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 struc- ture,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.