American Mineralogist, Volume 76, pages283-287, l99I

Structure model for kassite, CaTirOo(OH)t Prrrn G. Srr.r CSIRO, Division of Soils, Private Bag Number 2, Glen Osmond 5A5064, Australia Pnrnn R. Buspcx Departments of Geology and Chemistry, Arizona State University, Tempe, Aizona 85287, U.S.A.

AssrRAcr We have found the rare mineral kassite, CaTirOo(OH)r,in an unusual rock from Jose- phine Creek, Oregcn. Using electron-diffraction techniques,kassite was observedto have a b-centeredorthorhombic unit cell with dimensions4 : 0.908,b : 0.478, and c:0.523 nm. This cell is closely related to that given in the original description of kassite (Ku- kharenko et al., 1965) rather than the cell given in a more recent article by Evans et al. (1986). A crystal structure for kassite basedon that of CaSbrOuis proposed.

INrnooucrroN and alloys, some of which contain noble and rare-earth Kassite is a rare mineral that contains common ele- metals. Our attention was drawn to the kassite grains ments in simple proportions. In spite of this chemical becausethey gave energy-dispersiveX-ray (EDX) spectra simplicity, the literature describing kassite is confused. with a CalTi peak-height ratio of 1:2 rather than the l:l Kassite was first reported by Kukharenko et al. (1965), ratio exhibited by the that is abundant in the who found it in speiimens from the Afrikanda massif on sample. Burton (1982), following an extensive petro- the Kola peninsula in the USSR, where it occursin mar- graphic study of the JosephineCreek specimen,conclud- iolitic cavities of alkalic pegmatites. They assignedkassite edjllat it originated in the Earth's mantle. a chemical formula of CailrO"lOH), and an orthorhom- Th9 chemical formula determined from EDX spectra bic unit cell with dimensioni a : 0.901(3), b : 0.957(3), and electron microprobe analysisfor the kassitefrom Jo- and c: 0.527(l)nm. A secondoccurrence of kassitewas sephineCreek is essentiallyCaTirO"(OH), (with Fe pres- reported from an iron deposit in Henan Province, China ent below 2 at.Vo)-Except for a halving of the , axis, the (yu et al., l9B2). Most of the properties attributed to unit-cell dimensions as determined by TEM are as re- kassiteby yu et al. agreewith those of Kukharenko et al. ported for kassiteby Kukharenko et al. (1965). The latter (1965); however, the Chinese workers described kassite result is contrary to that ofEvans et al. (1986). In.this paper we describe the results of a TEM inves- as optically uniaxial and suggested- that the unit cell should . be tetragonal tigation ofkassite from JosephineCreek and, by studying Subsequently,Evans et al. (1986) reported kassitefrom a synthetic analogue of kassite, propose a structure for the Diamond Jo quarry, which is in a nepheline syenite the mineral. This structure is similar to that of lucasite pegmatiteat the edgeof the Magnet Cove complex. Their (Nickel et al', 1987)' study showed that although the sample from the Dia- mond Jo quarry has a chemical formula that agreeswith Expnnrvtnxtl,r. kassiteas reportedby Kukharenkoet al. (1965),in most ThemajormineralsinourpieceoftheJosephineCreek other respects it is indistinguishable from (Ca- specimen are , , perovskite, and sphene.The FerTioO,r.4HrO), a mineral that also occurs in the Afri- total specimen available to us was a spheroid approxi- kanda massif (Kukharenko et al., 1959). Based on their mately I cm in diameter, and so we were severelylimited extensive crystallographic, petrographic, and gravimetric in the amount of material available for study. It was not evidence, Evans et al. concluded that samplesof kassite possibleto isolate the kassitefrom the specimenbecause and cafetite from the Afrikanda massif had been inter- ofthe small amount ofkassite and becauseit is in intimate mixed by the Russian workers during their X-ray analy- contact with the other minerals. Ilence, suitable samples sis. As a consequence of the work of Evans et al., the for TEM were prepared by crushing a small part of the cafetite and kassite entries have been interchanged in Set specimen and dispersing the resulting grains over holey- 39 of the Joint Committee on Powder Difraction Stan- carbon support grids. All TEM was done using a Philips dards (JCPDS) powder diffraction file (ICDD, 1989). 400T electron microscope fitted with a Tracor Northern We found several grains of kassite while doing a trans- EDX spectrometer and a Gatan electron energy-loss spec- mission electron microscopy (TEM) study of rutile in a trometer. Individual minerals were identified in situ using specimen from Josephine Creek, Oregon. The Josephine the EDX spectrometer.Approximately ten pm-sized grains Creek specimen contains an extraordinary assemblageof of kassite were identified and studied. unusual minerals, including a large collection of oxides X-ray diffraction (XRD) study of the synthetic sample 0003-o04x/91/01024283$02.00 283 284 SELF AND BUSECK: KASSITE, CaTi,O.(OH),

rx512

g'rLtr Ti(L) 1 ': t \l I

@ o c/) a \: c = (K) = o rf,( -\Ai o o \ \r \\l C) \!

Energyloss (eV)

Fig.2. EELSspectrum ofkassite from JosephineCreek.

atomic TtlCa ratio was determined to be 2.05 + 0.07. This result is an average over analyses from several dif- ferent grains ofkassite, and the uncertainty representsthe spreadin the individual results. The concentrations of the minor elements vary from U) grain to grain. In some grains Fe occurs as a minor com- c = ponent, while it is present below the detection limit in o others. Figure lb is an EDX spectrum from the grain C) containing the largest amounts of the minor elements. The Fe/Ti atomic ratio from this spectrum is approxi- mately 0.07, which is much lessthan the 0.50 Fe/Ti ratio expectedfor cafetite. Electron energy-lossspectrometry (EELS) was used to determine the light-element concentrations in kassite from MsL ^, JIA/\/\QrMn Josephine Creek. A kassite EELS spectrum is shown in 246 Figure 2. It contains large peakscorresponding to Ca, Ti, and O energy losses.Quantitative EELS analysesusing X-rayenergy (KeV) perovskite and sphene as standards confirmed the value of the TilCa atomic ratio as determined by EDX analysis. Fig. l. EDX spectraofkassite from JosephineCreek, Ore- By examining EELS spectra in the energy range 0-l gon.(a) Spectrumfrom a typical kassitegrain. (b) Spectrurnfrom keY it was possible to determine that O is the only ele- the grain containingthe highestquantities oftrace elements. ment with an atomic number in the range 3-10 that oc- curs in kassitein significant proportions. Determining the was done using a Philips PWl800 diffractometer and concentration of O proved problematic becausethe ex- pressedpowders. A Cambridge Instruments 5250 scan- tended structure of the Ti(L) energy-losspeak makes it ning electron microscope fitted with a Link EDX system difficult to obtain reliable background subtractions at was used for imaging and chemical analysis of the syn- regions in the spectrum around the O energy-losspeak. thetic specimen. Differential thermal analysis (DTA) of This difficulty leads to a large uncertainty in the value the synthetic sample was done on a Rigaku Thermoflex obtained for the O concentration. Using perovskite, 8100 seriesinstrument. sphene, and rutile as standards, the atomic O/Ca ratio was determinedto be 5.7 ! 0.6. CHnurcu- coMposrrroN During his study of the Josephine Creek specimen, Figure la shows a typical EDX spectrum of kassite Burton (1982) obtained electron microprobe analysesof from the Josephine Creek sample. Ca and Ti occur in kassitebut was unable to identit/ it. Our EDX and EELS major amounts, togetherwith < I wto/oCr and Mn. Quan- results are consistent with his electron microprobe anal- titative analyses of kassite were obtained by using the yses.The samecompositions and totals were obtained by other minerals in the specimenas standardsto calibrate Dawson and Smith (1977, specimen 5, Table 4) from an the detection efficiency of the EDX spectrometerfor the unidentified kimberlitic Ca-Ti mineral. The microprobe elementsof interest. Using perovskite as a standard, the results of both Burton and Dawson and Smith have low SELF AND BUSECK: KASSITE, CaTi'On(OH), 285 totals (8900/0).From the above experimental results we conclude that the chemical formula of kassite from Josephine Creek is essentially CaTirOo(OH)r. For the grain that had the highest concentrations of minor elements(Fig. lb) the chemicalformula is Ca(Ti, ,,Feo,r)o- 03 E7(oH)''3. Cnvsr.lr.r,ocRAPrrY Selected-areaelectron diffraction (SAED) was used to determine that kassitefrom JosephineCreek has a b-cen- tered orthorhombic cell with dimensions 4 : 0.908. D : 0.478, and c : 0.523 nm. By using Au evaporated onto the TEM grid for an internal standard, and by calibrating the diffraction camera length as a function of in-focus objective lens current (i.e., specimenheight), the cell di- mensions could be determined with an accuracyof +0.3010. The unit-cell dimensions measuredusing SAED agreewith Fig. 3. Proposedstructure for kassite.The structureis based those published by Kukharenko et al. (1965), except that on that of CaSbrOu. the b axis is halved in our measurements(0.478 nm as opposed to 0.956 nm). The SAED patterns were exam- repeat distance in the direction perpendicular to the close- ined for reflections that would indicate a doubling of the packedO layersis doubled relative to that of the CaSbrOu b axis, but no such reflections were found. structure. If this doubling were applied to the cell dimen- The D-centeringand halved b-axis length of the cell sions for kassitegiven above (i.e., double the b axis), then determined by using SAED requires a different set of in- the cell dimensions would be the same as those given by dices from those normally used for the XRD pattern of Kukharenkoet al. (1965). kassite. [See,for example, JCPDS powder diffraction file entry 20-243 (ICDD, 1989).1However, all lines that are SYNrrrprrc ANArrocuE indexed for the lattice proposed by Kukharenko et al. A synthetic analogueofkassite has been describedby (1965) can also be indexed by using the cell with b : Rudashevskil et al. (1977). This analogueis a by-product 0.478nm. of the Bayer processin Al smelting and, in order to in- The D centering of the orthorhombic cell means that vestigate the structure of kassite in more detail, we ob- the unit cell can also be describedby a primitive mono- tained a quantity ofthis by-product (known as flash-tank clinic cell with dimensionsa : 0.524, b : 0.478, c : scale).The scalewas found to be a three-phasemixture 0.523nm and 6 : I19.9'. The volume of this cell is such of kassite, cancrinite, and hematite. Rudashevskii et al. that there can be only one formula unit per unit cell. The reported that synthetic kassite-formedat temperaturesin lengths of the a and c axes of the monoclinic cell are the range 100-250 "C, with pressuresrangtng from 30 to nearly equal and the angle between them is nearly 120'; 70 atm. However, C. Cardille (Alcoa Australia Ltd.; per- hence the cell is very close to hexagonalin dimensions. sonal communication) pointed out that kassite does not qC, Convergent-beam electron diftaction from the [010] zone form below 180 and that below this temperature the of the monoclinic kassite cell showed the pattern to be cancrinite in the scaleis replaced by the related feldspath- not only geometrically nearly hexagonal in dimensions oid, noselite. but also that groups ofreflections that would be equiva- The sample was treated chemically to isolate the kas- lent in a truly hexagonal crystal [e.g., (100), (001), and site from the other minerals in the scale.The cancrinite (101), referencedto the monoclinic celll have intensities was dissolvedby leaving approximately l0 g of powdered that are similar for all crystal thicknesses.This similarity scale overnight in a solution of 50 mL of concentrated ofthe intensities and the length ofthe , axis suggeststhat HCI and 250 mL of HrO. The solution was then brought the structure of kassite could be based on a hexagonal back to neutral pH by repeatedly centrifuging, decanting close packing of O atoms. In addition, it is reasonableto the supernatant, and adding HrO. The treated powder expect that in such a hexagonal net the Ti atoms would was recoveredby drying in an oven at 105'C. Hematite lie in octahedral sites. was removed by leaving the acid-treated powder over- One such structure is that of CaSbrOu(Magneli, l94l; night in a solution of 7.5 g of sodium dithionite and 100 Wyckofl 1965). The corresponding structure for Ca- mL of HrO at 60 "C. The treated powder was again re- TirO4(OH), is shown in Figure 3. The cations of the two coveredby repeatedcentrifugation, decantation,addition different atomic specieslie on alternate basal planes. We of HrO, and finally oven drying. propose that kassite has a slightly distorted CaSbrOu An XRD trace of the acid-washedand dithionite-treat- structure, which is also similar to that of lucasite, Ce- ed scaleis shown in Figure 4a. The positions and inten- Tir(O,OH)6 (Nickel et al., 1987). It is interesting that in sities of the peaks in this pattern agree with those of Ru- Iucasite,because of a displacement of the Ce atoms, the dashevskii et al. (1977\. The unit-cell dimensions for the 286 SELF AND BUSECIC KASSITE, CaTi,Oo(OH),

tallographic parameters. A full structure refrnement will undoubtedly require single-crystalX-ray measurements, but unfortunately crystals large enough for such work have not as yet been found.

DrscussroN Our experiments strongly suggestthat kassite has a chemical formula of CaTirOo(OH), and a structure dif- fering from that of CaSbrOuby only a slight distortion from ideal hexagonalsymmetry. This structure is closely related to that oflucasite, except that the D axis ofkassite 40 50 60 70 80 90 as determined by SAED is half that of the lucasite struc- 2-THETA aNGLE (degl ture; however, Russian work report a taJ the original does doubled b axis. In the structure with doubled b axis, the Ca atoms would be displaced from the ideal CaSbrO. positions (sixfold coordination) to positions having eight- fold coordination (see Nickel et al., 1987). As with the 3 failure of Rietveld refinement, the true nature of the Ca coordination will only be resolved when crystals large

z enough for single-crystal X-ray diffraction are found. z The above conclusions for the structure of kassite are not in agreementwith those of Evans et al. (1986). Given the results of our study, it would seemthat the Russians did not necessarilyintermix samples of kassite and caf- etite. This leavesa dilemma as how to reconcilethe con- clusions of Evans et al. with our results and those of Ku-

''THETA ANGLE (dEq) kharenkoet al. (1965).It is known that kassiteand cafetite (b ) coexist,and Figure 227 ofKukharenko et al. (1965) shows Fig. 4. (a) ExperimentalXRD pattern for synthetickassite. cafetite with a tangled-fiber morphology lying next to kas- (Peaksresulting from hematite are marked H.) (b) Computer- site with a blocky morphology. Interestingly, this figure generatedXRD pattern for kassitebased on the CaSbrO.struc- is similar to the SEM photographs of Evans et al., who ture. describe two types of kassite-platy rosettes and yellow spherules.The crystallographicwork of Evans et al. was carried out only on the rosettes,and so it is possible that synthetic kassitewere determined from the XRD pattern the mineral with the cafetite structure has a rosette or tobe a: 0.524(l),D : 0.468(l),c : 0.524(l)nm and B tangled-fiber morphology while kassite has the more : 120.0(lf (equivalent to an orthorhombic cell with di- blocky spherule morphology. mensionsa: 0.908,b : 0.468,and c : 0.524nm). Evans et al. basedtheir results not only on Fe content Not all of the hematite was removed by the sodium but also on . Becauseof the nature of our dithionite treatment, as indicated by the presence of a sample we were unable to determine any morphological few hematite diffraction lines. The composition and ther- or crystal-habit properties of the kassite from Josephine mal characteristics of the synthetic kassite were deter- Creek. We are unable to offer an explanation for the dis- mined by EDX and DTA analysis, respectively. Our crepancy between the crystal habit of the sample of Evans analysesconfirmed the Russian results. et al. (which we would label cafetite) and that of cafetite By using the Rietveld refinement program of Hill and describedby Kukharenko et al. (1965). Howard (1986), we compared the XRD pattern in Figure From the results ofour study ofkassite from Josephine 4a to our proposed structure. A calculated pattern for the Creek, we conclude that kassite and cafetite should not structure, shown in Figure 4b, is in reasonable agreement be distinguished by Fe content but rather by crystallo- with the e*perimental pattern. The atomic positions for graphic properties. Even so, it seems that the status of CaSbrOuare not available, and so the positions used to kassite and the related mineral cafetite are destined to generate the pattern in Figure 4b were those for the iso- remain controversial until more specimens have been structuralcompound PbSbrO6as given by Wyckoff(1965). characterized. A Rietveld refinement of the kassite structure was at- tempted using the XRD pattern in Figure 4a. However, Acrxowr-nncurNrs the slight distortion of the unit cell from hexagonalge- ometry meant that many reflectionswere unresolved,and We thank J.M. Bfud for supplying the Josephine Creek sample, J.H. Burton for a copy of his M.Sc. thesis, W.V, Preiss and P. Ding for trans- so the effects of instrumental parameters (especially peak lations, H.T. Evans for supplying copies of M. Fleischer's translations of width) could not be distinguished from the effects of crys- two Russian papers, J. Clark and S. McClurc for SEM work, C. Cardille SELF AND BUSECK: KASSITE, CaTi,On(OI!, 287 for discussion of flash-tank scale, E. Nickel and I. Madsen for discussion Cafetite, a new hy&ous titanate of calcium and iron. Zapiski Vseso- oflucasite, R. Hill for supplying a copy ofa Rietveld refinernent program, yuznogo Mineralogicheskogo Obshchestva, 88, 441-453 (in Russian). and M. Phillips for a helpftrl review. This study was supported with the Kukharenko, A.A., Orlova, M.P., Bulakh, A.G., Bagdasarov,8.A., Rim- help of National ScienceFoundation grants EAR-8408 168, EAR-8708529, skaya-Korsakova, O.M., Neldov, Ye.I., tryinskiy, G.A., Seryeyev,A.C., and equiprnent grant EAR-8408 163 for the electron microprobe. Electron and Abakumova, N.B. (1965) The Caledonian complex of the ultra- microscopy was performed at the facility for HREM at Arizona State basic alkalic rocks and carbonatites ofthe Kola Peninsula and northern University, which is supported by the university and NSF. Karelia. Izdatelstvo "Nedra" Moskva, 368-375 (in Russian). Magneli, A. (1941) The crystal structure of PbSbrO6and isomorphous RnrnnnNces crrED compounds.Arkiv foer Kemi, Mineralogi och Geologi, l58, no. 3, 6 p. Nickel, E.H., Grey, I.8., and Madsen, I.C. (1987) Lucasite-(Ce),Ce- Burton, J.H. (1982) An extraordinary specimen derived frorn the earth's Ti,(O,OH)., a new mineral fiom Western Australia: Its description and mantle. M.Sc. thesis, Cornell University, Ithaca, New York structure. American Mineralogist, 72, 1006- l0 10. Dawson, J.M., and Smith, J.v. (1977) The MAFJD (mica-amphibole- Rudashevskii,L.S., Firfarova, I.8., and Tsekhobal'skay4D.I. (1977)Syn- rutile-ilmenite-diopside) suite of xenoliths in kimberlite. Geochimica thetic analoguesand refinement ofthe properties ofkassite and hydro- et Cosmochimica Act,. 41, 309-323. kassite. Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva, 106, Evans, H.T., Dwornik, E.J., and Milton, C. (1986) Kassite from the Di- Il4-120 (in Russian). amond Jo quarry, Magnet Cove, Hot Spring County, Arkansas: The Wyckofl R.W.G. (1965) Crystal structures,vol. 3, p. 36,t-366. Wiley, problem ofcafetite and kassite. American Mineralogist, 71, 1045-1048. New York. Hill, RJ., and Howard, C.J. (1986) A computer program for Rietveld Yu, S., Li, S., Uu, IC, Atuang,L., andLi,Z. (1982) A preliminary study analysis offixed wavelength X-ray and neutron powder diftaction pat- on the mineralogy of the Zraoanzhuang-type iron ore deposit Wuyang, terns. AAEC Research Establishment (ANSTO), Lucas Heights, Aus- Henan Province, China. Zrongguo Dizhi Kexueyuan Yichan Dizhi Ku- trata, researchreport AAEC/MI12. angchanYanjuiso Sokan, 5, 1-22 (in Chinese). ICDD-International C,enterfor Dftaction Data (1989), 1610 Park Iane, Swarthmore,Pennsylvania 1908l-2389. MlNuscnrrr RrcErvEDM,lv I I, 1990 Kukharenko, A.A., Kondratyeva, V.V., and Kovyatsina, V.M. (1959) MrNuscmgr AccEp'rEDNoverusER 14, 1990