1269
The Canadian Mineralogist
Vol. 37, pp.1269-12'13 (1999)

pb2*a(S6.O3S2-)O
stDPtETERS|TE,z(OHlz,A NEWTHIOSULFATE.BEARING
MINERALSPECIESFROMTSUMEB,NAMIBIA
ANDREW C. ROBERTSS
GeologicalSumeyof Canada,601BoothStreet,Ottawa,OntarioKIA 0E8,Canada
MARK A. COOPER eNo FRANK C. HAWTHORNE
Departmentof GeologicalSciences,Universityof Manitoba,Winnipeg,ManitobaR3T2N2, Canada
ALAN J. CRIDDLE ANDCHRIS J. STANLEY
Departmentof Mineralogy,TheNaturalHistoryMuseum,CromwellRoad,LondonSW75BD, tl.K.

CHARLESL. KEY
210-10 Exchange Street, Portland, Maine 04101, U.S.A.
JOHN L. JAMBOR
Depat'tment of Earth and Acean Sciences, University of British Columbia, Vancouver, British Columbia V6T IZ4, Canada

Aesrnecr
Sidpietersite,idealiy^Pb2*4(S6*03S2-)O2(OH)2,istriclinic,Pl,withunit-cellparametersrefinedfrompowderdata:
a7.447(4),b 6.502(4),c 1L206(4) A, ct 1l a.30(3),B 89.51(4),r 89.04(6.1',v 494 4t5) L),i:b:c 1.r453:l:1.7235,i = z. rt strongesreightl i n e s o f t h e X - r a y p o w d e r - d i f f r a c t i o n p a u e r n l d ] n A ( I ) ( h k l ) ) a r e : 1 0 . 1 3 ( 1 0 0 ) ( 0 0 1 ) ,5 . 9 3 ( 5 0 X 0 1 0 ) , 4". 4 0 1 ( 3 5 X 0 1 1 ) ,3 414(100)(003),3.198(80)(0221.2.8S9(35X023,211),
2 805(35X211,014)and2.622(40)(213).The mineral occurson a single
specimen collected from the 40th to 44th levels of the Tsumeb mine, Tsumeb, Namibia, as ill-formed, almost warty, nodular
massesand crystal groups, as earthy massesand, rarely, as free-standing to radiating bladed-crystal aggregates.Masses, groups
and aggregatesnever exceed 3 mm in longest dimension. It is the last mineral to form in an assemblagethat includes smithsonite,zincite, galena, sphalerite, ald very minor quarlz and greenockite Individuai subhedral to almost euhedral crystals are bladed,elongate [011], with a length-to-width ratio of approximately 4:1 and not exceeding 0 2 to 0 3 mm in maximum length. Forms are[001 ] major, and possible very thin { I 00J and {01 1I minor. Cleavageis perpendicular to [01 1]. The mineral is opaque (masses)
to transparent(crystals), beige-cream to off-white to colorless (crystals), with a cream to off-white streak,and a vitreous (crystals)
toearthy(masses)topearlyluster Sidpietersiteissectile,withanunevenfracture,andisnonfluorescent;H(VHNload25g)5767, H(Mohs) l'2; D (calc ) 6.765 g/cml for the idealized formula. In polished section, sidpietersiteis light grey in plane-polarizedreflected light in air, with ubiquitous brilliant white to yellow-brown intemal reflections; the phase is bireflectant and perhaps
slightly pleochroic.Averagedresultsof electron-microprobeanalysesyield PbO 89.55, 56+037.58, 52- 3.035,HzO [1.79], sum
101.955,less O = 52- -1.5 t, total [100.45] wt.7o. The empirical formula, derived from results of the crystal-structure analysis andelectron-microprobe analyses,is Pb2*40e(56*0eTOzsoS2-o97)02 se(OH)2g3,onthe basis of O + 52- = 8 and with (OU.y= 2.69. 1h"
infrared-absorption spectrum shows bands for structural (OH) and thiosulfate The mineral name honors Sidney Pieters,
Windhoek, Namibia, for his outstanding contributions to Namibian mineralogy over the last fifty years.
Keywords: sidpietersite,new mineral species,lead hydroxide-oxide-thiosulfate, X-ray data, electron-microprobe data, infrared
specffoscopy,Tsumeb mine, Tsumeb, Namibia.

Souvarnr
La sidpietersite, dont la formule id6ale serait Pb2*a(S6*O3S2)O2(OH)2, est triclinique, Pl; elle possbde-les parametres
r6ticulaires suivants, affin6es d partir d'un spectre obtenu sur poudre: a'7.447(4), b 6.502(4), c 1I.206(4) A, ct 114.30(3),P 89.51(4),1 89.04(6)', V 494.4(5) A3,a:b'.c1.1453:l:1.7235,2=2.Les huit raiesles plus intensesdu specffede diffraction X

IE-mail address: aroberts@NRCan.ec.ca

t270(m6thodedespoudres)[d en A(O(ftftl)] sont: 10.l3(100X001), s.93(50X010),4.401(35X011), 3.414(100)(003),3.198(80)(022),
2.889(35)(023,211), 2.805(35X211,014) et 2.622(40)(213). Le min6ral a 6t6 d6couvert sur un seul dchantillon pr6lev6 entre lequarantibme et le quarante-quatribmeniveau de la mine de Tsumeb, en Namibie. I1 se pr6senteen massesmal form6es, presqueverruqueusesou nodulaires, en groupes de cristaux et en massesterreuses,et, plus rarement, en aggr6gatsde cristaux lamellaires
isol6s ou radi6s. Les masses,groupes et aggr6gatsne d6passentpas 3 mm en longueur maximale Il s'agit du dernier min6ral ds'Otreform6 d'un assemblagequi comprend smithsonite, zincite, galene,et sphal6rite,avec quartz_etgreenockite accessoires.Les
cristaux individuels sont sub-idiomorphes d presqu'idiomorphes, lamellaires, allong6s sur [011], avec un rapport longueur I
largeur d'environ 4:l et ne d6passantpas 0.2 h 0.3 mm en longueur maximale. Les fo_rmesobserv6es sont {001J majeure,possiblement {100} trds mince, et {011} mineure. Le clivage est perpendiculaire d [011] Le min6ral est opaque (masses) dtransparent (cristaux), beige-crbme d blanchAtre d incolore (cristaux), avec une rayure crdme I blanchdtre et un 6clat vitreux(cristaux) ou terreux (masses)d nacr6. La sidpietersiteest sectile, avec une fracture in6gale, et non fluorescente; sa duretd (VHN
mesur6 avec une massede 25 g) va de 57 h 6i, et la duret6 sur l'6che1lede Mohs est de I d 2. La densit6 calcul6e est 6.?65 g/cmr
selon la formule id6ale. En lame polie, la sidpietersite est gris pale en lumibre polaris6e dans I'air, avec des r6flexions intemes
brillantes ubiquistes blanchesijaune brundtre. Elle pourrait 6tre l6gdrementpl6ochroique. Une moyenne des r6sultatsd'analyses
obtenues aveCune microsonde electronique donne iUO SS.SS,56*03 7.58, 52- 3.035, H2O |l.79l, total 101.955, moins O = 52
-1.5 1, 6gal d 100.457oen poids. La formule empirique, d6riv6e des r6sultatsd'une 6bauchede la stmcture cristalline et d'analyses
par microsonde 6lectronique, serait Pb2+ase(Scoq?OzgoS2-oqr)Ozos(OH)zo:, sur une base de O + 52- = 8 et (OH) = 2.0O'Le
spectred'absorption infra-rouge contient des bandes typiques du groupe (OH) et du thiosulfate dans la structure. Le nom honore
Sidney Pieters,de Windhoek, en Namibie, et signale sescontributions extraordinaires d la mindralogie de la Namibie au cours descinquante demidres ann6es.
(Traduit par la R6daction)
Mots-clds: sidpietersite,nouvelle espdcemin6rale, thiosulfate et oxyde de plomb hydroxyld, donn6esde diffraction X, donn6esde
microsonde 6lectronique, spectroscopieinfta-rouge, mine de Tsumeb, Namibie.

INrnooucrroN
The Tsumeb mine, Tsumeb, Namibia (lat. 19"S,long. 18'E) has been a prolific source of new mineral
species. Here we describe yet another new phase,
sidpietersite, from this classic mineral locality. Megascopically identified as a potentially new speciesby oneof us (C.L.K.), the nondescript phase occurs on and isassociatedwith pale pink to colorless crystals of smithsonite; this samplehad beenacquired from a local minerin 1991. This particular specimenwas deemedunusual
and worthy of further study for two reasons: (1) thesmithsonite crystals are coated with an unidentified (atthat time) opaque tan-colored crust, and (2) the smithsonite crystals grew on a crystalline galena-sphalerite
matrix. The combination of smithsonite with a sulfidebearing matrix at Tsumeb is, historically, a strong indicator that additional rare and unusual phases may be inclose association.This particular specimen was one ofcountless samplesof smithsonite collected in the early90s from the 4fth to 44th levels of the Tsumeb mine.
Subsequent X-ray powder diffraction, coupled withscanning electron microscopy (SEM) and energy-dispersion analyses,proved that this nondescript phase isindeed new to science.
The mineral is named sidpietersite in honor of
Sidney ("Sid") Pieters(b. 1920) of Windhoek, Namibia,
for his outstanding contributions to Namibian mineralogy. Mister Pieters has been prospecting, mining, collecting, cutting, buying and selling the fabulous mineraltreasures from this area for more than half a century,
and a particularly fine tribute to him has been written
by Key (1977). The mineral and mineral name havebeen approved by the Commission on New Minerals andMineral Names, IMA. Holotype material is housed in
the Systematic Reference Series of the National Mineral Collection at the Geological Survey of Canada,
Ottawa, Ontario, under cataloguenumber NMC 68076.
The polished section used for the attempted reflectancestudy and for the Vickers microhardness determinations
is preserved at The Natural History Museum, Great Britain. as BM1998. 36.
OccunnrNcB ANDAssocIATED MTNSRALs
Sidpietersite was identified on a single sample thathas been split for scientific study; the new mineral israre, and we estimate that onlyl-Z milligrams of pure
sidpietersite are currently available. The matrix of theoriginal piece consists of a mixture of fine-grained galena and sphalerite. Perched on the matrix, and by far
the most visually striking mineral in the assemblage,are
pale pink to colorless crystals of smithsonite, 7-8 mm
in size, that have been partly coated by an opaque tancolored crust of zincite. Minor greenockite and quartz
also are present on and in the matrix. Sidpietersite isconsidered to be secondary after galena; masseshave
been observedboth on the galena-sphaleritematrix andon the zincite-coated crystals of smithsonite, and it isobviously the last mineral to form in this environment.
We postulate that thiosulfate formation occurred during
an intermediate stage of oxidation as the assemblage
moved toward complete oxidation and subsequentformation of sulfate after initial deposition of the sulfide.
Sidpietersite is the first confirmed thiosulfate-bearing
mineral known to occur at the Tsumeb deposit.

1271

A NEW THIOSULFATE-BEARING MINERAL SPECIESFROM TSUMEB. NAMIBIA

Psysrcal PnopeRrrcs
Sidpietersite occurs as ill-formed, almost warty,
nodular masses and crystal groups, as earthy masses
and, rarely, as free-standing to radiating aggregatesof
bladed crystals (Figs. lA, B) Masses, groups and aggregatesnever exceed 3 mm in maximum dimension,and most are typically much smaller. Individual bladedsubhedralto euhedralcrystalsdo not exceedO2toO3
mm in maximum length, are elongate along [011], andhave a length-to-width ratio of approximately 4:1. Some
crystals are platy on {001}. Cleavage is perpendicular
to the elongation direction [011]. These crystals are too
small to determine crystal forms by optical goniometry;
single-crystal precessionstudiesindicate that the domrnant form is { 001 }, and other possible minor forms are{100} and {0ll}. Twinning was not observedmegascopically nor in X-ray single-crystal studies.The colorof the mineral varies from beige-cream to off-white tocolorless,dependingon the nature of the material. Crys-

tals are colorless, and the streakis cream-coloredto offwhite. The luster varies from vitreous (for crystals) toearthy (for masses) to somewhat pearly. Crystals aretransparent, whereas masses are opaque. Sidpietersite
is sectile,with an uneven fracture, and is nonfluorescent
under both long- and short-wave ultraviolet radiation.
The density could not be measuredbecauseof the small
size of available crystals and dearth of material. The
calculateddensity, on the basis ofthe idealized formula
and unit-cell parametersderived from an investigationof the crystal structure (Cooper & Hawthorne 1999), is6.765 glcm3.The mineral is very soft; crystals have an
estimated Mohs hardness of 2 or less. Three Vickers
microhardnessindentations (25 g load) gave a range of
57-67 oncomposite grains usedfor the attemptedstudyof reflectance; this translatesto a Mohs hardnessrangeof 1-2. These measurableindentations were perfect inshapeand only slightly fractured. In dilute HCl, there is
only an initial effervescence,after which the material
turns to a chalky white color over a few minutes.
X-Rav Cnvsrer-r.ocnapnv
Preliminary single-crystal precession photos of acrystal of sidpietersite showed that the symmetry is triclinic with space-groupchoices Pl or Pl. The crystal-

TABI-E I X.RAYPOWDER-DIFFRACTIONDATAFORSIDPIETERSITE
I*

d&*\

d]\&\

10212
100 10 13
639
6393
30
s924
50
5 93
5o2
so23
4 Err
4 81
5
' 35
439t
4401
J
4 160
4 r'71*20
3 681 3677
*20
3 517
3 513
* 100 34t4
3404
3 270
3 260
3
*80
3 198
3 t96
+30
3 118
3 rzt
2.994
30b , o1a
2.952
2.904
35
z 66'
2.a7s
2.809
Jf
I uut
2.801
*40
2622
2622
x20
2 575
2 580
120
2 557
2 553
2459
3
2465
2401
2409
I
*5
2343
2343
2298
2295
2293
l5b
2281
223s
3
2230
*20
2144
2143
120
2123
2122
2 089
5
2092

Ftc. 1. A. SEM photomicrograph of a free-standing crystalof sidpietersite Scale bar: 20 pm. B SEM photomicrograph of an aggregateof sidpietersite crystals Scale bar:
50 um

I*

hH
001
0l 1
010
otz
TrT
0ll
114
013
201
003
012
022
210
212
020
o2i
2ll
vr
ol4
211
021
004
221
222
220
223
220
-223
0l 5
023
2M
zo4

dA@)

l0
*3
*lo

1 915
1886
20

I tr)o

3

I uro

*10
*
15
+5
+10

I 816
| 784
1723
| 702

10b

1.665

I 64E
a3

1 630
1618

*3

I @o
3
r5

r <o,
| 575

)

I ))/

d|\&.),thl

2.074 2r3
2.057 0t4
1914 222
1886 %A
1.861 400t EsE 232
1.83E V26
l 837 4ol
I 817 214
| 784 zr4
1724 015
006| 702| 674 036
1665 zr3
233
I 661| 654 235
| 649 216
1645 227
403
| 645
231
| 639
1 630
0u
422
I 61t
1.611 M2
l610
231
-44
t.599
1.598 044
017
| 575
423
rsa
-420
1.560
1.560 o4T

I ss6 vrz

114.6 m DebyFschqs powda mm employingNi-fihoed Cu ndiation (lCuKa = L5417EA). Int@ities eEtimt€dvirually;b = broadline Not @rwt€d forshinkage ud no htmal stmdud used; * = lin6 usedfor mit+ll refif,@eftb 6.s02(4),c rr.206(4)A d 1Ia 30(3), p 89 51(a)' Y
tod*ed on a 7.44"1(4),E904(6)"

r2"/2structure determination (Cooper & Hawthorne 1999)
indicated that Pl is the correct space-group.The unitcell parameters,given below, are for the same orientation as in the structural study.
Unit-cell parameters,a 7.447(4), b 6.502(4), c
11.206(4).A,o 114.30(3),I 89.s1(4),r 89.04(6)",V
494.4(5) A', a:b:c 1.1453:l:1.7235,and,Z = 2, wererefined from22 powder reflections representingd valuesbetween4.401 and 1.575A for which unambiguousindexing was possible on the basis of the calculated
intensities derived from the crystal structure. A fully
indexed powder pattem is presented in Table 1. The
powder data are not similar to any other inorganic phaselisted in the Powder Diffraction File. Sidpietersiteis thesecond confirmed thiosulfate found in nature, the firstbeing bazhenovite (Chesnokov er al. 1987).
Csplrrsrnv
Sidpietersite crystals were analyzed with a Cameca
SX-50 electron microprobe, utilizing an operating voltage of 15 kV, a beam current of 20 nA, and a beam 5
pm in diameter. Natural galena was used as a standard
for both Pb and S. An energy-dispersionscanindicatedthe absenceof any other elementswith atomic number
greater than 9 except those reported here. Ca and Cl
were sought, but are below detection limits. Data werecorrected using the PAP procedure of Pouchou &Pichoir (1984, 1985). The valence statesfor both Pb andS, as well as the number of O atoms, Weredeterminedby crystal-structure analysis (Cooper & Hawthorne1999) prior to final interpretation ofthe electron-microprobe results. The paucity of pure material prevented
quantitative determination qf HzO. However, the presence of H as (OH) was confirped both by crystal-strucfure analysis and powder infrared-absorption study; theformula was therefore calculated to give 2(OH). The
averageofflve determinations(and standarddeviations)
gavePb 83.13(1.12)and S 6.07(0.10)wl Vo.After converting Pb to PbO, partitioning S to 56+and 52- in a 1:1
ratio, converting 56+ to 56+03, and calculating HzO togive 2(OH), the analysesgave:PbO 89.55,So+O37.58,
Sr- 3.035,H2O |.791, sum 101.955,lessO = Sz--1.51,
total [100.45] wt. 7o. With O + 52-= 8. the empiricalformula for this lead hydroxide-oxide-thiosulfate is
Pb2+a.6e(S6+6
The ideal fore7O2e6S21,e7)O2.os(OH)z.o.t.
mula, Pb2*a(S6+O3S2JO2(OH)2,
requires PbO 88.66,
s6to3 7.95, s2- 3.18,H2O 1.79,sum 101.58,lessO =

@
o

-.
o
a

100

(E
F
E
a

(s
80

u

lto
ilq
!o

\

N

o@
o
5
ID
q
OJ

4000

2000

3000

wcvenumber
lcm']
Ftc. 2.

Infrared-absorption spectrum for sidpietersite.

o@
o\
aJ

1000

A NEW THIOSIJ'I-FATE-BEARINGMINERAL SPECIESFROM TSUMEB. NAMIBIA

52- -1.58, total 100.00wt. Vo.Furtherdetailsreeardins
the thiosulfatetopology are addressedin the compani
ion crystal-structurepaper (Cooper & Hawthorne t999).
It is, however, without doubt that this new mineral
would have been described as a sulfate and not a thiosulfate if not for the structural and infrared-absomtion
results; this is another example of the use of crystalstructure analysis to determine the exact chemical formula of a complex mineral species (cl Hawthorne &Grice 1990).
INpRennnSpBcrnoscopy

1273

the erroneousreflectance data. The mineral is bireflectant
and perhaps slightly pleochroic (uncertainty due to thepossible directional contribution from internal reflections).
AcruowleocBl,rBvrs
The authors thank A. Tsai (GSC) for the SEM photomicrographsreproducedin Figure 1, M. Clarke (GSC)
for drafting Figure 2, E. Moffatt (Canadian Conservation Institute) for the infrared spectrum of sidpietersite,and Drs. John M. Hughes, Jeffrey E. Post and Robert F.
Martin for reviews of the manuscript.

The proceduresfor acquiring the infrared-absorption
spectrum for sidpietersiteare identical to those used to
RsFBReNcBs
obtain the spectrumof mcalpineite (Robertset al. 1994)
and are not repeated here. The sample was analyzed, CHrsNoxov,B.V., Por-yerov,V.O. & BusuverrN, A.F(1987):Bazhenovite
CaS5. 62510: . 6Ca(OH)z. 20H2Ousing a Bomem Michelson MB-l00 FTIR spectrometera new mineral.Zap. Vses.Mineral. Obshchest.
116,737equipped with a wide-band mercury-cadmium telluride743(in Russ.).
detector. The transmittance spectrum (Fig. 2) shows amedium sffong, somewhat broad and asymmetric band
Cooppn,M.A. & HAwrHorur's,F.C (1999):The structuretothat has peaks at 3508 and 3542 cm-t attributable to O(56*0352) 02 (OH)2,a novel
pologyof sidpietersite,
Pb2+4
H stretching in the hydroxyl groups. Characteristic
thiosulfatestructure.Can M ineral. 37, I2'75- 1282.
bandsfor thiosulfate are strong to medium strong, sharp,
and are centeredat 1156, 1063,971,709.630 and, HAwTHoRNE,
F.C. & Gnrcr, J D (1990):Crystal-structure
analysisas a chemicalanalyticalmethod:applicationto
527 cm-r
light elements.Can.Mineral-28,693-702
Oprrcar- Pnoppnurs
Krv, C.L. (1977):Sid Pieters:a personalitysketchMineral.
Rec.8(3),54
In polished section,the mineral forms compact clusters of elongatecrystals (aspectratio 1:8 to 1:12), the Poucsou,J.-L.& PrcHon,F. (1984):
A newmodelfor quantilargest individuals of which are about 5 X 40 pm intativeanalysis.l Applicationto theanalysisofhomogeneplane section. The cluster polished for optical study rs
oussamples.
ln Recherche
Adrospatiale3, 13-38.
0.2 X 0.4 mm. Its specularreflectance (low, about 12(1985): "PAP" (phi-rho-z) procedure
74Vo and.n about 2) is almost entirely "swamped" by& _for improved quantitative microanalysis. In Microbeam
brilliant white intemal reflections.Its surfacereflectanceAnalysis (J.T. Amstrong, ed.). San Francisco Press, Sanis light grey. Also present,where the cluster ofcrystals
Francisco, California ( I 04- 106).
is thickest, are yellow-brown intemal reflections. It is
impossible, because of the intemal reflections, to obRonnnrs, A C , Encrr, T.S., Cnroorr, A.J., JoNBs, G C.,
tain reliable reflectance data with the crystals available.
Wrrunus, R.S., CunBroN, F F., II & JENSEN,
M.C. (1994):
Measurementswere made in air and in oil as a matter ofMcalpineite, Cu3TeO6.H2O, a new mineral from theroutine but, as expected,there were substantialdiscrepMcAlpine mine, Tuolumne County, Califomia and fromancies between the refringencesin the two media. Thrs
the Centennial Eureka mine. Juab Countv. Utah. Mineral.
proved that a substantial componenr of diffuse reflecMag. 58,4l'7- 424.
tance was contributing to the measurements,and that
this component was greaterfor the immersed specimen. ReceiveelOctober 3 I, 1998,revisedmnnuscript acceptedJanuBecause of the above, we have chosen not to oublishary 18,1999.