Canadian Mineralogist
Vol. 23, pp."149-162(1985)
CHARACTERISTICOPTICAL DATA FOR COOPERITE,BRAGGITEAND VYSOTSKITE
ALAN
J. CRIDDLE AND CHRISTOPHER J. STANLEY
Department of Mineraloglt, British Museum (Natural History),
Cromwell Road, South Kensington, London, Great Britain SW7 sBD
monfent que cooperiteet pyrrhotine ne seressemblentpas.
Les descriptionsoriginellesde Schneiderhcihn(1929a,b)
Reflectance spectra for cooperite PtS and braggite portaient non pas sur la cooperite,mais probablementsur
(Pt,Pd)S from the type locality at Potgietersrus, South la braggite, encore inconnue d 1'6poque.L'erreur s'est
Africa, weremeasuredbetween2100
and 700nm. Thesespec- aggravdequand le "spectre" de rdflectance de la pynhotra, and qualitative descriptionsof the grains, completethe tine. extraite de concentrescens6scontenir de liacooperite,
characterizationof the minerals madein 1932by Bamister fut attribud d la cooperite.
& Hey. The mineral association, optical properties and
(Traduit par la R€daction)
reflectancespectraof cooperite,braggiteand vysotskitePdS
from the Minneapolis Adit area of the Stillwater complex,
Montana, are also described.The data for cooperiteand Mots-clds: cooperite, braggite-vysotskite,spectrede rdflecbraggite are comparedwith those for the type material; the
tance, 6valuationdescouleurs,caract6ristiquesoptiques'
data for vysotskite may, in the absenceof appropriate data
VHN, intervalle de composition, Potgietersrus,comfor the type material from Noril'sk, U.S.S.R., be consiplexe de Stillwater, min6raux du groupe du platine.
dered characteristic for the mineral. Electron-microprobe
data are provided for all ofthe grainsmeasuredinthe reflecIurnoructroN
tance study, and VHN for most of them. The extraordinary confusion that has surrounded the quantitative data
and qualitative descriptions of theseminerals is described.
Cooperite,braggiteand vysotskiteare important
Reflectancespectraand color valuesdemonstratethe false- sourcesof platinum and palladium in many of the
hood of the long-standing belief that cooperite resembles world's largest depositsof platinum-group minerals.
pyrrhotine. The original descriptions (Schneiderhcihn
It is thus surprising that their optical characteristics,
l929a,b) were not of cooperitebut probably of the then.
particularly those of cooperite and braggite, have
unknown mineral braggite. This error was compounded
when the reflectance'spectrum' of pyrrhotine, from the remainedill-defined sincetheir discovery.At present,
supposedlycooperite-bearingconcentrates,was mistaken theh identificationwith the microscopeis uncertain,
owing to erroneousand conflicting descriptiveand
for that of cooperite.
ABSTRAcT
Keywords: cooperite, braggite-vysotskite,reflectancespectra, color values,optical characteristics,YHN, compositional range, Potgietersrus, Stillwater complex,
platinum-group minerals.
SOMMAIRE
On a mesur6, entre 400 et 700 nm, le spectre de r6flectance de la cooperitePtS et de la braggite (Pt,Pd)S de la
localit6 type, Potgietersrus(Afrique du Sud). Cesspecfies,
ainsi que la description qualitative des gxains, complEtent
la caractdrisationdesdeuxesplcesamorc€een 1932par Bannister et Hey, On d€crit aussiI'associationmin6ralogique,
les propri6tds optiques et Iesspectresde rdflectancede cooperite, braggite et vysotskite PdS du secteur dit Minneapolis Adit du complexe de Stillwater (Montana). Les donn6essur la cooperite et la braggite sont compar6esi celles
desdchantillons types; vu le manque de donn€espertinentes sur le mat6riautype de Noril'sk (U.R.S.S.), les observations ici consign6espeuvent servir d caractdriserla vysotskite. On pr€senteles resultatsd'analysesd la microsonde
6lectronique pour tous les grains dont on a d6termin6 la
r6flectance, et la durete VHN pour la plupart d'entre eux.
On ddcrit la confusion extraordinaire qui 6mane des descriptions, tant qualitatives que quantitatives, de cesespdces.Les specfiesde rdflectanceet l'6valuation descouleurs
quantitative data in the literature.
We first ericounteredthis problem in 1980when
examiningsamplesfrom the MinneapolisAdit area
of the Howland Reef (Bow el ol. 1982), Stillwater
Valley, Montana. Here, unusually, all three minerals
occur in close associationbut, during our initial
investigation, we were unable to identify them
without the aid of the electronmicroprobe. Indeed,
at first, we believed cooperite to be a new mineral
and braggiteto be cooperite.Closerexaminationof
the literature revealedthat the earliestdescriptions
of cooperite (Schneiderh6hn l929a,b) were on
material of dubious authenticity. Thesedescriptions,
and in particular a suggestedsimilarity to pyrrhotine, are completelymisleading.Unfortunately, when
cooperiteand braggitewere characterized@annister
& Hey 1932), their optical characteristics were not
determined.It remainedfor Edwards et al. (1942)
briefly to describethe qualitative optical properties
of a type specimen of braggite. The erroneous
descriptions and reflectance data for cooperite
(Schneiderhdhn1929a,b, Schnederh6hn& Ramdohr
1931,Frick 1930)and the incompletedata for braggite hinderedthe identificationof the mineralsby ore
microscopy. Thus, although confirmation of their
149
150
THE CANADIAN MINERALOGIST
identity was certain by X-ray diffraction and
chemicalanalysis,the problem remainedof identifying the right mineral in the first instance.As a result,
until quite recentlycooperitewasregardedas a'rarity' (Ramdohr 1980),and braggitewas reported to
be a 'very rare mineral' (Picot & Johan 1982).
Attention was first drawn to the 'conflicting and
overlapping data' for braggite and cooperite by
Leonard et ol. (1969),who also noted that adequate
quantitative data were lacking. The situation
improved somewhat in the 1970s,when electron
microprobesand microscope-photometers
capable
of measurementsof spectralreflectancebecamemore
widely available. The first spectral-reflectancedata,
though restricted to four wavelengthsthat correspondto the perceivedbrightnessand color ofthese
minerals,werethoseof Cabri (1972)for braggiteand
Schwellnuset ol. (1976)for cooperiteand braggite.
More completespectraand descriptiveinformation
were reported by Picot & Johan (1977). Unfortunately, the spectraand descriptionsfor cooperite
aretransposedin their book and appearas for bragg$e (this mistake occurredat an editorial stage;Picot
& Johan, pers. comm.). Their description of
cooperiteis also in error; it correspondsneither to
cooperitenor to braggite. In 1981,Cabri reviewed
the situation and concludedthat the reflectancespectra of braggite neededconfirmation, that those of
cooperite neededredetermination, and that more
work was required on the qualitative optical properties of vysotskite.The latter, despitethe uncertain
propertiesof braggite,had beencharacterizedwith
minimal optical data in l962by Genkin &ZWaejntsev. In 1982,Picot & Johan publishedcharacteristic reflectance spectra and descriptive data for
cooperite. Their R spectra for braggite are also
characteristic, but their description of this mineral
does not correspondto thesespectra;in fact, it is
almost identical to the description of
Schneiderhdhn'scooperiterhat had previouslybeen
reiteratedin various forms in the diagnostictextsof
Farnham (1931),Ramdohr (1950and all later edi:
tions), Schouten(1962)and Uytenbogaardt& Burke
(1971).Correct reflectancevaluesdeterminedat the
four wavelengthsrecommendedby the Commission
on Ore Microscopy (COM) for braggite and
cooperitefrom the Rustenburgmine, SouthAfrica,
werereported,togetherwith brief descriptivenotes,
by Kingston & El-Dosuky (1982).Theseauthorsgo
someway toward resolvingthe confusion concerning the two minerals; however,their assertionthat
the incorrect description'of the color of cooperite
(tlat had becomeentrenchedin the literature) is more
applicableto braggitehas introduced a new source
of confusion. Further, their conclusionthat the original source of the mistaken indentity of cooperite
arosefrom a description of braggite-cooperiteintergrowhs is one that we cannot support.
Clearly, a thorough re-investigatioi of all three
minerals using procedures recornmendedby the
COM was overdue.In particular, the confusionsurrounding the color and brightnessof the minerals
required measurementof the dispersion of their
reflectancethroughout the visible spectrumand calculation from thesespectraof unambiguouscolorvalues.More than this, we consideredit essentialto
relate thesemeasurementsto the composition of each
grain that was measuredand to obtain X-ray data
for selectedgrains. We were fortunate in having at
our disposal the type specimensof cooperite and
braggite of Bannister & Hey (1932), as well as the
intergrown grains of cooperite, braggite and vysotskite from the MinneapolisAdit. The latter, fortuitously, occur in a matrix consisting predominantly
of pyrrhotine, pentlandite and pyrite; hencedirect
comparisonof cooperiteand pyrrhotine was possible. What follows, therefore, may be regardedasthe
completion of the characterizationof braggiteand
cooperite, with additional and more complete data
for vysotskite.
In addition to measuringthe reflectancespectra,
calculating color values, and measuring microindentation hardness(VHN) values, we examinedin
detail the apparentcolor, brightness,bireflectance
and reflectancepleochroismofthe mineralsin planepolarized light, and their rotation tints between
crossedpolars. This was done not only to complete
their characterization but to determine whether the
qualitative properties alone were sufficiently distinctive to be used as a meansof identification.
PROCEDURE
EXPERIMEI.ITAL
All of the Pt-Pd-Ni-S-bearing samplesfrom the
Stillwater Yalley examined (fourteen polished
mounts, ten polishedgrain-mounts,and five polished
thin sections)camefrom an unusually coarse-grained
pegmatiticgabbro from the first cross-cutof the MinneapolisAdit. Thesesampleswere selectedfrom a
suiteof specimens
BM 1981,134.The equipmentand
polishing procedure used for the mounts and grain
mounts were describedin Criddle & Stanley(1979),
Comminution, separationand mounting procedures
for the grain mounts are those describedby Cabri
& Laflamme (lW6), The samepolishingprocedures
wereapplied to the grain mounts of the type specimensfrom Potgietersrusas were used on the Stillwater samples.
The equipmentand proceduresusedfor measuring the reflectanceof all the samplesare as described
in the Appendix to Cabri et al. (1981), except that
x 16air and oil objectives,adjustedto provide effective numerical aperturesof 0.2, were used. Microindentation hardnessmeasurementswere made at
100-gramforce with a Leilz Miniload 2 hardness
tester equippedwith a Vickers indenter. The areas
l5l
OPTICAL DATA FOR COOPERITE. BRAGGITE AND VYSOTSKITE
tion and vocabularybetweenthe two observers,the
results were very consistent.
Initially, we were under the impressionthat the
rotation tints differed betweenthe two microscopes
but, on checkingour notes,it becameclearthat the
difference was not in the color but in the relative
intensity,or brightness.This phenomenonwasmost
marked in the polymineralic Stillwater samples,
wherethe eyewas further confusedby the rotation
tints of the associatedminerals.As a result, in the
subjective terminology applied to the strength of
anisotropy and rotation tints (Galopin & Herry 1972,
p. 243), the anisotropy of the samemineral could
be described,dependingon orientation and associ
ation, asweak, distinct, or strong. We emphasizethis
point becauserotation tints have recently been
recommendedas one of the most important diagnostic propertiesfor ore minerals@icot & Johan 1982).
selectedfor both sets of measurementswere first
checked for compositional homogeneity with the
electron microprobe, a Cambridge Instruments
MicroscanIX, operatedwith an acceleratingvoltage
of 20 kV and a beam current of 2.50 x 10-oAon
the Faradaycage.The pure elementsPt, Pd and Ni,
as well as FeS, were usedas standards.The results
were coriected using the Cambridge Instruments
ZAF programme V028.
For the qualitative examination of the optical
propertiesof the mineralswe used aLeitz Ortholux
Pol-BK microscope as well as the Zeiss MPM03
microscope-photometer.It was apparentthat even
when using the samelightbulbs, adjustedto the same
voltage, and hence the same nominal colortemperatureof ca. 3100K, the relative brightnessand
the intensity of the rotation tints were greaterwith
the Leitz than with the Zeiss equipment. This we
ascribeto the longer light-path of the microscopephotometer (it includesa modulator that extendsthe
light-path by some 16 cm). We recordedseparately
with both instrumentsour observationsconcerning
the appearanceof the minerals in plane-polarized
light and the sequenceof rotation tints between
crossedpolars and with the analyzeruncrossedby
3'. Allowing for slight differencesin coloqpercepT]IBIT
Oprtcal Pnopenrrss
Qualitative opticol datafor the type specimensfrom
Potgietersrus
Five discrete and monomineralic grains of
AIJISOTROPY
1.
AID
ROIAIIO]I
TINIS
cooPmrn
CRND
BRAGCITT
POUE:
PotgieteraruF
peenish
li6h!
pey
to
to
Eid-grey
yc'llorlsh
grey
purnlish
b.om
grcl
10 browniah
AlR
S1 i I lvater
Areenish
gey
to
bright
ar€y
Potgietrersros
grey
OIL
TIX-CRGSBD
to
and
to
bright
greeniah
St il lsater
ATALISER
Sey
nrcentsh
groy
brtAht
to
pal.
and n€talllc
bright
rhitish
pale
and
to
hrom-
yello{
to
b.om-
d.
blniah./purpltsh
purplish
subdued
broMleh
brcm
brlght
to
to
yollop
brom
vhitiah
to
Jellor
brom-
sllrducd
or
Itrle
slightlJ
grcl
to
Bid-grel
to
EIc.t
to
pinkiGh
brom
grcy
dark
purp)lsh
brcm
lale
BY 3C:
Potgleteraru6
blulFh
IIR
pale
St i I lrator
Pol aietartsrus
lalc
groentsh
rhite
to
slaie
Freoniah
to
grey
slAic
crenBy,
elighlly
bromiEh
bluish
or
or
hrorn
Frcnieh
rhite
cradn'.
to
lishl
to
to
AruI
nnd
brosn
Iellorish
lale
rcddish
to
Frey
{roonish
lighi
pinkiFh
groy
1o brilllonl.
rhlrleh
Fr.I
10 Ft.al-Froy
io
l.llot
1.
arat
rollorish
lo
lo
lellosish
nld-gret
liFht
to
Fr.I
sltahllf
to
did-qrcl
hlniFh
or
rlnte
ro
Bid-gral
LromiFh
nnlo
pey
purJrlish,/blnish
slron8
hrllliant.
tinkieh
ol t,
Sl i I lsnter
palc
ro
Arer
ntd-gry
JelloriFh
slighill
10 brilliant,
grey
purplish/hluish
and briaht
hright
srel
rhile
liFhl
mddish
1o sliFlllI
nid-Atrr
!.llorish
nii-
ar.t
PtknlloTl
WSOIT'KITT,
(.RNSED
pinkish
to
brom
briAhl
and
grey
rhttish
pale
grey
\[
NOIII6:
AIR
{lark
Lrom
vcrt
albdudrl
ta
rlark
bluo-Arfl
!.
tr) darh
nid-{rfr
liAhi
Frcorish
FrcI
liqhi
Arofnish
sr.l,/rlritc
nid-arcl
lo
to
rliFhlll
ar.cnish
rrf\
Sl i I lrtrtor
oll
lxALlsER
\/v.
tN-ClossrD
AIR
BI
dark
hrorn
10 dnr.h hlu.-gri
nnsro
lo
roddish
nid-ar(I
lo
pnrPlish
brorn
30:
rhoF
pale
trinnod,
light
bluo
briAht
to
.oddtevp[rplish
fam.
Nore
soEbre
{rey
rhcr.
to
un-
to
brohiFh
dark
liaht
arlt
to
groeniFh
bid-grcy
grer
Stillrarer
the
sre
tints
oa
ln
air
but
Flightly
&rk.r
hrtFhl
to
liAht
reddiah
arey
broh
to
to
puqliah
rich
pal€
reddiah
grel
brorn
lo
nid-Frer
r52
THE CANADIAN MINERALOGIST
cooperite from BM L932, L30L were polished in
mount E.700. In planeseclionall are anhedral,with
angular outlines and curved a16 angular embayments.They vary in sizefrom I to 1.5mm. In planepolarized light in air they appear pleochroic from
white or creamy white to bluish white. They do not
appearto be bireflectant (c/ quantitative properties).
In immersion oil, under otherwisethe sameconditions, they arc apparentlypleochroic and bireflectant from white (higherR) to bluish white. Between
crossedpolars their anisotropy in both media is
strong. Their rotation tints between crossedand
slightly uncrossedpolars are summarized in Table
Il.[',t,n'nxxilnnn
l. None of the grains is twinned.
FIc. l. Photomacrographof a polishedsectionfrom StillThree roughly equant grains of braggite fromBM
water. The braggite B prism is characteristically frac1932, l3M were polished in E.701. They are all
tured. At both ends of the prism and to one side are slightly lessthan 1 mm acrossand haverounded outcompositionally homogeneousareasof Wsotskite V. A
lines (one has small angular embayments).In planecolor Jilter wasusedto exaggeratethe differencebetween
polarized light, in air and in oil, braggite is white
groundmass
grey)
yellower
(dark
pyrrhotine,
the
of
pentlandite and chalcopyrite, and the whiter F!, Pd sul- and very sliehtly bireflectant; it lacks reflectance
fides. As a result, pyrite dispersedthroughout the pleochroism. Its moderately high reflectanceis similar to that of cpoperite.Betweencrossedpolars, its
groundmassalso appearswhite. The bar scaleis in rnillimetres.
anisotropy is distinct in air and is reduced, to weak
to distinct, in oil (Table l). Like cooperite,all of the
grains are untwinned.
Thus, tHe featuresthat enablethe two minerals to
be distinguished where isolated in separate grainmounts, are 1) the absencein braggiteofthe apparent
reflectance-pleochroism
of cooperiteand 2) the much
more strongly anisotropic character of cooperite. In
our grain mounts we did not have any pyrrhotine
for comparison but this was remedied in the Stillwater samples.
Qualitative optical dota for the minerals from
Stillwater
braggite B. The section is oriented for R, cooperite.
Plane-polarized light.
The Stillwater samplesoffer an ideal opportunity
to compare the optical properties of the three PtPd-Ni-S minerals with pyrrhotine and with many
of the other minerals with which they are commonly
associated. Details of the mineral association and
paragenesisof the Minneapolis Adit samplesare to
be reported separately;however,insofar asthey have
a bearing on the diagnosticpropertiesof the three
PGM, the associationwill be outlined here.
The three sulfides occur in a pegmatitic gabbro in
associationwith: major pentlandite,pyrrhotine, chalcopyrite and pyrite, minor cubanite and nickelian
mackinawite, and accessorygold and the PGM, e.9.,
moncheite,isoferroplatinum, kotulskite, keithconnite, palladian tulameenite and a number of other
unnamed minerals. Unusually large (by PGM standards)prismatic crystalsof braggile, up to 8 mm long
(Fig. l), are not uncornmon.All of them are fractured. They are compositionally homogeneous,
exceptthat a distinctive rim of slightly more highly
reflecting vysotskiteinvariably occursat the 'ends'
OPTICAL DATA FOR COOPERITE. BRACCITE AND VYSOTSKITE
153
of the prisms. In addition, as is shown in Figure l,
vysotskite may be separatedfrom, but is closely
associatedwith, the braggiteprisms. Cooperitewas
not found in associationwith theseeuhedra.It occurs
as subhedralto anhedralinclusions,from 30 to 250
pm in size @igs. 2,3), within large (up to 15 mm)
irregularareasofbraggite (Fig. 4). Vysotskite,which
was not found in direct contact with cooperite, is
invariably present as a selvedgeto braggite at its
boundarieswith the Fe, Ni and Cu sulfides(Fig. 5).
Texturally, theie is abundantevidencethat vysotskite
was the last of the three PG sulfidesto form and that
it replaced braggite: it forms thin selvedgesto fractures in braggite,'healed' fractures,and finger-like ffK,ff,ffilX-,nre,ffimn
projections, or penetrationtextures(Fig. O. In this Ftc. 4. Photomacrograph of an irregular area of intergrown braggite, vysotskite and cooperite from Stilllatter form, the vysotskitechangesin composition,
water, The centml and smallergrain of cooperiteis illusmoving toward that of braggite the further it penetrated in Figure 2, The bar scaleis in millimetres.
tratesinto the braggitefrom its boundaries$/ith the
other sulfides. In someinstancesthe changeappears
to have been sequential and is detectable optically
as a subtly steppedreduction in reflectanceand intensity of rotation tints. Compositionally homogeneous
vysotskite occurs as small and irregular areas near
the larger areasof braggite. Here, and in the thicker
selvedgesto braggite (Fig. 5), guided penetration and
Fff ,r +
vermiform textures are common. The dominant
;i;", t
interpenetrant minerals are pyrrhotine, pentlandite
.*,1j!,:.}1
and chalcopyrite,although gold and the Pt-Pd tellurides are not uncommon in theseveinlets,which
are usually compound.
Our examination of the qualitative optical
behaviorof the three mineralsfrom Stillwater illustrates how the assessment
of brightnessand the relative intensitiesof color and rotation tints of one
5. Oil-immersion photomicrograph of vysotskite V
mineralcould be influencedby the presenceof other Frc.
' selvedgeto braggite B with associatedpentlandite Pe,
minerals in the same field.
pyrrhotine Po and minor chalcopyrite. Note tle guided
In plane-polarizedhghq cooperitefrom Stillwater,
penetration textures of chalcopyrite, pyrrhotine and
unlike that from Potgietersrus,is bireflectant and
pentlanditein the vysotskite.Plane-polarizedlight.
apparently pleochroicfrom white (higherR) to bluish
white in both media. Braggite,which surroundsit,
appears slightly lower rbflecting, slightly ereenish
white against white cooperite, and slightly higher
reflecting and pinkish white againstthe bluish white
cooperite. The higher bireflectance and more
pronouncedpleochroismof the Stillwater samples
are, however, more apparent than real, as will be
demonstrated in the description of quantitalive
properties. In comparisonwith cooperite, pyrrhotine is significantly lessbright and pale brown to pale
milky brown in color. Betweencrossedpolars, the
anisotropy of cooperiteis strong, apparentlystronger
that that of the type specimens,and as strong, or
strongerthan that ofpyrrhotine. The rotation tints
of cooperitefrom both localitiesare virtually identical and differ from those of pyrrhotine, a differ- FIc. 6. Oil-immersion photomicrographof vysotskiteV
ence that is even more marked with the analyzer
selvedgeto twinned braggite B. Note the optical
uncrossedby 3' (Table l). Simple twinning was
behaviorof the vysotskiteas it penetratesthe braggite.
observedin only one small graiu of cooperite@ig.
Slightly uncrossedpolars.
154
THE CANADIAN MINERALOGIST
2), ard undulatory extinction, which may be due to
deformation, in one of the larger grains.
In plane-polarizedlight, the single crystals of
braggite are white and neither bireflectant nor
pleochroic;noneis twinned. A characteristicfeature
of the polycrystallineaggregatesof grainsthat constitute the larger anhedralareasof braggiteis polysyntheticand simple twinning and, to a lesserextent,
kinkbanding [c/. Schneiderhdhn (1929a,b) on
cooperite,and Kingston& El-Dosuky(1982)on braggitel. In theseareasbireflectanceis weakly perceptible. In fact, as will be demonstratedbelow, the
bireflectanceis minimal, I to 1.590in both media,
and lower than that of the Potgietersrusbraggite.
Nevertheless,this small difference in reflectanceis
within the visual thresholdof the eye,and discrimination is helpeil by the small-scaletwin lamellae
wherebright and lessbright lamellaeare jucaposed.
Betweencrossedpolars, anisotropyis distinct in air
for both the single crystals and the polycrystalline
areas(Table l). In oil, it is reducedto "weak" for
the singlecrystalsbut remainsdistinct (again,probably becauseof the juxtaposition of lamellaewith
different huesand relative intensities)in the twinned
areas.It shouldbe clearfrom the above,from Table
l, and from our comparisonof the appearanceof
cooperiteand braggite,that thereis little likelihood
of confusing the two, and it is equally improbable
that braggite could be confusedwith pyrrhotine.
TABLE2.
BRAGGITE
ANDVYSOTSKITE*
CHEM1CAL
CO||IPOSITION
OF COOPERI'TE,
r-t.X
pr
Pd
lli
46.1
44.5
45.2
0.6
o.5
1.5
0.6
o.8
0.?
0.?
0.8
0.4
0.7
13.9
74.7
13.0
13.8
13.6
78.6
?s.9
79.O
?9.7
n.g
80.6
6.5
0.o
0.9
6.6
6.7
5.5
0.2
o.7
o.2
0.1
0.2
0.2
92.7
63.2
01.2
14.4
76.4
7A.A
62.9
62.7
62.O
Pd
t{i
s
1m.3
99.1
[email protected]
99.5
100.3
.98
.99
.9?
.99
.99
.o1
.01
.0:t
.o1
.o2
.03
.oit
.o3
.03
.03
.94
.94
.w
.97
.96
74.9
74.7
14.4
14.5
74.A
15.9
[email protected]
70/J.7
1m.5
9S.7
Sa.8
100.2
.a6
.AA
.aa
.90
.8a
.92
.13 .OOA 1.m
.12 .006 .99
.14.0G.97
.12.O4.94
.12 .Ooa 1.m
.12.o04.96
4.4
4.4
4.2
17.3
r7.4
77.6
99.6
100.4
99.5
.60.4.16
1.OO
.60 .2? .1,1 1.OO
.58 .20 .13 1.00
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It has beensuggestedrecentlythat in the isomorphous solid-solution seriesfrom braggite (Pt,Pd)S
to vysotskitePdS, the lJamevlsotskite be restricted
to mineral$ containing less than l0 mole q0 PtS
(Cabri et ol. 1978).This followed from the observation that, at the time, no membersof the seriescontaining betweenl0 and 30 mole VoPtS wereknown.
In our investigation, whereasmost of the larger
grainsofvysotskite analyzedcontain about l0 mole
9o PtS, a significant number of homogeneousand
inhomogeneousareasand grainswere found in the
10-30 mole Vo PtS range. In this account, we will
limit ourselves to descriptions of those optical
propertiesof vysotskitethat are characteristicand
to othersthat might causeconfusionwith braggite.
The latter arise from a gradual change in these
properties as the composition moves from that of
one end-membertoward that of the other. A more
detailed account of the compositionalvariation in
the series,togetherwith textural relationships,will
be reported separately.
In plane-polarizedlight, in air and in oil, vTsolskile (containing l0 mole 9o or lessftS) hasa slightly
higher reflectancethan braggite and is a slightly
creamierwhite. Neither bireflectancenor reflectance
pleochroism is detectable visually. In isolation,
homogeneousvysotskite could be mistaken for
braggiteand vice verso.Similarly in intergro\rr'nareas,
where the constituents are inhomogeneous,it is
usually impossible to distinguish them by eye.
Betweencrossedpolars,vysotskiteis appreciablyless
anisotropicthan braggite, and its sombrerotationtints in air are evenmore subduedin oil (Table l).
Even more distinctive are the tints that are seenwhen
the analyzer is uncrossedby 3o (Table l). In untwinned areasthesetints are lessbright than those
of braggite but, when twinned, as is usually the case,
they are much brighter. Other, more unusual
phenomenawereobserved:a gradual 'bleaching' or
desaturation of the tints away from a saturated
centre, and a play of color resemblingiridescence,
or undulose extinction of randomly oriented
aggregatesof minute grains (no grain boundaries
were detectable,however,in theseareas).Possible
explanationsfor thesephenomenaare compositional
inhomogeneityand deformation.
To summarize,the qualitative optical properties
of cooperiteare sufficiently distinctive for it not to
be confusedwith other minerals.Those of braggite
and vysotskitesimilarly are distinctive,but they are
difficult to distinguish from eachother exceptwhere
near end-membercompositionsoccur together.
Quantitative optical dato
Reflectancespectraare reported for five grains of
cooperitefrom Potgietersrusand six from Stillwater,
threegrainsof braggitefrom both localities,and four
r55
OPTICAL DATA FOR COOPERITE, BRAGGITE AND VYSOTSKITE
grains of vysotskitefrom Stillwater; theseweremeasured at an interval of 10 nm from 400 to 700 nm.
Measurementswere made in air and in Zeiss oil Np
1.515againsta ZeissWTiC standardno. 314. In
addition, severalgrains of plrrhotine from Stillwater
were measured for comparison, principally with
cooperite.Every grain measuredwas also analyzed
by electronmicroprobe (Iable 2), and asmany grains
as practicablewere indentedto detetminetheir VHN
(Table 5). The identity of grains representativeof
each mineral was confirmed by X-ray diffraction.
The identification numbers quoted in all of the
Tables refer to individual grains in the different
polishedmounts. It would be impracticalto publish
the very large.amount of data that we obtained;
therefore, we have selectedfrom the completeset
reflectance $pectra that are representative of each
mineral (Table 3). Similarly, although the complete
data-setswereusedto calculatethe color values,we
have omitted the chromaticity co-ordinatesfrom
Table 4. The completeset are availablefor inspection at the British Museum (Natural History).
The reflectance spectra of. cooperite from Stillwater are similar to thoseof the type specimensfrom
Potgietersrus,but are lessdispersed(Fie. 7) and less
bireflectant (cl, qualitative observations).Throughout the visible spectrumthe bireflectancesin air and
in oil of the type specimensare 5-8 90, and those
of the Stillwatercooperite4-690 (Tables3,4). These
differencescan be attributed to differencesin comTABLE4.
LLWIW
REPRESENTATIVE
N AND@F SPECTMFORCOOPERITE,
PYRRHOTINE,
BMGGITEANDVYSOTSKIIE
TABLE3.
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VALUES
ANDVYSOTSKITE
FORCOOPERITE,
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156
THE CANADIAN MINERALOGIST
position: the Stillwater sampleshave consistently
higher Pd and lower Pt and Ni contentsthan those
from Potgietersrus(Table 2).
One of the grains from the type locality is isotropic
(E.700,O.The reflectancesof this grain confirm the
assignmentof Ro to the spectral curves of lower
reflectance.Hence, the sign of the bireflectanceis
positive.Earlier, we pointedout that cooperitefrom
Potgietersrus is apporently pleochroic when
immersedand cooperitefrom Stillwater is apparently
pleochroicin both media. The reflectancecurvesand
color values (Table 4) show that the dominant
wavelengths)r7 (in subjective terms, the hues) of
both Ro and Re, are nearly constant,and hencethe
mineral is, strictly speaking, not pleochroic. The
explanationfor the perceptionof blue and white hues
is that the higher reflecting component Rr, is closer
to the illuminant point and at most half as saturated
(in quantitative terms, the excitation purity, P"9o)
as the Ro component. Thus, what is perceivedas
reflectance pleochroism is nothing more than a
difference in brightness (quantitatively, the
luminance, Yslo)ard is properly a function of the
mineral's bireflectance.
Severalgrains of monoclinicpyrrhotine from the
samepolishedmounts weremeasuredby N.S. Oldfield. From thesemeasurementswe selectedrepresentative spectra(Table 3) and plotted them with those
of cooperite(Fie. 7). Inspectionof thesedata should
prove that there are no similarities betweencooperite
"*::i:::::f;:::;:';;;;;;;;:1"1"""
LO
488
458
5AA
qqr,
Lombdo
6ez
6,5A
7Zg
nm
Frc. 7. R and ,z.i?spectrafor cooperitefrom Potgietersrus(P, dotted) and Stillwater (S, continuous)comparedwith
pyrrhotine (py, dashed)from Stillwater,
t57
OPTICAL DATA FOR COOPERITE, BRAGGITE AND VYSOTSKITE
and pyrrhotine. That point is made evenmore convincing in Table 4; here the values are referred to
the two illuminants recommendedby the COM, C
and A, with correlatedcolor-temperaturesol 6774
and 2856K, respectively.It will be recalledthat our
qualitative observationswere made with respectto
a lamp run at about 3100 K. It follows that our
descriptionswill correspondmost closelyto the quantitative values for illuminant A. The brownish to
brownish white color of pyrrhotine is explainedby
its fairly high saturation or excilation purity of about
l4v/o in air, and its hue, or dominant wavelength,
of about 588 nm. The fact that darker browns are
seenin oil is a consequence
of it being more saturated, with a P"of 20c/0,and lessstrongly reflect-
la
4@A
454
5ZA
ing (by about l39o) than in air. In contrast, cooperite
hasdominant wavelengthsin the 490nm range,and
the reasonthat it doesnot appearmore than slightly
blue is due to its low excitation-puritiesof l-4.8t/0.
In terms of relative brightnessor luminance, the
bireflectanceof cooperiteimmediatelydistinguishes
it from pyrrhotine and, although Wo Ry for pyrrhotine is about the sameas Wo Ro for cooperite,
the two could not be confusedbecauseof the differencesin hue and saturation.
The reflectancespectraof three grains of braggite
from both localities (Fig. 8), plotted to the samescale
as those of cooperite (Fig. 7), illustrate that the
mineral is much lessbireflectant than cooperite.They
also show that, Iike cooperite, the Potgietersrus
55Z'
Lombdo
6AA
65A
7ZZ
nm
FIc. 8. R and,zR spectrafor 3 grains of braggitefrom Potgietersrus(dashedlines) and 3 grains from Stillwater (continuous lines).
158
THE CANADIAN MINERALOGIST
braggite is slightly more bireflectant than its Stillwater counterpart. Theseslight differencescan be
attributed, in the sameway as for cooperite, to compositional variation (Table 2): the Stillwater grains
are more Pd- and less Ni-rich than the type
specimens.
In terms of color, the undispersedRo spectra
indicate that the mineral will be white, lacking any
noticeablehue, but that it will take on the color
attributes of the light soluce, that is, it should appear
white to bluish white with a Csource and a creamier
white with the14 source.This fact is emphasizedby
the very low levelsof excitation purity ln Table 4.
The scatter of the dominant wavelengths is insignificant sincethe chromaticity co-ordinatesplot
within lessthan l9o of thoseof the illuminants. The
levels of excitation purity for R", are a little higher,
with the result that more consistent and reliable
dominant wavelengthsof about 570-580 nm are
obtained. The difference in the dispersionof the
reflectanceof the two vibration directionsaccounts
for the detection by eye of the small measured
bireflectance.One factor that is not readily explained
in terms of color scienceis the presenceof pinkish
and greenish tints to the white braggite where it is
in contact with cooperite.We can only speculatethat
as a result of differencesin the polishing hardness
of the two minerals, we were observingKalb lightlines.
Vysotskiteposedproblemsfor reflectancemeasurementssince,unlike the other two minerals, selection of compositionally homogeneousareas large
5A
wyaolek
I t€
I
.b.qggi!€
I
I
__
__*___*___;
__;_-_::a
2A
4ZA
l--+
454
5AA
6ZZ
552
Lombdo
+-+-
+
|.++-*
652
7ZZ
nm
FIc. 9. Representative
R and i-R spectrafor vysotskiteand braggitefrom Stillwater, Note the lower bireflectanceof
vysotskite.
159
OPTICAL DATA FOR COOPERITE. BRAGGITE AND VYSOTSKITE
enough for photometry required that a large number of compositebraggite-vysotskitegrainsbe analyzed by electron microprobe. As we have mentioned, the serieswas discoveredto be continuous;
however,grains suitable for photometry are, with
the exceptionof a near end-membervysotskite,all
of similar composition, that is, they contain abour
l0 mole 9o PtS (Table 2). Also, owing to the extremely subduedrotation-tints and weak anisotropy,
it wasnot possibleto be certain of the extinction positions of any of thesegrains. For this reason,it was
necessary
to usethe photometerto selectreflectance
maxima and minima for eachgrain. This was done
at 560 nm.
The four grains measuredhave bireflectancesof
lessthan 190throughout the visible spectrum,dis-
tinctly lower than that of braggite. The dispersion
of their reflectancealso differs from that of braggite, as can be seenin Figure 9. It was not possible,
becauseof the low bireflectance,to determinewhich
of the two vibration directionsmeasuredcorresponds
to Ro. This was assigned,by analogywith the Ro of
braggite,to the spectraof lower luminance.It will
be apparent from Figure 9 why vysotskite, when
adjacentto braggite,appearsa creamierwhite and
slightly brighter; the reflectancesof both vibration
directionsof all of the grains are higher than those
of braggiteabove520nm. This is confirmed by the
color valuesin Table 4; the luminancesof vysotskite
arc l-Zvlo higher in air and l-390 higher in oil than
thoseof braggite;the dominant wavelengthsfor the
.4 illuminant are nearly constant between 580 and
5A
4Zn
45A
5AA
55U
6AA
652
7ZA
Lombdo
nm
Ftc. 10. RepresentativeR and tmRspectrafor cooperite,braggiteand vysotskite.The ornament is the samefor each
pair of curvesand for both media. S and P for Stillwater and Potgietersrus,respectively.
160
THE CANADIAN MINERALOGIST
590 nm and, perhapsmore importantly, the excitation purities are 34Vo higher than those of braggite.
Thus there should be no difficulty in identifying
the end membersof the braggite-lysotskite series,
but it seemslikely that if grains intermediate in composition were measured,their reflectancespectra
would fall betweenthe extremesshownin Figure 9.
This conclusionis supportedby the qualitative observations ofgradational and steppedreduction in the
reflectanceof vysotskitereported earlier.
To concludethis section,we haveplotted representative spectrafor the three minerals @ig. l0), including examplesfrom the type specimensas well as for
material from Stillwater. Thesespectraare characteristicfor the minerals.In our opinion, whereasthe
qualitative properties that we have describedcould
suggestto the trained observerthe possibleidentity
of one or other of theseminerals,reflectancespectra provide the only reliable meansof confirming this
identifi cation optically.
skite. Pyrrhotine, by comparison,is softer than all
three.
Of the publisheddata, only thoseof Cabri (1981)
agreewith ours for braggite (his data for cooperite
were made at a different loading and so are not comparablewith ours). Comparedwith our results,Kingston & El-Dosuky (1982)obtained lower valuesfor
braggite (meanYHN1g9864) and higher valuesfor
cooperite (meanVHN1* 939). In fact, their braggrte
valuesmatch ours for cooperiteexceptthat the hardnessanisotropy of their mineral is much lower and
similar to our braggite. The only other data available are those of Rozhkov et al. (1962)for identations at an unspecifiedload. For this reasontheir
data cannot be regardedwith any confidence.
We have no reasonto doubt that our data are as
accurateas the techniquepermits; whereaswe do not
considerthe VHN to be useful as a 'primary' means
of identification, we believethat it has somevalue
as a support to identificationsmade on the basisof
refl ectancedeterminations.
Mic ro- indent atio n hardness
The published VHN values for cooperite and
braggiteare few and contradictory; there are none
for vysotskite.All of our measurements
weremace
with a Vickersindenterat a 100-gramforce. Grains
wereselectedfor their compositionalhomogeneity,
and the number of indentationsmadein eachgrain
wasgovernedby its size.Whereverpossible,at least
10 indentationsweremade, but fewer in the caseof
the smallergrains of vysotskiteand cooperitefrom
Stillwater (the actual number is shown as a subscript
to the mean VHN in Table 5). Our results are
reproducible,similar for the samemineral from the
two localities,and their precisionis good (Table 5).
In terms of micro-indentation hardness,cooperite
is softer than braggitebut in the samerangeasvysot-
TABLE5. VALUESOF MICRO.INDENTATION
HARDNESS
1oo
Ceperl.te
polqrgEnsnns
8.700,4
E.7Oo,5
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8.391,1
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E.396rd
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E.3e6,ef
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DtscussloN AND CoNCLUSIoNS
The main objectivesof this study were l) to complete the characterizationof braggiteand cooperite
made by Bannister& Hey (1932),our predecessors
at the BM(NH),2)to provide characteristicoptical
data for Wsotskite (acceptingthat the material at our
disposalis not from the type locality at Noril'sk),
3) to dispelthe extraordinary confusion that has surrounded braggite and cooperite and that was
developing around vysotskite and, finally, 4) to
attempt to explain how this occurred.
We haveshown that the qualitative propertiesof
cooperite are parti\ularly distinctive and those of
braggite and vysotskiteare sufficiently distinctive for
end membersof the seriesto be identifiedwith some
confidence.Reflectancespectrafor the three minerals
provide evengreater assuranceof a positive identification, and havethe addedadvantageof supplying
the data necessaryfor deriving unambiguouscolorvalues.In this, they removethe subjectivityof color
of relativebrightness
descriptionand the assessment
which, evenfor the experiencedmicroscopist,cause
confusion through the eye'sability to modify impressions of color and brightness depending on the
appearanceof the associatedminerals.
It only remainsfor us to try and explain why the
original descriptionsof cooperitewere so misleading. When Schneiderhdhndescribedcooperite in
1929,itwas consideredto be a platinum sulfarsenide
(Cooper 1928).This was revisedby Cooper (1929)
to PtS2,and it was not until 1932that Bannister&
Hey, using X-ray diffraction to identify individual
grains in the concentratesfor wet-chemicalanalysis, proved it to be PtS. Schneiderhdhn's(1929a,b)
OPTICAL DATA FOR COOPERITE. BRAGGITE AND VYSOTSKITE
descriptions of his mineral as white with a distinct
greyish yellow tint, with high reflectivity in air and
in oil, distinct anisotropy with grey-pink and greygreenrotation tints, and characteristicpolysynthetic
twinning, would not fit our description of cooperite.
This descriptionof the appearanceof his prismatic
grainsdoes,however,correspondto a mineral which,
at that time, was unknown: braggite. In the same
papers,Schneiderhdhnrecordedthe resultsof Frick,s
determinationl with the reflection-photometer ocular of the 'absolute reflectivities' of cooperite tn
green, orangeand red light. Thesevalues,37, 37 and
3690, respectively,are exactly the same as those
determined at the same time'for pyrrhotine (and
within 290 of those for chalcopyrite).In l93l the
description was changed by Schneiderhdhn &
Ramdohr; the reflectanceof cooperitewas described
assimilar to that of chalcopyriteand pyrrhotine, and
its anisotropy as weak. The reflectance values were
also revised to 41,34 and 34r/o (thesewere later
attributed to Frick by Ramdohr 1969).Frick, himself, reported (1930)slightly different valuesagain
of 37.8, 37.8 and 36.40/o.
Subsequently,
Ramdohr
(1950),describingSchneiderh6hn'sobservationsin
1929,stated that they were incorrect because2 or
3 different mineralswere consideredas one. Obviously, fifty yearsafter the event,we cannot be certain of what happened,but suggestthat the most
plausible explanation is:
l. Schneiderhiihnoriginally describedbraggite as
cooperite(braggite,although found in concentrates
from the samelocality by Bannister& Hey in 1932,
was unknown in 1929).
2, Theconcentratesexaminedby Schneiderhdhn
contained pyrrhotine, and it was pyrrhotine and not
braggite or cooperitethat was measuredby Frick
(1930),leadingto the apparentidentity of cooperite
and pyrrhotine. Frick's (1930)tablesof reflectance
for the opaque minerals, except for a consistent
underestimateof the reflectancein greenlight, conform well with reflectances obtained with modern
microscope-photometers.So the fact that he repeatedly. obtained reflectances that match pyrrhotine
more closelythan cooperite-braggitetends to confirm this error.
3. Schneiderhdhnacceptedthesevalues, and he
and Ramdohr in l93l noted the similarity between
cooperite and pyrrhotine. This was to remain firmly
entrenchedin the literature into the 1980s.
Kingston & El-Dosuky (1982)suggestedthat the
original mistake made by Schneiderhithn(1929)and
repeatedby Schneiderhdhn& Ramdohr (1931),Frick
(1930)and Ramdohr (1969),was that of confusing
braggitewith cooperiteand 'braggite-cooperiteinter€rowths' asbraggiteshowing'pleochroism'. Whereas
we considerthat Schneiderhdhn'soriginal description was that of braggite, not cooperite, this was not
a questionof mistakenidentity betweentwo known
r6l
minerals.As we have shown, the optical properties
of both minerals are distinctive.
The explanationfor the continuedmisidentification of the minerals and the mistakenbelief that they
are rare followed naturally from the acceptanceof
the original mistake. It would, after all, be impossible to identify cooperite from the 1929 or l93L
descriptions.What is more difficult to understand
is how the mistakewasperpetuatedwhen combined
optical and electron-microprobestudiesweremade
of the PGM in the 1970s.
The qualitative and quantitative data in the literature for vysotskite are few and contradictory. Near
end-membervysotskite,we haveshown, can be identified with some confidence. Its reflectanceis not
lower than that of chalcopyrite(Genkin 1968),it is
slightly higher, but nowherenear as high as reported
by Vyalsov (1973). The dispersionof his reflectance
spectrumfor vysotskiteis similar to that of our spectra but is inexplicably7-8Vohigher. Until suchtime
as type material from Noril'sk is thoroughly reexamined,our data for vysotskitecan be taken as
characteristic,asthey are for cooperiteand braggite.
ACKNoWLEDGEMENTS
A.J.C. is grateful to the Anaconda CopperCompany for providing accessto the MinneapolisAdit
and, in particular, to Alistair Turner, Marvin Ratcliff and Mike Zdepski for organizingthe samplecollection and for guidancein the field. We are grateful to Nick Oldfield, formerly a vacation studentin
this Department, for measuringthe reflectancespectra of pyrrhotine and collating the color valuesand
VHN. We thank Drs. A.C. Bishopand A.M. Clark
for their constructivecommentson this work. We
are particularly grateful tri Dr. R.F. Martin for
improvementsto the text. Drs. G.A. Kingston and
J.F.W. Bowles reviewedthe manuscript.
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