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 modifications,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 explained 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 pattern (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). This type of minerals, with [001] as the fiber axis; and any other phys- chain is not unusual in mineral structuresand, by differ- ical properties. ent degreesof polymerization, gives rise to the double As already stated,the structure model of Moore (1969), chain of caysichite (Mellini and Merlino, 1978), to the R : 0. 17, is plausible as far as the octahedralframework tetrahedral strip of carlosturanite (Mellini et al., 1985), is concerned.However, as stressedby Moore himself and and to a large group of framework silicates,including the later remarked on by Dunn (1979) and Compagnoni et feldspars(Smith, 1974). According to our model, the ToO,, al. (1983), a more complete and satisfactory model for chain connectsto the octahedral framework through its the distribution of the silicate tetrahedra in the pipelike unshared corners, sandwiching each 3 x I octahedral wall. channelsshould be proposed.The model must be able to Figure 4 shows the interconnection between one tet- lead naturally from the 3.2-A c' periodicity, which cor- rahedral chain and the octahedral framework, as seen respondsto the length ofthe octahedral edge,to the tre- along the I l0] direction, whereasFigure 5 presentsthe bled 9.6-A c period, without completely destroying the c' [001] projection, which is common to all the actual and translational pseudosymmetry, as required by the very possible structures in this family, showing the distribu- FERRARISET AL.: BALANGEROITEAND GAGEITE 385

oy b I .I

Fig.5. [001]projection of thecrystal structure ofbalangeroite Fig. 4. The connectionbetween tetrahedral chains and oc- and gageite,described with referenceto the a x b x c cell and tahedralframework, as seen along [ 10]with respectto thebasic showingthe tetrahedralchains within the channelsdefined by axbxccell. the 3 x I octahedralwalls and 2 x 2 octahedralbundles. tion of the tetrahedral chains in the channelsof the oc- parameters tahedral framework. The resulting ideal chemical content Table I reports the positional and thermal of a\ a x b x c unit cell is MorOu(OH)oo(ToO,r)o,where for all the atoms, together with the occupanciesof Mn M and T indicate octahedral and tetrahedral cations, re- and Mg in the M(l) and M(2) octahedralsites. The bond polyhedra, spectively. The six oxygen atoms not belonging to the distancesin the various calculatedon the basis normal tetrahedralchain correspondto anions coordinated to six of the coordinates reported in Table l, appear octahedral cations and located in the central part of the and-as regards those associated with the octahedral bundles.The hydroxyls conespond to anions coordinated framework-in good agreement with those found by to only three octahedral cations: their number could be Moore (1969). It seemsuseful to remark that the Mg and lower than 40, becauseof substitution of oxygen for hy- Mn contentsderived from the structural refinement com- droxyl when the valence of the coordinating cations is pare fairly well with those obtained from the chemical higherthan 2+. data. The parallel refinement of Moore's model, carried given (1969) An isotropic refinement of the gageitestructure, based out starting with the coordinates by Moore on this model and using the X-ray reflections measured and using the same 871 reflections, led to a reliability by Moore (1969),improved the R value from 0.17, ob- indexofR:0.079. tained by Moore, to 0. 155 for 612 reflections. Rrvrsnn CRYSTALCHEMISTRY As we found one exceptionally good crystal among those provided by E. Gerstmann, we proceededto a new inten- On the basis of the average analysesof balangeroite gageite(Dunn, sity-data collection in the a x b x c' cell of Moore. A (Compagnoniet al., 1983)and 1979)and present total of 1026 independent reflections were measured on assuming,according to the structure model, six- x x an Ital Structures four-circle automatic diffractometer, teen Si atoms in the a b c volume, the formulas gageite, with Zr-filtered MoKa radiation. The atomic coordinates obtained for balangeroiteand for respectively,are - calculatedon the basis of our model were refined, togeth- (Mgru,oFer'zfiFe3.1fln,,,Alo, rCa" orCro o, TL o,Loooesi,6o5s r, er with the isotropic thermal parameters, in the space (OH)rr' and (Mg,,rrFeo.,,Mn* nrCa" roZn^ 3)>o2 65Sir6O54 53- group Pnnm. In a few cyclesof least-squaresrefinement, (OH)oorr. good. the R index dropped to 0.049 for 871 reflections with The agreementwith the ideal cell content is The g/cmr F. > 5o(F").' calculateddensities are 3.098 and 3.599 for balangeroiteand gageite,respectively. These values compare to the observedones: 2.98 g/cm3 for balangeroite(Com- ' To obtain a copy of the observed and calculated structure pagnoniet al., 1983)and 3.584(Palache, 1928) and3.46 factors, order Document AM-87-333 from the Business Ofrce, g/cm3 (Dunn, 1979) for gageite.New measurementson MineralogicalSociety of America,1625I Street,N.W., Suite 414, Washington,D.C. 20006,U.S.A. Pleaseremit $5.00in advance balangeroite (holotype material, torsion balance) gave for the microfiche. valuesin the range2.96-3.10 g,/cm3with an averagevalue 386 FERRARIS ET AL.: BALANGEROITE AND GAGEITE

Srnucrun-c,L DETAILS:CrurN SHIFTSAND LAYER SEQUENCES The introduction ofthe four-repeat tetrahedral chains fairly well explains the chemical data and a number of physical properties, as well as the gross features of the electron-diffraction patterns. The structural results so ftu obtained are conveniently summarizedin Figure 5, which presentsthe projection of the whole structure along the fiber axis. It is our present aim to determine the preciseand de- tailed arrangement of the various modules in the two structural forms whose existencewe outlined in the pre- lrl01 vious chapters. In particular we want to derive (l) the a) b) way in which the tetrahedral chains attach to both sides Fig. 6. The two possibleTOT modules.Only the upper chain of a 3 x I octahedral wall-i.e., whether the two chains is drawn; filled circlesindicate the connectionpoints ofthe lower are placed at the same height, thus building a TOT com- chain with the octahedralwall. (a) Upper and lower T chains are plex module with a twofold symmetry axis (Fig. 6b), or shifted by c/3. (b) Upper and lower T chains are related by a are displaced by *c/3 one relatively to the other (Fig. twofold axis and have common z height. 6a)-and (2) the relationships among the various TOT complex modules that, by interconnection through the octahedral2 x 2 bundles,build up the two different polytypes. of 3.06 g,/cm3.The agreementis reasonable,taking into To these ends we shall consider, beyond the simple account the difficulty in accuratelymeasuring the density geometrical featuresthat we have so far discussed,more of fibrous material, becauseof the tendency to underes- subtlefeatures in the diffraction patterns,namely, the sys- timate density values,as well as, at least for balangeroite, tematic absencesthat appear in the patterns and the in- the occurrenceof intergrown chrysotile, as discussedin tensity distribution among the various classesof reflec- the last part ofthis paper. tions. We shall pay attention mainly to the monoclinic

Taele1. Gageitesubstructure

M(1):0.36Mn + 0.64M9 00 0.66(5) 2.00 M(2): 0.46 Mn + 0.54Mg 0.3376(1) 0.3840(1) 0.76(4) 4.00 M(3): 1.00Mn 0.4225(1) 0.1518(1) 0 0.99(3) 4.00 M(4): 1.00Mn 0.1016(1) 0.4991(1) 0 0.82(3) 4.00 s(1) 0.2109(2) 0.0967(2) 0.4568(15) 0.47(6) 2.67 s(2) 0.0671(2) 0.1948(2) 0.03s5(18) 0.54(6) 2.67 o(1) Y20 0 1.02(10) 2.00 o(2) 0.3313(4) 0.091s(4) V2 1.1 1(8) 4.00 o(3) 0.3420(3) 02845(4) V2 1.05(8) 4.00 o(4) 0.4918(3) 0.3988(3) Y2 1.07(7) 4.00 o(s) 0.3489(4) 0.4917(4) 1.22(8) 4.00 o(6) 0.1878(3) 0.3904(3) 0.84(7) 4.00 o(7) 0.0201(4) 0.3056(4) 1.17(71 4.00 o(8) 0.1922(10) 0 1425(10) 0 0.98(20) 1.33 o(e) 0.1640(6) 0.1823(6) 0.7479(29) 0.s3(13) 2.67 o(10) 0.1136(11)0.1919(10) 0.73(20) 1.33 Bonddistances (in A) M(1)-o(4)(x4) 2.151 M(3)-o(2)( x 2) 2.227 -o(5)(x 2) 2.089 -o(7)(x2l 2.201 M(2)-o(3)( x 2) 2.130 -o(1) 2.336 -o(5) ( x 2) 2.209 -o(3) 2.130 -o(4) 2.132 M(4)-o(1)(x 2) 2.267 -o(6) 2.068 -0(6) ( x 2) 2.179 s(1Fo(2) 1.664 -o(2) 2.155 -o(s) 1.663 -o(7) 2.270 -o(8) 1.664 s(2Fo(4) 1.649 -o(s) 1.637 -o(7) 1.643 -o(e) 1.628 -o(10) 1.683 Nofe: Atomic coordinates,isotropic temperature factors (in A'?)and multipliers,with, in parenthe- ses, the standard deviations. FERRARIS ET AL.:BALANGEROITE AND GAGEITE

al b) cl

Fig. 8. Schematicdrawings of the diferent polytypes,with indications,in C/3 units, of the relativeheights of the TOT modules.(a) Triclinic form. (b) Monoclinicform. (c) Hypothet- ical orthorhombicform.

consistsof (l l0) structural layers that follow each other according to the alternating stacking vectors i : B/2 + C/3 and tz : B/2 - C/3. According to this model, the a) generalstructure faclor Fro,is given by the expression Foo,: {2,f,expl2ni(hx, + ky, + lz,)l) '{l + exp[2zri(k/z+ l/3)]], (l) where xr, y,, and z, are the coordinatesof the atoms in the layer, with referenceto the A, B, and C cell parameters. Within the kinematical approximation, the second power of the last factor will modulate the overall intensity distribution, depending on the value of k/2 + l/3. In particular, the second power of the last factor vanishes when k : 2n -f I and I : 3n, leading to the systematic absencesactually observed. More generally, it may as- sume four distinct values: 0 for 3k + 2l:6n + 3 lfor3k+21:6n+2 3for3k+21:6n+l 4 for 3k + 2l: 6n. Although the observed intensity depends also on the first factor-the Fourier transform of the layer-in b) expression l, as well as on dynamical effects,the second factor will largely imprint the ovorall intensity distribu- Fig.7. Crystalstructure drawings of bothpolytypes. The dig- tion. It is satisfyingto find that the reflections are neatly its givethe height, in C/6 units,ofthe differenttetrahedral chains distributed in four classesaccording to their intensity, as with reference to thebridging oxygen atoms. The symmetryele- follows: mentsare indicated.(a) Balangeroiteand gageite2M; both the doubleI x B x C cell and primitive a^ x x b^ c- cell afe 1.* I^" indicated.(b) Gageite lTc; theprimitive triclinic cellis indicated. k:2n+l l:3n absent 0 l: 3n + I medium-strong,diffuse 3 k: 2n l: 3n verystrong, sharp 4 form, and the will analysis be carried out on its diffraction l:3n + I weak,diffuse I pattern, with referenceto the C-centered cell characterizedby translationperiods A:2a, B :2b, C: c (2a x 2b x c cell). where the terms in the 1"","column recall the values as- Inspection of the systematicabsences in the diffraction sumed by the second power of the modulating term in pattern of Figure 2 shows, besides the absencesdue to the expressionof the structure factor. Figure lb is, in this C-centering,additional systematicabsences for k: 2n + respect, particularly convincing, as it displays simulta- 1 and /: 3n. Theseare obviously not space-groupex- neouslythe four classesofintensities. tinctions and must result from a particular structural ar- A secondhighly compelling check of the validity of the rangement.An arrangementthat naturally leadsto the C- model lies in the fact that it naturally leads to a sound centeringand to systematicabsences of the kind observed structural schemefor the triclinic form. In fact by sub- 388 FERRARIS ET AL.: BALANGEROITE AND GAGEITE

Fig. 9. Fiber texture ofbalangeroite, as seenalong the [001] fiber axis. The white contrast lines in each fibril develop within the (100)plane. stituting the vector sequencetl2tft2 .. . , realized in the correct schemefor the assemblageof tetrahedral chains monoclinic phase,with the sequencetrtrtrtr . . . (or trtrtrt, and octahedralwalls. . . .), we obtain a structure with lattice parameters cor- Figure 7a summarizesthe results of the precedingdis- respondingto those observedfor the triclinic form. cussion by giving the representationofthe structural to- With regard to the internal structure of the slab, it is pology of balangeroiteand gageite2M. The level of the necessaryto consider the relative arrangement of the tetrahedral chains is denoted by the height, in C/6 units, complex modules in the slab and the relative heights of of the bridging oxygen atoms in both disilicate groups the tetrahedral chains that sandwich the 3 x I walls. placed along [001] that build up the repeat unit of the Thesefeatures can be deducedby taking advantageofthe chain. In the samefigure we have outlined both unit cells, non-space-groupsystematic absences that were observed, the C-centereddouble cell, with,4 and B translations,and in the Okl plane,for k + 2l : 4n t 2 and,in the l0l plane, the simple cell with a^ and b- translation parameters. for h + 2l : 4n + 2 (Figs. la, 2). Theseabsences were The symmetry elementsare also shown in the latter cell: observed in the diffraction pattern of Figure 2 and are twofold axes,inversion centersat levels 0 and 3, it c^/6 interpreted as being dependent on the internal arrange- units, and r glides at levels 1.5 and 4.5, in c-l6 units. ment of the basic structural layer. The resulting space-groupsymmetry is P2/n, with the The absencesfor h + 2l : 4n -t 2 may be easily ex- twofold axis in the c direction; it seemsuseful to remark plained if we assumethat in the basic layer, adjacentTOT that a nonstandardorigin has been chosenon the twofold complex modules are related through a pseudoglidenor- axis to maintain a closer relationship with Moore's cell mal to B and with translational component A/4 + C/2: and the basic a x b x c cell. the operation is describedas a pseudoglidebecause ofits Similarly, Figure 7b gives the structural arrangement partial character,as it does not refer to the whole crystal. in gageite lTc. The heights of the various tetrahedral Furthermore, the observed absencesfor k + 2l: 4n + chains are indicated, and we have outlined the unit-cell 2 imply that the two modules are related also by a similar translations a, arrd b,, both starting from the inversion pseudoglidenormal to A. The presenceof both pseudo- center at level 2, in c,/6 units, assumed as origin, and glides demands a twofold axis along [001] in each TOT ending on inversion centersat level 4. The resulting space module, which indicates that Figure 6b representsthe groupis Pl. FERRARISET AL.: BALANGEROITEAND GAGEITE 389

Figure 8 presentsschematic drawings where the composite TOT modules are representedby segmentsorient- ed as the projection ofthe octahedralwalls, with the relative heights of the modules indicated. The structural schemesfor both monoclinic and triclinic forms are com- paredwiththe hypotheticala x 6 x c structure,which is orthorhombic and has space-groupsymmetry Pnnm. Note that no electron-diffraction evidence was found for the existenceof individual domains with such a simple arrangement,characterized by the absenceof shifts in the relative height of the TOT modules.

B.clANcnnorrE AND GAGETTEm OD srRUcruRES The polytypic relationship betweenthe monoclinic and triclinic modifications may be conveniently treated in more formal terms according to the theory of OD structures (Dornberger-Schifl19 56, 1964, I 966), which consist of equivalent layers. The monoclinic structural type in balangeroiteand gageite 2M and the triclinic type in gageite lTc correspondto distinct ordered members of a whole OD-structure family that, according to the theory, includesthose structuresin which all the pairs of adjacent layersare geometricallyequivalent. Each structure, in the presentfamily, may be describedas consisting of equivalent layers, with symmetry 2/m and translation periods a^ : 19.42, c^ : 9.84 A (we report here the metrical values obtained for gageite).The "width" ofthe layer is b": b^/2: 9.7 A, and ,y- : 89.5".Adjacent layersare related by a vector tt : -4^/2 + b. + c^/3 or t, : -a^/2*b.-c-l3. The pairs of layers obtained in either caseare geometrically equivalent. Therefore any sequenceof t, and t, vectors defines a member of the family, and infinite ordered polytypes, as well as disordered forms, are thus possiblein the family. However, only two members exist with the maximum degreeof order, the so-called MDO structures according to OD-structure theory. These are the members in which all triples, quadruples . . ., and n-tuples of consecutivelayers are geometrically equivalent. They correspondto the vector sequencestltrtrtr . . . (or t2t2t2t2.. .) and tft2tfi2 .. . . As already discussed,the first sequenceis realized in gageite lTc with only one layer in the triclinic unit cell; the second is realized in Fig. 10. Chrysotilesubstituting for balangeroite.(a) Individ- junctions. gageite2M and, with a diferent chemistry, in balange- ual chrysotilefibers neucleate at triple (b) Growthof roite. Both gageite2M and balangeroitecontain two lay- chrysotiledevelops along the interfacesbetween two adjacent balangeroitefibers. (c) Advanced substitution ofchrysotile fibrils ers in the monoclinic primitive cell. for balangeroite. The symmetry characteristicsof the OD-groupoid family, which plays a role analogousto that ofthe spacegroup for a single ordered crystal, is given by the collection of where, as usual, the first line of the symbol gives the all the transformations of space that bring a layer into l,-POs, namely, the plane spacegoup of the single layer; coincidence with itself, tr-PO operations (PO : partial the secondline gives the set of o-POs. The brackets around operation), or with the next layer, o-POs.The description the second position of each line indicate that only the and derivation of the OD-groupoid families were dis- basic vectors a* and c- are translation vectors that cor- cussedby Dornberger-Schiff and Grell-Niemann (196 l) respond to the translations of the structural layer; the third and Dornberger-Schif and Fichtner (1972). In the pres- vector b" is not a translation vector. ent casethe OD-groupoid family is According to the symbolism, two successivelayers are P | (l) 2/m related through a twofold screw axis with translational { I (l) 2,,r/n,.,} component 2/6 c^ and also through a glide normal to c- 390 FERRARIS ET AL.: BALANGEROITE AND GAGEITE

Fig. I l. Contact betweenthe curled chrysotile layers and the balangeroitefibers. with translational components -a^/2 + b.. A pair of substructuredescribed in Moore's work (1969).For I + layers related through the screw operation 2r,, indicated 3n, the different members of the family present different in the symbol is geometrically equivalent to a pair of patterns, with diffuse streaksparallel to bH in the disor- layersrelated through a screw2r,r,where the translational dered membersand sharp spotsfor the ordered ones.The component is in the opposite direction. If 2r,, operation various diffraction patterns we studied were indicative of is followed by a 22,, operation, the first and third layer various degreesof structural order, from those with near- are at the same level, and the twofold axis of the single ly continuous streaks without maxima at all, to those layer is now valid for the whole structure; moreover, the presentingnearly sharp maxima in the reciprocal lattice o-PO n,., is continued, becoming a true n glide of the positions coresponding to the monoclinic and triclinic whole structure.The overall space-groupsymmetry of the polytypes. These two extreme casescorrespond, respec- resultingstructure is thereforePll2/ n. On the other hand, tively, to disordered sequencesoft' and t2 stacking vec- if 2r,, is followed by 2r,r, the first and third layers are at tors (aperiodic sequencesof 2r,, and 2r,r) and to large different levels (2/3 c^ apart), and the twofold axes are domains of either polytypes. In intermediate cases,the no longertotal symmetry elements;moreover the r,., glide wider the maxima the smaller the volume of the ordered is not continued from the second to the third layer, but domains and the larger the disorderedportion of the crys- transposedby l/3 c-. Only inversion centersnow act as tal. total symmetry elements, and the overall space group symmetryis thereforePI. Tnxrunr AND FAULT coNTRAST IN BALANGERoTTE Besidesthese two ordered polytypes, the MDO mem- The lattice imagesobtained along the fiber axis (Fig. 9) bers in the family, a potentially infinite seriesof ordered show that the balangeroitebundles consist of randomly structures may be conceived, corresponding to periodic rotated single fibers with cross sectionsin the range 500- sequencesof 2r,, and.22,,operations. Aperiodic sequences 5000 A. The outer shapeof the fibers is somewhat irreg- of these o-POs will lead to disordered members of the ular and largely determined by the interlocking fibers. family. However, surfaceswith simple indices, such as {100}, The different patterns corresponding to the different {010},{ll0}, {110},with referenceto the 2a x 2b x c structuresof the family presentidentical reflectionsof the unit cell, are the most common. These facescorrespond type / : 3n. These "family reflections" are always sharp to the most populated octahedralplanes in the structures. and correspondto the "superposition structure," just that Figure 9 showsalso the occurrenceof widespreadfault- FERRARIS ET AL.: BALANGEROITE AND GAGEITE 39r ing. Either within (100) or, lessfrequently, (l l0) planes, phic olivine, before finally being transformed into anti- white lines ofcontrast are present throughout the fibers. gorite. Therefore, balangeroite turns out to be an inter- Apparently, these faulted regions separate parts where mediate mineral within the retrogrademetamorphism of similar contrast patterns are present. The fault density the Balangeroultramafic body. In particular it would be decreaseswith decreasingcrystal thickness,perhaps sug- formed from orthopyroxenes through the reaction gestingthat the faults responsiblefor this contrasrare not 2lMgrSirOu+ 20HrO: MgorOu(OH)40(Si4OrJ4+ 26SiO, infinitely extended along the observation axis and that and would transform to chrysotile through the reaction the greater the thickness of the crystal, the higher the Mg,Ou(OH)oo(Si4O,J4+ l2SiO, + 8H,O: l4MgrSirO5- probability of seeing the fault contrast. Regarding the (OHT. structural nature ofthe faults, no conclusiveproofcan be given. Most probably the fault contrast arises from fea- AcrNowr,nncMENTs tures other than faulted polytypic sequences,since con- We are grateful to P. J. Dunn (SmithsonianInstitution) for kindly pro- trastdevelops on the (100)rather than on the (l l0) plane; viding specimensofgageite from Franklin (R6639, C6803); P. B. Moore moreover, indications exist that the fault does not affect (Chicago)who sent us the set of reflection intensiti€s collected in 1969 gageitefrom Franklin, from the whole and some beautiful crystals of selected the crystal thickness, as would be required by a specimenno. ll40 in the collection of Ewald Gerstmann, to whom we polytypic fault. Also, stacking faults parallel to the ob- extend our acknowledgments;R. Compagnoni (Torino) who provided us servation direction would not produce any important with oriented thin sectionsofholotype balangeroite;M. Pasero(Pisa) who contrast, when using the [001] bright-field conditions we collected the new set of intensity data and carried out the least-squares adopted for HR imaging. A possible explanation is the refinement. This researchhas been supportedby M.P.I. 400/ogrants. occurrenceof Wadsleydefects, with the insertion of structural modules other than the 3 x I octahedral wall and 2 x 2 octahedralbundles. One example might be 3 x 2 RnrnnnNcns modules within the overall regular tesselation,with con- Compagnoni, R, Ferraris, G., and Fiora, L. (1983) Balangeroite,a new certed vacant tetrahedral sites. fibrous silicaterelated to gageitefrom Balangero,Italy. American Min- eralogist,68, 214-219. Mpra,vronpHlc REACTIoNSrNvoL\arNG Compagnoni, R, Ferraris, G., and Mellini, M. (1985) Carlosturanite,a BALANGEROITE new asbestiformrock-forming silicatefrom Yal Varaita, Italy. American Mineralogist, 70, 767 -'1 7 2. In severalways the fiber texture in balangeroiterecalls Domberger-Schitr,K. (1956) On order-disorder structures (OD-struc- that found in another 9-A asbestiform silicate, carlostur- tures). Acta Crystallographica,9, 593-601. - anite (Compagnoniet al., 1985;Mellini et al., 1985).In (1964)Grundzuge einer Theorie der OD-Strukturen ausSchichten. particular, Abhandlungender DeutschenAkademie der Wissenschaften,Berlin. balangeroite is also intergrown with parallel -(1966) Lehrganguber OD-Strukturen. Akademie Verlag, Berlin. chrysotile fibers. The relative abundanceof the two min- Dornberger-Schifl K., and Fichtner, K. (1972) On the symmetry of OD- eralsis widely variable (Fig. l0). Figure I I showsin more structuresconsisting ofequivalent layers.Kristall und Technik, 7, I 035- detail the contact between balangeroite and chrysotile, l 056. (1961) with the latter encroachinginto balangeroite.The whole Dornberger-Schifl K., and Grell-Niemann, H. On the theory of order-disorder(OD) structures Acta Crystallographica,14, 167-177. sequenceof images can be explained by assuming that Dunn, P.J. (1979) The chemical composition of gageite:An empirical chrysotile was replacingbalangeroite. In the earliest stage formula. American Mineralogist, 64, 1056-1058. of replacement(Fig. l0a), chrysotile would develop just Gjonnes,J., and Moodie, A.F. (l 965)Extinction conditionsin the dynamic at the junction of three balangeroitefibers, where struc- theory ofelectron diffraction. Acta Crystallographica,19, 65-67. (1986) polygonal tural misfit is Mellini, M. Chrysotile and serpentinefrom Balangero maximum and fluid circulation is most serpentinite.Mineralogical Magazine,50, 30 l-306. favored. The chrysotile growth would continue along the Mellini, M., and Merlino, S. (1978) Caysichite:A double crankshaftchain boundaries between two balangeroite fibers (Fig. 10b), structure.Canadian Mineralogist, 16, 8l-88. and clustersof chrysotile fibers would be formed around Mellini, M., Merlino, S., and Pasero,M. (1984) X-ray and HRTEM study the first gowth centers (Fig. l0c), but the original bal- of sursassite:Crystal structure, stackingdisorder, and sursassite-pum- pellyite intergrowth. Physicsand Chemistry ofMinerals, 10,99-105. angeroite texture would be still identifiable. Finally, ex- Mellini, M., Ferraris, G., and Compagnoni, R. (1985) Carlosturanite: tensive replacementof chrysotile for balangeroitewould nnrew evidenceofa polysomaticseries including serpentine.American obscure the original textural relationships within balan- Mineralogist, 70, 773-7 81. geroite(Mellini, 1986). Moore, P.B. (1969) A novel octahedral framework Gageite. American The rru observations parallel Mineralogist,54, 1005-1017. closely those made by Palache,C. (1928)Mineralogical noteson Franklin and SterlingHill, New Compagnoniet al. (1983)using optical microscopy.They Jersey.American Mineralogist, 13, 297-329. observed balangeroitethat developed quite early in the Snith, J.V (1974) Feldsparminerals. Springer,Heidelberg. Balangero serpentinite and also balangeroite pseudo- Tak6uchi,Y., and Joswig,W. (1967)The structureof haradaiteand a note morphs after orthopyroxenein specimensfrom the Lanzo on the Si-O bond lengthsin silicates.Mineralogical Journal, 5, 98-123. massif. Chrysotile was formed later, as deducedfrom the M,lNuscnrvr REcETVEDMw 22, 1986 rnvr images,and was successivelycorroded by metamor- M.lNuscnrp'r AccEprEDNoverrser 20. 1986