Zairyo-to-Kankyo, 42, 225-233 (1993) 論文

Analytical Study of the Formation Process of Hemimorphite-Part I -Analysis of the Crystallization Process by the Co-precipitation Method-

Hideki Nagata*, Morio Matsunaga** and Kunisuke Hosokawa**

* Department of Biotechnology & Water Treatment, TOTO Ltd. ** Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology

In order to verify the mechanism of the formation process of hemimorphite (Zn4Si2O7(OH)2.H2O) in galvanized steel pipe for water service, the synthesis of hemimorphite was performed through the application of a co-precipitation reaction, which was named the co-precipitation method. Sodium metasilicate (Na2SiO3.9H2O) solution was mixed with Zn(OH)2 colloidal solution. The precipitate obtained from the mixture was aged in aqueous solution to crystallize it into hemimorphite. The samples which were aged for the prescribed periods were analyzed using X-ray diffraction, infrared spectroscopy and thermal differential analysis. It became clear from the analysis that at the initial stage of hemimorphite formation, dissolved silicate was adsorbed onto the colloidal particles of Zn(OH)2 and precipitated with Zn(OH)2 particles; at the next stage, the adsorbed silicate broke the structure of Zn(OH)2 to form an amorphous compound as a precursor of hemimorphite; and at the final stage, rearrangement of atoms proceeded over a long period to form the framework of the hemimorphite. Furthermore, it was suggested that the OH bond of the Zn-OH-Zn bridge in the hemimorphite was hard to form compared with the other bonds in the crystal.

Key words: galvanized steel pipe, water supply system, corrosion product, hemimorphite, dissolved silicate, adsorption, hydroxide, amorphous compound, crystallization, synthe- sis, co-precipitation.

1. Introduction change of the to 13-Zn2SiO4. It is known that hemimorphite (Zn4Si207(OH)2 This peak was sharp and strong and the tem- H20) is naturally found with other zinc miner- perature ranged from 630°C to 790°C. The als in zinc under the environment exothermic peak corresponds to the transition and is stable below 250°C1' . Thermal analysis of the crystal structure from jl-Zn2SiO4 to a- studies of hemimorphite2have shown that the Zp2SiO4, . This peak was fairly broad differential thermal analysis curve for hemi- and moderately strong, from 835°C to 971°C. morphite exhibited two kinds of endothermic Several diffraction crystallographic studies6 peaks and one exothermic peak between 100°C have revealed the crystal structure of hemi- and 1000°C. The first endothermic peak morphite and its structural changes with the corresponds to the loss of water of crystalliza- dehydration process. The framework of the tion. This peak was flat and weak. The tem- hemimorphite crystal consists of the combina- perature at which this peak appeared varied tion of an Si04 tetrahedron and/or a Zn(OH)03 in each paper and ranged from 175°C to 657°C tetrahedron. In the framework, two Si04 in all the papers. The second endothermic tetrahedra form pyrosilicate, Si207, and two peak corresponds to dehydroxylation and partial Zn(OH)03 tetrahedra form the zinc polyhedra, Zn2(OH)06. The water molecule iss located in * 2-1-1 , Nakashima, Kokurakita-ku, Kitakyushu, 802 Japan the cavity which is surrounded by 6- and 8- ** 1-1 , Sensuicho, Tobata-ku, Kitakyushu, 804 membered tetrahedra rings, Zn4Si2(OH)2016 and Japan Zn4Si4(OH)4020, respectively. The first dehydra- 226 Zairyo-to-Kankyo

(A) (B)

Fig. 1 IR spectra of aged samples. (A) aging time: 0 hr-6 days, (B) aging time: 15 days-120 days. tion occurs by expelling the water molecules crystallization process was elucidated using through the channelways which are formed by Fourier-transform infrared spectroscopy(FT-IR), interconnected cavities along the c-axis of the differential thermal analysis(DTA) and X-ray cell. The second dehydration induces the diffraction(XRD). rearrangement of ZnO4 and SiO4 tetrahedra and to the structural change to the j3- 2. Experimental Zn2SiO4 crystal. The amorphous compounds which were crys- Spectroscopic studiesiohave shown that tallized to hemimorphite were prepared based the specific bands, which were attributed to on literature data13' . Zinc sulfate aqueous the vibrations of the pyrosilicate groups, the solution (0.33 mol dm-3) was added drop by OH groups, or the molecules of water of drop with continuous stirring into 750 cm3 of crystallization, were observed between 4000 sodium hydroxide aqueous solution (0.6 mol cm-' and 250 cmrl in the Raman or infrared dm-3). A white precipitate of Zn(OH)2 formed (IR) spectrum of the hemimorphite crystal. when the zinc sulfate solution was added but We reported in a previous paper12' that disappeared slowly on stirring. When the hemimorphite was observed as a layer-like entire solution became translucent white, the compound on the interior surface of the addition of zinc sulfate solution was stopped. galvanized steel pipe in water service and the Subsequently, 50 cm3 of sodium metasilicate formation of the hemimorphite layer on the (Na2SiO3.9H2O) aqueous solution (0.33 mol dm-3) zinc coating prevented the anodic oxidation was slowly added to the Zn(OH)2 colloidal of zinc. In the present study, we obtained a solution with continuous stirring. The solution sufficient amount of the homogeneous pre- became milky white. Zinc sulfate solution cipitate mainly by the technique of Visser and was added further to the milky solution until van Aardt13' and aged it in aqueous solution the added zinc sulfate solution reached a total to crystallize it into hemimorphite. The of 100 cm3. The mixed volume ratio of zinc Vol. 42, No. 4 227

(A) (B)

Fig. 2 DTA curves of aged samples. (A) aging time: 0 hr-6 days, (B) aging time: 15 days-120 days.

sulfate solution to sodium metasilicate solution of the solution with that of the Zn/Si=2 mix- was determined to be Zn/Si=2, which value is ture. Each mixture contained five times the equal to the stoichiometric ratio of hemimor- concentration of the Zn or Si compared with phite. The pH of the milky solution, which the Zn/Si=2 mixture. The precipitates obtain- was about 13, was not adjusted. After the ed from the mixtures were aged in the same precipitate settled, the supernatant liquid was manner. All chemicals used in experiments discarded by decantation. The residue was were reagent grade and were dissolved in dis- transferred to a water bath after three cleaning tilled water. operations by decantation and was kept at 85°C. The white powder obtained before aging as A white powder was obtained by filtration well as after aging was analyzed using FT-IR, after the prescribed aging period and was dried DTA, and XRD. The DTA measurement was at 50°C in an air oven. In order to evaluate carried out in an N2 atmosphere with an a- the influence of the mixed volume ratio of zinc A1203 reference at 10°C/min from room tem- sulfate solution to sodium metasilicate solution perature to 1000°C. The FT-IR measurement upon the crystallization of hemimorphite, an was done with 13 mm~b KBr pellets at a wave- excess Zn mixture (Zn/Si=10) and an excess Si number resolution of 4 cm-1 between 250 cm-1 mixture (Zn/Si=2/5) were prepared, and those and 4000 cm-1. The XRD measurement was solutions were adjusted to pH 13.1 by addition done with CoKa radiation at a scan rate of of NaOH or H2SO4 solution to equalize the pH 2°/min between 28=10° and 90°. In order to 228 Zairyo-to-Kankyo interpret the DTA curves of the aged sample , some of the samples were analyzed by FT-IR and XRD after the DTA measurements . The filter cake obtained from the Zn(OH)2 colloidal solution before addition of silicate was analyzed using DTA, FT-IR, and XRD to confirm the formation of zinc hydroxide in the colloidal solution.

3. Results The precipitates obtained from the Zn/Si=2 Fig. 3 IR spectrum of -Zn(OH)2. mixture were aged for the prescribed periods between 0 and 120 days. Fig. 1 shows the representative IR absorption spectra of the aged Fig. 2 shows the representative DTA curves samples with the IR absorption spectrum of for the aged sample and the hemimorphite stand- the standard (STD spectrum) of hemi- ard mineral. The DTA curve of the hemimor- morphite obtained from Mina Ojuela, Mexico. phite standard (STD curve) exhibited a sharp, The precipitate before aging and its aged strong endothermic peak at 710°C and a broad samples exhibited similar IR spectra up to 18 exothermic peak at 925°C. No endothermic hrs of aging time. The spectrum of the 18 hr peak was observed below 600°C. For the unaged aged sample showed the three different broad sample, no distinct peak was observed except bands which have peak tops at 3355 cm-1, 925 for a broad endothermic peak around 115°C. cm-1, and 555 cm-1, and two very weak, broad After 18 hrs, the broad endothermic peak bands at 1655 cm-1 and 375 cm-1. It was ob- shifted to a slightly higher temperature and served after 24 hr aging that the 925 cm-' two exothermic peaks appeared, the first one broad band of the spectrum of the 18 hr aged at 700°C and the second one at 840°C. After samples split into another three bands at 1100 24 hrs, the broad endothermic peak spread out cm-1, 930 cm-1, and 870 cm -1, which bands are further around 160°C, and the two exothermic some of the specific bands of hemimorphite. peaks were observed at 720°C and 850°C. For The broad band at 555 cm-1 also split into the samples aged from 60 hrs to 30 days, the three bands at 600 cm-1, 560 cm-1, and 450 cm-1, two exothermic peaks were observed in the which also belong to the specific bands of range of 730-740°C and 840-870°C, respectively. hemimorphite and other specific bands appeared The broad endothermic peak was observed in at 680 cm', 370 cm', and 330 cm'. The the much higher temperature region from 195°C broad band at 3355 cm' shifted its peak to a to 270°C than that in the curve of the 24 hr higher wavenumber, 3475 cm', toward the aged sample. After 60 days, the low temper- specific band of hemimorphite. The weak, ature side of the first exothermic peak was broad band at 1655 cm' shifted its peak to a slightly deformed, and the second exothermic lower wavenumber and became sharp and dis- peak became weak and indistinct. These tinct. As a result of these spectral changes, changes in the exothermic peaks were also ob- the configuration of the IR spectrum of the served after 90 days. After 120 days, the first aged samples became analogous to the STD exothermic peak deformed and its top was spectrum. After 60 hrs, most of the specific rounded. The second exothermic peak shifted bands became sharp and strong, but the 600 to a higher temperature, 935°C, near the exo- cm' band split into another two bands at 615 thermic peak in the STD curve. The broad cm-1 and 595 cm-1, which were not observed endothermic peak was observed around 200°C. in the STD spectrum. Any significant spectral In the XRD measurements of the aged change was not observed from 60 hrs to 90 samples. no XRD peak was observed up to 18 days, and these two bands remained. After hrs of aging time. The major XRD peaks of 120 days, these two bands were unified and the hemimorphite had been observed since 24 hrs specific band at 605 cm-1 appeared. As a re- of aging time, and most of the specific peaks sult, the IR spectrum of the aged sample as- of hemimorphite had been detected since 60 hrs sumed almost the same configuration as that of aging time. of the STD spectrum. Fig. 3 shows the IR spectrum of the filter Vol. 42, No. 4 229

Fig. 4 IR spectra of aged samples from the excess Zn or Si mixture. cake obtained from the Zn(OH)2 colloidal solution before the addition of silicate. This spectrum showed quite different absorption bands from those of the spectrum of the prec- Fig. 5 DTA curves of aged samples from the ipitate from the Zn/Si mixture in Fig. 1 (A). excess Zn or Si mixture. This compound was identified as s-Zn(OH)2 from the X-ray diffraction pattern and showed of the unaged sample or 18 hr aged sample in a sharp, intensive endothermic peak at 140°C Fig. 1 (A), and this spectrum exhibited three on the DTA curve. broad bands at 3345 cm', 990 cm1, and 530 The precipitates obtained from the excess cm-1. The specific bands of hemimorphite ap- Zn mixture (Zn/Si=10) and the excess Si mix- peared after 3 days and became clear and ture (Zn/Si=2/5) were aged for the prescribed strong after 9 days. periods between 1 and 9 days. Fig. 4 shows Fig. 5 shows the representative DTA curves the representative IR spectra of the hemimor- of the hemimorphite standard and the aged phite standard and the aged samples obtained sample obtained from the excess Zn mixture from the excess Zn mixture and the excess Si and the excess Si mixture. The DTA curves mixture. As for the IR spectra of the excess of the excess Zn samples showed quite different Zn samples, although the spectrum showed the configurations from that of the STD curve and three major absorption bands which peaked at the aged sample in Fig. 2. In contrast, after 3425 cm', 905 cm-1, and 425 cm-1 after 1 day, 3 days, the excess Si sample exhibited DTA those bands did not change into the specific curves similar to that of the 18 hr aged sample in bands of hemimorphite even after 9 days. On Fig. 2 (A). The DTA curve exhibited a broad the other hand, after 1 day, the excess Si endothermic peak around 145°C, a sharp and sample showed an IR spectrum similar to that intensive first exothermic peak at 695°C, and 230 Zafryo-to-Kankyo

a second exothermic peak at 830°C. The cule and the 0-H groups. The broadening of second exothermic peak shifted to a higher this band would be attributed to the multiple temperature after 9 days. vibration states of the water molecule, since In the XRD measurement, the broad, weak it is located at uncertain positions in a large XRD peaks which belonged to the specific cavity as mentioned above. The sharp band peaks of ZnO had been observed for the excess at 1635 cm-1 was assigned to the H-O-H bend- Zn sample aged for 1 to 9 days, and the major ing mode of the water molecule. The follow- specific peak of hemimorphite appeared slightly ing bands : 1085 cm-1, 935 cm-1, 865 cm', 675 only after 9 days. For the excess Si sample, cm-1, 605 cm-1, 560 cm', 540 cm-1, 450 cm-1, no XRD peak was observed up to the 2 day and 260 cm-1, were assigned to the vibration aged sample, but the major specific peaks of of the pyrosilicate group. The residual three hemimorphite, which were weak and broad, bands in Fig. 1: 385 cm-1, 330 cm-1, and 285 had been observed from the 3 day aged sample. cm-1, were assigned to the vibrations of the zinc polyhedra group. 4. Discussion 4.2 Structural Change in the Crystallization 4.1 Crystal Structure of Hemimorphite Process and Identification of IR Spectrum As shown in Fig. 1, the IR spectrum of each The crystal structure of hemimorphite had sample before aging and after the initial stage been studied in detail by diffraction crystallog- of aging (18 hrs) exhibited several broad bands raphy. Hill et al.$' investigated the crystal in the same region of wavenumbers. The structure of hemimorphite using neutron dif- sample before aging should be formed through fraction. According to their paper, the frame- the process in which the dissolved silicate work of the hemimorphite crystal basically species were adsorbed on the colloidal particles consists of two kinds of tetrahedra, 5i04 and of zinc hydroxide because the silicate did not Zn(OH)03, which are interconnected by shared precipitate by itself in the employed alkaline oxygen atoms. In the framework, each tet- solution. Since no XRD peak was observed, rahedron dimerizes and forms the pyrosilicate those samples were composed of amorphous group, Si207, or the zinc polyhedra group, compound formed from the mixture of zinc Zn2(OH)06, respectively. Two kinds of bridg- hydroxide, silicate, and water. The broad ing bonds, Si-O-Si and Zn-OH-Zn, interconnect band at 3355 cm-1 must originate from the each two tetrahedra in these dimerized tetra- 0-H stretching of water, zinc hydroxide, hedra. The water molecule is located in the silicate, and hydrogen bonds between those large cavity which is surrounded by the con- molecules. The weak, broad band at 1655 cm-1 nected-tetrahedra rings and is constrained by must be assigned to the H-O-H bending of hydrogen bonds which are formed in 4 direc- water. The broad bands at 925 cm-1 and 555 tions to and from the 4 hydroxyl groups of cm-1 would originate from the stretching vi- the Zn-OH-Zn bridging bonds. Since those bration and the bending vibration of polymeric hydrogen bonds are relatively long, the water silicate, respectively. The weak, broad band molecule is not anchored in a rigid position at 375 cm-1 would originate from the stretch- but can vibrate in the fairly large space in the ing vibration of zincate or amorphous zinc cavity. hydroxide. Since the atoms of the amorphous Brunel and Viernel1' studied the IR spectrum compound do not have the periodic and rigid of hemimorphite and assigned its absorption configuration of a crystal, their molecules also bands strictly to the vibrational mode of have multiple vibration states. As a result, several molecular groups in the hemimorphite the IR spectrum of the compound should ex- crystal based on the vibrational mode analysis hibit the broad, smooth bands as shown in of a symmetry group mm 2, to which the vi- Fig. 1 (A). After 24 hrs of aging time, the brations of the pyrosilicate group belong. We above broad bands became sharp or split into identified the IR spectrum of the hemimorphite several sharp bands. These spectral changes standard on the basis of their report. The seem to occur at the same time in all of the broad band at 3450 cm-1 shown in Fig. 1 was broad bands. This situation would mean that assigned to the combined 0-H stretching mode the framework of pyrosilicate and zinc poly- of the hydrogen bond between the water mole- hedra was formed from the random arrange- Vol. 42, No. 4 231 ment of molecules and the adsorbed water was therefore, the bond was easy to break . The confined inside the cavity as water of crystal- first exothermic peak observed around 740°C lization-these structural changes occur in in the DTA curves of all the aged samples the compound at the same time . would correspond to the transformation to 9- The double sharp bands at 615 cm-' and 595 Zn2Si04. It is presumed that the structural cm-' in Fig. 1 would result from a splitting difference from the standard mineral caused by of the 605 cm-1 band because this band split the incomplete formation of the Zn-OH-Zn into two other bands at 80 K". The clear bridge induced the exothermic transformation splitting of this band at room temperature to j3-Zn2SiO4. The second exothermic peak in might be caused by a subtle difference in the DTA curves of all the aged samples, which crystal structure. corresponds to the 925°C peak in the STD On the other hand, the DTA curves of the curve, would correspond to the transition from aged samples showed a transition quite differ- 3-Zn2SiO4 to a-Zn2SiO4. In fact, it was con- ent from that of the IR spectrum. First, the firmed from the results of XRD and IR most important point is that the endothermic measurements that the samples obtained after peak as observed at 710°C in the STD curve DTA measurement, whose original sample was was not observed in the DTA curves of the aged for 3 days or 6 days, was a-Zn2SiO4, aged smples. Secondly, it is important that the willemite. It is considered that the shift of exothermic peak around 740°C was observed the second exothermic toward the higher tem- up to 120 days of aging, which was not ob- perature region and the deformation of the served in the STD curve. Thirdly, the ex- first exothermic peak at 120 days of aging time othermic peak as observed at 925°C in the in Fig. 2 (B) resulted from the structure of the STD curve was not observed up to 90 days of compound approaching that of the hemi- aging. Next, we will interpret these facts in morphite crystal. So far, it can be concluded comparison with the literature data. from the result of the DTA measurements that Many studies'' ~' have reported that the the aged specimens did not crystallize as a DTA curve of hemimorphite exhibited two complete crystal of hemimorphite even after kinds of endothermic peaks and one exothermic 120 days of aging time, especially with respect peak between 100°C and 1000°C. The first to the OH bond in the Zn-OH-Zn bridge. It endothermic peak was very flat'' ,4' , 5' The remains as a difficult question that, while the second endothermic peak was sharp and specific IR band at 3470 cm' corresponding to strong'' ~' . The exothermic peak was moder- the combined OH stretching mode of hydrogen ately strong'' . The second endothermic peak bonds between the hydroxyl group and the and one exothermic peak as well as the results water molecule appeared clearly, the specific reported in previous studies were observed in DTA peak corresponding to the loss of the the STD curve as in Fig. 2. However, the hydroxyl group was not observed. Without first endothermic peak, which was assigned to the hydroxyl group, the vibrational state of the loss of water of crystallization in the the OH stretching would have varied, and the literature''~e', was not observed. The reason absorption band corresponding to it should for this would be that the dehydration occurred have altered its shape. It can be estimated little by little over a wide range of temperature. from the configuration of the IR spectrum that According to the preceding studies''~6', the the structure of the hydroxyl groups and the endothermic peak at 710°C in the STD curve crystal water in the 120 day aged sample is corresponds to the loss of hydroxyl groups in nearly the same as that in the standard. The the crystal and the structural transformation narrow difference in wavenumbers at these of Q-Zn2SiO4. The exothermic peak at 925°C peak tops between 3470 cm-' and 3450 cm-' in the STD curve corresponds to the transition might reflect some difference in the crystal from jl-Zn2SiO4 to a-Zn2S1O4. structure. It may be worth mentioning that Accordingly, the reasons for the absence of Visser and van Aardt13' also observed no second the endothermic peak corresponding to the endothermic peak in the DTA measurement 710°C peak in the STD curve would be that for the synthetic hemimorphite. the OH bond in the Zn-OH-Zn bridge was not At the early stage of aging, some informa- formed or was otherwise formed incompletely; tion regarding the structural change of the 232 Zairyo-to-Kankyo aged sample can be obtained from the DTA oxide in the excess Zn sample. It can be curves. The DTA curve of an unaged sample estimated from these observation that the ex- exhibited a broad endothermic peak around cess Zn samples were mainly composed of 115°C and no exothermic peak as in Fig. 2 (A). particles of and solid silicate adsorbed The endothermic peak around 115°C would on the zinc oxide particles. correspond to dehydration of adsorbed water. Here let us consider through what process No exothermic peak means that any structural the amorphous compound as a precursor of transition did not occur in the heating process. hemimorphite was formed in the Zn/Si mixture. After 18 hrs of aging time, the DTA curve To begin with, it is considered that at the first exhibited two exothermic peaks at 700°C and stage, the dissolved silicate was adsorbed on the 840°C in addition to one broad endothermic surface of the colloidal particles of s-Zn(OH)2 peak around 135°C. The appearance of two and precipitated out. Next, some recomposi- exothermic peaks means that the phase transi- tion of the bonds would occur between the tion occurred in this compound on heating. molecules of zinc hydroxide and silicate. As It is estimated from this fact that some order- shown in Fig. 3, the IR spectrum of -Zn(OH)2 ing of atoms occurred in this compound after which was obtained from the colloidal solution 18 hrs of aging. Although the endothermic before an addition of silicate showed a con- peak around 135°C corresponds to the dehydra- figuration quite different from that of the pre- tion of adsorbed water as in the case of the cipitate obtained from the Zn/Si mixtures. unaged sample, the shift of the endothermic This fact indicates that the bonds among the peak to a higher temperature means that the molecules of -Zn(OH)2 were mostly broken by constraint on the adsorbed water was enhanced the added silicate and another structure was in this compound. The first endothermic peak formed. This recomposition of the bonds by observed above 200°C should correspond to the silicate must be required for forming the pre- loss of water of crystallization. cursor of hemimorphite. The excess Si sample 4.3 Formation Process of Amorphous Com- should therefore be easy to crystallize into pounds as Precursor of Hemimorphite hemimorphite. On the other hand, for the The composition of the precipitates formed excess Zn sample, such a recomposition of the from the Zn/Si mixture seems to influence the bonds would not occur sufficiently at the initial crystallization of hemimorphite significantly. stage. It is not clear whether the reason for After more than 3 days of aging time while this is insufficient added silicate or the inability the IR spectra of the aged sample from the of the silicate to break the structure of ZnO. excess Si mixture approached the STD spectrum, Further investigation is needed to solve this the IR spectra of the aged sample from the problem. The reason why much of zinc oxide, excess Zn mixture hardly approached the STD not zinc hydroxide, precipitated in the excess spectrum. In addition to that, while the DTA Zn mixture is attributed to the particles of curves of the excess Si samples showed a zinc oxide being rapidly crystallized out when transition similar to that in the earlier stage a significant volume of excess zinc sulfate solu- of aging in Fig. 2, the DTA curves of the ex- tion was added quickly to the milky solution cess Zn samples showed a configuration quite after zincate saturation. Because zinc oxide is different from that of the excess Si sample. less soluble than zinc hydroxidel4', zinc oxide Especially, the distinct endothermic peak as preferentially precipitated in the excess zincate shown in the DTA curve of the excess Si sam- solution15) ,16' ple, which corresponds to dehydration of the adsorbed water or the water of crystallization, 5. Conclusions was not observed in the DTA curve of the From the results and discussion of the co- excess Zn sample. This means that the excess precipitation method, the following conclusions Zn samples contained little water. Accordingly, can be drawn: the broad band observed at 3425 cm-1 in the 1) When the Zn(OH)2 colloidal solution was IR spectrum of the excess Zn sample would mixed with silicate solution, the dissolved correspond to the OH stretching of silicate. silicates were adsorbed onto the colloidal parti- The result of the XRD measurement indicated cles of Zn(OH)2 and co-precipitated in the that the crystalline compound was almost zinc mixed solution. Vol. 42, No. 4 233

2) In the precipitate formed through the References above process, the adsorbed silicate broke the 1) D. M. Roy & F. A. Mumpton: Econ. Geol., structure of zinc hydroxide and formed new 51, 432 (1956). bonds resulting in the amorphous compound 2) G. T. Faust: Amer. Mineral., 36, 795 (1951). as a precursor of hemimorphite. 3) M. aL. Palomar & A. Hoyos : Bol. Real Soc. 3) The amorphous compound as a precursor Espanola Hist. Nat. (Geol.), 64, 235 (1966). 4) J. Gotz & C. R. Masson: J. C. S. Dalton, 1134 of hemimorphite was easily formed from a (1978). mixed solution having the stoichiometric Zn/Si 5) R. Arana & J. Galvez : An. Univ. Murcia, ratio of hemimorpite or an excess Si mixed Cienc., 42, 125 (1983). solution but was barely formed from an excess 6) H. F. W. Taylor: Amer. Mineral., 47, 932 (1962). Zn mixed solution. 7) W. S. McDonald & D. W. Cruickshank: Z. 4) The precipitate from the excess Zn mixed Kristallogr., 124, 180 (1967). solution was composed of zinc oxide and ad- 8) R. J. Hill, G. V. Gibbs, J. R. Craig, F. K. sorbed silicate on the zinc oxide particles, and Ross, & J. M. Williams : Z. Kristallogr., 146, the structure of the zinc oxide precipitate was 241 (1977). 9) B. J. Cooper, G. V. Gibbs, & E. K. Ross: Z. hardly broken by the adsorbed silicate. Kristallogr., 156, 305 (1981). 5) In the amorphous compound, the rear- 10) H. Poulet & J.-P. Mathieu: Bull. Soc. fr. rangement of atoms proceeded over a long Mineral. Cristallogr., 98, 3 (1975). period to form the framework of the hemi- 11) R. Brunel & R. Vierne: Bull. Soc. fr. Mineral. morphite crystal. Cristallogr., 100, 14 (1977). 6) The framework of the hemimorphite 12) H. Nagata, M. Matsunaga, & K. Hosokawa: crystal consists of the combination of pyro- This Journal (Zairyo to Kankyo), 41, 816 (1992). silicate, Si2O7 and zinc polyhedra, Zn2(OH)O6, 13) S. Visser & J. H. P. van Aardt: "Corrosion and the complete formation of the OH bond Products on Galvanized Steel Pipes in Hot in the Zn-OH-Zn bridge requires a long time and Cold Water Systems with Special Ref- erence to The Zinc Hydroxide Sulfate Hy- compared with other bonds. drates and Zinc Hydroxide Silicate Hydrate (Hemimorphite)", CSIR Research Report, Acknowledgement BOU335, Pretoria (1977). This work was supported by the Ministry of 14) J. W. Mellor: "A Comprehensive Treatise on Education, Japan and TOTO Ltd. under a co- Inorganic and Theoretical Chemistry", IV, p. operative program. The authors are grateful 529, Longmans, Green and Co. Ltd., (1969). to Miss. Y. Anai for contributing to the volume 15) T.P. Dirkse: J. Electrochem. Soc., 128, 1412 of the experimental work. (1981). (Received October 9, 1992) 16) C. D.-Chouvy & J. Vedel: J. Electrochem. Soc., 138, 2538 (1991).