Canqdian Mlneralogist Vol. 14, pp. 143-l 8 (f976)

A REFINEMENTOF THECBYSTAL STRUGTUBE OF

F. C. HAWTHORNE Dept. of Earth Sciences,University of Manitoba, Winnipeg

ABSTRACT large distortions are unknown in arsenatescon- taining isolated tetrahedral AsOe& groups, and Three-dimensional counter-diffractometer X-ray as part of a study on the stereochemistryof the data and a full-matrix least-squares method have \uf*Onu groups, the structure of adamite was been used to refine the of adamite refined in order to see if such large distortions (ZnzAsOeOH; 8.530(2), c a 8.3M(2),, 6.041{o)A', actually do occur. Z -- 4) in the space-group Pnnm. The final R-factor for 613 reflections is 4.4Vo, Edge-sharing chains of ZnO4(OH), octahedra ex- ExpnnttusNrel- tend parallel to the c-axis, and are linked by As tetrahedra to form channels parallel to c. Pairs of The crystals used in this study were from edge-sharing ZnO4OH trigonal dipyramids occur Durango, Mexico. As there was very little ma- in these channels. The structure is similar to that of terial available, only a partial chemical analysis andalusite. Both bond-strength considerations and could be performed; the results are given in the infrared slbctrum indicate the presence of weak Table 1. Normalization to two non-tetrahedral bonding.

TABLE 1. SoMMATRE ZnO=56.59, Cu0.0.02, Fe0=0.01 rt.Z, l,lno trot detected I-es donn6es tridimensionnelles obtenues par dif- fomula unlt Zn2As04(0[) fraction des rayons X sur diffractomdtre ir comp- teur ainsi que la m6thode des moindres carr6s d a 8.304(2)i rr("'-r) 19s mattrice compldte ont 6t6 utilis6es pour affiner la f 8.530(2) clysta1 slze (m) O.25 structur€ cristalline de loadamine (Zn:AsOaOH; a c 6.047(l) Rad/Hono. Mo/c : r aze.:ij rotal no. of 782 8.304(2), b 8.530(2), c 6.047(l)4, z 4) dam ,lo.l,l lhm of >4o 6f3 le groupe spatial Pnnm. Ir r6sidu final pour les Space Croup No. jlobs I 613 rdflexions est R : 4.4Vo. i lt FLtra1 B (obeefvcd data) 4.47, f.Lnal Rg (obsorved Des chalnes d'octaCdres ZilOA(OIl)z i arOtes D"ul" 4,4t!4 data) 4.97, - communes sont align6es paralldlement i l'axe c et 1i=t.(l"obsl]n."r"l) / .l r"r"l -lr - l15. li6es entre elles par des tdtraedres As de fagon i t, wrll-l t t r l)2/r '," . w=r u I obs| calcr obs I,'1 I I former des canaux parallbles i c. Des paires de di- TeEl,criturc fnctor fom uscd: cxp -lO:, hOt:,. . ...) pyramides trigonales ZnOAOII i arOtes corlmunes .,=, occupent ces canaux. La structure est semblable i le celle de fandalousite. Irs valences de liaison et (As-O) speche infrarouge indiquent la pr6sence de ponts cations, together with the final distance hydrogbne faibles. obtained, indicates that the adamite used in this (fraduit par le journal) study closd approaches the ideal stoichiome- tric composition, ZnrAsOa(OFI). Single-crystal precession photographs exhib- INTRoDUcTIoN ited orthorhombic synmetry with systematic Adamite, ZnrAsOOH, is a supergene absenceshul, h*l:2n*L; Okl, k*l:2n*L; hAO, which occurs in the oxidation zone of - h:Zn*t; AkO,k:2n*L AOl,t:2n*1. This is bearing deposits. Its structure was solved by compatible with the Pnnm as found Kokkoros (1938) who showed that it was simi- by Kokkoros (1938), and this was satisfactory lar to that of andalusite (AlrSiO4O), as proposed for the refinement of the structure. Cell dimen- by Strrrnz. (1937). Howeveq unlike andalusite, sions were determined by least-squaresrefine- the tetrahedral part of the structure appeared ment of L5 reflections aligned automatically on to be extremely distorted with the As-O bonds a four-circle diffractometer and are presented varying between 1.59 and 1.81A in length. Such in Table 1, together with other information per- r43 r44 TIIB CANADIAN MINERALOGIST

TAILE 2. AToMTCpOSXTTOIS AND AXTSTROPTC rEMpEnATgtE rAcmR coBrrxcrBl|Ig lor lD.altlT? (0r, toa) "

F,, ftt 13rz 0tt 0:z 0.3653(2) 0.3642(2) Ll2 31Q) 44(2) 74(3) 1(2) 0 0 zn(2) 2 r/2 0 0.2526(2') 62(2) 57(2) 16(2) 0

0.2498(2) 0.2438(2) 0 32(1) 38(1) 60(2) 2(r) 0 0 0(1) (10) 0. 1044G0) 0. 1041 0 27 rJL) 37(11) 187(25) 0 0 o(2) 0.4237(9) 0.1478(10) 0 53(12) 53(12) 91(21) 4(10) 0 0(3) 1 0.2312(8) 0.3603(9) 0.2233(9) 51(e) 46(9) 75(14) 0 0 olt 4 0.3933(9) 0.1274(10) 31(9) 60(10) 110(19) 9(8) -5 (8) -28 (8)

TABLE3. SELECTED INTEMTOMTCDI5TANCE5 AND ANGLES FOR ADAMIIE TABLE4. MAGNII1IDESAND ORIENTATIONS OF THE PRINCIPALAXES OF THE IHERI|IALELLIPSOTOS As-0(1) 1.696(8)E ztr(L)-0 (L) (9)l 2.004 R.M.s. As-0(2) 1.660(8) zo(1) -0(1) 2.062(9) Angleto Angleto Angle to dJsplacmnt a-axJs ,-axis As-0(3) !.-694(6) x zn(1)-0 (3) 2.010(6) x c-axis zn(l)-on 2.034(9) 0.113(3)R2 Mean L.681 4(9)' s4(s)' 90" zn(1) 0.u7(2) 90 90 0 Mean 2.024 0.130(3) 86(9) 4<9) 90 zn(2) -0(2) 2.079(7)i x 0.110(2) 90 90 0 za(2, zn(2)-0 (3) 2.264(7) x 0.124(3) 41(3) 137(3) 90 0.165(3) (3) zn (2) -oE 2.050(7) x 43G) 47 90 0.104(2) 15(7) 105(7) 90 l.leao 2,131 As 0.105(2) 90 90 0 0.119(2) 7s(7) 15(7) 90 AB tetrahedron 0.097(19) 2(46) 88(46) 90 0(1) 0.117(18) 88(46) o(r)-o(2) 2.678Gr)E 0(1)-As-o(2) o 178(46) 90 105.8(3) 0.186(13) 90 90 0 0(1)-0(3) 2.776(9) 0(1)-As-0(3) 110.4(2) x 2 o(2)-0(3) 2.769(9) 0(2)-As-0(3) r.r.1..8(2) x 0.130(14) 90 90 0 0(3)-0(3) 2.700(9) 0 (3)-As-o (3) r06.6(3) 0(2) 0.132(17) 163(66) s3(66) 90 0.143(18) s3(66) 37(66) 90 Hean 2.145 llean 109.5 0.088(13) 97(r4) 4e(9) 41(11) 0(3) 0.727(L2) 153(18) 75(15) 112(18) 0.157(9) 116(18) 135(10) 57(r1) o(1)-o(1) 0.103(18) 3(20) 87(2) 90 "z.ttz(tt)A 0(1)-zn(r)-o(1) 75.2Q)o 0,143(13) 0(1)-0(3) 3.53s(9) x 0(l)-zn(l)-0(3) 90 90 0 t23.5(2) x 0.148(15) 93(20) 3(20) 90 0(1)-0(3) 2.999(8) 0(1)-zn(1)-0(3) 94.8(2) x o(1)-oH 2.884(11) 0(1)-ztr(1)-0n 9I.2(3) 0(3)-0(3) 3.347(8) 0(3)-zn(1)-0(3) LI2.7(3) 0(3)-oE 2.926(8) x 0(3)-zn(r)-0lt 92.1(2) x beginning and end of each scan, Graphite mono- Neatr 3.070 Ileatr 100.1 chromatized MoKc radiation (I :0.710694) Zn(2) octahedron was used. Two standard reflections were moni- 'z.azzgz1 tored o(2)-o(2) o(2)-zi(2)-o(2) 85.5(3) every fifty reflections to sheck for con- 0(2)-0(3) 3.053(9) x 0(2)-zn(2)-0(3) 89.2(2) x stancy of crystal alignment; no significant change 0(2)-0(3) 3.23s(8) r o(2)-zn(2)-0(3) 96,2(2) x 0(2)-o[ 3.039(r) x 0(2)-zn(2)-0H 94.8(3) x was noted during the data collection. A total of 0(3)-0H 2.842(9) x 0(3)-2tr(2)-0u 82.3(2) x 0(3)-oH 782 reflections was measured over one asvrnme- -?.qq4(rz)^3.116(8) x 0(3)-zn(2)-oE 92.3(2') x oH-on on-zn(2)-oH 86.3(3) tric unit out to a maximum 20 valae oi eS". Because Uean 3.016 l{eau 90.1 of the near-spherical shape of the crys- tal, absorption sdenotes was assumedto be solely a func- ghared ealge tion of theta, and absorption correciions for lpherical crystal shape (g,R :2,5) were per- tinent to the data collection and refinement pro- formed. The data were then corrected for l,o- cedures. rentz, polat'aation and background effects and reduced to structure factors. A reflection was The crystal used to collect the intensity data considered to be observed if its magnitude was was.an irregular equidimensional fragment with greater than four standard deviations based on minimum and maximum dimensions of. 0.245 counting statistics; application of this and O.26lmrn criterion respectively.It was mounted on a resulted in 613 0bserved reflections. Syntex P1 automatic diffracto,meter oDeratino ii the e-20 sc{ur mode *ith v";;bi;-t"ti.-fr"ri REFINEMENT 2..0-24.0o/min depending on the dr-@asep&rt- tion and the peak count through an angle of The atomic coordinates given by Kokkoros 2", with stationary background counts ut tne (1938) were used as initial parameters for the REFINEMENT OF THE STRUCTURE OF ADAMITE 145

least-squares program RFINE (Finger 1969). coefficient at this stagewas 0.5(3)x10i; inclu- Scattering factors for neutral atoms were taken sion of the O04 and 022 reflections slightly in- from Cromer & Mann (1968) and anomalous creasedthis value and produced a slight increase corrections from Cromer & Lieber- in the R-factor but did not affect the positions posi- man (tgZO). The refinement converged rapidly or tempetature factors. The final atomic to an R-factor (seeTable L) of. 6.2Vo for an iso- tions and anisotropic temperature factor coeffi- tropic thermal model. The two most intense re- cients are presented in Table 2. Interatomic dis- flections (004 and A22) wete significantiy less tances and angleswere calculated using the pro- than theii calculated values at this stage; it was gram ERRORS (L. W. Finger, pers. comm.) assumed that these were severely affected by ex- and are given in Table 3. The magnitudes and tinction and they were removed from the refine- orientations of the principal axes of the thermal ment. The temperature factors were converted ellipsoids were calculated with ERRORS and to anisotropic ffable 1) and a correction was are given in Table 4. made for isotropic extinction (Zachariasen1968) as a with the extinction coefficient included DrscussroN variable in the refinement. Full-matrix least- squares refinement of all variables resulted in The structure of adamite is shown in Figure oonvergence at R-factors of 4.4Vo (observed) 1. is surrounded by four in a and 5.'7Vo (all data), and R--factors of 4'9o/o distorted tetrahedral arrangement,with th^ebond (observed) and 5.67o (all data)*. The extinction lensths varyins between 1.66 and 1.70A. The tet;hedral dist-ortionis much less than that re- ported by Kokkoros (1938), and the range of *A list of observed and calculated structure factors 6ond lengths observedhere is conformable with of Unpub- has been deposited with the Depository that displayed by ,most other arsenates (e.g. may be obtained on request to: lished Data. Copies Poulsen & Calvo 1968; Krishnamachari & Calvo of Unpublished Data, National Scienge Depository 1973; Calvo & Neelakantan 1970); ex- Library, National Research Council of Canada, 1970, arsenate tetrahedra is gen- Ottawa, Ontario, Canade'. treme distortion of

Fro. 1. The structure of adamite projected down the c axis' t46 THE cANADTAN MINERALOGIST

erally encountered only in strucfures containing agebetween the two arsenatelevels at z:0 and b91t pyro-groups (e.g. NaaAszOz, 7/2. Leung & Calvo z : Zn(l) polyhedra at the same level share 1973; PbzAsrOr,C. Calvo, pers. comm.) and one this edge is extremely contracted as polyarsenate chains (e.g. -edge; a LiAsO,, Hilmer & result of relaxation of the edge-sharingconfigu- Dornberger-Schiff 1956). Zn(l) is surrounded bv ration to reduce tbe Zn(l)-Zn(l) repulsive inter- four atoms and one OH groop, urruog"d action. inatrigonal dipyramid. The three ihori equato-rial The polyhedral bonds are statistically equal and the axial bonds distortions observed in the adamite structure are elongate; in addition, Zn(l) is stightly dis. are similar to those found in andalusite placed out of the equatorial plane toiards the @urnham & Buerger 1961). In partic- ular, the axial OH qoup. Zn(2) is bonded to four oxygen octahed,ron is extremely elongated in both structures. atoms and two OH groups in a distorteO oJta_ Whereas this may be due to Iocal bond-strength hedral arrangement. The octahedron is strongly considerations in andalusite (Bu,rnham elongated with Zne) forming long axial boids &- Buerger 1961), this is not the pri- mary reason to the O(3) oxygens, and the OH ions. whish in adamite as a formal bond-streneth analysis lie in the equatorial plane of the distorted octa- shows net deviations of only +0.65, -0.08, -0.02 hedron, are arranged in a cls configuration. +0.07 and on O(1), O(2), OH ana O(3) Dominating the adamite structure are the respectively. Examination of Figure 1 shows that chains of edge-sharing Zn(2) octahedra that ex- a strong elongation of the axial oc- tahedral tend parallel to the c-axis (Fig. 2), alternately bonds will produce a significant increase in sharing the O(2)-O(2) and OH-OH octahedral the dimensionsof the channelsin the adamite structure. edges. Both the shared edge lengths and the This will tend to offset the contraction effect angles subtended by them at the Zn(Z) cations of the O(1)-O(l) edge that is shared be- tween are contracted in agreementwith pauling's third the adjacent Zn(l) trigonal dipyramids, as this rule @auling 1960). These chains are linked to- tends to reduce the size of the chinnel. Tiius gether by arcenate tetrahedra to form long chan- it is apparent that the long axial bonds of the octahedra nels parallel to the c-axis, as shown in Figure 1. are necessaryfor the Zn(l) dipyramid to Each channel is bounded by four chaini with be large enough to incorporat"'a" ite size "jto- the octahedra occuning in layers at 7 : ls and of Zm. 3/+; the linking tetrahedra occur at z : 0 and Examination of the r/z empirical bond-strength and alternately point towards and away from table for adamite Clable 5), calculated using ihe the centre of the channel. bond-strength curyes of Brown & Shainon The Zn(l) atoms lie in these channels in the (1973), shows that there is a deficiencv around plane of the arsenatetetrahedra, providing link- O(3) and an excessaround the OH anion. Al_ $oueh the discrepanciesare s,mall,they suggest the_presence of hydrogen bonding betweeri-OH and O(3); using the bond-strength curves of Brown & Shannon (1,973),it can be shown that p9 F:O(3) distance ot 2.27A produces almost ideal bond strength sums arounh OH ;d Aai. The occurrence of weak hydrogen bonding in adamite is also indicated by the infrared slec_ trum (Sumin de Portilla 1974).T\e -principal OH stretching frequency occurs at 35g0 cm-1. slightly lower than the typical stretching fre_ quencies (3650-3700 cm) exhibitea lv Oft groups not involved in hydrogen bonding. The anisotropic therrral model appears phvs_ 1c{]f reasgnable,the values obtainid Cfa$e'+) fdling -yttfir the range generally exhiUited ty well-refined crystal structures. Except for the O(3) anion, all atoms lie on speciai positions with one vibration direction tying paralil to the c-axis and the rernaining two constrained to the a-b plane. In general, the principal axes of the Fra, 2. Polyhedral representation of tle adamite thermal ellipsoids are oriented so as to involve structure showing the edge-sharing octaledral the least bond stretching in their maximum chains extending parallel to a vibration directions. REFINEMENT OF THE STRUCTURE OF ADAMITE t47

l large number of arsenate,vanadate, phos- TABLE5. BOND-STRENGTHTABLE FOR ADAMITE phate and silicate have the general stoichiometry ABXOc(Z) (e.g. Richmond 1940). zn(l) zn(z) group rich in both variety and This is extremely 0(r) .366 1.792 complexity of structure tyPes, maDy of which .434 the are a.syet uncharacterized.For this reason, 0(2) 3Aa^i 2.053 present discussion will be limited to arsenates OH .397 -379^+ 1.173 bf the form lll+rAsOn(OlD. Table 6 summarizes the chemical and crystallographic data available o(3) .427 .208 t.245 1.880 for theseminerals. Adamite and its Mn-analogue are isostructural whereas their dirnorphs 5.039 paradamite and are not. Switzer (1956) powder pattern for para- ieported an unindexed (1973) the culv€s of Blom & shamotr damite and suggested that it was isostructural * calculated ftoB with tarbuttite, the triclinic form of Zn,POq(OH); this was subsequently confirmed by Finney chains. HoweYer, the chains in the tarbuttite (1966).It was suggestedby Waldrop (1970) and structure consist of edge-sharingtrigonal bipyra- Moore & Smyth (1968) that sarkinite has the mids whereasthe chains in the adamite structure wagnerite structure (Coda et al. 1967) and this are of edge-sharing octahedra. Despite the re- has recently been confirmed by Dal Negto e/ al' duction of anion coordination number in para' (r97s). damite, the polyhedral packing is more efficient, structure. The ,most apparent difference between tle resulting in a slightly denser its dimorph sarkinite, adamite and paradamite (tarbuttite structure, In the case of eveite and exist even Cocco et al. L966) structures is the coordination far greater differences in structure is similar in both of the Za. In adamite, one Zn atom is octahe' though the cation coordination independent Mn drally coordinated by four oxygenl structures. Sarkinite has eight 9d lwo in octahedral and half hydroxyls in a cis arrangement, and the other atoms, half of which are The poly- Zn is loordinated by a trigonal bipyramid of in trigonal bipyramidal coordination. octahedra is considerably four oxygens and one apical hydroxyl. In the merization ofthe Mn than in eveite. Three tarbuttite-type structureo both independent Zn more complex in sarkinite three edges with atoms have trigonal bipyramid coordination; one of the four octahedra share to form a kinked in- Zn is coordinated by three oxygens, one apical three adjacent octahedra octahedron shares and one meridional hydroxyl (still a cis con- finite chain, and the fourth and one trigonal bi figuration) and the other Zn is coordinated by edges with two octahedra although the hydroxyls four oxygens and one meridional hydroxyl' This pyiamid. In addition, this is now au un- causes a-reduction of. the coordination number are in a cis arrangement, a shared edge as in of one of the anions from [3] to [2]; in tarbut- shared edge rather than the four trigonal bipyramids tite, this anion is situated close to eveite. T'hree of [2]-coordinate pairs as in eveite, while tho hydroxyl anion, suggesting that the occur in edge-sharing -bond' an edge with an octahedron' strength deiiciency is compensatedby hydrogen the fourth shares far greater edge-sharing char- bondLg. If this is the case in paradamite, the As a result of its shows a far more principal OH stretching frequency should be acter. the sarkinite structure of the polyhedra than that considirablv lower than that exhibited by its efficient packing eveite structure. dimorph ujamite. Polyhedron polymerization is exhibited by the and Mn,AsO4(OI{), Cu,- fairlv iimilar in both structures; both have edge- Unlike Zn"AsOd(OH) to exhibit only one poly- sharing pairs of Zn trigonal bipyra'mids and iso- AsOa(OID appears Although the structure of oli- lated arienate tetrahedra linking edge-sharing morph, .

TAB1,I6. MrNErAlsor ru u2/ Aso4(ou)cRouP

(8) (ot t"l Cr Space GrouP IomuLa unit at8t rrt) c Pnm 6'047(1) e0 90 90 ADAMTTE zd2As0.(oE) 8.304(2) 8'530(2) 104'3(1) 87.e(1) 103.2(1) PI ?&DAIIITE zn2As04(OH) 5.807(5) 6'666(5) 5'627<5) Pnm 6'21(!) 90 90 90 EvEtTE un2Aso4(oH) S.57(r) 8177(1) P2Lta 10'208(2) e0 108.9(1) 90 SARKINITE MrrAeo4(oll) L2.179Q) 13'596(2) ?tm(?) 5'95 90 90 OLMNITX CurAs04(01{) 8'22 8'64 148 THE CANADIAN MINERALOGIST

venite has been solved (Heritsch 1938), some DAL NEGRo,A., Crusererrn, G. & pozes, J. M. M. uncertainties exist concerning this structure. (1974): The crystal structure of sarkinite, Mnr- Heritsch (1937) originally determined the space AsOa(OH). Tscherrnak's Min. petrog. Mitt, 21, group of olivenite as pmmm; later. Heritsch 246-260. (1938) solved the structure, defined the space FTNGER,L. W. (1969): RFINE. A Forrran IV com- puter program group as Pnnm an.d indicated that the for structure factor calculation one re- and least-squares flection violating this symmetry refinement of crystal structures. was due to Ren- Geophys, Lab., Carnegie Inst. (unpubl.). ninger effects. (1940) Wash. Richmond determined FINNEv, J. J. (1966): The unit the group p2t2t2r cell of tarbuttite, space of olivenite as Subse- Zn-(POr)(OH), and paradamite, Znr(AsOn)(OH). quenfly, Beny (1951) determined the space Amer. Mi,neral. 51, 1219-1220. group as Pmnm, suggestingthat the spacegroup HERrrscH, H. (1937): Vorbericht uber rontgeno- given by Heritsch (1937) as D,,n(pmmm) was graphische Untersuchungen an Olivenit Cur(OH) actually a misprint for Dl'zx(pmnm). This seems (AsOn). Z. Krist. 98, 251-353. not to be the case as the space group symbol ( 1938) : Die Struktur des Olivenites, Cu1(OH)(AsO). gg, appears several times in both of Heritsch,s oa- Z. Krist. 466-479. -by HTLMER, W. (1956): g9rsl iq addition, the structure proposed & DonNnrncen-Scsrrr,., K. Dio Kristallstruktur von Lithium polvarsenate Heritsch (1938) is only compatible with the (LiAsOr)-. Acta Cryst. 9, 87-88. space group Pnnm. In view of the confusion Korronos, P. (1937): Uber die Strukrur von Ada- existing concerning the structure of olivenite. a min. Z. Krist. 94, 417-434. single crystal study is planned. KRrsHNAMAcHexr,N. & Cervo, C. (1970): Crystal- lographic studies of arsenates II. Crvstal AcrNowrSDGMENTs structure of Co6AsrOlu. Can. J. Chem. 48, 3'124- 3t31. The author would like to acknowledee the & --- (1973): Magnesium arsenate, cooperation of the Materials Research Inititute, Mg;AsrO6. Acta Cryst. B2g, 261.1-2613. McMaster University, Hamilton, Ontario in the LEUNo, K. Y. & Carvo, C. (1973): Crysral struc- ture and phase collection of the X-ray intensity data. Financial transformations of sodium di- arsenate, Na*As2Or. support was provided by the Can. l. Chem. 51, 2082-2089. National R.esearch Moonr, P. B. & SMyrH, ( Council of J. R. 1963) : Crystal chem- Canada and the Universitv of Man- istry of the basic manganes3 itoba. arsenates: III. The crystal structure of eveite, Mnr(OH) (AsO"). Amer. Mineral. 53, 1841-1845. RprpnBNcrs Peur,rNq L. (1960): The Nature of the Chemical Bond, 3rd ed. Cornell Univ. pr,ess. Ithaca. New BBnnv,L. G. (1951): Observationson , York. corawallite, euchroite, liroconite and olivenite. PoursrN, S. I. & Carvo, C. (1967): Crystal struc- Amer. Mineral. 36, 484-503. ture of Cu"(AsOr)r. Can BnowN, J. C'hem. 46, 917-927. I. D. & SnewNoN,R. D. (1973): Empirical RrcuuoNo, W. E. (1940): Crystal bond chemistry of the length-bond strength curves for oxides. phosphatesoarsenates and vanadates of Acta Cryst. the type :{29, 266-282. A2XO4(Z). Amer. Mineral Zb, 441-4j9 BURNHAMC. W. & Bunncen,M. J. 0961): Refine_ SrnuNz, H. (1936): X-ray and morphological com- ment of the crystal structure of andalusite.Z. parisou of andalusite, libethenite and adamin. Z. Krist. ll5, 269-290. Krist. 94A, 6O-74. CALvq C. & NrBr"areNrAN, (1970): K. Refinemenr SuvrrN or Ponru,re, Y. L (1,974): Infrared spec- of the structureof Mg2As2O..Can. l. Chem. 48, troscopic investigation 890-894. of the structure of some natural arsenates and the nature Cooco, G., p, of H-bonds in FeNnerr, L. & Z*rtzzr, F. (l966.t: their structures. Can. Mi.n, L2, 262-269, The crystal structureof tarbuttite.Z. Krist. ldl SwrrzEn, G. (1956): 321-329. Paradamite, a new zinc ar. senate from Mexico. Science 12S. 1039. Coul, A, Gursrrreru, G. & TenrNr, C. (1,967): Waronor, L. (1970): The crystal structure The crystal $ructure of tri- of wagnerite. Acc. Naz. ploidite and its relation to the structures of Li.ncei,Srrie VIII, 43, other ZIL-224. minerals of the triplite- group. Z Krist, Cnoryrn, D. T. & MarrrN,J. B. (1963): X_ray scat- 131, 1-30. tering factors computed from numerical Hartree_ ZacnenresrN, W. H. (1968): Extinction and Boor- Fock wave functions.Acta Cryst. A24, 32I_324. man effect in mosaic crystals. Acta. Cryst. A2g, & LTEBERMAN,D. (1970): Relativistic 421-424. calculation of anomalousscattering factors for x_ phys.58, Manuscript received October 1975, emended Feb- rays."/. Chem. 1891_18tS. ruary 1976.