Dehydration of Allophane and Lts Structural Formula

American Mineralogist, Volume 59, pages 1094-1098' 1974

Dehydrationof Allophaneand lts StructuralFormula

Yasuo Krrlclwe Departmentol Soitsand Fertilizers,National Institute ol Agricultural Sciences, Nishigahara,Kita-Ku, Tokyo I 14, Iapan

Abstract

The water in allophane from 2 localities in Japan {Misotsuchi bed at Iijima, Nagano, and Kanumatsuchi bed at Kanuma, Tochigi) was studied by thermal analysis, infrared absorption spectroscopy, and X-ray diffraction. The water in the allophanes was divided into adsorbed water and structural hydroxyl groups by resolving the differential of the thermogravimetric curves into two gamma distributions. Computed weight percentages of the types of water in the Misotsuchi and Kanumatsuchi allophanes are: adsorbed water 23.2 and. 23,0, respectively, and structural hydroxyl 10.6 and 11.2. Omitting the adsorbed water, the hydroxyl groups compose 13.9 and 14,6 wt percent of the allophanes. These values are close to the 13.96percent for kaolinite. The structural formulae obtained from these results and chemical analysesare, for Misotsuchi allophane,

Sir se(Alr s sFe o. oz)Or g(OH)a. ao(O.L4 o.zo' 3.78HzO and, for Kanumatsuchi,

Sir.m(Alr. s aFeo. or)Or eu(OH)a. zz(O M ore' 3.80HzO where M representsbases.

fntroduction In this paper,the water in allophaneis divided into The properties of water in allophanepose an in- the adsorbedform and structural hydroxyl groups, terestingand important problem. Since the work of on the basis of the results of thermogravimetryand Rossand Kerr ( 1934), many scientistshave pointed differential thermogravimetry. The changes of al- out that water in allophaneis releasedover a wide lophane with heating were examined by infrared temperaturerange from room temperatureto near absorption spectroscopyand X-ray diffraction. The 1000'C, and that it is difficult to distinguishbetween structural formula of allophane was obtained from adsorbedwater and structural hydroxyl groups. Re- chemicalanalysis and the resultsof thermal analysis. sults obtained from substitution of hydroxyls by Samplesand Experimental Methods fluorine, deuterium exchange, infrared absorption spectroscopy,and nuclear magneticresonance spec- Allophane samplesobtained from two weathered troscopy show that: (1) the structural hydroxyl pumice beds-the so-calledMisotsuchi bed in Iijima groups in allophane are unstable, compared with Town, Nagano Prefecture,Japan, and the Kanumat- thosein kaolin mineralsand others,(2) all the hy- suchi bed in Kanuma City, Tochigi Prefecture,Japan droxyl groupsare on the surface,and (3) there is a -were preparedas follows: First, the gel film, com- continuous exchangeof protons between adsorbed posed of "imogolite," was removed from weathered water and the structural hydroxyl groups (Egawa, pumice with tweezers.The weathered pumice was Sato,and Nishimura,1960; Wada,1966; Kitagawa, suspendedin HCI-acid solution at pH 3.5, and the 1.972).Birrell and Fieldes (1952) pointed out the fraction less than 2 p,m was siphonedoff according irreversibledrying of volcanic ashsoils. Misono et al to the Stokes' Law. The iron oxide impurities were (1953) perceivedthe hysteresisbetween dehydration removed by the dithionate-citrate system buffered and rehydration of volcanic ash soils. The phe- with NaHCO3, according to the method of Mehra nomenonwas explainedby Kitagawa (1971.) from and Jackson(1959). The treatedsample was dried his conclusion that allophane consistsof fine "unit for two weeksin a desiccatorover 58 percentH2SO4 particles"which are cementedtogether on drying. until the weight was constant.The relative humidity ro94 ALLOPHANE: DEHYDRATION AND STRUCTURAL FORMULA 1095

insidethe desiccatorwas about 30 percent,calculated are shown in Figure 1. The Tc curves revealedtwo from tables on phaseequilibrium (Sasaki, 1966). kinds of weight loss, a rapid one up to 200'C, and a The purity of preparedsamples was testedby three slow one above 200"C. The Drc curves revealeda methods.X-ray diffraction patterns of both samples sharp,intense peak near 9 min (90"C), and a broad, showedtwo broad, weak peaksnear 3.5 and 2.3 A weakpeak centered about 35 min (350"C). The Tc (Fig.2, A) which originatedfrom allophane(Sudo, and Drc curvesobtained with a samplewhose weight 1959). Reflectionsassociated with layersilicates were was about 30 mg hardly varied evenwhen the heating not found even in oriented specimens.Differential rate was changedto 5 or 2D"C/min in measurement. thermal curvesshowed a sharp exothermat 935oC These curves suggesttwo types of dehydrationin for the Misotsuchi sample and at 92O"C for the allophane: (1) dehydrationof adsorbedwater at Kanumatsuchi,an endothermnear 130oCfor both, low temperatures,and (2) "dehydroxylation" of and no intermediateendotherm for either. Theseare structural hydroxyl groups at high temperatures. characteristicsof allophane (Van Olphan, l97l). This division qf water in allophane has been sug- Moreover, imogolite and other impurities could not gestedpreviously by Watanabe and Sugo (1972). be recognizedin electronmicrographs. The structural hydroxyl groups in allophanemay be Significantalteration of allophane,caused by dry- releasedtoo slowly to give a detectableDre endo- ing for two weeks over 58 percent HzSOa,was not therm, no endothermbeing observednear 350oC in noticed in the X-ray diffraction pattern, infrared either allophane. absorptionspectrum, differential thermal curve, and The X-ray diffraction patterns of allophanefrom weightJosscurve. Misotsuchi and Kanumatsuchi are shown in Figure The weight-losscurve and its differentialwere ob- 2. With the rise of heating temperature,the broad tained by using a Rigaku-Denki Thermoflex thermal peak near 3.5 A of both allophanesshifts towards analyzerat a heating rate of l0'C/min in ordinary 4.0A, and the peak near 2.3A disappears.These atmospherefrom room temperatureto 1000'C. A changesare especiallynoticeable between 300 and specimenof about 30 mg was packed in a small platinum pan for measurement.The base-lineof the xt 00'C curveswas obtainedby usingoAl2O3. 6 The infrared absorption spectrum was measured in the range from 4000 to 4@ cm-l by a Hitachi EPI-G2 infrared spectrometer.The tablets for measurement were prepared by pressing a mixture of 1 mg of specimenwith 200 mg of KBr at2OOkg/cm2 2 afterheating at 110, 150, 200, 300, 400, and 600oC for 2 hours. The untreated specimenwas also ana- to lyzed. :30o o X-ray diftraction patterns of powder specimens 9 6) were obtainedwith CuKc radiation at 40 kV, 10 mA, or 0^ I I anda scanningspeed of 4" (20)/min. rO Silicon, (9" aluminum, iron, and titanium were de- F :. termined colorimetrically by the molybdenum blue, t aluminon, o-phenanthroline,and hydrogen peroxide KANUMATSUCHI methods,respectively. Magnesium and calcium were determinedby atomic absorptionphotometry. Potas- sium and sodium were determinedby flame photom- etry. Results and Discussion Thernml Analysis, X-ray Difiraction, and a0 Infrared Adsorption xl0 min The weight-losscurves (Tc curves) and their dif- Frc. l. The Tc (a) and DTG (b) curves of allophane sepa- ferential curves (Drc curves) for both allophanes rated from Misotsuchi and Kanumatsuchi. 1096 YASUO KITAGAWA

MISOTSUCHI KANUMATSUCHI reversed toward low wave-numbers with heating above 300oC. As the temperature was raised the maximum of the absorption band near 1000 cm-', which is associatedwith Si-O stretchingvibration in allophane,was also shiftedtoward higher frequencies up to 300oC. The data in Table 1 suggest that (1) adsorbedwater of allophane is releasedbelow 200"C, (2) the structural hydroxyl groups of allophane are releasedabundantly between 300 and 400oC, and (3) dehydration of adsorbedwater strengthensSi-O bondsin allophane. The infrared absorption spectra of allophane in the region ranging from 800 to 400 cm-1 were also changedwith heating(Fig. 3). The band at 570 cm-1 was unchangedat 300'C but weakenedat 400oC, 8765t,876 54 and the bandsat 72O and 450 cm-1 appearedclearly X I 00 cm-l X t 00 cm-I above 400"C. Although thesebands are not identi- FIc. 2. The changesof IR spectraof allophane separated fied, the spectraare noticeablychanged between 300 from Misotsuchi and Kanumatsuchiby heating: A, unheated; and 400oC, which is in agreementwith the pre- E, 300'C; F, 400"C,G,600'C. cedingdata. Staticmethods, such as the experimentson changes 400"C. Udagawa,Nakada, and Nakahira (1969) of infrared absorption spectraand X-ray diffraction attribute this changeto a changeof the co-ordination patterns produced by heating, cannot compare di- number of Al from 6 to 4 during the dehydration of rectly with dynamic methods such as Tc and Drc. allophane. However, the changesin infrared absorptionspectra The changesof infrared absorption bands with and X-ray diffraction patterns strongfy indicate the heating are shown in Table 1. As the heating tem- alterationof allophanewith dehydration. perature was raised to 300oc, the maximum of the band associated with O-H stretching vibrations Annlysisol DTG Curues shifted to higfrer wave-numbers,from 3470 cm-' at Dtc curves of allophane show two types of de- room temperatureto 3500 at 300'C in Misotsuchi, hydration: dehydration of adsorbedwater and de- and from 346O to 3520 cm-l in Kanumatsuchi allophane.This shift may be ascribedto the releaseof adsorbedwater. In fact, the band at 1.630cm-l asso- MISOTSUCHI KANUMATSUCHI ciated with O-H deformation vibration was greatly weakenedwith heatingabove 200oC; this wasnot so below 150"C. The maximumof the band associated with the O-H stretchingvibration in allophanewas fl A B A-l-IA\JN Tesre 1. Changes of IR Absorplion Bands Associated with l^VN c O-H Vibrations and Si-O Stretching Vibration in Ailophanes by Heating

Temp O-H vibrations Si-O viblati.on i+w stretching, cm_l defolnation* stretching, cm_1 (r) 12, (r) (2) (r) 12) ff Unheated 34?0 3460 r.0 r.0 980 970 L. 110 3480 3480 0.8 0,7 990 980 150 3480 3500 0.7 0.6 990 990 200 3500 3520 0.3 0.3 1000 990 4.0 3.5 2.3 A 4.0 3.5 2.3 A 300 3500 3520 0.2 0.2 r000 I000 400 3450 3410 0.1 0.1 r000 r000 patterns 600 0.0 0.0 1000 r000 Frc. 3. The changes of X-ray difiraction of allophane separated from Misotsuchi and Kanumatsuchi by rRelative intensity of O-H deformation vibration: untteated, 1.0. heating: A, unheated;B, ll0"C; C, 150'C; D' 200"C; E' (1) Misotsuchi allophane. (2) Kanumatsuchi allophane. 300"C;F, 400'C; G, 600"C. ALLOPHANE:DEHYDRATION AND STRUCTURAL FORMULA t097 hydroxylation of sftuctural hydroxyl groups. From the shapeof the Drc curves(curves b, Fig. 1), the right side of the peaks (9 and 35 min) originating from eachtype of dehydratiorrwere presumedto tail \- =_ toward the high temperafirreregion. Consequently,a bell-shap€dcurve was hot applied to the Drc peak. MISOTSUCHI The formula, ! : c*\-rgn/a, intafltation of which correspondsto a gammafunction, was assumedto simulateeach peak of the Dtc curve observed.The ^30 ro parameters,a, c, and N, were computed and are ol o G) shown in Table 2, where "curve I" indicatesthe I ctr UI parametersfor the function associatedwith the de- I tO hydrationof adsorbedwater and "curve II" thosefor ;0 3. structural hydroxyl groups. The multiple correlation t coeffi.cientbetween the bomputedand observedDrc t0 \-___ II = o curveswas R 4.997 for both samples.This value KANUMATSUCHI is nearly 1.0. Accordingly, the simulationof Drc 20 curvesof allophaneby the fbrmula is mathematically I significant. b The computed Tc and Drc curves of allophanes 30 '[.-\ l.ll are in Figure 4, I shown where the dashedlines show v- curvesI and II, and the solid lines indicate the curves 0 0 2 4 5 E- a and b synthesizedfrom their respectivecurves I and xl0 min II. The computedcurves accurately reproduce the observed curves. Curve I of Drc is drawn over the Frc. 4. The Tc (a) and Drc (b) curves of Misotsuchi and range frorn room temperatureto 300'C and almost Kanumatsuchi allophane calculated from I function: I, the with of adsorbed water; IL terminates near 200oC. Curve II is drawn over a curves associated the dehydration the curves associatedwith the dehydroxylation of structural wide range,from 100 to 700'C. The maximum of hydroxyl group; I + II, the curves synthesizedfrom I and eachDrc curveis about90oC for curveI, and about II. 320'C for curveII, in both allophanes. The areasof the Drc curveswere calculatedwith the continuedproduct of parametersc, a, and I(N), Omitting the adsorbedwater, the structural hy- and are shown in the bottom of Table 2; r(N) is droxyl groupscompose 13.9 percent(by weight) of (N- 1) ! and is obtainedfrom tablesof the gamrna the Misotsuchi allophane and 14.6 p€rcent of the function. These valuescorrespond to the amount of Kanumatsuchi. These percentagesare close to the adsorbedwater (curve I) and structural hydroxyl 13.96percent hydroxyl present in kaolinite,SizAlzOr groups(curve II); the weightpercentage of adsorbed (OH)4. Udagawa,Nakada, and Nakahira (1969) water is 23.2 for Misotsuchiand,23.0 for Kanumat- and Kitagawa (1973a) have previously suggested suchi allophane, and that of structural hydroxyl is the structural similarity of allophane to kaolin 10.5and 1I .2, respectively. minerals. Strrctural Formula of Allophane TAsrB 2. The Parameters of the Function ! = cxN-te-'r" Simulating the Dehydration of Allophane, and the Area Ossaka (1960) proposeda generalformula for Obtained from Its Inteeral allophane, (SiOz)1-2Al2Os'5HzO,from considera- tion of chemicalanalyses. Iimura (1969) proposed Misotsuchi Kanumatsuchi curve I curve II curve I curve 1I a structural formula (OH)0.8601.61Si1.3301.zsA12.6s Parame ters (OH)n.tr, on the basisof chemicalanalysis, thermo- N 2.1L 4.20 r.87 3.33 a 4.97 8.57 6.15 9.80 gravimetry,and the characteristicsof ion exchange. -3 -r l c 2.48xIO-2 2.99x10-'g 3.15xlo 1. l3xto The structural formulae of allophanes from r(N) 2.30x10r 1.62xr0q r.55x102 2,21^).C6 Area Misotsuchi and Kanumatsuchibeds were constructed c.a.f(N) 23.2 10.6 23.0 1r.2 from their chemicalcompositions (Table 3). The 1098 YASUO KITAGAWA

TesrB 3. Chernical Composition and Structural Formula of Relerences Allophanes Btnnrrr, K. S., euo M. Frprors (1952) Allophane in volcanic ash soils."I. SoiI Sci.3,156-166. chenical conposition*, Wt. t Ecewl, T., A. SATo,AND T. NrsHrMune (1960) Releaseof 12't OH ion from clay minerals treated with various anions, SiOz 28.31 29.r7 with special reference to the structure and chemistry of TiO2 0.40 0.42 AlzOs 34,4r 33.81 allophane. Nendo Kagaku no Shinpo (Adr:. CIay Sci., Fe203 0.55 u.50 M9o 0.08 0.04 Toky o), 2, 252-26:2. (In Japanese) tr tr IIMURA, K. (1969) The chemical bonding of atoms in Kzo 0,29 0.t5 Na 2O 1,91 1.48 allophane, the "structural formula" of allophane. Proc. 10,60 11.20 -H2O** 23,00 Int. Clay Conf., Tokyo, 1969, l, 16l-172. Krrecewe, Y. (1971) The "unit particle" of allophane. Am. Total 99.76 99.81 Mineral. 56, 46547 5. structural fornula (1972) An aspect of tlre water in clay minerals: An

(1) Si r. ge (ALr . gaFeo. o z)Or. gl (Ott)3. t 6(OM***) o' zo'3.78H2O application of nuclear magnetic resonance spectrometry to clay mineralogy.Am. Mineral.57,751-764. (2) Sir. ss (Alr. gsFeo. o z)Or. g s (oH)3. 72 (OM*r*) o. t o'3.80H2O (1973a) A short discussion on the chemical com- *Analystr The author. position **+H2o a4d -H2o obtaineal by analysis of DTG curves. of allophane based on the data of Yoshinaga in ***M represents bases. 1966. SoiI Sci. Plant Nut. 19,321-324. (I) From Misotsuchi bed, Iijima, Nagano, Japan. (2) Fron Kanunalsuchi bed, Kanuna, Tochigi, Japan. (1973b\ Substitution of aluminurn in allophane by iron. Clay Sci.4, l5l-154. MrHu, O. P., lNo M. L. Jlcrsox (1959) Iron oxide re- moval from soils and clays by a dithionate-citrate system total H2O was split into structuralhydroxyl (+HrO) buffered with sodium bicarbonate. Clays Clay Mineral. T, and adsorbedwater (-HrO) by the precedingmath- 317-327. ematical analysis of Drc curves. The molecular MrsoNo, S., S. TrnesewA, A. KrsHrrA, eNo S. Suoo (1953) water ratios of the total water o,r alumina are Studies on the soil system of volcanic ash soils in 5.51 for lapan. Bull. Nat. Inst. Agric. Sci., Tokyo, lapan, B'2, Misotsuchi and 5.66 for Kanumatsuchi.This agrees 95-124. (In lapanese) with the resultobtained by Yoshinaga(1966). How- Oxeoe, K., H. Monrr,rwe, J, Ossexe, eNo S. Iwlt (1972) ever, the ratios *HzO/AlzOs, 1.73 for Misotsuchi An asp€ct on the structure of allophane (abstr.). 16th and 1.86 for Kanumatsuchiare lower than the values Nat. Clay Conf., Matsuyama, lapan, p. 13. (In Japanese) Ossex-1,J. (1960) On hydro.alumina 2.31to 2.81 obtainedby Yoshinaga. the silicate minerals Recently,Okada from Mt. Asama. Nendo Kagaku no Shinpo (Adt:. Clay (1972) -tH2O/ et al statedthat the molecular ratio, Sci., Tokya), 2, 339-349. (In Japanese) AlzOa,of allophaneis about 2.00. Ross, C. S., eNo P. F. Ksnn (1934) Halloysite and allo- The structural formulae of the two allophanesare phane. U.S. Geol. Suru. Prof. Pap.1E5G, 135-148. quite similar (Table 3). Allophane consistsof Si, SrsAKI, T. ( 1966) Kagaku Binran (Handbook of Chemistry). Maruzen, Tokyo, p. (In Japanese) Al, Fe, H, O, and bases. 555-714. Iron is included as a con- Suoo, T. (1959) Mineralogical Study on Clays ol lapan. stituentof allophane,but Ti is omitted (Yoshinaga, Maruzen, Tokyo, p. 15l-162. 1966; Kitagawa, 1973b). Basesare believed to ex- Uoecewe, S., T. Nlxeol, .lNo M. Nlxenrne (1969) Mo- change for part of the hydrogen in structural hy- Iecular structure of allophane as revealed by its thermal droxyl groups of allophane.The sum of Al and Fe transformation. Proc. Int. Clay Conf., Tokyo, 1969, l, is setat 2.00. 151-160. VeN Orpnlx, H. (1971) Amorphous clay materials. ,Sci- ence,l7lr 9l-92. (1955) group Acknowledgments W,loe, K. Deuterium exchange of hydroxyl in allophane. SoiI Sci.Plant Nut. 12, 176-182. (1972) I wish to express my gratitude to Dr, y. Watanabe, Na- Wrrewrr4 Y., eNo S. Suco Di.furential thermogravimetry (abstr.), tional Institute of Agricultural Sciences, who spared no of clay minerals in soils Osaka Meet., (In trouble oftering valuable advice, and Dr. S. Hirosaki and ,Soc.,Sci. Soil Manure, lapan, lEr 27. Japanese) YosnrNece, (1966) Miss Y. Saito, National Institute of Agricultural Sciences, N. Chemical composition and some for their helpful advice on mathematical analysis. Further, thermal data of eighteen allophanes from ando soils and pumices. I wish to thank Mr. M. Momota and A. Hiro,se, Application weathered Soil Sci. Plant Nutr. 12, 47-54. Laboratory, Rigaku-Denki Ltd., for making the thermal Manuscript receioed,June 4, 1973; accepted analyses. for publication,April 5,1974.