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HRTEM Evidence of a Polysomatic Series Including Serpentine

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HRTEM Evidence of a Polysomatic Series Including Serpentine American Mineralogist, Volume 70, pages 773-781, 1985 Carlosturanite:HRTEM evidenceof a polysomaticseries including serpentine M,c,ncs[o MBrrrr.n C.N.R., C.S. GeologiaStrutturale e Dinamicadell' Appennino Via S. Maria 53,56100 Pisa,Italy Grov.awNtFgnnlnrs Dipartimentodi Scienzedella Terra, Uniuersitddi Torino uia S. Massimo22, 10123 Torino, Italy .lNo Ronsnro Coupl,cNotn Dipartimentodi Scienzeilella Tena, Uniuersitdilella Calabria CastiglioneScalo, 87030 Cosenza,ltaly Abstract Carlosturanite,ideally M21[T12O28(OH)4](OH)3..H2O,is a water and magnesium-rich, silicon-poor serpentineJikeasbestiform mineral. TEM techniquesrevealed the fiber texture, which consistsof randomly rotated fibers approximately 1000A in cross section intermixed with smaller chrysotile fibers. TEM also permitted the unit cell to be found and a plausible structural model to be proposed. The model is basedon the ideal structure ofserpentine,from which carlosturanitediffers by the presenceof infinite rows of vacanciesin tetrahedral sites.As a result, carlosturaniteis a chain silicate,characterized by the presenceof triple chains.These chains are related to the four-repeatsingle crankshaftchains of haradaite and could be formed by linking three har- adaitechains. This model explainsthe physicaland chemicalproperties of the mineral. Starting from the carlosturanitearrangement, a comprehensivediscussion of the possible carlosturaniteJike phases is given and the family name inophites is proposed. Inophites belongto a polysomaticseries, S.X, resultingfrom combination of serpentineS modules,with compositionM3T2O'(OH)*, and of X modules,with compositionM6T2O3(OH)19'H1O. Evidence of the existenceof the S4X, ,S6X and SrX structures as fault lamellae within carlosturanite,SrX, is given. Introduction hairpin filament, according to standard procedures (e.g. Carlosturanite is a new metamorphic rock-forming sili- Mellini, 1982a).TEM/EDS analytical data were gathered cate (Compagnoni et al., 1985),closely related to the ser- by an EDAX 707A multichannelanalyzer and offJine pro- pentineminerals. With X-ray diffraction methods,only the cessedby the ratio method (Champnesset al., 1981),using powder pattern and the existenceof a 9.44 fiber axis could experimentallydetermined Ko, values(Mellini and Meni- be ascertained,an X-ray based structural determination chini, unpublished).Both ion-milled petrographicthin sec- being preventedby the fibrous habit of the mineral. How- tions and ground fibers from the holotype specimenswere ever, transmissionelectron microscopy (TEM) techniques studied. (electrondiffraction, lattice imaging,TEM/EDS microanal- Image simulation was performed with the SHRLI set of ysis) have allowed us to elucidatethe crystal chemistry of programs (O'Keefe and Buseck, 1979),starting from the carlosturaniteand its relationshipswith structurally related structure model we propose. phasesand parageneticminerals. The aim of this paper is to describea structural model for carlosturanite that has The unit cell beenderived by analogywith serpentineand that is consis- Grinding of the fiber bundles produceselongated lath- tent with the observedphysical and chemicalproperties of shaped fragments.Due to the presenceof a good {001} carlosturanite. cleavage,preferred orientation ofthe fibers on the support film is obtained,and, with a few degreesof tilt, hkOelectron Experimental diffraction patterns with mm symmetry are obtained (Fig. The electron microscopy investigation used a Philips 1a).These patterns can be indexed on the basisof a:35.7, 400T instrument,operating at 120kV and equippedwith a b :9.44, with systematicextinctions for reflectionshaving 0003-{Mxl8s/0708-0773$02.00 773 7'74 MELLINI ET AL.: CARLOST()RilVNN: POLYSOMATIC SER/ES Fig. 1. (a) [001] electron diffraction pattern of carlosturanite; O) [010] electron diffraction pattern from a fiber bundle, showing spotty diffraction rings; (c) [010] electrondiffraction pattern from a singlefiber. h + k :2n * I. The short 9.4A translationis parallel to mask c glide planes, seem to be excluded.In fact, such the fiber axis, in keeping with the X-ray results (Com- reflectionsoccur within the indexed X-ray powder pattern pagnoniet al., 1985). (Compagnoniet al., 1985),and odd rows appear as strong The [010] view usually producesspotty ring diffraction as evenones in the electrondiflraction patterns. patterns, which can be interpreted as due to rotational A sinusoidal trend in the intensity of diflracted spots disorder around [010] of single crystalline fitrers (Fig. 1b). along a* (Fig. la, c) can be easilyrecognized, with maxima Nevertheless,in some cases single-fiber patterns as in occurring for h : 14n and for h: l4n + 7 where k is even Figure lc were obtained. These patterns possessno sym- and odd respectively.These intensity featuressuggest the metry higher than the inversion center and give lattice pa- presenceof a C-centeredsubunit cell with a" : 36'717: rametersa' : al2 (becauseof extinctions): 18.4,c :734 5.2,b : 9.4,c : 734, P : 101',which is characteristicof and P: 101". Carlosturanite therefore is monoclinic, the serpentineminerals, and a modulation of the serpentine -- a 36.7,b :9.4, c :73L, F : LOl", spacegroup CLlm, structure.For comparison,lizardite 1T hasa : 5.332,b : Cm,or C2. Possibledynamical diffraction effectsthat could 9.235,c : 7.n34 in the C centeredorthorhombic setting produceforbidden reflectionslfil for I :2n * 1, and would (Mellini, 1982b). MELLINI ET AL,: CARLOSTURANITE: POLYSOMATIC SERIES 775 The structure model properties,such as the fibrous habit and the thermal be- The close relationshipsbetween carlosturanite and l: I havior. Within these boundary conditions, two types of layer silicates,already suggested by the X-ray rotation pho- models can be tested: those formed by addition of M cat- tographs and powder patterns, also are supported by ions and those formed by removal of T cations relative to HRTEM images.For instance,Figure 2 shows a pair of serpentine.The most likely model belongsto the latter type randomly-rotatedcarlosturanite fibers as seenalong [010] and is shown in Figure 3. It is obtained by preservingthe and connectedone to the other by a narrow region where octahedral sheet of serpentine but introducing ordered curvedJayerserpentine is present.The (001)lattice fringes tetrahedral vacanciesand substituting hydroxyl anions or of the serpentineare smoothly connectedwith coherent water moleculesfor oxygenatoms in positionsnow bonded interfacesto the (001)lattice fringesofcarlosturanite in the to only one or no silicon atom. One-seventh of the two fibers,with no faults or dislocationsvisible at the con- [Si2O?]6- groups arrangedalong [010] rows are replaced tact betweenserpentine and carlosturanite.In this view, the by [(OH).HrO]5- groups having tetrahedral arrange- l: I tetrahedrafoctahedral layers of serpentine are seen ments of hydrogen-bondedwater moleculesand hydroxyl edgeon, and the coherencysuggests that similar, although anions. A similar mechanismoperates, for instance,in hy- not identical, layers are also presentin carlosturanite.In drogarnetsand sheetsilicates such as zeophyllite(Meilino, Figure 2, theselayers are stackedone upon the other in the r972\. c direction. The occurrenceof a 9.4A fiber axis in carlostu- Starting from a sevenfoldserpentine unit cell (a: 5.2 ranite leads us to assumea similar value in the coherent. x 7A), with a content intermixed serpentine. 7M6[T4Olo](OH)s' M42T2EO79(OH)56, Any model for carlosturaniteshould be consistentwith (1) a ratio ofoctahedral to tetrahedralcations of 1.75and a substitution of twelve hydroxyl anions and two water mol- water content of 17 wt.%, both higher than in serpentine eculesfor oxygen atoms and elimination of four T atoms where they are 1.5 and 13 respectively;(2) the observed (namelysubstitution of sixteenhydrogens for four T atoms) unit cell; (3) a serpentinelike structure (Cm symmetry). leadsto Furthermore,the model must eventuallvaccord with other M42T24056(OH)58(H2O)2 Fig. 2. Curved layer serpentine(s) coherently connectsadjacent carlosturanite (cst) fibers. 1',t6 MELLINI ET AL.: CARLOSTIIRANITE: POLYSOMAT IC SERIES T Fig. 3. Structuremodel for carlosturanite.Small filled circlesare hydroxyl anions,large filled circleswater molecules. Carlosturanitereveals a new way in which misfit relief is achievedin serpentine-likestructures (Wicks and Whitta- M42 [T24Os6(OH)8](OH)6o(HrO)' ker, 1975),by separatingthe tetrahedral sheet into strips for carlosturanite, with brackets enclosing the silicate while retaining a continuous octahedralsheet. In this way, strip composition. This ideal formula compareswell with flat tetrahedral-octahedrallayers are obtained instead of the empirical formula (Mg,Fe,Ti,Mn,Cr,E)ur I(SiADt, the curved layersthat occur in chrysotileand parachrysoti- Or8(OH)41(OH)30.H2O,Z :2, Eivenby Compagnoniet le (Wicks and Whittaker, 1975).Moreover, extensivesub- al. (1985).The fibrous habit of carlosturaniteis evidently stitution of hydrogensfor silicon atoms leads surely to a the result of the infinite silicatechains along the fiber direc- higher number of hydrogen bonds within the crystal. This tion. More evidencethat supports this structural model, higher number probably resultsin a higher density of inter- such as calculated density, thermal behavior, water loss layer hydrogen bonds, thus leading to further stabilization during heating and formation conditions can be found in of the flat layer structure (Mellini, 1982b)and to the ex- the companion paper by Compagrroniet al. (1985).Also tremely small
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