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Modulated Crystal Structures Of r63 Thz Canadian M irrcralo g ist vol.36, pp. 163-179(1998) MODULATEDCRYSTAL STRUCTURES OF GREENALITEAND GARYOPILITE: A SYSTEMWITH LONG.RANGE.IN-PLANE STRUCTURAL DISORDER IN THETETRAHEDRA SHEET STEPIIEN GUGGENIIEIM1 Deportmenl of Eanh and Environrnental Sciences, Unhtersity oflllinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60680, U.S.A. RICHARDA. EGGLETON1 Deparxnent of Geology, Australian National University, Canberra, ACT 0200, Australia Arsrxecr High-resolution transmission electron microscope (TEM) images confirm that greenalite and caryopilite are modulated 1:1 phyllosilicates. The ocrahedrally coordinated Fe (greenalite) and Mn (caryopilite) form trioctahedral sheets.Six-member rings of tetrahedra link to form triangular islands four or five tetrahedra across, with each island coordinatirg 1s sns ssrahedral sheet. Adjacent islands are inverted and link to the neighboring octahedral sheet, which results in a triply-intersecting colTugation for the tetrahedral sheet. Islands vary in numbers of tetrahedra about a mean dictated by the octahedral sheet dimension. Island separationsrange about a mean distance within ttre X-Iplane, with island alignment fluctuating as a function of lattice vectors defined by the octahedral sheet. The tetrahedra ttrus show limited short-range order (spanning to five octahedra), but long-range disorder. Linkages of tetrahedra between islands are apparently completely disordered. Because of this disorder, there is no definable unit-cell. Fourier calculations involving non-repeating structures cannot use unit-cell fractional coordinates and Miller indices. We calculated diffraction patterns by finding the real-space coordinates ^ofevery arcm in the model relative to a defined origin. The reciprocal space variable, d*, is sampled at intervals of0.005 A to build the continuous Fourier transform of the model. DiscreF polytypes of I Z and lM for greenalite and caryopilite, respectively, were identified. Where g'ains sqltain nixfires, the relative abundanceof each polyrype is related to composition, with the dominant polytype based s1 minimizing misfit be[ween the sheetsof octahedra and of tetrahedra. Stacking in greenalite and caryopilite is defined by the relative positions ofadjacent octahedral sheets and, therefore, limits on the displacements ofneighboring domains of silicate rings within (001) are possible. Domain boundary Iinkages, however, cannot be determined precisely by using either diffraction or imaging data. Keywords: greenalite, caryopilite, serpentines, modulated structures, superstructure, phyllosilicates, polytypism, Fourier synthesis of disordered structures, electron diffraction. Sovrvranp Les images obtenues par microscopie 6lectronique en transnission d haute r6solution confirment I'hypothbse voulant que la greenalite et la caryopilite sont des phyllosilicates l:1 modul6s. Les octaddres contenant le Fe (greenalite) et le Mn (caryopilite) forment des feuillets triocta6driques. Des anneaux de six t6trabdres forment des agencements en llots triangulaires quatre ou cinq t6traBdresde large, chacun des ilots 6rant coordonn6 d un feuillet d'octaedres. Des ilots adjacents sont inversds et rattaches au ferrilleg d'sataBdrcs avoisilant, avec comme r6sultat une modulation des feuillets de t6trabdres en trois directions. Ces llots contiennent un nombre variable de tdtraddres,mais le nombre moyen d6pend de la dimension du feuillet d'octabdres. kur s6parationmoyenne dans le plan X-I a aussi une variance associ6e,l'alignement des ilots 6tant 9&6 par la fluctuation des vecteurs rdticulaires d6finis par le feuillet d'octabdres. Les tetrabdres montrent ainsi un certain degr6 d'ordre (limit6 d une s6quence de cinq octabdres), mais sont d6sordonn6s sur une plus grande 6chelle. Les agencementsde t6trabdres entre llots seraient complbtement ddsordonnds.Vu ce ddsordre, il est impossible de d6finir une maille 6l6mentaire. Des calculs faisant appel d la transformation de Fourier de structures non rdp6t6esne pourraient utiliser I'indigage de Miller et les coordonn6es fractionnelles des atomes dans une maille. Nous avons calcul6 des spectresde dffiaction X en sp6cifiant les coordonn6esdes atomes dans 1'espaceen termes absolus par rapport d une origine fixe. La variable dans l'espace r6ciproque, d*, est 6valu6e d un intervalle de 0.005 A afin de construire une synthdse de Fourier continue du moddle. Nous reconnaissonsdans ce moddle les polytypes lT et lM de la greenalite et de la caryopilite, respectivement. Ot les grains contiement des melanges, la proportion de chacun des polytypes d6pend de la composition, le polytype dominant 6tant celui qui r6ussit le mieux I minimiser t E-mai I addres ses:xtal @uic.edu, rae653@ anugpo.anu.edu.au r64 TIIE CANADIAN MINERALOGIST le d6calage entre les feuillets d'octabdres et de t6ffabdres. La sdquence d'empilements dans la greenalite et la caryopilite ddpend a.lorsdes positions relatives des feuillets d'octaBdresadjacents et, donc, des limites dals les d€placementspossibles de domaines adjacents d'anneaux dans Ie plan (001). En revanche, il est impossible de pr6ciser les agencementsdans les zones inter-domaines en utilisant les donn6es de diffraction ou les imases obtenues. (Traduit par la R6daction) Mots-cl6s: greenalite, caryopilite, serpentines, structrues modul6es, surstructure, phyllosilicates, polytypisme, synthdse de Fourier des structures ddsordonn6es,diffraction d'6lectrons. INrnopucuoN Samples analyzed using micro-analyical lechniques from pure areas always show an excess of Si and In phyllosilicates, octahedrally coordinated cations a deficiency in octahedrally coordinated cations, on generally coordinate to either oxygen atoms or OH the basis of an ideal structure and composition of groups; the oxygen atoms belong also to the silicate serpentine,with a total cation charge of +14. A change tetrahedra. Thus, the apical oxygen atoms form a in configuration of the ring of tetrahedra, from 6-fold junction between a continuous octahedral sheet rings within the domains to other configurations at (= sheetof octahedra) and a tetrahedral sheet (= sheet domain boundaries, would explain the excess in Si of tetrahedra). The oxygen atoms have a lateral spacing and deficiency in octahedrally coordinated cations congruent with both the octahedra and the tetrahedra. (Guggenhe-imet al. 1982).^Thespacings o[ approxi- In kaolinite and the chrysotile variety of serpentine, for mately 17 4,23 A, and 30 A are relatedto domainsof example, where there are relatively small octahedrally rings of tetrahedra attached to a continuous octahedral coordinated cations such as Mg and Al, the tetrahedral sheet, with the smaller domain required for the sheet sheetis continuousbecause the Si-to-Si spacingsmatch with the lmger average octahedrally coordinated cation. the spacings from apical oxygen to apical oxygen The spacingsappear to be step-like and not continuous defined by the octahedra. Large cations such as (Guggenheim & Bailey 1989), thereby suggestingthat Fe2* and Mn2*, however, prevent such a fit. In these in caryopilite, the domains have a diameter of three structures, the anion-to-anion spacing of the common rings of tetrahedra whereas in greenalite, the number is junction cannot match the Si-to-Si spacings of the four. Although Guggenheim et al. (1982) presentedan tetrahedra; thus, an inversion of tetrahedra or some idealized model for the structure of greenalite, they other structural perturbation or modulation is necessary were unable to obtain high-resolution electron-optical to reset the spacing after a short run of tetrahedra. It images to substantiate the model. They did, however, is this misfit between the octahedral sheet and the produce a laser-optical diffraction pattern of hN tetrahedral sheet that appears to be the underlying prin- reflections that simulates the observed pattern closely. ciple in the formation of modulated phyllosilicates Serpentine-group minerals can also have adjacent (Guggenheim& Eggleton 1987, 1988). layers displaced to fonn different polytypic amange- Greenalite is the Al-poor and Fe2*-rich selpentine, ments. Although there are twelve ideal polytypes in and caryopilite is the Mn analogue. They form in the serpentines,these may be divided into four groups low-energy environments, commonly in either very of three polytypes each, depending on the occupancy of low-grade metamorphic rocks or in hydrothermal certain interstices among the octahedra (Bailey 1969)' veins or replacements.The minerals are almost always Where stacking disorder is severeoas in greenalite and fine-grained, and usually intermixed with quartz caryopilite, group-A structures and group-C structures and either Fe or Mn silicates or oxides, respectively. may be thought of as LM and 1T polytypes, respectively, Greenalite and caryopilite form a (homologous) series becauserandom stacking makes the polytypes within of closely related modulated structures (Guggenheim each group indistinguishable. Guggenheim et al. et al. 1982). Electron-diffraction patterns (Guggenheim (1982) determined that two polytypes (lM and 17) ue et al. 1982) derived from single crystals of both present in all samples of greenalite and caryopilite minerals show that a noncommensurate superlattice studied, with greenalite predominantly composed of the exists, which produces satellite reflections around 1T polytype, and caryopilite mostly consisting of k = 3n reflections in the hlco plane (Fig. 1). Spacings the lM polytype. Single-crystal X-ray patterns from an measured between the subcell and the superlattice
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