY, 30(5), 265•`276,1980
Compositional Variation of Pentlandites in Copper Sulphide
Ores from the Kamaishi Mine, Iwate Prefecture, Japan*
Naoya IMAI**, Tadashi MARIKO***, Hiroaki KANEDA****
and Yoshihide SHIGA**
Abstract: Pentlandite has been found as a widespread minor or trace mineral in contact-metasomatic type copper
and iron-copper ores of the Kamaishi mine. The mineral usually occurs as microscopic grains up to 500 ƒÊm across,
being relatively abundant in the massive sulphide ore with cubanite- chalcopyrite and pyrrhotite-chalcopyrite
assemblages.
In this study, chemical compositions of the Kamaishi pentlandites with various modes of occurrence have
been determined using an electron microprobe. The pentlandites vary in their compositions over a wide range from
cobaltian pentlandite to cobaltpentlandite, although Ni to Fe atomic ratios are usually close to unity with few
exceptions. Their optical and other physical properties including X-ray data are also presented. The relationship
between the physical and chemical data is examined. Finally the variation in chemical composition of the
pentlandites is discussed in correlation with the difference in mode of occurrence and mineral assemblage.
1. Introduction
Pentlandites in the Sudbury-type nickel-
copper sulphide ores are usually characterized
by low values of Co content. For example, the
Sudbury pentlandites contain 0.009•`2.10 wt.
% Co (HAWLEY, 1962; MISRA and FLEET,
1973), and those in nickel-copper sulphide ores
from the Kotalahti and Hitura mines, Finland,
contain 0.16•`1.31 % Co (PAPUNEN, 1970).
On the other hand, the Co-rich varieties of
pentlandite, ranging in their composition from
cobaltpentlandite*1 to cobaltian pentlandite*1
(49.33•`18.84 wt. % Co) have been described
by KOUVO et al. (1959) from Finnish copper
sulphide ores. Subsequently, a cobalt-pent-
ladite with 54.1 % Co has been reported in the
ores having a peculiar mineral assemblage of
sulphides-arsenides from the vein deposits of
the Langis mine, Cobalt-Gowganda area, Can
ada (PETRUK et al., 1969). In Japan, an occur
rence of cobaltian pentlandite (15.8 % Co) was
reported by H. IMAI and FUJIKI (1963) from the
vein deposits of the Komori mine, Kyoto. Also,
a cobaltpentlandite (Co/Fe atomic ratio•¬9)
was confirmed in the massive iron-copper sul
phide ore from the Shimokawa mine, Hokkaido
(KATO and SATO, 1963).
An occurrence of pentlandite in copper sul
phide ores from the Shinyama ore deposit of
the Kamaishi mine was already reported by
Tsusua (1961). TAKEUCHI and YAMAOKA (1964;
1965) also found the mineral in copper sulphide
Received March 8,1980, in revised form September 29,1980.
*A part of this study was presented at the Autumn
Joint Meeting of the Mineralogical Society of Japan, Society of Mining Geologists of Japan, and the Japanese Association of Mineralogists, Petrologists and Economic Geologists, held in Kagoshima City, on October 19,1976 (IMAI et al., 1976b).
**Department of Mineral Industry , School of Science and Engineering, Waseda University, Ohkubo, 3-4-1, Tokyo 160.
***Institute of Earth Science , School of Education, Waseda University, Nishi-Waseda 1-6-1, Tokyo
160.****Department of Mineral Development Engineer
i ng, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Tokyo 113.
Keywords: Cobaltpentlandite, Cobaltian pentlandite, Kamaishi mine, Nippo ore deposit, Ohmine mine, Shinyama ore deposit.
*1 In this paper, conforming to H. IMAI and FIKOLI's
proposal the name of cobaltpentlandite is adopted for natural pentlandite with Co/Fe>1 and Co/Ni>1 in atomic ratios, and that of cobaltian pentlandite for those containing Co more than 3 weight percent with Co/Fe<1 and Co/Ni<1 in atomic ratios (H. IMAI and FUJIKI, 1963).
265
266 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:
ores from the Ohmine mine (now included in
the Kamaishi mine and is called the Nippo
working). However, no precise determination
of their chemical composition has been reported
yet.
In the course of our study on copper sulphide
ores of the Kamaishi mine (MARIKO et al.,
1973; IMAI et al., 1973; MARIKO et al., 1974;
SHIGA, 1975; IMAI et al., 1976a), we have made
an attempt to determine the chemical composi
tion of these pentlandites by an electron micro
probe. A preliminary investigation revealed
that the Kamaishi pentlandites vary in their
composition over a wide range with respect to
Co content (IMAI et al., 1976b). This paper
sums up all the results obtained to date, further
referring to the relation between the chemical
composition and physical properties, the cor
relation between the chemical data and the
mode of occurrence and mineral assemblage,
and the local variation of Co content in the
Kamaishi pentlandites. Crystal chemistry of
them is beyond the scope of this paper, and it
appeared in a separate paper (IMAI et al.,
1980).
2. Location and Outline
of the Ore Deposits
The Kamaishi mine is well known as one of
the largest producers of iron and copper ores
in Japan. The mine office is located at Kasshi
in Kamaishi City, approximately at lat. 39•‹15'
N, long. 141 •‹40'E. A Mineralized zone which
is called "West ore belt" of approximately 5
km (N-S) by 1.5 km (E-W), is situated in the
eastern part of the Kitakami mountains.
The ore deposits are of contact-metasomatic
type, occurring in the skarn zones which replace
mainly the calcareous and pelitic sediments of
Permo-Carboniferous age as well as the pre
existing "porphyrite" near the contact with
intruded granodiorites. The K/Ar ages of these
granodiorites which might presumably repre
sent the igneous activity to which the ore
deposits are genetically related range from 115
to 120 m.y. B.P., placing the time of crystalliza
tion late in the Cretaceous (SHIBATA and
MILLER, 1962; KAWANO and UEDA, 1965;
1967).
The ore deposits include two main ore types;
iron oxide ore and copper sulphide ore. The
former consists principally of magnetite occa
sionally with some sulphides, while the latter is
composed of pyrrhotite with some chalcopyrite
and cubanite, and rarely with magnetite. Some
orebodies, however, consist of the mixture of
magnetite and sulphides having an intermediate
character between the above two, called "iron
copper ore". Most of the iron ores of the mine
have been mined from the Shinyama ore de
posit, and considerable amounts of copper sulphide ores have come from the Shinyama
and Nippo ore deposits. The Nippo ore deposit
which had been mined by Rasa Industry Co.
under the name of Ohmine mine prior to 1971,
lies at about 5 km nortwest of the Shinyama ore
deposit. Geologic profiles of the Shinyama ore
deposit and the Nippo Fourth orebody are
shown in Figs.1 and 2, respectively.
The Shinyama ore deposit, the largest in the
mine consists of an iron oxide orebody and
seven copper sulphide or "iron-copper" ore-
Fig.1 Geologic profile through the Shinyama working
(after the data by the mine staff).FeB: Iron ore-body, 2B to 5B: Second to Fifth ore-bodies. Sixth and Seventh orebodies do not appear in
this profile.
30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 267
Fig. 2 Geologic profile through Fourth orebody (4B) at the Nippo working (after the data by the mine staff).
bodies in massive garnet and/or clinopyroxene skarns.
The iron oxide ores consist mostly of magnetite, locally accompanied with the impregnations or veinlets of chalcopyrite, pyrite and hematite. The principal ore-forming metallic minerals in the sulphide ores comprise, on the other hand, chalcopyrite, pyrrhotite and cubanite. In some places of barren zones, a small amount of pyrite occurs as disseminations or stringers. Under the ore microscope, the principal sulphides are found to be associated with various minor or trace minerals. They include Ni-and/or Co-bearing sulphides such as pentlandite, "argentian pentlandite", Ni-and Co-bearing mackinawite, siegenite and smythite as well as magnetite, sphalerite, marcasite and hematite.
The Nippo ore deposit comprising five copper sulphide orebodies occur in "breccia skarn"
(TAKEUCHI. and YAMAOKA, 1964;1965) and in
massive clinopyroxene-garnet and garnet
skarns. The principal ore-forming metallic
minerals in the Nippo copper sulphide ores
are pyrrhotite, cubanite and chalcopyrite, lo
cally with small amounts of magnetite, pyrite,
bornite and molybdenite. Minor or trace
minerals observable under the ore microscope
are Ni- and/or Co-bearing sulphides (pent
landite, "argentian pentlandite", Ni- and Co-
bearing mackinawite, smythite, siegenite and
millerite), marcasite, wittchenite, chalcocite,
covellite, electrum, native bismuth and ilmenite.
3. Mode of Occurrence of Pentlandite
Although pentlandite is found as a widespread
minor or trace mineral throughout the Shin
yama and Nippo sulphide ores, the mineral
tends to be more abundant in the massive ores
than in the disseminated, stockwork and
breccia ores. In weakly mineralized parts of
the barren skarns, hornfelses and igneous rocks,
pentlandite is almost or completely absent.
The pentlandites now in question vary in
grain size over a wide range from about 10 ƒÊm
to as large as 500 ƒÊm across. Individual grains
and aggregates show various shapes; subhedral
to anhedral (or oval), dots, "flames", lenses,
blades or rods (lamellae) and spindles, rosetts,
and "veinlets". The finer grains tend to occur
as "flames", spindles, minute en echelon lenses,
dots and lamellae within pyrrhotite and cu
banite and rarely within chalcopyrite. On the
contrary, the coarser grains tend to occur as
aggregates within pyrrhotite or to occupy the
spaces along the boundaries between pyrrhotite
and other minerals such as cubanite and chal
copyrite.
On the basis of a detailed observation of more
than one hundred polished sections, the Kama
ishi pentlandites are classified into three types
with respect to their textural form; 1) inter
stitital grains, 2) fine particles within other
sulphides and 3) "veinlets".
Interstitial grains
Most of the interstitial grains of pentlandites
lie along the grain-boundaries of pyrrhotite,
and also occupy the space between pyrrhotite
and massive or lamellar cubanite and rarely
chalcopyrite. This type of pentlandite is rel-
268 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:
Fig.3 Photomicrographs of the polished sections, showing the mode of occurrence of Fe-Co-Ni
pentlandites in the copper sulphide ores from the Kamaishi mine.1 : Massive po-cp ore, Shinyama Fourth orebody, 450 mL, in air, Specimen no. 17; 2: Breccia
(stockwork) po-cp ore, Nippo Second orebody, 480 mL, oil imm., Specimen no. 2-9; 3: Massive po-cp ore, Nippo Fourth orebody, 250 mL, oil imm., Specimen no. 4-18; 4: Massive cp-cb-po ore, Shinyama Fourth orebody, 450 mL, in air, Specimen no. 39; 5: Disseminated or stockwork cp-cb ore, Nippo Third orebody, 380 mL, in air, Specimen no. 3-8; 6: Disseminated cp-bn ore, Nippo
Third orebody, 250 mL, in air, Specimen no. 102-4.Photomicrographs of 1 to 5 were taken under obliquely-crossed polars and 6 under one polar.Abbreviations; bn=bornite, cb=cubanite, cp=chalcopyrite, g=gangue minerals, pn=cobaltian pentlandite or cobaltpentlandite, po=pyrrhotite, sg=siegenite.
30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 269
atively coarse-grained usually 500 ƒÊm or less
in size, and subhedral to anhedral in shape
(Fig. 3-1). This texture is most common in the
ores in which Co- and/or Ni-bearing sulphides
are notably concentrated. The grains sub
mitted to X-ray diffractometry which will be
described later belong to this type.
Fine particles within other sulphides
Within pyrrhotite grains, pentlandite occurs
as dots and blades or rods 20•`50 ƒÊm long and
as rosetts consisting of minute particles with
diameter up to 30 ƒÊm. The pentlandite blades
or rods may be observed to orientate parallel
to {0001} plane of pyrrhotite (Fig. 3-2). In
some cases, spindles or "flames" of them occur
along the twinning plane of pyrrhotite (Fig.
3-3).
On the other hand, the mode of occurrence
of pentlandites in massive and lamellar or lath-
shaped cubanites shows characteristic features;
they usually occur as spindles and blades or
stringers with narrow widths of 10 ƒÊm or less
and lengths from 50 to 200 ƒÊm (Fig. 3-4). In
some grains of cubanite, one or two prominent
sets of parallel blades of pentlandite may be
observed (Fig. 3-5). On rare occasions,
irregularly-shaped anhedral grains of pentlan
dite as large as 50 ƒÊm in diameter are embedded
in massive cubanite.
In general, pentlandites in chalcopyrite are
very rare and usually small in grain size, being
less than 50 ƒÊm across. However, in the Nippo
sulphide ore having a peculiar mineral as
semblage of chalcopyrite-bornite-pentlandite-
siegenite-millerite, pentlandites are intergrown
with chalcopyrite, and they have been altered
partially or completely to siegenite without
exception (Fig. 3-6). Most of the pentlandites
belonging to this type are considered to have
been exsolved from the host sulphides."Veinlets"
The "veinlets" of pentlandite ranging from
5 to 70 ƒÊm in width and attaining to about 400
ƒÊ m in maximum length, occur frequently along
the twinning planes of pyrrhotite grains in
massive cubanite and rarely in chalcopyrite.
These "veinlets" as well as interstitial grains
are likely to have been formed by accumula
tion of exsolved pentlandites from the host
sulphides due to subsequent migration (BRETT,
1964; MARIKO et al., 1974).
In some of the Sudbury-type nickel-copper
sulphide ores, pentlandites contain numerous
inclusions of pyrrhotite and chalcopyrite as
small blebs or rods (e.g., HAWLEY, 1962). In
contrast, few inclusions of chalcopyrite have
been observed within grains of the Kamaishi
pentlandites, although some of which are
replaced partially or completely by siegenite
as well as Ni- and Co-bearing mackinawite.
As stated before, pyrrhotite is one of the
major sulphide components, with which pent
landites are intergrown. MARIKO et al. (1974)
have revealed that the pyrrhotites in direct
contact with pentlandites in the Kamaishi
copper sulphide ores comprise (a) troilite-
hexagonal pyrrhotite assemblage, (b) hexa
gonal pyrrhotite and (c) hexagonal-monoclinic
pyrrhotites assemblage. They have also found
that pentlandites are scarcely found and most
of them are usually altered partially or com
pletely to siegenite within the aggregates of
monoclinic pyrrhotite.
4. Chemical Analysis
For the qualitative microanalysis of the
Kamaishi pentlandites in thirty two ore speci
mens, an Akashi electron microprobe ("Tro
nalyser", TRA-25) with 25•‹ X-ray take-off
angle and two-channel detecting system was
employed. The qualitative spot analysis in
dicated that the measurable elements were Fe,
Co, Ni and S, while the contents of other
elements such as Cu and Ag were less than the
detectable limits of the instrument. Linear-
scanning profiles obtained by electron micro
probe traverses under the characteristic X-rays
of FeKƒ¿, CoKƒ¿, NiKƒ¿, and SKƒ¿ confirmed
the absence of marked compositional zoning
within single grains.
In order to establish the chemical composi
tion of the material, the quantitative micro
analysis was performed on twenty four grains
in seventeen ore specimens listed in Table 1,
using a JEOL-50A electron microprobe with
35•‹ X-ray take-off angle and two-channel
detecting system. Instrumental setting for all
the measurements were ; accelerating voltage :
270 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:
Table 1 Occurrence site, ore type and mineral assemblage of the copper and iron-copper ores from
the Kamaishi mine, containing pentlandites chemically analysed by electron microprobe.
*In this paper , minerals in each assemblage are arranged in order of decreasing abundance. Abbreviations in Tables 1 and 2 : ap=argentian pentlandite , bn=bornite, cb=cubanite, cp=chalcopyrite, mg=magnetite, mk=mackinawite, ml=millerite, pn=pentlandite, po=pyrrhotite , py=pyrite
, sg=siegenite.
Table 2 Electron microprobe analyses of pentlandites from the Kamaishi mine.
*The materials were X-rayed .
20 kV, specimen current : 1.4•~10-8 A on
Al2O3, size of beam spot : 4 ƒÊmƒÓ, the manner of
X-ray intensity measurement : fixed-time count
ing method for 10 sec. •~7 times, analyzing
crystals used: LiF for FeKƒ¿, CoKƒ¿ and NiK,ƒ¿
and PET for SKƒ¿. The following materials were
utilized as microprobe standards : homogeneous
synthetic troilite for Fe and S, synthetic millerite
for Ni and pure metallic cobalt for Co.
After the corrections for dead time and back-
30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 271
Table 3 Chemical data for pentlandites from the Kamaishi mine.
ground, count rates were processed by means of
SHOJI'S programs which involve the ZAF cor
rections for matrix effects (YUI and SHOJI,
1976). The corrections followed the procedures
outlined by SWEATMAN and LONG (1969).
Table 1 gives a brief description of the
Kamaishi copper sulphide ore samples, the
Fe-Co-Ni pentlandites in which were chemi
cally studied in detail by electron microprobe
and Table 2 presents the final results of the
quantitative microanalysis. Each analysis rep
resents the result of five or more spot analyses.
Also, Table 3 gives atomic percents of metals,
atomic ratios of Ni/Fe and ‡”M*2/S, and the
corresponding chemical formulae calculated on
the basis of eight S atoms.
Figure 4 illustrates the triangle diagram,
showing the variation in chemical compositions
of the Kamaishi materials. From this figure as
well as from Table 2, it may be seen that Ni/Fe
atomic ratios of the Kamaishi pentlandites do
not vary significantly ranging from 0.81 to
1.07 close to unity, except for the material in
specimen no. 102 (anal. no. 21). Their com-
positional range coincides nearly with that of
Fig. 4 Triangular diagram showing the compositional variation of the Kamaishi Fe-Co-Ni pentlandites, together with that of the Finnish materials given by
Kouvo et al. (1959), as expressed by atomic percent of metals.Dashed-line represents the composition for which
Ni/Fe atomic ratio is equal to unity. Dash-dot lines represent the boundaries among pentlandite, cobaltian pentlandite and cobaltpentlandite. Analysis no.
(3) in Table 2 is omitted in the diagram because of its overlapping with others.
*2 In this paper, the symbol M denotes the metals of the
first transition series.
272 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:
the Finnish pentlandites given by Kouvo et al.
(1959) as shown in Table 4.
Table 2 and Figure 4 indicate that pent
landites from the Shinyama ores are usually
characterized by the lower Co contents, cor
responding to cobaltian pentlandite, although
there is an exceptional occurrence of cobalt-
pentlandite from Seventh orebody which oc
cupies the nearest position to the Nippo ore
deposit.
On the other hand, in the Nippo ores, pent
landites vary in their Co contents from cobaltian
pentlandite to cobaltpentlandite. Higher Co
contents are seen in pentlandites in the breccia
and impregnated ores from Second and Third
orebodies emplaced in "breccia skarn" on the
upper levels and from upper part of Fourth
orebody replacing massive clinopyroxene or
clinopyroxene-garnet skarn on the deeper levels,
whereas lower Co contents are measured in
pentlandites from the lower part of Fourth
orebody.
The compositions of pentlandites are fairly
uniform regardless of the difference in the host
or associated minerals within a given ore
specimen. However, at a specific position of
any given orebody, pentlandites in the pyr
rhotite-chalcopyrite ores tend to have slightly
higher Co contents than those in the pyrrhotite-
cubanite ores.
Specimen no. 102-4 of the Nippo ores con
taining cobaltpentlandite rich in Ni with the
Ni/Fe atomic ratio of 1.74, is characterized by
the chalcopyrite-bornite-pentlandite-siegenite-
millerite assemblage, which is somewhat pe
culiar in the Kamaishi copper sulphide ores in
general as seen in Table 1. Of special interest is
that in the above Ni-rich cobaltpentlandite, the
‡” M/S atomic ratio (1.098) deviates signifi
cantly from the stoichiometric composition of
9/8 (1.125), and the metal-deficiency is ap
parent. This feature may be explained by the
electronic structures of Fe, Co and Ni atoms of
the first transition-metal elements (DONNAY
and SHEWMAN, 1972; RAJAMANI and PREWITT,
1973; IMAI et al., 1980).
5. Reflectance and Microhardness
The reflectance measurements in air for some
Fig. 5 Reflectance-dispersion curves for the Kamaishi Fe-Co-Ni pentlandites, as compared with that of a
normal Fe-Ni pentlandite in the Kamabuse-yama serpentinite, Kanto mountains.
* Analytical no. by electron microprobe as shown in
Table 2.
** Co/‡”M atomic ratio (Ni/Fe atomic ratio•¬1). K is
the curve for the Kamabuse-yama Fe-Ni pentlandite.
selected grains of the materials on freshly-
polished surfaces were performed with Olym
pus MMSK-RK multi-photometric micro
scope. All measurements were made against
WTiC standard provided by Carl Zeiss Jena
Co.
The reflectance-dispersion curves from four
grains of the Kamaishi materials are given in
Fig. 5, together with that of normal Fe-Ni
pentlandite in the Kamabuse-yama serpentinite
(IMAI et al., unpublished data) for comparison.
Also, the relationship between reflectance for
light with a wave length of 560 nm and Co con
tents is expressed diagrammatically in Fig. 6.
The reflectance evidently increases with in
creasing the Co/‡” Matomic ratio of the pent
landites, as has already been pointed out by
some previous investigators (e.g., BURNS and
30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 273
Fig. 6 Relationship between Co/‡”M atomic ratio and
reflectance (R percent) in some Kamaishi Fe-Co-Ni
pentlandites, together with those for the Langis co
baltpentlandite and the Kamabuse-yama Fe-Ni
pentlandite (ă=560 nm).
Number corresponds to the analytical no. in Table 2.
L refers to the data for the Langis material given by
PETRUK et al. (1969), and K represents the value of
the Kamabuse-yama material.
VAUGHAN, 1970; RAJAMANI and PREWITT,.
1973).
The microhardness for selected grains of the
present materials were determined with an aid
of the AKASHI MVK-C microhardness tester.
The results thus obtained are shown in Fig. 7,
in terms of Vickers hardness number (VHN)
versus Co content. Kouvo et al. (1959) showed
that microhandness increased with increasing
Co content. As shown in Fig. 7, this trend is
also confirmed in the present study.
6. X-ray Diffraction Analysis
X-ray diffraction analysis for selected grains
of the Kamaishi pentlandites having different
Co contents was conducted using standard
Debye-Scherrer camera of 114.59 mm diameter
and Fe-filtered CoKƒ¿-radiation (ƒÉ=1.7902
A). Minute amounts of the powder were col
lected from the polished surfaces with a steel
needle under the ore microscope and attached
to the edge of glass fibres. The samples thus
prepared were X-rayed. To eliminate the errors
due to film shrinkage, Straumanis film-position
was employed. The intensities of the resolved
lines were estimated by both visual method and
Fig.7 Relationship between Co/‡”M atomic ratio and
Vickers hardness numbers (VHN) in some Kamaishi
Fe-Co-Ni pentlandites. The VHN was measured at a
50g load. The number corresponds to analytical no.
in Table 2.
Table 4 Unit-cell dimensions of some Fe-Co-Ni pent
landites.
Source: * Present study, ** Kouvo et al. (1959), *** Rajamani
and Prewitt (1973), + Petruk et al. (1969).
microphotometry.
All pentlandites now in question have similar
X-ray diffraction patterns which are nearly
identical with those of normal Fe-Ni pent
landites in literature and are in harmony with
the space group Oh5Fm3m. The unit-cell dimen
sions were determined by means of BRADLEY
and JAY'S extrapolation against cos2Į using
least squares method and the results are given
in Table 4, together with those from other
sources.
The unit-cell dimensions of solid-solution
series along the Co9O8-Fe,•¬4 ,5Ni•¬4.5S8 join de
crease with increasing Co content, although
274 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:
KNOP et al. (1965) have found that the unit-cell
dimensions of synthetic Fe-Ni pentlandites are
larger than those of natural equivalents having
the same composition, and on heat treating the
lattices of natural one expand irreversibely.
Accordingly, the unit-cell dimensions of natural
Fe-Co-Ni pentlandites and those of synthetic
equivalents cannot be correlated with each
other (KNOP and IBRAHIM, 1961; KNOP et al.,
1965; PETRUK et al., 1969).
The relationship between unit-cell dimen
sions and Co contents in terms of the Co/‡”M
atomic ratio of naturally occurring pent
landites including the present Kamaishi ma
terials is shown in Fig. 8, from which linear
relationship between the two is recognizable.
The straight line obtained from the least squares
method may be expressed as follows :
a(A)=10.063-0.1447 (Co/‡”M).
This straight line extrapolates to Co/‡”M=0
with an intercept of 10.063 A which is close to
10.059 A of natural Fe-Ni pentlandite for
Fig.8 Relationship between Co/‡”M atomic ratios
and unit-cell dimensions in natural pentlandites.
Solid circles: Kamaishi Fe-Co-Ni pentlandites (Ni/
Fe atomic ratio•¬1). Open circles: Finnish pent
landites (Kouvo et al., 1959), Outokumpu pent
landite (RAJAMANI and PREWITT, 1973) and Langis
cobaltpentlandite (PETRUK et al., 1969).
which the Fe/Ni atomic ratio is equal to unity
(IMAI et al., 1980).The problems on the serious discrepancy re
cognized between the unit-cell dimensions of natural Fe(-Co)-Ni pentlandites and those of the synthetic equivalents with the same com
position were discussed in a separate paper (IMAI et al., 1980).
7. Summary and Conclusions
In summarizing the data on the Kamaishi
pentlandites given so far, the following conclusions may be drawn.
(1) As far as the Co content is concerned, the pentlandites now under investigation vary in their chemical composition over a wide range from cobaltian pentlandite to cobaltpentlandite, although the Ni/Fe atomic ratios are usually close to unity with few exceptions.
(2) In the copper sulphide ores from the Shinyama ore deposit, pentlandites are usually characterized by lower contents of Co, corresponding to cobaltian pentlandite. However, the materials from Seventh orebody (ironcopper orebody) situated near the Nippo ore deposit are characterized by high contents of Co, corresponding to cobaltpentlandite.
(3) In the copper sulphide ores from the Nippo ore deposit, the pentlandites from Second and Third orebodies in "breccia skarn" on the upper levels and from the upper parts of Fourth orebody in "massive skarn" on the deeper levels, are characterized by high values of Co content, corresponding to cobaltpentlandite. On the contrary, those from the lower part of Fourth orebody contain lower Co, corresponding to cobaltian pentlandite. Pentlandites in the Sudbury-type nickel-copper sulphide ores are characterized by low Co content. On the other hand, pentlandites associated with hydrothermal vein deposits of epithermal or mesothermal class belong usually to cobaltian
pentlandite or cobaltpentlandite (H. IMAI and FUJIKI, 1963; PETRUK et al., 1969). From this, it is supposed that the variation of Co content in
pentlandites from the Nippo ore deposit as noted above may represent the thermal gradient at the time of ore formation.
(4) The cobaltpentlandite in the Nippo ore
30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 275
with chalcopyrite-bornite-pentlandite-siegenite
-millerite assemblage represents a metal-defi
cient species (‡”M/S atomic raio=1.098) in
which Ni/Fe atomic ratio (1.74) deviates sig
nificantly from unity (Ni-rich cobaltpent-
landite). This agrees well with the statements
by the previous workers that Ni/Fe atomic ratio
of pentlandites is increased in general way with
increasing Ni content to the bulk compositions
of sulphide assemblage (KNOP et al., 1965;
GRATEROL and NARDRETT, 1971; HARRIS and
NICKEL, 1972; RAJAMANI and PREWITT, 1973).
(5) The compositions of pentlandites are
fairly uniform, regardless of the difference of
the associated sulphide minerals within a given
ore specimen.
(6) At a specific position of any given ore
body, the composition of pentlandites in the
pyrrhotite-chalcopyrite ores tends to have
slightly higher Co content than those in the
pyrrhotite-cubanite ores.
(7) The pyrrhotites in direct contact with
pentlandites in the Kamaishi copper sulphide
ores include (a) troilite-hexagonal pyrrhotite
assemblage, (b) hexagonal pyrrhotite and (c)
hexagonal-monoclinic pyrrhotites assemblage.
In the last case (c), pentlandite are scarcely
found and most of them have been altered
partially or completely into siegenite.
Acknowledgements : The authors are grateful
to Professor S. TAKENOUCHI and Dr. T. SHOJI
of the University of Tokyo, and to Professor R.
OTSUKA of Waseda University for their kind
advices.
Thanks are also due to Professor T. NAKA
MURA and Mr. I. KINOUCHI of Waseda Uni
versity for their skilled technical assistance in
electronprobe microanalysis. The authors are
also indebted to the high-speed digital com
puter, HITAC 8800/8700 system installed at
the Computer Centre of the University of Tokyo
in the course of the ZAF corrections of the
microprobe data (Project No. 0358643002).
The authors express their sincere thanks to the
mining geologists of the Kamaishi Mine Office,
especially to Dr. S. HAMABE for their kind as
sistance during the field works.
This research has been supported in part by a
Grant-in-Aid for Fundamental Scientific Re
search from the Ministry of Education, Culture and Sciences in Japan, especially by Project No. 1431015 (1976/1977) awarded to the first author (N. I.), and Project No. 236043
(1977/1978) awarded to Professor S. TAKENOUCHI.
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岩手県釜石 鉱 山産銅硫化物鉱石 中のペ ン トラ ンダイ トに
おける化学組成の変化
今井直哉 ・鞠子 正 ・金田博彰 ・志賀美英
要 旨:釜 石接触交代 鉱床 の銅 および鉄銅 鉱石 中にはペン
トランダイ トが広 く認 め られる.こ のペン トランダイ ト
は普通500μm以 下 の粒子 として産 し,キ ューバ鉱― 黄
銅鉱 および磁硫鉄 鉱―黄銅 鉱の組合せ を有す る塊状硫化
物鉱石 か ら比較的 多量 にみ いだされる.
種々 の産状 を示すペ ン トランダイ トのEPMAに よる
化学分析 による と,そ のNi/Fe原 子比 は,一 部 の例外 を
除 くとあま り変化せ ず,ほ ぼ1に 近 いが,Co含 有量の
変化 に富み,コ バル トペ ン トランダイ トか ら含 コバル ト
ペン トランダイ トにわた る.ま た反射率,微 小硬 度お よ
び格子定数 の測定 を行い,こ れ らと化学組成 との関係 を
検討 した.ペ ン トランダイ トの産状,鉱 物組合せお よび
産出箇所 と組成 との関係 につい ても考察 を加 えた.