TI{E AMERICAN MINERALOGIST, VOL. 52, JULY-AUGUST, 1967

PROPERTIES OF A SYNTHETIC -ALUMINUM CARBONATE HYDROXIDE AND ITS RELATIONSHIP TO MAGNESIUM-ALUMINUM DOUBLE HYDROXIDE, MA- NASSEITE AND HYDROTALCITE1

G. J. Ross ,lxn H. Kooeue Soil ResearchInstitute. Canad.aDepartment of Agriculture, Ollowa,Onlorio. Assrnecr A synthetic mineral prepared by titrating a mixed solution of MgClz and AlCl: with NaOH in a COz-free system and then diaiyzing the suspension for 30 days at 60'C was a hydrated Mg-AI carbonate hydroxide with the formula MgoAlzCOa(OH)re.4HzO and properties of manasseite and hydrotalcite. The synthetic mineral is hexagonal with a:6.14t, c:15.6lz A and Z:1. Dehydration and corresponcling contraction of basal spacingswere reversible up to 300"C and irreversible above 450'C. Above 300'C both OH water and COz were evolved, and at 450'C the structure was destroyed. A one-dimensional structure analysis indicates a structure consisting of brucitic layers alternating with hydrous aluminum layers.

INrnooucrroN When a solution containing MgClz and AlCh is titrated with NaOH, a double hydroxide of Mg and AI is formed having a characteristic struc- ture (e.g.,Feitknecht, 1942;Feitknecht and Gerber,1942). These authors showedthat other divalent such as Ca, Co, Mn, Cd in solution with Al or Fe3+also give doublehydroxides upon the addition of a base.Mort- land and Gastuche (1962) a\alyzedprecipitates prepared in this way and found that with a molar ratio of 0.7( Mg/(Mgf AD <0.8 a pure Mg-Al double hydroxide free of gibbsite or was formed. From a com- parison of X-ray powder diffraction data, they concludedthat the pure double hydroxide precipitated in two forms with slightly different param- etersand was different from the Mg-AI double hydroxidesdescribed by Feitknecht and Gerber (1942). The powder diffraction data of Mortland and Gastuche,however, are close to those given for manasseiteand hydrotalcite in Smith (1965). This similarity is not surprising since manasseiteand hydrotalcite could be consideredas Mg-Al double hydroxides,except for the substitution of one CO3for two OH ions. Thus it seemslikely that the so-calleddouble hydroxides prepared by titrating AlCh and MgCb are indeed closely re- lated to manasseiteand hydrotalcite. This study was undertaken, there- fore, to determine the composition and structure of Mg-Al double hydroxide and to assessthe relationship to manasseiteand hvdrotalcite.

I ContributionNo. 203. SYNTHETIC MAGNESIUM-ALUMINUM CARBONATE HYDROXIDE IO37

Pnrpan.qtoN ol' mr Pnocpnetr The precipitate was prepared by titrating with 1N NaOH a solution consisting of AlCh and MgCls having a total concentration of 0.1M and a Mg/(MgfAl) molar ratio of 0.8 The molar ratio of 0.8 was selected because preliminary experiments showed that at this molar ratio a precipitate free of brucite or gibbsite was obtained. The NaOH was added in a stepwise manner and the pH was measured after each addition to give a titration curve. A stream of COrfree nitrogen was passed through the solution during the titration to mini- mize contamination of the precipitate with COz. For comparison a solution of MgCl: was titrated with NaOH in a similar way. The buffering at pH 9.5, obtained when MgCl2 alone was titrated with NaOH, was due to the precipitation of Mg(OH): (t'ig. 1, curve 1). The buffering between pH 4.0 and 4.5, obtained with the MgClz and AlCl3 mixture, was due to the precipitation of AI(OH)3 (curve 2). The secondbuffering range between pH 7.7 and 8.5 was more than one pH unit below that obtained when Mg(OH)z was precipitated. This bufiering range corresponds to that obtained under similar conditions by Turner and Brydon (1962) and may be due to

me NoOH

Frc. 1. Titration curves of 0.1M solutions of magnesium chloride and aluminum-masnesium chloride. 1038 G -T. ROSS AND IT. KODAMA

adsorption bonding betrveenthe hyclroxides of Mg and Al with the latter as the negative compound (Treadwell and Bernasconi, 1930) The suspension,obtained from the titration of the solution containing both AICI3 and MgClz, was transferred to a polyethylene bag and dialyzed for one month at 60'C against distilled rvatel which was changed ctaily. The suspensionwas then freeze-driedand the re- sulting powcler used for the remainder of the study.

Cunrrrcar,ANerysus Part of the freeze-driedsample was dissolvedin dilute nitric acid and Al was determinedby the aluminon method (Robertson,1950) and Mg by the atomic absorption method. The Cl content was determined by X-ray spectromelryand CO: and H:O by dr1-combustion using WO3 to ensurecomplete combustion (Steyermark, l96t). The compositionof the precipitate prepared in this study is closeto those of the two polymorphs manasseiteand hydrotalcite with the formula, Mg6AhCOr(OH)16.4HrO(Table 1). The precipitatehad rela- tively high CC)2content, which presumablywas incorporatedfrom the air during dialysisand after freezedrying, Brandenberger(1933) and Mump- ton, et al. (1965),have noted the rapid carbonationof related minerals. The CI content was negligible,unlike the considerableCl contents of some double hydroxidesreported b;, Feitknecht (1942).

X-n.rv AN,rr,vsrs X-ray diffraction polvder patterns were obtained using Fe-filteredCo radiation with a 114.7mm camera containing the specimenin a thin- walledglass capillary (Table 2). SomeX-ray iine broadeningwas evident which is attributed to small particle sizeand imperlect crystallinity of the precipitate and to statistical distribution of COs in its structure, dis- cussedin a later section.Except for small differencesin (001)reflections (which are shown below to be dependenton relative humidity) the pow- der data of the precipitate are very closeto thosereported by Mortland and Gastuche(1962). As to the valuesfor manasseiteand hydrotalcite, the powder data of the precipitate prepared in this study agree more closelywith thoseof manasseitethan with thoseof hydrotalcitc indicat- ing that the precipitate belongs to the hexagonalrather than to the rhombohedral group. The X-ray powder diffraction lines of the pre- cipitate. therefore,were preliminarily indexed in the hexagonalsystem using the unit-celldata for manasseite,a:6.13 A and c:15.37 A. 1lhe unit-cell dimensionsof the precipitate, calculated using a pair of well- defined small spacings(1.541 and 1.498A; of tLe indexed pattern, are 'ftre a:6.14rA and c:15.6lzA. data in Table 2 showgood agreement betweenthe observedQr,p values (Qu"t:l/d'or) and the pmz valuescal- culated on the basisof the derived cell dimensions. SYNTHETIC MAGNESIUM-ALUMINUM CARBONATE HYDROXIDE 1039

TaerE 1. Cnonrcel ANALYSES

Wt.''k* Precipitater Manasseite2 Manasseiter Hydrotalcitea

MgO 38.2 36.76 39.38 39 84 Fero: o.2o o.21 o 90 AlzOs 16.5 19.65 16.59 15, 16 COz 6.4( t 0.3) 6.98 7.48 7.26 cl 0.1(+0.01) H:O 38.4 36.13 36.34 3tr.83 Rem. 0.35 0.14

Total 99.6 100.07 100.00 100.13

* Based on air-dry weight. I Precipitate prepared in this laboratory. 2 Sample from Snarum, Norway. Pure sample, but contains gibbsite. Rem. is SiOz. Frondel (1941) 3 Analysis of sample 2 recalculated to 100/s after deduction oI0.350/6 SiOz and 6.38/6 gibbsite (on the basis of 6 Mg atoms per cell). Frondel (1941). a Sample from Snarum, Norway. Presumed to be a mixture of hydrotalcite and manas- seite. Rem. is insoluble. Manasee (1915).

Tnonlral Awar,vsrs The thermal stability of the precipitatewas investigatedby DTA and TGA. The DTA was carried out at a heating rate of lO'C/min (Morita and Rice, 1955).The TGA was obtained at a heating rate of 5"C/min with an analytical balance and a Fisher Balance Assembll' mounted over a cylindrical furnace containing a suspendedPt dish. The balance was calibratedwith an equal weight of calcinedsample. The DTA curve of the precipitateis similar to that given for hydrotal- cite (Mumpton, et al., 1965,p. 1904),although its endotherm peaksare Somewhatdifferent in magnitude and occur at slightly different tem-

T.tnn 3. Cnr-r CoNrnNrs

Precipitatel Manasseite2 Mg6ALCOa(OH)16.4H2O

Mg 6.0 60 6 AI 2.O 2.O 1 C 0.9 1.1 I H 24.4 24.9 24 o 23.O 23.6 23

I Calculated from analysis 1, Table 1. 2 Calculated from analysis 3, Table 1. G. "T. J?OSSAND I.I, KODAMA

peratures(Fig. 2). The significanceof thesedifferences is not clear since specificdifferences in operating conditionsare not known. The first two endotherms with peaks at 248"C and 267"C in the DTA curve of the precipitate are assignedto the loss of water of crystallization, although some absorbedwater could be included in this loss.This reaction accounts for the weight loss between 25oC and 200"C in the TGA and DTGA curvesshown in Figure 3. The third endothermwith its peak at 447oCis attributed to the loss of both OH water and COz from the structure and correspondsto the weight lossbetween 300oC and 450"C in the TGA and DTGA curves(Fig. 3).

roo 2oo 3oo 600 7oo auo ,r^ rlllrrrr ra,too

Frc. 2. Difierential thermal analysis curves of the precipitate and hydrotalcite.

Additional thermal data were obtained by dry combustion of the samplefor COzanalysis at increasingtemperatures (Steyermark, 1961). Thesedata were plotted in Figure 4 and show that nearly all of the COz was evolved between 400o and 600'C. This agreeswith the results for manasseiteobtained by Manasse(1915).

Elrncr or Rrrarrvn Hulrrlrry AND HEAT oN BASALRnlr-ncrroNs The effect of relative humidity on basal reflectionsat room tempera- ture was investigated by X-ray analysisof an unheated oriented specimen at 40 percent and 85 percent relative humidity. The effect of heat on basal reflectionswas examinedby heating oriented specimenson glass SVNTHETIC MAGNNSIUM-ALUMINUM CARBONATE HYDROXIDN 1041

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600 7oo 8oo T.rp.rotrr" "l

Frc. 3. Thermogravimetric and difierential thermogravimetric analysis curves of the precipitate. slidesto different temperaturesin the furnace of a TGA apparatusat a rate of S"C/min. After cooling over P2Os,the oriented specimenswere X-rayed at closeto 0 percentrelative humidity with a Philips Difiractom- eter. As shownin Figure 5, an increasein relative humidity from 40 percent to 85 percent at 25oCsignificantly increased basal spacings.Heating to 180oCcaused a slight decreasein basal spacing,but at 150"C and 180"C an additional first-order reflection appearedat a lower basai spacing-. From 220'C to 300"C only one first-orderbasal reflection at about 6.6 A was present.After heating at 450oC,no peaks were evident in the dif- fractograms indicating destruction of the . After heating, the oriented specimenswere rewetted, dried overnight at room temperature,and then X rayed at room temperatureand 40 per- SYNTH]JTICMAGNESIUM-ALUMINUM CARBONATE HYDROXIDE 1043 cent relativehumidity. AII the specimens,except the oneheated to 450oC, gave first-order basal reflectionsat the originai spacingwith no loss in intensity (Fig. 5). These resultsindicate that the amount of water of crystallizationde- terminesthe basal spacingbetween 7.9 A and O.OA. the 7.9 A spacing representsthe fully hydrated phase,and the 6.6 A spacingcharacterizes the fully dehydrated phase. Dehydration appearsto be completed be- tween 180"C and 220"C and is reversible up to 300oC indicating no ap- preciabledestruction of crystal structure at this Lemperature.Branden-

T,q.eln 4 ONr-DrurNsroxer, Srnucrunn F.qcrops (Fcor) eNo rnr Drntl'u ANn Onsrnltnl IntnNsnrns lon eN Ioeer, Moorr,t or rnB Pnoposnl Srnuctuln

I I cosz20 F""r" 1"'r"-- (F*r")' f- sin29cos d

002 -59.2 100 100 004 +88.5 53 60 006 +91.8 23 40 008 +39 9 2 6* 0.0.10 - 6.0 0.03 0.o.12 +113.4 7 6 0.0.14 -39.9 I 0.0.16 +61.8 3 5+

* Includes intensity of a prismatic reflection. f Based on the chemical formula Mg6AlzCO:(OH)ro.4HuO berger (1933) also concludedthat the X-ray diffraction propertiesof a similar hydrated Ca-AI double hydroxidevaried with its content of water of crystallization.

INln.{nno AssonprroN ANar,ysrs Infrared absorptionspectra were obtained with a Perkin-Elmer Model 21 Spectrophotometeron KBr pellets containing 0.20 percent of the sample.The spectrum of the precipitate (Fig. 6) is like the spectrum of hydrotalcite publishedby Mumpton, et al., p. 1903(1965). In both pat- terns the stretching vibrations of water are representedby a band at 3480 cm-1 and a shoulder at about 3000 cm-l; the stretching vibrations of carbonateare shown by bands at about 1480 cm-l and 1380cm-].

Car,curerroN ol STRUCTURALFoRMULA The formula of the precipitate was calculated using the data of chemi- cal and thermal analyses.Based on the break in the TGA and DTGA curves at 200oC(Fig. 3) and on the strong but reversiblecontraction in 1044 G. "T.ROSS AND H. KODAMA

! q)

o U oN (J

o

o G

Temperoture ("C)

Frc.4. Evolution of carbon dioxide from the precipitate heated to various temperatures. basal spacingsbetween 180oCand 220"C (Fig.5), the weight lossbelow 200oCwas attributed to absorbedwater and water of crystallization,and that above 200oC to OH water and COz. Becausethe amounts of ad- sorbedwater and water of crystallizationwere not accuratelyknown. the formula was first calculated on the basis of the data in Table 1 recal- culated as percent of the sampleweight at 200oC.The weight lossbelow ll0"C (3.27a)wasthen assignedto absorbedwater and excludedfrom the calculations.The weight lossbetween 110oCand 200'C (l2.l7o) was at- tributed to water of crystallization and correspondsto four H2O mole- culesin the formula. The calculatedformula for the precipitate (Table 3) agreesclosely with the experimentalvalues for manasseiteand they both conform to the generalformula, MgoALCOg(OH)16.4HrO. SYNTHDTIC MAGNESIUM-ALUMINLTM CARBONATE HYDROXIDE 1045

-o)6( .g u o 7. o- tt, o o lo 7.30 -ctc, o

iI ! It)

(, 6.90 -o o

Temperolure ('C)

Fro. 5. Efiect of relative humidity (solid triangles), heating (solid circles) and rehydra- tion (open circles) on basal spacings of the precipitate. Data from difiractograms of oriented specimens giving a regular sequence of basal reflections

Srnucruner, ANer,ysrs As shown in Table 2, the largestbasal reflectionoccurred at the half height of the c-dimension which indicates a hexagonal subcell with di- mensions,a:6.14 A and c:7.79 A. llt ir subcellcontains the half mole- cule MgaAI(OH)s(COt+.2H2Owhich requiresthe repetition of an equiv- alent atom array at a distance oI 7.79 A and implies that the distribu- tion of COsions must be statistical. The thermal data and the results of the dehydration experiment (Figs. 2, 3 and 5) indicated that, except for the loss of H2O below 200oC,the thermal decompositionof the precipitate was similar to that of brucite in 1046 G. -T. ROS.SAND II . KODAMA

WAVETENGTH(MICRONS}

FREQUENCY(CM_')

Frc. 6. Infrared spectra of the precipitate ancl hydrotalcite. that the OH was evolvedbetween 300oC and 450"C. When oxygen ions are in hexagonalclosest packing, as in brucite, the hexagonalbase with edge6.2 A permits four oxygensin eachlayer. Three layersof four close- packedoxgyens would give a subcellheight of 7.20A as comparedto the observed7.79 h value. A structural model was assumed,therefore, con- sistingof a brucitic laver containing3 n4g and 8 OH (two layersof close- packed oxygen ions) and a looselvstacked layer containing 1 Al, + COa and 2 HzO per subcell.A diagram of this model is shown in Figure 7. Basedupon the proposedstructure, a one-dimensionalstructure analy-

A+f(cor)+zHro

40H 3Mg 40H

o{ v?@ P r---- Al+j(COr)+zHrO

'8 40H

Fi r-- 3Mg o{ _- R"i 40H @to TE { _+_J_ iL Al+f(CQ)+zH2O

Frc. 7. Diagram of the proposed structure of the precipitate. SYNTIIETIC MAGNESIUM-ALUMINUMCARBONATE HYDROXIDE 1047 sis was used and the structure factors (F""r")and the intensities(1",r".) were calculated.The agreementbetween the 1".r".and 1o5".values (Table 4) supports the validity of the proposedstructure.

CoNcrusroNs X-ray diffraction, chemical, thermal and infrared analysesshowed that the precipitate preparedby titrating mixed solutionsof AICL and MgCb and dialyzing the suspension(Mortland and Gastuche, 1962) was not a pure hydrated Mg-Al double hydroxide, as has generallybeen be- lieved,but a hydrated Mg-Al carbonatehydroxide with similarproperties and the sameformula asmanasseite and hydrotalcite,MgaAlzCOa(OH) 16 .4H2O. A one-dimensionalstructure analysisindicated a structure consisting of brucitic Iayers alternating with hydrous aluminum layers.

Rrlrnluces

Bn.q.NonNsnncrn, E. (1933) Kristallstructurelle Untersuchungen an Ca-Aluminatehydra- ten. Schw,eiz M'inerol, Petrogr. Mift 13,569-570. Fnrtrnecnt, W. (1942) Uber die Bildung von Doppelhydroxyden zwischen zwei- und dreiwertigen M etallen H el,o.C him. A cta 25, 555-569. -- and M. Gerber (1942) Zur Kenntnis der Doppelhydroxyde und basischeDoppel- salze. III Uber Magnesium-Aluminiumdoppelhl'droxyde. Heh. Chim. Acta 25, l3l- r37. l'noNnnl, C. (1941) Constitution and polymorphism of the pyroaurite and sj6grenite grotps. Amer. Mineral 26,295-375. Mawesse, E. (1915) Idrotalcite e piroaurite. Atti Soc lioscana Sc. Nat., Proc. Verb. 24, 92-105. Monrtl, H aNo H. M Rrcn (1955) Characterization of organic substancesby differential thermal analysis. Anal. Chem.27, 336 339. Montre.No, M. M aNo M. C. Gasrucux, (1962) Cristallisation d'hydroxides mixtes de magn6sium et d' en milieu dialys6. C. R. Acad,.Sci Pari's,255,2131-2133. Muumox, F. A., H. W. Je,r'rr, aNo C. S. Tnolrlsox (1965) Coalingite, a new mineral from the New Idria serpentine, Fresno and San Benito Counties, Ca\ifornia. Amer. Mineral,. 50, 1893 1913. Roetnrson, G. (1950) The colorimetric determination of aluminum in silicate materials. I. Sci. Food.Agrie. l,59-63. Snrru, J. V. (ed.) (1965) Index (inorganic) tothePorvderDifiraction h'ile. Pub. Amer. Soc. Test.Moler. PDIS l5i Srrvrnu,trr, A. (1961) Quantitatioe Organi.c Micro Analysis. Second ed , Academic Press, New York. Tnnalwtr.l, W. D. aNo Eo. BnnNescoNr (1930) Versuchezur elektrometrischen Titration von Aluminium- und Magnesiumion nebeneinander. Ilelt Chim. Acta 13, 500-509. Tunnen, R. C. eNo J E. Bnvoox (1962) Formation of double hydroxides and the titration of clays. Science 136 (3521), 1052-1054.

Manuscript receiaed,,fuly 22, 1966; accepted.Jor publicotion, September8, 1966.