American Mineralogist, Volume 78, pages 827-834, 1993

Bernalite, Fe(OH)r, a new mineral from Broken Hill, New South Wales:Description and structure

Wrr-r-r,q.Nr D. BrncnDepartment of Mineralogy and Petrology, Museum of Victoria, 285 Russell Sreet, Melbourne, Victoria 3000, Australia

Ar,r,.q.N PnrNcDepartment of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia 5000, Australia

AnlvrrN Rpr-r,nnInstitute for Inorganic Chemistry, University of Hamburg, 6 Martin-Luther-King Platz, 2000 Hamburg 13, Germany

Hnlvrur W. ScnNr.qlr.nInstitute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurilch, Switzerland

Ansrnacr

Bernalite is a new iron hydroxide from the Proprietary mine at Broken Hill, New SouthWales, Australia. The mineral occurs as flattened pyramidal to pseudo-octahedral crystalsup to 3 mm on edge with concretionary goethite and coronadite. The crystals are darkbottle green, with a vitreous to adamantine luster. Bernalite has a pale green streak and isbrittle with no cleavage but an uneven to conchoidal fracture and a Mohs hardness of 4.Optical data are incomplete due to the effects of twinning; the indices of refraction are inthe range 1.92-1.94. Chemical analysis gave FerOr, 65.53; SiOr, 2.99; ZnO, 1.13; PbO,2.70;HrO,25.2; COr, 1.0; total 98.55 wto/o. The simplified formula is very close to Fe(OH),.Bernalite is pseudocubic but single-crystal studies gave an orthorhombic cell with a :

7 .544(2), b : 7 .560(4), c : 7 .558(2) A, v : 431.0(3) .4... The density is D-.". : 3.32(2)g/em3, D"ur,: 3.35 g,/cm3. The crystal structure was solved by Patterson methods, but fullrefinement was not possible because of the effects of polysynthetic twinning. The bestrefinement in space grotp Immmwfih Z: 8 gave R : 0.106, R*: 0. l12, using a set of785 reflections, of which 462were considered observed lI > 3o(Dl. Bernalite has a dis-torted ReOr-like structure, consisting of a three-dimensional network of corner-connectedFe(OH). octahedra.

The strongest lines in the X-ray powder pattern are [d.0.(A), 1","- hkl] 3.784 (100) (200,020,002);2.676 ( r5) (220,202,022);2.393 (16) (310, 130, 013, 031, 301, 103) ; 1.892(10) (400, 040, 004) ; r .692 ( r7) (420,240,204,402,024,042);1.545 (9) (422,242,224).The name is for the British crystallographer J. D. Bernal (1901-1971).

INrnonucrrou

While visiting the Proprietary mine at Broken Hill, NewSouth Wales, Australia, in the early 1920s, R. T. Sleecollected specimens containing dark green octahedralcrystals on concretionary goethite and coronadite. Sleepassed the specimens on to the noted Australian geologistFrank Stillwell for identification. Stillwell (1927) suggest-ed that the crystals were limonitic replacements of arse-nopyrite coated with scorodite. The specimens were sub-sequently deposited in the research collections of theDepartment of Geology at the University of Melbourne,where they remained unnoticed for many years. In 1990the collections of the Department of Geology were trans-ferred to the Museum of Victoria, and during this processthe specimens were rediscovered and examined. Prelim-inary examination by powder X-ray diffraction and elec-tron microprobe analysis suggested that the mineralforming the octahedral crystals was a new member of the

0003-o04x/9 3/0708-08 27$02.00

stottite group. A single-crystal structure determination wasundertaken to resolve fully the composition and stoichi-ometry of the mineral. The new species is ferric trihy-droxide and has been named bernalite in honor ofJohnDesmond Bernal (1901-1971), eminent British crystal-lographer and historian ofscience. Bernal made outstand-ing contributions across the field of crystallography, in-cluding the investigation of the crystal chemistry of ironoxides and hydroxides (Bernal et al., 1959). The mineraland the name were approved by the Commission on NewMinerals and Mineral Names. Type specimens (cotypes)are in the collections of the Museum of Victoria, Mel-bourne, and the South Australian Museum, Adelaide.

OccunnrNcn

At least two specimens containing bernalite, which wereoriginally one piece, were collected in the upper levels ofthe Proprietary mine at Broken Hill, New South Wales.

827

828 BIRCH ET AL.: BERNALITE

TaBLE 1, Chemical analysis of bernalite

Bernalite.(wt%)

Fe(OH)3(wt%)

Bernalite**(atomic

proportion)

FerO"sio,ZnOPboHrOCO,

Total

65.532.991 . 1 32.70

25.21 . 0

98 55

74.73

25.27

100.0

0.920.060.010.013 . 1 20.03

Fig. l. Pseudo-octahedral crystals of bernalite on goethitefrom the Proprietary mine, Broken Hill, New South Wales. Scalebar is 3 mm.

Unfortunately, accurate locality data for the material werenot recorded, so the exact occurrence in the Proprietarymine sequence cannot be established. However, the na-ture of the mineral and the goethite-coronadite matrixare consistent with the specimens having come from thenear-surface portion ofthe oxidized zone. At the Propri-etary mine the secondary zone extends to depths ofup to150 m from the original surface (Birch and Van der Hay-den, 1988; King, 1983). All this area of the lode, withinthe original Proprietary mine leases, has been opencut,and no further specimens of bernalite, either recently col-lected or in old collections, are known.

Prrvsrcar, AND oPTICAL PRoPERTIES

Bernalite occurs as flattened pyramidal to pseudo-oc-tahedral crystals up to 3 mm on edge (Fig. l). The crystalfaces are rough and slightly concave. The outer surface isyellow green with a resinous luster, but the inner portionsare transparent, dark bottle green with a vitreous to ad-amantine luster. A thin layer of dark brown fibrous goe-thite occurs on most crystals, which gives them a glossy,nearly black appearance. The streak ofbernalite is applegreen. In thin section the crystals are yellowish bottle greenand when viewed under crossed nicols show a fine-scalecrosshatched texture due to polysynthetic twinning. Thecrystallographic orientation of the twinning could not bedetermined, but it is probably on { 100}, {010}, and {001}.Strings of small goethite inclusions occur roughly parallelto the outer margins of the crystals. The mineral has noprominent cleavage or parting but is brittle with an un-even to conchoidal fracture. The Mohs hardness is 4, andthe density, measured by suspension in a mixture of di-iodomethane and chloroform, is 3.32(2) {cm' (mean offive determinations), which is within experimental errorfor the calculated density 3.35 g/cm3 for the empiricalformula (see below). Complete optical properties couldnot be determined because of the twinned nature of themineral. Grain mounts gave refractive indices in the range1.92-1.94. It was not possible to determine more than

Nofe. Bernalite of Proprietary mine, Broken Hill, New South Wales.- Average of eight microprobe analyses. HrO and CO, determined sep-

arately on a 7-mg sample.-. Atomic proportions calculated on the basis of one cation.

one refractive index on any grain, and therefore opticalorientation could not be established.

Crrrtrarsrnv

Bernalite was analyzed using a Jeol electron micro-probe operating at 15 kV with a specimen current of 20nA. The mineral is quite stable in the electron micro-probe beam. The standards employed were hematite (Fe),wollastonite (Si), sphalerite (Zn), and galena (Pb). No oth-er elements with atomic number >8 were detected bywavelength spectroscopy. HrO and CO, were determinedseparately using a Perkin Elmer 240 Automatic C/H/Nanalyzer on a 7-mg hand-picked sample. Eight micro-probe analyses were obtained, for which the average isgiven in Table l. The empirical formula calculated onthe basis of one cation is [(HrO)oor(Co.)oo.-Pboo,]'(FeAirSi.06znoo,XOH)r ru, with the simplified formula be-ing Fe(OH),.

The small amount of Si in the analysis has been as-signed to the same site as the Fe, an octahedral site inthis structure. White and Nelen (1973) and Kampf (1982)adopted a similar assignment in their respective descrip-tions of the closely related minerals tetrawickmanite andjeanbandyite. The presence of Si in octahedral coordi-nation with O in minerals and compounds formed at lowtemperature and pressures is exceedingly unusual, al-though a small number of synthetic compounds and atleast one mineral are known (Finger and Hazen, 1991).An alternative explanation is that the bernalite crystalscontain submicroscopic inclusions of silica. Other, per-haps less likely, locations for the Si (e.g., a tetrahedral siteassociated with the A cation site) are also possible. How-ever, the limitations associated with effects of twinningin fully refining the crystal structure and the rapid decom-position of the mineral in the electron beam of the elec-tron microscope unfortunately prevent clarification of therole ofSi in bernalite.

Pownnn X-RAY DTFFRACTIoN

The powder X-ray diffraction pattern was recorded us-ing a Philips automatic powder diffractometer with Fe-filtered CoKa radiation (I : 1J9026 A;. Line intensities(peak areas) were estimated from the diffractometer trace.

BIRCH ET AL.: BERNALITE 829

TABLE 2. Powder X-rav diffraction data for bernalite

ilt. 4"" (A) 4"" (A) hkt

3.7842.6762.3932 .1852 0231 .8921 .6921.545

Note: The pseudocubic cell parameters refined from the data above area : b : c : 7.568(1) A; V : 433 4(2) A3

The powder pattern was initially indexed on a cubic unitcell, and least-squares refinement was undertaken usingthe eight reflections with d > 1.4 A (2d < 80"). The cubiccell has a : 7.568(l) A. Subsequent structural studiesusing single-crystal X-ray diffraction methods showed thesymmetry to be orthorhombic, with the true cell onlyslightly distorted from a body-centered cubic, perovskite-like structure. The powder pattern indexed on the ortho-rhombic (pseudocubic) cell is presented in Table 2.

Mossn,c.usn sPEcrRoscopy

The room temperature Mdssbauer spectrum of berna-lite was recorded on a 20-mg hand-picked sample. Thespectrum is shown in Figure 2, and the spectral param-eters are given in Table 3. The isomer shifts are typicalfor Fe3+ and show that all Fe in bernalite is present inthe Fe3* rather than the Fe2* oxidation state. The spec-trum was fitted with two magnetically ordered fields ofsix lines and one quadrupole split doublet. Because theisomer shifts and the electric-field gradients for the mag-netic fields are very similar, the spectrum probably in-volves only one site, which is not fully ordered at roomtemperature. The isomer shift and quadrupole splittingof the small central doublet suggest that it is related tothe same site (J. Brown, personal communication). Un-fortunately the specimen was accidentally lost before lowtemperature spectra were recorded. In view of the verylimited amount of material on the type specimens, it wasdecided not to remove further material to complete theMiissbauer studies.

A considerable number of Mdssbauer studies have beenundertaken on iron oxy-hydroxides and iron hydroxides;however, none so far shows magnetic ordering at roomtemperature (Mathalone eI al., 1970; Au-Yeung et al.,1984; Coey and Readman,19731, Persoons et al., 1986).

TrrnnNr.q.L ANALYSTs

Differential thermal analysis revealed that bernalite de-hydrates to form hematite. This strongly exothermic re-action occurs in a single step between 190 and 200'C:

2Fe (OH) r -Fe rO .+3HrO.

The dehydration of bernalite was also observed in aPhilips CM 30 transmission electron microscope oper-ating with an accelerating voltage of 300 kV. When ex-

0

100t c

1 6

61 01 7o

3.7842.6762.3932.1 8520231 .8921 .6921.545

200, 020,002220,202,022310, 130, 301, 103, 031, 013222321 , 312, 213,231 , 123, 132400, 040, 004420, 402, 240, 204, 024, 042422,242,224

9 E- - r " t , | | ' t j | ' t | i l r | L ' r l- r 5 - 1 0 - 5 0 5 1 0 1 5

V e l o c i t y ( m m / s )

Fig. 2. M6ssbauer spectrum of bernalite recorded at roomtemperature.

posed to the focused electron beam, bernalite underwentdehydration within seconds to form microcrystalline he-matite. No orientational relationship between the originalbernalite lattice and the hematite was observed.

Cnvsr.l,r, STRUCTUREExperimental

Crystal fragments suitable for detailed study were soughtby examination under polarized light. Although most didnot extinguish uniformly, three fragments showing onlyslight undulose extinction were selected. Precession andWeissenberg photography showed a distribution of strongand weak reflections assignable to a pseudocubic body-centered perovskiteJike lattice with observed reflectionsh + k + I : 2n. However, some very weak reflectionsthat violated this condition were also observed. Inten-sity data were obtained from all three fragments using aCAD-4 single-crystal diffractometer, and the structure wassolved by Patterson methods using SHELXS-86 (Shel-drick, 1986). However, as no satisfactory refinementemerged, further measurement of intensities was under-taken on one of the crystal fragmenis, and a full sphereof data was measured within the limits -13 < h < 131--13 < k < 13 -13 < I < l3 .Atota lof 10760ref lect ionintensities was obtained using an ,'t-20 scan technique.The scan rate varied between 1.03 and 16.48'lmin, and

TleLe 3. Mossbauer parameters for bernalite at room temper-ature

lsomer shift(mm/s)

Electric fieldgradient Area (%)

Magnetic field pa.ameters0.36 0 010.37 0.01

45.051 .0

lsomer shift(mm/s)

Quadrupolesplitting(mm/s) Area (%)

0.61

Field 1Field 2

0 3 4 4.0

830 BIRCH ET AL.: BERNALITE

the maximum measuring time per reflection was 80 s.Three standard reflections were monitored every 3 h ofmeasuring time, and no loss of intensity was noted. Theorientation of the crystal was checked every 400 reflec-tions through remeasurement of the standard reflections.Some 4409 reflections having negative intensities wereexcluded from the data set. A numerical absorption cor-rection based on six indexed but not perfect crystal faceswas applied with the program SHELXT6 (Sheldrick,1976). Data were also corrected for Lorentz and polar-ization efects. The Laue symmetry of the data set wasfound to be 2/m which suggested the space groups P2,/m,1112, or l7lm. Data reduction of 6351 reflections ledto a set of 2637 unique reflections (R-".r. : 0.0722) wrththe assumption of the monoclinic space group P2r/m. Ofthis data set only 785 reflections were considered ob-served with I > 3o(I), o(I), on the basis ofcounting sta-tistics.

Structure refinement

Refinements of the structure were undertaken in anumber of space groups, including Pt, Ill2, IlIm, andP2r/m. All refinements were characterized by splitting ofthe O positions, suggesting that the crystal fragment usedfor data measurement was twinned.

Orthorhombic symmetry was clearly shown by unit-cell projections calculated by STRUPLO84 (Fischer, 1985)for geometrically correct FeOu octahedra (O-O distancesapproximately 2.7 L). On this basis two trial models forthe bernalite structure were derived in the orthorhombicspace group Pmmn. An electron density peak on the pe-rovskite,4 cation site in difference Fourier maps was ten-tatively assigned to HrO on the basis of the chemicalanalysis and included in the structural models. The twomodels differed only in the O positions, and both sug-gested higher symmetry than Pmmn. The difference per-pendicular to the c direction between the r glide of Pmmnand the mirror of the body-centered space group Immmwas found to be minimal when comparing O positions ofthe octahedra. This small difference was in accordancewith 90 very weak reflections, which did not belong tothe body-centered cell. The reflections were in the paritygroups oee : 24, eoe : 17, eeo : 21, and ooo : 28,where o represents odd and e even values offt, k, and l.

In order to continue the refinement in a higher sym-metry the absorption correction and the data reductionwere repeated in space group Immm. The 3852 reflec-tions were merged for the body-centered space group, andthe weak reflections inconsistent with the space groupwere omitted, giving 785 unique reflections (R*..r. :0.1069), of which 462 were significant with / > 3o(D.The R value excluding the O position for that of HrO atthat stage of the refrnement was 130/0. The electron den-sities for the two O atoms associated with HrO in thePmmnmodels were 6.85 and6.54 e/A3, respectively. Theywere included in the refinement together with split O pa-rameters. Fe was refined with anisotropic displacement

parameters, whereas all O atoms were refined with iso-tropic displacement factors. Four reflections were omit-ted because secondary extinction was suspected. The re-maining 458 reflections were used in the final refinementofone Fe position and three octahedral O positions withsite occupancies of 0.5 and the O of the HrO moleculewith a site occupancy of 0.125. This suggested an ide-alized formula, [Fe(OH),]r'2H2O. However, it is likelythat the small amount of Pb found in the analysis wouldalso occupy the A site accounting for part ifnot all ofthescattering of the HrO site. Refinements of the occupancyof the A site were undertaken, but the results were incon-clusive, as it was not possible to identify positively thecontents of the site. Attempts were also made to refinethe occupancy of the Fe site in accord with the stoichi-ometry found by chemical analysis; however, such re-finements were characterized by marked increases in R,and the isotropic temperature factors for O remained un-realistically low.

Because other closely related metal hydroxide struc-tures with cubic symmetry are known [e.g., In(OH)r, whichhas space group IM3, (Christensen et aI., 1957)1, an at-tempt was also made to refine the bernalite structure inIm3. As the data are not cyclically permutable, R-".r" wasvery high (R-"*" : 0.22) after data reduction and resultedin266 unique reflections, ofwhich 163 were consideredobserved with 1 > 3"(4. According to the cubic sym-metry the refinement needs only one O atom position forthe FeOu octahedra. Again, other split positions near theO were seen in the difference Fourier map. The R valueofthis refinement converged at 0.094, for 163 reflectionsand nine varied parameters. Although this may appear tobe a better refinement, fewer parameters are involved,and the HrO/Pb position cannot be accommodated inthis cubic model without destroying the symmetry. Thedistances between O and the O in HrO ranged between1.96 and 2.04 A, whereas the result of the orthorhombicrefinement shows distances between the O of HrO andthe O of octahedra of 2.68 and2.72 A, which is in agree-ment with H bonding of the form O-H"'OH or HOH. . . oH .

The value of R for the refinementin Immm is 10.60/0.This high value of R reflects a number of problems as-sociated with the bernalite structure. The chosen body-centered structure models represent only two of at leastsix orientations in the orthorhombic cell. It is clear thateven the preferred data set represents an intergrowth oflattices in all three crystallographic directions in the unitcell. The fact that the FeOu octahedra are slightly rotatedfrom the idealized perovskite structure explains the easewith which microtwinning may occur. Twinning of thepseudocubic unit cell results in the Fe sites being super-imposed and in the splitting of the O positions. Addi-tional uncertainties arise from the unresolved nature ofthe contents of the A cation and the probable measure-ment errors in the absorption corrections rendering thetemperature factors associated with the O positions un-realistically low.

BIRCH ET AL.: BERNALITE 8 3 1

TABLE 4. Crystal data and results of crystal-structure refinementfor bernalite

Taele 6. Final atomic coordinates, thermal parameters, and bondlengths for bernalite

Positional oarameters for the atoms(site occupancy factor 0.50)

U^o lu,JtA')

Crystal sizeUnit-cell dimensions

a (A)b (A)c (A)v (41

zSpace groupD,*" (g/cml4'" (g/cm"1p (cm ')

RadiationMoKa

Collection limit20(MoKal

Unique reflectionsReflections / > 3oTransmission factors

MaxMin

0.10 x 0 .12 x 0 .025 mm

7.s44(2)7.s60(4)7.558(2)

431.0(3)I

Immm3.32(2)

o o . / u

0.71073 A

0.81790 49620.1060.112

0.250 0.2500.s00 0.178(5)0.1s5(2) 0.190(2)0.318(3) 0.s000.000 0.500

0.250 0.0049(5)0.308(4) 0.001(4)0.500 0.001(2)0.323(3) 0.001(3)0.s00 0.0064(10)

Feo102o3Ow

Fe-O distancesFe-O1Fe-O2Fe-O3

2.014(1 1)1.984(5)2.035(71

In conclusion, structural models found in all lowersymmetry refinements showed only small deviations frombody-centered orthorhombic symmetry. Despite the rel-atively high R values for the final orthorhombic refine-ment in space group Immm, we feel it is the best, albeitan imperfect, model for the structure of bernalite. It isnot possible to refine completely the structure with theavailable crystals. Experimental details for the most sat-isfactory refinement are summarized in Table 4, and thefinal list of observed and calculated structure factor am-

/Votej O-Fe-O bond angles range between 86.0(9) and 90.4(7f'

plitudes is given in Table 5.' The final atomic parametersand their standard deviations for the best body-centeredorthorhombic refinement, estimated assuming 0.25 HrOin the,4 cation site, are given in Table 6.

Structure description

The structure ofbernalite is depicted in Figures 3 and4. The structure consists of FeOu octahedra, which arelinked by corner sharing to form a three-dimensional net-work containing large l2-fold coordinated cavities, which

80"/ d 5

462

BF_

' A copy of Table 5 may be ordered as Document AM-93-530from the Business Ofrce, Mineralogical Society of America, 1 I 30Seventeenth Street NW, Suite 330, Washington, DC 20036,U.S.A. Please remit $5.00 in advance for the microfiche.

Fig. 3. Stereoscopic view ofthe bernalite structure projected down a showing the Fe-O linkages within Fe(OH)6 octahedra and

between octahedra. HrO molecules and Pb atoms may fractionally occupy large coordinated sites within the structure and are linked

to the O ofthe octahedra by H bonding.

832 BIRCH ET AL.: BERNALITE

b

Fig. 4. Schematic diagram showing the structure of bernaliteprojected down [001] showing the corner-sharing linkage and therotation of the Fe(OH)u octahedra.

may accommodate a small amount of HrO or Pb. Thestructure is derived from the perovskite or ReOr-typestructures (Hyde and Andersson, 1989). In the bernalitestructure the octahedra are rotated or tilted about each ofthe three fourfold axes ofthe octahedra in such a way soas to reduce the symmetry from cubic (123) to ortho-rhombic (Immm). The distortion of the perovskite orReOr-type structures by the tilting and rotation of theoctahedra has been extensively studied, and the symme-try relationship established (Deblieck et al., 1985). TheA calion site of the perovskite structure in bernalite ispartially occupied; however, the exact chemical nature ofsite is unresolved. It may contain up to 0.25 of a HrOmolecule or a mixture of a small amount of Pb and H,O.

TABLE 7. Hydroxide minerals structurally related to bernalite

The small amount of Zn found during the electron mi-croprobe analysis would occupy the octahedral site.

The Fe-O distances fall within a small range, 1.984(5)-2.035(7) A, wittr a mean of 2.011 A, and the O-Fe-Obond angles show only small departures from 90"; theyrange from 86.0(9) to 90.4(7)'. The Fe-O bond lengthsare typical for Fe3*-O contacts in ferric oxysalts (see, forexample, Szymanski, 1 988).

Rgr,,lrroNsrrlP To orHER MTNERALS

Bernalite is fundamentally different from the ferric oxy-hydroxide minerals goethite, lepidocrocite, and akaga-neite in a number of ways, and these differences can bedirectly related to structural differences. Whereas in ber-nalite the octahedra are joined by corner sharing, the fer-ric oxy-hydroxides have structures based on pairs ofedge-sharing octahedra, which link by corner sharing. Edgesharing results in large distortions to the octahedral co-ordination, with a shortening of some of the Fe-O bondsand a lengthening of others; for example goethite has threeFe-O bonds at l.95 A and three at 2.09 A (Szytuta et a1.,1968). In contrast, bernalite has nearly perfect octahedralcoordination, and this regularity results in a small crystal-field stabilization energy and the absorption oflight beingshifted to the red end of the spectrum. Hence bernalite isgreen rather than red or yellow, like the ferric oxy-hy-droxide minerals.

It is clear from the Mtissbauer and structural data thatbernalite and amorphous Fe(OH), gels are structurallydistinct. There is considerable evidence suggesting thatthe Fe(OH)3 gels are structurally related to gibbsite andthus have structures based on sheets of edge-sharing oc-tahedra (van der Giessen, 1968; Au-Yeung et al., 1985;Coey and Readman, 1973).

There are a number of metal hydroxide minerals thatalso have the distorted perovskite or ReO.-type structure,including those in the stottite and schoenfliesite groups(Fleischer and Mandarino, I 99 I ), together with sdhngeiteand dzhalindite (Table 7). Ross et al. (1988) refined thestructure of stottite, FeGe(OH)u, in a tetragonal space

Composition Symmetry Reference

SdhngeiteDzhalinditeBernalite

TetrawickmaniteStottiteMopungiteJeanbandyite

BurtiteNataniteMushistoniteSchoenfliesiteVismirnoviteWickmanite

Ga(OH)3In(OH)gFqoH)3

MnSn(OH)6FeGe(OH)6NaSb(OH)6(Fe,Mn)Sn(OH)6

CaSn(OH)6FeSn(OH)u(Cu,Zn,Fe)Sn(OH)6MgSn(OH)6ZnSn(OH)6MnSn(OH)6

a : 8 . 1 2 8 , a e 9 0a : 7 . 6 9a : 7.705-7.735a : 7 . 7 5 9a : 7 . 7 2a : 7 .873

S6hngeite grouplm3 a :7 .47cub i c a :7 .95lmmm a :7 .54 , b :7 .60 , c : 7 58

Stottite groupP42 ln a :7 .787 , c :7 .797P4" l n a :7 .55 , c :7 .47P42 ln a :7 .994 , c :7 .859P42 ln a : c : 7 648

Schoenlliesite group

Strunz (1965)Genkin and Murav'eva (1963)this work

White and Nelen (1973)Strunz and Giglio (1961)Williams (1985)Kampf (1982)

Sonnet (1981)Marshukova et al. (1981)Marshukova et al. (1984)Faust and Schaller (1 971 )Marshukova et al. (1981)Moore and Smith (1967)

R3Pn3mPnSmPn3mPn3mPnSm

BIRCH ET AL.: BERNALITE 833

group and, like the present authors, encountered consid-erable problems with twinning and uncertainties in spacegroup assignment. Jeanbandyite is the other metal hy-droxide ofthis structure type that contains significant Fein the ferric oxidation state (Kampf, 1982), and it is pos-sible that a compositional field between bernalite andjeanbandyite may exist in nature. Considerable solid so-lution of the divalent cations has been reported for someof the schoenfliesite group minerals (Marshukova et al.,1984; Nefedov et al., 1977), and it appears that the fullextent of chemical substitutions in the metal hydroxideminerals of this structure type is yet to be established.

For the present, bernalite is assigned to a group withs6hngeite and dzhalindite, as the stottite and schoenflie-site groups have mixed occupancy ofthe octahedral site.Bernalite appears to be the only orthorhombic metal hy-droxide mineral, the others being cubic or tetragonal.However, it must be pointed out that many of the struc-tures have not been studied in detail.

PlnacrNBsrs

The oxide and hydroxide mineral relations of the Bro-ken Hill deposit have not been thoroughly investigated.The dominant minerals in the near-surface gossan arecoronadite and goethite, and there are reports oflepido-crocite and chalcophanite, but the existence of othermanganese and iron oxides or hydroxides can only bepredicted. The chemical and structural complexity of theoxidized zone (Plimer, 1984; van Moort and Swenson,1981) suggests that a wide range of microenvironments,some of them possibly unusual, was present at times dur-ing its formation. Bernalite appears to occupy such a mi-crochemical niche.

At an early stage in this investigation, it was consideredthat bernalite may have been one of the long sought-after,naturally occurring green-rust compounds. However, it isclearly distinct from these synthetic pyroauriteJike phases(Taylor, 1973). Because of direct association with con-cretionary goethite, which pre- and postdates it, bernalitealmost certainly crystallized from a ferric hydroxide gel.A variety ofthese gels has been synthesized over a rangeofpH by either precipitation from solutions offerric saltsby addition of a base, or by oxidization of Fe2* in basicaqueous solutions (Au-Yeung et al., 1985). Magnetic andX-ray diffraction properties of the products vary, oftendepending on particle size, but none matches those ofbernalite.

It is not possible to distinguish among these alternativepossible origins for bernalite. Whereas the environmentofthe oxidized zone strongly suggests that all near-surfaceFe would already be Fe3* when the bernalite precursorprecipitated, it is possible that reaction of the aqueoussalts with a strong oxidizing agent such as HrO' (Wil-liams. 1990) could have occurred on a limited scale. Sim-ilarly, difiLsion of atmospheric O, into concentrated Fe2+-bearing solutions could also result in the precipitation offerric hydroxide phases (Thorber, 1975).

In assessing the formation and stability of bernalite,several factors may be considered.

1. An upper temperatufe limit of 50-60 'c is likelyfrom the geological setting. There is no geological evi-

dence for any higher temperature, such as may have

occurred during metamorphism.2. Estimates of the pH during formation of bernalite

cannot be constrained on the basis ofthe published syn-thesis conditions for ferric hydroxide gels, in which thepH ranged from 4.5 to 9.

3. The bernalite crystals contain small but significantamounts of Pb. Si, and Zn. These are presumably, withthe possible exception ofSi, in solid solution, suggestingthey were originally absorbed or scavenged elements thatwere not expelled during crystallization of the gel. Theymay have played a role both in the initial precipitation

of the gel and in stabilizing the bernalite crystal structure.Preliminary attempts to synthesize bernalite by crys-

tallization from Fe(OH), gels have been unsuccessful, andthe synthesis of this mineral appears to pose a consider-able challenge.

AcrNowr-nncMENTS

The authors are grateful to Joan Brown, Department ofPhysics, Mon-

ash University, Melboume, for conducting the Mossbauer analysis and

for providing some ofthe relevant references Pat Kelly assisted with the

electron microprobe analysis, which was undertaken in the Department

of Geotogy, University of Melboume. Powder X-ray diffraction data were

obtained with the assistance of Mark Raven and Phil Slade ofthe CSIRO

Division of Soils. Adelaide. We also wish to express our gratitude to the

referees of this and a previous version of the paper. Their comments

helped us to clarify a number of important aspects of the crystal chemistry

of bemalite.

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MeNuscnrsr RECETvED Aucusr 21, 1992M,q.Nuscnrvr AcCEFTED Mancn 4. 1993