American Mineralogist, Volume 80, pages 841-844, 1995 Crystal structure of hillebrandite: A natural analogueof calcium 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 Mineral Sciences,Smithsonian Institution, Washington, DC 20560, U.S.A. ABSTRAgI The crystal structure 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-dimensional network of Ca-O polyhedra that accommodateswollastonite-type Si-O tetrahedral chains. Each of the wollastonite-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 calcium silicate hydrate (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 minerals 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 concrete 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 Space group 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.