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Crystal Structure Refinement of Lawsonite

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Crystal Structure Refinement of Lawsonite American Mineralogist, Volume 63, pages 3l l-315, 1978 Crystalstructure refinement of lawsonite WnRNTRH. Baun Department of Geological Sciences, Uniuersity o.f Illinois Chicago, Illinois 60680 Abstract 6rAl, Lawsonite,Ca " SirrlO7(OH)r.HrO, is found in low-temperature,high-pressure meta- morphic rocks. It is a hydrous counterpart of anorthite. A specimenfrom the type locality, Tiburon Peninsula,Marin County,California, was used for an X-ray study:a :8.795(3),b: 5.847(l),c : 13.142(6)A,V: 675.3A,Z - 4, Dcat.:3088 g.cm-3,space group Ccmm. Refinementof 865 F"m gave an R : 0.0256.The structure is basedon a three dimensional framework generatedby cross-linkingribbons composedof edgesharing singlechains of Al coordination octahedraand lateral bridging silicategroups The openingsof the framework accommodatethe Ca atomsand the watermolecules. The observedmean distances are Si ' - O : 1.633,Al' O : 1.913and Ca' - O - 2.421A.Thehydrogen atoms parriciprre in bent and bifurcatedhydrogen bonds. Individual cation-anion distances conform well to the exLendedelectrostatic valence rule. Introduction unit cell constantswere refinedfrom the settingof20 The crystal structure of lawsonite, CaAlrSirO, reflectionsmeasured on a singlecrystal X-ray diffrac- (OH)r.HzO, was determinedby Wickman (1947) tometer. The resulting cell constants agree closely in space group C222,. The Si-O bond lengths in with thosereported by Davis and Pabst(1960). Re- the SirO, group as found by Wickman rangedfrom ciprocal space was searchedin a sphere of radius 1.59to 1.72A. Subsequently,the structurewas re- sind/tr : 0.844-'. The total number of measured fined by Rumanova and Skipetrova (1959). They reflectionswas I1,268,which wereaveraged to yield found that it could be describedin the centric space 1,928unique reflections.Of these 1,063had an in- group Ccmm and that the Si-O distanceswere lying tensity of lessthan two sigma, and were not used in in a narrow range:between 1.65 and 1.69A.Recently the refinement. The systematic absences(hkl only Pabst( 1977)pointed out that lawsoniteis denserthan presentwith I -t k : 2n; and Oklonly with I : 2n) are anorthite,its anhydrousequivalent. The volume per consistent with space group Ccmrn as noted by oxygenatom in lawsonireis 18.74A,while in anor- Rumanova and Skipetrova. The successfulrefine- thite it is 20.944. Lawsoniteis one of severalespe- ment confirms this choice. Lorentz-polarizationcor- ciallydense hydrous silicates found in the Franciscan rectionswere applied,but becauseof the small linear Formation,California. It is typicalof a low-temper- absorption coefficientand the size of the crystal, an ature, high-pressuremetamorphic paragenesis. absorption correction was not necessary.Scattering factors were taken from the Internqtionql Tablesfor Experimental X-ray Crystallography (1974). The refinement of the 865 observedunique structure factors started with the A clear, almost colorless specimen of lawsonite parametersreported by Rumanova and Skipetrova from the type locality, Tiburon Peninsula, Marin (1959)and refinedin a few cyclesto an R (:>llF'ol County, California (USNM R3922)was divided into -lrcll />lrol) of 0.027(with anisotropictem- two parts; one was ground into a spherewith a diam- perature factors). A difference synthesis revealed eter of 0.04 cm, the other was used for a microprobe likely positionsfor the hydrogenatoms. Upon fur- analysis.Data were collectedon an automatic four- ther refinementincluding the hydrogen atoms an circle diffractometer (trAgKa = 0.560834), using R of 0.0256was achieved.The positional(Table l) proceduresdescribed by Baur and Khan (1970).The and the thermal (Table 2) parameterswere used to 0003-004x/78/0304-031l$0200 3t I 3t2 BAUR,' REFINEMENT OF LAWSONITE Table I Lawsonite,positional parameters* 0.33 TiOr, traces of MnO and KrO. The sum in- cluding 2HrO is 98.54 percent. This corresponds to Caon.,(Al,.ourFeo.o36Tio or3)Si2.orrOr(OH)r. HrO and shows that the crystal is close to the ideal composi- Ca 4m 0.3330s(s) 0 \ tion CaAlrSirO?(OH)r.HrO.This analysiscorrobo- A& 81 ,, ,4 0 si 8m 0.98040(s) 0 0.13298(3) rates observations made on lawsonitesfrom other localities(see Davis and Pabst,1960; Ernst eI al., l97O). o(r) 4m 0.049s(2) 0 \ o(2) 16 1 0,3788(1) o.27 26 (r) 0.11690(6) o(3) 8n 0.1378(1) 0 0.06s05 (8) o(h4) 8m 0.6391(1) 0 0.0479(1) o(ws) 4m 0.6091(2) 0 t4 Description of structure H(w) 8n 0.639 (4) 0 0.re4(3) The individual non-hydrogenatom positionsdeter- 11(oh) 8n 0.587(6) 0 0.0s6(4) H(w)geon. 8n o.673L 0 0,1905 mined in this refinement differ on the average by H (oh) el . 8n o.5329 0 0.0704 0.05A lrom the positionsfound by Rumanovaand Skipetrova(1959). The singlelargest discrepancy was * Estimated staDdard deviations are listed in paren- for which has shiftedin theses in units of the least significant digit. The observed O(h4) the courseof numbers of equivalent posj-tions and their point syn- the refinementby 0.134. The averagechange in the metries are given for every atom. For atons H(w)- geom and H(oh) . see text. individualCa-O, Al-O, Si-O, O(h4)-O and O(w5)- EI O distancesis 0.05A. The structure can be conveniently described as composedof edgesharing chains of Al coordination calculate a final set of structure factors (Table 3'). octahedra(with Al at y : 0.25 and 0.75) arranged The root-mean-squarethermal displacementsand parallelto the [010]direction (Fig. l). The octahedra their orientationsare recordedin Table 4. in thesechains sharethe edgeO(3)-O(ha). The O(2) Crystal data vertices(which are not in the sharededges) of neigh- boring octahedra are laterally bridged by silicate Cell constants4 : 8.795(3),b : 5.847(l), c : groups which alternateat heights_t : 0 and 0.5. The l3.la2(6)A; V : group 675.8A3;space Ccmm, D"^t" symmetry of this ribbon of octahedraand tetrahedra : 3.0889.cm'; Z : 4: p(AgKa) : 8.05cm-l; : trrR is 2, and its compositionis AlrSiO,o(OH)r.Similar 0.161. A microprobe analysisyielded (weight per- ribbons have been found in vauxite (Baur and cent):31.09 Al,O3, Fe,O,, 17.18 37.75SiO,,0.90 CaO, Rama Rao, 1968) where their composition is 'To AlFePO,(OH),. Gottardi (1968)has described analo- obtain a copy of this table order Document AM78-067 gous ribbons in ilvaite, epidote, pumpellyite and irom the BusinessOffrce, Mineralogical Society of America, 1909 K Street,N.W., Washington,D.C.20006. Pleaseremit $l 00 in chevkinite.He makes the point that theseoctahedral advancefor the microfiche. chainswith lateral bridging tetrahedracan be consid- Table 2 Lawsonite,anisotropic thermal parameters* for non-hydrogenatoms, isotropic thermal parameters[or hydrogenatom" Atom Bt, or B -22R R -12R R, R IJ "23 Ca 0. 00300(s ) 0. 00530(10) 0 .00133(2) 0 0 0 A,Q, 0. 001s0(s ) 0. 00309(1r) o. 00106(2) -0. 00012(s ) -0. 00007(2) 0.00003 (4) Si 0.00140(4) 0. 00343(10) 0. 00099(2 ) o. 00002(2) 0 o (1) o.ooz3(2) 0 .0081 (4) 0.0009(1) 0 0 0 o(2) 0.0024(1) 0.oo47(2) 0.0014 (1) -0. 0008(1) -0. 0003(1) o. 0004(1) o(3) 0.001-7 (1) 0.oo42(2) 0.0013 (1) 0 0. 0003(1) 0 o(h4) 0.0019 (1) o .0048 (2) 0. 0018(1) 0 0. 0004(1) 0 o(w5) 0.0040(2) 0.0226(7) 0.0014(1) 0 0 0 H(w) 2.6(O.9) H(oh) 7.0(1.6) * The anisorropi.c rhermal paramerers are defined by exp[-(Brrt.2 + g2|k2 a B3zL + 2ltZhk + 2Btgfrl + }lzSkl)] BAUR, REFINEMENT OF LAWSONITE 313 Table 4. Lawsonite, thermal ellipsoidsx mined here, with those reported by Haga and Take- uchi (1976)for ilvaite(in parentheses):8.795(8.818), Angle with axis Atom Axis Displacement 5.847(5.853)and 13.142(13.005 )4. Strunzconcluded [1oo] [oro] [oo1] that lawsoniteand ilvaite are isostructuraldespite the fact that their spacegroups are different (Pcmn for caI 0.096(r) 900 90 ilvaite versusCcmm for lawsonite).The similarity in 2 0.108(1) 90 90 0 3 0. r09(1) 090 90 the b cell constant is due to the length of the repeat A.t 1 0.o72(r) 64(ro) 26(10) 89(2) unit of the octahedral-tetrahedralribbons. It corre- 2 0.07 7 (1) rs4(9) 6s(10) 97(2) 3 0.097(1) e6(2) 88(2) 7(2) sponds to the height of two octahedra sharing an edge. Similar repeat units ranging in length from 5.6 si1 0.074(1) 2(2) 90 92(2) 2 0.077(1) 900 90 to 5.9A can be observedin epidote,pumpellyite, per- 3 0.093(r) 88(2) 90 2(2) rierite and chevkinite.The similarity of the other two o(1) 1 0.091(3) 90 90 0 2 0,095(3) 090 90 cell constants,however, is fortuitous becausethe ar- 3 0.rr8 (3) 900 90 rangement of the ribbons relative to each other is o(2) 1 0.081(2) 53(4) 37(4) 89(4) clearly different in lawsonite and ilvaite as has been 2 0.095(2) r27(4) 62(5) 130(3) shown by Gottardi (1968). 3 0.119(1) r2t-(2) 67(2) 40(3) o(3) 1 0.078(2) 16(3) 90 r06(3) The relatively high density of lawsonite as com- 2 0.08s(2) 900 90 pared to its anhydrouscounterpart anorthite (Pabst, 3 0.108(2) 74(3) 90 16(4) 1977) is related to the fact that Al is octahedrally o(h4) 1 0.083(2) rs(2) 90 105(2) coordinatedin lawsonitebut only four coordinatedin 2 0.09r(2) 900 90 3 0.L28(2) 75(2) 90 ls (2) anorthite.
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