American Mineralogist, Volume 64, pages 1243-1247, 1979

Senegalite, AI2(OH)3(H,O)(PO.), a novel structure type

Tenny D. KEEGAN, TaKAHaRU ARAKr AND PAUL B. Moonn Department of the Geophysical Sciences, The University of Chicago Chicago, Illinois 60637

Abstract

Senegalite,AIr(OH)'(HTOXPO.), Z : 4, orthorhombic, spacegrovp Y2rnb, a : 7.675(4), b : 9.711(), c : 7.635(4)A, is a new structure type based on chains of composition | [slAlt6lAl(OH)r(HrO)(Op).]or f,[t5t41tot41f*]where @is a coordinating and Op is phosphateoxygen. R:0.024 for 1288independent reflections. Two symmetry-equivalentchains run parallel to [01] and [T0U, each basedon distorted A(OH)3(HrO)(Op), octahedraland Al(OH)r(Op), trigonal bipyramidal edge-sharingdimers which further corner-link to completethe chain. Corner-linking eot tetrahedraknit neigh- boring chains to form an open sheet parallel to (010). Further tetrahedral corner-links to chainsrelated by the D-axialglide form an open polyhedral framework structure. Hydrogenbonds including OH ... OW, OH ... O and OW ... O rangingfrom2.'73 to 3.24A areproposed. Bond-distance averages arer5rAl(l)-O : 1.848A,t6lAl(2)-O : 1.899A,andtalP- O: 1.5344.

Introduction a small effect. Symmetry-equivalent reflection pairs Senegalite, a new and relatively uncommon min- were then averaged, and after applying Lorentz and eral, was first described by Johan (1976). He reported polarization corrections, 1288 independent F" were details are summarized in the composition AlrPOo(OH)..HrO, based on chem- obtained. Experimental ical analyses of material from the type locality at the Table l. Komondiako Iron Mine (magnetite ) in the Fa- Solution and refinement of the structure l€me River Basin of east Senegal, Africa. It occurs in the oxidation zone of the deposit, associated with tur- A three-dimensional Patterson map P(u, v, w) al- quoise, augelite, wavellite, and crandallite. Our in- lowed derivation of three independent atomic posi- vestigation was undertaken to determine the crystal tions assumed to be Al and P atoms, which consti- structure, ascertain the role of water in the species, tuted the basis of the weighted B-general synthesis and discern the relationship of this mineral to other (Ramachandran and Srinivasan, 1970, p. 96-119). phosphate hydrate structures involving Al3*. All independent atomic positions excepting hydro- gens thus obtained were then applied to full-matrix Experimental section least-squares refinement, the first two cycles with iso- A single crystal was obtained from a type-locality tropic temperature factors and the subsequent four specimen, provided by the Mineralogical Research cycles with anisotropic thermal parameters. The final Company of San Jose, California. All parameters refinement converged to R : 0.024 and R- : 0.038, were redetermined utilizing calibrated precession where photographs, yielding the results in Table l. Ex- - - P:- >ll4l l4ll anrr p -: [xl4l l4l)'l'" tinction criteria, in agreement with the earlier study, ^ p1p"; .rru '-" >.lr.r hemimorphic development of the crystals, and the I I success of the structure refinement established the with w : o-' of ,F". Refinement minimized w(F" - acentric groap F2rnb. Careful measurement of the F")'. crystal and application of the Gaussian integral Scattering curves for Al'*, Po and O- were ob- method (Burnham, 1966) permitted application of tained from Ibers and Hamilton (1974, p. 149), and absorption correction which, owing to low linear ab- anomalous corrections, Lf ", for Al and P sorption coefficient and favorable crystal shape, was from Cromer and Liberman (1970). Positions of hy- 0003_004){/ 7 9/ | | l2-1243$U.00 1243 t244 KEEGAN ET AL.: SENEGALITE

Table l. Experimental details for senegalite Table 2. Senegalite. Atomic coordinate parameters f

(A) Crystat Cell Data Aton

a, A 7.675(4) b, ^ s.7rt(4) A1(r) \ 0.1e887(7) o.s22s (1) c, A 7 635(4) A1(2) 0.8832(1) 0. 34948(8) 0.6620(r) Space group P2rltb P 0.sr93(1) 0.482s2(6) 0.6868 7 (8) Fomula A1, (0H) 3(H2C) (P04) o(1) o.7t7s(3) 0 .4818 (2) 0.7206(3) o(2) 0.43rr(3) (2) 0 (3) (B) 0.s6rs .837s IntensityMeasurments 0(3) 0.446s(3) 0.3349(2) 0.6882 (3) (2) 0.37(lid), 0.104(llb), 0.28(llc) o(4) 0.484s(3) o.sso2 0.sI16(3) Rotation axis a[100] Max (sine)/l 0.80 0H(r) o.72ss(3) o.2ss2(2) 0.s134(3) Scan speed, deg per nin 1.0 0H(2) 0.0423(3) 0.42s4(2) 0.8294(2) Base scan width, deg 3.2, widening to 6 8 at higher 1eve1s oH(3) 0.83r0(3) 0.24s2(2) 0.8698(2) Background counts Stationary, 20 s at beinning and end of scan Reciprocal space coverage hk9. and hkQ. with ll = 0-11 Radiation MoKor (l 0.7093 A), graphite nonochionator ol1l 0. 0521(3) 0.2018 (2) 0.60s0 (3 ) TndFhFhianr F- 1288 tEstinatecl standard enors in parentheses refer to the last disit . (C) Absorption Correction

u, cn-' 8 11 tancesand angles,Table 4 the anisotropicthermal vi- Gaussian integrol inteflal 7 Ttansnission factor range 0 857-0.925 bration parameters,Table 5 the parametersfor the ellipsoidsof vibration, and Table 6r the structurefac- drogen atoms were sought on difference Fourier tors. maps after final refinement,but residual peaks were rTo a copy of Table 6, order Document AM-79-lll interpretedfor OH(l) and OH(2), the othersremain- obtain from the BusinessOffice, Mineralogical Societyof America, 2000 ing ambiguous.Table 2 presentsthe refined atomic Florida Avenue, N.W., Washington, D. C. 20009.Please remit coordinates,Table 3 the polyhedral interatomic dis- $ | .00in advancefor the microfiche.

Table 3. Senegalite. Bond distances and angles t

A1(r) A1(2) P

m 6r;-o621 t1) t.788(2) Ar(2)-o(4)(2) r.B2o(z) P-o(4) I.sr4(2) Ar(r) -oH(1) 1.8r6(2) Ar(2) -o(r) 1.860(2) P-o(2) 1.s39(2) (3) A1lr;-onJs; 7.828 (2) Ar(2)-oH(1) r.8ss(2) P-o(3) 1.s3s(2) A1(1 ) -0 (3) 7.872(2) A1(2) -oH(2) 1.91s(2) P-o(1) 1.s46(2) (3) A1(1)-oH(2) I .93s(2) Ar(2)-oH(3) r.s24(2) average I .534 average 1.848A A1(2) -ol\' r . e8r(2) O-P-O'angles (') average 1.899 apica I - equatori al 0-{r-4 (.) o (2)-0 (3) 2.48r(3) 107.4(1) (3)-6116r, oHlz; {s) 2.406 (3) * 7s.4(7) o(1)-o(4) 2.48s(3) 108.8(1) o(3) -oH(r) 2.63s(3) 91.3 (1) oH(2)-oH(3) 2.406(s)* 77.6(r) o(1)-o(2) 2.498(3) r08.2(1) (3) (2) oH(1) -or112; 2.6s r (3) 8s.e(r) o(4) -olll 2.620(3) 87.0(1) 0(3)-o(4) 2.sos(3) r10.3(r) (1)-o(s) otz) 2.66s(3) e3.4(r) oU)Q) -rxe) 2.6s2(3) e0.4 (r) 0 (2)-0 (4) 2.s24(3) lrr .s (r) (1)-6",r,(s) o6zy 2.68s(3) 92.2(1) @(l)-ow 2.6s4(3) 86.4 (1) 0(r)-0(3) 2.s26(3) 1r0.6(r) (3) o(3) -0H13; 2.702(3) s3.8(r) oH(3)-OW 2.673(3) 86.4(r) average 2.5O4 109.5 average 2.625A 90.0 0(1)-oH(2) 2.681(3) s0.s (r) bonds o(1)-0H(3) 2.707(3) 91.3 (r) equatorial o(1)-oH(r) 2.770(3) s2.4O,) ouJz1...tow(l) 2.731(3) (1)-q11r; (3) (2) (1) o6z1 3.018 (3) r13.r(r) o(r)-o(4) 2.72s(3) ss.s(1) oHisl " 't661; 2.789(3') (1) (2) oqzl -oH6ry 3.1s3(3) r22. o (7) o(+)(2)-on(r) 2.7s0(s) es.s(1) ol|I(1)...>oH(3) 2.840(s) (3) (2)...to,r, on611 -oH 1s; 3.220(3) r24.t(t') oH(2) -olll 2.767(3) 90.s(1) oH1rl tt) 3.tsz(3) aveTage 3.130 LLg.7 oH(1)-ofr(3) 2.84r(3) s6.1(1) ow(I)...ro(z) 3.242 (3) average 2.682 90.0 api ca I Angles with 0l'I (3) (r) o {sy -oH 621 772.s(L) onqz; -ow -o 121 10s. 7(r) (2) oH(z)-ow(r) -611 Js; 10s.7(1) (2) olzy-ow(1) -6s131 r22.2(L)

fEstinated standard errors refer to the last digit. The equivalent positions (referred to Table 2) as superscripts are (1) = x, L"+y, 92'z; (2) = \+x, -y, -zi G) = L.+7,1a-y, \+2. *shared edge between polyhedra. KEEGAN ET AL.: SENEGALITE 1245

Table 4. Senegalite. Anisotropic-thermal vibration parameters parallel to (010). The tetrahedra further link to (x 10,)f chains related by the b-axial glide to form an open polyhedral framework (1976) At on Bzz structure. Johan re- ports an imperfect (010) .Each tetrahedron Ar(1) 2.4 (r) 1.16(s) 2.8(1) 0.r (r) -0.3(r) (2) 0.0(1) Ar 2.2(r) 1.3s (6) 2.8(1) 0.1(1) -0 3(1) -0 1(1) is associatpdwith three chains,sharing two of its cor- P 2.24(8) r.1r(s) 2.69(7) 0.r(1) -0 2(8) 0.0(l) nerswithin one chain as seenin Figures I and2. 0t1) 2.r(3) r .7(r) 6.s (3) -0.I (2) -0.8(2) 0.4(2) o(2) 4 0(3) r .7(1) 4.6(2) -0.1(2) -0 8(2) 1.0(2) All hydroxyl groups are the points of linkage 0(3) 3.8(3) 1. s (2) 4.7(3) -0.6(2) 0.e(2) -0.3(2) 0(41 s.7(3) 2 8(2) 4.0(3) 0.8(2) 0.7(2) -1.r (2) within a chain. The water (OW) of senegalitecoordi- 161Al oH(r) 2.6(3) 2.7(2) 4 3(3) 0.7(2) -0.2(2) r.2(2) nates to one and is therefore not zeolitic in na- 0H(2) 2.4(3) 1.8(r) 4.0(2) -0.3(2) -0.4(2) 0.4(2) 0H(3) 3 r(3) 1.s(r) 3 8(2) 0.s(2) -0.6(2) -o.s (2) ture; it probably accountsfor the endothermicreac- at 250"C reported ow 4.s(2) 2.r(2) 6.0(3) 0.4 (2) | .2(2) 0.r (2) tion by Johan. rcoefficients in_the-erpression exp-[811h2 * Bzzk2 * BzJ2 + Z|tzhk + zpl3n& + zF23x(1. bstlhaled Standafd errors refer to the last dieit. Bond distancesand angles Bond distancesand anglesare presentedin Table Description of the structure 3. The averageP-O : 1.534A is typical for values Senegaliteis basedon two kinds of aluminum-ox- found for mineral phosphates.Average P-O : ygen coordination polyhedra which alternately share l.5l9A was found in the chemically-relatedaugelite edgesand corners to form infinite Al-O polyhedral (Araki et al., 1968).The Al-O octahedral distances chains. Two orientations of symmetry-equivalent average1.899A, close to l.89lA averagein augelite. chainsrun parallelto [0] j and [T0l]. The distorted trigonal bipyramidal averageis 1.848lr Figure I illustrates the chain parallel to [l0l] and (1.8324in augelite). its circumjacent phosphate tetrahedra. The Al-O The Al-O trigonal bipyramid is substantiallydis- polyhedra include a distorted A(OH)3(H,O)(Op), torted, but the polyhedron is not regular and lies be- octahedron(Op: phosphateoxygen) and a distorted tween the trigonal bipyramid and square pyramid Al(OH).(Op), trigonal bipyramid. The two poly- (seeStephenson and Moore, 1968,for a discussionon hedra share the OH(2)-OH(3) edge to form dimers five-coordinate polyhedra). With fixed Al-O dis- which further corner-linkat OH(l) to completethe tancesthe trigonal bipyramid would have one apical chain. The chain composition is thus O-A1-O' angle of 180", three equatorial angles of f,[t5t41tet41,OH)3(H,OXOP)o],or l[t'rAltu1Al6,] where 120", and six apical-equatorial angles of 90o. The @is a coordinatingoxygen. most significant angular distortion, OH(2)-AI-OH(3) As seenin Figure I the associatedtetrahedra cor- : 79.4" arisesfrom the OH(2)-OH(3) shared edge ner-link to neighboring chainsto form an open sheet between Al(l) and Al(2), an edge distance OH(2)-

Table 5. Senegalite parameters for the ellipsoids ofvibration*

VL VLD B,8rzA2 Aton oia B, 812A2 A1(1) o.o74(2) 74(s) 162(r0) 84(6) 0.s6(1) 0(4) 0.08s(s) 110(4) sr (4) 46(4) (3) 0.082(2) 1.11 144(8) 107(10) 121(6) 0. 128(4) rr2(16) s4(r2) 136(4) 0.0ss(2) 127(6) 93(4) s2(6) 0.137(4) r49(r3) nr(r2) 88(13) A1(2) (2) 0.078 3r(26) Lr7(s7) 76(16) 0.s6(1) 0H(1) 0.076(s) 40(7) r22(s) 68(6) 0.080(2) 0.88(3) 110(3s) 14e(34) LL2(r2) 0.101(4) sl (8) 63(6) 130(6) 0.09s(2) 113(s) 103(s) 27(s) 0.1s2(3) 81(4) 44(4) 48(s) P 0.07r(2) 70(7) lss(7) 84(4) 0.s2(r) 0H(2) 0.082(s) 26(20) 64(22) 86(r2) o.72(3) 0.082(2) 1s2(8) r11(7) r07(8) 0.0s0(4) rLr(24) 3s(20) 116(71 0.0s0(1) r0s(8) 90(4) 1e(8) 0.1r3(3) r0s(6) 6s(7) 26(7) o(1) I 0.076(s) r0(3) 8s(17) 80(4) 0.88(3) oH(3) 0.076(s) tr2(11) 2s(7) 7s(s) o.72(2) 2 0.08e(4) e0(r7) 8(4) s8(4) 0.092(4) 140(r0) r01(12) 727(8) 3 0.140(3) 100(3) 82(3) 13(3) 0.rrs(4) r2r(7) rr2(s) 40(7) o(2') I 0.076(s) 80(6) 14e(s) 6r(4) 0.88(3) OW 0.096(4) r27(14) 32(16) 81(8) r.07(3) 2 0.r04(4) Ls2(7) 111(7) 108(7) 0.108(4) 43(r2) s9(r6) r17(6) 0.r30(3) r16(7) 68(4) 3s(4) 0.14r(3) 63(s) 83(4) 28(s)

0(3) I 0.080(s) 66(8) 24(8) 8s(7) 0.86(3) 2 0.100(4) 43(8) r0s(r0) r27(6) 5 o.L27(4) s7(6) ros(4) 37(6)

ri =Lith,principal axis; = lt ms mplitude; ei.a, eib,oic = angtes (deg) between the ith principal axis and the cell axes 4, b and. c, 1246 KEEGAN ET AL; SENEGALITE

Fig. l. Polyhedral diagram of the senegalite structure down the b axis. Note the Al-O polyhedral chains running parallel to [101]. The POa tetrahedra are stippled and link above and below to Al-O chains running parallel to [01]. Atom heights are given as fractional coordinates in y. The atoms are labelled according to Table 2.

OH(3) : 2.406A correspondingto the shortestedge distancefound in the structure. Hydrogen bonds Since the hydrogen positionscould not be located by difference synthesis,Seometrical considerations were used. The hydrogen-bondingcriteria of Baur (1970) suggestedfive candidatesfor hydrogen-bond participation. These bonds are featured in Figure 3 and are complicatedby the fact that it is not possible to select a specific projection owing to within-layer and interlayer bonds. Table 7 outlines the bond length-bond strength relationships following the guidelinesof Baur. The most seriousdeviations from electrostaticneutrality, O(4), Ap" : -0.250; and OW, Ap" : +0.333, are manifestedin net bond shortening and lengthening respectively,compared with poly- hedral averages. The (PO.) ,O(l) and O(3), receivebonds from OH(3) and OH(l) respectively, since OH(l) "'O(3) : 3.13and oH(3) "'O(l) : 2.79A. O(2) receives one bond from the water molecule, Polyhedral of the senegalite structure down the Fig. 2. diagram OW. which donatesa secondbond to OH(3) and re- a axis. Atom heights are given as fractional coordinates in x and are labelled according to Table 2. Polyhedral vertices which link ceives one bond from OH(2). This arrangement at the next level are broken. The POa tetrahedra are stippled. createsa network ofhydrogen bonds and contributes KEEGAN ET AL: SENEGALITE 1247

b=0 -. Table 7. Senegalite.Relations in bond strength-bondlengthr c=u ::?

Coordinatj-ng Cations Bond Length Deviations

Anions Al (r) Ar (2) p H6 Hs apo At (1) A1(2) p

o(1)-1r - t -0.083 - -0,039 +0.012 0(2)1-l - I +0.017 -0.060 - +o.OOs 0(3)1-I - I +0.017 +0.024 - +0.005 o(4)-11 -0.250 _ _0,079 _0,020 (2) 0H oH(r) I r 1 - -0,061 -0,032 _0.004 l+1.04 cH(2) I 1 I - -0.067 +0.087 -0.066 cr(3) 1 1 I 1 +0.100 -0.020 +0.025 -1-21+0.333 - +0,082

"apo ir ttt" aeu-i*ioie tt" uona-stte"gfh ;* f;';ffi"rtty (p. = ,.000) A bond-length deviation is the polyhedral average subtracted fr;n the individual bond distance.

A curious octahedral-tetrahedral corner-sharing tetramer [FIAI,(OHXH,O),F4(PO4),]has been noted for morinite by Hawthorne (1979). He showed a complexstructure genealogy which relatedmany dis- b=tL tinct structure types, including augelite, but there c=0 doesnot appearto be any analogywith senegalite. Fig. 3. Local environment of proposed hydrogen bonds in senegaliteshown as a portion of Fig. 2. Sincebonds link to levels Acknowledgments above and below, alternate atom heights are given as well. The This study was supporredby NSF grant EAR-75-19483(Geo- 4124"edge-sharing dimer is emphasized. chemistry) and a Materials Research Laboratory grant (NSF) awardedto the University of Chicago.

to increased rigidity between and within two svmme- References try-equivalentpolyhedral chains. Araki, T., J. J. Finney and T. Zoltai (1968)The crystalstructure of Discussion augelite.Am. Mineral., JJ, 1096-1103. Senegaliteis a new structure type Baur, W. H. (1970)Bond length variation and distorted coordina- and demon- polyhedra strates tion in inorganic crystals. Trans.Am. Crystallogr. As- the significance of structure analysis in soc.,6,129-155. classifying crystal structures and expressinga sen- Burnham, C. W. (1966) Computation of absorption corrections, sible structural formula. The chemicallv-similar and the significanceof the end effect.Am. Mineral., SI, 159-167. augelite,Alr(OH)3(POo) was formerly believedto be Cromer, D. T. and D. Liberman (1970)Relativistic calculation of anomalous scattering phys., a dehydrated version of senegalite, factors for X-rays. "/. Chem. 53, Alr(OH)3 I 89l-1898. (HrO)(PO4),but, aside from presence the of 5- and Hafihorne, F. C. (1979) The crystal structur€ of morinite. Can. 6-coordinated Al'* by oxygens,the structural prin- Mineral., 17,93-102. ciples of the two are quite different. Araki et al. Ibers, J. A. and W. C. Hamilton (Eds.)(1974) Intemational Tables ( 1968) demonstrated that augelite is based for X-ray Crystallography, Vol. IV. Kynoch press, Birmingham. Johan,Z. (1976) . on an edge-sharingtetramer of composition La senegalite,AI2PO4(OH)3 H2O, un nouveau mineral.Lithos, 9, 165-171. [t5rAl,t6rAl2(OH)"(Op)Jwith an Al:4 ratio of l:3.5, Ramachandran,G. N. and R. Srinivasan(197O\ Fourier Mahods and is locally more condensedthan senegalitewith in Crystallograplry. Wiley-Interscience, New York. the M:{ ratio of l:4. It appearsthat rearrangement Stephenson,D. A. and P. B. Moore (1968)The crystalstructure of through breaking numerousbonds is necessaryto es- grandidierite, (Mg,Fe)Al3SiBO".Acta Crystallogr.,824, lslg- 1522. tablish any structural similarity between the two minerals,and senegaliteis hardly a hydrated version Manuscript received, March 5, 1979; of augelite. acceptedfor publication, July 3, 1979.