American Mineralogist, Volwne 63, pages 814-823, 1978

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

GoRnoN E. BnowN

Department of Geology, Stanford UniuersityS t anfo rd, C alfo rnia 94 3 0 5

lno Joe,N R. Cr-enr

U. S. GeologicalSuruey345 Middlefield Road

Menlo Park, Califurniq 94025

Abstract

The crystal structure of the rare playa mineral hydrochlorborite from the Salar Carcote,Antofagasta, Chile-the second world occurrence-has been solved by direct and differenceFourier methods and refined to a conventional R of 0.050, including hydrogen positions. Ourcrystal data agree with those previously 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 formulaunit contains one less water molecule than previously assigned, resulting in a calculateddensity of 1.841 rather than 1.876 g cm-s. The structure has isolated borate polyanions, eachcomposed of two borate tetrahedra and one borate triangle, cornerlinked to form a three-membered ring, with a side borate tetrahedron corner-linked to the triangle l3:(2T + A) + Tl;this group is unique among borate polyanions reported to date. The polyanions are cross-linked by hydrogen bonds to water molecules and by CaOdOH)'(HrO), polyhedra, four ofwhich share two corners and an edge to form four-membered chains. The most unusualfeature of the structure involves the Cl anion, which is not bonded to Ca as expected, butinstead is hydrogen-bonded to eight oxygens (three hydroxyls and five water molecules) atdistances ranging from 3.182 to 3.3954. To our knowledge, this is the largest number ofhydrogens bonded to Cl reported to date in an inorganic structure. The average BIII-O, Brv-O, CavIr I -O, and Clvr l r . . .O d is tances are 1.367,1.474,2.478, and3.296A, respect ive ly . Thegood {001} cleavage and tabular morphology parallel to {001} are consistent with the struc-ture, wherein slabs of composition 2{CadHrO)'[BsOr(OH){'OB(OH)']} '*, which are parallelto {001}, are hydrogen-bonded to [Cl(HrO)r] '- anions. This arrangement is also consistentwith the mineral's reported slow solubility in water at23oC and its observed seasonal nature.The formation of this mineral may result from a reaction between associated ulexite and halitewhen pH increases. However, its apparent rarity suggests that such a reaction takes place onlyunder unusual circumstances.

Introduction

The p laya minera l hydroch lo rbor i te , 3CaO.CaCL.4BzOs'2lH2O, was f i rst reported from China(Ch'ien and Chen, 1965; Ch'ien et al., 1965), al-though no detailed locality information was given bythese authors. In 1966, a second occurrence of thismineral was found in northern Chile. In this locality,hydrochlorborite is associated with ulexite, halite,and clay minerals. This occurrence was described by

0003-004x/78/09 10-08 14$02.00 8 14

Hurlbut et al. (1977), who also gave crystallographic,physical, optical and chemical data for the mineral.Among the more interesting observations made byHurlbut et al. were the slow dissolution in water (at23'C) and the seasonal nature of hydrochlorborite;the mineral apparently occurs in this locality onlyduring the dry season of the year. In order to shedlight on its slow rate of dissolution and to determinethe structural role of water in this mineral, we under-

BROIYN AND CLARK:

took the structure determination and refinement re-ported below. Our investigation shows that the chem-ical formula, 3CaO.CaCl2. 4B2Os.22H2O, assignedby the studies reported above, has one water moleculetoo many, the correct formula having only 2l watermolecules (Brown and Clark, 1977).

Experimental and computational details

The crystal selected for intensity measurementswas provided by R. C. Erd, U. S. GeologicalSurvey,and came from the Salar Carcote, Antofagasta,Chile, the locality described by Hurlbut et al, (1977).The specimen was prismatic, having {110}, {21l}, and{001} forms, and dimensions 0.15 X 0.2 X 0.2 mm.The c* axis of the crystal was oriented parallel to thed-axis of a Picker Fncs-I diffractometer, and theorientation was refined by least-squares fit of theangular coordinates of 35 automatically centered re-flections in the ?i range 30o to 40o. The resulting cellparameters [a 22.783(4), b : 8.749(l), c :I 7.06 I (3)A, P : 96.696(5)", V : 337 7 .5 Aal agree wellwith those reported by Hurlbut et al.la : 22.783(3),b : 8 . 7 4 5 ( r ) , c : r 7 . 0 6 6 ( l ) , { , B : 9 6 . 7 0 5 ( r ) o , V :3376.9(3)Asl; however, Hurlbut's values were used inall further calculations because of their higher preci-sion. Space grotp 12/a, assigned by Hurlbut et al. onthe basis of systematic absences on precession photo-graphs and morphology, was assumed to be correct.

A total of l44l nonzero symmetry independentintensities in the angular range 5o to 40o 20 wercmeasured using an os-N scan mode, a scan range of2.0" plus the a,-a, dispersion, graphite mono-chromatized MoKo radiation, and a solid-state de-tection system. Two standard reflections, at 0o and90o x, were monitored every 30 reflections andshowed no significant variation in integrated intensityduring the 3 days required for data collection. Of theobserved intensities, 188 were less than 3o(1), whereo(/) is the standard deviation of the intensity (1) ascalculated using the formula of Corfield et al. (1967)and an instrumental instability constant of 0.04.These data were corrected for Lorentz and polariza-tion factors, the latter assuming a 50 percent ideallymosaic monochromator crystal; no corrections weremade for absorption (p : 9.00 cm-l, MoKa) orextinction effects. After refinement was completed,no systematic discrepancies between strong observedand calculated structure factors were found, in-dicating that there were no serious extinction effects.

The resulting structure factors were converted tonormalized structure factors,.E, using programs fromthe University of Rochester Crystallographic Pro-

HYDROCHLORBORITE 8 1 5

gram Library. For this calculation and the refine-ments discussed below, neutral atomic scatteringfactors were calculated by using the analytical ex-pression and coefficients of Doyle and Turner (1968)and the anomalous dispersion values of Cromer andLiberman (1970); for hydrogen, the SDS coefficientsgiven by Ibers and Hamilton (1974, Table 2.28) wereused. The chemical formula applied at this stage wasthe one reported by Hurlbut et al. (1977). The statisti-cal distribution of lEl, R, and lE - I I confirmedthe centrosymmetric space group 12/a.The signs ofthe three largest E-values were fixed as positive, and aset of the 100 largest and 50 *o E s was used in thetangent formula program MulrnN (Main et al.,l97l) to search for solutions. The solution that hadthe best figure of merit proved to be the correct oneand yielded an E-map having 40 major peaks. The l8strongest peaks were assigned to the 2 calcium, Ichlorine, and l6 of the l8 oxygen atoms of the asym-metric unit. Structure-factor calculations based onthis assignment gave a conventional residual, R, of0.260. The positions of the four borons and tworemaining oxygens in the asymmetric unit were lo-cated by a three-dimensional difference Fourier syn-thesis and by study of a structure model. Two of theoxygen positions assigned using E-maps were incor-rect; however, in retrospect, the E-maps frorn thecorrect Mut t,,rN solution were found to show allnonhydrogen atoms of the asymmetric unit, plus sev-eral spurious ones,

Seven cycles of full-matrix isotropic refinementwere computed using RrtNr (Finger and Prince,1975), resulting in a conventional, unweighted,Fl : >

l l4 l - lF " l l /> l r , l o f 0 .071. Twe lve add i t iona lrefinement cycles having anisotropic temperature fac-tors reduced the unweighted R to 0.058 (R = 0.073,all data); l8 ofthe 1253 structure factors greater than3op were not included in the refinement because lAFlwas larger than an arbitrary cutoffvalue. The averageshift/error at this stage of refinement was 0.03r. Thestructure model from the isotropic refinernent wasused to compute another three-dimensional differ-ence Fourier synthesis, which, together with crystalchemical considerations, provided approximate loca-tions for the 2l hydrogen atoms in the asymmetricunit. Six additional cycles of refinement were carriedout in which positional parameters and an isotropicthermal parameter of each hydrogen were allotred tovary, and the positional and anisotropic thermalparameters of the 25 nonhydrogen atoms were fixedat their refined values after cycle 19. The finalunweighted R was 0.050 for the 1244 observed ciata

8 r 6 BROWN AND CLARK:

with f' greater than 3op; nine observations were ex-cluded from these cycles because ofpoor fit. The finalunweighted R for all l44l data was 0.063. A finalthree-dimensional difference Fourier was featureless,confirming that the chemical formula is indeedCa.BrOruCl, .2lH2O.

The final positional parameters, isotropic temper-

HYDROCHLORBORITE

ature factors for all atoms in the asymmetric unit,and anisotropic thermal parameters for all non-hydrogen atoms are l isted in Table l, together withthe estimated standard error of each parameter. Ob-served and calculated structure factors from the finalcycle of refinement (cycle 25) are compared in Table2. and the orientations and dimensions of the thermal

Table l. Atomic coordinates and thermal parameters for hydrochlorborite

Struc tura lro le I

Coordinates2r a

Equivalentlso t rop ic B

( b 2 B r rTherml paraneters x 10a

3

B z z B s s B t z B i g Bzs

Ca(1 )ca(2)

B (1 )R ( 2 )B (3 )B (4 )

CI

0 (1 )o (2 )o (3 )o (4 )

oH (1)0H(2)oH (3)oH (4)oH(5)oH (6)oH(7 )

ri20 (1)H2o(2)Hzo (3)Hzo(4)H20 (5 )n2o (6 )HzO(7)

H (1 )n (2 )E I/ ?.7

n(4)H(s)H(6 )H ( 7 )

H (8 )H (e )H(10)H (11 )H (12 )11(r.3 )H (14 )

H ( l s )H (16 )H (17 )E(r-8)H (19 )H (20 )H (21 )

r lng r ( r ,r lng T(2)

s i d e T ( 4 )

r (1) -Ar (2 ) -aa-r (4 )r (1 )r (1 )r (2 )r ( 2 )r (4 )r (4 )r (4 )

0H (1)oH (2)oH(3)oH(4)oH (s)on (6 )orr(7)H 2 O ( 1 )H2O (1)E2o(2)H2O(2)n2o (3)Hzo (3)H2O (4)

Hzo(4)H2O(5 )H2O(5 )H2O(6)Hzo (6 )H2o(7)E2o(t)

0. 03304 (8)0.2t377 (8)

0 . r 5 9 2 ( 4 )0 .1897 ( s )0. 0884 (4 )

- 0 . 0107 (4 )

0 . 133s ( r )

0 . 1939 (2 )0 . 1007 (2 )0 . t 2 9 3 ( 2 )0 . 0336 (2 )

0 . r 47 s ( 2 )0 . 1934 (2 )0 .2056 (2)o .23L8 (2)

-o .o57 5 (2 )-0 .0302 (2 )0 . 0110 (3 )

0 . 0s47 ( 3 )0 . 0610 (3)0 .0s61 (2 )0 . 2 3 1 9 ( 3 )0. 1358 (3)0 . 1637 ( 3 )0 . 1 7 4 5 ( 3 )

0 .158 ( s )0 . 1s9 (6 )o . 2 4 s ( 5 )0 . 2 4 2 ( 4 )

-0 .103 (7 )-0 . 061 (6 )0 . 0 s 0 ( s )

0 . 0 6 4 ( 8 )0 .038 ( s )0 . 037 (6 )0 . 098 ( 6 )0 . 086 (4 )0 . 0 7 9 ( 5 )0 . 211 (6 )

0 . 2 7 7 ( 5 )0 . L 4 2 ( 9 )0 . 093 (6 )0 . l s 7 ( 5 )0 . L 7 4 ( 6 )0 . 18s (5 )0.1-s8 (8)

o . L 7 r 1 ( 2 )0 . 0 s 7 s ( 2 )

0 . 31s0 (11 )o .4666 ( r 2 )0 .4902 (11 )0.5596 (r2)

0 . 6 3s6 (3 )

0 . 3289 (6 )0 . 3 9 0 4 ( 6 )0 . s 3 0 1 ( 7 )0 . sss6 (7 )

0 . 1 4 9 8 ( 6 )0 . 3 7 6 4 ( 6 )0 . 4 3 s s ( 7 )o .5793 (7 )0 . 6 5 3 4 ( 7 )0 .4001 (7 )o.6323(7)

o . 2 1 4 2 ( 8 )o . 9 L 4 5 ( 7 )o .5759 (7 )

- 0 . 1 6 0 1 ( 7 )0 . 8 s 8 s ( 7 )0 .1488 (7 )o .9943 (7 )

0 .120 ( r s )0 . 4 3 1 ( 1 6 )0 . 4 s 1 ( 1 2 )0 . 6 6 8 ( 1 1 )0 . 670 (16 )0 . 3 7 9 ( 1 6 )0 . s 7 0 ( 1 3 )

0 . 1s8 (16 )o .257 (L4 )0 . 813 (16)0 . 877 (16 )o.564 (r2)0 . 633 (17 )

-0, 17 3 (16 )-0. 183 ( 13)0 . 7 8 s ( 1 6 )o . 84s (16 )0 . r 1 r ( 15 )0 . 241 (18 )r . 048 (16 )0 . 907 (16 )

0 .1619 (1 )0 . 2956 (L )

0. 1914 (6 )0 . 3166 (6 )0 . 2 4 6 9 ( 6 )o . 7 7 L 4 ( 6 )

0 . 0 0 8 4 ( 1 )

o . 2 6 9 3 ( 3 )0 . 188s (3 )0 .3076 (3 )0 . 2414 (3 )

0 . 1803 (3 )0 .1312 (3 )0 . 4012 (3 )0 .2887 (3 )0 . 1 9 7 5 ( 3 )0 . 1s07 (3 )0 . 1 0 2 1 ( 3 )

0.0269 (3)0 . 1354 (4 )0 . 4 2 1 8 ( 3 )0 . 3917 (3 )o , 2 7 6 7 ( 3 )0 .4039 (3 )0 . 0443 (3 )

o . L47 (7 )0 . 096 (8 )0 . 4 1 8 ( 6 )0 . 328 (5 )0 . 1s3 (9 )0 .102 (8 )0 . 0 8 8 ( 6 )

0 . 003 (9 )- 0 . 0 0 3 ( 7 )0 . 1 1 7 ( 8 )0 . 160 (8 )0 . 385 (6 )o . 4s6 (8 )0 . 4 2 3 ( 8 )

0 . 4 2 2 ( 6 )o . 2 1 1 ( 8 )o . 294 (8 )0 .440 (8 )0 . 4 0 e ( 8 )0 . 004 (8 )0 . 046 (8 )

8 . 6 ( 7 ) - 2 . 0 ( 9 )8 . 1 ( 7 ) 0 . s ( 8 )

s (4 ) -4 (s )11 (s ) - 4 (6 )14 (4 ) - 1 ( s )10 (4 ) 1 ( s )

1 4 . 1 ( 9 ) 5 ( r )

5 ( 2 ) 2 ( 3 )L ( 2 ) 7 ( 3 )

r 4 ( 2 ) 7 ( 3 )1 3 ( 2 ) 2 ( 3 )

5 (2 ) - 3 ( 3 )8 ( 2 ) - r ( 3 )3 ( 2 ) 1 ( 3 )

10 (2 ) - 4 ( 3 )13 (2 ) - 6 ( 3 )L7 (2 ) - 1 ( 3 )8 ( 2 ) - 3 ( 3 )

L3(2) 8(4)22 (3 ) - 4 (3 )r 2 ( 2 ) 7 ( 3 )r3(2) -1(3)23(3) o(3)1 1 ( 2 ) - 1 ( 3 )L2 (2 ) - 5 (3 )

2 . 4 ( 4 ) r ( 1 )2 . 3 ( 4 ) 2 ( r )

1 ( 2 ) 6 ( 6 )3 ( 3 ) s ( 7 )2 ( 3 ) t ( 7 )1 ( 3 ) r ( 7 )

2 . 6 ( 6 ) 2 < 2 )

0 ( 1 ) - 1 ( 4 )-1 ( 1 ) - 1 ( 3 )-3 ( 1 ) - r 4 ( 4 )3 ( 1 ) - 15 (4 )

3 ( 1 ) - e ( 4 )4 ( 1 ) 0 ( 4 )o ( 1 ) - 2 ( 4 )4 ( r ) - 2 ( 4 )3 ( 1 ) 13 (4 )1 ( 1 ) 8 ( 4 )3 ( 1 ) 6 ( 4 )

7 ( 2 ) e ( 4 )2 ( 2 ) - e ( 4 )4 ( 1 ) - 3 ( 4 )0 ( 1 ) 2 ( 4 )0 ( 2 ) 6 ( 4 )7 < 2 ) 2 ( 4 )4 ( 2 ) - 8 ( 4 )

1 . 1 4 ( 4 ) 7 . s ( 4 ) 3 1 ( 3 )L . 1 4 ( 4 ) 6 . 8 ( 4 ) 3 8 ( 3 )

0 . 8 ( 2 ) s ( 2 ) 3 r ( r 6 )r . 5 ( 2 ) e ( 3 ) 4 4 ( 1 7 )0 . 8 ( 2 ) 4 ( 3 ) 01 . 1 ( 2 ) 5 ( 2 ) 4 4 ( 7 6 )

2 . 0 9 ( s ) 1 2 . 8 ( 6 ) 6 8 ( 4 )

1 . r ( 1 ) 8 ( 1 ) 36 (9 )0 . 8 ( 1 ) s ( 1 ) 4 o ( e )1 .6 ( 1 ) s ( 1 ) 68 (10 ). 1 . 6 ( 1 ) 8 ( 1 ) 4 e ( e )

1 . 1 ( 1 ) r o ( 1 ) 2 1 ( 8 )1 . 4 ( 1 ) 8 ( 1 ) 5 4 ( 9 )1 .4 ( r ) 11 (1 ) 54 (9 )1 . 6 ( 1 ) e ( 1 ) 58 (9 )1 . 7 ( 1 ) 8 ( 1 ) 71 (10 )1 . 7 ( 1 ) 8 ( 1 ) 4 7 ( 9 )1 . 6 ( 1 ) 1 2 ( 1 ) s r ( 9 )

2 . 8 ( 1 ) L 9 ( 2 ) 1 0 3 ( 1 1 )2 . 3 ( r ) 14 Q) 52 ( 10 )2 . 1 ( 1 ) 10 (1 ) e8 (11 )2 . 0 ( 1 ) r 2 (2 ) 64 (10 )2 . 7 ( r ) 1 1 ( 1 ) 4 6 ( 1 0 )2 . r ( r ) 1 6 ( 1 ) 6 2 ( 1 0 )2 . s ( 1 ) 1 4 ( 2 ) 1 0 1 ( 1 1 )

4 ( 3 )6 ( 3 )r (2 )r (2 )e (s )6 ( 4 )3 ( 2 )

11 (7 )3 ( 3 )7 ( 4 )5 ( 3 )r (2 )s (3)5 ( 3 )

2 ( 2 )l s ( 7 )8 ( 4 )3 ( 2 )4 ( 3 )8 ( 4 )

14 (6 )

'See F lg . I . T = te t rahedron, A = t r iang le , T-TH atona are assoclated wlth the OH or H2O as noted.

2Nunber ln parentheses is one standard devlation;0 . 3 1 5 0 ( 1 1 ) , r e a d 0 . 3 1 5 0 I Q . O O L I , e t c . ( ' I

sThemal parameters ia the expression, exp { -l-

L i=l

= l inking one tetrahedrol to aaoth.er, etc.

f o r 0 . 0 3 3 0 4 ( 8 ) ,3 - 1I h . r ' . e . . 1 ' .

j= l nrnJ r I

a t 0 .

read 0 .03304 t0 .00008, fo r

The thermal ell ipsoid of B(3)

was nonposltlve definite, so g2 was arbitrari ly set

BROWN AND CLARK: HYDROCHLORBORITE

Table 4. Bond distances and angles in the borate polyanion of hydrochlorborite

8 1 7

Atons I Distance2 Atomsl Dis tance2 Atorns l D ls tance2 Atomsl

(i) (i) (i) ri,rDistance2'

B(1 ) -o (1 )-o(2)-oH(1)-oH(2)

average

B(2) -o(1)-o (3)-oH(3)

-on(4)

average

average of12

CentralaEom

1 . 4 7 0 ( 1 1 )1 . 483 (11 )r .477 (Lr)1 .464 (11 )

7 . 4 7 4

r.459 (r2)r.476(t2)1 . 471 (11 )

L.49r(r2)

7 . 4 7 4

r . 4 7 4

o(1) -o (2)o (1) -oH (1)o (1) -oH (2)o (2) -oH (1)o(2) -oH(2)oH(1) -oH (2)

average

o (1 ) -o (3 )0(1) .oH(3)o (1) -oH (4)

o(3) -oH(3)

o (3) -oH (4 )oH(3) -oH(4)

average

average of1 8

2 .453 (7 . )2 . 346 (7 )2 .393 (7 )2 . 3 7 0 ( 7 )2 . 433 (7 )2 . 4 3 6 ( 7 )

2 , 4 0 5

2 . 4 3 L ( 7 )2 . 42 r ( 7 )2 . 3 6 4 ( 8 )

2 . 368 (7 )

2 . 432 (7 )2 . 4 2 7 ( 8 )

2 . 4 0 7

2 . 4 0 6

B (4) -o (4 )-oH(s)-oH(6)-oH(7)

average

B (3) -o (2 )-o (3 )- 0 ( 4 )

average

B ( 1 ) - B ( 2 )

B (1 ) -B (3 )B ( 2 ) - B ( 3 )B (4 ) -B (3 )

average

r . 471 (11 )1 . 4 5 s ( 1 r )1 .494 (L2 )1 . 4 7 8 ( 1 1 )

L . 4 7 4

1 . 3 7 8 ( 1 1 )I . 3s6 (11 )1 . 3 6 6 ( 1 1 )

1 . 3 6 7

2 . s4 rQ .4 )

2 .490 ( r 4 )2 .480 ( r 4 )2 .539 (L4)

2 , 512

o(4) -oH(5) 2 .289(8)o(4) -oH(6) 2 .4L4(8)o(4) -oH(7) 2 .466(8)o H ( s ) - o H ( 6 ) 2 . 4 s 9 ( 8 )oH(5) -oH(7) 2 .389(8)oH(6) -o r1(7) 2 .423(8)

average 2.407

o(2) -o (3) 2 .396(7)o ( 2 ) - o ( 4 ) 2 . 3 5 e ( 7 )o ( 3 ) - o ( 4 ) 2 . 3 4 6 ( 7 )

average 2.367

B ( 1 ) o ( 1 ) , o ( 2 )o ( r ) , o H ( 1 )o ( 1 ) , o H ( 2 )o ( 2 ) , o H ( 1 )0 ( 2 ) , o H ( 2 )

oH(1) ,oH(2)average

B ( 2 ) o ( 1 ) , 0 ( 3 )o ( 1 ) , o H ( 3 )o ( 1 ) , 0 H ( 4 )o ( 3 ) , o H ( 3 )o ( 3 ) , o H ( 4 )

oH(3) ,oH(4)average

9 ( 1 ) B ( 1 ) , B ( 2 )o ( 2 ) B ( 1 ) , B ( 3 )

B ( 4 ) o ( 4 ) , o H ( s ) I 0 2 . 9 ( 7 )o ( 4 ) , o H ( 6 ) 1 0 9 . 1 ( 7 )o(4) ,oH(7) rL3 .4(7)

o H ( s ) , o H ( 6 ) 1 1 3 . 0 ( 7 )o H ( 5 ) , o H ( 7 ) 1 0 9 . 1 ( 7 )o H ( 6 ) , 0 H ( 7 ) 1 0 9 . 3 ( 7 )

average 109.5

B ( 3 ) o ( 2 ) , o ( 3 ) L 2 2 . 4 ( 8 )o ( 2 ) , o ( 4 ) 1 1 8 . 5 ( 8 )0 ( 3 ) , 0 ( 4 ) 1 1 9 . 0 ( 8 )

r 1 ( o o

0theratoms

Angle( ' )

r 12 .4 (7 )1 0 s . 5 ( 7 )1 0 e . 3 ( 7 )106 .4 (7 )1 1 1 . 3 ( 7 )1 1 1 . 8 ( 7 )109 . 4

111 . 8 ( 7 )1 1 1 . s ( 7 )1 0 6 . s ( 7 )1 0 6 . 9 ( 7 )1 1 0 . 1 ( 7 )1 1 0 . 0 ( 7 )109 . 5

r20 . 4 (7 )121 . 0 ( 6 )

Cent ra laEom

0theraEoms

Angle( ' )

0 ( 3 ) B ( 2 ) , B ( 3 ) 1 2 2 . 2 ( 7 )o ( 4 ) B ( 3 ) , B ( 4 ) 1 2 7 . 0 ( 7 )

lA11 ato." at E, y, z (Table I'Number in parentheses ls one

1 .47010 .011 A - e t c .

)s tandard dev ia t ion ; fo r 1 ,470(11) , read

vibration ellipsoids for calciums, chlorine, and oxy-gens, calculated using L. W. Finger's unpublishedEnnon program, are given in Table 3'.

The distances, angles, and associated standard er-rors for the borate polyanion and Ca polyhedra re-

I To receive copies of Tables 2 and 3, order Document AM-78-087 from the Business Office, Mineralogical Society of America,1909 K Street, NW, Washington, DC 20006. Please remit $1.00 inadvance for the microfiche.

ported in Tables 4 and 5 were calculated using Ennonand the refined boron, calcium, and oxygen positionsand variance-covariance matrix from refinementcycle 25. Several additional cycles of refinement werecarried out during which the positional and ani-sotropic thermal parameters of all cations were fixedat their refined values and the positional and thermalparameters of chlorine, oxygens, and hydrogens wereallowed to vary. The resulting positional parametersand variance-covariance matrix were used to com-

BROWN AND CLARK' HYDROCHLORBORITE

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BROWN AND CLARK: HYDROCHLORBORITE

Fig. l . Stereoscopic-pairv iew(*btowardsreader)ofselectedport ionsof thehydrochlorbor i testructurewithspheresof arbi t rarysizefor atoms, showing the Cl-H bonding, some Ca-O polyhedral chains, and the borate polyanions. The O. . H bonds are not drawn, butmost can be noted with reference to Table 6. Drawing from Onrsp (Johnson, 1965).

8 1 9

p u t e t h e H - O , H - C l , H . . . O , H . . . C l , a n d C l . . . Odistances and associated errors reported in Table 6.Because the errors associated with the H positionsand these bond lengths are relatively large, the re-ported H positions and distances involving H shouldbe considered as approximate. The large errors aredue in part to the fact that our intensity data extendonly to 40" U and the number of parameters to berefined is large compared with the number of dataavai lable.

Structural description

As shown by the stereographic-pair view in Figurel, the structure of hydrochlorborite consists of iso-lated borate polyanions which are linked to eachother by Ca polyhedra and hydrogen bonding to

Fig. 2. View along b of one [B'O,(OH). . OB(OH), ] , - polyanionin hydrochlorbor i te, c approximately hor izontal , thermal e l l ipsoidsof 50 percent probability except for unlabelled hydrogen atoms(spheres ofarbi t rary s ize). Drawing f rom Onrnp (Johnson, 1965).

form slabs stacked sandwich-like along c. These slabsare held to each other by hydrogen bonding to Clanions. The Cl anions do not coordinate Ca, as wasexpected, but instead are held in the structure byeight hydrogen bonds to oxygens. The Ca polyhedraare linked together in groups of four through thesharing of two corners and an edge to form shortchains approximately parallel to a; these groups donot link to form infinite chains parallel to b as pre-viously suggested in the preliminary report of thiswork (Brown and Clark, 1977).

The borate polyanion

Each polyanion is formed by corner-sharingamong two borate tetrahedra and a triangle, produc-ing a triborate ring. This triborate polyanion type isthe most frequently reported in borate structuresstudied to date and has been given the notation 3: (27+ A ) by Christ and Clark (1977). However, in hydro-chlorborite, this basic unit is modified by attachmentof a tetrahedron to the non-ring oxygen of the boratetrangle (Fig. 2), and the notat ion becomes 3:(2T +A)* T. This is the first such modification to the 3:(2T +A) polyanion reported to datez.

The only other modified triborate group known atthis time is the 3: (3T) + T polyanion in the structureof uralborite, Ca2[B3O3(OH),.OB(OH)s] (Shashkinet al., 1970; Simonov et al., 19771' Christ and Clark,1977). Ten other borate structures having the iso-lated, unmodified 3:(2T * A) polyanion have beendiscussed by Christ and Clark (1977).

'z Modif icat ion by attachment of a tr iangle, 3:(2T + A) * A, wasreported in the structure of a synthetic magnesium borateMgBnO, 9HrO (Abdullaev and Mamedov, 1969), but the structureis probably wrong (Wan and Ghose, 1977).

820 BROWN AND CLARK:

In hydrochlorborite, B(3) is trigonally coordinatedby oxygen anions which lie approximately in a planewith B(3) ar an average distance of 1.367,{. Thisaverage BIII-O distance is considerably shorter thanthe average BIV-O distance of 1.474A for the threeborate tetrahedra. The shorter BIII-O bonds are pre-sumably related to the stabilization of the la'L and 3e'molecular orbitals of [BOs]8-, which are composedof boron 2p and oxygen 2p atomic orbitals, relativeto the 3tz molecular orbital of [BOn]u- (Vaughanand Tossel l , 1973). The B(l)rV-O(2)-B(3)I I I -O(3)-B(2)Iv-O(1) ring is essentially planar except for puck-ering of the B(l)-O(l)-B(2) linkage. The angle fromthe ring to the attached tetrahedron, B(4)IV-O(4)-B(3)ttt, is 127.0", somewhat larger than the averageB-O-B angle of 121.2' within the ring. The experi-mental location of protons in this hydrated borate isin complete agreement with the rules of Christ andClark (1977). Those oxygens attached to only oneboron add a proton to form hydroxyl ion, and alloxygens not associated with the borate polyanion aredoubly protonated, forming water molecules. Asmight be expected, the equivalent isotropic temper-ature factors for oxygen and hydroxyl ions are signifi-cantly lower than those for the water molecules(Table I ),

The calcium polyhedra

The two crystallographically distinct Ca cations,Ca(l) and Ca(2), are each coordinated by two oxy-gens, three hydroxyls, and three water molecules atdistances ranging from 2.336 to 2.681A (Table 5), thegrand mean CavrII-O distance being 2.474A. Eachpolyhedron shares two O-O edges with borate tetra-hedra, averaging 2.350,4 in length relative to the un-shared edges, which average 3.236A. As expected, theO-Ca-O angles opposite shared edges are narrowerthan those opposite unshared edges; the cation-cat-ion repulsions across shared edges result in signifi-cant distortions of the polyhedra. Two Ca(2) poly-

C a 1 C a 2

Fig. 3. An isolated four-membered chain of calcium polyhedrasharing corners and an edge in hydrochlorborite, viewed along c.Adapted from a drawing ofOnrnp (Johnson, 1965).

HYDROCHLORBORITE

hedra share an O-O edge, 3.0454, and each shares acorner with a Ca(l) polyhedron so that a four-mem-bered chain is formed, oriented approximately paral-lel to a (Fig. 3). These chains are joined to othersthrough borate polyanions to form ribbons parallelto a. Adjacent ribbons are joined together throughhydrogen bonding to form slabs that are orientedparal lel to {001}.

Chlorine coordination and hydrogen bonding

The Cl anion is hydrogen-bonded to eight oxygens,five associated with water molecules and three withhydroxyls, all at distances ranging from 3.182 to3.3954, the average ClvIIl-O distance being 3.2964(Table 6). The eight H. ' 'Cl distances average 2.431.,a value that agrees reasonably well with two H' ' 'Cl

distances of 2.41 and2.49A reported in the structureof hilgardite (Ghose and Wan, 1978). The occurrenceof six hydrogen bonds to chlorine has been reportedin several structures, but, to our knowledge, hydro-chlorborite is the first inorganic structure in which asmany as eight H. . .Cl bonds occur. Its averageO. . .Cl distance is somewhat longer than the averageof 3.194A reported for s ix O. . 'Cl in MgCl, . l2H2O(Sasvari and Jeffrey, 1966) where that chlorine doesnot coordinate to Mg but is hydrogen-bonded only.

The Cl(OH)'(HrO)u polyhedra in hydrochlorboriteoccur as isolated pairs, sharing an edge, HrO(4)-HrO(4)', the long axis of the dimer being orientedalong a (Fig. I ). These dimers connect the slabs ofborate polyanions and Ca-polyhedral chains dis-cussed earlier, literally holding the structure togetherthrough Cl. . .H-O l inkages.

Each proton in hydrochlorborite is bonded to onlyone other anion, either chlorine or oxygen, in addi-tion to its own associated hydroxyl- or water-oxygenatom. Of the l3 hydrogen bonds to water oxygens(Table 6),2 can be considered strong, O . . .O < 2.7 A,and I I weak, O. . .O > 2.7 A. Of the lat ter, only onemust be considered very weak, i.e. )3.0A.

The seven water molecules can be classified follow-ing the scheme devised by Chidambaram et al. (1964)and modified by Ferraris and Franchini-Angela(1972), which is based on the coordination of lone-pair orbitals on the water oxygens. HrO(l), HrO(2),and HzO(S) each have one Ca cation oriented ap-proximately along the bisectrix of the lone-pair orbit-als on the water oxygen, and hence are classified aslD. HrO(3), HrO(4), and HrO(6) each bond to oneCa cation and accept one hydrogen bond, both lyingalong the lone-pair orbitals, and so are classified as2H water molecules. The seventh water molecule,

C a 1 '

BROWN AND CLARK HYDROCHLORBORITE

Table 6. Hydrogen bonds in hydrochlorborite

821

Donoroxygen

aE.om

Hydrogenaton o fdonor Acceptor a toml

Dis tance2

o . . . o 0 - HAng1e2 (o )

H . . . 0 0 - H . . . 0

(.&)

H ( 1 9 )

H ( 2 0 ) o H ( 3 ) :

oH(3 )3

e r 2

- 9 5 12 - a

2 . 6 8 s ( e ) 0 . 8 s ( 1 5 )

2 . 6 9 3 ( 8 ) 0 . 8 8 ( 1 3 )

1 . 8 s ( 1 s ) 1 6 8 ( 1 3 )

1 . 8 8 ( 1 3 ) l s 2 ( 1 3 )

average 2 strong bonds 2 , 6 8 9 0 . 8 6 r . 8 6 r60

Hzo (o)

Hzo(t)

Hzo (s)

H2o(2)oH(s)oH(1)

H2o(2)

oH(3)

H2o (s)

H20 (1)

0H(4)

Hzo (s )

H2o (1 )

H ? O ( 5 )1 - 3

* t A - i t

i , t - y , Z

H (12 )

H (10 )

H (s ) H ro (6 ) :

H (1 ) H ro (7 ) :

H (11 )

H (3 ) H ro (7 ) z

H ( 1 7 ) o H ( 6 ) :

H ( 9 ) 0 H ( 7 ) :

H(4) Hzo(4) t

H (16 )

o ( 3 )

oH(7)

e ' r + a ' ; - zx, y-1, z

2 . 7 4 0 ( 8 ) 1 . 0 0 ( 1 0 )

2 . 7 4 e ( 8 ) 1 . 0 7 ( 1 3 )

2 . 805 (8 ) 1 . 21 (13 )

2 .81e (8 ) 0 .69 ( r2 )

2 . 8 2 9 ( 8 ) 0 . 9 6 ( 1 3 )

2 . 8 4 7 ( 8 ) O . 9 2 ( r o )

2 . 856 (8 ) 1 . 06 (13 )

2 . 8 4 9 ( 8 ) o . 7 2 ( r 2 )

2 . 8 7 1 ( 8 ) 1 . 0 3 ( 1 0 )

2 . 9 2 7 ( 8 ) 0 . 6 6 ( 1 3 )

I . 7s ( 11 ) 170 (8 )

1 . 70 (13 ) 168 (1 3 )

1 . 6 1 ( 1 3 ) 1 6 6 ( 1 3 )

2 .74 (L3 ) 167 (13 )

2 .07 ( r 3 ) 134 (11 )

1 . 9 3 ( 1 0 ) 1 7 4 ( 1 0 )

1 . 87 (13 ) 153 (13 )

2 . r 4 G 2 ) r 7 4 Q 2 )

1 . 8 7 ( 1 0 ) 1 6 3 ( 7 )

2 .32 ( r3 ) l - 5s (13 )

I- a ' V1 "z -z

s, A+7, z

o (3 )

10 bonds 2 . 8 2 9 0 . 9 3

Hzo( : ) t 2 .46 ( r3 )

average a l l 13 bonds 2 . 8 2 9 0 . 9 0 L . 9 7

0 . . . c l H . . . C l 0 - H . . . C LH20(6)

Hzo(4)

oH(2)

H2O(4)

Hzo(s)

Hzo(7)

oH(7)

oH(6)

H (18 )

H ( 1 4 )

H ( 2 )

H( rs )H (13 )

H(2L)

H ( 7 )

H ( 6 )

C I :

c I :

1q' i

-A'7

a' V

-A'

C17 1V - E ' 6

* ' 2 -c '

c1

c1

v L . & , t - q , 4

148 (13)

16s (12)

14s (10)

1s8 (9 )

L57 (12)

158 (13)

r-34 (8)

173 (13)

1t + z7z*"

L-A' V

-z1 . "t * z

3 . 1 8 2 ( 6 ) 0 . 7 4 ( 1 3 ) 2 . 5 3 ( 1 4 )

3 . 2 2 0 ( 6 ) 0 . 8 8 ( 1 4 ) 2 . 4 3 ( r 4 )

3 . 2 7 4 ( 6 ) 1 . 0 5 ( 1 4 ) 2 . 3 6 Q 4 )

3 . 2 8 3 ( 6 ) 1 . 0 0 ( 1 1 ) 2 . 2 7 ( r L )

3 . 3 2 s ( 7 ) o . e o ( l s ) 2 . 4 8 ( L 5 )

3 . 3 1 0 ( 7 ) 0 . 8 5 ( 1 5 ) 2 . s 0 ( 1 5 )

3 .376(6) 1 .10(11) 2 .52( r r )

3 . 3 9 s ( 6 ) 1 . 0 4 ( 1 5 ) 2 . 3 6 ( r s )

average 8 bonds 3 . 2 9 6 o . 9 4 z . c J 155

nexc neares tne ighbor , HrO(2) 3 . 7 6 9 ( 6 )

lAtoms at r , U, z (Table 1 ) unless t ransformed as

- ^1f91P.r in parentheses^is one standard deviar ion1 . 8 5 ( 1 5 ) , r e a d 1 . 8 5 l O , I 5 L - e t c .

n o t e d .

f o r 2 . 6 8 5 ( 9 ) , r e a d 2 . 6 8 5 t 0 . 0 0 9 4 ; f o r

HrO(7), has two hydrogen acceptor atoms formingbonds along the lone-pair orbitals and so is of Clasi2E.

The relative importance of hydrogen bonding inthis structure is il lustrated by the contribution ofhydrogen to the bond strength summations for theoxygen atoms (Table 7). Because large errors are

associated with the hydrogen positions, as discussedearlier, the bond strengths involving hydrogen havebeen obtained using the O.. .O distances and theassociated bond strengths proposed by Zachariasen(1963), recently shown to be the best availablemethod for evaluating hydrogen bonding in boratestructures (Ghose et al., 1978). However. to our

822 BROWN.AND CLARK:

Table 7. Bond strengths (s) in hydrochlorborite

te (valence unlts)

C a - O O - B 0 . . . 0

HYDROCHLORBORITE

buI et al. (1977) suggested that when the water tablein the Salar Carcote intersects the clay layer contain-ing crystals of hydrochlorborite, these crystals dis-solve. A mechanism for the reformation of hydro-chlorborite as the water table drops could involve thesimple type of polyanion equilibrium suggested byChrist et al. (1967) to explain transformations withchanging pH and activity of HrO among hydratedNa, Ca, and Mg borate minerals. The association ofhydrochlorborite with ulexite, NaCaIBuOu(OH).].5H2O, and halite permits the following hypothesis. Ifwe assume that total boron in the system remainsconstant but that the activities of Na+, Ca2+, H+,Cl-, and HzO can be varied independently, followingthe suggestions by Christ et al., the transformationbetween ulexite and hydrochlorborite may be ex-pressed in terms of the polyanions as

4[8,O6(OH)6]'- f 3(OH)- + 4H,O

: 5[B3O'(OH)4. OB(OH)3r-

This reaction suggests that increasing pH in the pres-ence of halite and water favors the formation ofhydrochlorborite from ulexite. However, the appar-ent rarity of this mineral also suggests that otherunaccounted-for factors may prevent or invalidatethis simple reaction.

AcknowledgmentsWe wish to thank several colleagues: R. C. Erd, U. S. Geological

Survey (U.S.G.S.), Menlo Park, Cali fornia, for suggesting thiswork and for providing the sample ofhydrochlorborite; Charles L.Christ, U.S.G.S., Menlo Park, for his encouragement, helpful dis-cussions, and review of the manuscript; Subrata Ghose, Universityof Washington, Seattle, for giving us information on hilgardite inadvance of publication; and Wayne A. Dollase, University ofCalifornia at Los Angeles, for review of the manuscript. Judith A.Konnert, U.S.G.S., Reston, Virginia, and Mark P. Taylor, Stan-ford University, California, are thanked for providing assistancewith several of the computer programs used. This study was sup-ported in part by NSF grant EAR-74-03506-A01 (Brown).

References

Abdullaev, G. K. and K. S. Mamedov (1969) Crystal structure ofmagnesium diborate, Mg[8.O"(OH)'].6HrO. Materialy, 1969,28-32 (in Russian; Eng. translation by G. Soleimani, U. S. Geol.Surv.).

Brown, G. E. and J. R. Clark (1977) The atomic arrangement ofhydrochlorborite, Car[BsOs(OH). 'OB(OH)s]Cl. THzO (abstr.).Am. Crystallogr. Assoc. l|'inter Mtg Prog. and Abstr. Series 2,5u ), 40.

Brown, I . D. and R. D. Shannon (1973) Empir ical bond strength-bond length curves for oxides. Acla Cryslallogr., A29,266-282.

Chidambaram, R., A. Sequeira and S. K. Sikka (1964) Neutron-diffraction study of the structure of potassium oxalate mono-hydrate: Lone-pair coordination of the hydrogen-bonded watermolecule in crystals. J, Chem. Phys., 4l, 3616-1622.

0 (1)o (2 )o (3 )o (4 )

on(1)oH(2)oH(3)oH(4)0B(s)oH(6)on(7)

H20 (1)H p (2 )H2O(3)n p (4 )s r(s)H p(6)H20 (7)

c1

1 .54 0 -44L . 7 L 0 . 2 5L . 7 7r . 7 6 0 . 2 2

0 . 7 4 0 . 4 5 0 . 8 0o . 7 7 0 . 2 7 0 . 8 80 .76 0 .81o . 7 2 0 . 3 0 0 , 8 20 . 7 9 0 . 2 8 0 . 8 00 . 7 1 0 . 2 6 0 . 8 80 . 7 4 0 . 8 8

0 . 2 6 1 . 7 30 . 2 8 1 . 5 6o .25 1 .65o . 2 3 1 . 7 50 . 2 5 1 . 6 60 , 2 7 L . 6 2

r . 6 2

0 . 3 8

0 . 5 1

0 . 180 . 43

0 .090. 170 .190 . 2 00 ,38

1 . 9 81 . 9 62 .Lst . 9 8

2 .00L . 9 22 . 0 81 . 841 . 8 72 .032 . 0 5

1 . 991 , 84r . 992 . L 52 . 102 . 0 92 . O 0

1 .00

Bond strengths obtained as follows:(I) B-0, ca-O--Donffiy and Allmnn (1970);

Brom end Shemon (1973)(2) O. . .O- -Zachar lesen (1963, Tab le 9 '

assunlng s for o-II = 1 - s for 0'..0)(3 ) 0 . . .C1, asssLng s = 0 .125 fo r ech o f 8

0 . "C l and assoc la ted O- I t s - 0 .875

knowledge, no Cl. . .O bond strength-bond lengthrelationship has been proposed. Therefore, we haveassumed that a reasonable Cl . . .H strength is ap-proximately 0. 125 valence units in the present case,although obviously this value should vary somewhatwith distance. Using this value and assuming thebalance. 0.875 v.u.. for the associated H-O bondstrength, summations ranging from 1.84 to 2.15 areobtained for the oxygen atoms (Table 7). Consideringthe assumptions involved, these values seem satisfac-tory and do confirm the importance of hydrogenbonds in this structure and their correct assignmentas acceptor and donor.

Relationship of crystal structure to cleavage, morphol-ogy, solubility, and paragenesis of hydrochlorborite

The good {001} cleavage, tabular morphology par-allel to {001}, slow dissolution in water at 23oC, andthe reported seasonal nature of this evaporite mineralcan be rationalized in terms of the crystal structurewh ich cons is ts o f s labs o f compos i t ion2{CadHzO)g[BsOa(OH)4. OB(OH)a]]1+, oriented par-allel to {001} and bonded through hydrogen to[Cl(HrO)r]r- anions. Water apparently breaks thesehydrogen bonds and conceivably those holding to-gether the ribbons of borate polyanions and four-membered Ca polyhedral chains parallel to a. Hurl-

BROI,YN AND CLARK:

Ch'ien, T.-C. and S.-C. Chen (1965) Brief note on prel iminaryresults of study of a new borate mineral-hydrochlorborite_(Ca.B'O'uClr.22HzO). Sci. Sinica, 14, 945-946 (in Russian;translat ion by M. Fteischer, U. S. Geol. Surv.).

-, S -N. Ma and H.-C. Liu (1965) Hydrochlorbonre, a newhydrous chlor-borate mineral. Acta Geol. Sinica, 45, 209-216(from Mineral. Abstr., 19, l2B [1968], and Chem. Abst., 64,I 5584e).

Christ, C. L. and J. R. Ctark (197'l) A crystal-chemical classifica-tion of borate structures with emphasis on hydrated borates.Phys. Chem. Minerals, 1, 59-8'l .

- , A. H. Truesdell and R. C. Erd (1967) Borate mineralassemblages in the system NarO-CaO-MgO-BrOr-HrO. Geo-chim. Cosmochim. Acta, 3l , 313-33i .

Corf ield, R., R. J. Dodens and J. A. Ibers (1967) The crystal andmolecular structure of nitridodichlorobis (triphenylphosphine)rhenium (V), ReNCidP(C"Hu),),. Inorg. Chem.,6, lg7-204.

Cromer, D. T. and D. Liberman (1970) Relat ivist ic calculat ion ofanomalous scattering factors for X-rays. ,/. Chem. phys., 53,l 89r - l 898.

Donnay, G. and R. Al lmann (1970) How to recognize Or-, OH ,and HrO in crystal structures determined by X-rays. Am. Min_e r a l . , 5 5 . 1 0 0 3 - 1 0 1 5 .

Doyle, P. A. and P. S. Turner (1968) Relativistic Hartree-Fock X-ray and electron scattering factors. Acta Crystallogr., A24,390-39'.7.

Ferraris, G. and M. Franchini-Angela(1972) Surveyof thegeome-try and environment of water molecules in crystalline hydratesstudied by neutron diffraction. Acta Crystallogr., B2g, 3572-3583.

Finger, L. W. and E. Prince (1975) A System of Fortran IV Com-puter Programs for Crystal Structure Computations. Nat. Bur.Stand. Tech. Note 854.

Ghose, S. and C. Wan (1978) Hilgardire, Ca.[BuO,]Cl.H,O: apiezoelectric zeolite-type pentaborate. Am. Mineral., 63, inpress.

HYDROCHLORBORITE 823

and J. R. Clark (1978) Ulexite, NaCaB,O"(OH)o. 5HrO: structure refinement, polyanion configuration, hydrogenbonding, and fiber optics. Am. Mineral., 63, l6Ll7l.

Hurlbut, C. S., Jr., L. F. Aristarain and R. C. Erd (1977) Hydro-chlorborite from Antofagasta, Chile. Am. Mineral.,62, 147-150.

Ibers, J. A. and W. C. Hamilton, Eds. (1974) International Tablesfor X-Ray Crystallography, lY. Reuised and Supplementary Ta-bles to Volumes II and III. Kynoch Press, Birmingham, England.

Johnson, C. K. (1965) Onrrp. A FonrnlN thermal-ellipsoid plotprogram for crystal-structure illustrations. Oak Ridge Natl.Lab., O RNL-3794, reuised.

Main, P., M. M. Woolfson and G. Germain (1971) MULTAN, AComputer Progrsm for the Automatic Solution of Crystal Struc-/ares. University of York Printing Unit.

Sasvari, K. and G. A. Jeffrey (1966) The crystal structure ofmagnesium chloride dodecahydrate, MgCl. l2HrO. Acta Crys-tallogr., 20,875-881.

Shashkin, D. P., M. A. Simonov and N. V. Belov (1970) Crystalstructur'e of uralborite, Ca,[B.O.(OHLl. Sou. Phys.-Dokl., 14,1044-1046.

Simonov, M. A., Yu. K. Egorov-Tismenko and N. V. Belov (1977)More precise determination of the structure of uralborite,Ca,[BnOn(OH),]. (in Russian) Dokl. Akad. Nauk SSSR, 234,822-825.

Vaughan, D. J. and J. A. Tossel l (1973) Molecular orbital calcu-lations on beryllium and boron oxyanions: Interpretation of X-ray emission, ESCA, and NQR spectra and of the geochemistryof beryl l ium and boron. Am. Mineral. , 58,765-770.

Wan, C, and S. Ghose (1977) Hungchaoite, [Mg(H,O)6BoOE(OH)ol. 2HrO, a hydrogen-bonded molecular complex.Am Mineral., 62, 1135-1143.

Zachariasen, W. H. (1963) The crystal structure of monoclinicmetaboric acid. Acta Crystallogr., 16, 385-389.

Manuscript receiued, March 29, 1978; acceptedfor publication, May 26, 1978.