American Mineralogist, Volume 81, pages238-243, 1996

The crystal structure of namuwite, a mineral with Zn in tetrahedral and octahedral
coordination,and its relationship to the synthetic basic zinc sulfates
Lnn A. Gnolr
Department of Geological Sciences,University of British Columbia, Vancouver, British ColumbiaY6T lZ4, Canada

Ansrnlcr
The crystal structureof namuwite, (Zn,Cu)oSOo(OH)6.4HrO,
trigonal, a: 8.331(6),c: 10.54(1)A, V : 634(l) 43, P3, Z :2, has been solved by direct methodsand refined
to an R index of 7.90/ousing MoKa X-ray data. The fundamental building unit of thestructure is a gently modulated sheet of edge-sharing(Zn,Cu) octahedra parallel to (001),
with one vacancy for every six occupied octahedralsites.The sheetsare decoratedon bothsidesby SOoand [Zn(OH).(HrO)]r tetrahedra;the latter are located above and below the(T-O-T) layersare held
vacant octahedralsites.Adjacent tetrahedral-octahedral-tetrahedral
together by hydrogen bonds to interstitial HrO molecules.
In synthetic ZnoSOo(OH)u'4HrO successiveoctahedral layers are displaced by th.la +bl. In namuwite, this is not the case,which suggeststhat the presenceof Cu in the octahedral sites prevents this from happening. Namuwite undergoesa reversible transition,
probably to the pentahydratephase,at approximately room temperature.
INrnooucrroN
Namuwite, (Zn,Cu)oSOo(oH)5.4Hro, was first described by Bevins et al. (1982) from the Aberllyn leadmine in Wales, where it occurs with hydrozincite on a
breccia composed of slate fragments cementedby sphalerite, quartz, calcite, and ankerite. Namuwite has also
been described from the Dyfngwm, Dylife, and Smallcleugh mines in Wales (Livingstone et al. 1990); fromVincenza, Italy (Bertoldi et al. 1984); Laurion, Greece(Rewitzer and Hochleitner 1989); Tsumeb, Namibia(Schnorrer-Kdhler 1986); and from several localities inGermany (Schnorrer-Kbhler et al. 1988). The material
studied by Bevins et al. (1982) occurred as aggregatesof
perfect to subperfecthexagonalplatesup to 60 pm across.
Their indexing of powder X-ray diffraction data was performed on the basisof a hexagonalcell with a: 8.29 andc : 10.50 A. Systematicabsencessuggestedthat possible
space groups are P6, P6, P6/m, P622, P6mm, P6m2,
P62m, and,P6/mmm (Bevinset al. 1982).
The samples used in this study are from Granetal in
the Harz Mountains, Germany. They occur with two
habits: as tabular crystals with a hexagonal outline andas needlelike crystals with a trigonal outline and perfectcleavageperpendicular to the needle axis. The crystals
were previously studied by the late Renate von Hodenberg, who found evidenceof a phasetransition between
0 and 45 "C, in which the c dimension increasedfrom
10.4 to 10.9 A (W. Krause,personalcommunication).
This work is part of a long-term study of the copper
oxysalt minerals. Namuwite is of particular interest becauseit is hydrated, and our understanding of hydrated
structures has generally progressedat a slower rate than
our understanding of anhydrous compounds, as is evi102-0238$05.00
0003-004x/96l0

dent from the compendium of copper oxysalt structures
presentedby Eby and Hawthorne (1993)'

crrnvrrclr-

ANALYSESElectron microprobe analyseswere conducted with a
CamecaSX-50 instrument equippedwith one energy-dispersion (ED) spectrometer and four automated wavelength-dispersion(WD)spectrometers.EDspectrashowed
no elementsother than Zn,Cu, S, and O. All WD analyseswere conducted with an acceleratingpotential of 15
kV and a sample current of 10 nA. Counts for each element were collectedfor 20 s. The beam was intentionallydefocusedto a diameter of 25 pm to minimize damage
to the sample. Back-scattered electron (BSE) imaging
showed no compositional zoning or intergrowths. The
standardsused were sphalerite (Zn, S) and covellite (Cu).
Data reduction was achieved using conventional ZAF
techniques.The analyses,shown in Table l, are normalized on the basis of five cations per formula unit. The
high totals associatedwith the analysesof this study are
due to the loss of HrO through volatilization during datacollection. The low totals of the analysesfrom Livingstone et al. (1990) suggestthat one ofthe cations is underdetermined; by comparison with the other analyses,
this is probably Zn.The analysisof Bevins et al. (1982)
used atomic absorption spectrometry (AAS, for Zn andCu) and thermogravimetric analysis (TGA, for S andHrO).
Analyses of the crystals used in this study show approximately 20 ato/o
Cu in the Zn sites.This is appreciably
less than the 3lo/oreported by Bevins et al. (1982) andthe 43o/ogiven by Livingstone et al. (1990), which are

238

239

GROAT: CRYSTAL STRUCTURE OF NAMUWITETABLE1. Chemicalcompositionof namuwite
Livingstoneet al.

ZnO
CuO
SO"
HrO
Total
Zn
Cu
b

o

Thisstudy-(1990r.

53.02
13.16
14.04
25.03+
105.25

32.28
23.48
13.73
21.77+
91.26
2.30
1.7'l
0.99
13.99

3.28
0.83
0.88
13.77

Bevins et al.

1800(1s82)t
37.8
22.0
14.9
24.5
99.2
2.51
1.49
1.00
14.34

1600
't400
.

1200

o
z
u

1000

F

z-

800

Notej Oxides are in weight percent; analyses are normalized on five
cations per formula unit.
- Average fiveofelectron microprobe analyses.
". Average of three electron microprobeanalyses.
t Zn and Cu by AAS; SOs and H.O by TGA.
* Calculatedby stoichiometry,assumingfour H"O moleculesperformula
unit.
os@@
rOSO

s@o
oFN
N O @
- N

both higher than the maximum of 33o/oreported by Glibert (1977) for synthetic(Zn,Cu)oSOo(OH)6.
4H,O.
Srnucrunn

DETERMTNATToN

Numerous crystalsof both tabular and needlelikemorphologies were mounted on a Nicolet R3rn automatedfour-circle diffractometer equipped with a molybdenumtarget X-ray tube (operating at 55 kV, 40 mA) and agraphite crystal monochromator mounted with equatorial geometry. Random orientation photographs of thecrystals showed faint rings (similar to those seenin photographsof microcrystalline aggregates)
and a small number of broad, very diffuse spots. In most casesthe diffractometer was unable to center on these reflections.
However, random orientation photographsofone tabular
crystal showed reflectionsthat appearedless diffi.rsethan
those of the others, and the centers of nine of these reflections were determined. Least-squaresrefinement ofthese reflections produced hexagonally constrained celldimensions,a : 8.331(6)and c : 10.54(1)A.
Intensity data were collected in the 0-20 scan mode,using 96 stepswith a scanrangefrom [2d(MoKa,) - 1.1]
to 120(MoKar) + l.lF and a variable scan rate between
3.91 and 29.3/min dependingon the intensity of an initial one-second count at the center of the scan range.
Backgrounds were measured for one-half the scan timeat the beginning and end of each scan. Two standard
reflections were monitored every 46 measurementsto
check for stability and constancy of crystal alignment.
Eventually, it was discovered that the intensities of reflections collectedovernight were much lower than thosecollected during the day. The count rate (Fig. l) would
begin falling every day at approximately 23:00, reaching
minimum levels of approximately zero counts per second
about 6 h later. By 07:00, all intensities had returned totheir normal daytime levels.
It was obvious that the crystal was undergoing a reversible transition. Random orientation photographs
taken in the middle of the night showeddiffuse rings andt@NOOOTO
NNNOOt60
TSFOoNbO-NN

TIME

Frctrnn 1. Time (hours:minutes)
vs. intensity(countsper
second)for checkreflections131(top) ar'd 320 (bottom),fromthe first namuwitedatacollection.
a few very weak spots. The difractometer was unable tocenter on any ofthese reflections.
Further investigation revealed that every room in the(new) building had a heat source controlled by a centralcomputer. To save energy, these were turned off every
day at 20:00. By 05:00, when the heat sourceswere turnedon again, the room temperature was at a minimum, andit would take about 2 h for the room to warm up to thenormal daytime temperatureof approximately 24'C. Reflection intensities would begin falling at about 23:00,
when the room temperaturewas approximately 22'C,
and reach a minimum at about 05:00, when the room
was coldest.Presumably,the building would heat up rapidly
once the heat sourceswere turned on, which would account for the return to normal intensity levels by 07:00.
After completion of the first data collection, a second
data collection was initiated at a different time of day toobtain reflectionsmissing from the original data set. The
final merged dalaset comprised 2065 reflections representingtwo asymmetric units from 3-60' 20. Scalefactors
were refined independently for data from eachcollection.
Absorption corrections had no effect on the R (merging)index (presumably becausethe reflections were so weak)
and were not used. After correction for Lorentz, polarization, and background effects,the data were averaged
and reduced to structure factors. The R index for the
averagingprocedurewas 8.10/0.Of the 1242 unique reflections, 493 were classedas observed, i.e., with magnitudes greater than six standard deviations on the basisof counting statistics.Miscellaneousinformation pertaining to data collection is given in Table 2.
The SiemensSHELXTL PC system of programs wasused throughout this work. Scatteringcurves for neutral
atoms together with anomalous dispersion coemcients

240

GROAT: CRYSTAL STRUCTURE OF NAMUWITETABLE2. Miscellaneousinformationfor namuwite
a (A)
c (A)
v(A")
Spacegroup
Crystalsize(mm)Cellcontent

8.330(1)10.540(2)634(1)

n0.08 x 0.08 x 0.112[(Zn,Cu)oSO.(OH)6.
4H,O]

Radiation;mono
Uniqueintensities
N o .o f l F . l > 6 o
R.* (%)
F.*" (%)

MoKc;graphite
1242
485
7.9
9.3

were taken from the International Tablesfor X-ray Crys- in a trigonal antiprism). However, attempts to take pretallography, volume 4 (Ibers and Hamilton 1974).R and cessionand Gandolfi photographs of these crystals were
R* indices are ofthe conventional form and are expressed unsuccessful,in that no reflections were recorded on thefilms, even after extremely long exposuretimes.
as percentages.
Intensity distributions and systematic absencessugDrscnrr"rroN oF THE srRUcruRE
gestedthe spacegroup P3. The structure was solved bydirect methods; all three cation positions were revealed
Coordination polyhedra of cations in the namuwite
by the solution with the best figure of merit. Five of the structure are shown in Figure 2. The S atom, at special
six anion positions were located on difference-Fourier position 2dwith symmetry 3, is tetrahedrally coordinatedmaps, and the structure was refined to an R index of by four O atoms. The mean S-O distance (1.45 A) isfor an isotropic displacementmodel. Conversion similar to that reported for other sulfate minerals. The
19.50/oto anisotropic displacementfactors for the cations result- Znl aIom, at special position 2c with symmetry 3, is teted in convergenceat an R index of 9.2o/o.The final anion rahedrally coordinated by three OH groups and one HrO
position (OW2) was located on a difference-Fouriermap, molecule. The bond lengthsare statistically identical; theBecauseof the poor mean value of l.95 A is very closeto the I4lZn-Odistanceand the R index was reducedto 7.9o/o.
quality of the data, attempts to refine anisotropic dis- (1.97 A) calculatedusing ionic radii from Shannon(1976).
placement factors for the anions were unsuccessful.The The OH-Znl-OH and OH-Znl-OW anglesare 112.9and
structure was also refined in the spacegroups Pl and P3 105.7', respectively.Becauseof this, the tetrahedraarewith no improvement to the final R index or to the struc- slightly flattened parallel to c. The Znl site must be comtural model. Refinement of the occupancy of the OW2 pletely occupied by Znz+ becauseCu2+does not occur inposition suggestedthat the site is fully occupied. Bond- tetrahedral coordination (Eby and Hawthorne 1993); in
valencecalculationswere usedto identify the anionic spe- addition, Zn2* is thought to prefer tetrahedral over occies (Table 3). Positional coordinatesand anisotropic and tahedral coordination. In sclarite, another mineral withequivalent isotropic displacementfactors are given in Ta- both fourfold- and sixfold-coordinated Zn2+, the tetra'ble 4. Selectedinteratomic distancesand anglesare given hedral sites are fully occupied by Znz+, and the defrcit inin Table 5. The observedand calculatedstructure factors the octahedralsitesis made up by Mn2* (Grice and Dunn
are presentedin Table 6.'l 989).
Note that needlelike crystals from Granetal may have
The Zn2 atom, at the general position 69, is coordia different structure than the one described here. The nated by five OH groups and one O atom (O1) forming
morphology (trigonal prismatic) suggeststhat the space a distorted octahedron. Bond lengths from Zn2 to thegroup is different (the presenceof a 3 axis would result four equatorial and one apical OH groups show little
variation (2.02-2.10 A and 2.16 L). However, the dis' A copyof Table6 maybe orderedasDocumentAM-96-604 tance between the Zn2 cation and the apical Ol atom isfrom theBusinessSocietyof America,1015 much greater (2.43 A). In general, when the Zn2+ ion
Ofrce,Mineralogical
DC 20036,U.S.A. adopts octahedralcoordination the result is a regular polEighteenth
StreetNw, Suite601,Washington,
Pleaseremit $5.00in advancefor the microfiche.
yhedron; however,the electronmicroprobe analysesshowthat approximately one-third of the Zn2 sitesin the structure contain Cu2+.It is anticipated that the distortion of
the Zn2 octahedron results from the Jahn-Teller effect.
TABLE
3. Bond-valencet
arranoementin namuwite
TheZn2 octahedra share edgesto form Cdlr-type sheets
Zn1
Zn2
Total
parallel to (001) (Fig. 3), with one vacancy for every six
o1
1.8(2)
2.2(2)
0'135(7)'3+
occupied octahedral sites. Lengths ofth-e five shared oc02
1.s7(9),3
1.s7(9)ltahedraledgesvary from 2.685 to 2.972 A, and the lengths
1.16(2)oH10.41(1)0.37(1)ofthe sevenunsharededgesrange from 3.002 to 3.100
0.38(2)
A. Becauseof the distortion introduced by Cu'z+in the1.11(4)oH20.50(3)'.I
0.33(2)Zn2 sites, the layers are slightly corrugated. The sheetso.28(2)ow1
0.51(6)0.51(6)are decoratedon both sidesby SOoand ZnlSo($: unow2
specifiedanion) tetrahedra(Fig. 4). Each SOotetrahedron
Total
2.01(8)1.91(4)
6.5(3)
sharesan apical Ol atom with three octahedra,and each. Calculatedfrom the curvesof Brown (1981).
Znl6, tetrahedron sharesthree apical OH2 groups with

241

GROAT: CRYSTAL STRUCTURE OF NAMUWITETABLE4, Positionalcoordinatesand anisotropicand equivalentisotropicdisplacementfactors
uu
Zn1
Zn2
S
01
02
oH1
oH2

ow1
ow2

ur"

us

u8

0
0
0.1672(6)299(14) 138(11) 138(11) 621(36)78(10) 669(17) -e(13)0.7134(4)0.0006(4)285(9)
88(10)
2/50h271(28) 271(28) 557(76)0.2768(141 366(31)2hh0.1429(26) 242(58)0.8282(29)0.5007(29)0.3239(20)626(58)-0.3730(19)0.1083(19) -0.0861(12) 216(27)-0.1891(22)0.0609(21) 0.1169(14) 294(341000.3519(41) 645(101)
0.7173(62) 0.1077(64) 0.6031(36) 179O(172)00.1318(4)

ur"

U,"

0
-e(15)
0

69(6)
28(s)
136(14)

Notei 4 values were multipliedby 104

three of the six octahedra surrounding the vacant octahedral site. In effect, each vacant site sharestwo (opposite) faceswiLhZr'lA4 tetrahedra.
The three OH2 moleculesforming the baseof the ZrLlOA
tetrahedron are involved in hydrogenbonding to the basal Ol atoms of the sulfate molecules. The tetrahedraloctahedral-tetrahedral(T-O-T) layers in namuwite areheld together by hydrogen bonds to the interstitial OW2
molecules. Details of the hydrogen bonding remain obscure,although the bonding probably involves 02 atoms(2.97 and 3.05 A from OW2), the OWI molecule(2.96
A from OW2), or both. It is possible that the hydrogenbonding involves some disorder, as in the case of syn(Bearet al. 1986).
theticZnoSO4OHr.5HrO
The [Zn(OH)r(HrO)] tetrahedron presentin namuwite
also occursin syntheticZnr(OH)r(NO3)2.2H2O(Stiihlin
and Oswald 1970).Sheetsof edge-sharingoctahedrawith
one vacancy for every six filled sites are also apparent in
the chalcophanite structure (Post and Appleman 1988).
Other minerals with both octahedrally and tetrahedrally
coordinated essential Zn include hydrozincite (Ghose
1964),sclarite(Griceand Dunn 1989),and simonkolleite(Allman 1968). In each case the fundamental building
unit ofthe structureis a sheetofoctahedra with vacancies
that share upper and lower faces with Zn6o tetrahedra.
The ratio of filled to vacant sites is 4:2 for hydrozincite
and 4: I for sclarite and simonkolleite. The layersare held

together by CO, groups in hydrozincire, Zngo and CO,
goups in sclarite, and by a layer of HrO molecules insimonkolleite.
Nllvruwrrt

AND THE syNTHETrc BAsrc zrNc
SULFATES

The basic zinc sulfates have the general formula
Zn,SOoOHr.mHrO, where | < n < 7 and 0 < m < 5.
Namuwite is isomorphous with synthetic ZnoSOuOHr'
4HrO. Both compoundshave similar a-cell dimensions,but the c parameterin namuwite is somewhatlarger.Frias
Ferreira da Rocha and Glibert (1972) and Gliben (1977)
gavevaluesof 10.274and 10.32A forZnoSOoOH)r'4HrO.
The reported values for namuwite are 10.50 (Bevins etal. 1982)and 10.54 A (ttris study). The differencecould
be due to the presenceofCu, which distorts the octahedral sheet.However, Glibert (1977) found that the presenceof Cu in synthetic (Zn,Cu)oSOoOH2'4H2Ohad little
effecton c; with 6 atoloCu, c: 10.318(6)A, and with 3l
ato/oCu, c : 10.316(7) A.
The structureof Zn.SO,OH2'4H2Ohas not been described.However, the structureof ZnoSOrOHr'5H'O wassolved by Bear et al. (1986). Precessionphotographs(A)andangles(') in namuwite
TmLe5. Interatomic
distances
Z11-OH2 x 3
Zn1-OWl(zn1-o)1.96(2)
1.95(4)
1.95

Zn2-O1"
Zn2-OH1o
Zn2-OH1"
Zn2-OH1o
Zn2-OH2b
Z12-OH2"\Zn2-O)

2.43(212.02(11
2.06(1)2.05(2)2.10(2)2.16(2)2.14

s-ol
S-O2x 3

1.41(3)
1.46(2)
1.45

(s-o)

OH2-ZI1-OH2. x 3
OH2-Zn1-OW1x 3(O-Zn1-O)

112.9(s)105.7(5)109.3

01.-Zn2-OH1b
O1"-zn2-oH1.
01"-Zn2-OHld
O1.-Znz-OH2hoHlb-Zn2-OH1d
OHlb-Zn2-OH2b
OHlb-zn2-OH2.
oHlaZn2-OH1d
OH1"-Zn2-OH2b
OH1LZn2-OH2.
oHld-Zn2-OH2"
OH2b-zn2-OH2.
<O-Zn?-O\

83.3(6)94.3(6)82.6(6)84.9(6)95.1(8)84.7(6)100.3(6)81.7(7198.1(6)82.2(6)99.0(7)93.5(9)90.0

O1-S-O2x3
O 2 - S - O ?x 3

1 0 9 . 9 ( 01 )109.0(1.l)(o-s-o)109.5

N o t e : E q u i v a l e npto s i t i o n sa: : l z , 4 t , 2 ; b : 7 , y + ' t , 2 ; c : r ,+ 1, zi d : y, y - x, zi e : y, x - y, zil : y + 1, x - y, z.

x - y

Frcunr 2. Details of the coordination polyhedra rn namuwite; S and Znl are line shaded,Zn2 is dot shaded,O 1 and 02
are open circles, OHI and OH2 are crosshatched,and OWI ispartially line shaded.

242

GROAT: CRYSTAL STRUCTURE OF NAMUWITE

Frcunn3. The structureof namuwiteprojectedonto (001).
The S tetrahedraare shadedwith crosses,the Znl tetrahedrawith parallellines,and theZn2 octahedrawith dots.
showed that the true symmetry is triclinic, with a pronounced pseudohexagonalsubstructure.Refined cell parameters, from X-ray powder diffraction data, were a :8 . 3 5 4 ( 2 ) , 0: 8 . 3 5 0 ( 2 )c, : l l . 0 0 l ( 2 ) A , o : 9 4 . 4 r ( 2 )P,
: 82.95(2),7 : 119.93(2)".The structurewas refined in
the spacegroup Pl to an R index of 3.lol0.The results
show that the anhydrous componentsofthe structure aresimilar to those of namuwite, with successiveoctahedral
layers aligned almost vertically, such that the Zn tetrahedra are almost directly above and below one anotherin adjacent layers. The environments of the interlayer
HrO positions in the pentahydratedo not correspond to
the environment of the OW2 site in namuwite.
Bear et al. (1986) also reported on the structure of
ZnoSOoOHr.3HrO,formed by dehydration of the pentahydrate. Precessionphotographsshowed a doubling of
the unit cell; refined cell parameters,from X-ray powder
diffraction data, were a : 8.367(3),b : 8.393(3),c :l 8.569(5)A 1i.e.,2 x 9.28A), o : 90.29(3),B : 89.7r(3),:120.53(3)'. Dehydration was accompaniedby twin7
ning, tilting and misorientation of layers and a gradualbreakdownin the long-rangeorderingbetweenlayersalongc. The structure was refined in the spacegroup 11 to an
R index of l3o/o.The results show that the centering isdue to a displacementofsuccessive octahedral layers by
t/zla + bl. The environments of the interlayer HrO positions in the trihydrate do not correspondto the environment of the OW2 site in namuwite.
Bear et al. (1986) found that unlike the trihydrate,
ZnoSOoOHr.4HrO occupies a very narrow stability region and is extremely difficult to prepare in a pure form.
The formation of the tetrahydrateas an intermediatephase(between the penta- and trihydrates) was recorded in aseriesofprecession photographs.Thesewere indexed using an /-centered triclinic cell with pseudohexagonalgeometry,a x b x 8.35 A, c = 20.70A (1.e.,2 x 10.35
A), o * 0 = 90, ^y = I20". The precessionphotographs

Frcunr 4. The structureofnamuwite projectedonto (100);
shadingas in Figure3. The OW2 atomsare shownas crosshatchedcircles.
showed that in the synthetic tetrahydrate, successiveoctahedral layers are displaced by t/zfa + Dl (Bear et al.
1986). In namuwite, the layers are not displaced. Thissuggeststhat the presenceof Cu in the octahedral sites
prevents displacementof the layers with dehydration, effectively maintaining the anhydrous part of the pentahydrate structure. This may also explain the stability of thenamuwite structurerelative to that of ZnoSOoOH2'4H2O.
Is namuwite triclinic? The unconstrained cell dimensionssuggestthat this may be the case:a: 8.331(5),b :8.334(4),c: 10.543(8)A, o : 90.26(5),B : 89.79(5),7: 120.00(4)",on the basis of only nine reflections.However, refinement of the namuwite structurein Pl resulted
in an R value of 8. l0lo(cf. 7 .9o/oin P3) with no significant
deviations from the trigonal model. This may indicatethat the presenceof Cu in the sheetsresults in higher
symmetry, although in most copper oxysalt structuresthepresenceof Cu results in reduced symmetry becauseof
the Jahn-Teller efect (Eby and Hawthorne 1993).
Trrn onrryoRATroN-rryDRATroN TRANsrrroN
Hydration-dehydration transitions in synthetic zinc
sulfate compounds were studied by Jacob and Riquier(1969). They synthesizeda compound with cell dimensions similar to those publishedfor ZnoSOo(OH)6'4HrO(a : 8.35 and c : 10.326A; Uut which they describedas
the(2.5-3.5)Zn(OH),'ZnSOo.(2-2.2)H,O.Irrespectiveof
hydration state,Jacob and Riquier (1969) found that thec dimension increasedto 10.86 A in a humid environment at 25 "C. When dried at ambient temperatures,thec dimensionreturnedto 10.369A. thls correspondswith
the observation by von Hodenberg of a transition in namuwite between 0 and 45 'C in which c changesfrom
10.4 to 10.9 A (w. Krause, personalcommunication) andwith the tetra- to pentahydratetransition studied by Bear

GROAT: CRYSTALSTRUCTUREOF NAMUWITE
et al. (1986, 1987).Jacoband Riquier (1969)alsofoundthat with continued heating the c parameter fell to 9.27
A, which is similar to that reported by Bear et al. (1986)
for the trihydrate. After prolonged heating at 100-150'C, this was reduced to 6.98 A, which is comparable tothe thickness of an individual T-O-T layer in namuwite(approximately 7.4 A).
Bear et al. (1987) found that lower hydrates could beformed from ZnoSOoOHr.5HrOby isothermalheatingor
by lowering the partial HrO pressure.They discoveredthat the tetrahydrate formed at 32-34 oC, or at P",o of
5-6 mm Hg. The trihydrate formed at 48-50 oC, or at apartial HrO pressureof 1.5-5.0 mm Hg. A monohydrate
appearedat a partial HrO pressureof < 1.5 mm Hg. Continued heating resulted in the formation of intergrowth
structuresat 60-100 "C, a hemihydrateat 100-l15'C,
and the anhydrate at l7 5 "C. The c parameters for thehemihydrate and the anhydrate were 7.084 and 7.088 A,
respectively. Rehydration to the pentahydrate from thetri- or tetrahydratewas relatively rapid, but from the lower hydrates it took severaldays.
The presenceof Cu in the namuwite structuremay serve
to lower the temperature of the transition from the tetrahydrate to the pentahydrate. There may also be an
upper limit beyond which the dehydration-rehydrationtransition is irreversible. When the sample used in thisstudy was reexaminedseveralyears after the data collection, it was discovered that the originally tabular crystal
had split into many very thin, curved plates.Attempts toobtain an X-ray difraction photograph from the aggregate with a Gandolfi camera were unsuccessful.
AcxNowr,nocMENTs
The author thanks F.C. Hawthome for suggesting the project and forthe use of his equipment. W. Krause supplied the crystals,and M. Raudsepphelped with the electron microprobe analyses.The manuscript was
improved by commentsfrom T.S. Ercit and constructivereviews by J.D.
Grice and J.E. Post. This work was supported by a researchgrant fromthe Natural Sciencesand Engineering Research Council of Canada.

RrrnnrNcns crrro
Allman, R. (1968) Refinementofthe structue ofzinc hydroxide chloride,
Zn,(OH),Cl, l H,O. Zeitschrift fiir Kristallographie, 126, 417-426 (irtGerman).
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243

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