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The Crystal Structures of Whewellite and Weddellite: Re-Examination And

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The Crystal Structures of Whewellite and Weddellite: Re-Examination And American Mineralogisl, Volume 65, pages j27-334' 1980 The crystal structuresof whewelliteand weddellite: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 whewellite;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 mineral 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 calcium, 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 minerals 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 crystal structure, 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 cleavage 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.
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