THn AlrERrcAN MruERALocrsr JOURNAL OF THE MINERALOGICAL SOCIETY OF AMERICA

Vol.48 JANUARY-FEBRUARY, 1963 Nos. 1 and 2

THE CRYSTAL STRUCTURE OF CARMINITE

J. J. FrNwov,LDepartment of Geology,(Iniaersity of Wisconsin, Madison, Wisconsin.

ABSTRACT Carminite, 8[PbFer(AsOa)r(OH)r], is orthorhombic Cccm or Ccc2,a:16.595, b:7.580, c:12.295, all + .005 A. The structure was determined by means of Patterson, Fourier and difierence maps and refined by least squares. The final R-factor is 10.8/6. Both centric and noncentric cycles were attempted, but the noncentric cycles converged essentially to the centric structure. The structure consists of octahedra of oxygen and hydroxyl about iron connected to each other by an edge at the mirror plane and by a corner at a two fold axis. The octahedra extend unbroken parallel to the Z-direction in projection on (010) and are connected together in the X-direction by arsenate tetrahedra. Coordination about the lead atoms is eight fold. The structure of carminite is very similar to that of brackebuschite. INrnooucrroN Since the original determination of the space group and unit cell of carminite, PbFez(AsOa)r(OH)r,by Rosenzweigand Finney (1959),simi- larities betweencarminite and other minerals have been observed.Re- cently Strunz (1960) has shown that carminite and palermoite, SrAl2 (POD2(OH)2,are isotypic and further that thesetwo mineralsbear some relation to the minerals of the descloiziteseries such as conichalcite, CaCu(AsOa)(OH),and duftite, PbCu(AsO) (OH). Donaldsonand Barnes (1954)have determinedthe structure of pyrobelonite,PbMn(VOt(OH), another member of this family having the same generalformula. The cell dinrensionsof palermoite(a:7.31, b:15.79, c:11.53 A; ut. similar to those of carminite (o:16.595, 6:7.580, c:12.295 A), and the space group is the same (Mrose, 1953). A closerelationship appears to exist with brackebuschite,Pb2(Mn,Fe) (VO4).HrO(Donaldson and Barnes,1955). The cell dimensionsof bracke- buschiteare a:7.681, b:6. 155,c: 16.524A, p:93"+5' lor spacegroup B 27/m. If the D-axisof brackebuschiteis doubled the dimensionsare almost identical with those of carminite. Ilowever brackebuschiteis monoclinicand carminite is orthorhombicand the Pb: Fe ratio is 1 :2 for carminite and the Pb: (Mn,Fe) ratio is 2:l Ior brackebuschite.Evidently

I Present address: Department of Geological Engineering, Colorado School of Mines, Golden. Colorado. 2 J' J. FINNEY the changeof the Pb:(Mn,Fe) ratio in favor of the small cation is not sufficient to cause distortion of the structure beyond an adjustment to orthorhombic symmetry. Donaldson and Barnes have concluded that though some relationship exists between brackebuschiteand the descloizitefamily, brackebuschite cannot be considereda memberof that group of minerals' Throughout this investigation, the formula used for carminite is that determined by Foshag (1937) and confi.rmedby LeMesurier (1939)' ExpBnrruBurAr-WoRK The sample of carminite, from Mapimi, Mexico, used for the original unit cell urrd rpu." group determination was used also throughout this investigation. The experimental work was carried out with a crystal having the dimensions.043X.300x.048 mm correspondingto the crys- tallogiaphic axes @,b and crespectively.Data werecollected by means of multiple film pack Weissenbergphotographs oI the h\I, hII, h2l, h3l and 0&l reiiprocal levels. Before collection of the Oil data the crystal was cut approximately in the shape of a cube. AII intensities were estimated by compurison of known intensities with the reflections on the Weissenberg photographs and corrected for the Lorentz and polarization factors. The iurg. i.t*t absorption coeffi.cientof 381/cm for molybdenum radiation necessitatedapplication of absorption corrections. Transmission factors for cylindrical crystals were used for the hol, hItr, h2l and fr31reflections and transmission factors for spherical crystals were applied to the Okl reflections. The cell dimensions were redetermined by the 0- method of Weis-2, Cochranand Cole(1943) as: a:16.595, 6:7.580, c:t2'295 all * '005A' The calculated density is 5.46. No measured density was obtained from this sample but LeMesurier (1939) observeda specificgravity of 5.22' The cell formula is PbsFero(AsOr)re(OH)m. Only those reflectionsappear which obey the following restrictions: hkl tufk:2n okl' (k:zn) t:2n h0l (h:Zn) tr:2n

The only two space groups compatible with these restrictions ate Cccm and Ccc2 confirming the original determination. Foshag observed the morphology of carminite to be holohedral and on this basis spacegroup Cccm was chosenfor the initial structure determination. The equivalent positions lor Cccmare listed in Table 1. Srnucrune DorBnurNarroN An examination of the z0l intensities showed the following nonsystem- atic distribution, where h:2n and l:2n becauseof space group re- CARMINITE

16 (m) i,,tIz; i, v, i1-z 8 (') 8 (ft) 8U) 8 (d) 8 (h) 8 (g) 4 (J) 4 (e) 4 (d) 4 (') 4 (b) 4 (a) quirements.Reflections having hll:4n weregenerally strong and those having h*l:2n were generallyweak. Becausethe lead atoms are the greatestcontributor to the structure amplitudes and thus probably determine the majority of phases,these atoms were thought to be locatedat r:i, r:I.However, if the lead atoms occupiedan eight fold position such as (g) r,0,tor (k) i,L,z reflec- tions in which Z*l:2n wonld appear strong whereasthey actually ap- pear as weak reflections.If, insteadof an eight,foldposition set, the lead atoms are placedin two four fold positionssuch as (o) 0,0,| and (/) t'f,0 generalagreement is found between observedand calculatedstructure amplitudes. Ln h\l Patterson projection was calculated and the resulting map is reproducedin Figure 1A. The peak at |,| representsthe lead-leadvector and is producedby a combinationof one set (o) or (D)with anotherset (e) o. f). The remaining large peaks can then be explained as lead-arsenic and lead-ironvectors. These are appropriatelydesignated in Fig. 1A. The Iead atoms were placed arbitrarily in position set (o) and ff). With this arrangementthe arsenicatoms are situatedin (g) r,0,] wherer: .21 and (l) x,y,Owhere r:.04. The sixteeniron atoms wereplaced in (rz) with the r and z parameters some multiple of f . Three dimensional Fourier sections were calculated in the Y-direction using the contributions of the lead atoms only and the resulting maps confirmed the above relationships. The iron parameters-were fixed as r: .38,y: .12 andz:.135. In addition, the 1 parameterfor Aszwas fixed as y:.75. Differencemaps in the Y-direction were calculated.For these maps the contributions of all metal atoms were subtracted and the posi- tions of all oxygen atoms and hydroxyls were determined. The final Fourier projectionon (010)is shownin Fig. 1B. J. J. FINNEY

Fro. 1A. (left) Patterson map of carminite projected on (010). Contours are at equal but arbitrary intervals except the origin and Pb-Pb peaks which are twice the contour interval of other peaks. B. (right) Final Fourier map of carminite projected on (010) Contour interval is 10 e/42 except for the peaks representing lead atoms which are 20 ef f*. Zero and negative contours are dashed. X indicates the location of oxygen and hydroxyl.

For the initial structure determinationthe temperaturefactors B:1.0 for lead and B: .50 for arsenicand iron were applied to the scattering factors. During all stagesof the structure determination and refinement the scattering factors for lead were taken from James and Brindley (1931),for arsenicand oxygen were those determinedby Beryhuis et al. (1955)and for iron the valuesobtained by Wood and Pratt (1957) were used. Hydroxyl was assumedto have the scattering power of oxygen. AII curves were modified for the ionic state. In addition, a real dispersion correctionof -4.0 electronswas applied to the scatteringfactor for lead.

Rolrxplrnltt oF THE Srnucrunn Refinement of the structure of carminite was effectedby use of the method of least squares.The parametersobtained from the Patterson, Fourier and difierence maps were used as starting parameters for this refinement.Twenty-two positionalparameters, twelve isotropictempera- ture factors and five scale factors were allowed to vary and the entire 39X39 matrix was computed. All reflectionswere weighted equally ex- cept nonobservedreflections which were weighted zero. Table 2 is a Iist of the R-factors resulting from the first and final cycles for each reciprocal level. CARMINIT'E 5

The standard deviationsof the positional parameterswere calculated from the formula: orr,:/IraF;'^ 1'r' \a;i.,,,) where m is the number of observations,s is the number of variables and D;; is the diagonal term of the inverse matrix. The final positional param- etersand temperaturefactors together with the standard deviationsare

Teslr 2. R-Facronsrnou Frnsr aNnFtlrar, Cvcr,rs ol Rrrrr.rnv.rxrlon Elcn Rrcrpnocer, Lrwr,

First cycle Final cycle Reflections

hot n1 n .o97 73 iltL .205 .112 130 hzl .223 .098 l6 NJL .289 .t4l 102 0hI .266 .070 20

Overall .239 .108 Total observed 403 Nonobserved 2r0

Total 613

Iisted in Table 3. Large fluctuations of some thermal parameterswere observedand the generally large values of the standard deviations of the temperature factors make the thermal parameters unreliable. Becauseof the eccentricitiesof the thermal parametersand their stand- ard deviations,the correlationmatrix was calculatedfor the last cycle.Of the 74l nondiagonal terms of the half-matrix in the correlation matrix, 98 coefficientsare larger than .100 and 13 are larger than .300 though none is larger than .520. Thus interactions of thermal parametersare not the primary cause of the large standard deviations and erratic behavior of sometemperature factors. Rather they are partially causedby the domi- nant role of the heavy atoms in the structure and perhaps also by the actual location of As1,Oz, Oaand OH1 off of the mirror plane. This possi- bility is discussedin a later section.

DrscussroN oF TrrE Srnucrunr Schematicdiagrams of the structure of carminite projected on (010) and (100) are shown in Figs. 2 and 3. The arrangementof the oxygen atoms around arsenicatoms is tetrahedral. Within the Asr tetrahedron the six O-O distancesrange Irom 2.77 to 2.92 A, with a meandistance of 2.84A. The 0-0 distancesof the Asztetrahedron ranqe from 2.59to 2.72L 6 J, J, FINNEY

Tasm 3. FrNer, PosrrroxAr- PARAMETERs,Tnrtlrnnrunn Facrons exrr SlaNo.a.to DrvrettoNs or Posrrtoxnr- Pauunrnns lon CenurNrtn

Parameter Parameter

fr .000 .000 Oz JC. .0949 .0031 v .000 .000 v .5317 .0120 z .250 .000 z .000 .000 B 11 .09 B 2.24 I .01

r .250 .000 o: .1136 .OO24 v .750 .000 v -.1055 .0111 z .000 .000 z .000 .000 IJ .36 .08 B -1 .60

li .0427 .0006 Oq T. .1525 .0023 v .7403 .ffi29 v .174t .0104 z 000 .000 z .24rr .0035 B 1.68 .21 B .79 .69

.2t1r .0005 o; .2721 .0022 v .000 .000 v -.or29 .0086 z .250 .000 z .1450 .0030 B 70 .lJ B .58 .56

.3778 .0005 OHr fi .1656 .0023 ) .1309 .0021 v .2589 .0115 z .1360 .0006 z .000 .000 B .40 .11 B -1 '55

Or fi .0182 .0026 OHz fi .4303 .0031 v .2404 .0098 v .000 .000 z .1153 .0031 z .250 .000 B .92 .66 B 90 .97

I Thermal parameter tends negative.

with a mean of 2.68 h. The mean As-O distancesare 1.74 Lforthe Asr tetrahedron and 1.64 A for the Asz tetrahedron. However becausethe standard error of the As-O bond-lengthis greater'than.10 Athisdiffer- enceis not significant. The sixteen iron atoms are located in one sixteen fold position set. They have six nearestneighbors forming an almost regularoctahedron at a mean distancefrom the iron atom of 2.02A. Of the six, four are oxygen atoms and two are hydroxyl groups. Or, Or, On and 05 are the oxygen atomsand OHr and OHzare the hydroxyls. Only Or is not bondedto iron. Its distanceto the iron atoms ol 3.97 A is significantly larger than the meanof 2.02A. CARMINITE 7

Coordination about the two crystallographically distinct lead atoms is eight fold. The eight neighborsabout Pbr are made up of four eachof Or and O+while the coordinationabout Pbz is composedof^two each of Oa and Ozand four of Or. The meanPbr-O distanceis2.67 A and the mean Pbz-Odistance is 2.69A. table 4lists all bond lengthsand anglesand the standard deviationsof the bond lengths. The structure of carminite consistsof networks of linked octahedra of oxygen and hydroxyl about iron extending parallel to the c-axis. The individual octahedra are joined together on an edgeat the mirror plane at z:0,+ and at a vertex at the two fold rotation axis at y:0,+ and z:f,,f;. The networks are in turn linked to arsenate tetrahedra. AII four vertices of the Asr tetrahedron are joined also to the vertices of the Fe-O(OH) octahedra, but only three of the vertices of the Asr tetrahedron are so joined. 03 is not bondedto an iron atom but only to lead and arsenic.The arsenategroups appear to sit atop one another if the structure is viewed as in Fig. 2. However,as seenin Fig. 3 theseare not in contact.The Or-Or distanceof the two overlying Asr tetrahedrais 3.37 A and the Or-Ordis- tanceof the two overlying As2tetrahedra is 3.57A. Becausethe Fe-O(OH) octahedrashare an edgeon one sideand a ver- tex on the other it was thought that the structure actually belongsin spacegroup Ccc2.In the event that carminiteis noncentricneither Oznor Or lie on a mirror plane and instead of only Or, both Or and Oawould be bonded to Fe. The environment about the two crystallographicallydis- tinct arsenicatoms would thus be similar in all respects.Some evidence for this arrangement appears in the results of the least squares refine- ment. As stated previously the temperature factors of As1,Oz, Oa and OHr behaved erratically during the courseof the refinement, but the fact that these eccentricities are noticed only in the atoms lying on the mirror plane makesthe existenceof the mirror somewhatsuspect. In order to ascertain whether Ccc2 is the true space group or not, a noncentric model of carminite was proposed and a least squares refine- ment was attempted. Thermal parameter interactions becameextreme and it was found necessaryto hold temperaturefactors constant at an arbitrary value. The R-factor decreasedto 13/6 and within the limits of error the structure refined to the original model. It was also found that the systematic absencescharacteristic of space group Cccm and Ccc2 could be reproducedaccidently by use of a noncentric model in space groups.C222rand, C222. Agreement of observedand calculated structure factors for the h\l, OhI and hh\ reflections was excellent but no agreement was found in the three dimensionaldata. No evidencecontrary to the existenceof the mirror plane was observed in the Fourier and difierence maps. Still, on the basisof uniformity of environment it is thought that the structure is slightly noncentric. 8 "T. J, FINNEY

T,c.sr.n4. Ixrnnlrourc DrsuNcrs, Born Axcrrs alln Suunnno DsvreuoNs or DtsraNcss lon Canurxrrn

Bond length Bond length

AsrOr (2) 1.7sA .10 As:-On Q) 1.64A .10 _Oz 180 .13 -os Q) t.64 .09 -Oe 1.66 .11 Mean t.& Mean 1 JA

Or-Or 2.84 .13 Or-Or 2.65 .14 or-oz Q) 2.92 .16 O.Os Q) 2.71 .13 OrO: (2) 2.80 .15 O.Or Q) 2.72 .13 O:-Oa 2.77 .17 OtOu 2.59 .12

Mean 2.84 Mean 2.68

Fe-Or 2.O0 .10 OrOz 2.92 -Oz 2.ls .13 O.O+ 2.76 -o4 2.03 .09 O.OHr 2.83 -Os 2.07 .09 OrOHz 2.73 -OHr 2.00 .12 Oz-OHr 2.38 -oH: 1.90 .09 Oz-OHz 3.10

Mean 2.02 Oz-Os 2.84 O.OHr 3.o1 Or-OHz 2.80 OrOs 3.03 Os-OHr 2.82 05-oH2 2.87

Mean 2.85

Pbror @) 2.48i\ .09 Pbr-Pbr >4.00 -or (4) 2.86 .10 _pbz >4.00 Pbz,oz (2) 306 . l.) -Ast 3.72 .03 -Or (2) 2.51 .11 -As: 3.50 o1 -os (4) 2.56 .08 -Fe J. /J .02 Pbr-Pbz >4.00 Asr-Asr 3.9r .04 -Asr 3.M .03 -Asz >4.00 -Asz 3.67 .01 -Fe 3.11 .04 -Fe 3.95 .02 -Fe J. JJ .04 Asz-Asr >4.00 Fe-Fe 3.M .03 -Fe 3.26 .02 -Fe 3.44 .03 -Fe J..)O .02 rOHz-OHz 2.44 .13 oHr-oH1 2.80 .16

I Calculated using *6nr: .4265, CARMINlTD

'Irsrn 4-(Continued)

Angle Angle Angle

Or-Asr-Or 108.5" O;Fe-Oz 89 .9" O:-Fe-OHz 100 8o OrAsrO: 110.50 OrFe-Or 86.50 Or-Fe-Os 98.0" Or-Asr-O: 110.60 Or-Fe-OHr 90.00 Or-Fe-OHr 98.0o OrAsr-Oz 110.6" OrFe-OHz 88 .90 Or-Fe-OHz 90.9o Or-Asr-O: 110.5. Oz-Fe-Os 85 .2" Or-Fe-OHr 87.7" Or-AsrO: 106.10 OrFe-OHr 70.2" Os-Fe-OHe 92.6o

Mean 109.50 Mean 89 .90

Or-Asz-Or 107.40 Or-Fe-Os l75.lo Oo-Asz-Os (2) 111.6. Oz-Fe-Or 167.7" OrAs:-Os (2) 111.1" OHrFe-OHr 171.00 Or-Asz-Os 103.90 Mean 171.20 Mean 109.50

SrunanrrrBs To BRAcKEBUSCHTTE Figure 4 is a projection of the structure of brackebuschiteon (100) with the b-axisdoubled and the origin shifted for easiercomparison of the two structures.Brackebuschite contains two crystallographicallydiffer- ent lead atoms which are designatedPbrr and Pbzr,in Fig. 4. Carminite, on the other hand, can be thought of as having only the site equivalentto Pb15occupied, though in space group Cccm these are two four fold posi- tions. The manganesesites are designated,in brackebuschite,as Mn16 and Mn26,but only Mn16is occupied.The vacant positionsare markedby X1,1,26in Fig. 4. In carminite, both sites are occupied by iron atoms though symmetry requires a different distributiol of 1' parameters (Fig. 2). In their determination of the structure of brackebuschite,Donaldson and Barnes (1955)tentatively located water moleculesabove and below the plane of the four oxygen atoms around Mnrr. This position is marked X6s in Fig. 4. The structure of carminite shows that hydroxyl is located in this position as well as in the position occupiedby Pbzuin brackebusch- ite. This producessix fold coordinationabout the iron atoms in carminite and would be equivalent to six fold coordinationabout both Mnrn and Mn26 in brackebuschite if Pb2bis considered as OH. Radical distortion has not taken place in carminite becausethe site formerly occupied by Pbzr in brackebuschiteis now occupiedby OH, which is approximately the same size as Pb, and the existing octahedralsite Mn2b though now filled has not changed size or environment. The bond lengths between 10 J. J. FINNEY

2 4 6 o o

Oo.Ce PbFeAsOOH r-.ruI--- ol? Angsf roms

Fro.2. The structure of carminite projected on (010). The o-axis is vertical.

Pbru and its nearestoxygen neighborsin brackebuschiteare 2'58, 2'76, 3.02,2.64and 2.83A and thosefor the equivalentposition oHr in carmi- nite are2.38,2.89,3.24,2.82 and 2.83A. An intermediate structure type may exist in which a small cation is added to the vacant Mnzr, site of brackebuschite with a corresponding removal of the large cation from the Pb2r,position to balance charges, with or without changesin the water content. CAKMINITE tl

62,38 88,r2

2A 7A 87Or3 63032 247

e 85 e o r5 50 6^r) Qo e

(, O o Ce t,o FeASOOH E:..:.:.::.==]I::=:= ot2 Angslroms

Frc. 3. The structure of carminite projected on (001). The o-axis is vertical.

A table listing observed and calculated structure factors for carminite has been de- posited as Document No. 7428 with the American Documentation Institute, Auxiiiary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. Copies may be secured by citing the document number, and remitting $1.25 for photoprints or $1.25 for 35 mm. microfilm. Advance payment is required.

AcrNowrBocMENTS The author is indebted to Dr. A. Rosenzweigof the University of New L2 J. .T. FINNEY

Pbrb

\ 7s \_,/ r )\ V\ \ \ I \ \,\ I X Ax( Mn2b ( (

( A ( \..2

) X OH / \ ()

\ t1 ,/ t ( ) ( /\ A\ ( ) ( OH V7 \'/ ( W \

Cooc PbMnVO r----.------.-1 ol2 Angslroms

Ftc. 4. The structure of brackebuschite projected on (100). c sin p is vertical and the b-axis is doubled. CARMINITE 13

Mexico who supplied the sample of carminite and gave permission to continue the original work, to Dr. S. W. Bailey of the University of Wis- consin for his advice during the course of the investigation, to D. T. Cromerand to A. C. Larson of the Los Alamos ScientificLaboratory, Los Alamos, New Mexico for their aid in the computation of Fourier and difierence synthesesand portions of the least squaresrefinement. Com- puter time was graciously supplied by that Laboratory. The Numerical Analysis Laboratory at the University of Wisconsin aided in preparation of programs, data processingand final refinement of the structure. This investigation has been supported by a grant from the National Science Foundation.

RrlrnnNcns

Brncnurs, J., I. M. H.a.aNeemr,, M. Porrnns, B. O. Loopsrnn, C. H. Ma,ccrlt.q,vnv .q.ro A. L. Vnnr+rNo,t,ll (1955) New calculations of atomic scattering factors. Acto Cryst. 8, 478,483. DoNamsoN, D. M. aNo W. H. BenNns (1955) The structure of the descloizite and adelite groups, III brackebuschite. Am. Mineral,. 40, 597-613. Fosreo, W. F. (1937) Carminite and associated minerals from Mapimi, Mexico. Am. Mineral,. 22, 479-484. Jews, R. W. .lNo G. W. Bnrlor.ev (1931) Some numerical calculations of atomic scatter- ing factors. Phil. Mag. (7),12,8t-112. Lnwsunren, C. R. (1939) Carminite and budkinite from the Ashburton District. Jour. Roy. Soc. Western Australio,25, 237. Mnosr, M. E. (1953), Palermoite and goyazite, two strontium minerals from the Paiermo Mine, North Groton, New Hampshire (abs.). Am. Mi.nerol.38, 354. RosrNzwnrc, A. ,tmo J. J. FrNrrv (1959) The unit cell of carminite. Am. Mineral'. 44, 663-665. Stnuxz, H. (1960) Isotypie Palermoit-Carmitit. Neues f ahr. Mineral. 1960,49-62. Wrrsz. O.. W. CocrneN ero W. F. Cor.n (1948) The accurate determination of cell dimen- sions from single crystal x-ray photographs. Actra Crysl. 1, 83-88. Wooo, J. H. eNo G. W. Pnarr (1957), Wave functions and energy levels for Fe as found by the unrestricted Hartree-Fock method. Phys. Rea.107, 995-1001.

Manuscr'ipt recei,red,July 19, 1962; accepl,ed,Jor publication, October 1, 1962.