Clay Minerals (1998) 33, 363–367 NOTE Synthesis of a regularly interstratified 2:1 margarite and beidellite (34 A˚ phase) Regularly interstratified mica-smectites and The conditions of composition and temperature chlorite-smectites have been synthesized by Ueda under which the 34 A˚ phase formed as a single & Sudo (1966), Frank-Kamenetskij et al. (1972), phase were fairly limited. The compositions ranged Matsuda & Henmi (1973, 1974)and by Eberl from margarite to margarite and beidellite in a ratio (1978). Only a few studies have been carried out, of 2:1. The temperatures were 400À5008C, and run however, on regularly interstratified minerals whose durations were >2 weeks. The 25 A˚ phase often spacings exceed 30 A˚ (Lazarenko & Korolev, 1970; formed during short runs despite the optimal Sato & Kizaki, 1972). compositions for formation of the 34 A˚ phase. Regularly interstratified minerals with basal Experimental results at 4508C are shown in a ˚ ˚ spacings of 25 A and 30 A have been synthesized triangular diagram of CaOÀAl2O3ÀSiO2 with from kaolinite by means of hydrothermal treatments starting compositions (Fig. 1). As a Ca starting ˚ and by the addition of various oxides or carbonates material, CaCO3 formed the 34 A phase within a (Matsuda & Henmi, 1974, 1983). In these studies a shorter time than did CaO. Compositions richer in phase with a basal spacing of 33À34 A˚ was Ca and Al than ideal margarite led to margarite accompanied by a 25 A˚ mineral after hydrothermal formation at 400À5008C, and to the formation of treatment of kaolinite plus calcium carbonate. The the 25 A˚ phase and boehmite at lower temperatures. present report describes briefly the formation and Oxalic acid was not effective in the formation of some mineralogical properties of a 34 A˚ phase that the 34 A˚ phase. has been synthesized in an almost pure state. Experimental methods Hydrothermal treatments were undertaken by standard test-tube type equipments. Mixtures of natural kaolinite (Shokozan, Japan), alumina gel, and the reagents CaO, CaCO3 or oxalic acid were used as starting materials. Fifty milligrams of starting mixture and 30 ml of distilled water were sealed in a gold or silver capsule (3 mmf).Theywereheatedat 1kbarfor3À67 days. The products were identified by X-ray diffraction (XRD)with Cu radiation. Formation of the 34 A˚ phase The 34 A˚ phase was often formed at 400À5208C together with a 25 A˚ phase (1:1 regularly inter- F IG. 1. Product phases at 4508Cinthe stratified margarite and beidellite; Matsuda & ˚ ˚ CaOÀAl2O3ÀSiO2 plot. Solid circle = 34 A phase, Henmi, 1983). The 25 A phase was synthesized open circle = 34 A˚ phase + margarite-beidellite (25 A˚ , within the wide temperature range of 350À5208C, Matsuda & Henmi, 1983), open square = margarite, whereas smectite formed at temperatures <3508C, open triangle = margarite-beidellite (25 A˚ ), solid boehmite formed at 350À4508C from aluminous triangle = margarite-beidellite + boehmite, open compositions, and anorthite and margarite formed at rhombus = anorthite, solid rhombus = margarite- temperatures >4508C. beidellite + anorthite. # 1998 The Mineralogical Society Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/33/2/363/3297775/363.pdf by guest on 24 September 2021 364 Note XRD of the 34 A˚ phase glycolated conditions. Average basal spacings under various conditions were calculated from basal Oriented aggregates of the 34 A˚ mineral were reflections in the range up to 358 2y, except for examined by XRD (Cu-Ka). The XRD patterns of the 1st order (Table 1). Table 1 shows that the oriented aggregates that were air-dried and glyco- phase possesses one smectite-like layer in the lated and glycerolated are shown in Fig. 2. The structure. Assuming that the thicknesses of glyco- phase was observed to have more than 15 basal lated smectite are 17 A˚ , the thickness of the non- reflections when air-dried and over 25 others under expandable layer in the mineral is ~19.5 A˚ , which FIG. 2. XRD patterns of the 34 A˚ phase. Peak positions are in A˚ ; Cu-Ka radiation. Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/33/2/363/3297775/363.pdf by guest on 24 September 2021 Note 365 TABLE 1. Basal 001 and 060 spacings of the 34 A˚ TABLE 2. Chemical composition and structural formula phase (A˚ ). of the 34 A˚ phase. Condition of sample Interlayer cation Spacing (A˚ ) SiO2 33.95 TiO2 0.37 2+ Air-dried (RH 60%)Mg 34.60 Al2O3 41.65 + K 31.47 Fe2O3 0.08 After heating at 6008CCa2+ 29.1 MgO 0.66 Ethylene glycol Mg2+ 36.43 CaO 7.94 + K 36.21 Na2O 0.53 2+ Glycerol Mg 36.98 K2O 0.44 + K 33.03 P2O5 0.04 H O(+)7.54 Greene-Kelly test 37.08 2 H2O(-)6.92 060 1.478 Total 100.12 Tetrahedral Si 7.33 Al 4.67 corresponds to two layers of a mica-like compo- Charge (À4.67) nent. Therefore, the XRD data show that the phase is a regular interstratification of two mica-like Octahedral layers and one smectite-like layer. Al 6.04 The phase has an 060 spacing of 1.478 A˚ and is Mg 0.03 Fe3+ 0.01 thus dioctahedral in nature. Sum 6.07 Charge (+0.21) Chemical composition of the 34 A˚ phase (Sum)( À4.46) ˚ A monomineralic 34 A phase containing very Interlayer fixed small amounts of amorphous or low crystalline Ca 1.86 phases was analysed chemically. Interlayer cations Na 0.22 were saturated with Mg2+ and chemical analyses K 0.12 were performed by wet methods. Sum 2.20 The results of chemical analysis and the number of Charge (+4.06) cations based on O30(OH)6 are shown in Table 2. The calculation assumes that the phase is composed Interlayer exchangeable Mg 0.18 of two margarite layers and one smectite (beidellite) Charge 0.36 layer. Octahedral sites are mostly occupied by Al, (Sum)(+4.42) and the layer charges mainly originate from Al-for-Si substitution in the tetrahedral sheets. As seen in CEC (mEq/100 g)28.5 Table 2, the chemical compositions of the expand- able and non-expandable layers are Mg0.18Al2 (Si3.64Al0.36)O10(OH)2 and (Ca1.84Na0.22K0.12) (Al4.04Mg0.03Fe0.01)(Si3.69Al4.31)O20(OH)4,respec- tively. Interlayer cations within the mica-like layers Electron microscopy are mostly composed of Ca. The Ca/(Na+K+Ca) ratio is 0.84, which is in agreement with the value Electron micrographs of the sample were taken for natural margarite (Deer et al., 1962). The with a Zeiss EM 902 and are shown in Fig. 3. The compositional formula of the 34 A˚ phase agrees sample consists of particles that range in form from fairly well with a 2:1 mixture of margarite, rectangular to ribbon-shaped, and that are somewhat CaAl2(Si2Al2)O10(OH)2, as non-expandable layers, irregular in shape. These images are similar to those and beidellite, Ex0.33Al2 (Si3.67Al0.33)O10(OH)2 (in of rectorite (Brown & Weir, 1963; Matsuda, 1984). which Ex indicates a monovalent exchangeable High-magnification lattice images (Fig. 3b)show cation), as expandable layers. that the periodic layer structures, which have a Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/33/2/363/3297775/363.pdf by guest on 24 September 2021 366 Note FIG. 3. Electron micrographs of the 34 A˚ phase; a = typical images of the 34 A˚ phase, b = high-magnification lattice images of the sample. Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/33/2/363/3297775/363.pdf by guest on 24 September 2021 Note 367 spacing of ~30 A˚ , consist of one layer of REFERENCES dehydrated smectite and two layers of margarite. Brown G. & Weir A.H. (1963)The identity of rectorite Conclusions and allevardite. Proc. Int. Clay Conf. Stockholm, 1, 27À34. A regularly interstratified 2:1 composition of Deer W.A., Howie R.A. & Zussman J. (1962)Sheet margarite and beidellite with a basal spacing of silicates. Rock-forming Minerals, vol. 3, Longmans. 34 A˚ was synthesized in an almost pure state by Eberl D. (1978)The reaction of montmorillonite to hydrothermal treatment at 400 5008C and at 1 kbar À mixed-layer clay: the effect of interlayer alkali and pressure from mixtures of kaolinite, calcium oxide alkaline earth cations. Geochim. Cosmochim. Acta or carbonate, and alumina gel. Optimal starting 42,1À7. compositions consist of margarite and margarite/ Frank-Kamenetskij V.A., Kotov N., Goilo E. & beidellite in a ratio of 2:1. Klotchkova G. (1972)Some aspects of structural X-ray diffraction and electron microscopy indi- transformations of clay minerals under hydrothermal cate that the phase is a regular interstratification of conditions. Proc. Int. Clay Conf. Madrid, 303À312. two mica-like layers and one smectite-like layer. Lazarenko E.K. & Korolev Yu.M. (1970)Tarasovite, a There is no report of natural occurrences of 2:1 new dioctahedral interlayered mineral. Zapiski Vses. regularly interstratified margarite-beidellite, but it Miner. Obshch. 99, 214À224. may possibly form in the calcareous and aluminous Matsuda T. (1984)Mineralogical study on regularly environment in which margarite occurs. interstratified dioctahedral mica-smectites. Clay Sci. 6, 117À148. Matsuda T. & Henmi K. (1973)Hydrothermal behaviors ACKNOWLEDGMENTS of an interstratified mineral from the Ebara mine, The author wishes to thank Emeritus Professor K. Hyogo Prefecture, Japan. J. Clay Sci. Soc. Japan 13, Henmi of Okayama University for constructive 87À94 (in Japanese with English abstract). suggestions. Professor K. Nagasawa of Tokoha Matsuda T. & Henmi K. (1974)Syntheses of inter- Gakuen Hamamatsu University kindly read through statified minerals from kaolin with addition of the manuscript. I am indebted to Mr Ann-Fook Yang of various cations.