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Electron-Diffraction and Electron-Microscopy Study Of American Mineralogist, Volume 72, pages 382-391, 1987 Electron-diffraction and electron-microscopystudy of balangeroiteand gageite:Crystal structures, polytypism, and fiber texture GrovaNNrFnnru,nrs Dipartimento di Scienzedella Terra, Universiti di Torino, Via S. Massimo 22, 10123 Torino, Italy M,lncpr,r-o MBr,r,rNr C.N.R., Centro di Geologia Strutturale e Dinamica dell'Appennino, Via S. Maria 53, 56100 Pisa, Italy Srrr',c.NoMBnr-rNo Dipartimentodi Scienzedella Terra, Universiti di Pisa,Via S. Maria 53, 56100Pisa, Italy ABSTRACT Electron-diffraction and transmission-electron microscopy (rervr)investigations were carried out on the fibrous minerals balangeroite and gageite.The studies indicated for balangeroitea monoclinic unit cell with a- : 19.40,b^: 19.40,c- (uniqueaxis) : 9.65 A, 7* : 88.9"; two polytypic modifications were found for gageite:gageite 2M, monoclinic, isostructural with balangeroite,cell dimensions a^ : 19.42,b^: 19.42,c^: 9.84 A, "),- : 89.5o,and gageitelTc, triclinic, unit-cell dimensionsa,: 14.17,b,: 14.07,c,: 9.84 A, a, : 76.5", 0, : 76.6','y, : 86.9'. Chemical and physical data pointed to the presenceof four-repeat silicate chains (T), running in the channelsof the octahedralframework describedby Moore ( I 969) and placed on both sides of Moore's octahedral walls (O) to give rise to composite TOT modules, interconnectedthrough octahedralbundles. The correspondingideal crystal-chemicalfor- mula is M4rO6(OH)40(SioOrJ4,with M denoting cations in octahedralcoordination, mainly Mg in balangeroiteand Mn in gageite. The monoclinic and triclinic structural arrangementsmay be describedas consisting of equivalent layers, characterizedby translation periods a^ and c- and width b. : b^/2. Adjacent layers are related by the vector tr : -t^/2 + b" + c*/3 or the vector t, : -a^/2 + b. - c-l3. The layer sequencetft2t12... is realizedin the monoclinic modifi- cations,whereas the sequencetltrtr . (or trtrtr. .) is realizedin the triclinic modification. The symmetry properties of the two arrangements,the peculiar featuresof their diffraction patterns, and the appearance of diffuse spots in various patterns are discussed and ex- plained on the basis of the OD (order-disorder)theory. Finally, reu evidencefor the replacementof balangeroiteby chrysotile during retrograde metamorphism is given, and the role of that mineral in the metamorphism of ultramafic rocks is discussed. thickness (2 x 2 bundles). Each module sharesits free IncrnonucrroN cornerswith the doubly sharedcorners of the other mod- Gageiteis a fibrous manganesesilicate that was found ule. The resulting octahedral framework contains [001] in low-temperaturehydrothermal veins at Franklin, New pipelike channelsthat, accordingto Moore (1969), house Jersey. Although the complete structure determination disordered silicate tetrahedra. was hampered by the disordered nature of the mineral, Although the topology of the octahedral framework Moore (1969) was able to outline a substructure, with seemedto be quite well defined, the proposed number referenceto an orthorhombic cell with a : 13.79, b : and the disordered arrangementof the tetrahedra in the 13.68,c' : 3.279 A, spacegroup Pnnm (a x b x c' cell channelswere probably an artifact of the assumedsub- hereafter),disregarding the indications for a trebled c pa- structure. In fact, as regardstheir number, Dunn (1979) rameter that were suggestedby the occurrenceof addi- derived, on the basis of new chemical analyses, tional diffirse and weak reflections. (Mn,Mg,Zn)o0Si,sO50(OH)40as the chemical content of a The substructure consists of two kinds of interlinked cell with trebled c parameter; this empirical formula modules, both of which are built up by chains of edge- does not agree with the crystal-chemical formula sharing octahedra:walls three chains wide (3 x I walls) (Mn,Mg,Zn)4r(Si,rO36)[Ou(OH)or]proposed by Moore and bundles that extend two chains both in width and ( I 969). 0003{04xl87/0304{382$02.00 382 FERRARIS ET AL.: BALANGEROITE AND GAGEITE 383 Fig. l. Electron-diffraction patterns (c* vertical). (a) Balangeroiteu00l; the reflections are indexed with referenceto the 2a x 2b x ccell. The strongestspots,/:3n, correspondtothe a x b x c'cell. (b) Balangeroite[1I0]; the reflectionsare indexedwith referenceto the 2a x 2b x c cell. The 003 spot, kinematically forbidden, arisesby dynamical effects(Gjonnes and Moodie, 1965) and vanisheswith a changein orientation (e.g.,Fig. la). The four different classesof reflections(absent, weak, medium, strong)may be clearly observed in this diffraction pattern. (c) Gageite lTc [110]; the reflections are indexed with referenceto the primitive triclinic cell. More recently,Compagnoni et al. (1983) describeda fraction(sao) techniques. As balangeroiteand gageitedevelop new fibrous silicate, balangeroite,the Mg-dominant an- pronounced{ I l0} cleavageon grinding,controlled tilting exper- alogueofgageite. From rotation photographs,which imentswere performed with the [001]fiber direction as tilt axis. [001] pat- clearly indicated a trebled c parameter, and by least- In this way,the recombinationofseveral [zv0] diffraction ternsdisplayed an overallview of the wholereciprocal lattice. squaresfitting of the X-ray powder-diffraction pattern, Viewsalong [001] were also obtained for balangeroite,starting theparameters a: 13.85,b: 13.58,c:9.65 A 1a " from suitablethin sections. b x c basic cell, hereafter)were obtained. On the basis of the actual chemical data for balangeroite,and taking into account both the structure model of Moore (1969) and Psnunocrr-Ls AND TRUECELLS the empirical formula of Dunn (1979) for gageite, the Up to this point, we have introduced two different unit following unit-cell content was proposedfor balangeroite: cells,the a x b x c' cell of Moore (1969)and the a x (Mg,Fe,Mn,tr)orSir5(O,OH)ro. b x c cell of Compagnoniet al. (1983). Although both As a result of electron-diffraction and transmission- cells have been supersededby the results of our electron- electron microscopy (rervr)studies of balangeroite and diftaction study, we shall frequently refer to the a x b x gageite,we have revised the interpretation of the crystal c cell, so closely related to the structural model of Moore structuresof these minerals and their polytypic relation- (1969), as a uselirl referenceframe. Crystallographic planes ships, and we have consideredin more generalterms the and directions in the preceding as well as in the subse- order-disorderphenomena in the whole structural family. quent pageswill be referred to that cell, unlessotherwise stated. ExprnrlrBNTAL DETATLS Figure la shows a [100] electron-diffraction pattern Gageitefrom Franklin,New Jersey,U.S.A., was kindly pro- taken of balangeroite.Incidentally, such a pattern is al- vided by Mr. EwaldGerstmann through the courtesyof P. B. most identical to the [010] pattern, owing to an overall Mooreand by the SmithsonianInstitution through the courtesy pseudotetragonalsymmetry. The sharp, strong spots in P. Dunn. of J. Orientedthin sectionsof holotypebalangeroite that pattern correspondto the a x b x c' cell of Moore. wereobtained from R. Compagnoni. A similar distinction between weak reflections with / : Electrondiffraction and imagingwere carried out in a Philips reflectionswith /: 3r is evi- 400T electronmicroscope, as describedby, e.g.,Mellini et al. 3n t I and strongsubcell (1984).Ground specimens ofbalangeroite and gageite were stud- dent in Figure lb, which showsthe [ 10] diffraction pat- ied, but the only ion-thinnedspecimens were of balangeroite, tern. From a series of similar [zu0] diffraction photo- becauseof the morelimited availabilityof gageite. graphs, the whole pattern of balangeroite was obtained Electron-diffractiondata wereobtained by selected-areadif- (Fig. 2). The corresponding direct lattice may be de- 384 FERRARIS ET AL,: BALANGEROITE AND GAGEITE + * * o o + l=2n X l=2n+l X * )( t( all preaent B.l o o I = I o 3n!t *--- x + A. Fig.2. Thediftaction pattern ofbalangeroite and gageite 2M, with referenceto the 2a x 2b x ccell (i.e..A x B x C cell\. scribed in terms of a metrically orthorhombic2a x 2b x c C-centeredcell, or, otherwise,a primitive cell with unit- cellpammeters a- : 19.40,b^: 19.40,c^: 9.65A, ?- : 88.9'and related to the basic a x b x c cell through the rransformarion marrix ( l I0/ l l 0/00 1) (Fig. 3a). A similar difraction pattern occursfor gageite,and the correspondingprimitive direct cell possessesparameters a^ : 19.42,b^ : 19.42,c^ -- 9.84A, y- : 89.5'.How- ever, the electron-diffraction study ofgageite showed the frequent occurrenceofa secondelectron-diffraction pat- tern (Fig. lc) besidesthat formerly mentioned. The cor- respondingdirect cell hascell parametersa.: 14.17,b,: 14.07,c,:9.84 A, d, : 76.5,P,:76.6o,7, : 86.9"and is related to the basic a x b x c cell through the trans- formation matrix (l0h/01Y3/001) (Fie. 3b). It will be shown that the two distinct difraction patterns corre- spond to two polytypic modifications of gageite,which, on the basis of the symmetry properties and structural b) relationships that we are going to discuss, may be con- Fig. 3. (a) Geometricalrelationships among the a x b x c veniently called gageite2M and gageite lTc. cell,the 2a x 2b x c C-centeredcell and the correspondinga- x b^ x c^primitive cellfor balangeroiteand gageite 2M. (b) Geo- Trrn nnnrrroNAr, MoDULE: THE FOuR-REpEAT metricalrelationships between the triclinic cell ofgageite lTc and SILICATE CHAIN thebasicaxbxccell. Any proposed structure model for balangeroite and gageitemust take into account (l) the previous structural strong intensities of the diffraction spots corresponding work of Moore (1969) on gageite;(2) the two different totheaxbxc'cell. patterns that we observedby electron diffraction; (3) the We assumedthat the open [001] channels within the relationships connecting pseudocellsand true cells; (4) octahedral framework are occupied by a continuous chain the chemical data; (5) the infrared spectrum ofbalanger- of silicate tetrahedrawith 9.6-A periodicity. Sucha struc- oite (Compagnoni et al., 1983), typical of chain silicates tural unit correspondsto the four-repeat crankshaft chain in the range900-1 100 cm-'; (6) the fibrous nature of both of haradaite (Tak6uchi and Joswig, 1967).
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