Clay Science 12 Supplement1, 64-68 (2005)

Interlayer Structure in Sodium

T. Kogure a,b *, Y. Banno c and R. Miyawaki d

a Dept. Earth & Planetary Science, Grad. School of Science, The University of Tokyo,Tokyo, 113-0033, Japan bCREST , Japan Science and Technology Agency (JST), 4-1-8 Honchou Kawaguchi, Saitama 332-0012, Japan Inst. of Geology and Geoinformation, Geological Survey of Japan, AISI, Tsukuba,Ibaraki, 305-8567, cJapan Dept. of Geology, The National Science Museum, Tokyo, 169-0073, Japanand d

(Received March 18, 2005. Accepted May 25, 2005)

ABSTRACT

In this paper we present our recent results with respect to the crystal structures of two trioctahedral sodium micas, aspidolite and wonesite, and discuss the nature of the interlayer structure in sodium micas. High-resolution transmission electron microscopy (HRTEM) and electron diffraction analyses of these micas indicate the existence of a large layer offset, i.e., lateral shift between the two tetrahedral sheets across the sodium-bearing interlayer region. The amounts of the layer offset are about 0.9A (aspidolite) and 1.25 A (wonesite), and their direction is one of [100], [110] and [110]. These directions are occasionally disordered. By combination of the intralayer shift in the 2:1 layer and the layer offset, ordered aspidolite has monoclinic ([100] layer offset, C2/m) and triclinic ([110] or [110] layer offset, C1) cells with one-layer periodicity. Both structures were identified in powder X-ray diffraction patterns and probably the triclinic structure is more common. This is the first report that structural variations are generated in micas by the combination of the intralayer shift and layer offset. The layer offset in wonesite is close to [110] and the structure is one-layer triclinic (Cl). These results give us an insight that sodium micas (preiswerkite, , aspidolite, wonesite, etc.) can possess various amounts of the layer offset, depending on the cavity space in the tetrahedral sheet that is primarily determined by the ditrigonal rotation angle. Therefore the composition in the 2:1 layer can drastically change the crystallography (e.g., powder diffraction patterns) of the sodium micas. In conclusion, sodium micas are not the simple analogue of potassium micas but belong to a group with a very unique character. Key words:Sodium , Aspidolite,Wonesite, Interlayer region, Layer offset, Ditrigonal rotation

INTRODUCTION percent of the a-axis length at most.1) 2) Consequently, the lateral displacement between the adjacent 2:1 layers in micas Mica is a common mineral group with diverse is almost identical with the intralayer shift. By contrast, compositions and origins. The basic structure of micas and , in which no interlayer cations exist, consists of 2:1 or TOT (tetrahedral-octahedral-tetrahedral) have a considerable amount of shift (close to a/3) between layers, and interlayer cations between the adjacent 2:1 layers. the T sheets across the interlayer region to reduce repulsion The two tetrahedral (1) sheets in a 2:1 unit layer are laterally between opposing basal oxygen atoms and tetrahedral shifted by about a/3 (intralayer shift) where a is the length of cations.3) the a-axis (5.1•`5.4 A), to form octahedral sites between The most common element for the interlayer cations them. As six directions are possible for the intralayer shift, in micas is potassium but sodium or divalent large cations micas appear in several different polytypes by the choice and can replace it. For instance, paragonite is the sodium combination of these directions. On the other hand, the two analogue of , a dioctahedral potassium mica. A T sheets belonging to different 2:1 layers across the number of analyses confirmed that the interlayer structure in interlayer region are hardly staggered because the interlayer paragonite is almost the same as that in muscovite, although cations are accommodated in the cavities formed with the the basal spacing is considerably decreased (ca. 0.3 A) due two opposing tetrahedral six-fold rings. Accurate structure to a smaller ionic radius of sodium than that of potassium.4 5) In contrast analyses of micas revealed that the two T sheets across the , few works have been reported for the interlayer region are slightly shifted in real mica structures structures of sodium-bearing trioctahedral micas, mainly due

(termed layer offset'), but the amount of the shift is a few to the lack of good specimens. Probably the structure 65

analysis of preiswerkite by Oberti et al.61is the only reliable one. a Recently we have investigated and reported the structures of two trioctahedral sodium micas, aspidolite7)and wonesite.8) Aspidolite is known as the Na analogue of (previously called Na-phlogopite) and wonesite is an interlayer-deficient sodium mica reported by Spear et aI.9)The structure analyses of the specimens were impossible due to fine intergrowth with phlogopite and/or talc. We have investigated the specimens mainly by using HRTEM, electron diffraction and X-ray diffraction (XRD) using a Gandolfi camera. These results have revealed interesting and important nature of the interlayer structure in trioctahedral sodium micas.

EXPERIMENTAL b Aspidolite-phlogopiteinterleaved mica was obtained froma rocksample (a registeredspecimen of the Geological Surveyof Japan,GSJ M 35151-1)from a graniticcontact aureolein Kasuga-mura,Gifu Prefecture,central Japan.10) Electronmicroprobe analyses showed that the composition for aspidolite is (Nat.77Ko,22)(Mg4.53Alo.84Feo.47Tio.ov) (Si5.11Al2s9)020F0.0.(OH)3.93.This is close to the midpointof aspidolite (NaMg3A1Si3O1o(OH)2)- preiswerkite (NaMg2A1Al2Si2O10(OH)2)series formed by Tschermak substitution,but slightlyclose to aspidolite.Hence this sodiummica shouldbe called aluminianaspidolite. Rock fragmentscontaining wonesite from the PostPond Volcanics, Vermont,USA, were donated from the NationalMuseum of Figure 1. (a) Filtered HRTEM image of interstratified aspidolite-phlogopite recorded down [ 100]. The inset at the bottom-right is the magnified Natural History, Washington,D.C. (catalog # NMNH image of a portion. "T" and "0" indicate the positions of tetrahedral and 145724)and fromDr. S. Guggenheim,University of Illinois octahedral sheets respectively. The white bars connect the dark spots at at Chicago.Electron microprobeanalyses of wonesite the left-hand and right-hand tetrahedral sheets in a 2:1 layer. Note the indicatedalmost the samecomposition as reportedby Spear two tetrahedral sheets across the interlayer regions except those with asterisks are staggered. The interlayer regions with asterisks are et a1.9) interpreted to be potassium-occupied. (b) Filtered HRTEM image Specimens for TEM examinationalong [hk0] recorded down [310]. Note the two tetrahedral sheets across the directionswere prepared by usingthe methoddescribed in interlayer regions are slightly staggered except those with asterisks. Kogure.111HRTEM examination was performedat 200 kV usinga JEOLJEM-2010 with a nominalpoint resolution of 2.0 A (C5= 0.5 mm).High-resolution images were recorded previous work'') can be referred to for the interpretation of on films at near Scherzerdefocus. Successfulimages the HRTEM images of micas. The structure in the right-hand recordedon the filmswere digitized using a CCDcamera for in Figure la has one-layer periodicity and the two T sheets imageprocessing. Noisy contrast from amorphous materials in a 2:1 layer are not staggered, indicating phlogopite-1M on the specimensurfaces was removedusing the rotational observed along [100] (the intralayer shift is parallel to the ltering technique12) implemented withfi Gatan electron beam). The contrasts of the two T sheets across the DigitalMicrographversion 2.5. 3) interlayer regions are also not staggered, in accordance with Micafragments were attached to a thinglass fiberfor the normal phlogopite structure where the layer offset is XRDmeasurements. XRD "powder" patterns were obtained negligible. On the other hand the structure around the left of witha Gandolficamera of 114.6mm in diameteremploying the figure, which is identified as aspidolite by EDS analysis, Ni-filteredCuKa radiation.The patternswere recordedon has also no stagger between the two T sheets in a 2:1 layer, an imagingplate and processedwith a Fuji BAS-2500 but the two T sheets across the interlayer regions are bio-imaginganalyzerand with a computerprogram by considerably staggered. This stagger is also observed in the Nakamuta.4) image along [310] (Figure lb). From these images it is evident that the structure of the interlayer region in RESULTS AND DISCUSSION aspidolite is considerably different from that in phlogopite, with a layer offset similar to paragonite but the amount of Identification of large layer offsets in sodium micas the offset is considerably larger. Although it is possible to estimate roughly the amount Figure 1 shows filtered HRTEM images of the and the direction of the layer offset from these HRTEM interstratified phlogopite-aspidolite, recorded with the images, more quantitative estimation can be made using electron beam parallel to (a) [100] and (b)[310]. Our diffraction techniques. A triclinic C-centered cell (C1) with 66

Figure 3.(a) Experimental XRD pattern from a wonesite crystal acquired with a Gandolfi camera.(b) Calculated "powder" pattern using new cell

parameters with a large layer offset.(c) Calculated pattern using crystal parameters in Spear et al. 9) with no layer offset. For the calculation, a Figure 2. Schematic figure for the interlayer structure in aspidolite, showing pseudo-Voight function (the same ratio of Gauss and Lorentz functions) two tetrahedral sheets and sodium ions between them. The upper sheet is with a half-width of 0.35•‹is assumed for the peak profile. Notice that shifted by 0.9 A along [1101. Sodium ions are assumed to exist at the major peaks in the experimental pattern are reproduced in the calculated

center of the staggered six-fold rings above and below the sodium ion pattern in (b), suggesting existence of the large layer offset.

cell parameters of a=5.30, b=9.18, c=9.88 A, ƒ¿=94.4,B =97 .8,ƒÁ=90•Kwas derived from the selected area electron a diffractions (SAED) along two zone axes at the aspidolite region. The lateral displacement between adjacent layers calculated from these cell parameters is -0.253a-0.083b. If we assume that the intralayer shift in aspidolite is exactly a/3 as that in phlogopite-1M, the layer offset - at the interlayer region is 0.080a-0.083b. From this result, the amount of the layer offset is 0.9 A and the direction of the offset is almost [110]. figure 2 illustrates the proposed interlayer structure in aspidolite. The position of sodium b ions is assumed to be the center of the staggered two tetrahedral rings. The direction of the offset in aspidolite is the same as those found in pyrophyllite and talc3), and in paragonite5); the two T sheets are shifted from each other as an apex oxygen atom of the ditrigonal oxygen ring is separated from the center of the cavity (figure 2). Similarly we have investigated the wonesite specimen by HRTEM and SAED, and a larger layer offset of 1.25 A along the direction of almost [110] was identified. 8) figure 3 Figure 4. Filtered HRTEM images of aspidolite recorded down [100] where shows the experimental XRD pattern from a wonesite (a) a random mixture of two stagger directions at the interlayer regions and (b) non-staggered interlayer regions are observed. The interlayer fragment using a Gandolfi camera (figure 3a), a calculated regions with the asterisk in (a) are occupied by potassium. pattern using our model with the large layer offset (b), and that using crystal parameters in Spear et al.9l with no layer offset (c). The pattern in (b) explains the experimental data readily found in HRTEM images of aspidolite and wonesite. far better than that in (c). This large offset is rather close to In Figure 4a, a mixture of staggers to up and down is that (1.55 A) in talc. Veblen15l reported the exsolution observed, which must corresponds to the layer offsets along lamellae of talc in the wonesite specimen, where the 2: 1 [110] and [110]. Furthermore, some interlayer regions show layers are continuous across the two phases. This is no stagger although their basal spacing is the same as that of reasonable by considering the close layer offsets between staggered layers (Figure 4b). One may suspect that the wonesite and talc. non-staggered interlayer regions correspond to the normal mica structure with negligible layer offset, but it is more Structural variation in sodium micas by the layer offset reasonable to suggest that they are [100] layer offset. In this As a pseudo three-fold axis runs through the center of case the direction of the layer offset and intralayer shift is the tetrahedral six-fold ring, it is expected that the layer the same, which results in a monoclinic unit cell. If the offset occurs not only along [110], but also along [110] and amount of the layer offset to [100] is the same as that to [I00]. These different directions of the layer offset were 67

[110], the cell parameters are a=5.3 0, b=9.18, c=10.12A, a =105 .3•Kand the space group is C2/m. We haveƒÀ actually b observed an XRD pattern from aspidolite that can be explained this monoclinic unit cell.7 Consequently aspidolite has two one-layer polytypes, aspidolite-1A ([110] or [110] layer offset, Cl) and aspidolite-1M ([100] layer offset, C2/m). From XRD and TEM examinations, probably the triclinic structure is more common than the monoclinic one. These two structural variations in aspidolite can be called polytypes according to the official definition of polytypism, 16)if the direction of the layer offset is ordered. However, this polytypism is an exceptional one among micas and most phyllosilicates for which the structural Figure 5. (a) The relationship between d(C-O), the distance from the center variations are generated with interlayer shifts with the of the oxygen hexagonal ring to the closest oxygen atoms, and the layer amount of zero, a/3 or b/3 (if idealized in some cases) to offset in four sodium micas. ƒ¿'s for aspidolite and wonesite were calculated from the composition for the 2:1 layer using the formula by certain directions. Weiss et al.17)(b) Definition of d(C-O) and the equation to calculate d(C-O) from the averaged neighboring O-O distance (d(O-O)b) and the General principle of the layer offset in sodium micas ditrigonal rotation angle (ƒ¿). From the above results layer offset is definitely an important parameter in the structure of sodium micas. Here, we discuss the general principle involved in determining the The location of sodium ions in aspidolite and amount of the layer offset. The driving force to produce the wonesite is not known and precise structure analyses are layer offset is repulsion between facing basal oxygen atoms required. However, if sodium ions are assumed to be located across the interlayer regions. However, the amount of the at the midpoint of the two centers of the staggered hexagonal layer offset is limited by the interlayer cations to be rings (Figure 2), then two oxygen atoms indicated by the accommodated in the cavity space of the oxygen hexagonal arrows in the figure are closest to the sodium ion. It is likely rings. In the case of potassium micas, the ionic radius of that this Na-O distance limits the amount of layer offset. The potassium is so large compared to the cavity space, even if closest Na-O distance reported for paragonite is 2.496 A,5) the hexagonal rings are not reduced in size by the ditrigonal and those calculated for aspidolite and wonesite are 2.45 and rotation, that no layer offset forms. In contrast, sodium ions 2.42A, respectively. Detailed discussion is not possible are so small that allow a considerable amount of layer offset because the values for aspidolite and wonesite are derived if the cavity space is large. The cavity space can be from several inaccurate and uncertain parameters, e.g. a. expressed by the distance between the center of the However, the closer Na-O distance in wonesite than that in hexagonal ring to three oxygen atoms that move closer to the paragonite may reflect the deficient occupancy of sodium at center by ditrigonal rotation (d(C-O), see Figure 5). d(C-O) the interlayer sites. is expressed as;

CONCLUSION where d(O-O)b is the average distance between neighboring Our investigations have revealed that sodium micas basal oxygen atoms and a is the ditrigonal rotation angle for are not the simple analogue of potassium micas as one might the tetrahedral sheet. We consider four sodium micas here: expected. Instead, sodium micas belong to a group with a preiswerkite, paragonite, aspidolite and wonesite. The unique character. In most phyllosilicate groups (potassium rotation angles and layer offsets for preiswerkite and micas, chlorites, 1:1 phyllosilicates, etc.), the amount of the paragonite were reported from their structure analyses.5), 6) interlayer shiftis a discrete, nearlyfixed value (zero, a/3 or Although rotation angles for aspidolite and wonesite are not b/3).18)In contrast, the amount of the interlayer shift can be known, they can be estimated from the compositions using continuous in sodium micas and is strongly dependent on the the formula proposed by Weiss et al.17) d(O-O)b is easily composition of the 2:1 layer. The obtained results here are calculated from the content of aluminum in the tetrahedral fundamental for the identification, structure analysis, and sites, using the formula and table reported also in Weiss et al. 17) The rotation angles reported for preiswerkite and certain applications of these materials.

paragonite are 20.0•‹ and 16.2•‹ respectively, and those ACKNOWLEDGEMENTS estimated for aspidolite and wonesite are 10.5•‹ and 6.3•K respectively. Next, d(C-O) was calculated from the above We are grateful to T. Kamiya and S. Yamada for equation and plotted with the amount of layer offset for each donating the aspidolite specimen, and to S. Guggenheim mica (Figure 5). No layer offset exists in preiswerkite 2), (University of Illinois at Chicago) and P. J. Dunn because of its small cavity space by a large a, originating (Smithsonian National Museum of Natural History) for the from the complete Tschermak substitution. For the other donation of the wonesite specimens. We also thank T. sodium micas, the amount of layer offset is almost Takeshige (the University of Tokyo) for preparation of the proportional to d(C-O) or the cavity space, as shown in the TEM specimens. Transmission electron microscopy was gure. fi carried out in the Electron Microbeam Analysis Facility of 68

Department of Earth and Planetary Science, the University of Tokyo.

REFERENCES

1) Bailey, S. W.(1984) in Micas, Reviews in Mineralogy Vol.3 (S. W. Bailey, Ed.), Mineralogical Society of America, Washington, D. C., 13-60. 2) Brigatti, M. F. and Guggenheim, S.(2002) in Micas: Crystal Chemistry & Metamorphic Petrology, Reviews in Mineralogy and Geochemistry Vol.46 (A. Mottana, F. P. Sassi, J. B. Thompson, JR., S. Guggenheim, ed.), Mineralogical Society of America, Washington, D. C., 1-98. 3) Evans, B. W. and Guggenheim, S.(1984) Hydrous Phyllosilicates (exclusive of micas), Reviews in Mineralogy Vol.19 (S. W. Bailey, Ed.), Mineralogical Society of America, Washington, D. C., 225-294. 4) Sidorenko, O. V.., Zvyagin, B. B., Soboleva, S. V.(1977) Sov. Phys. Crystallogr., 22, 554-556.

5) Lin, C-Y. & Bailey, S. W.(1984) Am. Mineral., 69, 122-127.

6) Oberti, R., Ungaretti, L., Tlili, A., Smith, D. C., Robert, J-L.(1993) Am. Mineral., 78, 1290-1298.

7) Kogure, T., Banno, Y. and Miyawaki, R.(2004) Eur. ‚i. Mineral., 16, 891-897.

8) Kogure, T, Miyawaki, R. and Banno, Y.(2005) Am. Mineral., 90, 725-731.

9) Spear, F. S., Hazen, R.M. and Rumble, D.(1981) Am Mineral., 66, 100-105.

10) Banno, Y, Miyawaki, R., Matsubara, S., Makino, K., Bunno, M., Yamada, S. and Kamiya, T.(2004) Eur. J. Mineral., 16, 177-183. 11) Kogure, T.(2002) in Micas: Crystal Chemistry & Metamorphic Petrology, Reviews in Mineralogy and Geochemistry Vol.46 (A. Mottana, F. P. Sassi, J. B. Thompson, JR., S. Guggenheim, ed.), Mineralogical Society of America, Washington, D. C., 281-312. 12) Kilaas, R.(1998) J. Microscopy, 190, 45-51. 13) Kogure, T. and Banfield, J. F.(1998) Am. Mineral., 83, 925-930.

14) Nakamuta, Y.(1999) J. Mineral. Soc. Jpn., 28, 117-121

(in Japanese with English abstract). 15) Veblen, D. R.(1983) Am Mineral, 68, 554-565. 16) Guinier, A., Bokij, G. B., Boll-Dornberger, K., Cowley, J. M., Durovic, S., Jagodzinski, H., Krishna, P., De Wolff, P. M., Zvyagin, B. B., Cox, D. E., Goodman, P., Hahn, Th., Kuchitsu, K. and Abrahams, S. C.(1984) Acta Cryst., A40, 399-404. 17) Weiss, Z., Rieder, M., and Chemielova, M.(1992) Eur. J. Mineral., 4, 665-682. 18) Durovic, S.(1992) in International Tables for Crystallography Vol.C (A. C. J. Wilson, ed.), Kluwer Academic Publishers, Dordrecht, Netherlands, 667-680.