American Mineralogisl, Volume 65, pages j27-334' 1980

The crystal structuresof whewelliteand :re-examination and comparison

VITToRIo Tszzorl AND CHIARA DotvtBNEcgBrrt C.N.R. Centro di Studioper la Cristallografia Strutturale c/o Istituto di Mineralogiadell'Universitd, Pavia' Italy

Abstract

Whewellite, CaCrOo'HrO, and weddellite, CaCzO+'(2 + x) HrO where x = 0'5, occur in sediments,in plants, and in urinary stones' : Their crystal structureshave been refined to R : 0.033and R 0.032respectively, using new setsof X-ray diffraction data, collectedon a single-crystaldi-ffractometer. Refinedcell parametercare'. Y2,/c, a:6.29O(l), D: l4'583(l),c: 10'116(l)A'B: : 109.46(2)",Z : 8 for ;I4/m, a : 12.371(3),c :7'35't(2)A, Z 8 for weddellite' po- During refinementof whewellite,three out of four H atomscould be located,and split weddel- sitions,p".tiutty occupied,for the two independentwater moleculeswere found. In lite refinement, it was possible to locate all the H atoms, and a split position for the "zeolitic" water was found: a maximum water content of 2'5 HrO was confirmed' The comparisonof the structuresexplains the relationshipsexisting between some repeats A pos- of the two mineralsand showsthe differencesbetween the Ca coordinationpolyhedra. two sible correlation betweenthe structural featuresand the mechanismof formation of the speciesis suggested. The symmetry and planarity of the oxalate groups are discussed'

Introduction X-ray diffraction data. Table I presentsthe experi- menial details. On the crystalsof both speciesthree Whewellite and weddellite are hydrated oxalates standard reflections monitored at three-hour inter- of , respectively CaCrOo' H'O and vals showedless than 3.5 percent intensity variation CaCrOo'(2+x)HrO (with x = 0.5), found naturally in plant tissues,in sedimentsas a mineral of organic origin, and in urinary stones.About 70 percentof hu- man urinary stonescontain whewellite and/or wed- dellite, either singly or mixed with other components, mostly phosphates,uric acids,or urates. The frequency with which the two are of figures by Germain et al. (1971), S'u5ingef a/' found in associationin natural sourcessuggested a (1g62),andJohnson (1965) were employed' The scat- re-examination and a comparison of their crystal iering curves for neutral atoms given by the Inter- structureas the first stageof an investigationon their national Tablesfor X-ray Crystallography(1974) were geneticrelationshiPs. used.High-precision methods using an coscan of four Previous structure analyseswere carried out by intense reflectionsfrom each of severallattice rows Cocco (1961)and Cocco and Sabelli (1962)for whe- provided accuratedata for unit-cell parametercom- wellite (two-dimensional photographic data, R : putations. 0.14) and by Sterling (1965)for weddellite (three-di- mensionalphotographic data, R : 0.13). Resultsof the structure refinements

Experimental VVhewellite of the cell axes Single crystals of whewellite and weddellite were The orientation and the dimensions of the two alterna- obtained from urinary calculi. A Philips PW ll00 (Table l) are consistentwith one et al' (1965) single-crystaldi.ffractometer was used to collect the iive orientations proposed by Arnott 0np3-ffi4x/ 80/ 0304-0327$o2.oo 32'.1 328 TAZZOLI AND DOMENEGHETTI: WHEWELLITE AND WEDDELLITE

Table l Crystal and diffraction data from them). The occupancyof thesepositions W(10) W(20) Pnoperty Whewellite Weddell ite and and, in turn, that of the main positions W(l) and W(2) were refinedwith other cyclesof least squares,together with the Fo Fmula CaC2O4.HZO caC^O,.. (2+x) H^O coordinatesand the ani- z1 z sotropic thermal factors of the non-hydrogenatoms. (x < 0.5) Fixed isotropic Space gnoup Pz t +/n temperaturefactors were usedfor the /c atomsH(ll), H(21), H(22), a (A) 6. 2eo(I ) 12.37 t(31 W(10), and W(20). The b (A) 14.583(| ) final atomic parametersare shown in Tables2 and 3;

c (A) lo. il6(l) 7.3s712) bond lengths and angles in Table 4. The observed B(') to9. tt6(zl and computed structure factors are compared in Table v(a3) 476.22A 1 125.927 5.' The final value of the discrepancyindex z was R : 0.033for the 2864 observedreflections. dimensions(mm) Cnystal O.36xO.36xO.20 O.3OxO.25xO.ZO lleddellite Radiation (A) l\4oKo,l=O.?1069 Mo Kc,i=o.Z1069 Monochrcmator gnaphite graphite The values of the cell parameters(Table l) agree

p {..-l) r3.o ro.S well with thoseof Sterling(1965). The atomic coordi- Scan width (.) 2.5o t.oo natesof this author, with the exclusionof those rela- -l) scan speed (os o. 1".-l o.o25o s-l tive to oxygen of the "zeolitic" water, were used to tlrange(') 2-4o 2_go start the refinement. During the first cycles, carried Maximum (sinfll/L o.so444 o.?o353 out using isotropic thermal factors, the examination -kh Measuned reflections 5l5Z t?99 (h k l, l) of the difference Fourier map confirmed the presence lndependent reflections 5152 Sg9 of zeolitic water, with incomplete occupancy,in the Observed reflections 2A64 60z channel running along the four-fold axis, with (whit l) 3o t) coordinatessimilar to thosegiven by Sterling. In the last stagesof refinement,difference Fourier maps al- lowed us to locate the hydrogen atoms of the two moleculesof non-zeoliticwater and to find a split po-, (spacegroup F2,/c). The valuesgiven for c and by B sition W(30) for the oxygen of the zeolitic water, the Cocco (1961) are not consistentwith y\r/c orienta- occupilncy of which was refined alternately with that tion assumedby this author, but seemto agreebetter of the main position W(3) (he distancebetween the with a P2,/n oientation (Leavens,1968). two positions was 0.574). Fixed isotropic temper- The , re-determinedby means of ature factorswere usedfor the atomsH(5), H(6), and the direct methods,is consistentwith that published W(30). The final atomic parametersare given in Ta- by Cocco (1961)and by Cocco and Sabelli (1962). bles 6 and 7, lengthsand bond anglesin Table 8. The Isotropic temperaturefactors were usedin the first observed and computed structure factors are com- least-squarescycles of the refinement; successively pared in Table 9.'The final value of the discrepancy the atomswere treatedanisotropically. A pseudo-cell index wasR : 0.032for the 889observed reflections. with D': b/2 (the intensitiesof the reflectionswith /c even are far greaterthan thoseof the reflectionswith Discussionand comparisonof the structures k odd) makes the atoms of the pairs O(l)-O(3), o(2)-o(4), Ca(r)-Ca(2), C(3)-C(4), 0(6)-0(7), lMhewellite O(5)-O(8), and W(l)-W(2) pseudo-equivalent(see In whewellite (Fig. l) the coordination polyhedra Fig. I and 2). In order to avoid correlation effects of the pseudo-equivalentatoms Ca(l) and Ca(2) are among the variables during calculations, the coordi- distorted squareantiprisms: in eachof them sevenof nates and thermal factors of the first or second atom the oxygensbelong to five oxalic groupsand one to a of a pair were refined separately, as were the scale water molecule. Each Ca polyhedron shares three factor and the coefficient of secondary extinction edgeswith three adjacentCa polyhedra. In this way (Zachariasen,1963). From a differenceFourier map, three of the four hydrogen atoms were located and I two maxima, double the height of that of the hydro- To receivea copy of Tables5 and 9, order Document AM-?9- 116 from the BusinessOffice, Mineralogical Society gens, were interpreted as split positions of the of America, two 2000 Florida Avenue, NW, Washington,DC 20009.please remit water molecules(respectively 0.73 and 0.394 away $1.00in advancefor the microfiche. TAZZOLI AND DOMENEGHETTI: WHEWELLITE AND WEDDELLITE 329

Fig. l. Whewcllite, sectionalmost parallel to (100).

Fig. 2. Whewellite, sectionparallel to (010). 330 TAZZOLI AND DOMENEGHETTI: WHEWELLITEAN D WEDDELLITE

Table 2. Atomic coordinates, occupancies, and equivalent polyhedral layers are formed parallel to (100), isotropic temperature factors of whewellite* the good plane. Within the layers, Ca atoms oc-

Atom Occ. v/6 g*ta') cur at the vertices of hexagons which have at their center one of the two independent oxalic groups. c(l) I o.9e3z{z) o.32ot( t) o.2452(2\ 0.80 These oxalic groups lie in the (100) plane with the c(2) I I, OOO9(2) O.4270(11 o.2492(1) o.77 c(3) I 0.5 189(2) 0. 1266(l) o. rs12(l) t. 07 C(l)-C(2) bonds nearly parallel to the D axis. The c(4) I o.4505(2) o. I t73(l) o.3l3l(l) t. o8 layers are connected to one another through the sec- o(l) r 0. 9756(2) O.2826( I ) o.1322(l) l, l6 ond series of oxalic groups and the water molecules o(2) I t.0066(2) o.465e(l) o.1395(t) 1.14 (Fig. 2): oxalic groups alternate with the water mole- o(3) | o.s799(2) o.2ele(l) o. 3sso( I ) l. t7 o(4) I 1.Oo7312) 0.4658(l) 0.36r4(t) 1.21 cules and form ribbons lying in the (010) plane and o(s) I o.3614(2) O.l4rs(l) o. 069o(I I 2.20 running along c. Bonding 0(6) I o.72tt5(2) O.1227(tl o, t974( I ) l. l6 between the pseudo-equiv- o(7) I 0.2438{2) o. tzzgltl o.295?(tl l.3r alent water molecules W(l) and W(2) and the o(e) I o.6073(21 O. l06S(l) o.4264(1) 2.24 pseudo-equivalent oxygens O(8) and O(5) is through ca(t) I 0.9676(l) 0.1243(l) o. 0546(I ) o.67 the pseudo-equivalent hydrogen atoms H(l l) and ca(2) I o.ee68(l) o.1236(l) o. 4357(r ) 0.66 H(21). The bond between the two water molecules w(l) o.8s o. 3e32(3) O.345s( I ) o. to22(21 1.81 takes place through hydrogen H(22). The hydrogen w(2) 0.86 o.5e l3(3) 0.3829(3) 0.3908(2) 4.4A atom H(12), which probably does not take part in hy- Hill) I o.4a7 0.372 O.05 I ( 5. oo) drogen bonds and which, in any case, cannot respect H(21) I o.5 ro 0.364 0.426 ( 5. oo) H(221 1 0.530 0.367 0.320 ( s. o0) the pseudo-symmetry with H(22) for steric reasons, was not identified. w(lo) o. 15 0.38S O.396 0.099 (r, so) w(20) o. 14 0.584 0.409 0.392 (4.5o) Weddellite Standard deviations in panentheses; equivalent isotropic tempe- In weddellite nature factons aftep Hamilton (1959). the calcium coordination polyhedron is a distorted square antiprism but, of the eight oxy-

Table 3. Analysis of the anisotropic thermal parameters in whewellite*

n. m. s. U.a u.b U.c Atom I U.a Uio U.c I I c(r) o.062 96 172 84 o(5) 0. r05 t07 a2 143 o. t02 r la azil o. tlo t7 89 126 o. 126 15 I 86 99 o.246 9l 172 97 c(2) o. o7o 98 l7o o(6) o. o88 27 9l r36 o. l03 86 898 0. 114 63 97 47 o. 117 154 89 96 o. 154 94 173 94 c(s) o.096 22 89 r3z o\z) o. o85 170 e8 60 0. ltr OU 88 42 o. 117 80 86 30 o. t38 OU 17A 89 o. 170 9t 176 do c(4) o. oee 37 89 146 o(B) o.104 32 90 141 o. I 12 53 e6 56 0. l09 5U 95 52 o. 143 OT 176 89 o.250 93 175 93 o( l) o. oBo 72 I 15 155 ca(r) o. 074 59 90 r68 o. t02 7e 25 ll5 o. o92 t02 14 93 o.165 r58 86 92 o. t07 r46 r04 lol o(2) o. oBl OU 6t 150 ca(2) o. 072 58 90 167 o. t00 ro0 29 6l 0. o95 89 I 90 0. 163 155 88 95 0. r05 r48 89 lo2 o(3) o. o82 70 69 159 w( l) o.t12 63 t3 162 o. 098 93 2r 69 o. 14l 2A lol a4 o. t68 r60 85 9l 0. 19l 93 t6 | t07 o(4) o.082 74 t24 t46 wel 0. l09 c0 92 r65 o.105 72 35 124 o.123 34 oo 76 0.168 t56 a4 94 o.379 90 177 88 * Root mean squane thenmal vibnations along the ellipsoid axes (A) and angles (o) between the cnystallo- gnaphic axes and the pnincipal axes(U.)of the vibFation ellipsoid.E.s.d. of n.m.s.nangefnom o.ool to 0. oo3 331 TAZZOLI AND DOMENEGHETTI: WHEWELLITE AND WEDDELLITE

occupancies, and equivalent Table 4. Interatomic distances (A) and bond angles (o) in Table 6. Atomic coordinates, of wedf,ellitet whewellite (standard deviations in parentheses) isotropic temperature factors 2) z/c pH(A Di stances Atoms D i stanc es Atom Occ. v/o

I ) 2.452(2\ ca(r)-o(r) 2.434\21 ca(2)-o( 0.4464(2l O.241sl2t o. lo53(3) t-23 -o(3) 2. 46 1(Z) -o(2) 2.42O\2't _o(4) -o(2') 2.425(2) 2.449(2\ o(1) o.3564(r) 0.2458(l) O. ls29(2) tlrt6 -o(4') -o(3, 2.$a(21 2.435(21 o(21 o. 23s5(2) o. 4634( I ) O. tTee(z| 2.7e -o(5) 2.446\1) -o(6) 2. t+5011) _o(z) 2.42511') -o(6) 2.427(1'r 0.1993(l) O.30ll(l) o.o o.9? -o(7) z.481ltl -o(8) 2.432\1t -w(r) 2.542\2'J _w(z) 2.3ee(2) w(l) r o. l49O(3) o. | 145(2) o.0 2.75 w(z) 1 o.ote2(21 o.3841(3) 0.o 2.97 7.20 Avenage 2.442 Avenage 2.444 w(3) 0.58 0.o 0.o o.6s3o(le)

H(l) I 0. 166 o.07l o. lo5 ( 5. oo) ( H(2) I 0.350 0.018 o.r15 5. oo) c( r)-c(2) 1,563(2) c(3) -c(4) 1.538(2) -o(5) 1.25412J c(r)-o(r) 1.25412) c(3) w(3O) O. r 7 O.O o. o o.24o ( z. Oo) -o(3) 1.24s121 -o(6) 1.250(2\ c(2)-o(2) 1.257\2) c(4) -o(7) 1.25512) -o(4) 1.2s712) -o(s) 1.24e(2) + Standard deviations in panentheses; equivalent isotrcpic tem-

oenature factons aften Hamilton (1959). Avenage 1.255 Average t.252

w( 1)-o(s) 2.654(2\ H(21)-o(s) l. e6 -w(2) 2.ezolzl -w(2) o.77 gens,six belong to four oxalic groupsand two to wa- w(2)-o(5) 2.683(2) with respectto whewellite, calcium is 2. 11 ter molecules: H(22)-w( l) one more H(rl)-o(e) 1-7o -w(2) O.73 coordinated to one less oxalic group and -W(l) o.98 water molecule.If we assumethat in solution the for- mation of calcium oxalates and their precipitation hydration Angl es Angl es take place by substitution, in the Ca'* sphere, of the water molecules with oxalate ions, o(r)-c(r)-o(3) 127.55\17) o(5)-c(3)-o(6) 126.7s(16) weddellite should be the first stageof this process. o(l)-c(l)-c(2) I 16,07( r4) o(5)-c(3)-c(4) ll5.el(13) o(3)-c( I )-c(2) ll6.38(r4) 0(6)-c(3)-c(4) 117.25(13) Each polyhedron of calcium in weddellitemakes a o(2)-c(2)-o(4) I 26. 33(1 7) o(z)-c(4)-o(8) 127.07(t7l two adjacentpolyhedra (Fig. 3), changingthe o(2)-c(2)-c( I ) 1 16.74\14) o(?)-c(4)-c(3) il6.5e(14) link to o(4)-c(2)-c( r) I r6. e2(r 4) o(8)-c(4)-c(3) I r6.32( l4) layers found in whewellite to chains running along c H(21)-w(2)-H(22) 97. Z (axis 4). The link between these chains is made by

Table 7. Analysis of the anisotropic thermal paramet€rs in weddellite*

tth Atom n. m. s. U.a u.b n. m. s. U.a U.c I I I I I

90 o. lo9 80 95 169 w(r) o.143 95 174 c o 0. | 18 15 100 79 0. 152 90 90 84 90 o. rM t0l 168 d0 o.247 174 90 o. 106 [30 e7 40 w(2) o. 153 r53 63 o(t) o 0. r23 42 lo2 50 0. r83 90 90 r53 90 o. l7 l l02 167 9C o.236 r16 90 o(2) o. roe 89 139 49 w(3) o.2a7 e7 3 to3 t3 90 o. l3l OJ 49 41 o.2a7 90 180 o.277 173 UO oc o.330 90

Ca 0.104 90 90 180 0.109 tl6 26 90 ( o. I te) 153 116 90

o (A) angles (o) between the cnystallo- Roo, ...r squane thenmal vibnations along the ellipsoid axes and nangefnom gnaphicaxesandthepnincipal axes(U.)of thevibnationellipsoid. E.s.d. of n.m.s.

o. o0 I to 0. o04 332 TAZZOLI AND DOMENEGHETTI: WHEWELLITE AND WEDDELLITE

Table 8. Interatomic distances (A) and bond angles (") in The repetition around the four-fold axis of the Ca weddellite (standard deviations il parentheses) chains and the oxalate-waterribbons generate a channel (Fig. a) in which zeolitic water occnrs in two Di stanc es Distances alternativeand very closepositions, W(3) and W(30). ca-w(r) 2.391(3) o (2)-w(l) 2. e | 8(3) The distancebetween two W(3) water moleculesre- -w(2) 2.453(3) -w(2) 2. 866(3) -O(21x2 2.446(2\ lated by a symmetry center correspondsto that of a -o(r) x2 2.461(2l H (l) -o(2) 2.04 strong hydrogen bond; the closestcontacts of water -o(lflx2 -w(r) Z.50t(2\ o.96 W(30) occur with four non-zeolitic W(l) waters.The H (2) -o(2) r.86 Avenage 2.45A -w(2) l. os sum of the occupanciesof the two alternative posi- : c- c 1.549(4) w(3) -w(3) 2.6e (3) tions W(3) and W(30) turns out to be Occ 0.58 + -o(2) x 4 3.303(2) 0.17 : 0.75, which, taking multiplicity into account, C-o(l) x 2 1.252(3\ -W(l) x4 3.293(e) -o(2) x2 t.246(3) brings the contribution of zeolitic water to 0.37;thus w(30)-w(30) 3.53 -O(2) x 4 3.35 the formula of the weddellite examined is Average 1.249 -w(r) x 4 2.92 CaCrOo'2.37H2O. Sterling (1965) observed that the multiplicity of the position of the zeolitic water limits the maximum water content of weddellite to 2.5HrO; however, G6rard et al. (1968)later postulatedthe existence Atoms Angl es Angl es of weddellitewith three moleculesof water. on the basis o(2)-c-o( t) 126.67(1al H( t)-w( l)-H( t) 106.s4(271 of thermal analysesof sampleskept in water-vapor- o(2)-c-c 116. 15(l2) H(2)-w(2)-H(2) l07. I t(29) o(r)-c-c r't7. t7lt1l controlled atmosphere.In order to checkthis hypoth- esis,a systematicresearch of possiblecavities existing in the structure was carried out; two "holes" (with . .oxalate-water-oxalate. means of (. . .) ribbons coordinates0, 0.50, 0.25 and 0, 0, 0) were identified, lying in planesparallel ro (100). In theseribbons the but they appear too small to hold water molecules, water molecules W(l) and W(2) and the oxygens which in this casewould have too short contacts(ess O(2) are bonded through hydrogensH(5) and H(6). than 2.354) with other water molecules.Therefore a

Fig. 3. Weddellire,secrion parallel to (100). TAZZOLI AND DOMENEGHETTI: WHEWELLITE AND WEDDELLIT:E 333

Fig. 4. Weddellite, section aknost parallel to (001). greater degreeof hydration implies another crystal- lengthsand anglesin eachgroup were submittedto a line phase,which could be that of the trihydrate hy- significancetest (Stout and Jensen,1968, p. 419421). pothesizedby someauthors (Walter-Levy and Lanie- It was found that: piece, 1964; G6rard et al., 1968; Doremus et al., (a) in whewellitethe oxalic group C(l), C(2) . . . ap- r9't6). pears to be planar, with a symmetry not higher than 2mm, and the oxalic group C(3), C(4)... Planarity and symmetry of the oxalic groups appears to be significantly non-planar, with a The deviations of the atoms from the least-squares symmetry 1; ptane (Table l0) and the difference among the bond (b) in weddellite the oxalic group appears to be

Table 10. Deviations from planarity in oxalic groups

- Weddell ite Whewellite - Plane I Whewellite Plane Il

Equat ion of l€st- 5.gg22x-o.9465y-o.06842-o,2622x+14.42o2y+1.22412-o.7o45x+12.35o9y+o.o0ooz- .the -2.2035=O -3'2891=0 - squares plane -5.5155=o

0.9888 0. l2l0 o. 0569 0.9984 0. oooo Dinect ion cosines o.943r-0.0649-0.0068 o.o417 c( l) o. ooe(2) (A) of c(l) - o.003(l) c(3t -o.o2o(r) Displacements r) 009(2) c\21 o.ool(r) c(4) -o.olr(r) c(l o. atoms fnom the Plane -o.oo2(z\ o(l) - o.005(1) o(5) o.02l(r) o(l) ') - o. oo2(2) o(21 o. 005( I l o(6) -o.oo3(r) o( I - o(3) o.006(r) o(7) -o.oos(r) o(21 o. oo2(2) - o(4) - o. 006(r) o(8) o.ot2(r) o(21) o.oo2(21

+ Refenred to the cnystal axes; x'y' z are fFactional coondinates 334 TAZZOLI AND DOMENEGHETTI: WHEI,YELLITE AND WEDDELLITE

Table 11. Significant repeats in whewellite and weddellite compoundson which investigationsare in progressin this laboratory. References Arnott, H. J., F. G. E. Pautard and H. Steinfink (1965) Structure ofcalcium oxalatemonohydrate. Nature, 208, 1197-1198. Busing,W. R., K. O. Martin and H. A. Levy (1962)Onrls, a For- tran crystallographicleast-squar€s program. U.S. NaL Tech.Inf. Serv.,ORNL TM-305. Cocco, G. (1961) La struttura della whewellite.Atti Accad. Naz. (seeFig. 2) Lincei Rend. Cl. Sci.fu. mat. nat., 31,292-298. - and C. Sabelli (1962) Affinamento della struttura della whewellite con elaboratoreelettronico. Atti Soc. Tosc. Sc.Nat., (seeFrg. 3) A,3-t2. Doremus,R. M., G. L. Gardner and W. McKai (1976)Crystalliza- tion of calcium oxalate in various media and urolithiasis. Col/a- quium on renal lithiasis,18-32. The University PressofFlorida, Gainesville. Gdrard, N., G. Watelle-Marion and A. Thrierr-Sorel(1968) Etude sous pressionde vap0ur d'eau control6e de la d€shydratation planar, with a symmetry not higher than 2mm des oxalatesde calcium hydrat€s.Bull. Soc. Chim. France, II, 43674378. and not lower than m (crystallographic symme- Germain, G., P. Main and M. M. Woolfson (1971)On the appli- try); mmm symmetry as proposed by Sterling cation of phaserelationships to complexstructures. III. The op- (1965)must be excluded,consistent with the re- timum use of phaserelationships. Acta Crystallogr.,A27, 368- markable difference between the environments 376. and bondsof O(l) and O(2). Hamilton, W. C. (1959)On the isotropictemperature factor equiv- alent to a given anisotropic temperature factor. Acta Crystal- Conclusions logr.,12,609-610. Hornstra, J. and B. Stubbe(1972) PW ll00 Data ProcessingPro- The comparisonbetween the crystal structuresof gram. Philips ResearchLaboratories, Eindhoven, Holland. weddellite and whewellite reveals marked sinilar- International Tablesfor X-ray Crystallography(1974) Vol. IV. Ky- noch Press,Birmingham. ities, which are in agreementwith the frequent asso- Johnson,C. K. (1965)Ontrp. Oak RidgeNational Laboratory Re- ciation of the two minerals and with the easy trans- port ORNL-3794. formation of the former into the latter. Leavens,P. B. (1968)New data on whewellite.Am. Mineral., 53, A simple schemeof linear dimensions(Tdble I l) 455463. showsand explains,with the help of the figures,the Lonsdale,K. (1968)Epitaxy as a growth factor in urinary calculi and gallstones.Nature, 217, 56-58. relationships existing between some significant re- North, A. C. T., D. C. Phillips and F. S. Mathews(1968) peats of A semi- the two compounds(in particular the ratio empirical method of absorption correction. Acta Crystallogr., c*"aa/b*r,.*: l/2) and displays the distortion of the A24.351-359. structure resulting from the dehydration of weddel- Sterling, C. (1965) Ciystal-structure analysis of weddellite, lite to whewellite, which appearsclearly by observ- CaC2Oo' (2+r)H2O. A cta Crystall o gr., I 8, 9 17-92 l. Stout, G. H. and L. H. (1968) ing, along the repeats and Jensen X-ray StructureDetermina- 12011*h"* [100]*.oo,the lroz. Macmillan Publishing Co., New York. changeof the hydrogen bond system. Walter-Levy, L. and J. Laniepiece(1964) Sur la thermolysedes All the structural details given here could be of hydrates de I'oxalate de calcium. C.R. Acad. Sci. Pais, 259, someinterest for the study of the growth morphology 4685-4688. of whewellite and weddelliteand the possibleepitax- Zachariasen,W. H. (1963) The secondaryextinction correction. Acta Crystallogr.,I 6, 7 l|.r--7 ial relations betweenthe two minerals hypothesized 14. by Lonsdale(1968) in urinary stones;in addition the structural relations will aid in the understandingof Manuscript received, January 16, 1979; the kinetics of formation and transformationof these acceptedfor publication, September 12, 1979.