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 iron. 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 crystal system, 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