The Crystal Structure of Lizardite 1T: Hydrogen Bon{S and Polytypism

American Mineralogist, Volume 67, pages 587-59E,1982

The crystal structure of lizardite 1T: hydrogen bon{s and polytypism

Mencelro MBrtrNr

C.N.R., C.S. Geologia Strutturale e Dinamica dell'Appennino Via S. Maria 53, 56100 Pisa, ItalY

Abstract

The occurenceof lizardite lTfrom Val Sissone,Italy, is reported. Electron microprobe analysis leads to the doubled unit cell content (Mgs.seFeo2isFeSioAlo r+) (Sir eeAlo.rr)O r o.oo(OH)e.0o' The unit cell parametersare d : 5.332(3)A,c = 7.T3()]\ spacegroup is P3lm' The crystal structure has been refined, using 209 symmetry independentreflections, to R : 0.031. The refined model is not far from the idealized geometry of the serpentinelayer. : -3'5"). The ditrigonal distortion of the six-memberedtetrahedral ring is small (a No buckling of the brucite-like sheet is observed. The occurrenceofboth negativeand positive a valuesin serpentineminerals is explained on the basis of stacking sequenceand hydrogen bonds. Namely, the best hydrogen bond system is attained by rotation of the bridging oxygens belonging to the tetrahedral sheet' The direction ofrotation dependson the way in which subsequentlayers are stacked one upon the other. The substitution of trivalent atoms for magnesiumand silicon leads to .i.ong", hydrogen bonding between adjacentlayers, thus promoting the formation offlat- layer structures and increasingthermal stability.

Introduction symmetry, but Krstanovic (1968)refined his model in the space group Cm. Subsequently' Wicks and The well-known classification of the serpentine Whittaker (1975),in discussingKrstanovic's refine- minerals was developed by Whittaker and Zuss- ment, stated that lizardite 1T "is in fact orthorhom- mann (1956).The schemeis basedon the recogni- bic and only pseudo-trigonal". tion of cylindrical layers in chrysotile, corrugated Wicks and Whittaker (1975) and Krstanovic layers in antigorite, and flat layers in lizardite. A (1980) have discussed the distortions of both the recent review of the crystal structure of the serpen- tetrahedral and the octahedral sheetsof a very pure tine minerals was given by Wicks and Whittaker lizardite 1?with limited substitutions for silicon or (1975). The most serious handicap to full under- magnesium. According to these authors, the com- standing of the serpentine structures is the low position of lizardite 1T from RadusaMine results in degree of three-dimensionalorder present in these a large misfit between the tetrahedral and octahe- minerals. For instance, the previous two-dimen- dral sheets, and produces shifts of the atoms away sional determinations of the crystal structure of from the ideal positions. In particular, they noted lizardite, by Rucklidge and Zussman (1965)and by buckling of the plane of the magnesiumatoms and Krstanovic (1968),led to discrepancy factors of 18 various shifts, along [001], of the oxygen, silicon percent and 19 percent respectively, in spite of the and magnesium atoms. In this present paper, I fact the flat-layer lizardite structure would seem to report a more regular model for the crystal structure be the most promising for X-ray diffraction analy- of lizardite lZfrom Val Sissone,ltaly, that contains sis. significant aluminum and iron substitution. The Lizardite from Kennack Cove (Rucklidge and structure is not very different from the idealized Zussman, 1965)is composedof domainsof 1? and geometry of the serpentine layer and was refined disordered 2H polytypes. Lizardite from Radusa using diffraction data obtained on euhedralcrystals Mine (Krstanovic, 1968) is composed of the 1? of lizardite 1T. The different composition of the Val polytype. According to Rucklidge and Zussman Sissone lizardite 1T, with its different structural (1965), the average crystal structure has trigonal constraints, limits comparison with the lizardite lT 0003-004K82/0506--0587$02.00 587 MELLINI: LIZARDITE IT

structure of Krstanovic (1968),but does provide cm3 in volume. The fragment is composedmainly of important structural details of a composition inter- saccharoidaldolomite with abundant yellow orange mediate between the slightly substituted lizardite lT clinohumite. A vein of lizardite crystals, up to I mm of Krstanovic and more aluminous structures such thick, cuts through the rock fragment. According to as the 9Zpolytype ofJahanbaglooand Zoltai(1968). the collector, Dr. Edgar Huen, the fragment was detached some years ago from an erratic block in Occurrence the Val Sissonemoraine. Subsequentattempts to Few data about the occurrenceof lizardite lTin locate the block have been unsuccessful. Val Sis- Val Sissone,ltaly, are available.All the observed soneis in the contactzone,composed of carbonate, material occurs in a small rock fragment nearly I calc-silicate rocks and amphibolites, that is associ- ated with the Bergell granite (Wenk and Maurizio, 1970; Trommsdorf and Evans, 1972 and 1977). Calcite, dolomite, diopside,amphiboles, spinel and chondroditeoccur in this zone. The crystals are light yellow in color, and their crystal habit ranges from trigonal plates to truncated trigonal pyramids, depending on the crystal thickness. A similar habit has been observed for the related mineral cronstedtite (Fe3*Fe3+) (SiFe3+)O5(OH)a(Steadman and Nuttall. 1963). The lizardite crystals are equant and reach 0.2 mm in diameter. Examination by SEM shows the pres- ence of euhedral crystals as well as less regular aggregates. One of these aggregatesis shown in Figure la, together with a more regular, rather thin, truncated triangular pyramidal crystal. The mor- phology is consistent with the point group 3m. The micrograph 5O u,m shows that the aggregatesconsist of several individual (001) plates, stacked one upon the other. Sometimes, adjacent plates are slightly rotated around [001]. Finally, the rims of the thin outer plates appear not absolutely flat, but partially bent. Larger crystalsof lizardite 1?have also been found. Figure lb is a light optical photograph of a large crystal of lizardite lT, whose astonishing, larger dimension reaches2 mm. It is a detached crystal, found by Dr. Edgar Huen in the same environment where the rock fragment was collected, and has the sameproperties as the vein crystals. Chemicalanalysis The preliminary qualitative chemical investiga- tion, by energy dispersive analysis during the SEM observation, revealed abundant silicon and magnesium with lower contents of iron and aluminum. The quantitative chemical data were gatheredby an ARL-sEMe electron microprobe, equipped with crystal spectrometers,using 15 kV accelerating voltage, 20 nA specimencurrent, an approximate 5 pm diameter electron beam and standard Bence Fig. l. a) Crystals of lizardite lT, observed by SEM; b) Albee correction procedures. The estimated stan- Photograph of a large crystal of lizardite lf. dard deviations on oxide contents. obtained from MELLINI: LIZARDITE IT 5E9

Table l. Electron microprobe analysis of lizardite 1? from Val ly coordinated. The addition of 0.33 water mole- Sissone.Italv cules to compensate for the water assumedlost during analysis produced the slightly idealized for-

vrelght percent atoms per cell mula + + gAls.s7) sio2 1.83 (Mg2.7eF e2 6.6aFe3s. 1 M9o 40.9 2.19 (Sir.arAlo.rz)Os.oo(OH)a.oo. A1 0.24 -O- g tt203 * 2.9 0.10 The calculated densities are 2.575 and 2.630 * FeO 1.0 0 .04 cm-3 for the actual and the idealized formula, re- F not detected spectively, while the experimental value' determined by the heavy liquid method, is 2'58(1) g 89.1 -. *t* cm H20 10.9 Comparison with the chemical data discussedby (1970) the most Total iron was shaleal between the two oxldation Whittaker and Wicks shows that

states to assure full occupancy of the cationic interesting feature of the Val Sissone lizardite is sites and charge balance. probably the high Al2O3content (4'5 wt.Vo)-Similar values have been obtained by hydrothermal synthe- *rF calculated by dlfference sis of one layer and similar, as well as higher values, li- counting statistics, range from 0.3 to 2.5 relative have been reported for synthesized multi-layer are percent, depending on the abundance of the ele- zardite (Caruso and Chernosky, 1979). They lizar- ment. As four analyseson two different grains did known also for naturally occurring multi-layer and not show variations in oxide contents larger than I dite (Jahanbaglooand Zoltai,1968 and Wicks (Frost, 1975; relative percent for more abundant elements and Plant, 1979)and single-layer lizardite larger than 5 relative percent for less abundant Dungan, 1979). elements, the analyses were averaged to produce the data reported in Table 1. X-ray crYstallograPhY The ferrous and ferric contents were calculated Lizardite 1? from Val Sissone produces very from the determined iron value by assuming full sharp diffraction spots. Streaking or diff-useness, occupancy of the cationic sites by silicon, magne- indicating partial disorder, are rarely observed. sium, iron and aluminum (five cations per unit cell) Weissenbergand precision photographs' as well as and charge balance. This statementis basedon the intensity data collected by single crystal counter assumption that, for each silicon atom which is methods lead to an unambiguousdetermination of substituted by a trivalent cation, one trivalent cat- the Laue symmetry 3m. No systematic extinctions ion must be substituted for the divalent magnesium were observed, thus leading to the possible choice in octahedral coordination. The water content, cal- between P3lm and PSIw space groups, with cell by difference from 100 weight percent, of culated parametersa - 5 3L, c - 7.24. The centrosymme- 10.9 wt.% is lower than the expectedof 13 wt.Vo. tric space group was ruled out, based on the non- The water deficiency may be due to unknown centrosymmetric arrdngementof atoms in the ser- analytical errors, as well as to dehydration of lizar- pentine structure and on the point gloup suggested dite under the electron beam, with a subsequent by morphological observation. Overexposed rota- biased, slightly high determination of the other tion photographs do not show any c periodicity oxides. Unfortunately, the shortage of material larger than 7.24. prevented an independentdetermination of the wa- A tiny euhedral crystal of lizardite (nearly 0.1 x ter content. 0.1 x 0.15 mm3),with a trigonal truncatedpyrami- The analytical data lead to a formula calculated dal habit, was chosen for the refinement of the unit on the basis of charge balance and five cations per cell parameters a.ndfor collection of the intensity unit cell of data, on a Philips PW 1100single crystal automated (Mgz.zgFeE5aFefi joAo.ot; difractometer, equipped with graphite-monochro- matized molybdenum Ka radiation. The unit cell (Sir.srAlo. rz)O5.33(OH)3.3a,parameters, refined by least squares fitting of the where all the iron atoms are assumedas octahedral- crystal setting for 25 reflections, converged to the 590 MELLINI: LIZARDITE IT

powder Table 2. X-ray diffraction pattern of lizardite lTfrom Val alent sets in the Laue groupjm, was measuredfrom Sissone.Italv 3 to 25'O using a O-2O scan and a scan width of 1.40'. Intensity data for l2l reflections were not hkl collected oDs, calc. d"4"g this step, as a pre-sean showed - Ipeak 2!C;( Ibr.r, with Io..r and Iu."x 7.2't6 001 indicating peak intensity and background intensity, 4 .628 100 m respectively. No crystal decay 3.890 3. 895 10'l m or instrumental drift was observed. 3.624 3.519 oo2 ns The data were corrected for Lorentz 2.850 2.850 102 and polarization factors. No absorption correction 2 .668 2.659 't'to was made, owing to the small dimensions of the 2.506 2.504 11'l crystal and the low absorption coefficient. Syrn- 2 .418 2.413 003 metrically-equivalent reflections were averaged to 2 ,152 2.148 112 ms produce an unique set of 209 reflections; the dis- 1 ,948 1 .948 202 crepancy factor R"q : f2ou (nLiwi (n-F,1211>^rr11n_ 1.790 1.'t90 113 ms 1)2iwiFi')lVz,calculated among the n symmetryre- 1.748 1.747 2'to lated reflections,was 0.04. 1,70'l 1 .699 211 The trial model, derived 1.676 1.669 203 from the idealized structure p3lm, 1.574 2't2 in the space group was easily refined with - 1.539 1.541 300 m isotropic temperature factors to R : >llfl" 't : .505 1.507 301 lF.lllEl4l 0.047.The introductionof two indepen- 'l 1 .498 .498 't14 dent hydrogenatoms, located by a Fomap,lowered 't.4't8 1.418 302 the R to 0.042.Further refinement,using isotropic 't .33s 220 temperature factors for the heavy atoms and aniso- 1.314 1.312 221 tropic temperature factors for the hydrogen atoms lo^weredthe R to 0.031. No peak higher than 0.3 eA-3 was observedin the final Af'syithesis. Scat- values a: 5.332(IA, r : 7.233{@)A.These values tering factors for neutral atoms were taken from the are not significantly different from those obtained International Tables for X-Ray Crystallography, by least squaresfitting of the X-ray powder diffrac- vol. IV (1974). Final atomic thermal and positional tion pattern, a : 5.338(2)Aand c : 7.238(3)4.fne parameters are reported in Table 3. Observed and pattern (Table 2) was obtained with a Gandolfi calculated structure factors are given in Table 41. camera (57.3 mm in diameter) and cobalt Ka radia- I To tion on a single crystal, previously identified by receive a copy of Table 4, order Document AM-82-201 from the Business rotation and Weissenbergphotographs. Office, Mineralogical Society of America, 2000 Florida Avenue, N.W., Washington, D.C. 20009. please A total of 338 reflections, belongingto two equiv- remit $1.00 in advancefor the microfiche.

Table 3. Final atomic positional and thermal parameters.Esd's are in parentheses

ul'l uzz u-^ uzr ut: ulz

't si /3 2/3 o.o766(7, s0(4) 50(4) 116(8) 25(2\ 0 0 M9 0.3327(s) 0 '122(7J c.4596(1) 32(41 32(4) 16(2) 7 (9) 0 ol 1/3 2/3 0,3000 96(12) 96(12) 68(21) 48(6) 0 0 o2 0.5087(11) 0 -0.0036(9) 130(12) 130(12) 136(17) 6s(6) 48(20) 0 o3 0.5654(10) 0 0. s93s(7) 9s(9) 95(9) 91(1s) 48 (5) -3 (20) 0 o4 0 0 .t61 0.3088(14) s8(17) 58(17) (34) 29(8) 0 0 U 0 0.709(5) 0.026(5) H4 0 0 0.199 (s) 0. 001( s)

Anisotroplc temperature factors (xl04) are of the form: (u,rr'2a*2+ urrk2b*2* u,ar2.*2* "*pf-zrr2 2ul2hka+b*+2u,rhr.a*c*+zurrt lb*"*)l MELLINI: !'IZARDITE IT 591

Structure descriPtion (Mgs.ssFe35rFe35oAlo.to)

The main features of the crystal structures of the (si3.66,A10.34)oro.oo(oH)s.oo' serpentineminerals are well established(Wicks and Whittaker, 1975).As shown in Figures2 and 3, 1:l for comparison with the popular formula layers, consisting ofa silicate tetrahedral sheet and (Mgo- (Si4- *Al*)Oro(OH)s. "Al*) a brucitelike magnesium octahedral sheet, alter- The deviations from idealized geometry for the For nate along [001]. The linkage between adjacent Val Sissonelizardite structure are rather small. layers is strengthened by hydrogen bonds. Al- instance, the ditrigonal tetrahedral sheet is charac- though the general arrangementis known, the pres- terized by remarkable hexagonal pseudosymmetry ence of stacking disorder in most lizardites has (Fig. 2) becausethe rotation of tetrahedra towards : prevented the determinationof the three dimension- the ditrigonal configuration is quite small (a al arrangement of atoms within the layer. Because -3.5"). of the absence of random stacking shifts the lizar- The presenceof several symmetry constraints, dite 1? from Val Sissoneappears to be truly trigo- due to the trigonal symmetry, leads to the existence nal, at least within the limits of the present refine- of a few (x,y) planeswhere the atoms are located' It ment (R : 0.031). No indication of orthorhombic is therefore possible to locate both the basal oxygen (Wicks and Whittaker,1975)or monoclinicsymme- plane and the silicon plane of the tetrahedral sheet try (Krstanovic, 1980)was recorded. In spite of the and both the magnesiumplane and an outer oxygen trigonal symmetry, the Val Sissonelizardite lT can plane of the octahedral sheet. Moreover, the oxy- be alternately described by a C-centeredmetrically gen atoms common to both the tetrahedral and orthorhombic unit cell, for easier comparison with octahedral sheet do not deviate too much from the the data available for lizardite and the other serpen- geometricalplane. Indeed, the-z heightsof Ol and tine minerals. Its parameters are a : 5.3324, b : b4 uto.r differs only by 0.064Ainstead of 0.3A, as 9.235A, c : 7.834. The unit cell content may be reported by Krstanovic for the magnesium rich doubled to lizardite 1?. Also, no buckling of the plane of

Fig. 2. I : I layer of lizardite lT, viewed along [Ol]. 592 MELLINI: LIZARDITE IT

a-b I

Fig. 3. Crystal structure of lizardite 1T, viewed along [10]. Dotted lines representhydrogen bonds.

magnesium atoms was found in the Val Sissone aggreementwith the results of the statistical analy- lizardite lT. sis of Si-GSi configurations by O'Keefe and Hyde (1978). Silicon coordination Only one independent silicon atom is present per M agnesium coordinatio n unit cell. It forms three long bonds (1.6464) tith The coordination polyhedron around Mg is a the bri$SinS 02 oxygen atoms and one short bond trigonal antiprism (flattened obtahedron). It is rep- (1.6164) with the non bridging Ol oxygen atom resented in Figure 4 by the corresponding Schlegel (Table 5). The averageSi-O bond distance,1.6394, diagram. The diagram is obtained by contraction of is larger than expected for unsubstituted silicon the, upper face of the polyhedron and enlargement tetrahedra (Baur, 1978)but is in agreementwith the of the lower face, so as to show all the polyhedral chemical data, which suggestsubstitution of silicon edges. The thickness of the octahedral sheet along by aluminum. The possible substitution by iron [001]ranges from 2.064 (measuredbetween Ol and seemsunlikely and, in fact, Rozensone/ al. (1979) 03 atoms). The value is lower then the 2.204 did not find trivalent iron in tetrahedral coordina- previously reported for lizardite lTand is closer to tion during the Mossbauer analysis of several lizar- the thickness found in other serpentine minerals dites. Moreover, assuming a linear relationship (Wicks and Whittaker, 1975)and brucite (2.114). between the averagetetrahedral bond distance and The bond distancesrange from 2.02llto 2.082Afor the aluminum content and taking into account the the Mg-OH bonds and are 2.l2lA long for both the different coordinations of the oxygens, the ionic Mg-O bonds formed with the silicon bonded Ol prewitt radii reported by Shannon and (1969)indi- oxygens(Table 5). Inspectionofthe bond distances cate an aluminum site population of 0.11. There- and GO distances shows that the octahedral atom justified fore, it seems to assumethat the tetrahe- is shifted from the center of the octahedrontowards dral site is occupied by nearly 0.9 silicon atoms and the outer 03 plane. This feature can be related to 0.1 aluminum atoms. the donor behavior of 03 atoms in the hydrogen Finally, the Si-GSi configuration is rather regu- bonds that develop between adjacent layers. lar and the non-bondedSi-Si distances.the Si-O-Si The average bond distance, 2.0674, is smaller angles and the related Si-O bond distances are in than the value of 2.104 computed from the ionic MELLINI: LIZARDITE IT radii given by Shannon and Prewitt (1969). The Table 5. Bond lengths (A) an0 bond angles (") in lizardite 1T smaller dimensions of the coordination polyhedron Esd's are in Parentheses' are in keeping with the chemical data, which sug- gest aluminum and trivalent iron for substitution by si 1-o1 1.61 5 (s) M9-o3 2.021 15\ magnesium. 1 .545(3) x3 -o3 2.026(5) x2 avera9e 1.639 -o4 2.082(5) Hydrogen bond system -o't 2.'121(3) x2 Two crystallographically-independenthydrogen average 2.067 atoms are present in the crystal structure. Both o2-o2 2.66'7(5\ hydrogen atoms were located in the Fo map and (3) o2-o1 2.682(s) sl-si 3.078 their positions were refined. One of them, H4, which is located in the center of the ditrigonal ring, 02-si1-o2 108.3(3) st-o2-st 138.6(3) does not form any hydrogen bond. The other one, o1-s11-O2 110.6(3) H3, links adjacent layers through hydrogen bonds o3-H3 0.84(6) o4-H4 0,19 (6\ formed between the donor 03 and the acceptor 02 oxygen atoms. The hydrogen bond is weak, 0.094 'v.u. porting the substitution of trivalent atoms in the two according to Lippincott and Sch-roder(1955), sites. due to the long O2-O3 distance(3.03A). However, All the results of the refinement, particularly the the relative weaknessof the singlehydrogen bond is bond distances and bond strength balance, support balanced by the high number ofbonds (one hydro- the crystal chemical formula gen bond per 8.2A2). (Mgs.ssFe65*Fe35oAlo.to) Bond strength balance (si3.66,A10.34)o16.6s(oH)6.e6, The bond strength calculations (Table 6) were made according to Brown and Wu (1976),using the previously proposed on the basis of the chemical same parametersR1 : 1.622and N : 4.290for both data. tetrahedral and octahedral cations. Taking into account the hydrogen bond contribution, the sums of Polytypism and hydrogen bonding bond strengths reaching the anions closely fit the Ditrigonal distortion and hydrogen bonding theoretical values and the discrepanciesare always lower than 0.07 v.u. On the other hand, the sums of As previously stated, the six-memberedtetrahe- bond strengthsreaching the cations are significantly dral ring is distorted toward the ditrigonal config- higher than two and lower than four, for the octahe- uration, due to alternate rotation of the bridging dral and tetrahedral site, respectively, further sup- oxygen atoms. In the present case the oxygen atoms move away from the nearest magnesium atoms within the samelayer and toward the hydrox- o3 yl groups ofthe adjacent layer. The rotation can be describedby the value a : -3.5o, where the minus sign means movement away from the atoms in octahedral coordination. In the nomenclature proposed by Franzini (1969)for the mica layer, negative rotations colTespondto I layers and positive rotations correspond to A layers. B layers occur infrequently in layer silicates and, in the serpentine minerals, they have been previously reported for only the 9-layer serpentine(Jahanbagloo and Zoltai, 1968;o : -8o).A layersoccur in two amesites2H2, from Saranovskaie (Steinfink and Brunton, 1962; Anderson and Bailey, 1981;a : +14.7') and from Antarctic (HalI, 1974; Hall and Bailey, 1979; a : Fig. 4. The octahedral site, representedby Schlegeldiagram. + 13.6'and * 14.5').There are also other structures Heavy lines represent shared edges. of serpentine minerals where A layers have been 594 MELLINI: LIZARDITE IT

Table 6. Bond strength balance (v.u.), calculatedaccording to Brown and Wu (1976)and Lippincott and Schroder (1955).

si Ilydrogen bond )c' bution

o1 0 . 3'l6x3 1.016 1.964 1 .964 0.316 'l O.939x2 .878 +0.094 1 .972 0.389 't 0.386 .1 61 -0.094 1.067 0. 386 0.343x3 1 .029 1 .o29

3.833

Ic, = sms of bond strengths to the anlon. = )c' sums of bond strengths to the anion. after correction for the hydrogen bond. )a., = sms of bond strengths to the cation, reported. These are the Unst-type serpentine(lizar- ditrigonal distortion of the tetrahedral sheet also is dite 6?1) and its Mg-Ge synthetic analogue(Hall et supported by the available data on layer silicates al.,1976), amesite6R2 (Steadman and Nuttall, 1962) other than serpentineminerals. In fact,.in chlorite and the lT, 6R2, 321 polytypes of cronstedtite the movement of the bridging oxygen atoms always (Steadmanand Nuttall, 1963).Unfortunately, these leadsto strongerhydrogen bonds, by B layersin the last six structure analysesare less accuratebecause case of Ib-chlorite and by A layers in the case of Ia of poor experimental data. Their R factors range and Ilb-chlorite (Shirozu and Bailey, 1965).On rhe from 0.15 to 0.21, the refinementsdo not take into other hand, only A layers occur in mica (Baronnet, account cation ordering(Bailey, 1975)and, in some 1976)or in talc (Rayner and Brown, 1973),where no cases, the A configuration has been assumed by hydrogenbond can form. analogy with other structures. As a consequence, the details of these refinements are less reliable. Experimental and theoreticalre sults In the four structures for which three-dimensional refinements are available, the bridging oxygen Petersonet al. (1979)performed molecular orbital I - atoms always move toward the hydroxyl groups calculations on the clusters [Mg:Fo(OH)7Si6O6F6] belonging to the adjacent layer. Therefore, eitherA and [MgaFr2(OH)3Si3OrFs]6-.The clusters were or B layers are present, and stronger hydrogen used to model the serpentine I : I layer. By defini- bonding is achievedin these serpentinestructures. tion, the model does not take into account interlav- Similar movement towards the hydroxyl groups er effects.Assuming 1.62Aas the value of the Si-b occurs also in the Unst-type serpentineand its Mg- bond distance, minima in the total energy curve are Ge synthetic analogue (Hall et al., 1976),amesite found at a: -2.1o anda: -3.9", for the trimeric 6R2 (Steadmanand Nuttall, 1962),cronstedtite 321 and the tetrameric unit, respectively. The results fit (Steadmanand Nuttall, 1963).The oppositemove- the experimental data for the Val Sissonelizardite ment, away from the hydroxyl groups, has been 1T, which has an averageSi-O distanceof 1.639A reported for cronstedtite 1?and 6Rz(Steadman and and a : -3.5". Nuttall, 1963). Unfortunately, the details of these When the calculation is performed for a tetrahe- six last structure analyses cannot be confidently dral sheetwith greatersubstitution, using 1;68Aas used in this discussion.It is conceivablethat, as the average tetrahedral bond distance, two minima shown by the best four availablestudies, the pres- are found. The lower minimum is at o : -12.3" and ence of A or B layers in the serpentine minerals the higher energy minimum is at a : 112.7" for the depends on which configuration leads to more effi- trimeric unit and at a: -13.8'and c : *10.6'for cient interlayer hydrogen bonding. The importance the tetrameric unit. Assuming that the energy of interlayer hydrogen bonding with regard to the curves and their minima are not model dependent MELLINI: LIZARDITE IT 595 but physically sound, some important remarks can layer shifts, Figure 2 clearly shows that shorter be made: OH-O distances will result in B layers and longer (1) B layers are expected for tetrahedral sheets OH-O distances will result in A layers if the stack- -r with lower substitution for silicon. This theoretical ing sequencedoes not include 60" or 180"rotation result fits the experimental data for the Val Sissone (r rotation accordingto Bailey, 1969,and Wicks and lizardite 17, which has a : -3.5'. Whittaker, 1975).Otherwise, the presenceof the r (2) B layers are expected also for tetrahedral rotation leads to shorter OH-O distances by A sheets with greater substitution for silicon. Actual- layers and to longer OH-O distancesby B layers. ly, they occur in the 9Jayer serpentine (Jahanbag- Therefore, B layers must be expected when the loo and Zoltai, 1968), which has an average Si-O stacking sequencedoes not include r and A layers bond distanceof 1.664 and a : -8'. However, the when the stacking operator includes r. If the possi- -rbl3, existence of a higher energy minimum suggeststhe ble interlayer shifts (a13, as they were defined possible existence of A layers for aluminum-rich by Bailey, 1969, and Wicks and Whittaker, 1975) disorderedflat-layer serpentines.The choiceofthe are also taken into account, similar relationshipsare positive, or negative, rotation might then be deter- derived. The complete results are given in Table 7. mined by interlayer effects, namely by the stabiliza- The layer configurationsreported in Table 7 were tion energy associatedwith the formation of hydro- obtained assuming that the formation of hydrogen gen bonds. Although A layers occur in amesite2H2 bonds controls the direction of the movementof the (Hall and Bailey, 1979;Anderson and Bailey, 1981), bridging oxygen atoms in flat-layer serpentines, no close comparison can be made, as the disordered which is indicated by the few, reliable data on these CNDO/2 model by Peterson et al. (1979)is not minerals. The Table 7 can be eventually used to strictly applicable to the ordered layer of amesite predict the direction of movement of the bridging 2Hz. oxygensin other polytypes, whose structureis not (3) Assuming that A and B layers may exist known. More complex polytypes, not included in independently in aluminum-rich flatJayer serpen- Table 7, can be analyzed by examination of the tines, the simultaneousoccurrence of both A and B stacking sequenceand reduction of the long, com- layers within the same complex polytype may be plex sequenceto the sum of shorter sequences'as expected. in the case of the 9-layer serpentine (Jahanbagloo and Zoltai, 1968). Polytypic sequenceand ditrigonal distortion Misfit relief and interlayer bonding A close relationship between the A or B nature of the layer in the serpentineminerals and the stacking The aluminum content is a very important param- operator can be found. Disregardingpossible inter- eter in the crystal chemistry of the serpentine

Table 7. Relationship between the stacking sequenceoperator and ditrigonal distortion of the tetrahedral sheet'

* Interlayer, layer Polytypes Examples shtft configuration

Val Sissone None B 1T Lizardite 1T from

t -"1)H Not knorvn

a/3 A 1M, 2t'r1 , 3T1 Not known

r+a/3 B 2Or t 2Mat 6H,, Not known

b/3 B 3R,2T,6P'2,3'.r2 Not known Amesite 2H^ (saranovskaie and Antarctic) r+b/3 A 2H2, 6P'1 z

5R3, 6H2 Not known b/3+t

(for examPle' tF More complex polytYPes can be analyzeal by examination of the stacking sequence a/3+r, -a/3+r", in the 9-layer serpentine (Jahanbagtoo and Zo1tai' 1958) the sequence ls "none,

repeated three times. MELLINI: LIZARDITE IT

minerals. Although no sharp compositional break tution of Al3+ (or any other suitable trivalent cat- between flatJayer and curved-layer serpentinesor ion). between single-layer and multi-layer lizardites has In the first case, stabilization must be achieved been observed (Jasmundand Sylla, l97l and 1972: by dimensional fitting of the two sheets. In the plant, Wicks and Whittaker,1975;Wicks and 1979), second case, the modified chargedistribution inside the greater substitution of aluminum for silicon and the layer must also be taken into account. This was magoesiumleads to more stable structures (Caruso first noted by Gillery (1959), who stated that "the and Chernosky, 1979)and promotes flat-layer lizar- octahedral part acquires a positive charge and the (Wicks dites and Whittaker, 1975).The aim of this tetrahedral a negative charge and the layer as a section is to explain how aluminum stabilizes the whole is neutral. The layer surfaceswhich face each structure. It is not my purpose to assert here that other have opposite chargeand this provides a force aluminum is an essential component of flat-layer of attraction between them which increases with structures, or that it must be absentin curved-layer increasing aluminum content. Hence the basal spac- serpentines. ing decreaseswith increasing aluminum content". It is generally acceptedthat the usual disorder in Subsequently,Steadmal and Nuttall (1963)invoked serpentineminerals is due in part to the mismatch in some kind of electrostatic interaction between the dimensions of the larger octahedral and smaller successivelayers in the different polytypes ofcron- tetrahedralsheets (e.g., Radoslovich,1962; Wicks stedtite. The existence of electrostatic interaction and Whittaker, 1975). Progressive substitution by between the 1:1 layers of kaolinite-like minerals larger cations for tetrahedral silicon and by smaller was supported also by Cruz et al. (197D, who cations for octahedral magnesium leads first to treated the minerals as condenser plates with sur- reduction of the misfit and then to larger dimensions face chargesdue to hydroxyl dipoles.Giese (1973) of the tetrahedral sheet over the octahedral sheet. showed that the electrostatic interaction in kaolin- The effectivenessof such a mechanismfor relief of iteJike minerals could be explained assumingthat misfit have been given by several authors. For long hydrogen bonds are largely electrostatic in instance,Roy and Roy (1954)took advantageof the nature. Finally, Chernosky (1975) revived the hy- largel ionic radius of germanium (0.a0A instead of pothesisproposed by Gillery (1959)and once again prewitt, 0.26A of silicon, according to Shannonand related the reduction of the c parameter to the 1969)and synthesizeda Mg3Ge2O5(OH)aserpentine increasing charge difference between the two that crystallized in euhedral hexagonal plates and sheets.The hypothesisby Gillery did not seemto produced sharp X-ray diffraction patterns. Many receive general acceptance. Several papers deal papers deal with the relationship betweenthe Al2O3 only with the geometrical fitting or misfitting be- content and the cylindrical or flat nature of the tween the two sheets of the layer, and do not take (e.g., layers Roy and Roy, 1954;Nagy and Faust, into account what happens between successive 1956;Chernosky, 1975;Thomas et al.,1979). Caru- layers. so and Chernosky (1979) described the increasing Other evidence seemsto support the hypothesis thermal stability of lizardite with the increasing of the existence of an "electrostatic" interaction Al2O3 content. Highly refined structure models which increaseswith an increasing substitution by resulting from a high degree of three-dimensional trivalent cations. One argument is based on the order have been produced only for aluminum-rich basal spacing.The value increasesfrom nearly 7.34 serpentine minerals (Jahanbagloo and Zolta| 196g; in chrysotile polymorphs to 7.46A in the Mg4e Hall and Bailey, 1979;Anderson and Bailey, 1981; serpentine,in agreementwith the larger ionic radius present the work). Therefore, it seemswell estab- of germanium (0.404 instead of 0.26A for silicon, lished that the aluminum content (and, as a rule, according to Shannon and prewitt, 1969).On the any suitable substituent) is a very important factor other hand, the basal spacing is reduced to 7.08A in the crystal chemistry of the serpentineminerals. when aluminum (ionic radius 0.394) substitutesfor In spite of the evidence for such a mechanismfor silicon (e.9., Chernosky, 1975)and to nearly 7.lA relief of the misfit by fitting of dimensions, care is when iron (0.494) substitutesfor silicon (steadman required with regard to the interpretation of these and Nuttall, 1963 and 1964). Although some ac- data. We must distinguish, for instance, between count must be made for the smaller size of the the mechanism accompanying the substitution of octahedrally-coordinated aluminum and iron with Gea+ for silicon and that accompanyingthe substi- respect to the octahedrally-coordinated magne- MELLINI: LIZARDITE IT sium, a definite decrease of the basal spacing is en the bond they form with hydrogens. The bond observed. This indicates that interlayer bonding is valence balance is retained by formation of hydro- stronger than in Mg-Si or Mg-Ge serpentines. gen bonds with undersaturatedtetrahedral oxygen A secondindication that the interactionsbetween atoms. The structure, as a whole, has a lower adjacent layers in lizardite are stronger than in energy, thus justifying the increasingthermal stabil- unsubstituted serpentine is given by their IR spec- ity of substituted lizardite (Caruso and Chernosky, tra. Farmer (1975) reports that the substitution of r97s). silicon by aluminum, compensatedby replacement of magnesium by aluminum, gives two broad OH Acknowledgments stretching absorption bands at lower frequencies Valuable criticism from the referees,Dr. M. Cameronand Di' than in chrysotile and antigorite. This indicates, F. J. Wicks, is acknowledged.The specimenof lizardite and data according to Farmer (1975), a stronger hydrogen about its occurrencewere suppliedby the collector Dr. E. Huen' gratefully for providing the bond. A similar line of evidence is obtained from Dr. P. Orlandi is acknowledged and the preliminary energy dispersive and (1975), crystals of lizardite the IR data given by Heller-Kallai et al. who scanning electron microscoperesults (obtained at the Istituto di report similar IR absorption bands only for those Mineralogia dell' Universitd di Firenze). Many thanks are due to specimens of serpentine which contain aluminum Prof. P.F. Zanzzzi, Istituto di Mineralogia dell' Universith di (according to them, these are five lizardites and two Perugia, for collecting X-ray diffraction intensity data. Last but of the Istituto di Mineralogia aluminous antigorites). not least, I thank the colleagues dell' Universiti di Modena, who made available to me their A further line of evidence can be found in the electron microprobe laboratorY. results of the present study. The average tetrahedral bond distancein lizardite lTfrom Val Sissone, References 1.6394, doesnot differ too much from the expected value, 1.6254, of a tetrahedronthat containsonly Anderson, C. S. and Bailey, S. W. (1981)A new cation ordering pattern American Mineralogist, 66, 185-195. silicon (Shannon and Prewitt, 1969; the value is in amesite-2ll 2. Bailey, S. W. (1969) Polytypism of trioctahedral I : I layer calculated taking into account the different coordi- silicates.Clays and Clay Minerals, 17,355-371' nation numbers of the oxygen atoms). The differ- Bailey, S. W. (1975) Cation ordering and pseudosymmetryin ence in bond distance does not seem large enough layer silicates. American Mineralogist, 60' 175-187' I to support the idea that the high three-dimensional Baronnet, A. (1976) Polytipisme et polymorphisme dans les 'l.T micas. Contribution a I'etude du role de la croissancecristal- order of lizardite may result from such slightly line. Thesis. UniversitE d' Aix Marseille III. enlarged dimensions of the tetrahedral sheet. Baur, W. H. (1978) Variation of mean Si-O bond lengths in Therefore, it seems reasonableto suppose that silicon-oxygen tetrahedra.Acta Crystallographica,B34, 175 l- electrostatic interactions between adjacent layers, r756. based on hydrogen bonds, exist and lead to stronger Brown. I. D. and Wu, K. K. (1976) Empirical parametersfor -oxygen bond valences. Acta Crystallogra- interlayer bonding. The mechanismcan be formally calculating cation phica, B32, 1957-1959. described as substitution by trivalent cations for Caruso, L. J. and Chemosky, J. V' (1979)The stability of lizzr- both tetrahedral silicon and octahedral magnesium, dite. Canadian Mineralogist, 17, 757 J 69. with a resultant excess of negative charge on the Chernosky, J. V. (1975)Aggregate refractive indicesand unit cell tetrahedral sheet and positive chargeon the octahe- parametersof synthetic serpentinein the systemMgO-Al2O3- Mineralogist, 60, 200-20E. dral sheet. The negative charge is localized on the SiO2-H2O. American Cruz, M., Jacobs, J., and Fripiat , J. J. (1972)The nature of the basal oxygen atoms of the tetrahedral sheet' The cohesion energy in kaolin minerals.International Clay Confer- outer octahedral oxygens drain electrons from the ence. Madrid. 35-43. more electropositive hydrogen atoms. These be- Dungan, M. A. (1979)A microprobe study of antigoriteand some come positively charged, weakly acidic hydrogens. serpentine pseudomorphs. Canadian Mineralogist, 17, 771- The separation ofpositive and negative chargeson 784. Farmer, V. C. (1975)The infrared spectra of minerals. Mineral- the two faces prevents curling of the layer and ogical Society Monograph, 4, London. produces stronger interactions between adjacent Frost, B. R. (1975)Contact metamorphismof serpentinite,chlo- layers, thus leading to flat structures. ritic blackwall and rodingite at Paddy-Go-EasyPass, Central An alternative formulation can be given if Paul- Cascades,Washington. Journal of Petrology, rc' n2-:313. (1969) The A and I mica layers and the crystal ing's rule is taken into account. The valencesof the Franzini, M. structure of sheet silicates. Contributions to Mineralogy and outer tetrahedral oxygens are undersaturated and Petrology, 21, 203:224. the valences of the outer octahedral oxygens are Giese, R. F. (1973)Interlayer bonding in kaolinite, dickite and oversaturated. Therefore octahedraloxygens weak- nacrite. Clays and Clay Minerals, 2l' 145-149. 598 MELLINI: LIZARDITE IT

Gillery, F. H. (1959)The X-ray study of synthetic Mg-Al serpen- Rucklidge, J. C. and Zussmann,J. (1965)The crystal structure of tines and chlorites. American Mineralogist, 44, 143-152. the serpentinemineral, lizardite Mg3Si2O5(OH)a.Acta Crystal- Hall, S. H. (1974)The triclinic structure of amesite.M.S. Thesis, lographica,19, 381-389. University of Wisconsin, Madison. Shannon, R. D. and Prewitt, C. T. (1969)Efrective ionic radii in Hall, S. H. and Bailey, S. W. (1979)Cation ordering pattern in oxides and fluorides. Acta Crystallographica,B.25, 925-946. amesite. Clays and Clay Minerals, 27, 241-247. Shirozu, H. and Bailey, S. W. (1965)Chlorite polytypism: IIL p., Hall, S. H., Guggenheim,S., Moore, andBailey,S. W. (1976) Crystal structure of an orthohexagonaliron chlorite. American The structure of Unst-type 6Jayer serpentines. Canadian Mineralogist, 50, 86E-885. Mineralogist, 14, 314-321. Steadman,R. and Nuttall, P. M. (1962)The crystal structure of Heller-Kallai, L., Yariv, Sh., and Gross, S. (1975)Hydroxyl amesite. Acta Crystallographica, 15, 510-511. stretching frequencies of serpentine minerals. Mineralogical Steadman, R. and Nuttall, P. M. (1963)Polymorphism in cron- Magazine, 40, 197-200. stedtite. Acta Crystallographica, 16, 1-8. International Tables for X-ray Crystallography, vol. IV (1974). Steadman,R. and Nuttall, P. M. (1964)Further polymorphismin Kynoch Press, Birmingham. cronstedtite. Acta Crystallographica, l7, 404-406. Jahanbagloo,J. C. and Zoltai, T. (1968)The crystal structure of a Steinfink, H. and Brunton, G. (1956)The crystal structure of hexagonalAl-serpentine. American Mineralogist, 53, 14-24. amesite. Acta Crystallographica,9, 487492. Jasmund,K. and Sylla, H. M. (l97l) Synthesisof Mg- and Ni- Thomas,J. M., Jefferson,D. A., Mallinson,L. G., Smith,D. G.. antigorite. Contributions to Mineralogy and petrology, 34, g4- and Crawford, E. S. (1979)The elucidation of the ultrastruc- 86. ture of silicate minerals by HREM and X-ray emissionmicro- Jasmund, K. and Sylla, H. M. (1972)Synthesis of Mg- and Ni- analysis. In L. Kihlborg, Ed., Direct Imaging of Atoms and antigorite: a correction. Contributions to Mineralogy and Molecules in Crystals. The Royal Swedish Academy of Sci Petrology,34,346. ences.Stockolm. Krstanovic, I. (1968)Crystal structure of single-layerlizardite. Trommsdorf, V. and Evans, B.W. (1972)Progressive metamor- Zeitschrift ftir Kristallograpttte, 126, 163-169. phism ofantigorite schist in the Bergell tonalite aureole(Italy). Krstanovic, I. (1980) Crystal structure of flat-layer serpentine American Journal of Science, 272, 423437. minerals. Abstracts of the 26th International GeologicalCon- Trommsdorf, V. and Evans, B. W. (1977)Antigorite ophicarbon- gress, Paris. ates: contact metamorphismin Valmalenco, Italy. Contribu- Lippincott, E. R. and Schroder,R. (1955)One dimensionalmodel tions to Mineralogy and Petrology, 62, 423437. physics, of the hydrogen bond. Journal of Chemical 23, l}W- Wenk, H. R. and Maurizio, R. (1970)Geological observations in I 106. the Bergell area (SE-Alps). IL Contact minerals from Mt. Nagy, B. and Faust, G. T. (1956)Serpentine: natural mixtures of Sissone-Cimadi Y azzeda(A mineralogicalnote). Schweitzer- chrysotile and antigorite. American Mineralogist, 39, 957-9j5. ische Mineralogische und PetrographischeMitteilungen, 50, O'Keetre, M. and Hyde, B. G. (1978)On Si-O-Si configurationin 349-354. silicates. Acta Crystallographica,B.34, 27 -j2. Whittaker, E. J. W. and Wicks, F. J. (1970)Chemical differences Peterson,R. C., Hill, R. J., andGibbs, G. V. (1979)A molecular- among the serpentine"polymorphs": a discussion.American orbital study of distortions in the layer structures brucite, Mineralogist, 55, 1025-1047. gibbsite and serpentine. CanadianMineralogist, 17, 7 03J ll. Whittaker, E. J. W. and Zussmann,J. (1956)The characterization Radoslovich, E. W. (1962)The cell dimensionsand symmetry of of serpentine minerals by X-ray diffraction. Mineralogical layer lattice silicates. IL Regressionrelations. American Min- Magazine, 31, 107-126. eralogist, 47, 617-636. Wicks, F. J. and Plant, A. G. (1979)Electron-microprobe and X- Rayner, J. H. and Brown, C. (1973)The crystal structure of talc. ray-microbeam studies of serpentinetextures. CanadianMin- Clays and Clay Minerals, 21, 103-114. eralogist,17, 785-E30. Roy, D. M. and Roy, R. (1954)An experimental study of the Wicks, F. J. and Whittaker, E. J. W. (1975)A reappraisalof the formation and properties of synthetic serpentinesand related structures of the serpentineminerals. CanadianMineralogist, layer silicate minerals. American Mineralogist, 39, 957-97 5. 13.227-243. Rozenson,I., Banninger,E. R., and Heller-Kallai, L. (lg7g) M