The Gopiapite Problem: the Grystal Structure of a Ferrian Copiapite

American Mineralogist, Volume 58, pages 314-322, 1973

The GopiapiteProblem: The GrystalStructure of a FerrianCopiapite

Luce, FeNrnNr, ANroNro NuNzr, PrBn FnaNcnsco ZtNezzt, lNo ANNe Rosn Zlxzlnr Institute of Mineralogy, Uniuersityof Perugia,06100 Perugia, Italy

Abstract

Crystal structure, thermal and chemical data are reported for a ferrian copiapite from Cali- fornia, with a cell content (nooFeo*3*Alos3*Znoe2+)Fs*s*(SO,)u(OH)r*'18.78Hzo, lattice parameters:a-7.390(8),b-IE.2l3(lO),c=7.290(8)A,"-93'40(15)',8-102"3(25)', - - t 99"16(15)'and space group P1; p calc - 2.123 e/cm'; p obs 2.A8-2.15.The crystal structure was solved with Patterson and Fourier methods and refined to an R value of 0.074 for 1435 independent observed reflections which were measured photometrically on integrated Weissenbergphotographs. The main features of the atomic arrangement are: (1) chains formed by SOn tetrahedra and Fe(OH)(ILO)rO" octahedra, (2) isolated octahedra at the origin of the cell, and (3) water molecules not linked directly to cations and contributing to a complex system of hydrogen bonding. The structure is somewhat different from that of a magnesiocopiapite from Alcaparrosa (Chile) reported by Siisse (1970). Lattice parameters of the material studied by this author seem to indicate that Alcaparrosa crystals are actually pseudocopiapite. The possibility that copiapite and pseudocopiapite are two stereoisomerically different mineralogical species is suggested. Structural analysis accounts for the wide range of isomorphous replacement and large varia- bility in water content of copiapite.. When the X position is occupied by a trivalent cation, as is the case of copiapite under study, the occupancy is statistically 2/3, jtrstifyine the constant O/X ratio with values near to l, where O represents +2 (one oxygen equivalent) and X = 2 (cationic charge y occupancy of the cation) for all cations occupying the X site, as observed by Berry G947).

Introduction (1833) andlater named by Haidinger(1845). Mel- (1890) proposedthe following The present paper concerning the structure of a ville and Lindgren Me'z-O'2FezOs' ferrian copiapiteis a contribution to the crystalchem- chemical formula for copiapite: proposedfor some istry of iron sulfates.It follows two papers on the 6SOs'20 HzO. In 1938 Bandy formulae of the crystal structuresof roemerite (Fanfani et al., 1970) natural copiapitesseveral chemical indicate a and butlerite (Fanfani et al., I97l). type, Me2.Fe+(SOr)o(OH)ynH2O,to ln 1'947Berry, Severalwidely occurring basic hydrated iron sul- considerablerange in water content. on 42 copiapite fates, with variable chemical composition, are on the basis of chemical analyses the general grouped under the name copiapite. samplesfrom different countries,derived whereX is one The mineral occurs frequently in associationwith formula,X(OH)2R43.(SO+)a'nHzO, (i.e., charge of 2* for the other secondaryminerals such as melanterite,aluno- oxygen equivalent a total elementsNa, K, Cu, gen, fibroferrite, halotrichite, botryogen, butlerite, X site) of one or more of the some- amarantiteand other sulfates.Generally it is a result Fe2*,Mn, Mg,Zn, Al, Fe3*;R3. is mainlyFe3*, of the oxidation of iron sulfides.Conditions for the times A13',and the value of n is 20. The namesfer- formation of copiapite and other sulfateshave been ricopiapite, ferrocopiapite, magnesiocopiapite,alu- discussedby Bandy (1938) for the depositsof Al- minocopiapite, cuprocopiapite and zincocopiapite caparrosaand Quetena (Northern Chile), where the were proposedfor varietieswhere X wasmainly Fe3*, mineral is one of the earliestbasic minerals originat- Fe", Mg, Al, Cu, and Zn respectively.This large ing from the oxidation of pyrite ores. variation in the chemical composition appears to Copiapitewas first describedand analyzedby Rose causechanges in the physicalproperties of copiapite. 314 THE COPIAPITE PROBLEM 315

For example,as Berry pointedout, opticallythe va- The differencein elementsbetween copiapite and rieties of copiapite fall into three distinct groupsdif- pseudocopiapitewas ascribedpresumably to varia- fering from one anotherin the valuesof the principal tions in composition,but Berry could not carry out indices of refraction according to the chemistry of any chemical analysis on Ungemach's crystals of the X site: when X : Cu, the highestindices are pseudocopiapitebecause of the minute amount of exhibited;when X = Fe2*or Mg, the indicesare at material at his disposal.In 1947, in the paper on a minimum; copiapiteswith X = Fe3*or Al3* have compositionand opticsof copiapite,Berry measured intermediateindices. the refractiveindices of pseudocopiapitecrystals and From a crystallographicpoint of view, the min- suggestedon their basisthat pseudocopiapiteis in eral was first consideredorthorhombic, then mono- reality a ferro- or a magnesiocopiapite,rather than a clinic, until Ungemach(1935) and, about the same ferricopiapiteas assumedby Ungemach.Since the time, Palache (quoted in Palache, Feacock and propertiesof pseudocopiapitelie within the observed Berry, 1946) discoveredits triclinic symmetry;co- or expectedrange of those of copiapite,the name piapite was assignedto the pinacoidalclass on the "pseudocopiapite"was not considereduseful as in- basis of an examinationof the very large number of dicative of a mineralogicalspecies by Berry. forms observed.In 1939 Peacock (quoted also in Available crystallographicdata for copiapitesare Palacheet al., 1946) showeda closeagreement be- reportedin Table9. tween his morphologicaland X-ray data on Chuqui- Since the space group of copiapite is Pi, it is camata (Chile) crystalsand the morphologicaldata interestingto note that from a structuralpoint of view by Ungemachon SierraGorda (Chile) materialre- and assuminga fully ordered atomic arrangement, ferredto the standardsetting: X must occupy a special position in the cell at an equal (Peacock) aib:c : 0.4058:I :0.4039, inversioncenter. Assuming then the O/X ratio one, as inferred by Berry (1947) and confirmed ot : 93o50',I : 102"10',I : 99"21L'; to probable one for a : 7.34,0: 18.19,c : 7.2g4,, by Garavelli (1955) as the most the X position is completelyoccupied only a : 93o51',0 : l0l'30', I : 99"23'; copiapites, whenX is representedby a divalentcation. When X is (Ungemach)a:b : c : 0.4005:l :0.3971, a trivalent cation, the corresponding position is : : 102"08', : a 93"58i', 0 I 98o50'. required to have 2/3 ocotpancy,forming copiapites Comparablelattice parameterswere reported for an with lattice vacancies.A completelyordered structure aluminocopiapite(Jolly and Foster, 1967) and for can rule out the limiting chemicalformula a zincocopiapite(Kuang et al., 1964). . In his study on morphology of copiapite crystals, xr*Rn3*(son)u(oH),nHro Ungemachrecognized an aberrantvariety frorn Tierra proposedby Berry for some copiapitescontaining a Amarilla (Chile), which is visibly indistinguishable certain amount of alkali metals. From a structural from copiapite, but gives different crystallographic point of view O/X ratios differing from the value of I elements,the most significantdifierence occurring ir1 can be explained,assuming that a changeOH- = HrO the value of angle '. Ungemachnamed this variety can occurin the structurewithout apparentchanges in "pseudocopiapite." Berry ( 193 8) studiedUngemach's the atomic arrangement. crystalsof this variety, confirmingthe morphological In 1970Siisse published the preliminary results of elementsand measuringthe cell constants.The ele- the crystal structureof a copiapitefrom Alcaparrosa ments of pseudocopiapiteaccording to theseauthors (Chile)with latticeparameters: a : 7.37,b : 18.89, are: c : 7.42i+,a: 9lol8',A : 10224',? : 98o59'and HrO. (Ungemach)a:b : c : 0.3938: I :0.3951, schematiccell content MgFen(SOn)u(OH)r'20 Therefore the structural study of a ferrian copia- a : 91"18*',A : 102"4',I : 98"59'; pite was undertakenin the attempt to contribute to (Berry) a:b : c : 0.4007: I :0.4005, a better knowledgeof crystallochemicalrelationships : : 102"22', : a 91o22',0 "y 98'50'; in this widespreadclass of sulfateminerals. from morphologicaldata; and: ChemicalData (Berry) a : 7.26,b : 18.67,c : 7.454., a : 90o58',g : 104"35',.y : 98o36', A chemical analysison a samplefrom Coso Hot from X-ray data. Spring, Inyo County, California, was carried out be- 316 FANFANI, NUNZI, ZANAZZI, AND ZANZARI

fore undertaking the structural work. The material for the analysiswas carefully selectedfrom a speci- men consistingof bunchesand clustersof extremely tiny yellowcrystals. The resultsare listed in Table 1, alongwith thosekindly suppliedby ProfessorGara- velli for material from the samelocality. The chemical formula derivedfrom the analyticaldata is:

(n o.arFeo.ns'*Alo.rrt*Zno or'*; . Fen't1son;u(oH),on. I g.TgHro correspondingto an intermediate term between a ferri- and an aluminocopiapite.The mineral has an O/X rutio very close to 1 and can be classifiedas a typical copiapite with lattice vacanciesaccording to Berry (1947).

Thermal Data Thermogravimetric(rc) differential thermogravimetric (orc) and differentialthermal analysis (nrl) curves for Coso Hot Spring copiapite are shown in roo 2oo oo0 4oo soo 600 700 goo goo rooo oC Figure 1. All endothermic peaks corespond with Fro. 1. Thermogravimetric (rc), differential thermo- loss-in-weightsteps. From the rc curve the mineral gravimetric (orc) and difierential thermal (ou) curves of appearsto losewater gradually.At 380'C dehydra- copiapite. Rate of heating: lO"C/min.; thermocouple Pt/Pt tion is completed,in agreementwith data reported { 10 percent Rh; reference material AlzOs. by Scharizer(1927) on copiapitefrom Alcaparrosa and by Garavelli( 1955) on materialfrom Elba. The 2.04 water moleculesand can be partially attributed Drc curve showsfour distinct peaksroughly centered to the lossof OH groups. at 135o, 185",225o and 360'C. The threelower The behavior in dehydrationin our sampleis not temperature peaks are relatively sharp, while the in completeagreement with that found by Garavelli higher temperaturepeak is quite broad. Assuming (1955). The orl curve,too, is slightlydifferent from for our samplea total water contentof 19.80mole- that reportedby Cocco (1,952) for an elban copia- cules accordingto the analysis,the water loss in the pite. Thesediscrepancies in thermal behavior are not first three stepsis 6.94, 6.37 and.4.45 molecules. surprising considering the wide range in composi- The broad peak at 360'C correspondsto the lossof tionallimits and crystallogaphicproperties.

SingteCrystal X-Ray Study Tlsrs 1. Analysis of Coso Hot Spring Copiapite Experimental. A well-formedtabular crystalwith g in oxides CelI content dimensions0.18 x 0.19 x 0.04 mm, parallelre- (a) (b) (b) (a) spectivelyto the c, a, and b axes,was employedfor Fe2O3 29.33 29.oL + o.43 o.38 X-ray data collection with CuKo radiation. Cell di- Al2o3 o.99 L.Z3 o.29 mensionswere measuredfrom oscillation and basal ZnO O.2O o. 03 FeO o.34 o.06 Weissenbergphotographs, rotating the crystal around CUO o. or o.oo the a and c axes.Photographs were calibrated with so3 39.62 39.85 Ag powder, and the following values were obtained HzO 29.57 29-33 r.3 4.OO 4.OO by least-squaresrefinement, starting from 20 values SO, 6. OO 6. OO of high anglereflexions: TotaI 99.7L 99.77 OH 2.O4 z.t3 H2o r8.78 r8.55 a : 7.390+ 0.008A,o : 93"40'+ 15' (a) - Present work. : p : (b) - Garavelli (private conmunlcation). b 18.213+ 0.010A, 102"3'+25' c : 7.290+ 0.008A,t : 99"16'+ 15' THE COPIAPITE PROBLEM 317

Theseresults are in good agreementwith the values reportedby Palacheet aI. (1946). The theoretical density for the cell content determinedby the chemical analysisis 2.123 g/cm"; experimentaldensity for copiapitesis in the range2.08-2J.5 g/cm1,Palache etaL (1963). For the structural study intensity data from hkO to hk5 and from Okl reciprocal lattice layers were collectedand photometricallymeasured on integrated Weissenbergphotographs. 1435 independentreflec- tions with intensity above the limit of observation were measuredon a total of 2950 collected reflec- Frc. 2. The crystal structure of copiapite viewed along [100]. tions. Data were correctedfor the usual geometrical factors. Transmissionfactors for the absorptioncor- refinement. At this stage a difference Fourier map (p : rection 179.8cm-1, for CuK") werecomputed was computed to try to confirm the likely positions by a direct analyticalprocedure using a programwrit- of hydrogen atoms from geometrical and electro- (1965). ten by De Meulenaerand Tompa Squared static considerations.The poor resolutionof the syn- structure factors were then put on the samerelative thesis did not allow assignmentof a physicalmean- scale by comparison of the common reflections oc- ing. Therefore hydrogenatoms were not included in curring on the Okl photograph. calculations.The refinementwas then continued to a final R valueof 0.074.The weightingscheme used StructureDetermination and Refinement - wasas follows: wL/2:1for lFol < alFmin.l;wr/z The structureproposed by Siisse (1970) for a la Fmin.l/Fol for lFol > 4lFmin.l. Unobservedre- magnesiancopiapite from Alcaparrosa was used as flection were excluded from calculations.Observed the startingpoint of our investigation.The calculated andcalculated structure factors are listed in Table2.' valuesof the structurefactors were not in agreement (conventionalR index equal to 0.65). The difter- Discussionof the Structure encesin chemicalcomposition and lattice parameters The structure of copiapite projected along the betweenAlcaparrosa copiapite and Coso Hot Spring a axis is shownin Figure 2. Table 3 givesthe atomic copiapiteled us to supposethat large differenceshave coordinates and Table 4 shows the anisotropic to occur in the atomic arrangement. thermal parametersand the isotropic equivalentscal- A three-dimensionalPatterson function was then culatedaccording to Hamilton (1959). The distinc- computed to determinethe positions of iron ions in tion between oxygen atoms, hydroxyl groups, and the cel[. By successiveFourier maps, three sulfur water moleculesis made on the basis of electrostatic atoms and twenty-threeoxygen atoms werelocated in valencerule. For sake of comparison,the labelling the asymmetricunit. The X site at the inversioncenter of atoms is maintainedequal to that reported by 0, 0, 0 was consideredoccupied by a hypotheticalion Stisse ( l97O). with scatteringfactor 1/3 f^v* + 2/3 /p""* and a Interatomic distances and angles arc listed in total occupancy of 0.69, in agreement with the Tables5 and 6. chemical analysis. The scattering factors for Al3*, The two crystallographicallynon-equivalent Fe3* Feu*, S and O were taken from InternationalTables ions (both in general positions) lie in distorted for X-ray Crystallography(1962). oxygen octahedra of the same configuration and A first refinementof the structurewas carried out shareone vertex. In both octahedrathe ligands are: by electron-densitymaps to an R value of. 0.22. The refinementwas then continued by least-squares 'To obtain a copy of Table 2, listing observed and calcu- method, employing a block-diagonalprogram writ- lated structure factors of copiapite, order Neps Document ten by Shionofor the tsN{ 1130. During the first five Number 02061. The present address is Microfiche Pub- cycles,positional parameters and individual isotropic lications, Division of Microfiche Systems Corporation, 305 New York, N.Y. 10017. Please remit in thermal parameters were varied, and the conven- East 46th Street, advance $1.50 for microfiche or $5.00 for photocopies. tional R value was reducedto 0.10. Further reduc- Please check the most recent issue of this journal for the tion in R was achievedin two cycles of anisotropic current address and prices. 318 FANFANI, NUNZI, ZANAZZI, AND ZANZARI

Tesre 3. Final Atomic Coordinates with Their Standard bond length between sulfur and oxygen bonded to Deviations in Parentheses* iron (1.49 A) is significantlylonger than the average distancebetween sulfur andunsharedoxygen atoms (r.44A). o o o The pair of Fe octahedra sharing the hydroxyl Fe(r) .7848(3) .3r35(r) .5524(4) Fe (2) . s990(3) .67r8(r) . 80s8(4 ) group is also linked by S(2) and S(3) tetrahedra s (1) . 832s(s ) .7398(2) .2L9s(6) to form a cluster of two octahedra and two tetra- (2) s .8r70(s) .4L7s(21 .2r88(6) two adja- s (3) . 6402(s ) .1927(2) . r9s3(6) hedra. The S(1) tetrahedronconnects o (r) .7357(r5) .67so(5) .07r4(16) cent clusters to produce a chain which runs in o (2) .6930(r4) .7 698(7 ) .2963(t7l the t1011 direction. The chain is shown sche- o (3) .0483(r4) .292r(7) .6322(r7 | o (4) .0527(16) .2022(7 | .8s47(r8) matically in Figure 3, the chain compositionbeing o (s) .52or(r4) .39rO(6) . ro85(15) [Fe,(OH) (H2O)4 (O2SO2 ), (OSO3)]"-'. o (5) .8295(L4) .4886( 6 ) .32o8(16) The main featuresof copiapitestructure are: (1) o (7) .8774(13',t .3626(6) .3474(r5) o (8) .9350(r4) .4228(7 | .0842(r6) chainsof SO+ tetrahedraand Fe(OH) (HrO)rOt o(e) .s5s5(18) . r1s9(7) .1687(r7) octahedra(2) isolatedoctahedra at the origin and o(ro) .sog9(r4) .7637(5',) .863s(r6) (3) six water moleculesper cell not linked directly o(rr) .792r(14) .2r3r (7) .o980(r6) o (r2) .7r85(14) .2128(6) .4053(r6) to cations and contributing to the complex system oH(r3) .4626(r4) .5604(6) . s4s4(16) of hydrogenbonding presentin the structure. HrO(r4) .68s9(rs) .2699(7 (L6',, Features (2) and (3) are common to other - (rs) | .7 673 .8685(16) .4142(7) .7r38(r8) iron sulfates.For example,isolated octahedra and (r6) .2076 (27) .9474 (LL) .o66s(28) (r7) .6933(r5) .5740(6) .7598(17) free water moleculeswere found in roemerite(Fan- (r8) (28) .or58 .9855(r2) .7338(28) fani et al., 1970), and in coquimbite and para- HrO(I9) (15) - .830r .7282(6',) .7309(r6) (201 .17r3(26) .0925(rr) .o473 (29) coquimbite(Robinson and Fang, l97l). (21) .7 660(23) .9r96(9) .4381(241 The systemof hydrogenbonding has beenderived (22',) .5280(r4) .s508(6) .2767(16) (231 . 63s5(31 ) .o743 (L2l . s949(3r) by optimizingthe criteriareported by Baur (1972); * hydrogenbond lengthsare listed in Table 5. The Occupancy factors are equal to I for all atoms except for the X ion, for which it is o.69. resulting oxygen electrostaticbalance is reported in Table 8, on the assumptionthat the donor atom re- ceives0.83 a.u. (5/6) of the bond strengthdonated three oxygen atoms belongingto three SO+groups, by the hydrogen atom, while the acceptor receives two water molecules,and one hydroxyl group which O.l7 u.u. (1,/6).The resultsin the caseof copiapite representsthe common vertex. Average distances are of course over-simplifiedand approximate,be- are 1.95 A for Fe-OH, 2.00 A for Fe-O and2.04 A cause the system of hydrogen bonds is somewhat for Fe-H2O disordered,especially in the zone around the origin. The site i at the origin of the cell is surrounded For example, when the X position is empty, the octahedrallyby six water moleculesat a meandistance water moleculessurrounding it probably rearrange, of 1.93A. fnis site is occupiedrandomly by Al't settingup H-bondsalong the edgesof X octahedron. and Fe'* (the atomic ratio Al/Fe is l/2) to a total Furthermore,these water molecules,as well as the occupancyof 0.69. "free" moleculesHzO(21) and.H2O(23), which It is interestingto note that all the Al presentis participatein the sameH-bond system,seem to be in the X position, a result analogousto that found only statisticallypresent, as shownby the abnormally in coquimbite (Fang and Robinson, 1970; Giace high value of their thermal parametersand by the vazzi et al., 1970), wherethe Al presentin the cell water content revealedbv chemical analvsis. preferentially fills a site surroundedoctahedrally by six water molecules. SOa groups are tetrahedral with S-O distances rangingfrom 1.40 Io 1.52A. Each SOatetrahedron shares two corners with two Fe octahedra, the other two vertices being unshared. As already c pointed (Fanfani out in roemerite et a1.,1970) this Frc. 3. Arrangement of octahedra and tetrahedra in fact affectssulfur-oxygen distances; thus, the average copiapite chains. THE COPIAPITE PROBLEM 319

Tler,e 4. Anisotropic Thermal Parameters, with Standard Deviations in Parentheses and Equivalent Isotropic Temperature Factors after Hamilton*

-22 Btz -t3A -23 e (421 IJ

x .or58(r2) .oo32(2) .or66(r7) .oor2(s) .oo38(rr) .0006(5 ) 3. 54 Fe(I) .oo9o(4) .oo25(r) .oro2(6) .ooo7(2) .ooo8(4) .ooos(2) 2.40 Fe(2) .oo9r(4) .oo25(r) .oo97(5) .ooo9(1) .oorr(4) .ooo4(2) 2.42 s(1) .oo72(6) .oo24(r) .0068(ro) .ooo9(2) .ooo9 (6 ) .0006(3) r.95 s (2) .oo?6(6) .oo20(1) .oo86(ro) .ooo6(2) .oo2r(6) ,ooo9(3) r.95 s (3) .oo9o(6) .oor9(r) .oo79(ro) .oorr (2) .ooo7(5) .ooo3(3) r.97 o(r) .or4r (2r) .oo2r(4) .oo99 (2't) .ooo6(8) -.oor9(r9) .ooo4(8) 2.65 o(21 .oo95(r9) .oo35(4) .or55 (30) .oor3(8) .oo44(r9) .ooo4(9) 3.ro o(3) .oo79(18) .OO37(4) .or42 (291 .oor5(8) -.ooo6(r8) .oor4(9) 3,rr o(4) .or49 (241 .oo29(41 .ors4(3r) .ooo3(9) .oor4(22) .oor2(ro) 3.35 o(s) .oo95(r8) .oo33(4) .oo52 (251 -.oooo(7) .oo2r(r5) .ooor (8) 2.45 o (6) .oro2(r9) .oo18(4) .or50(29) .oor5 (7) .oor3(r9) .oor6(9) 2.46 o (7) .oo8s(r8) .0026(4) .oo84(26) .oor4(7) .oorr(r7) .oor4(8) 2.23 o (8) .oo83(r8) .oo33(4) .oo9s(28) .oor3(7) .oo3o(r7) .ooo9(9) 2.55 o (9) .o26L(29) .aO27(4) . oo94( 30) -.ooor(23) -.ooo2(9) 3.54 o (ro) .oo86(r9) .oo29(4) .croo(28) -.ooor(r8) .ooor(9) 2.55 o(rr) .oo94(r9) .oo4o(5) .oo93(28) .ooo9(8) .oo39(r8) .oorl (9) 2.92 .orr9 (20) .oc20(3) .oro9(27) .oor7 (7) .ooo9(r8) .oorr (8) 2.36 oH(r3) .oo83(r8) .oo3o(4) .orr8 (28) .oor7(7) .ooo3(r8) .oor5(9 ) 2.63 H^O(14) .ott2(2r) .oo28(4) .oo9t (30) -.oooj (8) .oor6(r9) .ooo5(9) 2.66 ' (t5) .or39(2r) .oo3r(4) .or50(3 r ) .oo22(8) .OO_?O(20) .oor2(ro) 3.r9 (r6) .o373(49) ,0066(8) .o389(55) .oo23(r8) -.ooo9 (42) -.oo26(r8) 8.33 (r7) .or 14(20) .oo25(4 ) .or36(29) .oor8(7) .ooro(r9) .ooo9(9) 2.73 (18) ,o5oo(50) .oo7r (9) .0369 (s7 ) .ooo9(2r) .or16(44) .ooo3(r9) 8.99 H?O(19) .oL27(2Ol .OO27(41 (28) .ooo3(8) .oo45(r8) -.ooo5 (9) 2.'t7 - .orr5 (201 .c302(43) .oos3(8) .oso't (521 .oor2(r6) -.oo22 (431 .oor5(r8) 8.rr (2r) .o315(39) .OO49(7) .o32r(4s) .oor3(r4) .oo5r(33) -.oo16(r4) 6.56 (22') .or39(20) .oo28(4) .or44(29) .oo22('11 .oo72(r9) -.oooo (9) 2.94 (23) .0524(6r) .006r(9) .0430(6r) .oo40(20) .0086(50) .oo43(19) 8.97

The anisotropic temperature factcrs are in the = * * + + * 2ozg!l)] r exp [-torr!2 Bzz\2 0ss]2 2orzh\ 2Br:E

The complex formed by X octahedra and deviationsproduce a different reciprocal orientation H2O(2I) and H,zO(23) moleculesbridges two iron- of octahedraand tetrahedra,even if the packing of sulfur chains through hydrogenbonds. Furthermore polyhedra is not greatly affected. In the atomic adjacent chains are interlinked by hydrogen bonds, arrangementthe main changeconcerns the orienta- both directly and through the HzO(22) molecules. tion of the X octahedronand the location of water The structure accounts for the perfect {010} moleculesH2O(22) betweenthe chains. Of course cleavage. these displacementsaffect the system of hydrogen bonding, which difters noticeably between the two ComparisonWith AlcaparrosaCopiapite structures. and Conclusions Thesestructural difierencescould be expectedon The crystal structure analysisof Coso Hot Spring the basis of the different chemical cell content and copiapiteshow analogiesand difterenceswith respect lattice parameters.Since crystal elementsfound by to that reported by Stisse(1970) for Alcaparrosa Siisseclosely agree with those for pseudocopiapite copiapite.Configuration of ligands about iron and X by Ungemachand by Berry, the magnesiocopiapite octahedrais equal for both structures.The general investigated by Siisse may be a pseudocopiapite. atomic arrangementis similar, the structureproposed This agreeswith the suggestionsof Berry-based by Stisse consisting of chains of sulfur and iron on optics-that pseudocopiapite is probably a coordination polyhedra, isolated magnesium octa- magnesio-or a ferro-copiapite.On this assumption hedra, and free water moleculesbuilding up a com- copiapiteand pseudocopiapitecan be consideredtwo plex system of hydrogen bonds. However, a com- stereoisomericallydistinct mineralogical species.In parison of atomic coordinates reveals significant confirmaton of this hypothesis,we observethat, de- changesin the positions of all atoms: deviations spite the large range in composition of copiapites, mainly concern the x and z coordinates,while y a continuousvariation in lattice parametersbetween coordinatesare approximatelyin agreement.These the limits found for copiapite and pseudocopiapiteis 320 FANFANI, NUNZI, ZANAZZI, AND ZANZARI

TesLn 5. Bond Irngths, with Standard Deviations structural chain motif, is fully explained. This confirms the observations on other iron sul- preferen- Atoms bondedx sond lengths (i) phates, that Al-Fe substitution occurs tially in X(tIrO)u isolated octahedra. -Hô x (r) (r6)(rrr) 1.93(2) *x : -njo(re) (rr) | .97 (2)xx 2) The ratio O/X 1., regarded as the most -H;o (20)(r) I.90(2)xx probable one for copiapites on the basis of a Fe(r)(I)-o(3) (Iv) 2.o2(L) large number of chemical analyses, is struc- -o(7)(r) r.98(r) -o(r2)(r) 2.OO(r) -oH(r3)(v) r.95(r) -Hô(r4) (r) 2.o2(t) T.Lsr,B6. Bond Angles, with their Standard Deviations -uzntrq\ /rt 2.os(t) Fe(2) (I)-o(1) (vI) r.98(1) -o(s) (v) 2.06(r) Atom trio forming angle+ AngIe (degrees) -o(ro)(r) r.9s(r) -oH(r3)(r) r.94(r) HÔ(r6) (rrr) -x(r)-H-O(r8) (rr) QA ? /Al -HÔ z -c"ot2o) (r7) (r) 2.Os(r) "2'', (tl 90.o(9) -H;o(re)(r) 2.04(r) H2O(r8)(rr) -x(r) -H2o(20)(r) 93.o(9) s(r)(r)-o(r) (r) r.52(r) o(3) (rv)-Fe(1)(I)-o(7) (r) 8s.2(5) -o(2) (r) r.43(r) -o(r2)(r) 86.9(s) -o (3) (v) r.45(r) -oH(r3)(v) L74 .2 (5) -o (4)(v) r.45(1) -Hô(r4)(r) 96.7(51 s (2)(r) -o (5) (r) r.49(r) _H,ofr5)"2"' (r) ao ? rql -o(5) (r) r.43(r) o(7)(I)-Fe(I) (I)-o(r2) (I) vJ.r(5, -o (7)(r) r.47(r) -oH(r3)(v) 89.o(s) -o (8)(r) r.44(r) -nô(r4)(r) 176.2(5) _H"o(15) s(3)(r)-o(9) (r) r.40(r) "2-'- (r) 87.9(s) -o(10)(v) r.47(r) o(12)(I)-Fe(I) (I)-oH(r3) (v) -o(rr)(r) r.46(r) -Hô(r4)(r) 90.3(5) -o(r2)(r) 1.s2(r) -Hio(r5)(r) r75.o(s) * oH(r3)(v)-Fe(I) (I)-Hô(r4) (r) 89.o(s) The Roman numerals designate coordinates for _H"o(r5)(r) 90.3(5) the atoms as follows: "2" H2o(14)(I)-Fe(r) (I)-H2o(Is) (I) 88.9(5) (rl x, y, z (vII) 2-x, L-y. L-z o(I) (vI)-Fe(2) (I)-o(s) (v) 85.o(s) (II) x, y-L. z-I (VIII) r-x, I-y, -z -o(10) (r) 90.9(s) (III) x, y-1, z (Ix) x-I, y, z _oH(r3)(r) r7s.6(5) (IV) I+x, y, z (x) l-x, 2-y, I-z -H"O(r7)(r) 86.4(s) (v) l-x, l-y. L-z (xI) x, y, -I+z -H;o(re) (r) 92.o(s) (VI) x, y, I+z o(s) (v)-Fe(2)(r)-o(ro) (r) 9r.r (s) _oH(13)(r) 92.8(s) ** Two such symnetricaLly equivalent bonds are -HÔ(r7)(r) 87.2 (5) present. -Hio(rg) (r) (5) z L75.2 o(ro) (I)-Fe(2) (r)-oH(r3)(I) 93.o(5) -H,O(r7) (r) r76.9(s) -H;o (re) (r) 92.6(5) not reported in literature. This can be seen in oH(I3) (I) -Fe (2) (I) -H,o (r7) (I) 89.7(s) -H;o(1e) (r) 89.9(5) Table 9, where crystal data for several copiapites Hzo(rZ)(I)-Fe(2) (1)-H2o(19)(I) 88.9(s) are listed. this Moreover, table shows the impossi- o(r)(r)-s(r) (r)-o(2) (r) ro9.4(7) bility, until now, of relating chemical data with the -o(3)(v) 105.8(7) -o(4) (v) rL2.4(7) two different sets of lattice parameters. o(2)(r)-s(r) (r)-o(3)(v) ro9.o(7) However, in our opinion, the problem of the -o(4) (v) relationships between chemical data and atomic o(3)(v)-s(1) (r)-o(4) (v) ro9.5(7) arrangementstill remainsopen. It would be interest- o (s)(r) -s (2)(r) -o (6)(r) ro9.6(7) -o(7)(r) rro.9(7) ing to verify with new data whether the copiapite -o{8)(r) ruo.f(/, and pseudocopiapitestructures are typical of certain o(6)(r)-s(2) (r)-o(7) (r) rro.9(7) -o(8)(r) r1r.2(7) compositionsor whether they merely representtwo o(7)(r)-s(2) (r)-o(8) (r) 107.6(7) polymorphic forms. o(9)(r)-s(3) (r)-o(10) (v) rro.3(7) The following crystal chemicalconsiderations can -o(rr)(r) r13.3(7) -o(12)(r) r07.o(7) be derived from the results of the structural analysis o(ro)(v)-s(3) (r)-o(rr) (r) rro.r (7) of Coso Hot Spring copiapite: -o (r2) (r) 107.6(7) 1) the possibility of a wide range of substitution, o(rr) (r)-s(3)(r)-o(12) (r) ro8.2(7)

mainly in the X position, where the coordination x see footnote to TabJ-e 5. polyhedron is only weakly connected to the THE COPIAPITE PROBLEM 321

Tasre 7. Supposed Hydrogen Bond Lengths Tenr,r 8. Balance of Electrostatic Valences for Oxygen Atomsx

Presumed hydrogen bond* Length -H -H"' . "H- Total (r) H Hro(22) (r) 2.764 oH(13) -o o(r) 3/2 | /) 2 H2O(14) (r) B (2)(v) 2.7 L o (2) 3/2 L/6 x2 2-L/6 H O (II) (VI) o (3) 3/2 r/6 2+L/6 2.70 o (4) 3/2 r/6 2-L/3 (rs) (r) H o (6)(vrr) 2 .68 H2o o (s) 3/2 r/6 2+r/5 H o (8)(vr) 2.63 o (5) 3/2 L/5 x2 2-r/6 H2o(r6) (r) H O(9) (VIII) 2.94 o(7) 3/2 1/) o (8) 3/2 t/6 x2 2'r/6 H HrO(23) (v) 2.58 'o o (9) 3/2 1/5 x2 2-L/6 H2o(r7) (r) H (8) (vrr) 2.73 o(ro) 3/2 L/2 H H^o(22) (v) 2.69 o(1r) 3/2 I/6 x2 2-L/6 o(r2) 3/2 t/2 r/6 2+L/5 H2O(18) (r) H H;o(21) (rx) z.oL H H;o(21) (x) 2.7 L oH(r3) I/2 x2 5/6 2-r/6 'o Hro(la) r/2 5/6 x2 2+r/6 Hzo(r9) (r) n (3) (v) 2.99 - (r5) r/2 5/6 x2 2+I/6 H o (1r)(vrr) 2.8r (r6) L/3 5/6 x2 2 (r7) H2o(20) (r) E o (4)(xr) 2.67 t/2 5/6 x2 2+1/6 (r8) L/3 5/6 x2 2 H o(e) (r) 2.82 H.O(19) L/2 5/6 x2 2+I/6 ' H2o(2r) (r) H o (2)(r) 2.78 (2o\ r/3 5/6 x2 2 n H^o(23) (v) 2.95 (2r) 5/6 x2 r/6 x2 2 (221(r) H 'o (5)(vrrr) (22) 5/6 x2 r/6 x2 2 HzO 3.03 (23) 5/6 r/6 x2 2+r/6 H o (6)(r) 2.7 4 H2o(23) (r) H o (12)(r) 2.99 X was assmed to be one oxygen equivalent.

x See footnote to Table 5. 3) The water content limit of copiapite is 20 mole- turally confirmed; when X is a trivalent cation, cules per cell. The occurrenceof "free" water lattice vacanciesare observed to maintain the moleculesjustifies the possibility of partial de- total positive chargein the X position at approx- hydration, as suggestedby chemical analyses, mately 2. without sigrificant damage in the ftamework.

TesLB 9. Crystallographic Data for Copiapites

Morphologlcal data X-ray data Ref I

a:b:c ati) u r8l c til - -o. -4 (so4) (oH) 20.o H2o 90- 6 2. 70. o.4oo5:r:o.397r 93"581' rOZ"O8, gSoSo' (r) (".3]ozoro. r.l* rso4)(oH) 2o.r arr.j*rrl 6 r .z:. "z? o.4058:l:o.4039 g30SO, rO2'rO, 99"2tl' 7.34 r8.r9 7.2a 93-5r' rOr'30' gg"2l' (2',, (rro. ) rel+zo (soa) (oH) r 7 . 30 H2o rs."3]oz"ro. o6cuo.o4Ko. onuao. o, '1.285 r . to. 7.35 r8.r6 93050' 10ro30' 990221 (3) (M9o.s9coo.o3Fej+orcuo.orato.ro)(rej+rrlro.rr)e(so4)6(oH) Hzo 2.24.19.46 7.25L r8.r6r 7.267 94"OO' rO2"L7' 97"581 (4) (oH) 18.7I H2o trel+nraro.rrzno.o,) rel+ tso4) 6 2,o4. 7.390 18.213 7,290 93040' rO2003r 99'r6' (5)

pseudocopiapite o.3938;rrO.39519r'r81' rO2"04',98'59' (6)

pseudocopiapite - '7 O.4OO7:I:O.4OO59f 22' tOZ"ZZ' 98'50' 7 .26 18.67 .45 goosg' ro4':5' gBoge' (7't

MsFe4(OH) (SO4) H2O 2 6.20 7 .3'l 18.89 7 .42 9r"rg' toz"zt' 98o59' (8) (soa) tref,+orero.ar) trel+rraro.or)a e{oH) r. rr. rs. eo nro '1,3r2 r8.650 ?.337 91'r2' ro2oo5' 98"49' (9)

* Referencei (I) From Slerra corda, ChiLe (Ungflach,1935). (2) From Chuquicilata, Chtle (Palache et aI.,I946).Analyst conyer. (l) rion Tsaidm Basln, chtna (Kuang et aI..1964).ch4ical fomula recalcirlated lsswing 6 SO, groups in the unit ceII. (4) From Mosquito Fork. Alaska (Jolly and Foster, 1967). lSi rrom Coso Hot Sprlng, CalifornLa (present study). {6) From Tleira Anarilla' Chite (Ungflach,1935).No analysls avallable. (7) Fron Tlerra Amarilla, chlle (Berry,l938). No anltysis avallable. (8) rrom Alcaparrosa, Chile (slls- se,l97O).llo detall"d lnalysis avallable. The cell content is probably idealized. (9) From Ritorto, Italy (Garavelli, prlvate comunicatlon). 322 FANFANI, NUNZI, ZANAZZI, AND ZANZARI

Temperature factors seem to indicate that the Gttcovt'zzo, C., S. MnNcnETTr, AND F. Sconomr (190) more movable of them are }{20(21) and The crystal structure of coquimbite. Rend. Acc. Naz. H2O(23), and the three moleculesbonded to the Lincei, Serie VIII, 69, 129-140. HerorNcr,n,W. (1845) Handbuch der bestimmendenMin- X cation. This result agreeswith the data by eralogie. Vienna. Siissefor Alcaparrosa crystals. Hrunrou, W. C. (1959) On the isotropic temperature factor equivalent to a given anisotropic temperature Note Added in Proof, In 1972 Siissepublished factor. Acta. Cry stallogr. 12, 609-610. the final results of his study on copiapite (2. Kris- International Tables for X-ray Crystallography (1962). tallogr. 135, 34-55). Since no sigrificant differ- Vol. III. Birmingham, Kynoch Press. encesfrom the preliminary study occur, considera- Jorrv, J. H., rNo H. L. Fosren (1967) X-ray diffraction tions developedin the present paper are still valid. data of aluminocopiapite. Amer. Mineral.52, 1220-1223. Ku,ulc, Tu-Cnrr, Hsr-LrN Lr, HsreN-Tr HsIrn, lNo Snu- Acknowledgment SnrN YrN (1964) Zincobotryogen and zincocopiapite, two new varieties of sulfate minerals. Ti Chih Hsueh Pao, This study was supported by the Italian Consiglio Nazion- 44r 99-101, From Amer. Mineral. 49, 1777. ale delle Ricerche. Mervrrrr, W, H., eNp W. LrNmner (1890) Contributions References to the mineralogy of the Pacific Coast. U.,1. Geol. Sura. Bull. 61,23-26. BlNov, M. C. ( 1938) Mineralogy of three sulfate deposits of PeLrcnr, C., H. BenuAN, AND C. FnoNoBr, (1963) The Northern Chile. Amer. Mineral. 23, 669-:760. system of mineralogy, VoI. IL lohn Wiley & Sons, Inc., Brun, (1972) W. H. Prediction of hydrogen bonds and New York. hydrogen atom positions in crystalline solids. lcta M. A. Pelcocx, eNo L. G. Brnnv (1946) Crystal- gr. C ry stallo B.28, 1456-1 465. lography of copiapite. Unio. Toronto Studies, Geol. Ser. (1938) BERRy,L. G. On the pseudocopiapiteof Ungemach. s0,9--26. Uniu. Toronto Studies Geol. Ser. 41, 7-lE. RoarNsoN,P. D., eNo J. H. FeNo (1971) Crystal structures (1947) Composition and optics of copiapite. Uniu. and mineral chemistry of hydrated ferric sulphates: II. Toronto Studies, Geol. Ser. 51, 2l-34. The crystal structure of paracoquimbite. Amer. Mineral. Cocco, (1952) G. Analisi termica differenziale di alcuni 56, 1567-1572. solfati. Per Mineral.2l, 103-141. Rose, G. (1833) Ueber einige in Siidamerikavorkommende DB MeureNlER, J., eND H. Toupe (1965) The absorption Eisenoxydsalze.Ann. Phys. 27, 309-319. correction in crystal sturcture analysis. Acta Crystallogr. Scnlnrzen, R. ( 1927) Beitriige zur Kenntnis der chemischen 19, 1014-1018. Konstitution und der Genese der natiirlichen Ferrisulfate. FrNreNt, L., A. Nuxzr, eNo P. F. Ztxtzzr (1970) The Z. Kristallogr. 65, 335-360. crystal structure of roemerite. Amer. Mineral. 55. 78-89. Sijsss, P. (1970) The crystal structure of copiapite. Neues _ ( AND 197I ) The crystal structure of lahrb. Mineral. Monatsch. 196E, 286-287. butlerite. Amer. Mineral. 56, 7 5l-7 57. UNceuecn, H. (1935) Sur certains mindraux sulfat6s du FeNo, J. H., eNo P. D. RosrNsoN (1970) Crystal struitures Chili. Bull. Soc. Franc. Mineral SE.9T-221. and mineral chemistry of hydrated ferric sulphates: I. The crystal structure of coquimbite. Amer. Mineral. 55, 1534-1540. Genevettl, C. L. (1955) Ricerche su alcuni solfati del giacimento ferrifero di Terranera (Isola d'Elba) , Rend. Manuscript receiued, luly 13, 1972; accepted lor publica- Soc. Mineral. Ital. ll, 149. tion, Norsember21, 1972.