American Mineralogist, Volume 74, pages 1147-1 151, 1989 Crystallization of silica gel in alkaline solutions at 100 to 180 'C: Characterizationof SiOr-Y by comparisonwith magadiite Hlluro Muursnr Department of Chemistry, Fukuoka University of Education, Akama, Munakata, Fukuoka 8 I I -41, Japan AssrRAcr Sior-f, an intermediate product of the transformation of silica gel to quartz, was syn- thesized from KOH or NaOH solution containing SiO, and NaCl with a molar ratio of SiOr:HrO:Na*:OH-equal to l:100:2-4:0.5at 100-180'C. In almostall reactionprocesses, SiOr-)z appearedas the first crystal phaseand then was transformed into SiOr-Xr, cristo- balite, and f,nally quartz. The characterizationsbased on the chemical and physical data for SiOr-/ showedthat Sior-y is magadiite (NarSi,oOrr.xHrO,x: 9-ll), which was discovered at Lake Magadi (an alkali lake), Kenya, and has been considered to be an important precursor of inorganic bedded cherts. INrnooucrroN et al. (1973).Lagaly et al. (1975) have sinceshown that Hydrothermal crystallization of amorphous silica was natural and synthesized magadiite is a sodium silicate investigatedin the 6inary systemSiOr-HrO at high tem- with the compositionNarSi,oOrn'11HrO. perature and high pressure,and the transformatlon via We also obtained the product identified as SiOr-/in some metastable phases (cristobalite, keatite) to quartz Naollt KgH, and NarCO, solutions containing NaCl at 100-180'C pressure was observed6eat, teS+; Carr and Fyfe, 1958).When at the autogenous of an autoclaveat alkali-metal or alkaline-earth salt was added in aqueous a given temperature.In this paper, the transformation of media, the transformation to quartz occurred at lower amorphous to crystal phasesunder hydrothermal condi- temperatures with various intermediate phasesthat al- tions was further examined on the basis of the run du- ration,temperature,andconcentrationofalkali-metaland ways contain some alkali ions and ro u.. not crystal- phasesin the one-componentSiO, system. alkaline-earth salts. The preparation condition of SiOr-Y Heydemann (1964) observedthat amorphous silica in was determined from the experimental observations,and KOH solution crystallizesvia SiOr-Xand cristobalite to its synthesiswas attempted. The nature of SiOr-I was qurirrzat 100-250 "C, and characterizedthe nature and determined and compared with that of the other silicates. propertiesof SiOr-X. Mitsyuk et al. (1973, 1976) de- Theresults showedthat SiOr-Yis indeedmagadiite. The details chemicalproperties and X-ray scribedmany intermediate phasesthat appearat 150-2g0 -ofthe the difraction .C when alkali-metal or alkaline-earth salt is added to an data of SiOr-Xr, from which it was determined to be a alkaline medium. Sior-y, which is one of the interme- quasi-kenyaite, will be reportedin a subsequentpaper. diate crystal phases,was formed in 0.5-2o/oKOH solu- tions containing NaCl in the temperaturerange 150-180 ExpnnrnanNTAl- DETAILS 'C at 0.5-4.0 kbar. This is a sodium silicate that has the Hydrothermal experiments on the transformation of compositionNarO'2lSiOr'l2HrO-NarO.29SiOr.l4HrO, silicagel werecarried out in the temperaturerange 100- dependingon the nature of the reaction medium. This 180 'C in l-2 wto/o(0.18M4.36M, M = mol/L) KOH compoundwas found to have a high capabilityfor cation solutionscontaining 0.2, 0.5, 1.0, and 2.0M NaCl. We exchange.Kitahara et al. (1982)and Muraishi and Kita- usedTeflon bottleswith a volume of 15, 50, and 100cm3 hara ( 1986) observedthe effectsof seedcrystals, some asreaction vessels. Usually a I 5-cm3bottle wasused, and metal oxides, and alkali-metal salts on the crystallization 0.3 g of starting silica (Wako's silica gel Q-22, through of silica gel in KOH solution at temperaturesbelow 300 200 mesh) was put in the bottle together with 7.5 mL of oC, under which conditions SiOr-X, SiO2-Xr,and SiOr-I the solution. The Teflon bottle was closed with a Teflon mainly formed as intermediate phases. plug, and it was placed in a stainless-steelautoclave that On the other hand, Eugster (1967) described new hy- was designedto accommodate the bottle. After the au- drous sodium silicates,the mineral magadiite [reportedly toclave was kept at a given temperature for a definite NaSi'O'r(OH)3'3HrO)l and kenyaite [NaSi,,O,o,(OH).. time, it was quenchedrapidly with water. The reaction 3HrO)] from Lake Magadi, Kenya. It is thought that ma- products were taken out of the autoclaveand washedwith gadiite precipitates from alkaline lacustrine brines and NaOH solution (pH 10), since SiOr-I has good cation converts to kenyaite and finally to quartz (chert). Its syn- exchangebelow a pH of 10. Some of the conditions for thesishas been reported by McCulloch (1952) and Lagaly preparing SiOr-y are describedin Table 2. 0003-{04xl89/0910-1 I 47$02.00 lI47 r 148 MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS z 200'c a -+- c o s0 (h) -+- I 80'c c -:-. o 50 (h) --:- z <r-_ 160 : -:-- "c : 10 (d) -:- o z a 1400c o o '. 02168 0 s l0 (d) Reoctiontime / d Reoctiontime Fig. L Effectof NaCl concentrationin l0loKOH solutionin Fig. 2. Efect of temperature for a l0/o KOH solution con- thecomposition of SiOr:OH-:Na*: 3:l:l.l-11.0at 160'Con taining NaCl of 2 moUL on the crystallization sequence defined the crystallizationsequence of SiOr-I (4, SiOr-X,(Xr), cristo- in Figure I as a function of reaction time. balite(C), and quartz(Q) as a functionof reactiontime. Magadiite was synthesizedby heating mixtures of the lower temperatures(e.g., below 120 "C) in the l0loKOH- ratio of 9 mol of SiOr, 2 mol of NaOH, and 75 mol of NaCl or 0.5MNarCO, solutions.When NaOH is usedas HrO at 100 "C for 4 weeks,according to the method de- an alkali sourceinstead of KOH (i.e., using NaOH-NaCl scribedby Lagaly et al. (1975).As it is known that ma- solutions),a well-crystallizedSiOr-I is synthesized.When gadiite as well as SiOr-I exhibits a cation-exchangeca- KOH is used instead of NaCl (i.e., using KOH-KCI so- pability, synthetic magadiite was also treated with the lutions), SiOr-X, is directly formed and transformed into same method describedabove for SiOr-f. cristobalite, which then goesto quartz. This differenceis The reaction products were identified by the use of a understandablefrom the fact that SiOr-I is a sodium Rigaku xcc-20 X-ray difractometer. Chemical compo- silicate and SiOr-X, is formed as a combination of both sition of the products was determined by conventional sodium and/or potassium silicatesas describedbelow. gravimetric methods for silica and by atomic absorption Figure 2 shows the effect of temperature on the crys- for Na and K. The water content was measured bv a tallization of silica gel for a loloKOH solution containing thermogravimetric method. 2MNaCl. It has been reportedthat SiOr-yis formed in a temperaturerange of 150-180'C under high pressure Rnsur,rs AND DrscussloN (Mitsyuk et al., 1973).In our investigation,the period Preparation conditions of SiOr-Y and rnagadiite over which SiOr-Ywas stablewas found to increasewith It was observedthat SiOr- y, SiOr-Xr, and cristobalite, a decreasein the temperature, and it was synthesizedeven as intermediate crystal phasesappearing in the order list- at 100 'C. Comparatively well-crystallizedSiOr-I was ed, form in the 100-180 "C experimentsin lo/o(0.18M) obtained after a reaction time of I to 3 months at 100 KOH solutions containing various amounts ofNaCl. The 'C. Sior-y was stable for several months and slowly final product is quartz. The results of the reaction se- transformed into quartz. Thus, a lower temperature was quenceof 160'C experimentsare shown in Figure l. In advantageousfor the formation of SiO'-Icompared with this figure (and in Fig. 2 also), the schematicline breadth SiOr-Xr. In addition, the amount of solid phase in the for each phasewas determined on the basis of the inten- alkaline solutions was followed through the reaction sity of a main X-ray diffraction peak. Increasingthe NaCl term. The solid phasedecreased in the initial stageof the concentration induces the formation of SiOr-Y and cris- reaction and then increasedwith reaction time up to con- tobalite. Further addition of NaCl seemsto result in pre- stant amounts. These results suggestthat the transfor- venting SiOr-X, from appearing, in which situation mation processof silica gel to SiOr-I may be a dissolu- SiOr-Y is directly followed by cristobalite. The same ex- tion-precipitation process. perimental results were also observed for the reaction at The appearanceof SiOr-Ivaried as a function of NaCl MURAISHI: CRYSTALLIZATION OF SILICA GEL IN ALKALINE SOLUTIONS tr49 ct z 6 I Y,Xz E o (J o z Y,Xz o o X2 100 200 300 400 o Molorrotio HzO/OH- Fig. o 4. Chemical composition of the solutions required for u X2 c the formation of SiOr-Iat 100-150'C and magadiiteat 100'C. o The amount of silica in the solutions is SiO,/OH- : 2-6. (J x2 x2 x2 XXT X 100 120 1t0 160 180 Temperqture ('C ) and natural magadiitefrom Brindley (1969).As seenin Table l, there is little difference between the X-ray dif- Fig.3. Therange ofSiOr-Yappearance as a functionofNaCl fraction peaksof natural magadiite and those of the syn- concentrationand temperaturefor a l0loKOH solution.(The thesizedmaterial. The slight differencemay be causedby crystalsindicated with smallletters are minor phasesas deter- a variety of impurities included in natural magadiite. The minedby X-ray diffractionpatterns.) (X); SiOIX other abbre- patterns of the two kinds of synthesized silicates-ma- viationsas in Figure1. gadiite and SiOr-I-are similar to each other except for the differencesin intensity, 1, resulting from their differ- encesin crystallinity.