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 mea- surement 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 al- lophane 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 al- lophane.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.