Crystal Structure of Hydrochlorborits"C*Li'3,{""# )4 . OB

AmericanMineralogist, Volwne63, pages 814-823, 1978

Crystalstructure of hydrochlorborits"C*li'3,{""#)4 . OB(OH )'l Cl .7H2O , a seasonal

GoRnoNE. BnowN Departmentof Geology,Stanford Uniuersity St anford, Calfo rnia 943 0 5

lno Joe,NR. Cr-enr U. S. GeologicalSuruey 345 MiddlefieldRoad Menlo Park, Califurniq 94025

Abstract

The crystalstructure of the rare playa mineral hydrochlorboritefrom the SalarCarcote, Antofagasta,Chile-the secondworld occurrence-hasbeen solved by direct and difference Fouriermethods and refined to a conventionalR of 0.050,including hydrogen positions. Our crystaldata agreewith thosepreviously reported (monoclinic, 12/a, Z = 8, a = 22.183(3),b : 8.745(I ), c : 17.066(I )A, P : 96.705(l)"V : 3376.9(3)48,G : 1.8521;however, our formula unit contains one lesswater moleculethan previouslyassigned, resulting in a calculated densityof 1.841rather than 1.876g cm-s.The structurehas isolated borate polyanions, each composedof two boratetetrahedra and one borate triangle,cornerlinked to form a three- memberedring, with a sideborate tetrahedron corner-linked to the trianglel3:(2T + A) + Tl; this group is uniqueamong borate polyanions reported to date.The polyanionsare cross- linked by hydrogenbonds to water moleculesand by CaOdOH)'(HrO), polyhedra,four of which share two cornersand an edgeto form four-memberedchains. The most unusual featureof the structureinvolves the Cl anion, which is not bondedto Ca as expected,but insteadis hydrogen-bondedto eight oxygens(three hydroxyls and five water molecules)at distancesranging from 3.182to 3.3954.To our knowledge,this is the largestnumber of hydrogensbonded to Cl reportedto datein an inorganicstructure. The averageBIII-O, Brv- O, CavIrI-O,and Clvrlr...O distancesare 1.367,1.474,2.478, and3.296A, respectively. The good {001}cleavage and tabular morphologyparallel to {001}are consistentwith the structure,wherein slabs of composition2{CadHrO)'[BsOr(OH){'OB(OH)']}'*, which are parallel to {001},are hydrogen-bondedto [Cl(HrO)r]'-anions. This arrangementis alsoconsistent with the mineral'sreported slow solubilityin water at23oCand its observedseasonal nature. The formationof this mineralmay resultfrom a reactionbetween associated ulexite and halite whenpH increases.However, its apparentrarity suggeststhat sucha reactiontakes place only underunusual circumstances.

Introduction Hurlbut et al. (1977),who alsogave crystallographic, The playa mineral hydrochlorborite,3CaO. physical,optical and chemicaldata for the mineral. CaCL.4BzOs'2lH2O,was first reported from China Among the more interestingobservations made by (Ch'ien and Chen, 1965;Ch'ien et al., 1965),al- Hurlbut et al. werethe slow dissolutionin water(at thoughno detailedlocality information was given by 23'C) and the seasonalnature of hydrochlorborite; theseauthors. In 1966,a secondoccurrence of this the mineral apparentlyoccurs in this locality only mineralwas found in northernChile. In this locality, during the dry seasonof the year.In order to shed hydrochlorboriteis associatedwith ulexite,halite, light on its slowrate of dissolutionand to determine and clayminerals. This occurrencewas described by the structuralrole of waterin thismineral, we under- 0003-004x/78/0910-08 14$02.00 814 BROIYN AND CLARK: HYDROCHLORBORITE 815 took the structuredetermination and refinementre- gram Library. For this calculationand the refine- portedbelow. Our investigationshows that thechem- ments discussedbelow, neutral atomic scattering ical formula, 3CaO.CaCl2.4B2Os.22H2O, assigned factors were calculatedby using the analyticalex- by the studiesreported above, has one water molecule pressionand coefficientsof Doyle andTurner (1968) too many,the correctformula having only 2l water and the anomalousdispersion values of Cromerand molecules(Brown and Clark, 1977). Liberman(1970); for hydrogen,the SDScoefficients givenby Ibersand Hamilton(1974, Table 2.28) were Experimentaland computationaldetails used.The chemicalformula applied at this stagewas The crystal selectedfor intensity measurements the onereported by Hurlbut et al. (1977).The statisti- - wasprovided by R. C. Erd, U. S.GeologicalSurvey, cal distributionof lEl, R, and lE I I confirmed and came from the Salar Carcote,Antofagasta, the centrosymmetricspace group 12/a.The signsof Chile,the localitydescribed by Hurlbut et al, (1977). the threelargest E-values were fixed as positive, and a The specimenwas prismatic, having {110}, {21l}, and set of the 100largest and 50 *o E s wasused in the {001}forms, and dimensions0.15 X 0.2 X 0.2mm. tangent formula program MulrnN (Main et al., The c* axisof thecrystal was oriented parallel to the l97l) to searchfor solutions.The solutionthat had d-axis of a Picker Fncs-I diffractometer,and the the bestfigure of merit provedto be the correctone orientation was refined by least-squaresfit of the andyielded an E-maphaving 40 majorpeaks. The l8 angularcoordinates of 35 automaticallycentered re- strongestpeaks were assignedto the 2 calcium,I flectionsin the ?i range30o to 40o.The resultingcell chlorine,and l6 of the l8 oxygenatoms of the asym- parameters[a 22.783(4),b : 8.749(l), c : metric unit. Structure-factorcalculations based on I 7.06I (3)A,P : 96.696(5)",V : 3377 .5 Aal agree well this assignmentgave a conventionalresidual, R, of with thosereported by Hurlbut et al.la : 22.783(3), 0.260.The positionsof the four borons and two b :8.745(r),c: r7.066(l),{,B :96.705(r)o, V: remainingoxygens in the asymmetricunit werelo- 3376.9(3)Asl;however, Hurlbut's values were used in catedby a three-dimensionaldifference Fourier syn- all furthercalculations because of theirhigher preci- thesisand by studyof a structuremodel. Two of the sion.Space grotp 12/a, assignedby Hurlbut et al. on oxygenpositions assigned using E-maps were incor- the basisof systematicabsences on precessionphoto- rect; however,in retrospect,the E-maps frorn the graphsand morphology, was assumed to becorrect. correct Mut t,,rN solution were found to show all A total of l44l nonzerosymmetry independent nonhydrogenatoms of the asymmetricunit, plus sev- intensitiesin the angularrange 5o to 40o 20 werc eral spuriousones, measuredusing an os-N scanmode, a scanrange of Seven cycles of full-matrix isotropic refinement 2.0" plus the a,-a, dispersion,graphite mono- were computedusing RrtNr (Finger and Prince, chromatizedMoKo radiation, and a solid-statede- 1975),resulting in a conventional,unweighted,Fl : > - tection system.Two standardreflections, at 0o and ll4l lF"ll/> lr,l of 0.071.Twelve additional 90o x, were monitored every 30 reflectionsand refinementcycles having anisotropic temperature fac- showedno significantvariation in integratedintensity tors reducedthe unweightedR to 0.058(R = 0.073, duringthe 3 daysrequired for datacollection. Of the all data);l8 ofthe 1253structure factors greater than observedintensities, 188 were less than 3o(1),where 3opwere not includedin therefinement because lAFl o(/) is the standarddeviation of the intensity(1) as waslarger than an arbitrarycutoffvalue. The average calculatedusing the formulaof Corfieldet al. (1967) shift/errorat this stageof refinementwas 0.03r. The and an instrumentalinstability constant of 0.04. structure model from the isotropic refinernentwas Thesedata werecorrected for Lorentz and polariza- used to compute another three-dimensionaldiffer- tion factors,the latter assuminga 50 percentideally enceFourier synthesis, which, together with crystal mosaicmonochromator crystal; no correctionswere chemicalconsiderations, provided approximate loca- made for absorption(p : 9.00 cm-l, MoKa) or tions for the 2l hydrogenatoms in the asymmetric extinction effects.After refinementwas completed, unit. Six additionalcycles of refinementwere carried no systematicdiscrepancies between strong observed out in whichpositional parameters and an isotropic and calculated structure factors were found, in- thermalparameter of eachhydrogen were allotred to dicatingthat therewere no seriousextinction effects. vary, and the positionaland anisotropicthermal The resultingstructure factors were convertedto parametersof the 25 nonhydrogenatoms were fixed normalizedstructure factors,.E, using programs from at their refined values after cycle 19. The final the Universityof RochesterCrystallographic Pro- unweightedR was 0.050for the 1244observed ciata 8r6 BROWN AND CLARK: HYDROCHLORBORITE with f' greaterthan 3op;nine observationswere ex- ature factors for all atoms in the asymmetric unit, cludedfrom thesecycles because ofpoor fit. Thefinal and anisotropic thermal parameters for all non- unweightedR for all l44l data was 0.063.A final hydrogen atoms are listed in Table l, together with three-dimensionaldifference Fourier was featureless, the estimatedstandard error of eachparameter. Ob- confirming that the chemicalformula is indeed served and calculated structure factors from the final Ca.BrOruCl,.2lH2O. cycle of refinement(cycle 25) are compared in Table The final positionalparameters, isotropic temper- 2. and the orientationsand dimensionsof the thermal

Table l. Atomic coordinates and thermal parameters for hydrochlorborite

Equivalent 3 Structural Coordinates2 lsotropic B Therml paraneters x 10a I role ra (b2 Brr Bzz Bss Btz Bi g Bzs

Ca(1) 0. 03304(8) o.L7r1(2) 0.1619(1) 1.14(4) 7.s(4) 31(3) 8.6(7) -2.0(9) 2.4(4) r(1) ca(2) 0.2t377(8) 0.0s7s(2) 0.2956 (L) L.14(4) 6.8(4) 38(3) 8.1(7) 0.s(8) 2.3(4) 2(r) B(1) rlng r(r, 0.r592(4) 0. 31s0(11 ) 0. 1914(6 ) 0.8(2) s(2) 3r(r6) s(4) -4(s) 1(2) 6(6) R(2) rlng T(2) 0.1897(s) o.4666(r2) 0. 3166(6 ) r.5(2) e(3) 44(17) 11(s) -4(6) 3(3) s(7) B(3) 0. 0884(4 ) 0.4902(11) 0.2469(6) 0.8(2) 4(3) 0 14(4) -1(s) 2(3) t (7) B(4) side T(4) -0. 0107(4 ) 0.5596(r2) o.77L4(6) 1.1(2) 5(2) 44(76) 10(4) 1 (s) 1(3) r(7) CI 0.133s (r ) 0. 63s6 (3) 0.0084(1) 2.09(s) 12.8(6) 68(4) 14.1(9) 5(r) 2.6(6) 2<2) 0(1) 0. 1939(2) 0.3289 (6 ) o.2693(3) 1.r (1) 8(1) 36(9) 5(2) 2(3) 0(1) -1(4) o(2) r (1)-A 0. 1007(2) 0.3904(6) 0. 188s(3) 0.8(1) s(1) 4o(e) L(2) 7(3) -1 (1) -1 (3) o(3) r(2)-a 0.t293(2) 0.s301(7) 0.3076(3) 1.6(1) s (1) 68(10) r4(2) 7(3) -3 (1) -r4 (4) o(4) a-r (4) 0.0336 (2 ) 0. sss6(7) 0.2414 (3) .1.6(1) 8(1) 4e(e) 13(2) 2(3) 3(1) -15(4) oH(1) r (1) 0.r47 s (2) 0.1498(6) 0.1803 (3) 1.1(1) ro(1) 21(8) 5(2) -3 (3) 3(1) -e (4) 0H(2) r (1) 0.1934 (2) 0.3764(6) 0.1312(3) 1.4(1) 8(1) 54(9) 8(2) -r(3) 4(1) 0(4) oH(3) r (2) 0.2056 (2) 0.43ss(7) 0.4012 (3) 1.4 (r) 11(1) 54(9) 3(2) 1(3) o(1) -2(4) oH(4) r(2) o .23L8(2) o.5793(7) 0.2887(3) 1. 6(1) e(1) 58(9) 10(2) -4 (3) 4(r) -2(4) oH(5) r (4) -o.o575(2) 0.6534(7) 0.1975 (3) 1. 7(1) 8 (1) 71(10) 13(2) -6 (3) 3(1) 13(4) oH(6) r(4) -0.0302(2) 0.4001(7) 0. 1s07(3) 1.7(1) 8(1) 47(9) L7(2) -1 (3) 1(1) 8(4) oH(7) r (4) 0. 0110(3) o.6323(7) 0.1021(3) 1.6(1) 12(1) sr(9) 8(2) -3 (3) 3(1) 6(4) ri20(1) 0. 0s47( 3) o.2142(8) 0.0269(3) 2.8(1) L9(2) 103(11) L3(2) 8(4) 7(2) e(4) H2o(2) 0 . 0610(3) o.9L45(7) 0.1354 (4 ) 2.3 (r) 14Q) 52( 10) 22(3) -4(3) 2(2) -e(4) Hzo(3) 0.0s61(2) o.5759(7) 0.4218(3) 2. 1(1) 10(1) e8(11) r2(2) 7(3) 4(1) -3(4) Hzo(4) 0.2319(3) -0.1601(7) 0.3917 (3) 2.0 (1) r2(2) 64(10) r3(2) -1(3) 0(1) 2(4) H20(5) 0. 1358(3) 0.8s8s(7) o,2767(3) 2.7(r) 11(1) 46(10) 23(3) o(3) 0(2) 6(4) n2o(6) 0. 1637( 3) 0.1488(7) 0.4039(3) 2.r(r) 16(1) 62(10) 11(2) -1(3) 7<2) 2(4) HzO(7) 0.1745(3) o.9943(7) 0. 0443(3) 2.s(1) 14(2) 101(11) L2(2) -5(3) 4(2) -8 (4) H(1) 0H(1) 0.158(s) 0.120(rs) o.L47 (7 ) 4(3) n(2) oH(2) 0. 1s9(6 ) 0.431(16) 0. 096(8) 6(3) E I/ ?.7 oH(3) o.24s(5) 0.4s1(12) 0.418(6) r(2) n(4) oH(4) 0.242(4) 0.668(11) 0. 328(5) r(2) H(s) oH(s) -0.103(7) 0.670 (16 ) 0. 1s3(9) e(s) H(6) on(6) -0. 061(6 ) 0.379(16) 0.102(8) 6(4) H(7) orr(7) 0.0s0(s) 0.s70(13) 0.088(6) 3(2) H(8) H2O(1) 0.064(8) 0.1s8 (16 ) 0.003 (9 ) 11(7) H(e) H2O(1) 0.038(s) o.257(L4) -0.003(7) 3(3) H(10) E2o(2) 0. 037(6 ) 0 . 813(16) 0.117(8) 7(4) H(11) H2O(2) 0.098 ( 6) 0. 877(16) 0. 160(8) 5(3) H(12) n2o(3) 0. 086(4) o.564(r2) 0. 385(6) r(2) 11(r.3) Hzo(3) 0.079(5) 0.633 (17 ) o.4s6 (8 ) s(3) H(14) H2O(4) 0.211 (6) -0, 173 (16 ) 0.423(8) 5(3) H(ls) Hzo(4) 0.277(5) -0. 183( 13) 0.422(6) 2(2) H(16) H2O(5) 0.L42(9) 0.78s(16) o.211(8) ls (7) H(17) H2O(5) 0.093 (6 ) o. 84s(16) o.294 (8) 8(4) E(r-8) H2O(6) 0.ls7(5) 0. r1r (15) 0.440(8) 3(2) H(19) Hzo(6) 0.L74(6) 0. 241(18) 0.40e(8) 4(3) H(20) H2o(7) 0. 18s(5) r. 048(16 ) 0.004 (8 ) 8(4) H(21) E2o(t) 0.1-s8(8) 0. 907(16) 0.046 (8) 14(6)

'See Flg. I. T = tetrahedron, A = triangle, T-T = linking one tetrahedrol to aaoth.er, etc. H atona are assoclated wlth the OH or H2O as noted. 2Nunber ln parentheses is one standard devlation; for 0.03304(8),read 0.03304 t0.00008, for 0.3150(11), read 0.3150 IQ.OOLI, etc. (' I 3-1 sThemal parameters ia the expression, exp -l- h.r'.e..1' The thermal ellipsoid of B(3) { I rI . L i=l j=l nrnJ was nonposltlve definite, so g2 was arbitrarily set at 0. BROWN AND CLARK: HYDROCHLORBORITE 817

Table4. Bond distancesand anglesin the boratepolyanion of hydrochlorborite

I Atons Distance2 Atomsl Distance2 Atornsl Dlstance2 Atomsl Distance2' (i) (i) (i) ri,r B(1)-o(1) 1.470(11) o(1)-o(2) 2.453(7.) B(4) -o (4) r. 471(11) o(4)-oH(5) 2.289(8) -o(2) 1 .483(11) o (1)-oH (1) 2 .346(7 ) -oH(s) 1. 45s(1r) o(4)-oH(6) 2.4L4(8) -oH(1) r.477(Lr) o (1)-oH (2) 2.393(7) -oH(6) 1.494(L2) o(4)-oH(7) 2.466(8) -oH(2) 1.464(11) o (2)-oH (1) 2.370(7) -oH(7) 1.478(11) oH(s)-oH(6) 2.4s9(8) o(2)-oH(2) 2 .433(7) oH(5)-oH(7) 2.389(8) oH(1)-oH (2) 2.436(7) oH(6)-or1(7) 2.423(8)

average 7.474 average 2,405 average L.474 average 2.407

B(2)-o(1) r.459(r2) o (1)-o (3) 2.43L(7) B(3) -o (2) 1. 378(11) o(2)-o(3) 2.396(7) -o(3) r.476(t2) 0(1).oH(3) 2.42r (7) -o (3) I . 3s6(11) o(2)-o(4) 2.35e(7) -oH(3) 1. 471(11) o (1)-oH (4) 2.364(8) -0(4) 1.366(11) o(3)-o(4) 2.346(7) -on(4) L.49r(r2) o(3)-oH(3) 2.368 (7 ) average 1.367 average 2.367 o (3)-oH (4 ) 2.432 (7 ) oH(3)-oH(4) 2.427(8) B(1)-B(2) 2.s4rQ.4)

average 7.474 average 2.407 B(1) -B (3) 2.490(r4) B(2)-B (3) 2.480(r4) average of r.474 average of 2.406 B(4) -B (3) 2 .539 (L4) 12 18 average 2 ,512

Central 0ther Angle Central 0ther Angle aEom atoms (') aEom aEoms (')

B(1) o(1),o(2) r12.4(7) B(4) o(4),oH(s) I02.9(7) o(r),oH(1) 10s.5(7) o(4),oH(6) 109.1(7) o(1),oH(2) 10e.3(7) o(4),oH(7) rL3.4(7) o(2),oH(1) 106.4(7) oH(s),oH(6) 113.0(7) 0(2),oH(2) 111.3(7) oH(5),oH(7) 109.1(7) oH(1),oH(2) 111.8(7) oH(6),0H(7) 109.3(7) average 109.4 average 109.5

B(2) o(1),0(3) 111.8 (7) B(3) o(2),o(3) L22.4(8) o(1),oH(3) 111.s(7) o(2),o(4) 118.5(8) o(1),0H(4) 106.s (7) 0(3),0(4) 119.0(8) o(3),oH(3) 106.9(7) r 1(o o o(3),oH(4) 110.1(7) oH(3),oH(4) 110.0(7) average 109.5

9(1) B(1),B(2) r20 .4(7) 0(3) B(2),B(3) 122.2(7) o(2) B(1),B(3) 121.0 (6) o(4) B(3),B(4) 127.0(7)

lA11 ato." at E, y, z (Table I 'Number ) in parentheses ls one standard deviation; for 1,470(11), read 1.47010.011A- etc. vibrationellipsoids for calciums,chlorine, and oxy- portedin Tables4 and5 werecalculated using Ennon gens,calculated using L. W. Finger'sunpublished and the refinedboron, calcium,and oxygenpositions Ennon program,are given in Table3'. and variance-covariancematrix from refinement The distances,angles, and associatedstandard er- cycle25. Severaladditional cycles of refinementwere rors for the boratepolyanion and Ca polyhedrare- carried out during which the positionaland anisotropicthermal parameters of all cationswere fixed I at theirrefined values and the positionaland thermal To receive copies of Tables 2 and 3, order Document AM-78- parameters 087 from the BusinessOffice, Mineralogical Society of America, of chlorine,oxygens, and hydrogenswere 1909 K Street, NW, Washington, DC 20006. Pleaseremit $1.00 in allowedto vary. The resultingpositional parameters advance for the microfiche. and variance-covariancematrix were usedto com- 818 BROWN AND CLARK' HYDROCHLORBORITE

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a aa€9n909 9AAOnAA4 NO tr^ do< nnorqN6N 9@6AiOOd 60NO 9v oo$6a@F6 F o6d9N€O@ NO o o+{ss$h4 60$$$sn€ s nil6N d NQ NNNNNNNN NNNNNNNN {sti

N .\N .\N N NNN I trl - *\N --\NS N ll+- N^ VN Ed Sr 5 -\Sr F\N 55-S1 OE rl dv @o o -\N -\N | -\s NUd x l>t r* @ s H -llO Xd ^^r\N^l .. .. ,* \xrx ONNoi^ o xl o ovv.ovi Nîô"^ -\N *\N .\N @ oddsotv vnv9^î d o doorioo C)vOvNO$v dtl-\Nstt Nld NlE V N V iE ^^9^nS- o !^^q^^ sNvîlvv^ odNrOiN vvodvoot 6 ivvxvv T !d NV E N NV €d(!r6d< OO.\N9OU 6l tl 6d d6 o! d BROWN AND CLARK: HYDROCHLORBORITE 819

Fig. l. Stereoscopic-pairview(*btowardsreader)ofselectedportionsofthehydrochlorboritestructurewithspheresof arbitrarysize for atoms, showingthe Cl-H bonding, someCa-O polyhedralchains, and the borate polyanions.The O. . H bonds are not drawn, but most can be noted with referenceto Table 6. Drawing from Onrsp (Johnson, 1965). putethe H-O, H-Cl,H...O, H...Cl, andCl...O form slabsstacked sandwich-like along c. Theseslabs distancesand associatederrors reported in Table6. are held to eachother by hydrogenbonding to Cl Becausethe errors associatedwith the H positions anions.The Cl anionsdo not coordinateCa, aswas and thesebond lengthsare relativelylarge, the re- expected,but insteadare held in the structureby portedH positionsand distancesinvolving H should eighthydrogen bonds to oxygens.The Ca polyhedra be consideredas approximate.The largeerrors are are linked togetherin groupsof four through the duein part to the factthat our intensitydata extend sharingof two cornersand an edgeto form short only to 40" U and the numberof parametersto be chainsapproximately parallel to a; thesegroups do refinedis largecompared with the numberof data not link to form infinitechains parallel to b as pre- available. viously suggestedin the preliminaryreport of this work (Brownand Clark, 1977). Structuraldescription As shownby the stereographic-pairview in Figure The boratepolyanion l, the structureof hydrochlorboriteconsists of iso- Each polyanion is formed by corner-sharing lated borate polyanionswhich are linked to each amongtwo boratetetrahedra and a triangle,produc- other by Ca polyhedraand hydrogenbonding to ing a triboratering. This triboratepolyanion type is the most frequentlyreported in borate structures studiedto dateand has been given the notation 3: (27 + A ) by Christand Clark (1977). However, in hydrochlorborite,this basicunit is modifiedby attachment of a tetrahedronto the non-ringoxygen of the borate trangle(Fig. 2), and the notation becomes 3:(2T +A) * T. This isthe first such modification to the 3:(2T + A) polyanionreported to datez. The only othermodified triborate group known at thistime is the 3: (3T) + T polyanionin the structure of uralborite,Ca2[B3O3(OH),.OB(OH)s] (Shashkin et al., 1970;Simonov et al., 19771'Christ and Clark, 1977).Ten other borate structureshaving the isolated,unmodified 3:(2T * A) polyanionhave been discussedby Christand Clark (1977).

Fig. 2. View along b of one [B'O,(OH). .OB(OH),],- polyanion 'zModification by attachmentof a triangle,3:(2T + A) * A, was in hydrochlorborite,c approximatelyhorizontal, thermal ellipsoids reportedin the structureof a syntheticmagnesium borate of 50 percent probability except for unlabelled hydrogen atoms MgBnO,9HrO (Abdullaev and Mamedov, 1969), but the structure (spheresofarbitrary size).Drawing from Onrnp (Johnson,1965). is probablywrong (Wan and Ghose,1977). 820 BROWN AND CLARK: HYDROCHLORBORITE

In hydrochlorborite,B(3) is trigonallycoordinated hedrashare an O-O edge,3.0454, and eachshares a by oxygenanions which lie approximatelyin a plane cornerwith a Ca(l) polyhedronso that a four-mem- with B(3) ar an averagedistance of 1.367,{.This beredchain is formed,oriented approximately paral- averageBIII-O distanceis considerablyshorter than lel to a (Fig. 3). Thesechains are joined to others the averageBIV-O distanceof 1.474Afor the three throughborate polyanions to form ribbonsparallel boratetetrahedra. The shorterBIII-O bondsare pre- to a. Adjacentribbons are joined togetherthrough sumablyrelated to the stabilizationof the la'Land 3e' hydrogenbonding to form slabsthat are oriented molecularorbitals of [BOs]8-,which are composed parallelto {001}. of boron 2p and oxygen2p atomic orbitals,relative Chlorine coordinationand hydrogenbonding to the 3tz molecularorbital of [BOn]u-(Vaughan and Tossell,1973). The B(l)rV-O(2)-B(3)III-O(3)- The Cl anionis hydrogen-bondedto eightoxygens, B(2)Iv-O(1)ring is essentiallyplanar except for puck- five associatedwith watermolecules and threewith eringof the B(l)-O(l)-B(2) linkage.The anglefrom hydroxyls,all at distancesranging from 3.182to the ring to the attachedtetrahedron, B(4)IV-O(4)- 3.3954,the averageClvIIl-O distancebeing 3.2964 B(3)ttt,is 127.0",somewhat larger than the average (Table6). Theeight H. ' 'Cl distancesaverage 2.431., B-O-B angleof 121.2'within the ring. The experi- a valuethat agreesreasonably well with two H' ' 'Cl mentallocation of protonsin this hydratedborate is distancesof 2.41and2.49A reported in the structure in completeagreement with the rulesof Christ and of hilgardite(Ghose and Wan, 1978).The occurrence Clark (1977).Those oxygens attached to only one of six hydrogenbonds to chlorinehas been reported boron add a proton to form hydroxylion, and all in severalstructures, but, to our knowledge,hydro- oxygensnot associatedwith the boratepolyanion are chlorboriteis the firstinorganic structure in whichas doubly protonated,forming water molecules.As many as eight H. . .Cl bonds occur. Its average might be expected,the equivalentisotropic temper- O. . .Cl distanceis somewhatlonger than the average aturefactors for oxygenand hydroxylions are signifi- of 3.194Areported for sixO. . 'Cl in MgCl,.l2H2O cantly lower than those for the water molecules (Sasvariand Jeffrey,1966) where that chlorinedoes (TableI ), not coordinateto Mg but is hydrogen-bondedonly. The Cl(OH)'(HrO)upolyhedra in hydrochlorborite The calciumpolyhedra occur as isolatedpairs, sharingan edge,HrO(4)- The two crystallographicallydistinct Ca cations, HrO(4)', the long axis of the dimer beingoriented Ca(l) and Ca(2),are eachcoordinated by two oxy- alonga (Fig. I ). Thesedimers connect the slabsof gens,three hydroxyls, and threewater moleculesat borate polyanionsand Ca-polyhedralchains dis- distancesranging from 2.336to 2.681A(Table 5), the cussedearlier, literally holding the structuretogether grand meanCavrII-O distancebeing 2.474A. Each throughCl. . .H-O linkages. polyhedronshares two O-O edgeswith boratetetra- Eachproton in hydrochlorboriteis bondedto only hedra,averaging 2.350,4 in lengthrelative to the un- one other anion,either chlorine or oxygen,in addi- sharededges, which average 3.236A. As expected,the tion to its own associatedhydroxyl- or water-oxygen O-Ca-O anglesopposite shared edges are narrower atom. Of the l3 hydrogenbonds to water oxygens than thoseopposite unshared edges; the cation-cat- (Table6),2 canbe considered strong, O . . .O < 2.7A, ion repulsionsacross shared edges result in signifi- and I I weak,O. . .O > 2.7A. Of thelatter, only one cant distortionsof the polyhedra.Two Ca(2) poly- mustbe consideredvery weak, i.e. )3.0A. The sevenwater molecules can be classifiedfollow- et al. (1964) Ca 1' ing thescheme devised by Chidambaram and modified by Ferraris and Franchini-Angela (1972),which is basedon the coordinationof lone- pair orbitalson the wateroxygens. HrO(l), HrO(2), and HzO(S)each have one Ca cation orientedap- proximatelyalong the bisectrixof thelone-pair orbitals on the water oxygen,and henceare classifiedas lD. HrO(3),HrO(4), and HrO(6) eachbond to one Ca1 Ca 2 bond,both lying Fig. 3. An isolatedfour-membered chain of calciumpolyhedra Ca cationand acceptone hydrogen sharingcorners and an edgein hydrochlorborite,viewed along c. alongthe lone-pairorbitals, and so are classifiedas Adaptedfrom a drawingofOnrnp (Johnson,1965). 2H water molecules.The seventhwater molecule, BROWN AND CLARK HYDROCHLORBORITE 821

Table6. Hydrogenbonds in hydrochlorborite

Donor Hydrogen oxygen aton of Distance2 (.&) Ang1e2 (o) aE.om donor Acceptor atoml o...o 0-H H...0 0-H...0

Hzo(o) (19) H oH(3) 2.68s(e) 0.8s(15) 1.8s(1s) 168(13) 3 1 Hzo(t) H(20) oH(3): er -95 2.693(8) 2 2-a 0.88(13) 1.88(13) ls2(13)

average 2 strong bonds 2,689 0.86 r. 86 r60 Hzo(s) (12) H o(3) 2.740(8) 1.00(10) I . 7s(11) 170(8) H2o(2) H(10) oH(7) 2.7 4e (8) 1. 07(13) 1. 70(13) 168(1 3) oH(s) H(s) Hro(6): e' r+a';-z 2. 805(8) 1 . 21(13) 1.61(13) 166(13) oH(1) H(1) Hro(7): x, y-1, z 2.81e(8) 0.69(r2) 2.74(L3) 167(13) H2o(2) (11) H H?O(5 ) 2.829(8) 0.96(13) 2.07(r3) 134(11) oH(3) 1-3 I H(3) Hro(7) z -a'V 2.847(8) O.92(ro) 1.93(10) 174(10) H2o(s) H(17) 1" oH(6): *t A -it z-z 2. 856(8) 1 . 06(13) 1. 87(13) 153(13) H20(1) H(9) 0H(7): i, t-y,Z 2.849(8) o.72(r2) 2.r4G2) r74Q2) 0H(4) H(4) Hzo(4) t s, A+7, z 2.871(8) 1.03(10) 1.87(10) 163(7) Hzo(s H(16 ) ) o (3) 2.927 (8) 0. 66(13) 2.32(r3) l-5s(13)

10 bonds 2.829 0.93 (1) H2o Hzo(:)t 2.46 (r3)

average all 13 bonds 2.829 0. 90 L.97

0...cl H...Cl 0-H...CL 1 H20(6) H(18) q' -A' 1 i t+z 3.182(6) 0.74(13) 2.53(14) 148(13) 7 7 Hzo(4) H(14) CI: a' -A' V z*" 3.220(6) 0.88(14) 2.43(r4) 16s(12) oH(2) H(2) C1 3.274(6) 1.05(14) 2.36Q4) 14s(10) 71 H2O(4) H(rs) -A' L -z V-E' 6 V 3.283(6) 1.00(11) 2.27(rL) 1s8(9) 1." Hzo(s) H(13) cI: *' -c' 2 t*z 3.32s(7) o.eo(ls) 2.48(L5) L57(12) Hzo(7) H(2L) c1 3.310(7) 0.85(15) 2.s0(15) 158(13) oH(7) H(7) c1 3.376(6) 1.10(11) 2.52(rr) r-34(8)

oH(6) H(6) vL. &, t-q, 4 3.39s(6) 1.04(15) 2.36(rs) 173(13)

average 8 bonds 3.296 o.94 z.cJ 155

nexc nearest neighbor, HrO(2) 3.7 69 (6)

lAtoms at r, U, z (Table 1 ) unless transformed as noted. in parentheses^is - ^1f91P.r one standard deviarion for 2.685(9) , read 2.685 t0.0094; for 1.85(15), read 1.85 lO,I5L- etc.

HrO(7), has two hydrogenacceptor atoms forming associatedwith the hydrogenpositions, as discussed bondsalong the lone-pair orbitalsand sois of Clasi earlier,the bond strengthsinvolving hydrogen have 2E. beenobtained using the O...O distancesand the The relative importanceof hydrogenbonding in associatedbond strengthsproposed by Zachariasen this structure is illustratedby the contributionof (1963), recently shown to be the best available hydrogento the bond strengthsummations for the method for evaluatinghydrogen bonding in borate oxygen atoms (Table 7). Becauselarge errors are structures(Ghose et al., 1978).However. to our 822 BROWN.AND CLARK: HYDROCHLORBORITE

Table7. Bond strengths(s) in hydrochlorborite buI et al. (1977)suggested that whenthe water table in the SalarCarcote intersects the clay layercontain- te (valence unlts) ing crystalsof hydrochlorborite,these crystals dis-

Ca-O O-B 0...0 solve.A mechanismfor the reformationof hydro- chlorboriteas the water table drops could involve the 0 (1) 1.54 0-44 1.98 o(2) L.7L 0.25 1.96 simpletype of polyanionequilibrium suggested by o(3) L.77 0.38 2.Ls o(4) r.76 0.22 t .98 Christ et al. (1967)to explaintransformations with on(1) 0.74 0.45 0.80 2.00 changingpH and activityof HrO amonghydrated oH(2) o.77 0.27 0.88 L.92 Na, Ca, and Mg borateminerals. The associationof oH(3) 0.76 0.81 0.51 2.08 oH(4) o.72 0.30 0,82 1.84 hydrochlorboritewith ulexite, NaCaIBuOu(OH).]. 0B(s) 0.79 0.28 0.80 1.87 5H2O,and halite permits the followinghypothesis. If oH(6) 0.71 0.26 0.88 0.18 2.03 on(7) 0.74 0.88 0.43 2.05 we assumethat total boron in the systemremains H20(1) 0.26 1.73 1. 99 constantbut that the activitiesof Na+, Ca2+,H+, Hp(2) 0.28 1.56 1,84 H2O(3) o.25 1.65 0.09 r. 99 Cl-, and HzOcan be varied independently, following n p(4) o.23 1.75 0. 17 2.L5 the suggestionsby Christ et al., the transformation sr(s) 0.25 1.66 0.19 2.10 Hp(6) 0,27 L.62 0.20 2.09 between ulexite and hydrochlorboritemay be ex- H20(7) r.62 0,38 2.O0 pressedin termsof the polyanionsas c1 1.00 4[8,O6(OH)6]'-f 3(OH)- + 4H,O Bond strengths obtained as follows: (I) B-0, ca-O--Donffiy and Allmnn (1970); : 5[B3O'(OH)4.OB(OH)3r- Brom end Shemon (1973) (2) O...O--Zacharlesen (1963, Table 9' This reactionsuggests that increasingpH in the pres- assunlng s for o-II = 1 - s for 0'..0) (3) 0...C1, asssLng s = 0.125 for ech of 8 ence of halite and water favors the formation of 0."Cl and assoclated O-It s - 0.875 hydrochlorboritefrom ulexite.However, the appar- ent rarity of this mineral also suggeststhat other unaccounted-forfactors may prevent or invalidate knowledge,no Cl...O bond strength-bondlength this simplereaction. relationshiphas been proposed. Therefore, we have assumedthat a reasonableCl . . .H strengthis ap- Acknowledgments proximately0. 125 valence units in the presentcase, We wishto thankseveral colleagues: R. C. Erd, U. S.Geological althoughobviously this valueshould vary somewhat Survey(U.S.G.S.), Menlo Park, California,for suggestingthis CharlesL. with distance.Using this value and assumingthe work and for providingthe sample ofhydrochlorborite; Christ,U.S.G.S., Menlo Park, for his encouragement,helpful dis- balance.0.875 v.u.. for the associatedH-O bond cussions,and reviewof the manuscript;Subrata Ghose, University strength,summations ranging from 1.84to 2.15are of Washington,Seattle, for givingus informationon hilgarditein obtainedfor theoxygen atoms (Table 7). Considering advanceof publication;and Wayne A. Dollase,University of the assumptionsinvolved, these values seem satisfac- Californiaat Los Angeles,for reviewof the manuscript.Judith A. Reston,Virginia, and Mark P. Taylor,Stan- tory and do confirm the importanceof hydrogen Konnert,U.S.G.S., ford University,California, are thankedfor providingassistance bondsin this structureand their correctassignment with severalof the computerprograms used. This studywas sup- asacceptor and donor. portedin part by NSF grant EAR-74-03506-A01(Brown).

Relationshipof crystalstructure to cleavage,morphol- References ogy, solubility,and paragenesisof hydrochlorborite Abdullaev,G. K. and K. S. Mamedov(1969) Crystal structure of The good {001}cleavage, tabular morphology par- magnesiumdiborate, Mg[8.O"(OH)'].6HrO. Materialy, 1969, allelto {001},slow dissolutionin waterat 23oC,and 28-32(in Russian;Eng. translation by G. Soleimani,U. S.Geol. thereported seasonal nature of thisevaporite mineral Surv.). Brown,G. E. and J. R. Clark (1977)The atomicarrangement of can be rationalizedin terms of the crystalstructure hydrochlorborite,Car[BsOs(OH).'OB(OH)s]Cl. THzO (abstr.). which consists of slabs of composition Am. Crystallogr. Assoc. l|'inter Mtg Prog. and Abstr. Series2, 2{CadHzO)g[BsOa(OH)4.OB(OH)a]]1+, oriented par- 5u), 40. allel to {001} and bonded through hydrogento Brown,I. D. and R. D. Shannon(1973) Empirical bond strength- anions.Water apparentlybreaks these bond lengthcurves for oxides.Acla Cryslallogr.,A29,266-282. [Cl(HrO)r]r- Chidambaram,R., A. Sequeiraand S. K. Sikka(1964) Neutron- hydrogenbonds and conceivablythose holding to- diffractionstudy of the structureof potassiumoxalate mono- getherthe ribbonsof borate polyanionsand four- hydrate:Lone-pair coordination of the hydrogen-bondedwater memberedCa polyhedralchains parallel to a. Hurl- moleculein crystals.J, Chem.Phys., 4l, 3616-1622. BROI,YN AND CLARK: HYDROCHLORBORITE 823

Ch'ien,T.-C. and S.-C.Chen (1965)Brief note on preliminary and J. R. Clark (1978)Ulexite, NaCaB,O"(OH)o resultsof studyof a new boratemineral-hydrochlorborite_ . 5HrO:structure refinement, polyanion configuration, hydrogen (Ca.B'O'uClr.22HzO).Sci. Sinica, 14, 945-946(in Russian; bonding,and fiber optics.Am. Mineral.,63, l6Ll7l. translationby M. Fteischer,U. S. Geol.Surv.). Hurlbut,C. S.,Jr., L. F. Aristarainand R. C. Erd (1977)Hydro- -, -N. S Ma and H.-C. Liu (1965)Hydrochlorbonre, a new chlorboritefrom Antofagasta,Chile. Am. Mineral.,62,147-150. hydrouschlor-borate mineral. Acta Geol.Sinica, 45, 209-216 Ibers,J. A. and W. C. Hamilton,Eds. (1974) International Tables (from Mineral. Abstr., 19, l2B [1968],and Chem.Abst., 64, for X-Ray Crystallography,lY. Reuisedand SupplementaryTa- I 5584e). blesto VolumesII andIII. Kynoch Press,Birmingham, England. Christ,C. L. and J. R. Ctark (197'l)A crystal-chemicalclassifica- Johnson,C. K. (1965)Onrrp. A FonrnlN thermal-ellipsoidplot tion of borate structureswith emphasison hydratedborates. program for crystal-structureillustrations. Oak Ridge Natl. Phys. Chem.Minerals, 1, 59-8'l. Lab., ORNL-3794, reuised. -, A. H. Truesdelland R. C. Erd (1967)Borate mineral Main, P.,M. M. Woolfsonand G. Germain(1971) MULTAN, A assemblages in the systemNarO-CaO-MgO-BrOr-HrO. Geo- ComputerProgrsm for the Automatic Solutionof Crystal Struc- chim.Cosmochim. Acta, 3l , 313-33i. /ares.University of York PrintingUnit. Corfield,R., R. J. Dodensand J. A. Ibers(1967) The crystal and Sasvari,K. and G. A. Jeffrey(1966) The crystal structureof molecularstructure of nitridodichlorobis(triphenylphosphine) magnesiumchloride dodecahydrate, MgCl. l2HrO. Acta Crys- rhenium(V), ReNCidP(C"Hu),),.Inorg. Chem.,6, lg7-204. tallogr.,20,875-881. Cromer,D. T. andD. Liberman(1970) Relativistic calculation of Shashkin,D. P., M. A. Simonovand N. V. Belov(1970) Crystal anomalousscattering factors for X-rays. ,/. Chem.phys., 53, structur'eof uralborite,Ca,[B.O.(OHLl. Sou.Phys.-Dokl., 14, l 89r-l898. 1044-1046. Donnay,G. andR. Allmann(1970) How to recognizeOr-, OH , Simonov,M. A., Yu. K. Egorov-Tismenkoand N. V. Belov(1977) and HrO in crystalstructures determined by X-rays.Am. Min_ More precisedetermination of the structure of uralborite, eral.,55.1003-1015. Ca,[BnOn(OH),].(in Russian)Dokl. Akad. Nauk SSSR,234, Doyle,P. A. and P. S. Turner(1968) Relativistic Hartree-Fock X- 822-825. ray and electronscattering factors. Acta Crystallogr.,A24,390- Vaughan,D. J. and J. A. Tossell(1973) Molecular orbital calcu- 39'.7. lationson berylliumand boron oxyanions:Interpretation of X- Ferraris,G. andM. Franchini-Angela(1972)Surveyof thegeome- ray emission,ESCA, and NQR spectraand of the geochemistry try and environmentof water moleculesin crystallinehydrates of berylliumand boron.Am. Mineral.,58,765-770. studied by neutron diffraction.Acta Crystallogr.,B2g, 3572- Wan, C, and S. Ghose (1977) Hungchaoite,[Mg(H,O)6 3583. BoOE(OH)ol.2HrO, a hydrogen-bondedmolecular complex. Finger,L. W. and E. Prince(1975) A Systemof FortranIV Com- Am Mineral.,62, 1135-1143. puter Programsfor Crystal Structure Computations.Nat. Bur. Zachariasen,W. H. (1963)The crystalstructure of monoclinic Stand.Tech. Note 854. metaboricacid. Acta Crystallogr.,16, 385-389. Ghose,S. and C. Wan (1978)Hilgardire, Ca.[BuO,]Cl.H,O: a piezoelectriczeolite-type pentaborate. Am. Mineral., 63, in Manuscript receiued,March 29, 1978;accepted press. for publication,May 26, 1978.