American Mineralogist, Volume 80, pages 841-844, 1995

Crystal structure of hillebrandite: A natural analogueof silicate hydrate (CSH) phasesin Portland cement

YoxcsHlx Dl'r Garber ResearchCenter, Harbison-Walker Refractories, l00l Pittsburgh-McKeesportBoulevard, West Miffilin, Pennsylvania15122, U.S.A. Jrrrnrv E. Posr Department of Sciences,Smithsonian Institution, Washington, DC 20560, U.S.A.

ABSTRAgI The of hillebrandite, CarSiO3(OH)r, was solved and refined in space groupCmc2,, a: 3.6389,b: 16.3ll, c : 11.829L, to R : 0.041using single-crystal X-ray data. The structure consistsof a three-dimensionalnetwork of Ca-O polyhedra that accommodateswollastonite-type Si-O tetrahedral chains. Each of the -typ€ chains is an averageof two symmetrically equivalent chains related by the mirror plane perpen- dicular to a. In a given structural channel ofthe Ca-O polyhedral network, only one chain orientation can be occupied to give reasonableSi-O distances.The 03 and 04 sites cor- responding to each vacant Si2 site are occupied by OH groups to achieve chargebalance. The wollastonite-type Si-O tetrahedralchains in the hillebrandite structure resemblethose reported for many (CSH) phases.

Irvrnooucrrox brandite and comment upon the structural relationships hillebrandite with other CSH phases. Hillebrandite, CarSiOr(OH)r, is one natural member of of the CaO-SiOr-HrO ternary system,which includes nu- merous natural and synthetic calcium silicate hydrate ExpnnrnnnNrAl- METHoDS (CSH) phases,most with a common unit-cell axis of about After an exhaustiveexamination of many hillebrandite 3.64 or 2 x 3.64 A and a fibrouscrystal habit alongthis samples,a fragmentof a specimen(NMNH 95767 -7) frorn axis. Many of these compounds are the major hydration Velardena, Durango, Mexico, was found to be suitable products of Portland cement and the main strength-form- for structure determination. Microscope examination un- ing phases in steam-cured mortar cement. Despite the der polarized light and systematicprecession photograph- great interest in the atomic arrangements of the CSH ic studies revealed that the selectedcrystal fragment con- phasesby cement chemists as a key to understandingthe tained about l0o/omisaligned hillebrandite fibers, but it hydration mechanism and strengthdevelopment of Port- was the best crystal found from the samples.The over- land cement paste, understanding of the CSH structures exposedprecession films taken along a, b, and c axesdid is far from satisfactory, even after decadesof intensive not show any evidence of the superstructurereflections research.To date, no material suitable for single-crystal suggestedby earlier investigators (Heller, 1953; Ishida et X-ray study has been synthesized.Natural that al.,1992). The systematicabsences on the films suggested are analogousto the CSH phaseshave been subjectedto possible spaceg.roups Cmc2r, Cmcm, and Ama2. numerous crystallographicinvestigations. Unfortunately, Intensity data were collected on an Enraf Nonius CAD4 significant progressin those studiesalso has been limited diffractometer utilizing graphite-monochromated MoKa by lack of crystals suitable for single-crystal diffraction radiation for the diffraction sphere.Unit-cell parameters studies. Powder X-ray diffraction studies (Heller, 1953) (Table l) were refined using diffraction angles from 25 have shown that hillebrandite resembles a synthetic automatically centered reflections (20 : 20-36'). Other B-CarSiOo.H,O phaseand the CSH(II) cementphases de- details of data measurement are presented in Table l. scribedby Taylor (1986), Gard and Taylor (1976), and Examination of the intensity and orientation standards others, which are the main components in and showedno significant deviations. The intensity data were Iime-silica products formed during the binding process. corrected for Lorentz and polarization effects, and ab- As part of a larger study of cement-relatedminerals, we sorption effectswere corrected utilizing + 180'd-scans at present here the details of the crystal stmcture of hille- 10" intervals for six reflections. After converting the in- 0003404x/95l0708-0841$02.00 841 842 DAI AND POST: CRYSTAL STRUCTURE OF HILLEBRANDITE

TABLE1. Crystaland experimentaldata TleLe 3. lnteratomic distances (A) for hillebrandite

Crystal size (mm) 0.040x0.060x0.150 Cal-O1 2.2s(31 Ccm2, (no. 36) O2(x 2) 2.4O(2) z o 04 (x 2l 2.34(21 Cell parameters o6 2.41(3) a (A) 3-6389(8) Mean z.Jo b (A) 16.311(5) Ca2-O1(x 2) 2.38(2) c (A) 11.829(3) o3 2.43(4) v(4") 702.1(3) 05 (x 2) 2.46(2) a limit (") 1-30 o4 2.66(4) Reciprocalspace +h, +k, +l o6 2.62(3) Standards Mean 2.48 Orientation 3 per 400 refls. Ca3-O2 2.36(3) Intensity 3per5h 03 (x 2) 2.37(2) Total data collected 6807 OO(x 2) 2.38(21 Unique data 1056 o5 2.43(3) F > 2o, data 325 Mean 238 R.",r" (% on F*") b.5 sil-o2 1.60(3) R(%) 4.1 o5 1.61(3) R.(%) 4.5 07 1.62(5) Goodness-of-fit 0.86 o8 1.62(2) Final differencemaps (e/A3) Mean 1.61 (+) 0.69 si2-o3 1.74(4) (-) 1.05 o4 1.63(4) 07 (x 2l 1.64(5) Mean 1.66 tensity data to structure factors, symmetry-equivalent re- flections were averaged.In the final cycle of full-matrix least-squaresrefinement, only the reflections with .F > least-squaresrefinement was made by refining positional 2o"were used,and the observedreflections were weighted parameters,anisotropic displacement factors for cations proportionally to oi2, with a term to reducethe weighting and isotropic displacement factors for anions, the scale of intensereflections (Fair, 1990). factors, and a secondary extinction factor (a total of 59 Crystallographiccalculations were made using the pro- parameters). The isotropic displacement factors for an- gramsof the Molen (Fair, 1990)package on a VAX3l00 ions were employed in the final refinement becausethe workstation. The E statistics strongly indicated the ab- poor diffracting quality of the crystal resulted in low ob- senceof a center of symmetry, and subsequentstructure serveddata vs. variable ratios (-5.5). The resultsof the determination revealed that Ccm2, (no. 36) is the correct final cycle ofleast-squaresrefinement are recorded in Ta- spacegroup. Direct methods were used for phase deter- ble l. mination. The subsequentelectron density map revealed The positional parameters and equivalent isotropic three Ca, one Si (Sil), and severalO positions. These displacement factors are given in Table 2. Table 3 lists positions are all located on the 4a (0,y,2) specialposition selectedinteratomic distances.Anisotropic temperature on the mirror planes (Table 2). Fourier difference map factors and observed and calculated structure factors are calculations located the 07, 08, and half-occupied Si2 listed in Tables 4 and 5, respectively.' sites.The refinement, then, convergedto R = 0.045 with all cations anisotropic and all anions isotropic. The un- Srnuct-nr DEscRrPrroN usual anisotropic displacementfactors ofthe Sil site and The hillebrandite structure consists of a three-dimen- abnormal Si I -O distancessuggest splitting of the site and sional network of Ca polyhedra connectedto form tun- displacement from the mirror plane onto the 8b (x,y,z) nels parallel to [l00] (Fig. l) that accommodate isolated generalposition. The two disordered Sil sites,related by wollastonite-type SiOotetrahedral chains (Fig. 2). the mirror, are about 0.5 A apart. The final full-matrix Ca{O,OH) polyhedral network In the asymmetric unit, there are three independent TABLE2, Atomicparameters for hillebranditestructure Ca positionscoordinated by six (Cal, Ca3) and seven(Ca2) Occu- anions. Each Ca-(O,OH) polyhedron sharestwo edgesat pancy x (A1 U* x: 0 and x: 0.5 with two neighboringCa polyhedraof Ca1 1.0 0 0.2164(2) 0.064 0.0137(5) the samekind to form three continuous Ca-(O,OH) poly- -0.0479(3) Ca2 1.0 v2 0.3992(2) 0.0114(s) parallel Ca3 1.0 Y2 0.0490(2) 0 1888(2) 0.0118(5) hedral columns to [00]. Cal-(O,OH) and Ca3- si1 0.5 0.430(2) 0.0863(3) -0.1019(4) 0 011(1) (O,OH) columns share common edgesto form a double si2 0.5 v2 0-3727(5) 0.2134(6) 0.008(1) chain of Ca-(O,OH) octahedralfour-membered rings par- o1 1 0 0 0.3091(6) - 0.0812(8) 0.014(3) 02 1.0 Y2 0.12s9(6) 0.0198(8) 0.013(3) allel to [00]. The Ca2-(O,OH)columns link the double o3 1.0 Y2 0.4637(7) 0.138(1) 0.024(3) o4 1.0 Yz 0.2964(7) 0.1251(9) 0.019(3) -0.1123(8) o5 1.0 1 0.4890(6) 0.010(3) ' A copy of Tables 4 and 5 may be ordered as Document AM- 1.0 o6 0 0.1370(6) 0.2361(8) 0.013(3) 95-591 from the BusinessOffice, Mineralogical Societyof Amer- 07 0.5 0.364(4) 0.1328(7) -0.205(1) 0.014(3) ica, ll30 SeventeenthStreet NW, Suite o8 0.5 0 0.110(1) -0.127(2) 0.014(3) 330, Washington, DC 20036,U.S.A. Pleaseremit $5.00in advancefor the microfiche. DAI AND POST: CRYSTAL STRUCTURE OF HILLEBRANDITE 843

Fig. 2. The [010] projectionof two equivalentSi3OE tetra- hedralchains (one filled black, the otherin white)in eachstruc- tural channel.Si2. Sil, andO atomsare shown as large to small circles,respectively.

does not affect the distribution of the Si and O atoms in the next channel;no evidencewas observedfor long-range ordering of the SiOo chains in different channels,which would result in doubling ofthe a axial length as suggested by Heller (1953)and Ishidaet al.(1992). Our preliminary electron diffraction investigations along a, b, and c axes on the specimen used in this study showed weak stria- t/zainthe reciprocal plane, suggesting Fig. l. Perspectiveview of hillebranditestructure along the tions at a* and b* a axis.Cal-, Ca2-,and Ca3-(O,OH) polyhedra are shaded with only short-range ordering of the SiO4 dreierkette chains light-to dark-graycolors, respectively. The Si-O tetrahedra plot- possible in a given crystal domain. Further TEM work is ted in ball-stickpatterns form wollastonite-typechains accom- warranted to understand the mechanism of short-range modatedin the structuralchannels parallel to thea axis(stippled ordering. circlesare Sil and blackcircles are Si2). OH groups On the basis of Pauling's electrostaticvalence rule, the Cal + Ca3 chains by edge sharing to form an infinite results of bond-valence calculations (Table 6) were used comrgated sheet normal to b (Fig. l). Between two cor- to distinguish O atoms, OH groups, and HrO molecules rugated sheets,the Ca2 polyhedra of one sheetshare cor- in the refined structure as suggestedby Donnay and All- ners with the Ca3 octahedra ofthe other sheetto form a mann (1970).Of the eight O sites,the Ol and 06 appear three-dimensionalnetwork of Ca-(O,OH) polyhedra.This to be OH groups, which account for two-thirds of the type of arrangement of the Ca-(O,OH) columns yields HrO content in the chemical formula. In fact, there are structural channelsbetween the comrgated sheetswhere positive electron-density anomalies near the Ol and 06 the SiOochains reside and reinforce the linkagesbetween sites on the final difference map. Nevertheless,the data the sheets. set available does not allow positive assignmentof the H SiOn tetrahedral chain positions(Nelson and Guggenheim,1993). The fully oc- cupied 03 and 04 sites, however, bond to a half-occu- There are two talSisites (Sil, Si2) in the channels;each pied Si2 site (Fig. 2, Table 6). Apparently, when the Si2 site is one-half occupied.The SiO" tetrahedralink to each site is occupied,the 03 and 04 sitesachieve normal bond other to form infinite wollastonite-type chains, the so- valences.When the Si2 site is vacant, however, the 03 called dreierketten.Figure 2 showsthe paired (SilOo) and and 04 sites are substantially underbonded, suggesting bridging (Si2O4)tetrahedra kinked so as to repeat at in- OH groupsat the 03 and 04 sites'Thus, the 03 and 04 tervals of three tetrahedra.These wollastonite-type chains sitesare half-occupiedby O atoms (Si2 site occupied)and were also found in foshagite(Gard and Taylor, 1960) and half-occupied by OH groups (Si2 site vacant), accounting (Hamid, l98l) and have beenspeculated to for the remaining one-third of the HrO in the structure. occur in many other calcium silicate hydrate phaseswith The anisotropic coordination environments around 03 a common short unit-cell translationof -3.64 A or 2 x and 04 may explain the large displacementfactors of the 3.64 A(Gard and Taylor, 1916 Garder al.,1977;Taylor, sites relative to other O sites (Table 2) and the abnor- 1986, 1992; Richardson and Groves, 1992; Cong and mally long Si2-O3(1.74 A)bond. Nevertheless,the X-ray Kirkpatrick, 1993). In each structural channel (Figs. I diffraction data do not support positional disorder for the and2), there are two symmetrically equivalent dreierket- 03 or 04 sites. ten related by the mirror plane perpendicular to a; how- ever, only one dreierkette can exist in a given channel to Rrr-lTEo srRUcruRES give reasonableinteratomic distances;that is, in any giv- en channel the Si and O atoms are ordered into one set Numerous X-ray diffraction studies and recent NMR of the dreierkette sites. This ordering schemeapparently studies have shown that the majority of CSH phasesin 844 DAI AND POST: CRYSTAL STRUCTURE OF HILLEBRANDITE

TABLE6, Bond valences (vu) for hillebrandite

o3 o6 07 '"- Cal 0.42 0.31 J 0.36'.- I 0.30 2..t5 Ca2 0.33"- J 029 0.15 0.26,'-I o.'17 1.79 Ca3 0.35 0.34"+ I 0.29 0.33"- J 1.98 't.07 ,, si1 1.04 1.01 1.01 I 4.19 si2 0.73 0.98 't.76 0.96"'- 3.63 2 1.08 2.O4 1.70 1.85 1.13 1.97 2.02 0.97. 0.87. Note: constantsarefromBreseandO'Keeffe(1991); 'Bond-valence thearrowsindicatethebond-valencesummationdirection. sums of 03 and 04 when Si2 sites are vacant.

steam-cured Portland cement have wollastonite-type among cement minerals as well as synthetic CSH phases dreierketten in their structures, characterizedby a com- with various Ca-Si ratios. mon translation of about 3.64 A or 2 x 3.64 A in one cell direction (Taylor, 1986, 1992: Richardson and AcxNowleDGMENTS Groves, 1992; Cong and Kirkpatrick, 1993). Natural The Smithsonian Institution is acknowledgedfor support through a minerals in the CaO-SiOr-HrO system can be arbitrarily Smithsonian Postdoctoral Fellowship to Y.D. Y.D thanks Harbison- divided into two groups: one with the common short axis Walker Refractoriesfor supporting the final srageof manuscript prepa- of about 3.64 A or 2 x 3.64 A (hillebrandire,, ration. TEM work was conductedat the Johns Hopkins University with assistancefrom Huifang Xu Thorough and constructive reviews of the tobermorite, foshagite,nekoite, etc.) and the other with- manuscript were provided by GeorgeE Harlow and Carl A. Francis. out this feature (afirillite, reyerite, , jaffeite, etc.). The phases in the first group, hereafter called cement R-nrnnnNcnscrrED minerals, are analogousor similar to the CSH phasesin Albeni, A., and Galli, E. (1980)The structureof nekoite,C-arSi6O,5.7H,O, steam-curedPortland cements. a new type ofsheet silicate.American Mineralogist, 65,1270-1276. The Ca-Si ratios of the cement minerals vary from 2:l Brese,N.E., and O'Keeffe, M. (1991) Bond-valenceparameters for solids. (hillebrandite) to l:2 (nekoite), which is within the range Acta Crystallographica,847, 192-197. '?O zeSi (2.5:I to l:2.5) of phases Cong,X.D., and Kirkpatrick, R.J. (1993) and MAS NMR study synthetic summarizedby Taylor p-C,S (1986) of hydration and the structure of calcium-silicatehydrates Ce- and Richardsonand Groves (1992). Among the ment and ConcreteResearch, 23, 1065-1077. numerous cement minerals only five structuresare known Donnay, G., and Allmann, R. (1970) How to recognizeO, , OH and experimentally: hillebrandite (this study), tobermorite HrO in crystal structuresdetermined by X-rays. American Mineralo- (Hamid, l98l), foshagite(Gard and Taylor, 1960),oken- gist,55,1003-1015. Fair, (1990) ite (Merlino, 1983),and nekoite(Alberri C.K. Molen: Structure determination system.Enraf-Nonius and Galli, 1980). Delft Instruments X-ray Diffraction B.V., the Netherlands. The wollastonite-type SiOo and Ca-(O,OH) polyhedral Gard, J.A., and Taylor, H F W. (1960) The crystal structure of foshagite chains parallel to the short axis are the common struc- Acta Crystallogmphica,I 3, 785-7 93. -(1976) tural units found in these minerals. The arrangementsof Calcium silicate hydrate (II) C'C-S-H(ID"). Cement and -67 the structural units vary with the ratios of Ca-Si. In ne- Concrete Research.6. 667 8. Gard, J.A., Taylor, H.F.W., Clifl c., and Lorimer, (1977) koite and , : G.W. A re- Ca-Si l:1.8 and 2.0, respectively, examination of jennite. American Mineralogist, 62, 365-368. two wollastonite-type SiO, chains are linked together to Hamid, S.A. (1981) The crystal structure of the I I A natural tobermorite form double chains. Some of the double chains intercon- Ca, ,5[SijO?5(OH)r 5] I HrO. Zeitschrift fiir Kristallographie, I 54, | 89- nect to form SiOo tetrahedral sheets.Between these tet- I 98. (1953) rahedral sheetsare the remaining Heller, B.A. X-ray investigation of hillebrandite Mineralogical SiOodouble chains and Magazine, 30, 150-1 54. all Ca-(O,OH) chains. In foshagiteand tobermorite, Ca- Ishida, H , Mabuchi, K., Sasaki, K., and Mitsuda, T. (1992) Low-rem- : Si l:l and l:0.8, respectively;instead of SiOotetrahe- perature synthesis of B-Ca,SiOofrom hillebrandite. Journal of the dral sheets, Ca-(O,OH) polyhedral sheets sandwich the AmericanCeramic Society, 75, 2427-2432. Merlino, S. (1983)Okenite, Ca,oSi,.Oou SiOo wollastonite-type chains and the remaining Ca- l8HrO: The first exampleof a chain and sheetsilicate American Mineralogist, 68,614-622. (O,OH) chains. : In hillebrandite, with Ca-Si 2:1, the Nelson, D.O., and Guggenheim,S. (1993) Inferred lirnitations ro rhe ox- Ca-(O,OH) polyhedral chains interconnect into a three- idation of Fe in chlorite: A high-temperaturesingle-crystal X-ray study. dimensional network in which the SiOodreierketten chains American Mineralogist, 78, | 197-1207 . are accommodated. Thus, there are at least three major Richardson,I.G., and Groves,G.W. (1992)Models for the composition and structure of calcium silicate hydrate (C-S-H) gel structure types among the in hardened tri- cement minerals with Ca-Si calcium silicatepastes. Cemenl and ConcreteResearch. 22, l00l-1010. ratios varying from 0.5 to 2.0, representedby nekoite Taylor, H.F.W. (1986) Proposedstructure for calcium silicatehydrate gel. (0.5),tobermorite (0.8),and hillebrandite(2.0). The Ca- Journal of American Ceramic Society,69, 464-467 -(1992) Si ratios that mark the transition points from one type to Tobermorite, jennite and cement gel. Zeitschrift fiir Kris- tallographie, 202, 4 -50. another are not known. Further structure investigations l of other known cement minerals are essential for ad- M,rrlt;scRrpr RECETVEDJuxe 23, 1994 vancing our understandingof the structural relationships M,c.NUScRrFrAccEprED MencH 29. 1995