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 chem- ical 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 thermogravi- metric (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