American Mineralogist, Volume60, pages435440, 1975

Joaquinite:The Natureof its Water Contentand the Questionof Four-CoordinatedFerrous Ironr

Gsoncs R. RossunN Department of Geological and Planetary Sciences, Califurnia Institute of Technology, Pasadena,CaliJornia 91125

Abstract

The polarizedoptical and infraredspectra of orientedcrystals of joaquinitefrom San BenitoCounty, California, have been obtained. The infraredspectra have polarized absorp- tion bands at 3561, 3500, and 1607 cm-r, which are assignedto the vibrationsof crystallographicallyordered molecular water in joaquinite.The opticalspectra show features dueto neodymimumand divalent . The nearinfrared absorptions near 1000 and 2100 nm are discussedin termsof six-coordinatediron(ll) ratherthan the four-coordinatediron(II) proposedfrom X-ray structuralstudies.

Introduction per formulaunit (basedon two Ba).Laird andAlbee (1972)found that electronmicroprobe analyses to- Joaquiniteis a rare silicatemineral best known taled to less than 100 percent and attributed the from its associationwith benitoiteand neptuniteat deficiencyin the total from 100 percentto water. the DallesGem Mine, San BenitoCounty, Califor- They concludedthat hydroxideis presentto the ex- nia.Its structuralproperties have recently been deter- tent of about3.3 OH per two Ba.They presented in- mined (Cannillo, Mazzi, and Rossi, 1972:Laird and frared spectralevidence to substantiatethe conclu- Albee, 1972).Cannillo and coworkersproposed a sionthat thereis crystallographicallyoriented OH in unit cell formula of BaoFerRelTi4O4[SidOlLaird joaquinite.Cannillo et al (1972)elected not to assign "]n; and Albee,and Semenovand coworkers(1967) have hydroxideto the structure;instead, they suggested proposedsimilar formulae but haveincluded hydrox- that hydroxide (or water) could be occurringin ide iohs.Cannillo's structure determination suggests cavitiesin the structurebut hasno structuralrole. that joaquinite containsfour-coordinated iron(lI) A studyofthe opticaland infrared spectra ofsingle in an unusual coordinationgeometry. Four-co- euhedralgrains of joaquinitehas been conducted to ordinatediron(Il) is rare in silicateminerals. The characterizeboth the four-coordinatediron and the electronicspectroscopy of four-coordinatediron(ll) role of the OH content.The resultsof that studyare is accordinglypoorly examined. Square planar Fe(Il) reportedherein. A comparisonof thespectra of four- hasbeen studied by Burns,Clark, and Stone (1966) in coordinatediron(Il) in a variety of mineralswill gillespite and by Rossman (in preparation)in appearat a laterdate. eudialyte,whereas mineralogical tetrahedral Fe(lI) has been studied in spinels (Slack, Ham, and ExperimentalDetails Chrenko,1966; Mao and Bell, in preparation)and The joaquinitespecimens used in this studycame staurolite(Burns, 1970). The spectroscopyof other from the vicinityof the DallesGem Mine nearNew four-coordinationgeometries has not beencharacter- Idria, SanBenito County, California. Their identity ized in silicatemineral systems. was verifiedthrough morphology,mineral associa- The role of OH in the structureof joaquinite tion, and the infrared spectra.They consistedof remainsunsettled. Semenov et al (1967)reported 1.5 roughlyequidimensional, yellow-brown crystals 0.6 wt percentHrO in their analysisof Greenlandjoa- to 1.2 mm in size which, although clear and quiniteand concludedthat therewas one hydroxide transparent,had several fractures within the crystals. Crystals were first oriented by external mor- I Contribution No. 2465, Division of Geological and planetary phology,ground to the appropriatethickness and Sciences,California Institute of Technology. polishedwith 0.3 trrmAl2O3 porvder. In all cases, 436 GEORGER. ROSSMAN sectionsfrom the center of the crystal were used.The Analytical optical spectraof self-supportingthin slabswere ob- Electron microprobe analyseswere obtained by tained aI 23"C with a Cary l7I spectrophotometer Arthur Chodos with an automated Mec v equipped with dual calcite polarizers. The crystals microprobe for the specimenswhose spectra are il- were then further ground and polishedto a thickness lustrated.Reporting total iron as FeO, the Bxa sam- appropriate for the infrared spectra which were ob- ple had 3.29 wt percentFeO (averageof 4 points) and tained at -28oC with a Perkin Elmer 180 spec- the Bxo sample had 3.49 percent FeO (average of 3 trophotometer equippedwith a common-beamwire- points). More detailedelectron microprobe analyses grid polarizer. Absorbances are defined as log I"f I; e of several specimens from this locality are reported values are in liters mole-tcm-t. by Laird and Albee (1972), who report FeO values Optical and X-ray Orientation (ZFe as FeO) from 3.76 to 4.55 wt percent. No evidenceof trivalent iron was found in the optical The optical orientation of joaquinite was originally studiesreported below. presented by Palache and Foshag (1932), who reported a : X, b: Y, : Z and c an orthorhombic The Water Content cella:9.61 A, b : 10.45A, c : 22.4 A.This conven- tion was followed by Semenov et al (1967). Both The near-infraredoptical spectrum(Fig. l) shows Laird and Albee (1972)and Cannillo et al (1972)have for 7-polarization a sharp band at 1935 nm and a demonstratedthat joaquinite has a monoclinic unit weak band at -1440 nm. These absorption bands cell which can twin on the (001) plane to form an respectively occur at wavelength regions in which the orthorhombic cell expandedalong the monoclinic c* combination mode, stretch-plus-bend,of molecular axis. Cannillo and coworkers designated the axes in HrO, and the first overtone of the symmetric stretch the orthorhombic cell, following the precedent of of water occur. Their existenceimplies the presence Palacheand Foshag,setting ao. : 9.58 A b.. : 19.59 of molecularwater in joaquinite, and the pronounced A, and co.: 88.84A, which is four timesthe c dimen- polarization of these bands means that the water is sion specifiedby the earlier workers. They designated structurally ordered.These observations motivated a the axesof the rnonoclinicunit cell following the con- study of the single-crystalinfrared spectrain the water ventions for the monoclinic , in- and hydroxide absorption regionsusing the methods terchangingthe orthorhombic a and b axes,so that of Labotka and Rossman Q91\. 4- = 10.50A, b- : 9.58A and cm : 11.78A. Laird The infrared spectrum of powdered joaquinite and Albee followed the same convention for the (Laird and Albee, 1972) showed O-H stretching monoclinic cell with 4* : 10.51A, b- : 9.66 A, and modes at -3560 and 3500 cm-'. These frequencies c- : I 1.82A, but chosean orthorhombic cell-ao, : are in the range in which the O-H stretchingmotions 10.48A, bo,: 9.66 A, and c,, : 22.26A.-whose a of hydroxide ions in minerals are commonly found. and b axescoincide with the monoclinic a and b axes. Because these frequencies are higher than the O-H The orientation convention usedin this report was stretching frequenciesof both liquid and coordinated dictated by the crystals used in the experiments. water, and becausethese two absorption bands are Crystals were cut according to external morphology lnarro* compared to those of liquid water, it would and the optical orientations of Palacheand Foshag be straightforward to assign them to weakly (1932) to yield cuts with centered Bxa and Bxo hydrogen-bondedhydroxide ions which, presumably, figures. Extinction was sharp and uniform through- are crystallographically ordered. out the crystal along the apparent orthorhombic The polarized crystal infrared spectral data (Fig. 2) directions, indicating that the crystal used for provide additional important information regarding the Bxo slice is submicroscopically twinned to the nature of the hydroxyl content:(l) The 3561cm-' produce an orthorhombic crystal on the scale of band is 7-polarized and the 3500 cm-' band is 0- the optical experiment.Orientations were, therefore, polarized. This confirms that the O-H units are chosen to follow the convention of Palache and crystallographicallyordered. (2) The 1607 cm-' ab- Foshag but modified by the axial lengths reported by sorption band is B-polarized.This feature can only be Laird and Albee; thus a : aor : 9.66 A, 0 : b,, : assignedto the zz deformation of molecular water. 10.48A, and 7 : co, : 22.26 A where co, is in the Molecular water in the gas phasehas been shown to direction of the monoclinic c*-. have three infrared-active fundamental absorption JOAQUINITE

JOAOUINITE I I I I I I \ Lu \ I \ O I z. v.o^^ \ I \ < I \ m I \ E. I \ l\-;-'l 8 o.o m {/\ o.2 \

800 t200 t600 2000 24c0 WAVELENGTH(nm) Ftc l. Polarized optical absorption spectrum ofSan Benito County, California,joaquinite crystals at 295K. 0.20 mm thick Bxa crystal for a,8,0.19 mm thick Bxo crystal for 7. bands. They correspondto the z, deformation at 1595 forces.Discrete hydroxide ions are absent.This cm-', the z, symmetric stretchaI 3657 cm-', and the modelis consistentwith the proposalof Cannrlloet al zr antisymmetricstretch at 3756cm-1 (Benedictand (1972)that the hydroxyl or water resideswithin Plyler,l95l;Benedict, Claasen, and Shaw,1952). The cavitieswithin the structurein a fashionsimilar to polarization of the molecular water absorptions is beryl. readily predicted from group theory (Nakamoto, The spectroscopicand thermal propertiesof the 1963).The v, and the z, modes are both expectedto water in joaquiniteare similarto thoseof waterin be polarized in the direction of the two-fold axis of beryl (BerAl,Si6O,s).Wood and Nassau(1968) have the molecule whereasz3 is polarized in the plane of used infrared spectroscopyto study isolatedwater the molecule.The assignmentof the 3561cm-' band moleculestrapped in the channelsof beryl. The to z, and the 3500 cm-' band to y, is consistentwith general featuresof the spectra of water in beryl, theseconstraints. (3) The 1935nm band (5168cm-'), which they reported,are similar to those of joa- polarized along co",is assignedto the combination v" quinite,but richerin structuredue to the presenceof * zsof molecular HrO. The u2enetgy is known, so by two structurally distinct types of water and to the algebraic difference, the stretching frequency, zr, presenceof low-energyhindered rotational motions must be at 3561 cm-r. It should be 7-polarized as is that are coupledwith the stretchingand bendingof the combination v, * zr, which in fact it is. This con- the water molecules.In beryl, the water is tightly firms the assignment of the zs band. Other features trappedin the channelsby alkali ions and was not can be assignedas overtones,such as the 3190 cm-' releasedfrom some crystalsin heatingexperiments B-polarized band, which is 2u2. All of the spec- until a temperatureof 1350'Cwas reached. troscopic features which arise from O-H units can be The Drn curveofjoaquinite (Semenovet al, 1967) explained in terms of molecular water. shows an endothermat 770"C, which has been The structural model of the water in joaquinite suggestedto be dueto the releaseof water.Release of consists of molecules well-isolated from each other HrO at such a high temperatureis generally at- oriented parallel to (100) with their symmetry axis tributedto the dehydroxylationof hydroxidegroups parallelto [010].They are essentiallyunbonded, lack- in the mineral rather than due to releaseof pre- ing significanthydrogen bonding and coordination to existingmolecular HrO. The fact that the wateris un- a cation through ,instead being held in cages able to escapeuntil this temperatureis reachedin- by physical entrapment and long-range electrostatic dicatesthat it is tightly trappedin joaquinite. GEORGER. ROSSMAN

WAVELENGTH(MICRONS) 2.5 46

356tii3502 JOAQUINITE

+I U.J O o.9 z m E. o a o.6 m

o.3 \------,-\.- I -- o.o 4000 3500 3000 2500 t800 WAVENUMBER(CM-I) Ftc. 2. Polarizedinfrared absorptionspectra ofSan BenitoCounty, California,joaquinite;crystals l8 pm thick, at 305K. For clarity, the a and B spectra in the 4000-2500cm-r region have been displaced ver- tically 0.05and 0.10 units respectively,and in the 1800-1400cm-'region 7 andB havebeen displaced0.l0 and 0.05 units resoectivelv.

The inclusion of hydroxide ion in the formula of can be made up by inclusionof Na+, K+, and Li+ joaquinite no longer appears necessary.When the plus minor amounts of other divalent cations found detailedchemical analyses of Laird and Albee (1972) in the analyses.The joaquinite formula proposedby are compared to the idealizedformula for joaquinite Laird and Albee suggestedthat a substitutionoccurs proposed by Cannillo et al (1972) from the X-ray betweenFe2+ and Na+ on a six-fold site as well as a structural data, Ba.TioFerREaSiruOur,it is found that substitutionof Sr into the rare earth site. An idealized the total cation charge 105.3(based on 16 Si) from formula of San Benito joaquinite is accordingly the microprobe analysescompares favorably with the statedas Ban(RE,Sr)o(Fe,Na)rTioSir.Our.xHrO, where anion charge of 104 demanded by 52 oxygen ions. .r is a variable quantity roughly equal to 2. This agreementmust be consideredto be close,par- ticularly because Laird and Albee's data refer to Optical Spectrumand Discussion averaged analysesover numerous points of an in- homogeneoussample and include 0.4 Li+ based on The polarizedoptical spectrum of joaquinite(Fig. site-occupancy considerations rather than direct l) showstwo prominentregions of absorptionnear analyses.Even closer agreementwas obtained (total 1000nm and2lOp nm. Numeroussharp and relative- charge 104.7) by examining unpublished data of ly weakfeatures occur through the range400-900 nm Laird and Albee taken on a singlepoint on their joa- whichare rare earth absorption bands, predominant- quinite sample.The role of the in joaquinite ly thoseof Nd8+,which is the most abundantspec- still requires clarification. Iron is present to an extent troscopicallyimportant rare earthelement found in well below the two atoms per formula unit in the the San Benitojoaquinite. These bands will not be idealizedformula. The deficiencyof atoms, however, consideredfurther. The sharp, narrow absorption JOAQUINITE 439 featureat 1935nm in 7-polarizationhas beendis- and the M(2)-O(5) and M(2)-O(2) bonds elongated cussedin connectionwith the water content. The to 2.21A. charge-transfertail which begins to rise at about The spectrumof joaquinite bears a surprisingly 700 nm risesto at least 350 nm without additional close resemblanceto the enstatite spectrum. The features.lt has its greatestabsorbance in the visible spectraof both mineralshave prominent ab$orption regiorlin the 7-polarizationand, as such,is responsi- bandsnear 1000 and 2000 nm, whichare polarized in ble for the pleochroismof joaquinite. differentdirections. The energies of thejoaquinite ab- The two broad near-infraredbands are both sorption bandsare roughly l0 percentlower than polarizedand shift slightly in band maxima with those of enstatite.The prominent 506 nm spin- polarization.The intensities decrease in theorder a at forbiddenband ofenstatite has a correspondingband 1070nm ) 0 * I bothat 955nm. The components of in joaquinite at 523 nm. The resemblanceis sur- the lowerenergy band are polarized 0 at 2080nm ) 7 prising becauseit would be expectedthat four- at 2270nm )) a which is effectivelyabsent. These coordinatedC, iron(II) would have absorption two bandsare interpreted as absorption bands of the bandsat a much lower energythan six-coordinated Fe2+.The intensities of thebands are within the range C' Fe(II) in a fashionsimilar to 4/9 energyreduction commonlyencountered with Fe(II). In the polariza- betweenOr and Za Fe(II). One structuralaspect of tion of maximumband intensity,€roro and erogrboth the iron siteis disturbing.If the siteis viewedas an equal 24. octahedronfrom which two equatorialligands have The X-ray structuraldata indicatethat the optical beenremoved, the two axial oxygensare bent toward spectrumshould be interpretedin termsof four coor- the remainingtwo equatorialoxygens, placing the dinatedFe(II). In theirstructure determination, Can- iron outsidethe volumeof the distortedtetrahedron nillo el a/ foundtwo Fe-O(4)bond distancesof 2.16 delineatedby connectingthe positions ofthe four ox- A andtwo Fe-O(8)distances of 2.21A (oxygenposi- ygenligands. tion designationsfrom Cannillo et al, 1972)'The These observationssuggest that the iron is six- O-Fe-O bond anglesdepart greatly from both the coordinated in a highly distorted octahedral site idealtetrahedral angle of l09o and from the square resemblingthe M(2) site of enstatite.ln view of the planarangle of 90".The iron is locatedin a siteof C, structuralcomplexity, which resultsfrom twinning symmetry which can be approximatedas an oc- and the questionsremaining regarding the detailsof tahedralsite from which two oxide ions havebeen the iron site,detailed analysis of the joaquiniteop- completelyremoved. The remainingoxygens have tical spectrumis not warranted,but mustawait X-ray O-Fo-O bond anglesnot greatlychanged from the structuraldata from untwinnedcrystals. octahedral values. They are O(8)-Fe-O(8) 173", Acknowledgments O(a)-Fe-O(a)94o, two O(a)-Fe-O(8)at 8l', andtwo O(a)-Fe-O(8)at 95". Thesedetails of the iron coor- Mr. Buzz Gray, of the Dalles Gem Mine, provided most of the joaquinite specimensused in this study. ProfessorFiorenzo dinationpolyhedron must be considered with caution Mazzi providedadditional structural data regardingthe iron site in view of high R value (l6vo),which was due to the from his X-ray stfucturedetermination, and Jo Laird suppliedin- poor quality of the crystalused in the structuredeter- formationregarding her X-ray studiesand chemicalanalyses and kindly reviewedthe manuscript. mination. In C2,symmetry the degenerateE and I electronic References states of tetrahedral and square-planar four- BeNrorcr.w. s.,rNo E. K. plyr,En(1951) Absorption spectrum coordinated iron will be split so that five states-two of watervapor and carbon dioxide in theregion of 2.7microns. uAr,uAr, uBr, and 582-exist. A detailed analysisof the J. Res.'Natl.Bur. Stand. 46, 246-265. (1952) splitting of energy states of Fe(II) in Cr, geometry in -, H: H. Crnrsen,rNp J' H' Snrw Absorptionspec- tj"-:.f-:i*' vaporbetween 4'5 andl3 microns'J' Res'Natl' enstatite has been presentedby Runcima,l s"nguitu, and Marshall (1973).The qualitatiu. tvrl""ti ,fiT..ti|%:'il.t|.'lt;^-., rNpA. r. Sroxe(re66) vibronic aspectsof that analysis are applicable to the joa- polarizationintheelectronicspectraofgillespite,amineralcon- quinite problem, because the iron site reported for tainingiron(ll) in square-planarcoordination. Inorg. Chem 5, joaquinite corresponds to the M(2) site in enstatite 1268-78' rT;.;; Appticatiansof crvstatFietd with the M(2)-o(3) and M(2)-o(6) bands (oxygen ;.tli,r!1".r*,^rricat position designations from Runciman et al) c^,u*"J", e., F. Mr,zzt,nrpG. Rossr(1972) The structure type elongated to the point that the O(3) and 0(6) ions are of joaquinite.Tschermaks Mineral. Petrogr. Miu. 17,233-246. no longer within the coordination sphere of M(2), Lnnorr,r, T. C., er.roG. R. RosstrlrN(1974) The infrared 440 GEORGER. ROSSMAN

pleochroism of lawsonite: The orientation of the water and SeveNov.E. t., V. I. BuKrN,Y. A. Blllsuov, euo H. SdnrNsrN hydroxide Broups Am. Mineral. 56, 799-806. (1967)Rare earthsin mineralsof the joaquinitegroup. Am. Letno, J., AND A. L. Alser (1972) Chemical composition and M ineral.52, l'762-1769. physical, optical,.and structural properties of , nep- Slecr, G. A., F. S. Hrpt.lxo R. M. CsrEuro (1966)Optical ab- tunite, and joaquinite, Am. Mineral. 57, 85-102. sorptionof tetrahedralFe'+(3du) in cubic ZnS, CdTe, and Nrrnuoro, K. (1963) Infrared Specta of Inorganic and Coordina- MgAIrO.. Phys Reu. 152,376402. tion Compounds.J Wiley and Sons, New York, p. 8l-84. Woop, D. L., lNo K. Nlssrru ( 1968)The characterizationof beryl PrLlcHr, C., nuo W. F. Fosnlc (1932)The chemicalnature 01 and emeraldby visible and infrared absorptionspectroscopy. joaquinite. Am. Mineral. 17, 308-312. Am. M ineral.53,'l'17 -800. RuNcur,rlN,W. A., D. SeNcuere, AND M. Mlnsunll (1973)The polarized spectra of iron in silicates. I. Enstatite. Am. Mineral. Manuscript receiued,September 20, 1974: accepted s8, 444-450. for publication,January 14, 1975.