American Mineralogist, Volume 63, pages 160-171, 1978
Ulexite, NaCaBsOu(OH)r.sHrO:structure refinement, polyanion configuration, hydrogen bonding,and fiberoptics
SuBnnre GHosn, Cun'Nc WnN Department of Geological Sciences, Uniuersity of l(ashington Seatt le, Washington 98I 95
eNoJoeN R. Cmnx U. S. GeologicalSuruey,345 Middlefeld Road Menlo Park, California 94025
Abstract
The crystalstructure of ulexite,NaCaBsOe(OH)6.5HrO, triclinic, PT. has been refined to a conventionalR:0.046 usingleast-squares methods, partially block-diagonal, for 49ll reflectionscollected on a single-crystaldiffractometer with graphite-monochromatizedMo radiation.AII l6 hydrogenatoms in theasymmetric unit werelocated, and all form hydrogen bonds.Refinedcellconstantsare:d=8.816(3),b=12.870(7),c=6.678(l)A,a:90.36(2)", B : 109.05(2)',7 = 104.98(4)",V : 688.4(4)A3,2 :2, density(calc) = 1.955g/cm3. The structurecontains isolated pentaborate-type polyanions composed of three tetrahedraand two triangles,plus chainsof Na octahedraand chainsof Ca polyhedra,all cross-linkedby polyanionbonds to thecations and by a networkof hydrogenbonds. Average bond distances (A) are:B-O, tetrahedrall.4T5,triangular 1.367; Na-O,2.421;Ca-O,2.484;hydrogen bonds, O-H 0.80,H . . .O 2.08,O. . .O 2.852.The octahedraland polyhedral chains are parallel to c, theelongation direction, and causethe fibrous habit of ulexitecrystals that is essentialto the opticalfiber bundles.The hydrogenbonds rangefrom strongwith a minimum O..'O distanceof 2.595(3)A to veryweak and possibly bifurcated with O. .'O distancesof 3.082(a) and3.194(3) A. The watermolecule environments are normal; each contacts at leastoneNa or Ca on its lone-pairside. Bond strengthscalculated using the observedO-H and H.'.O valuesagree fairly well with thoseobtained from an empiricalrelationship. A good cleavage parallelto {120},rather than to {010}as previously suggested, breaks hydrogen and Na-OH(5) bondsonly; a reproduciblefracture surface is parallelto {001}.
Introduction by Weichel-Mooreand Potter(1963), who conclude their paper with the sentence"The discoveryand According to Dqna's Systemof Mineralogy(Pa- study of ulexiteshow that it is possibleto find in lacheet al.,195l, p. 345-348),ulexite has been recog- Naturea devicealmost as good as a man-made[opti- nizedas a validspecies since about 1840. Its formula, cal] fibre bundle." NarO'2CaO'5BrOs'16HrO, assignedby chemist Thecrystallography of ulexitewas studied by Mur- George Ludwig Ulex for whom the mineral was doch (1940)and reexaminedby Clark and Christ named,has been associated with the mineralalmost (1959),who alsogave indexed X-ray diffraction pow- as long. Ulexiteoccurs in saltplayas and dry saline der data. Clark and Appleman(1964) solved the lakesand hasalso beenfound associatedwith gyp- structure,but publishedonly a brief reportbecause sum deposits.Frequently it occursin bundlesof fi- the data weretoo numerousto be refinedeconomi- bers matted togetherto resemblea cotton ball, and cally at that time. Eventoday the dimensionsof the this habit is knownas "cottonball ulexite." Less fre- structurerefinement are too large to be treatedeco- quentlythe fibers are oriented in parallelbundles that nomically by the usual [ull-matrix least-squares transmitan optical image with remarkableclarity method.In the presentwork with newdata, we have and resolution.This phenomenonwas investigated beenable to determineall the hydrogenpositions and
0003-004x/78l0 l 02-0 l 60$02.00 I 60 GHOSE ET AL.: ULEXITE
Table L Crystallographic and optical data for ulexite below3o (/), whereo (/) is the standarddeviation of the intensity(1) as calculatedfrom the countingsta-
Ctn1 e fnL Lo gr aphi c da t a, Optical data tistics.The intensitydata were corrected for Lorentz and polarizationfactors. No correctionswere made Triclinic, Pl Biaxial positive (+) for absorption(p : 29.73crn-t, MoKa) or extinction = (oH)6.5H2o1 Z 2 [NaCaBso6 effects.At the end of the refinement(see below) no = a 8.816(3)i a = r.asl ) seriousdiscrepancies between strong observedand b =I2.e1o(7) B=1.506>** c = 6.67A(Ll 'r = I.s2e I calculatedstructure factors with low 2d valueswere a:b:c = O.6850tl:0.5189 2V' caLc. = 79o found, indicatingthat there were no seriousex- o = 90.36(2). tinction effects. B = I09.05(2) . y = r04.98 (4) . 9p v = 688.4(4) i3 Hydrogenlocations and refinementof the structure Y 11
Table2. Atomiccoordinates and thermalDarameters for ulexite
a (iz)t C+m'at'n-1 Coordinatee* ThemaL ptrmet.r" roTe* a -eq, uzz utz uzg
Ca 0,I422(L) 0. 0256(1) 0.3042 ( 1)l L.59 O.O2O20.0115(2) 0.0103(2) o.u-zs(2) 0.0020(2) 0.0017(2) -0.0003(1) Na 0.4774(I) 0. 501s(1) 0.2438(2)] 4.06 0.0514 0.0316(6) 0.023s(6) o.0266(5) 0.0100(s) 0.0096(5) 0.0012(4)
B(1) central T 0.0506(3) o.2oo2(2) 0.668s (4 )l r.9r o.0242 0.012(1) 0. 012(1 ) 0.015(1) 0.00s6(9) 0.0016(9) 0.0009(9) B(2) r 0.3464(3) o.27OO(2) 0.8ee5(4)l 1.90 0.0241 0.013(1) 0,0r3(1 ) 0.013(1) 0.004(1) 0.0027(9) 0.0007(9) B(3) ^ -0.1891(3) o.2240(2) 0.782r(4)12.25 O.O28s 0.015(1) 0.015(r) 0.016(1) 0.007(r) 0.004(1) -0.0009(9) B(4) T 0,2344(3) o.0737 (2) 0.7834(4)j L.92 O.0243 0.013(1) 0. 01r(1) 0.01s(1) 0.0050(9) 0.0037(9) 0.0021(9) B(s) A -0.1737(3) o.2697(2) 0.4276<4)l2.10 O.0266 0.015(1) 0. 01s(1 ) 0.013(r) 0.006(1) 0.0021(9) -0.000s(9)
o(1) T-A 0.1985(2) 0.2890(1) 0.7929 G)11 2.2L 0.0280 0. 0r23(8) 0. 0107(8) 0.0219(9) 0. 0043(6 ) -0.000s ( 7) -0. 0012( 6) o(2) T-I 0.1024(2) 0. 1066(r) 0.6208(2)lt.72 0,0218 0. 01rl (7) 0.0121(7) 0.0118(7) 0.00si (6) 0.00r1 (6) -0. 0008( 6) o(3) T-A -o.0289(2) o.2424(t) 0.4664(3 )l 2.4r 0.0305 0. 0r-38(8) 0.0180(8) 0.0141(8 ) 0.0087(7) 0.0039 (6) 0. 0044( 6) -0.0650(2) (6) o(4) T-T 0.1677(r) 0,7840(3 ) i 2.36 o,0299 0,0147(8) 0. 01s2(8) 0. 0146(8 ) 0,0082(7) 0.0064(7) 0.0030 o(s) T-A 0.3701(2) 0. 1692(1) 0,9112(3)l 1 01 0.0244 0.0123(8) 0. 0114(8 ) 0.0170(8) 0.0036(6 ) 0.002s(6 ) 0. 0007(6) o(6) T-A -o.259r(2) 0.2602(2) 0.s667(3)l 3. 10 0. 0393 0. 0160(8 ) 0. 028(1) 0. 0122(8 ) 0. 0134(8 ) 0.0031(7) 0.001r(7) -0.0008(7) oH(l) T o.2992(2) 0. 0085(r ) 0.6683(3)2.80 0.0355 0.0158(9) 0.0r9s(9) 0.0194(9) 0.0100(7) 0.ooso(7) oH(2) r -o.LL32(2) 0, 3228(1) 0.9369 (3) o.0323 0.0227(9) 0.0131(8) 0.01s2(8) 0.o014(7\ 0.0023(7) -0.0009(7) oH(3) r 0. 1s80(2 ) 0. 0068(1) 0.9265(3) 2.25 0,0285 0.0153(8) o.or26(8) o.017s(8) 0.0059(7) 0.0037(7) 0.0028(6) oH(4) T -0. 3183(3) 0.1469(2) 0.8361(3) 4.15 0.0525 0.020(1) 0.0163(9) 0.039(1) 0.0046(8) 0.0167(9) -0.0002(8) oH(5) A o.4716 (2) 0.3s97(1 ) 0.002s(3) 2,25 0.0284 0.0136(8) 0.0124(8) 0.0211(9) o.oo33(i) -o.oo17(7) -o.ooo4(7) oH(6) A -0.2383(2) 0,3130(2) o.2405(3)4,23 0.0535 0.024(1) 0.03s(1) 0.0128(8) o.0202(9) o.oos2(7) o.oos6(7)
H20( 1) 0 .1462(2) 0.2L06(2) o.2240(3)3,r7 0.04p2 0.020s(9) o.o22(L) 0.0196(9) 0.010s(8) 0.0064(8) 0.0031(7) H2o(2) o.4293(3) 0.1021(2) 0.3302 (4 ) 3.44 0.0436 0.018(1) 0.029(1) 0.026(1) o.ooo1(9) 0.0007(9) 0.0018(9) H20(3 ) 0.4707(3) 0.3s86(2) 0.4870(4) 1 7? 0,o472 O.024(r) o.o22(r) 0.031(1) 0.0079(9) 0.0067(9) -0.0028(9) H20(4) 0. 1925(3 ) o.4794(2) 0.18s0(4)3.94 0.0499 0.028(1) o.o22(L) 0.034(r) 0.0040(9) 0.001(1) -0.000(r) H2o(5) o.2252(3) 0.476s(2) 0.6107(4) 5.07 0.0642 0.040(1) 0.017(1) 0.041(1) 0.008(1) 0.0r3(r) 0.00s4(9)
H(1) oH(r) o.397(4) o.022(3) 0.698(s) 2,O 0.02s(9) H(2) oH(2) -0. 134(4 ) 0.3i3(3) 0.878 (s ) 2.4 0.03(1) H(3) orl(3) 0.193(4) -0. 045(3 ) 0.9is (s ) 2.3 0. 029 (9) H(4) oH(4) -0.379(s) 0.169(3) o.872(6) 2.7 0.03(1) H(5) oH(5) 0. s55(s) 0. 342(3 ) 0.078 (6 ) 3,4 0.04(r) H(6) oH(6) -0.189(5) 0. 313(3 ) 3.4 0.04(1)
H(7) r{2o(1) 0. 086(4 ) 0.230(3) 0.28s (5 ) 1.9 0.024(9) r{(8) H2o(1) 0.092(7) 0. 218(4 ) 0.088(9) 7.1 0.09(2) H(9) H2o(2) 0.439(6) 0.L27(4) o.229(8) 5.9 0.07(2) n(10) H2o(2) 0.513(s) 0.134(3) 0.418(6 ) 3,3 0.04(1) H(11) H2O(3) 0.392(5 ) 0.316(3) o.42r(6) 1' 0.04(1) H(12) r{20(3) 0.33s(3) 0.s13 (6 ) 3.4 0.04(1) n(13) H2o(4) 0. r-20(6) o. 431(4 ) 0.r13 (7) 5.1 0. 06(1) H(14) H2o(4) 0. 192(s) 0.473(3) 0.294<6) 2,5 0.03(1) H(15) I{zo(5) o.2o2(5) o.422(4) 0.664(6) 3.8 0. 05(1) H(r6) 1r2o(5) 0. 191(s) 0, 521 (4) o.662(6) 4.O 0. 05(r)
*See Fige. 4, 5. I=tetraTedtn, L=t?iangle, I-L=LLnkLng tetmhedton to trLot4le, etc. H at@6 fte resoeiated tLth the desig- rcteA Ufl Ol H^U. 4*Nwber ln puentheses is me stedild deuLatLm; for 0.1422(1) read 0.1422t0.0001, etc. +Bea.=8r2i.' Tenpenatwe-factor erpressi.m, ecpl--it2(u11h2a*24lr"k2b*2+Ur3l,2ca2+2lJ1zhka*b*cos ya+2tJ13h\.a*cacoaB*+ 2u23ke.l2^b*c6su*)1. Nmber in pmntlesee is me stqtdayd deutattn; for 0.0115(2) lead 0.0115L0,0002, etc. servedreflections, and 0.175 for the 1246reflections deviations in unit-cell dimension measurementsbut with 1 < 3o(/). The averageshift/error at this stage should be appliedconservatively because ofthe block was 0.27. method that had to be used for the refinement.The The final positional and thermal parameters are average estimated standard deviations in Ca-O, listed in Table 2. Observedand calculatedstructure Na-O, and B-O bond lengthsare 0.002,0.003,0.003 factors are given in Table 3.' The bond lengths and A, respectively, and in O-Ca-O, O-Na-O, and angleswith standarddeviations were calculated using O-B-O bond angles,0.07o, 0.07o, and 0.20o,respec- the program BoNu-e (Stewart et ol., 1972)and are tively. The standard deviationsin O-H, H . . .O bond listed in Tables 4-9. The estimated standard devia- distancesand O-H . . .O anglesare 0.04, 0.04 A, and tions in bond lensthsand anslesinclude the standard 4o, respectively.
' To receivea copy of Table 3, order document AM-78-063 Discussionof the structure from the BusinessOffice, Mineralogical Society of America, 1909 (1964), K Street N.W., Washington, DC.20006. Pleaseremit $1.00 in As describedby Clark and Appleman the advance for the microfiche. structure contains three kinds of groups: isolated GHOSE ET AL: ULEXITE r63 pentaboratepolyanions, Ca-coordination polyhedra, and Na-coordination octahedral chains, all cross- linked by hydrogen bonds.We now note that the Ca- coordination polyhedra also share edges to form chains (Fig. 1). These chains are separatefrom the Na-coordination octahedralchains (Fig. 2); the pen- taborate polyanionssandwich between the two kinds of chains to help link them together (Fig. 3). Both kinds ofchains run parallelto c, the fiber-axisoptical direction. The polyanions (Fig. 4) are the pen- Fig. l. View of the distorted hexagonal bipyramid of oxygen taboratetype with three tetrahedraand two triangles, atoms surrounding Ca and the chain formed by edge-sharing.The *b* with *a approximately vertical, fc 5:2L 'l 37",A = B (Christ and Clark, 1977),formula view is looking down horizontal to the right (ct. Table T) l8506(oH)6F-. pentaborate Stereochemicalconfiguration of the the triangular B-O bond has more covalentcharacter polyanion than the tetrahedral B-O bond. Within the B(4)- The [B,O.(OH).]3- polyanion found in ulexite is tetrahedron, the B(4)-OH(3) distance, 1.496 A, is formed by two six-membered boron-oxygen rings significantly longer than the B(4)-OH(l) distance, joined through a common tetrahedral boron atom 1.466A. This differenceis most likely due to the fact (Fig. a). Each ring consistsof the central BOn-tet- that OH(3) is further bonded to two.Ca cations rahedron, a BOr(OH)r-tetrahedron,and a BO,(OH)- whereas OH(l) is further bonded to only one Ca triangle sharing corners.The polyanion hasthe point cation. A similar differencein the B-OH distancesis symmetry 1 and the two,BOdOH)2 tetrahedraare in observedwithin the B(3)O{OH), tetrahedron,where crs-configuration with respect to the central borate the B(3)-OH(2) distance,1.492 A',is longer than the tetrahedron. B(3)-OH(4), 1.449A; in this case,OH(2) is the ac- The two boron-oxygen rings are nearly per- ceptor of two hydrogen bonds, one strong, whereas pendicularto eachother, the anglebetween the mean OH(4) is an acceptor of one weak hydrogen bond planesbeing 88.6o.Within eachring the averageB-B plus possibly one other very weak one (Table 8). separationis very similar (2.498 and 2.505 A); how- Neither of thesetwo OH groups is bonded to cations ever, the separationof 2.545A betweentwo tetrahe- other than boron; therefore,the hydrogen bonds ap- dral borons is significantlylarger than the separation parently play a significant role in determining their of 2.480 A between a tetrahedral and a triangular non-bridging B-OH distances.The temperaturefac- boron (Table 4). Both rings are nearly planar with tors of these two OH groups are the highest of any internal angles averaging 117.9" in ring I polyanion atoms in ulexite (Table 2). The averageof to(l)-B(l)-o(2)-B(4)-o(s)-B(2)1,and 118.4'in six O-H distancesis 0.80 A and the averageB-O-H ring2 [O(3)-B(1)-o(4)-B(3)-0(6)-B(5)].The maxi- angle,I l5o. mum deviation of a boron atom from the mean plane This same polyanion is found in linked form in in ring I is -1.40 A, in ring 2 *0.31 A;hence, ring 2 severalother structures,although none of thesehas is more nearly planar than ring 1. The average tetrahedral and triangular B-O dis- tances are in good agreement with similar values found in other borates.However, within eachborate triangle or tetrahedron, there are variations in B-O distances(Table 5) which are significant.Within the borate triangles,the (non-bridging) B-(OH) bond is significantlylonger (average1.383 A) than the bridg- ing B-O bonds (average 1.359A). Within the three borate tetrahedra the B-O distances(average 1.488 A) involving oxygens further bonded to triangular Fig. 2. View of the octahedronof oxygenatoms surrounding Na are distinctly longer than those(average 1.459 borons and the chains formed by edge-sharing.The view is looking down A) involving oxygens further bonded to tetrahedral +bx with +a approximately vertical, *c horizontal to the right (cl boron atoms. This result is a reflection of the fact that fable 6) t64 GHOSE ET AL,' ULEXITE
q
d
-
o 6
E E
q o o i
d a
o
o zd
6 ; a i
O o ;
6
o
o a
bo v = q
oo =
o/]. 3€ 66 5- a^ 96 goXE )J -ES o}| >d
d_ r6;-F t GHOSE ET AL.: ULEXITE t65 beenrefined by modernmethods. Simple chains com- posedof this polyanion,linked Dia an oxygenatom commonto a triangleof onepolyanion and a tetrahe- dron of the next (Christ, 1960),are found in the structureof probertite,NaCaBuO'(OH[' 3HrO (Kur- banovet al., 1963).Similar chains, modified by at- tachmentof a sidetriangle, are found in thestructure of kaliborite,HKMgz[B'O?(OH)s. OB(OH)r]r.4HrO (Corazzaand Sabelli,1966; Christ and Clark, 1977). The same polyanions,further linked into sheets (Christ, 1960),are found in thestructure of heidornite, NarCa'ClIBuOs(OH)r][SOo],(Burzlaff, 1967). The anhydrousanalog of the ulexitepentaborate-type polyanion,[BuO,r]"-, (5:24 + 3?n),is found in gar- relsite, NaBaaSizBzO,.(OH)o(Ghose et al., 1976). Fig. 4. The borate polyanion [B'O6(OH).]8- and its bond Other kindsof pentaborate-typepolyanions are dis- distances. cussedby Christ and Clark (1977). not too different in value, 2.421 A and The polyhedralchains tancesare 2.484A, respectively,the quite differentspatial ar- Thesechains dominate the structureand are un- rangementsaround the two cationsare striking (Figs. doubtedlyresponsible for thefibrous habit of ulexite. l-3) and clearly demonstratetheir structuraldis- Na is octahedrallycoordinated by four watermole- tinction. The Na-Na approachesare substantially cules and two hydroxyl ions, the latter from two closerthan those for Ca-Ca,3.4 A comparedwith 4.0 differentpolyanions. As Figure 2 shows,the octa- A, undoubtedlydue to the differencein cation hedra share edges OH(5)-OH(5)' and H,O(3)- charge.The Na temperaturefactor is considerably HrO(3)'to form chainsparallel to c. The four edge- higherthan that of the Ca (Table2), probablybe- sharingatoms are arranged around the central part of causeit is lighter,less highly charged, and has a lower the octahedron,water molecules HrO(4) and HrO(5) coordination. forming the vertices.As might be expected,the tem- perature factors for these vertex atoms are high Table 4. Ring angles,planes, and deviationsfrom ring planesfor (Table 2). The distancesand anglesfor the octahe- the borate PolYanion in ulexite dron (Table6) aresomewhat distorted from thoseof a regularoctahedron. The relationshipin a regular Ring Ring atone B-O-B mglee : octahedron,edge distance \/2 (center-vertexdis- B(1)-o(2)-B(4)- B(1)-o(2)-B(4) L2o.L(2)" o(s)-B(2)-o(1) B(4)-o(5)-B(2) L20.2(2)" tance), gives3.424 A using the averageNa-O dis- B(2)-o(1)-B(1) rzL.5(2)" tance, slightly longer than the observedaverage of B(1)-o(3)-B(s)- B(1)-o(3)-B(s) LzL.e(2)' 3.415A. o(6)-B(3)-o(4) B(s)-o(5)-B(3) r2o.3(2)" B(3)-o(4)-B(1) 122.7(2)" Ca is coordinatedby threeoxygen atoms and three Paraneters of planea* deflned by ring oxygena hydroxyl ions from threedifferent polyanions, plus ABCD two watermolecules. The latterare not thosecoordi- -4.10514 8.81817 -3.522L4 -1.56667 natingNa; the two polyhedralchains are separate in , 1.79183 9.48293 2.43L22 3.38038 the structure(Fig. 3). Figure I showsthat the Ca Angle between ring planes, 88.58o polyhedroncan be describedas a distortedhexag- Deoiatim frcn rtng-plotee defined by tluee onVgaa onalbipyramid, atoms OH(3)-H,O(2)-OH(l)-O(2)- Ringl . Rlng2 (A) (i) O(2)'-OH(3)'constituting the central part, and O(4)', Atom Deviation Aton Deviation HrO(l) forming the vertices.Edges OH(3)-OH(3)' B(1) +0.87 0 B(1) +0.234 B(2) -o,542 B(3) +0.306 and O(2)-O(2)' are sharedto producethe poly- B(4) -1.405 B(5) -0. 094 hedralchains parallel to c. Threeedges of tetrahedra oH(1) -1.841 oH(4) -o.525 oH(s) -u. otJ oH(5) -0.254 in the polyanionsare also edgesof the Ca polyhe- lhe eqmtions of the planes in direet space are of dron; theseare noted in Table 7 whereall the dis- thefomle+BU+Cz=D' uhene n, A, z are the atofrLc tancesand anglesare listed. eoordirctes in L units dtd D te the di.etance of the plme the origin in A units. Although the averageNa-O and Ca-O bond dis- fron t66 GHOSE ET AL,' ULEXITE Table5. Bonddistances and angles in theborate polyanion [B'O"(OH)u]. of ulexite tdtc g)c 1rygens of Orygens of DLstotee* Atans Angle(o)* ArryLe(o)* d) 0-B-0 arryLe 0-B-0 mgle (3.) Tetrahedra B(r_) -o (r) 1.486(3) o(1), o(2) 110.3(2) o(2), o(4) 109.0(2) B(L)-B(3) 2.5s2(s) -o(2) r.462(4) o(1), o(3) l-06.4(2) o(3), o(4) 110.8( 2) -B(4) 2.s37(4) -0(3) r..481(3) o(1), o(4) rLr.2(2) -B(2) 2.476(4) -o(4) r.4s2(4) o(2), o(3) r.oe. r-(2) -B(5) 2.48L(4) Average L.470 B(3)-B(5)' 2.48L(4) B(4)-B(2) 2.48o(4) B(3)-o(4) 1.456(4> o(4), 0(6) 1r_0.5(2) 0(5),oH(4) 110.8 (2 ) Average 2.501- -o(6) 1.498(3) o(4),oH(2) 111.0 (2) oH(2),oH(4) 1r.1.5(2) -oH(2) L.492(3) o(4),oH(4) 10s.9(2) -oH(4) L.449(4) o(6),oH(2) r.07.0 (2) Average r.474 B(4)-o(2) 1.466 (3) 0(2), o(s) 111.6 (2) o(s),oH(3) 109.8(2) -o(5) 1.493 (3 ) o(2) ,oH(1) r.05.1(2) oH(1),oH(3) r-10.4(2) -oH(1) 1. 466(1) o(2),oH(3) r_08.3(2) -orr(3) r_.496 (3) o(5) ,oH(1) LLO.7(2) Average r_.480 Triangl-es B(2)-o(1) 1.3s2(3) 0(1), o(5) L23.s(2) o(3), 0(6) L23.8(2) B(s)-o(3) 1.3s6(4) -o(s) 1.366(4) o(1) ,oH(5) 7L6.4(2) o(3),oH(6) l1e.3(3) -o(6) 1.361(4) -oH(s) 1.386(3) o(5),0H(5) L20.0(2) 0(6) ,oH(6) 1r-6.9(3) -oH(6) 1.380(3) Average 1.368 t 359.9 r 360.0 Average L.366 *Nzrnber in patentheses is one standad deu'Lation; fo! L.486(3t yead J,.486!0.003, ete. Hydrogen bonds cut-offvalues for hydrogen bonds (e.g.Brown, 1976, As expectedin this kind of structure,all the hydro- suggests3.1 A). Thus a final decisionabout the H(l) genatoms form bondsto oxygenatoms and thereis bonding cannot be made unambiguouslyon the basis evenone possiblybifurcated bond. All the oxygen of availabledata. The valuesin Table 8 show that the atomsexcept O(2) participatein the hydrogenbond- extremevalues of distances,both short and long, and ing. Most of thesebonds are illustrated in Figure5, maximum bending are all associatedwith hydroxyl andthe relevantvalues are listed in orderof increas- ions, although the averagevalues for all hydroxyl ing O...O distancein Table8. FollowingBrown ions do not differ significantly from the average val- (1976),the hydrogenbonds can be dividedinto two ues for the water moleculesexcept for O. . .O (ave.), classes:four strong,2.595-2.711 A andeither l2 or 13 2.883 A hydroxyls,' 2.833A HzO. The strong bonds weak,greater than 2.13 A,. Of theweak bonds, those are important in linking polyanions to one another greaterthan 3.0 A shouldprobably be classifiedas and to the cation chains. One of the strong bonds, very weak. H(l) makestwo contactsthat suggesta OH(5).'.OH(6), links triangle hydroxyls of poly- bifurcatedbond. In both contactsthe H...O dis- anions adjacent along a, and another links triangle tance is greaterthan the 2.4 A proposedby Baur hydroxyl OH(6) to tetrahedralhydroxyl OH(2) of a (1972)as an upperlimit, but in viewof the associated polyanion adjacent along c. The other two strong errors in our study we includeboth in the Table. bonds link water moleculesHrO(l) from the Ca poly- Chargebalance considerations (see Bond strengths) hedron and HrO(5) from the Na octahedron to ring suggestthat OH(4),being slightly charge-deficient, is oxygen atoms of the same polyanion. a more likely recipientthan HrO(2), which is fully The hydrogen atoms are all located about 0.8 A satisfiedby other bonds.However, the OH(l)... from their associatedoxygen atoms, the averageO-H OH(4) distance,3.194(3) A, is actuallylonger than being 0.80 A as previouslynoted. Correcting for rid- . 'H,O(2) the OH(l). distance,3.082(4) A, andboth 'zUsingOH(l).'.OH(a); with OH(l)...H,O(2)the average is are pushingthe limits that havebeen suggested as 2.865A. GHOSE ET AL.: ULEXITE t67 ing motion(Busing and Levy,1964) does not change Table 7. Bond distances and angles for the Ca polyhedron the averagevalue. Despite the high estimatedstan- ulexite darddeviations for individualvalues, this averageof l6 determinationsshould be a reasonablygood one Polyhedral edges Atons * Distmcet OEVgen Distpnel for X-ray work. It is 0.16-0.18A shorterthan the rfrt atona (A) O-H values determined from neutron diffraction studiesuncorrected for thermalmotion (Baur,1972, Ca-H2o<2) 2 .474(2) o(2)-oH(1), 2.342(3) Table2). The H'..O bondvaries from an average -on(1) 2 .4r7 (2) edge, B(4) T -o(4) ' oH(3)'-o(2)r, 2.40r (3) , 2.418(2) 1.85A for the four strongbonds to 2.12A for ll r,u,r-z edge, B(4)' T weak (excluding Neu- -H2O(1) 2 .438(2) 0H(3) -H2o( 2) 2.9s4(3) bonds the two longestones). -oH(3) ' oH(1)-H2o(2) 2.979(4) , 2.sr3(2) tron studiesgive H" .O approximately1.88 A for E,y, I-z o(2)-o(2) 2.979(3' -o(2) 2.5L5(2) shared edge" water molecules(Ferraris and Franchini-Angela, -o(2)r, oH(3)-or{(3) 3.208 (3) 2.s72(2) 7,!,L-z shared edge" 1912). -0H(3), 2.s84(2) Average 2.810 The averageH-O-H angleof the watermolecules a,A, L- z (Table with 108' (Ferraris Average 2.484 l{20(1)-H20(2) 3.0s7 (4) is 105' 9) compared and n20(1) -o (2) 3.067(3) H2o(r)-oH(3) ' 3.2r8 (3) ca-ca' , 4.L24 V,/,1--z H2o(1 ) -oH(3) 3.324 (3) Table 6. Bond distances for the Na octahedron in l{20(1 ) -oH (1) 4.085 and.angles 3. 961 | e'!'ts H2o(1)-o(2) 4,4TL Average 3.527 AngLes Oetahedral edges o(4)'-0(2) 2.372(3) 111,36' CalCa-Ca" edge, B(1)' T Distmcel 2xAgen Dietmet IO2.00o " ca-OH(3)-ca" o(4)'-on(3) 3.097(3) rlt qtma (A) 108'32o Ca-O(2)-Ca' o(4)'-oH(3)' 3.157(3) o(4)'-oH(1) 3.4s7(3) Na-H2O (4 ) 2. 3s1(3 ) oH(5)-oH(5) 3.532(3) o(4) 'i-H2o(2) 3.944 | -n2o(3) , shared edge o(4) '-o(2) 4.310 2.383(3) "t, I-r,L-a ,I-z H2o(3)-H2o(3) 3.s36 (3) -r{20(5)|, shared edge Average 3.390 2.418 (3 ) I-c,L-A ,l-z H2o(3 ) -oH (5 ) 3.256(3) -oH(5) 2.428(3) q2o(3)'-oH(5)' 3.423(3) Average of 18 3.242 -H2o(3) 2.464(3) r-Hzo(1) -oH(5)' Average o(4) 4.733 , 2. 48O(2) l-x,L-A z , n2o(5)'-oH(s) 3.262(3) 0-Ca-0 mgles AVerage 2,42L H2o(5)'-n2o(3), 3.243(4) OaAgen Ang1,et Orygen Anglel Hzo(5)t-Hzo(3) 3.2L4(4) atons (") atone (e) Na-Na', ? ?r q/2\ ttzo(5)'-os(5)l 3.46e (3) l-x,l-A ,l-z ' -Na"' Average 3.297 o(2),ou(l) 56.65(7) o(4),,o(2) s6.66(7) - - 3.408(2) 0H(3) s6.33(6) o(4) 76.44(6) L+, L-! ,2 (7) ' H2o(4)-H20(3)' 3.234(3) oH(3),n2o(2)"0(2)' 72.37 o(4) "oH(3)1oH(3) 7e.62(6) H2o(4 -H2o(3) (4) 76.13(8) o(4) ',oH(1) 91.30(6) AngLes ) 3. 360 oH(1),H2o(2) H20(4)-oH(5)' 3.68e (3) o(2),o(2)' 7L.68<6) o(4) 109.44(8) Hzo(4) -oH (5 78.00(6) o(4) 121.80(6) Nar-Na-Na" L66.67(6)' ) 3.762(3' oH(3),oH(3)' "H2o(2) Na-oH(5)-Na" 87.95(8) Average 3.511 Average 58.53 "0(2)Average 89.2L Na-tt2O(3)-Na' 86.27(9)" Average of 12 3.415 H20(r),H20(2) 78.10(8) Average of 18 83,49 H2o(1),o(2) 76.49(8) H20(4) -H2o(s) 4.738 H2o(1),oH(3)' 8r.08(6) o(4) ].54.L2(6) r{20(1),oH(3) 82.82(7) "H20(1) O-Na-OarryLes H2o(1),oH(1) 114.57(6) H2o(1),oH(2)| 123.35(8) O&Agcn Angle+ tuygen AngLet 92.14 atffis (") atma (o) Average *AtmLc as in Toble 1 mle88 trmsfomed oH(s),oH(5)| 92.0s(8) H2o(4),H20(3)'86.2(1) coordinates frm noted.. H2o(3),H2o(3)' 93.73 (9) H2o(4),H2o(3) 88.4(1) the lable 1 Daluea u puentheses ie me stmdard detiatin; H2o(3),oH(5) 83.44 (e) H2o(4),oH(s)' 9e.5s(9) tNwnber in fu | 2.414(2) ?ead.2.414t0.002, etc. H2O(3)',oH(5) 89.4s (9) r{2o(4),oH(5) 103.8s (9) Average 89.67 Average 94,50 H2o(5)',oH(5) 84.s9 (9) Average of 12 89.90 Ilzo(s)r,Hzo(3)' 8s.0(1) 1 (Baur, l97 2, T able Hzo(5)',Hzo(3) 82.34(e) H2o(4),H20(5) 166.8(1) Franchini-A ngela, 1972) and 09' H2o(5)',oH(5)' 90.19 ( 9) 2) reported from neutron studies. Following the Average 85.s3 classificationscheme proposed by Chidambaram et ql. (1964) as revisedby Ferraris and Franchini-An- *Atmic eoordirctee re i.n Iable 1 mless ttmsfomed frm the Iable 1 DaLues u noted. gela(1972), all the water moleculescan be assignedto tYmber in puntheses is me etmlard deuiatim; for 2, i.e. conlacting two cations on the negative 2.351(3) read 2.351!0.003, etc. Class side,with the possibleexception of HrO(2), wherethe GHOSE ET AL.' ULEXITE Fig.5. A view ofthe ulexitestructure nearly along thea*-axis, showingsome ofthe hydrogenbonds and selectedatoms. Adapted from Onrre drawings (Johnson, 1965). presenceof an acceptedhydrogen bond is ambiguous results.One of these usesthe empirical valuesgiven as previouslydiscussed. HrO(3) is coordinatedto two by Zachariasen (1963) for hydrogen bonds in borate Na cations and thus is Class2,Type A, with two lone structures, and the good results show that these re- pairs directed toward two monovalent metal ions. main today quite satisfactoryfor this kind of struc- Water molculesHrO(4) and HrO(5) are Class2,Type ture. The other set of valuesis from direct calculation G, one lone pair being directedtoward a monovalent of strengths using our X-ray values for O-H and metal ion and the other toward a proton. HzO(l) is H. . .O in the generalequation presentedby Brown Class2, Type H, with one lone pair directedtoward a and Shannon(1973), s : so(R/Ro)-N,with sochosen divalent metal cation and the other toward a proton. as 0.825 (strength of an averageO-H bond), Ro : If HrO(2) acceptsthe long bifurcated hydrogen bond, 0.80 A (averageO-H distancein ulexite), and N : it is also Class 2, Type H. If it does not accept this 1.6 (empirical constant). It is encouragingto note bond, it is classifieda Class l', Type J, with a divalent that the sums obtained using this direct calculation metal ion along a lone-pair orbital, in view of the are in generalsatisfactory and thus it may, after all, anglesinvolved (Table 9), which are not appropriate be possible to obtain hydrogen-bond strengths di- for the metal ion being located along the bisectrixof rectly from good X-ray results. We tried applying the lone-pair orbitals. valuesobtained from the curve for valenceus. H' . .O distancesgiven by Brown (1976,Fig.3), but whatever Bond strengths the reason, those values do not produce sums as As these values were compiled, it became evident satisfactory as the ones obtained from Zachafiasen that the strengths assigned to hydrogen bonds were (1963). Finally, with respect to the possible bifur- crucial to the sums. In Table l0 we prbsenttwo setsof cated hydrogen bond involving H(l), the bond GHOSE ET AL' ULEXITE 169 Table 8. Hydrogen bonds in ulexite Table 10. Bond strengths (.s)for the oxygen atoms ln ulexlte o (1)* (2)* s' Dnor (2)* s' Dmo? Total Dffio? Hldrogen Acceptor Distme (A) An4Le( )+ onqm st aceepted otAgen atom* of oryge! u e 0...0 o-H H...0 o-H...o atm H* H-DM6 atm* donor qton* B o,25 2.0! os(6) 6 oH(2) 2.s9s(3) 0.87(5) 1.73(5) r70(4) o(1) 15 0.20 L,96 r,A,z-I 1.84(4) 163(4) n20(1) 7 o(3) 2.668(3) 0.8s(4) o(2) 1.98 none 1. 98 r{20(5) 15 o(1) 2.700(3) 0.7e(4\ r.92(4) 166(5) o.26 2,O2 (5) r.92(4) 171(4) oH 5 or{(6) 2.7aLG) 0.79(4) o(3) 7 o.22 1qc r+L,A, d 0.17 2.O2 8 0.18 2.O3 (4) o(4) 1.85 H2o(4) 14 H2o(5) 2.163(4> o.73(4) 2.04(4) 171 0.12 0.17 2.00 L6 oH(6)" 2.840(4) 0.83(s) 2,13(5) 145(4) H2o(5) o(s) 1.7r 4 0.15 0.17 9 2.O3 c,1-A,I-2 0.16 0.17 2.05 H2O(2) 9 o(5) 2.856(3) 0.77(6) 2.12(5) 159(5) (6) r.72 10 0.17 0.18 12 2.O1 r,A,2-L 0 oH(2) 2 r12o(4)' 2.864(4) O.79(4) 2.tO(4) 162(4) 0. 90 t.94 e,I-A,L-z oH(1) r.04 0.84 none r.88 H20(r) 8 o(4) 2.871(3) 0.8e(s) 2.04(s) r-s5(5) 0. 83 0.31 0.17 2.02 r,A, z-I oH(2) 0. 71 0.84 6 0.24 0.17 13 1.96 0.84 L.97 (3) H2o(3) 12 o(6) 2.88r(4) 0.84(5) 2.04(5) 173 oH(3) 1,13 0.78 none r, 91 a+l,A,z 0. 88 0.16 0.08 + 1A' 2.882(3) 0.78(4) 2.11(4) 166(6) H2o(4) i3 oH(2), oH(4) 0.80 0.90 3 0.18 0.13 1 2.Or r,A,z-I o,76 2.O4 2.15(3) 1s9(4) H2o(2) 10 o(5) 2.90r(3) 0.78(3) oH(5) 1.28 0.84 none 2.r2 a+l,A,z 0.69 0.24 0.19 Z.O9 oH(4).' 2.906(3) 0.83(4) 2.09(4) r.66(3) oH(3) 3 oH(6) 0.97 0.72 5 0.20 0.17 16 2,06 e,A,2-z H2o(3) 11 H2o(1) 2.959(3) O.76(3) 2.2O(3) r72(s) 0.74 0.83 0. 14 1.98 n2o(1) o.27 0.75 0.70 11 0.16 r.8g o(5), 3.031(3) 0.75(5) 2.31(4) 130(2) 0.83 0,84 1.95 oH(4) -r + x-L,y,z H2O(2) 0,28 0.88 0.86 none 2.02 , ? rlro(2)" 3.082(4) O.79(4) 2.52(4) 129(3) 0.86 0.83 2.M r-o,a, L-z 0.90 0,75 none 2.OL oH(1) '-? n2o(3) 0,3s oH(4) 3.r94(3) 0.79(4) 2.48(3) 1s2(3) 0.83 0.78 0.17 ]. 98 l+fi,A, z H2o(4) 0.20 0.86 0,96 2 0,18 2,29 0.75 0.8r o.zT 1 . 96 0.18 1,98 *Atmie coord;.rctes 48 in TahLe 2 mI.ess twsfomed 6 noted. H2o(5) 0.18 0,84 0,78 L4 +Nwlber ia puentheees is ore st@dud deuiqti@; fov 2'595(3) vead 2.595!0.003, etc. "Bord strengths (ualnee unite) obtai'ned as follms' (1) s' = ts(Na) + t''(Cd + ,e(B) usLng aoerage frcn Bram Table9. Anglesaround the watermolecules in ulexite water oavgen, HAdrogen Anglee (o) clreeificatim* atm atme * n-n2u-d H-H20-C2,C2-H2O-C3 c2 c3 H-H20-C3 that best balanceis ob- 1 ca H(11) 100(5) I07(2) 107(1) strengthsclearly indicate urass z, 99(2) tainedwhen H(l)' ' .OH(4)is considereda hydrogen Type It lls (3) 125(3) bond and HrO(2) is assignedno chargefrom this 2 ca ?ll(I)rt I02(5) II3(2) ? 120(1) source,as hasbeen done in Table 10.Nevertheless, Class 2, ?9r(5) pieceof evidenceand cannot be Type H ?r 10 r37(4) this is only one ?83(4) consideredsufficient to resolvethe question. 3 11 Na Na' 111(4) 101(3) 86.3 (1) Class 2, L32(4) Cleavage,twinning, and fiber oPtics Type A I2 112(3) 109( 2) Murdoch (1940)reported two good cleavagesfor 4 13 Na H(2)' 106(5) 125(4) 110(1) ulexitb,one "very perfect" parallel to {0_10}and an- ura6s z! 1r0(4) Type G 100(3) other "not quite so good" parallelto {lT0}.He also (4) 103 reporteda poorcleavage parallel to {110} and an easy 5 15 Na H(1"4) 106(s) 94(3) 100(1) perpendicularto the elongation Class 2, 121(3) cross-fracturenearly Type G 116(2) direction.Examination of the structure(Figs. 3' 5) 118(3 ) showsthat the reportedcleavages do not seemrea- sonable,because they requirebreaking both Ca-O *Classificatton md. rctatim frm Eermris m-d. wmehini- AweLq- (1972). andNa-O bonds.Therefore, we havereexamined the lsee tert; H(1) mag not be fotuin4 a hydrogen bonl'. rf it find that a good ie rct, H2o(2) ie Clase 1.', rype J, A ? indieatee unbigww cleavageof somesingle crystals and desigwtime. cleavagealong the elongationis in fact parallel to 170 GHOSE ET AL. ULEXITE {120}.This is much more reasonablein terms of the scope. Clearly the crystalsare indeed randomly ori- structure, as only hydrogen bonds and Na-OH(S) ented about the fiber axis (a point that is confirmed bonds are broken between structural units. We did by X-ray diffraction precessionphotographs). How- not find other cleavagesbut do confirm a fracture ever, the crystals are not packed solidly, because surfaceparallel to {001}. spacesabout 0.5 micrometerin sizesurround them. If Murdoch (1940) found twinning "rare in separate one end of such a bundle is placedin a colored liquid, crystals,but rather common in the aggregate"and he the color can be observedtravelling capillary-fashion, gave {010}and {100}as well-determinedtwin planes. apparentlyalong thesespaces (R.C. Erd, U.S.G.S., Consideration of the structure suggeststhat such personalcommunication). Presumablythe spacesdo twinning could develop when mistakes in the chain not affect the light transmission.We do not know stacking occur as crystals come together during whether the individualswithin the spacesare actually growth. single crystals. To our knowledge, no experiments The excellent light transmission by some ulexite have been made to determine whether ulexite fibers crystalswas explainedby Weichel-Moore and Potter might be artificially arrangedto eliminatethe random (1964)in terms of fiber optics.They designateas core cross-sectionalarray and maximizethe core-to-cladd- the crystals aligned along the fiber direction c with ing index difference.A curious featureshown in Fig- index 7 approximately 1.529, and they show that ure 6 is the presenceof light and dark bands across cladding resultsfrom random orientationsofcrystals someindividuals. The dark bandsmight be dueto the about the fiber direction, producing a core-to-cladd- presenceof minute inclusionsof clay particles,but it ing index differenceranging from 0 to a maximum of is not clearwhy theseshould congregateinto bands. 7-d :0.038 (Table l). Their Figure 2 is a photo- We have not investigatedanother curious observa- micrograph of a ulexite fiber bundle taken between tion about ulexite single crystals. Murdoch (1940) crossedpolarizers at a magnification of fifty. In our and R. C. Erd, U.S.G.S.(personal communication) Figure 6 we show two views of the natural surfaceof note that some clear singlecrystals of ulexite remain a similar ulexite fiber bundle taken at much higher clear as long as they are storedwith othersbut, when magnifications with the scanning electron micro- isolated, appear to alter. Is there a loss of water, an Fig. 6 Two views of the same part of a natural surfaceon a ulexite fiber bundle, taken with a Cambridge 180 scanningelectron microscope by R. L. Oscarson, U S. Geological Survey. Distance between scale marks is 3 micrometers: (a) magnification 990X (b) magnification2590X. GHOSE ET AL.: ULEXITE 17l actualalteration to probertite,or decomposition?Fi- Corazza, E. and C. Sabelli (1966) The crystal structure of kalibo- CI Sci. mat' nat',41, nally, what characteristicsof their formationpro- rile. Accad Naz Lincei, Rend fs., crystalsfound at Bo- 527-552. ducedthe excellentlong clear Cromer, D. T. and J. T. Waber (1965)Scattering factors computed ron, California?The localityis apparentlyunique in from relativistic Dirac-slater wave functions ' Acta Crystallogr', this regard. 18,104-109 - and D. Liberman (1970) Relativistic calculation of anoma- lous scatteringfactors for x-rays.-/. Chem.Phys, J'i, l89l-1898' Acknowledgments Donnay, G. and R Allmann (1970)How to recognizeO'z-' OH-' We thank R. C. Erd, U.S. GeologicalSurvey, for information' and HrO in crystal structures determined by X-rays. Am' Min- discussions, and assistance in determining cleavages.The SEM eral..55.1003-1015 pictures were taken by R. L. Oscarson, U.S. Geological Survey. Evans. H. T, Jr. (1948) Relations among crystallographicele- Figures 3-5 were prepared by Floyd Bardsley, University of Wash- menLs Am. Mineral., JJ, 60-63. ington, Seattle.We are indebtedto C. L. Christ, U.S. Geological Ferraris, G. and M Franchini-Angela(1972) Survey of the geome- Survey, and Gordon-€. Brown, Stanford University, for dis- try and environment of water moleculesin crystallinehydrates cussion and manuscript review. For information about fiber optics, studied by neutron diffraction. Acta Crystallogr',828' we thank R. E. Newnham, The PennsylvaniaState University, R 35'12-3583. A. Young, Georgia Institute of Technology, and E. J. Weichel' Finger, L. W. (1969) Determinationsof cation distributions by Moore. Underhill Center, Vermont. least-squaresrefinement of X-ray data. Carnegie Inst Wash' Year Book, 67, 216-21'7. Ghose, S., C. Wan and H H. Ulbrich (1976)Structural chemistry References of borosilicates.I. Carrelsite,NaBarSirBrO*(OH)a: a silicobo- polyanion Acta Crystallogr', Baur, W H.(19'12) Prediction ofhydrogen bonds and hydrogen rate with the pentaborate [BuO,r]"- atom positions in crystalline solids Acta Crystallogr., 828, 832. 824-832. plot 1456-r465. Johnson,C. K. (1965) Onrep: A ForrnnN thermal-ellipsoid illustrations' Oak Ridge National Brown, t. D. (1976) On the geometry of O-H.. O hydrogen program for crystal-structure revised bonds.,4 cla Crystallogr., A 32, 24-31. Laboratory, O R N L-3794, N. V. Belov(1963) Crystal - and R. D. Shannon(1973) Empirical bond-strength-bond- Kurbanov, M, I. M. Rumanovaand 3H,O Russian]' length curves for oxides Acta Crystallogr., A29,266-282. structure of probertite, CaNa[B,O"(OH)4] [in 152, ll00-1103' Burzlaff, H. (1967) Die Struktur des Heidornit CarNarCl(SOo), Dokl Akad. Nauft SSSR, ulexite Am. Mineral , 25, Neues Jahrb. Mineral. Monatsh', 157-169. Murdoch, J. (1940) Crystallographyof B,O"(OH),. '754-762. Busing,W. R. and H. A. Levy (1964)The effectof thermal motion (1951) Dana's Systemof on the estimation of bond lengths from diffraction. Acta Crystal' Palache,C., H Berman and C Frondel Wiley' New York' logr., I 7, 142-146. Mineralogy,Tth ed., Vol. 2. John H. L. Ammon, C. Dickinson and S' Chidambaram, R., A. Sequeiraand S. K. Sikka (1964)Neutron- Stewart,J. M., G. J Kruger, of crystallographicprograms diffraction study of the structure of potassium oxalate mono- R. Hall (1972) The X-ray system Maryland, College Park, Mary- hydrate: lone-pair coordination of the hydrogen-bonded water . . Tech Rep. 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