Suessite, Fe3Si: a new mineral in the North Haig ureilite

American Mineralogist, Volume 67, pages 126-131, 1982
Suessite,Fe3Si:a new mineral in the North Haig ureilite
Kleus Kr,rr, J. L. Bemr-Byl
Department of Geology and Institute of Meteoritics
Universityof New Mexico, Albuquerque,New Mexico 87i,31
euo L. H. Fucss
Argonne National Laboratory
Argonne, Illinois 60439
Abstract
The North Haig olivine pigeonite achondrite (ureilite) is a polymict breccia consisting of
major olivine, low-Ca pyroxene and an intergranular carbonaceousmatrix. Olivine and
low-Ca pyroxene vary widely in composition,coveringthe rangesobservedin all known
ureilites. Minor granularenstatiteclastswith diopsideexsolutionblebs, dkin to enstatite
achondrites,were also observed. Native metal in ureilites is normally kamacite of variable
Ni content,in somecaseswith up to ZVoSi in solid solution.However, kamacitewith trace
amounts of Si is extremely rare in North Haig, only a few l-2 p.m grains within silicates
were observed.Instead,the commonmetallicphaseis approximatelyFe3Si,a new mineral
which we have named suessitein honor of ProfessorHans E. Suess.Suessiteoccurs as
minor anhedral vein fillings in interstitial cracks, in silicates, and in the intergranular
carbonaceousmaterial and rangesin size from 1 pm blebs to elongatedgrains about 30 x
150 pm in size. Suessiteis cream white in reflectedlight, isotropic, ferromagnetic,and
shows no cleavage.Reflectancein percent (determinedby G. A. Desborough)at the 4
standardwavelengthsof 470, 546, 589and 650 nm for 2 grainsis 48.5(5),51.6(3),53.5(7),
50(2), and 49.7(5\, 53.4(4), 54.5(6), 52(l), respectively. Analyses (by EMX) indicate
presenceof dominatinglow-Ni and less common high-Ni varietiesof suessite(in wt.Vo,
meanin parentheses):
Fe 84.7,83.1(8a.2);Ni 1.6,a.5 Q.5); Co 0.21,0.27(0.23);Cr 0.10,
0.04(0.08);Si 15.3,13.7Oa.7);P 0.06,0.17(0.10).Thus, suessite
is (Fe, Ni, Co, Cr)zsa_t
M
(Si, P)f.0, mean(Fe, Ni, Co, Cr)2e6(Si,P)r.0,very closeto Fe3Si.X-ray powder diffraction
shows that suessitepossessesa similar structure to alpha-Fe(kamacite)and the solid
solution alloy (Fe3Si),, in displaying three lines (relative intensities in parentheses):
(in A). rne calculatedcell sizeis 2.841t0.0024N : 22.%Ar.
2.005(10),
1.42(l),1.160(3)
Thus, suessiteis the Si-rich end memberof the cr solid solutionregionof the Fe-Si phase
diagram with composition close to Fe3Si. Silicates, particularly olivine, in ureilites have
core compositions of Fo.to-gzand thin (<100 p,m) rims of essentially Folso formed by
reduction and reaction with the carbonaceousmatrix material. We suggestthat Si and Fe
liberated in this reduction process formed suessite,possibly also by reaction with
preexisting kamacite.
Introduction
The North Haig olivine-pigeonite achondrite
(ureilite), a meteorite found in 1961 in Western
Australia(McCall and Cleverly, 1968),is a brecciated rock that consistsof olivine and low-Ca pyroxene (pigeonite), embeddedin an intergranular carrPresent address: Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, Tucson,
Arizona 85721.
0003-004x/82/0102-0126$02.00
bonaceous matrix. Olivine and pigeonite vary in
composition over wide ranges and, in fact, cover
the compositional ranges of all other ureilites combined (Berkley et al., 1978a,b,1980).North Haig
also contains minor fragments of granular enstatite
with diopsideexsolutionblebs, akin to someenstatite achondrites.Thus, North Haig consistsof material of essentially all varieties of ureilites, in addition to minor amounts of clasts of non-ureilite
origin, and is therefore properly classified as a
polymict-brecciatedureilite.
126
127
KEIL ET AL,: SUESSITE
All ureilites contain a metal phase, kamacite, that
occurs intimately associatedwith the intergranular
carbonaceousmatrix or as decorations along fractures and cleavageplanes within silicate grains.
This kamacite has variable Ni contents and up to
about 3.8 wt.% Si in solid solution (Berkley et al.,
1980).However, electron microprobe analysesof
the metallic phase in North Haig indicate that
kamacite with trace amounts of Si in solid solution
is extremely rare, and only a few l-2 pm diameter
grains were observed enclosed within silicate
grains. Rather, the metallic phase in North Haig is
Fe3Si, a new mineral that also contains minor
amountsof Ni(2.5),Co(0.23),Cr(0.08),and P(0.10;
all avg. wt.%).In the presentpaper,we describethe
propertiesof this new phaseand discussits origin.
We have namedthis new mineral suessite,in honor
of Dr. Hans E. Suess, Professor of Chemistry,
University of California San Diego, La Jolla, California, USA, in recognitionof his outstandingcontributions to the field of cosmochemistryand meteoritics (Keil et al., 1980). The mineral and name
have been approved by the Commissionon New
Minerals and Mineral Names of the International
Mineralogical Association.
Occurrenceand physical properties
Suessitegenerally occurs as anhedralvein fillings
in interstitial cracks and fissures between silicate
grains (Fig. 1). Most commonly it is found in the
carbonaceousmatrix, interstitial to silicate grains,
rather than within them. Suessite ranges in grain
sizefrom I pm blebs to linear vein fillings (Fig. 1),
occasionally up to several hundred pm in length.
Other opaquesin North Haig are rare troilite which
occurs in blebs about 1-50 pm in diameter, both
interstitially to and within silicate grains, as well as
extremely rare l-2 pm diameter blebs of low-Ni
kamacite.Kamacite has less than 1 wt.% Ni and
only trace amounts of Si (Table 1) and occurs
exclusivelywithin silicategrains,not in the interstitial carbonaceousmatrix. In our sections,suessite
was not observed in direct contact with either
troilite or kamacite. North Haig is a meteorite find
and, thus, has been subjectedto considerableterrestrial weathering. Since metallic phases tend to
most readily weather to hydrous ferric oxide, it is
difficult to estimate the pre-terrestrial abundanceof
suessiteand kamacite. However, in our sections,
suessiteis by far the most abundantmetallicphase,
whereaskamacite is extremelv rare.
Fig. l. Suessite(white) as vein fillings and blebs in the North
Haig ureilite. Silicates (olivine, pigeonite) are grzy; carbonaceousmatrix is black. Hydrous ferric oxide veins (light gray)
are of terrestrial weathering origin. Reflected, plane polarized
light. Scalebar equals100pm.
In polished section, reflected light and air, suessite displays a cream white color under the microscopeand is isotropic. Reflectancedatafor suessite
for the four standardwavelengthswere determined
by G. A. Desborough, using the Zeiss standard
WC-005.For two grains, the reflectanceof suessite
is as follows (in %o):grain 1: 470-48.5(5),54e
51.6(3),589-53.5(7),650-50(2); srain2: 470-49.7(5),
546-53.4(4),
589-54.5(6),650-52(1) (numbersin parenthesesare the number of measurements),Suessite shows slightly less relief under the microscope
than associatedsilicate grains. The mineral does not
display a discernablecleavageand is strongly ferromagnetic,i.e., is significantlyaffectedby the magnetic field of a small hand magnet.
Composition
The compositionof suessitewas determinedwith
an automated anr EMX-sM electron microprobe,
using an accelerating potential of 15 kV and a
sample current of about 0.03 pA. Standards were
pure Fe, Si, Ni, Co, and Ca, and P in natural
apatite. Synthetic Fe3Si, prepared by annealing
under pressure of stoichiometric amounts of pure
Fe and Si, was employedas a secondarystandard.
Resultsobtained by analyzing suessiteagainstpure
Fe and Si or the synthetic Fe3Si compared very
well, after appropriate corrections for differential
matrix effects were made. Corrections for instrumental and differential matrix effects were made
following the procedures described by Keil (1967),
using the Tracor Northern ZAF correction procedure for differential matrix effects.
128
KEIL ET AL.: SUESSITE
Table l. Electron microprobe analysesof suessite,minor troilite and extremely rare kamacitein the North Haig ureilite (inwt.%).
Numbers in brackets are numbers of grains analyzed.
Suessi te
Troi I i te
H i g hN i
L o wN i
i te
Kamac
Mean
8 3 . r ( 8 t . 7 - 8 s . 8) t20l
4 . s ( 2 . e - 5 . 4 ) [20]
84.2167)
2,s 167l
0 . 2 1 (o . l 6 - 0 . 3 0 ) [ 3 2 ]
0 . 2 7 (0 . 2 0 -0 . 3 2 )[ l l ]
0.23[43]
0.10
0,24
0 . 1 0 ( < 0 . 0 2 0- . 2 5 ) [ 3 0 ]
0 . 0 4 (0 . 0 2 -0 . 0 5 )I 8 l
o.08[38]
0.12
0.08
JT
r5.3 (r4.6 -r6.4 ) [26]
1 3 , 7( 1 2 . 6- 1 4 . 3) I l 5 ]
1 4 . 7[ 4 r ]
n .d .
.0.07
P
0 . 0 6 ( <0 . 0 2 - 0 . 1 2 ) [ 2 6 ]
0 . r 7 (0 . r 0 -0 . 2 5 )Il s]
0 . r 0 [ 4 1]
n.q.
n.d.
FE
84.7 (83.0-88.8 ) [47]
Ni
1.6(0.s-2.4)1471
LO
Cr
)
TOTAL
n.d.
n . d.
r0r.97
101.78
60.8
<
n,d.
'l0l
.81
98.5
0.51
0.05
n.d.
J/.3
98.52
99.40
A t o m i cr a t i o s o f ( F e , N i , C o , C r ) : ( S i , P )
2.84 : 1
3.14:1
Compositionof suessiteis shown in Table I and
Figures2-4. Suessitecontainsmajor Fe, Ni and Si
and minor to trace amounts of Co, Cr and P.
Although compositions vary from grain to grain,
analysesindicateexistenceof two varietiesof suessite,onewith low Ni (avg.l.6wt.Vo),low Co and P,
and high Fe, Cr and Si; and one with high Ni (avg.
4.5Vo),high Co and P, and low Fe, Cr and Si.
Random analysesindicate that low-Ni suessiteis
about2.5 times as abundantin our sectionsof North
Haig as is high-Ni suessite.The compositionalgap
betweenlow- and high-Ni varieties is at about 2.7
wt.% Ni (e.g., Fig.2), but both low- and high-Ni
2.96:1
suessitevary slightly in composition from grain to
grain,coveringa rangeof 0.5-6.4 wt.% Ni (Table 1;
Figs. 2-4). Nickel apparentlysubstitutesfor Fe, but
the correlationis far from perfect (r : 0.56; Fig. 2).
Cobalt correlateswell with Ni (r : 0.69; Fig. 3) and
increases with increasing Ni (and decreaseswith
increasingFe), and P substitutesfor Si (r : 0.85;
Fig. 4). Chromium is slightly higher in low-Ni than
in high-Ni suessite(avg. 0.10vs. 0.04 wt.Vo,respectively), but precision of routine microprobe analy-
oo
oo
o
oo
-oo
r =O.56
^oo
N
,
ae
,
"
o
-/ai",'
z
,/t,o
^o,/o
z
a/
r = 0.69
/o
o
o
Fe (wl %)
Fig. 2. Nickel and iron contents of 47 grains of low-Ni and 20
grains of high-Ni suessite from the North Haig ureilite, as
determinedby electron microprobe analysis. Boundary between
low- and high-Ni suessite is at -2.7Vo Ni. Diagonal line is a
multivariate best-fit regression line; r : statistical correlation
coefficient.
Co (wf.%)
Fig. 3. Nickel and cobalt contents of 32 grains of low-Ni and
I I grains of high-Ni suessite from the North Haig ureilite as
determined by electron microprobe analysis. Symbols as in
Fig. 2.
t29
KEIL ET AL.: SUESSITE
Structure
X-ray powder diffraction studiesof suessitewere
performed by scratching small amounts of powder
from the interiors of several of the largest analyzed
o
grains. The powder was picked up on a greaser=O85
g
coatedglassfiber mount. The X-ray powder pattern
oo
(FeKctradiation) was obtained using diamond pow;e
der as an internal standard for determining d-spac'
ings (Table 2).
a/,
The resulting X-ray patterns show that suessite
possesses
a similar structureto alpha-Fe(kamacite)
and to the solid solutionalloy (FerSi)..in displaying
only three lines as comparedto six for the intermetallic stoichiometric compound Fe3Si (BiF3-type;
b.c.c., p.712, Hansen,1958)(Table2). The calculated cell size for suessiteis 2.841t0.0024 g :
P (wf. %)
22.%4, which correspondsto 18-20 atomic perFig. 4. Silicon and phosphorouscontents of 26 grains of lowcent of Si in pure Fe (Pearson,1958),in approxiNi and 15grains of high-Ni suessitefrom the North Haig ureilite
mate agreementwith the analyzed grains (Table 1).
as determined by electron microprobe analysis. Symbols as in
Fig.2.
The combined effect of impurities of Ni, Co, Cr and
P on d values is not known, but this may causethe
values for atomic percent Si predictsesat these low concentrationsis too low to reveal discrepancyin
the
basis
of the d valte and measured by
ed
on
any systematic compositional correlations. In the
microprobe.
electron
highly reduced environment in which suessite
Our X-ray diffraction data seem to suggestthat
formed, it appears likely that chromium occurs in
is not the intermetallic compound FesSi
suessite
the divalent rather than the trivalent state. The
(BiF:-type),
although it very closely approaches
atomic ratios for suessitewere therefore calculated
but rather a solid solution in the
this
composition,
on the basisof (Fe, Ni, Co, Cr):(Si, P), with the
However,
the X-ray data are not
Fe-Si.
system
atomic proportionsof Si + P : 1. Theseratios are
lines of the intermetalThe
3
missing
unambiguous.
2.84:l for low-Ni suessite,3.14:l for high-Ni suespattern are very weak
the
lic
in
suessite
compound
site, and 2.96:l for the weightedmean. The struchave seen them
that
we
would
it
is
doubtful
and
tural formulae are as follows:
even if they occurred, becauseof the high background in the suessitepattern (note that we were
Low-Ni suessite
'fable
(Fe2.77sNis.saeCoe.667Crs.004)(Si,P)
r.o
2. X-ray powder diffraction data for suessitein the North
Haig meteorite compared with the intermetallic compound
High-Ni suessite
Fe3Si,and Fe:Si solid solution alloy and a-Fe.
(Fe2.e76Nis.
Cro.oo2xSi,P)r
.0
1s+Coo.or
Mean suessite
FerSi
Suessite
(Fe2s67Ni6.632Cos.ss3Crs.0o4XSi,P)l
0
(Feasi)., (3)
N . H a l g ( 1)
BiF3-type (2)
aR
Simplified,meansuessiteis therefore(Fe2.eNi6.1)
Si
or nearly Fe3Si.Thus, suessiteis compositionally
identical to the Si-rich end member of the cr solid
solution region of the Fe-Si phase diagram, namely
the compoundclose to Fe3Si,with minor substitutions of Ni and trace substitutionsof Co, Cr, and P.
That Si-rich boundaryin the Fe-Si system,according to Hansen(1958),is near 26 atomic percent Si,
very near the 25.1atomic percent Si calculatedfor
meansuessite.
2.005
r
t0
al
r
aff
3.26
0.6
2.82
0.3
1.994
r0
1.70
03
r
aff
2.027
I .998
'|
.42
I
l.4t
1.5
| .413
I .5
I 433
I .160
3
l.lst
3
I .154
3
t.170
(l)
Norelco powdercamera, diil.
57.3 m,
Fe K radiation, iln filter;
Diamond
i n t e r n a l s t a n d a r d ; i n t e n s i t i e s ( I ) a r e v l s u a l . a = 2 . 8 4 1 r O . O O f2f
( a - Fe = Z AeOi).
(z) - (4) Calculated pattems by B. Tanl.
IO
3
130
KEIL ET AL.: SUESSITE
able to separateonly minute amounts of the phase 1968)is now consideredto be (Nf,Fe)s(Sf,P)2
(Wai,
from the meteoriteand, hence,long exposuretimes 1970).
were necessary).Furthermore,the syntheticFe3Si
The new mineral suessite, Fe3Si, also formed
prepared by hot pressing of pure Fe and Si in the under highly reducing conditions. Silicates in ureistoichiometric proportions also gives an X-ray pat- lites, particularlyolivine, have thin (<100 pm) rims
tern identical to that of suessite and with the 3 in contact with the carbonaceousmatrix that are
weakest lines of the intermetallic compound miss- essentially FeO-free, whereas the main portions of
ing. Thus, the possibility exists that the particular the grains have moderatelyhigh FeO contents (e.g.
,
annealinghistory accountsfor the absenceof these Berkley et al., 1976, 1978a,b,1980).Specifically,
lines in the synthetic and natural phase. Finally,
olivine rim compositions are essentially Fo1so,
accordingto Hansen's (1953)review of the Si-rich whereas the remainder of the grains is about
end memberof the a solid solution region of the FeForc-gz. We and others have suggestedthat the
Si phase diagram, there is a controversy as to
FeO-free rims formed by reduction of iron by the
whether or not the disorder-order transition occurs carbonaceous
matrix. We proposethat suessitealso
within a homogeneousphaseof the simple bcc (a) formed in this process,due to reductionof iron and
type. However, there is agreementthat the Si-rich stoichiometricallyequivalentsiliconliberatedin the
boundary of this region is at 26 atomic percent Si reduction process. Possibly, reaction of liberated
near Fe3Si. We therefore define suessiteas that iron and silicon with preexisting kamacite may also
phase.
have taken place in the formation of suessite.
Hence, only the few minute grains of kamacite
Discussion
occurring within silicate grains were sufficiently
Silicidesare stableunder exceedinglylow oxygen shieldedto remain as kamacite and did not react to
partial pressuresand, thus, are not likely to form in form suessite.
crustal rocks of the earth. Nevertheless. several
It is curious,however,that the polymict-brecciatterrestrial occurrencesof silicides have been report- ed ureilite North Haig is the only one that contains
ed. The first study describesFeSi and FeSi2from suessite,whereas the other seven studied by us
placers and drill core samplesof sandstonesof the (Berkleyet al.,1980) containkamacitewith <0.03Poltava series,Donets Region (Gevork'yan, 1969; 3.8wt.% Si. Specialconditionsmay have prevailed
Gevork'yan et al., 1969), but these occurrences during the formation of the North Haig breccia that
have been questionedby Fleischer (1970, 1980). did not occur or only occurred to a lesser degree
Additionaloccurrencesof FeSi as well as of FeaSie, during the formation of the other ureilites. All
Fe2Si and Fe3Si2were more recently described ureilites show evidence for shock metamorphism,
from acid insoluble residuesof Lower Cambrian red but North Haig is by far the most severely shocked
limy sandstonesand limestonesfrom the Bazaikha ureilite. The formation of suessite may have ocRiver, western part of eastern Sayan (Novoseleva, curred during this extreme shock event which may
1975;Novoselevaand Bagdasarov,1979)and were have caused rapid increase in temperature with
attributed by these authors to an extraterrestrial concomitant reduction of silicate rims by carbon
origin due to ablation of extraterrestrial silicides from the matrix, followed by rapidly falling temduring passage through the earth's atmosphere. peratures.The varying compositionof suessite,the
However, this appears to be an unlikely mode of occuffence of only thin FeO-free rims around siliorigin, in view of the exceedinglylow abundanceof cates, and the presenceof some remnant kamacite
silicidesin meteorites.
suggestthat this process was indeed of rather
Only one silicide has previously been described limited duration and was probably followed by a
from meteorites, namely the mineral perryite, a rapid drop in temperature.
nickel silicide containing minor iron substitutingfor
Acknowledgments
Ni, and P for Si. The mineralwas found exclusively
as a very rare compound in highly-reduced meteorWe thank Dr. R. Skaggsand co-workers (Los damos Nationites such as irons, pallasites,enstatite chondrites al Laboratory) for preparation of the synthetic iron silicide used
and enstatite achondrites (e.g., Fredriksson and as a secondaryelectron microprobe standard, and Mr. B. Tani
(Argonne National Laboratory) for calculated X-ray patterns.
Henderson, 1965; Reed, 1978; Wai, 1970). The
We owe specialthanks to Drs. L. Bettenay (Western Australian
acceptedcomposition,originally given as (Ni,Fe)z Museum, Perth), R. Hutchison (British Museum, London) and
(Si,P) (Fredriksson and Henderson, 1965; Reed. R. S. Clarke, Jr. (SmithsonianInstitution, Washington,D. C.)
KEIL ET AL.: SUESSITE
for generouslysupplying us with material of the North Haig
ureilite. We are most grateful to Dr. G. A. Desborough(U.S.
GeologicalSurvey, Denver) for quantitativereflectancemeasurementsand to Dr. M. Fleischer (SmithsonianInstitution,
Washington,D. C.) for bringingto our attentionseveralRussian
referencesto terrestrial silicides. This work was supported in
part by the National Aeronautics and Space Administration,
(K. Keil, PrincipalInvestigator).
Grant NGL-32-004-64
l3l
Gevork'yan, V.Kh., Litvin, A. L. and Povarennykh, A. S.
(1969)Occurrenceofthe new minerals fersilicite and ferdisilicite. GeologicheskoZsurnal Akademia Nauk Ukrain SSR, 29,
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Hansen,M. H. (1958)Constitutionof Binary Alloys. McGrawHill, New York.
Keil, K. (1967)The electron microprobe X-ray analyser and its
applicationin mineralogy. Fortschritte der Mineralogie, 44,466.
Keil, K., Berkley,J. L. and Fuchs,L. H. (1980)Suessite,Fe3Si,
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