Electron-diffraction and electron-microscopy study of balangeroite and ...

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 Scienze dellaTerra,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- (unique axis) : 9.65 A, 7* : 88.9"; two polytypic modifications were found for gageite:gageite2M, 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-chemicalformula 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,whereasthe 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.

IncrnonucrroN Gageiteis a fibrous manganesesilicate that was found in low-temperaturehydrothermal veins at Franklin, New Jersey. Although the complete structure determination was hampered by the disordered nature of the mineral, Moore (1969) was able to outline a substructure, with referenceto an orthorhombic cell with a : 13.79, b : 13.68,c' : 3.279 A, spacegroupPnnm (a x b x c' cell hereafter),disregardingthe indications for a trebled c parameter that were suggestedby the occurrenceof additional diffirse and weak reflections. The substructure consists of two kinds of interlinked modules, both of which are built up by chains of edgesharing octahedra:walls three chains wide (3 x I walls) and bundles that extend two chains both in width and 0003{04xl87/0304{382$02.00

thickness (2 x 2 bundles). Each module sharesits free cornerswith the doubly sharedcorners of the other module. The resulting octahedral framework contains [001] pipelike channelsthat, accordingto Moore (1969), house disordered silicate tetrahedra. Although the topology of the octahedral framework seemedto be quite well defined, the proposed number and the disordered arrangementof the tetrahedra in the channelswere probably an artifact of the assumedsubstructure. In fact, as regardstheir number, Dunn (1979) derived, on the basis of new chemical analyses, (Mn,Mg,Zn)o0Si,sO50(OH)40 as the chemical content of a cell with trebled c parameter; this empirical formula does not agree with the crystal-chemical formula proposed by Moore (Mn,Mg,Zn)4r(Si,rO36)[Ou(OH)or] ( I 969).

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,Compagnoniet al. (1983) describeda new fibrous silicate, balangeroite,the Mg-dominant analogueofgageite.From [001] rotation photographs,which clearly indicated a trebled c parameter, and by leastsquaresfitting of the X-ray powder-diffraction pattern, t h e p a r a m e t e ras: 1 3 . 8 5 b, : 1 3 . 5 8c, : 9 . 6 5 A 1 a " 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 the empirical formula of Dunn (1979) for gageite, the following unit-cell content was proposedfor balangeroite: (Mg,Fe,Mn,tr)orSir 5(O,OH)ro. As a result of electron-diffraction and transmissionelectron microscopy (rervr)studies of balangeroite and gageite,we have revised the interpretation of the crystal structuresof these minerals and their polytypic relationships, and we have consideredin more generalterms the order-disorderphenomenain the whole structural family. ExprnrlrBNTAL DETATLS Gageitefrom Franklin,New Jersey,U.S.A.,waskindly provided by Mr. EwaldGerstmannthroughthe courtesyof P. B. Mooreandby the Smithsonian Institutionthroughthe courtesy of P. J. Dunn. Orientedthin sectionsof holotypebalangeroite wereobtainedfrom R. Compagnoni. Electrondiffractionand imagingwerecarriedout in a Philips 400T electronmicroscope, as describedby, e.g.,Mellini et al. (1984).Groundspecimens ofbalangeroite andgageite werestudied, but the only ion-thinnedspecimens wereof balangeroite, because of the morelimited availabilityof gageite. Electron-diffraction datawereobtainedby selected-area dif-

As balangeroite and gageitedevelop fraction(sao)techniques. pronounced on grinding,controlledtilting exper{ I l0} cleavage imentswereperformedwith the [001]fiberdirectionastilt axis. In this way,the recombinationofseveral[zv0]diffractionpatternsdisplayedan overallview of the wholereciprocallattice. starting Viewsalong[001]werealsoobtainedfor balangeroite, from suitablethin sections. Psnunocrr-Ls AND TRUECELLS Up to this point, we have introduced two different unit cells,the a x b x c' cell of Moore (1969) and the a x b x c cell of Compagnoniet al. (1983). Although both cells have been supersededby the results of our electrondiftaction study, we shall frequently refer to the a x b x c cell, so closely related to the structural model of Moore (1969), as a uselirl referenceframe. Crystallographic planes and directions in the preceding as well as in the subsequent pageswill be referred to that cell, unlessotherwise stated. Figure la shows a [100] electron-diffraction pattern taken of balangeroite.Incidentally, such a pattern is almost identical to the [010] pattern, owing to an overall pseudotetragonalsymmetry. The sharp, strong spots in that pattern correspondto the a x b x c' cell of Moore. A similar distinction between weak reflections with / : 3n t I and strongsubcellreflectionswith /: 3r is evident in Figure lb, which showsthe [ 10] diffraction pattern. From a series of similar [zu0] diffraction photographs, the whole pattern of balangeroite was obtained (Fig. 2). The corresponding direct lattice may be de-

384

FERRARIS ET AL,: BALANGEROITE AND GAGEITE

+

*

o

o X

)(

*

B.l o I

*

*---A. x

o

+

l=2n

X

l=2n+l

t(

all preaent

o

I =3 n ! t

+

Fig.2. Thediftactionpatternofbalangeroite 2M, andgageite with reference to 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 unitcell pammetersa- : 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 parameters correspondingprimitive direct cell possesses a^ : 19.42,b^ : 19.42,c^ -- 9.84 A, y- : 89.5'. However, the electron-diffraction study ofgageite showed the frequent occurrenceofa secondelectron-diffraction pattern (Fig. lc) besidesthat formerly mentioned. The correspondingdirect cell has cell parametersa.: 14.17,b,: 1 4 . 0 7c, , : 9 . 8 4 A , d , : 7 6 . 5 , P , : 7 6 . 6 o , 7 ,: 8 6 . 9 "a n d is related to the basic a x b x c cell through the transformation matrix (l0h/01Y3/001) (Fie. 3b). It will be shown that the two distinct difraction patterns correspond to two polytypic modifications of gageite,which, b) on the basis of the symmetry properties and structural relationships that we are going to discuss, may be conamongthe a x b x c Fig. 3. (a) Geometricalrelationships veniently called gageite2M and gageite lTc. cellandthecorrespondinga-x cell,the2a x 2b x c C-centered andgageite2M. (b) Geob^ x c^primitive cellfor balangeroite Trrn nnnrrroNAr, MoDULE: THE FOuR-REpEAT between thetricliniccellofgageitelTc and metricalrelationships SILICATE CHAIN thebasicaxbxccell. proposed Any 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 t o t h e a x b x c ' c e l l . 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 strucoite (Compagnoni et al., 1983), typical of chain silicates tural unit correspondsto the four-repeat crankshaftchain 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 differical 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 tetlead 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 destroyingthe 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-

385

FERRARISET AL.: BALANGEROITEAND GAGEITE

oy

b

I

.I Fig.5. [001]projectionof thecrystalstructureofbalangeroite Fig. 4. The connectionbetweentetrahedralchainsand oc- and gageite,describedwith referenceto the a x b x c cell and tahedralframework,asseenalong[ 10]with respectto thebasic showingthe tetrahedralchainswithin the channelsdefinedby axbxccell. the 3 x I octahedralwalls and 2 x 2 octahedralbundles. tion of the tetrahedral chains in the channelsof the octahedral framework. The resulting ideal chemical content of a\ a x b x c unit cell is MorOu(OH)oo(ToO,r)o, where M and T indicate octahedral and tetrahedral cations, respectively. The six oxygen atoms not belonging to the tetrahedralchain correspondto anions coordinated to six octahedral cations and located in the central part of the bundles.The hydroxyls conespond to anions coordinated to only three octahedral cations: their number could be lower than 40, becauseof substitution of oxygen for hydroxyl when the valence of the coordinating cations is higher than 2+. An isotropic refinement of the gageitestructure, based on this model and using the X-ray reflections measured by Moore (1969),improved the R value from 0.17, obtained by Moore, to 0. 155 for 612 reflections. As we found one exceptionally good crystal among those provided by E. Gerstmann, we proceededto a new intensity-data collection in the a x b x c' cell of Moore. A total of 1026 independent reflections were measured on an Ital Structures four-circle automatic diffractometer, with Zr-filtered MoKa radiation. The atomic coordinates calculatedon the basis of our model were refined, together with the isotropic thermal parameters, in the space group Pnnm. In a few cyclesof least-squaresrefinement, the R index dropped to 0.049 for 871 reflections with F. > 5o(F").'

Table I reports the positional and thermal parameters for all the atoms, together with the occupanciesof Mn and Mg in the M(l) and M(2) octahedralsites.The bond distancesin the various polyhedra, calculatedon the basis of the coordinates reported in Table l, appear normal and-as regards those associated with the octahedral framework-in good agreement with those found by Moore (1969). It seemsuseful to remark that the Mg and Mn contentsderived from the structural refinement compare fairly well with those obtained from the chemical data. The parallel refinement of Moore's model, carried out starting with the coordinatesgiven by Moore (1969) and using the same 871 reflections, led to a reliability indexofR:0.079.

Rrvrsnn CRYSTALCHEMISTRY On the basis of the average analysesof balangeroite (Compagnoniet al., 1983)and gageite(Dunn, 1979)and assuming,according to the present structure model, sixteen Si atoms in the a x b x c volume, the formulas obtained for balangeroiteand for gageite,respectively,are (Mgru,o Fer'zfiFe3.1fln,,,Alo, oesi,6o5s rCa"orCro o,TL o,Loo r,(OH)rr' and (Mg,, rrFeo.,,Mn*nrCa"roZn^3)>o2 65Sir6O54 53(OH)oorr. The agreementwith the ideal cell content is good. The calculateddensitiesare 3.098 and 3.599 g/cmr for balangeroiteand gageite,respectively.Thesevalues 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 BusinessOfrce, g/cm3 (Dunn, 1979) for gageite.New measurementson MineralogicalSocietyof America,1625I Street,N.W., Suite 414, (holotype material, torsion balance) gave balangeroite Washington,D.C. 20006,U.S.A. Pleaseremit $5.00 in advance for the microfiche. valuesin the range2.96-3.10 g,/cm3with an averagevalue


386

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 detailed arrangement of the various modules in the two structural forms whose existencewe outlined in the previous chapters. In particular we want to derive (l) the lrl01 a) b) way in which the tetrahedral chains attach to both sides possible Fig. 6. The two TOT 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 comchain 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 accuratelymeasuringthe density geometrical featuresthat we have so far discussed,more of fibrous material, becauseof the tendency to underes- subtlefeaturesin the diffraction patterns,namely, the systimate density values,as well as, at least for balangeroite, tematic absencesthat appear in the patterns and the inthe occurrenceof intergrown chrysotile, as discussedin tensity distribution among the various classesof reflecthe last part ofthis paper. tions. We shall pay attention mainly to the monoclinic

Taele1. Gageitesubstructure M ( 1 ) : 0 . 3 6M n + 0.64M9 M(2) : 0.46 Mn + 0.54Mg M ( 3 ): 1 . 0 0M n M ( 4 ): 1 . 0 0M n

s(1) s(2) o(1) o(2) o(3) o(4) o(s) o(6) o(7) o(8) o(e) o(10) (x4) M(1)-o(4) -o(5)(x 2) M(2)-o(3)( x 2) -o(5) ( x 2) -o(4) -o(6) s(1Fo(2)

-o(s) -o(8) -o(s)

00 0.3376(1) 0.3840(1) 0.4225(1) 0.1518(1) 0 0.101 6(1) 0.4991(1) 0 5) 0.2109(2) 0.0967(2) 0.4568(1 8) 0.0671(2) 0.1948(2) 0.03s5(1 0 Y20 V2 0.331 3(4) 0.091s(4) V2 0.3420(3) 02845(4) Y2 0.4918(3) 0.3988(3) 0.3489(4) 0.4917(4) 0.1878(3) 0.3904(3) 0.0201(4) 0.3056(4) 0.1922(10) 0 1425(10) 0 0.1640(6) 0.1823(6) 0.7479(29) 0 . 1 1 3 6 ( 1 1 )0 . 1 9 1 9 ( 1 0 ) (inA) Bonddistances 2.151 M(3)-o(2)( x 2) -o(7)(x2l 2.089 -o(1) 2.130 -o(3) 2.209 ( x 2) M(4)-o(1) 2.132 -0(6) ( x 2) 2.068 -o(2) 1.664 -o(7) 1.663 1.664 s(2Fo(4) -o(7) 1.637

-o(e)

-o(10)

0.66(5) 0.76(4) 0.99(3) 0.82(3) 0.47(6) 0.54(6) 1.02(10) 1.11(8) 1.05(8) 1.07(7) 1.22(8) 0.84(7) 1.17(71 0.98(20) 0.s3(13) 0.73(20)

2.00 4.00 4.00 4.00 2.67 2.67 2.00 4.00 4.00 4.00 4.00 4.00 4.00 1.33 2.67 1.33

2.227 2.201 2.336 2.130 2.267 2.179 2.155 2.270 1.649 1.643 1.628 1.683

Nofe: Atomic coordinates,isotropic temperature factors (in A'?)and multipliers,with, in parentheses, the standard deviations.

FERRARIS ET AL.:BALANGEROITE

AND GAGEITE

al

b)

cl

Fig. 8. Schematicdrawingsof the diferent polytypes,with indications,in C/3 units, of the relativeheightsof the TOT modules.(a)Triclinic form. (b) Monoclinicform. (c) Hypothetical orthorhombicform.

a)

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 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 assume four distinct values: 0 for3k + 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 expression l, as well as on dynamical effects,the second factor will largely imprint the ovorall intensity distribuFig.7. Crystalstructuredrawingsof bothpolytypes. Thedig- tion. It is satisfyingto find that the reflections are neatly its givetheheight,in C/6 units,ofthe differenttetrahedral chains distributed in four classesaccording to their intensity, as with reference to thebridgingoxygenatoms.The symmetryele- follows: mentsare indicated.(a) Balangeroite and gageite2M; both the doubleI x B x C cell and primitive a^ x b^ x c- cell afe 1.* I^" indicated.(b)GageitelTc; theprimitivetricliniccellis 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 analysiswill be carried out on its diffraction l:3n + I weak,diffuse I pattern, with referenceto the C-centered cell characterized by translationperiodsA:2a, B :2b, C: c (2a x 2b x c cell). where the terms in the 1"","column recall the values asInspection of the systematicabsencesin 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 systematicabsencesfor k: 2n + respect, particularly convincing, as it displays simulta1 and /: 3n. These are obviously not space-groupex- neouslythe four classesofintensities. tinctions and must result from a particular structural arA 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 systematicabsencesof the kind observed structural schemefor the triclinic form. In fact by sub-

b)

388


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 monoclinic phase,with the sequencetrtrtrtr . . . (or trtrtrt, . . .), we obtain a structure with lattice parameters correspondingto those observedfor the triclinic form. With regard to the internal structure of the slab, it is necessaryto consider the relative arrangement of the complex modules in the slab and the relative heights of the tetrahedral chains that sandwich the 3 x I walls. Thesefeaturescan be deducedby taking advantageofthe non-space-groupsystematicabsencesthat were observed, in the Okl plane,for k + 2l : 4n t 2 and,in the l0l plane, for h + 2l : 4n + 2 (Figs. la, 2). Theseabsenceswere observed in the diffraction pattern of Figure 2 and are interpreted as being dependent on the internal arrangement of the basic structural layer. The absencesfor h + 2l : 4n -t 2 may be easily explained if we assumethat in the basic layer, adjacentTOT complex modules are related through a pseudoglidenormal to B and with translational component A/4 + C/2: the operation is describedas a pseudoglidebecauseofits partial character,as it does not refer to the whole crystal. Furthermore, the observed absencesfor k + 2l: 4n + 2 imply that the two modules are related also by a similar pseudoglidenormal to A. The presenceof both pseudoglides demands a twofold axis along [001] in each TOT module, which indicates that Figure 6b representsthe

correct schemefor the assemblageof tetrahedral chains and octahedralwalls. Figure 7a summarizesthe results of the precedingdiscussion by giving the representationofthe structural topology of balangeroiteand gageite2M. The level of the tetrahedral chains is denoted by the height, in C/6 units, of the bridging oxygen atoms in both disilicate groups placed along [001] that build up the repeat unit of the chain. In the samefigure we have outlined both unit cells, the C-centereddouble cell, with,4 and B translations,and the simple cell with a^ and b- translation parameters. The symmetry elementsare also shown in the latter cell: twofold axes,inversion centersat levels 0 and 3, it c^/6 units, and r glides at levels 1.5 and 4.5, in c-l6 units. The resulting space-groupsymmetry is P2/n, with the twofold axis in the c direction; it seemsuseful to remark that a nonstandardorigin has been chosenon the twofold axis to maintain a closer relationship with Moore's cell and the basic a x b x c cell. Similarly, Figure 7b gives the structural arrangement in gageite lTc. The heights of the various tetrahedral chains are indicated, and we have outlined the unit-cell translations a, arrd b,, both starting from the inversion center at level 2, in c,/6 units, assumed as origin, and ending on inversion centersat level 4. The resulting space groupis Pl.

FERRARISET AL.: BALANGEROITEAND GAGEITE

389

Figure 8 presentsschematic drawings where the composite TOT modules are representedby segmentsoriented as the projection ofthe octahedralwalls, with the relative heights of the modules indicated. The structural schemesfor both monoclinic and triclinic forms are comparedwiththe 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,characterizedby the absenceof shifts in the relative height of the TOT modules. B.clANcnnorrE AND GAGETTE m 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-Schifl1956, 1964, I 966), which consist of equivalent layers. The monoclinic structural type in balangeroiteand gageite2M and the triclinic type in gageitelTc 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 definesa 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 gageite2M and, with a diferent chemistry, in balangeroite. Both gageite2M and balangeroitecontain two layers in the monoclinic primitive cell. 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 all the transformations of space that bring a layer into coincidence with itself, tr-PO operations (PO : partial operation), or with the next layer, o-POs.The description and derivation of the OD-groupoid families were discussedby Dornberger-Schiffand Grell-Niemann (196 l) and Dornberger-Schif and Fichtner (1972). In the present casethe OD-groupoid family is

P| {I

(l) (l)

2/m 2,,r/n,.,}

(a)IndividFig. 10. Chrysotilesubstitutingfor balangeroite. ual chrysotilefibersneucleate at triplejunctions.(b) Growthof chrysotiledevelopsalongthe interfacesbetweentwo adjacent balangeroite fibers.(c)Advancedsubstitutionofchrysotilefibrils for balangeroite.

where, as usual, the first line of the symbol gives the l,-POs, namely, the plane spacegoup of the single layer; the secondline gives the set of o-POs. The brackets around the second position of each line indicate that only the basic vectors a* and c- are translation vectors that correspond to the translations of the structural layer; the third vector b" is not a translation vector. According to the symbolism, two successivelayers are related through a twofold screw axis with translational component 2/6 c^ and also through a glide normal to c-

390


Fig. I l.

Contact betweenthe curled chrysotile layers and the balangeroitefibers.

with translational components -a^/2 + b.. A pair of layers related through the screw operation 2r,, indicated in the symbol is geometrically equivalent to a pair of layersrelated through a screw2r,r,where the translational component is in the opposite direction. If 2r,, operation is followed by a 22,, operation, the first and third layer are at the same level, and the twofold axis of the single layer is now valid for the whole structure; moreover, the o-PO n,., is continued, becoming a true n glide of the whole structure.The overall space-groupsymmetry of the resultingstructureis thereforePll2/ n. On the other hand, if 2r,, is followed by 2r,r, the first and third layers are at different levels (2/3 c^ apart), and the twofold axes are no longertotal symmetry elements;moreover the r,., glide is not continued from the secondto the third layer, but transposedby l/3 c-. Only inversion centersnow act as total symmetry elements, and the overall space group symmetry is thereforePI. Besidesthese two ordered polytypes, the MDO members in the family, a potentially infinite seriesof ordered structures may be conceived, corresponding to periodic sequencesof 2r,, and.22,,operations.Aperiodic sequences of these o-POs will lead to disordered members of the family. The different patterns corresponding to the different structuresof the family presentidentical reflectionsof the type / : 3n. These "family reflections" are always sharp and correspondto the "superposition structure," just that

substructuredescribedin Moore's work (1969).For I + 3n, the different members of the family present different patterns, with diffuse streaksparallel to bH in the disordered membersand sharp spotsfor the ordered ones.The various diffraction patterns we studied were indicative of various degreesof structural order, from those with nearly continuous streaks without maxima at all, to those presentingnearly sharp maxima in the reciprocal lattice positions coresponding to the monoclinic and triclinic polytypes. These two extreme casescorrespond, respectively, to disordered sequencesoft' and t2 stacking vectors (aperiodic sequencesof 2r,, and 2r,r) and to large domains of either polytypes. In intermediate cases,the wider the maxima the smaller the volume of the ordered domains and the larger the disorderedportion of the crystal. Tnxrunr

AND FAULT coNTRAST IN BALANGERoTTE

The lattice imagesobtained along the fiber axis (Fig. 9) show that the balangeroitebundles consist of randomly rotated singlefibers with cross sectionsin the range 5005000 A. The outer shapeof the fibers is somewhat irregular and largely determined by the interlocking fibers. However, surfaceswith simple indices, such as {100}, { 0 1 0 } , { l l 0 } , { 1 1 0 } ,w i t h r e f e r e n cteo t h e 2 a x 2 b x c unit cell, are the most common. These facescorrespond to the most populated octahedralplanesin the structures. Figure 9 showsalso the occurrenceof widespreadfault-


ing. Either within (100) or, lessfrequently,(l l0) planes, white lines ofcontrast are present throughout the fibers. Apparently, these faulted regions separate parts where similar contrast patterns are present. The fault density decreaseswith decreasingcrystal thickness,perhaps suggestingthat the faults responsiblefor this contrasrare not infinitely extended along the observation axis and that the greater the thickness of the crystal, the higher the probability of seeing the fault contrast. Regarding the structural nature ofthe faults, no conclusiveproofcan be given. Most probably the fault contrast arises from features other than faulted polytypic sequences,since contrastdevelopson the (100)ratherthan on the (l l0) plane; moreover, indications exist that the fault does not affect the whole crystal thickness, as would be required by a polytypic fault. Also, stacking faults parallel to the observation direction would not produce any important contrast, when using the [001] bright-field conditions we adopted for HR imaging. A possible explanation is the 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 modules within the overall regular tesselation,with concerted vacant tetrahedral sites. Mpra,vronpHlc REACTIoNSrNvoL\arNG BALANGEROITE

In severalways the fiber texture in balangeroiterecalls that found in another 9-A asbestiform silicate, carlosturanite (Compagnoniet al., 1985;Mellini et al., 1985).In particular, balangeroite is also intergrown with parallel chrysotile fibers. The relative abundanceof the two mineralsis widely variable (Fig. l0). Figure I I showsin more detail the contact between balangeroite and chrysotile, with the latter encroachinginto balangeroite.The whole sequenceof images can be explained by assuming that chrysotile was replacingbalangeroite.In the earliest stage of replacement(Fig. l0a), chrysotile would develop just at the junction of three balangeroitefibers, where structural misfit is maximum and fluid circulation is most favored. The chrysotile growth would continue along the boundaries between two balangeroite fibers (Fig. 10b), and clustersof chrysotile fibers would be formed around the first gowth centers (Fig. l0c), but the original balangeroite texture would be still identifiable. Finally, extensive replacementof chrysotile for balangeroitewould obscure the original textural relationships within balangeroite(Mellini, 1986). The rru observations closely parallel those made by Compagnoniet al. (1983)usingoptical microscopy.They observed balangeroitethat developed quite early in the Balangero serpentinite and also balangeroite pseudomorphs after orthopyroxenein specimensfrom the Lanzo massif. Chrysotile was formed later, as deducedfrom the rnvr images,and was successivelycorroded by metamor-

39r

phic olivine, before finally being transformed into antigorite. Therefore, balangeroite turns out to be an intermediate mineral within the retrogrademetamorphism of the Balangeroultramafic body. In particular it would be formed from orthopyroxenes through the reaction + 26SiO, 2lMgrSirOu+ 20HrO: MgorOu(OH)40(Si4OrJ4 and would transform to chrysotile through the reaction + l2SiO, + 8H,O: l4MgrSirO5Mg,Ou(OH)oo(Si4O,J4 (OHT. AcrNowr,nncMENTs We are grateful to P. J. Dunn (SmithsonianInstitution) for kindly providing specimensofgageite from Franklin (R6639, C6803); P. B. Moore (Chicago)who sent us the set of reflection intensiti€s collected in 1969 and some beautiful crystals of gageitefrom Franklin, selectedfrom the specimenno. ll40 in the collection of Ewald Gerstmann, to whom we extend our acknowledgments;R. Compagnoni (Torino) who provided us with oriented thin sectionsofholotype balangeroite;M. Pasero(Pisa)who collected the new set of intensity data and carried out the least-squares refinement. This researchhas been supportedby M.P.I. 400/ogrants.

RnrnnnNcns Compagnoni, R, Ferraris, G., and Fiora, L. (1983) Balangeroite,a new fibrous silicaterelatedto gageitefrom Balangero,Italy. American Mineralogist,68, 214-219. Compagnoni, R, Ferraris, G., and Mellini, M. (1985) Carlosturanite,a new asbestiformrock-forming silicatefrom Yal Varaita, Italy. American Mineralogist, 70, 767-'172. Domberger-Schitr,K. (1956) On order-disorder structures (OD-structures). Acta Crystallographica,9, 593-601. (1964)Grundzugeeiner Theorie der OD-Strukturen aus Schichten. Abhandlungender DeutschenAkademie der Wissenschaften,Berlin. -(1966) Lehrganguber OD-Strukturen. Akademie Verlag, Berlin. Dornberger-Schifl K., and Fichtner, K. (1972) On the symmetry of ODstructuresconsistingofequivalent layers.Kristall und Technik, 7, I 035l 056. Dornberger-Schifl K., and Grell-Niemann, H. (1961) On the theory of order-disorder(OD) structures Acta Crystallographica,14, 167-177. Dunn, P.J. (1979) The chemical composition of gageite:An empirical formula. American Mineralogist, 64, 1056-1058. Gjonnes,J., and Moodie, A.F. (l 965)Extinction conditionsin the dynamic theory ofelectron diffraction. Acta Crystallographica,19, 65-67. Mellini, M. (1986) Chrysotile and polygonal serpentinefrom Balangero serpentinite.Mineralogical Magazine,50, 30 l-306. Mellini, M., and Merlino, S. (1978) Caysichite:A double crankshaftchain structure.Canadian Mineralogist, 16, 8l-88. Mellini, M., Merlino, S., and Pasero,M. (1984) X-ray and HRTEM study of sursassite:Crystal structure, stackingdisorder, and sursassite-pumpellyite intergrowth. Physicsand Chemistry ofMinerals, 10,99-105. Mellini, M., Ferraris, G., and Compagnoni, R. (1985) Carlosturanite: nnrew evidenceofa polysomaticseriesincluding serpentine.American Mineralogist, 70, 773-7 81. Moore, P.B. (1969) A novel octahedral framework Gageite. American Mineralogist,54, 1005-1017. Palache,C. (1928)Mineralogical noteson Franklin and SterlingHill, New Jersey.American Mineralogist, 13, 297-329. Snith, J.V (1974) Feldsparminerals. Springer,Heidelberg. Tak6uchi,Y., and Joswig,W. (1967)The structureof haradaiteand a note on the Si-O bond lengthsin silicates.MineralogicalJournal, 5, 98-123. M,lNuscnrvr REcETVED Mw 22, 1986 M.lNuscnrp'r AccEprEDNoverrser 20. 1986