American Mineralogist, Volume 66, pages 154-168, I98I

The crystal structureof santaclaraite,lCaMnn[SisOr4(OH)l(OH).HrO: the role of hydrogenatoms in the pyroxenoid structure Yossxezu Ounsur Department of Geology, University of Pennsylvania Philadelphia, Pennsylvania I 9 I 04

AND LARRY W. FINGER

Ge op hy sic al L ab o r at o ry, Carne gie I nstitutio n of Was hin gt o n Vl/ashington, D. C. 20008

Abstract

A new , santaclaraite(CaoqoMn?LM&orFe2o*0,)s[Si5Or4(OH)XOH) . H2O, is triclinic with a : 10.273(4),0: l l.9l0(4),c : 12.001(6)A,a : 105.77(3),F : l l0.6a(3),r : 8?.13(3)", V : l3l7.0(8)A'; Z :4 for the 1T unit-cell setting.The crystal structureconsists of alternat- ing tetrahedral and octahedral layers. The tetrahedral layer is made up of infinite single chains of silicate tetrahedrawith a repeatperiod of five tetrahedra.The octahedrallayer in- cludesrows of ten octahedrawith adjacent octahedralrows displacedalong their length to form bandstwo or three octahedrawide. As isolatedunits, the tetrahedralchain and octahe- dral band of santaclaraiteare similar to the correspondingportions of the struc- ture. The structureof santaclaraite,however, ditrers in that (l) two adjacentchains (or bands) in a given layer are displacedby a half c translation,and (2) the octahedrallayer is rotated by a half turn in the plane parallel to the layer with respectto the adjacenttetrahedral layer. The three roles of hydrogen as hydrogen bond, hydroxyl group, and water moleculeare respon- sible for the above half-translation and half-rotation. Three octahedralsites, Ml, M2, and M3, are essentiallyoccupied by Mn atoms.The Ca atoms are orderedin M5, and the small amount of Mg is probably concentratedin M4. Differential thermal analysisand thermo- gravimetric analysisindicate that the dehydration of santaclaraiteoccurs at approximately 550'C.

Introduction The importance of octahedral cations in con- trolling the octahedral-tetrahedrallinkages in the Santaclaraite,a new mineral from the Franciscan pyroxenoid structure has previously been discussed formation, Santa Clara County, California, is struc- for three-tetrahedral-repeatpyroxenoids (Ohashi and turally related to rhodonite, babingtonite, nambulite, finger, 1978).The octahedralcations are identical in and marsturite (Ohashi and Erd, 1978).The mineral rhodonite and santaclaraite,so the different struc- description is given elsewhere(Erd and Ohashi, in tures must be due to efects producedby the hydro- preparation). Santaclaraite is chemically equivalent gen atoms. to rhodonite plus two water molecules, CaMn SirO,, Inesite.a double-chainsilicate with a five-tetrahe- (rhodonite) + 2HrO; its water content is less than dral repeat,has two crystal-chemicallydistinct types thet of inesite, .2.5H,O, CaMnr,[Si,O,.(OH)] but of hydrogenatoms, one as HrO and the other as OH- more than that of babingtonite,Car(Fe,Mn) (Wan and Ghose, 1975, 1978).The hydrogen in ba- Fe'*[Si,O,o(OH)],nambulite, (Li,Na)MnoISi, bingtonite (Araki and Zoltai, 1972) and nambutte Or4(OH)1,and marsturite, NaCaMnrISirO,o(Narita et al., 1975; Murakami et al., 1977) is be- (OH)], another recently discoveredpyroxenoid (Pea- lieved to form a hydrogenbond O-H ...O, on the cor er dt., l978al. basisof the short O-O distance,as in pectolite (Pre- 'A:.proved by the Commissionon New and Mineral witt and Buerger,1963; Tak6uchi and Kudoh, 1978). iiames, IMA. Santaclaraiteis unique among pyroxenoid miner- ac[3-.iAK / 8l /0 I 02-0 I 54$02.00 OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE als in that alkali atomssuch as Na and Li are not es- sential constituentsand also in that as many as four hydrogen atoms exist for each five silicons. Thus a Rhodonile detailed structural analysis of this mineral should P cell provide a better understandingof the role of hydro- gen in pyroxenoid structures. \.1 Experimental (< Unit-cellsetting \..t. Crystals of santaclaraiteare commonly prismatic and elongatedalong the zone axis of two well-devel- oped cleavagesthat intersectroughly at a right angle, t as in other pyroxenoidsand pyroxenes.Preliminary t f study with precession and zone-axis photographs oli showedthat the crystal was triclinic and that a trans- lation along the zone of the two cleavageswas ap- proximately l2A. This translation can be compared with the chain-identity period of five-tetrahedral-re- peat pyroxenoids,rhodonite and babingtonite(Table 1). The crystallographicc axis is chosenparallel to the zone axis. (ftkO)precession photograph, which The contains Fig. l. Comparison ofunit cells for santaclaraite and rhodonite. information on the structureprojected along the zone Triangles represent tetrahedral chains in rhodonite projected axis, is similar to the correspondingphotograph of along the chain direction. rhodonite. No similarities to rhodonite were ob- served,however, in other photographs of santacla- raite. of rhodonite, it is more convenient in discussing The unit cell of santaclaraiteis comparedwith that modular crystallography(Thompson, 1978) of pyrox- of rhodonite in Figure L Although the B-centered enoids to use a body-centeredcell. This 1l cell of cell of santaclaraitecorresponds to the primitive cell santaclaraite is comparable to the Cl setting for rhodonite and also to multiple cells for three-repeat- Table l. Comparison ofthe unit cells for santaclaraite, rhodonite, pyroxenoidsdiscussed by Ohashiand Finger (1978). and babingtonite The thicknessof the layers,d(100) of the.Il or Cl Pyroxene-type ce11 Pyroxenoid-type cell cell, is approximately equal in santaclaraiteand STC* RHD** gg5t src* RHD** gBuf rhodonite, whereasthe D axis of the /l or Cl cell, of the tetrahedralchains or of c] cl ei pI pI which is the separation a(A) L0.273(4') 9.444 9-'73L 15.610(4) 6.7O'7 6.1L9 the octahedralbands in a given layer, is muih longer b I1.91O(4) 1.0.540 10.410 7 -59I12) 1 .6A2 7 .509 santaclaraite(see Fig. l). In the initial analysis, c 12.001(6) 12.234 L2.245 12.00r(6) L2.234 12-245 in this longer separationwas erroneouslythought to be o(') ro5.?7(3) 108.68 rO8. 36 109.80(3) I1I.54 1r2.21 B 110.64(3) LO3.29 144.2'7 88-s9(3) 85.25 86.25 due to the presenceof water moleculesbetween the Y 87.13(3) 42.23 A4.94 99.94(2) 93.95 92.13 v (a') 13r?.o (s) 1167 -'7 Lt4l- .4 1317.0 (8) 583.B 57O.'7 octahedralbands.

,redv49E5 \ ffu, (110) (1ro) (100) (1oo) (r00) {lr0) (110) (1I0) (0I0) (010) (010) Data collectionand structuralanalysis tetrahedrar [oot] [oor] [ oor] Ioor] loor] [oorl chain A crystal0.l4 x 0.20x 0.28mm wasused for col- close-pack (1O0) (r00) (100) (210) (r10) (r10) Iayer lecting the X-ray diffraction intensitiesup to 650 in 20 for Nb-filtered MoKc radiation on an automated * Santaclaraite. This stualy. From least-squares lefinenent four-circle diffractometer.Integrated intensities mea- with twelve reflections centered on a sinqle crystal diffrac- Eometer. sured with an a-20 scan were correctedfor Lorentz ** Rhodonite. Calculatcd from teduced cell data given by peacor and Niizeki (1963) and polarization effects.Absorption corrections(lin- + Babinglonite. Ca]culated from reduced ceII data given by coefficient 47.2 cm-') were also ap- Araki and Zoltai (197:) ear absorption plied, using the numerical integration technique of 156 OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE

Burnham (1966a).A total of 3307 reflectionswith a could be developedfrom thesemodels, structure so- structurefactor greaterthan twice its estimatedstan- lution by direct methodswas abandoned. dard deviation was usedin the structureanalysis and The minimum-function method was then tried us- refinement. Atomic scattering factors for the fully ing possibleM-M inversionvectors. The resultswere ionized state (except O-) and dispersioncorrections essentiallythe sameas thoseobtained from the direct were taken from International Tablesfor X-ray Crys- methods:the three structural arrangementsdescribed tallography,Vol. 4 (p.99 and p. 149,respectively). above were also found, and the tetrahedral chain Wilson's N(Z) test strongly indicated the existence could not be located. In spite of incomplete results of a center of symmetry and thus a centrosymmetric from the direct and minimum-function methods, triclinic spacegroup was assumedin the subsequent however,the layer arrangementwith no atoms at the structureanalysis. Strong peaks, which form a nearly inversion center seemeda plausible part of the cor- trigonal pattern on the (100) Pattersonmap of the rect structure.This optimistic view was largely based body-centeredcell, indicate the close-packedar- on analogywith the known crystal structuresof other rangementof cations and oxygensparallel to (100). five-repeatpyroxenoids. The geometryofthe gap be- Direct methods of structure determination were tween the octahedralbands, therefore, was analyzed first attempted.From a total of 3307above-minimum in an attempt to fit a five-repeattetrahedral chain. reflections,109 reflections with E valuesgreater than Two casesare known of octahedral-tetrahedrallink- 2.5 were used as input for a computer program age: one in rhodonite and the other in the hydrous sIcMA2 of the symbolic addition method (Karle and phasesbabingtonite and nambulite (Tak6uchi, 1976). Karle, 1966).In addition to three origin-defining re- Two possible arrangementsfor the tetrahedral flections,signs of two reflectionswere assignedto ex- chain that fills the gap betweenthe octahedralbands pand the data set of determinedphases with a modi- in santaclaraite(Fig. 2) were derived. Both models fied versionof the tangentformula program (Brenner required 17 (not 15) oxygensbased on five silicons, and Gum, 1968).Essentially three kinds of solutions the samenumber of oxygensas in the chemical for- were obtained: (l) the layer arrangementwith no mula. This result was unexpectedbecause the inde- atoms on the inversion centers;(2) the layer an'ange- pendent water molecule or moleculessuch as those ment with one atom on the inversion center;and (3) found in inesite had also been anticipated in san- the displaced layer arrangementresembling inesite taclaraite. (Wan and Ghose, 1975, 1978).E maps for the first The arrangementshown in Figure 2b yielded a re- solution, calculated with approximately 500 reflec- sidual factor, R, of 55 percent after the scale factor tions, showed an octahedral band apparently con- was adjusted. A further refinement by the least- sisting of six unique octahedral positions, one of squaresmethod did not improve the R factor and re- which had to be wrong becauseonly five octahedral sulted in unrealistic Si-O distances.Successive Fou- cations were expectedfrom the chemical formula. rier syntheses,however, improved the structural Although the overall arrangementis diferent, as dis- model to an R of 43 percent. The structure refine- cussedbelow, each octahedral band would be like ment was smooth and straightforward after this the one in rhodonite if the sixth peak were ignored.If stage.Three cycles of least-squaresrefinement with the fifth peak were ignored, the band would be like the program RFINE2(Finger and Prince, 1975) that in nambulite (Narita et al., 1975;Murakami et brought the R factor to 23,7 and 4 percent.The final al., 1977\. R factor is 3.6 percent for 3307 above-minimum re- Between the octahedral layers there are several flections.2Atomic coordinatesand isotropic temper- peaksthat would be Si atomsif the structureis based ature factors are given in Table 2, and interatomic on alternating tetrahedraland octahedrallayers. An distancesare listed in Table 3. attempt was made to form a tetrahedral chain by connectingthese Si positions and oxygensfound in Problems in nearly close-packedarrays the octahedral layers. There were, however, no The process of structure analysis has been de- meaningful electron densitiesat the positions where scribedin detail, becauseproblems encountered with the bridging oxygenswere expected. The computer program MULTAN of Matn et al. 2 To receivea copy of the F" and table,order Document (1971)was also usedin an attempt to solvethe d AM- struc- 80-145from the BusinessOffice, Mineralogical Societyof Amer- ture by direct methods.All trial models were as de- ica, 2000 Florida Avenue, N.W., Washington,D.C. 20009.please scribedabove. Thus, becauseno reasonablestructure remit $1.00in advancefor the microfiche. OHASHI AND FINGER: STRUCTURE OF SANTACI-'/IRAITE l5'l

Fig. 2. Two possible arrangements of a tetrahedral chain between octahedral bands in the same layer. Note that a different location of the Sil tetrahedron results from reversing the chain direction. Initial values of coordinates for bridging , indicated by small circles, were derived from these diagrams. The arrangement that accounts for observations for santaclaraite proved to be that shown in Fig. 2b. santaclaraitemay have generalsignificance for anal- ing this structure have been studied by J. Karle, ysis of a structure based on a nearly closest-packed Naval ResearchLaboratory (personal communica- arrangement.In such a structurethere are many par- tion, 1979).He has concludedthat if six to eight sym- allel interatomic vectors. Thus, the portion of the bolic phaseshad been used in the initial stagesthe structurethat is basedon thesevectors can be found correct structurewould have been found. on the E maps or the minimum-function maps. The first problem is the existenceof false peaks at the Descriptionof the structure edge of the structural unit. Although the octahedral Figure 3 shows a perspectiveview of the basic rows end at M5 in the refined structure of santacla- structural units, the tetrahedral chains and the oc- raite (Fig. 4), the initial E maps contain an equally tahedral bands, both extending in the c-axis direc- strong peak at the position where a cation would be tion. Two tetrahedral chains cross-link two octahe- expectedif the octahedralrow extendedbeyond M5. dral bands by sharing oxygensthat are on the basal A secondbut more seriousproblem is that atoms facesoftetrahedra and at the edgeofthe octahedral that deviatefrom the closest-packedarray tend to be bands. The tetrahedral chains also attach, at the masked by strong modulation of closest-packed apical oxygens,to the central part of the octahedral atoms.In santaclaraite,for example,each octahedral bands. Therefore,a given tetrahedral chain bridges band can be approximated by a closest-packedar- two octahedralbands in the samelayer and also ties rangement,but the adjacentband in the samelayer is togetheroctahedral bands in neighboringlayers. The slightly displacedand is not on the extensionof such resulting structure consists of alternating arrange- closest-packing.In other words, oxygensin the gap ments of tetrahedraland octahedrallayers. betweentwo octahedralbands are not on the closest- The octahedrallayer of santaclaraiteis compared packednet. Reflectionsthat contain critical informa- with that of rhodonite in Figure 4. Each octahedral tion on these"out-of-net" atomsare outnumberedin band is remarkably similar in the two structures,but the phase determination by reflections to which every other band in santaclaraiteis displaced by a mostly "on-the-net" atoms contribute, because half c translation relative to the arrangementin phasesfor the latter group ofreflections can be deter- rhodonite. This displacement results in a slightly mined with a higher probability. wider gap betweentwo octahedralbands in santacla- The problemsin the use of direct methodsof solv- raite. 158 OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE

Table 2. Atomic positionalparameterst and isotropictemperature factorsof santaclaraite

Atom x y z B(L2l

Ml 0.0104f 0.03761 0.87646 0.64 M2 0.00534 0. ff3t6 0.62112 0.52 M3 0.02109 0.83376 0.63081 0.62 M4 0.00166 0 .242L8 0.1I8s3 0.56 M5 0.01957 0.31307 0.86949 0.70

si1 0.2092 0.5836 0.5776 0.41 si2 0.2203 0.5605 0.3564 0.44 si3 0.2256 0.4693 0. r2I2 0. 40 si4 0. 2r0r 0.5305 0.8810 0.40 si5 0. 1953 0.3531 0.5409 0.39

oAI 0.t270 0. I 910 0.8386 0.66 oA2 0. rll_5 0.8140 0.0865 0.55 oA3 0. 1071 0.02r0 0.3269 0.62 oA4 0.I22I 0.9s48 0.5760 0.60 oA5 0. 1366 0.1528 0.8169 0.56

oBI(oH) 0.L20+ 0 .6740 0.6437 0 .74 oB2 0.1316 0.7672 0.3327 0.73 oB3 0.1536 0.3399 0.0887 0.70 oB4 0.1363 0.6393 0.8435 0.87 oB5 0.1068 0 .2307 0.5620 0.62

ocl 0. 1740 0.5938 0.4398 0.72 oc2 0.1683 0.5646 0 . 2155 0. 58 oc3 0. r602 0. 5011 0.9873 0.63 oc4 0. f549 0.4076 0.7663 0.63 oc5 0. 1575 0.4485 0.5603 0.67

oDl (oH) 0. 104r 0. 0863 0. 0802 0.75 oD2(H2o) 0.I293 0-2764 0.3267 0.90

Estimated standard errors

M 0.00005 0.00004 0.00004 0 .0f Si 0.00008 0.00007 0.00008 0. 01 o 0.0002 0.0002 0 . 0002 0.03

* Based on Ii ceII (Table 1). Fig. 3. A perspectiveview of the santaclaraitestructure. and octahedral bands are displaced along The tetrahedrallayer Tetrahedral chains of santaclaraiteis compared broken lines to avoid superposition. with that of rhodonite in Figure 5. The major differ- encein the tetrahedralchain is in the C-shapedjoint formed by three tetrahedra Sil-Si5-Si4 (tetrahedral triplet), which is more open in rhodonite.Every other tetrahedralchain in the samelayer is again displaced Table 3. M-O and Si-O distances*(A) in santaclaraite by a c/2 translation.The most important differenceis

I,t2- M3- orientation of the chains with respectto the octahe-

oAl 2.140 oA3 2.320 cAI 2.248 oAI 2.349 cA2 2.3I9 dral layer next to the chain. Note the directions of oA2 2.188 oA4 2.264 0A3 2.215 oA2 2.228 0A5 2.302 oA3 2.2L9 oA4 2,t84 cA4 2.L52 oB3 2.I4A OB2 2.311 the b and c axesin Figures 4 and 5. The tetrahedral oA5 2.306 oA5 2.I1a oBr 2.t62 oB4 2.062 0B3 2.433 layer of santaclaraitemust be turned upside down in oDI 2.198 oB2 2 . A92 0B5 2. 143 oDl 2.141 OC22.494 oDl 2.175 oB5 2.158 0D2 2.312 oD2 2.298 0C3 2.451 the projection plane in order to obtain the orienta- oc4 2.599 tion found in santaclaraitewith respectto the octahe- Ave.2.204 2.199 2.205 2.204 2. 416 dral layer. If the tetrahedral-octahedralrelation in rhodonite is defined as parallel, then the relation can oAt 1.510 0A2 I.618 oA3 1.503 0A4 1.613 0A5 1.626 oB1 1.624 0B2 r.583 oB3 I.506 0B4 1.583 0B5 L591 be referred to as antiparallel in santaclaraite.The oc1 1.601 0c2 L625 oc3 1.633 0c4 1.65r oc5 1.661 oc5 1.638 0c1 1.667 oc2 1.655 0C3 1.654 0C4 L.642 antiparallel arrangementis also found in nambulite portion Ave.1.618 I.623 t.624 L 625 1.630 and babingtonite. A of the santaclaraite * Estinated stand4rd errors for individual M-O and Si-o structure showing the packing of octahedral bands bonds are 0.0024. and tetrahedralchains is given in Figure 6. OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE

Fig. 4. comparison ofoctahedral band afiangem€ntsin santaclaraiteand rhodonite. z;

Fig. 5. Comparisonoftetrahedral chain arrangementsin santaclaraiteand rhodonite. 160 OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE

Fig. 6. A portion of the santaclaraite structure projected from the -ar direction onto the close-packed layer plane. A lower layer shown on the left half of the diagram is related by a body-centering translation, th(t + b + c), to an upper layer shown on the right.

Cationordering (Table 4). Coordination polyhedra (Table 3) and site occupanciesare thus in good agreementfor the occu- The coordination polyhedra for Ml, M2, and M3 pancy of Mn in these three sites. The coordination can be describedas slightly distorted octahedrawith polyhedron for M4 is a more distorted octahedron averag€M-O distancesof 2.204,2.199and 2.2054, with a larger variation of M-O distances,ranging respectively.Refinements of cation occupanciesin- from2.062to2.349A(Table 3). When refinedwith a dicatethat Mn is the only cation in Ml, M2, and M3 linear combinationmodel of Mn2* (23 electrons)and

Table 4. Cation occupanciesin santaclaraiteand Mg-rich rhodonites

santaclaraite Mq-rich rhodonit3s

Moclel 1* Model 2** Natural Synthetic

Mn Mn M9 Mn Ca Mg Mg M9

MI 1.01 -0.01 r.01 -0.0r 0.89 0. l1 0.696 0.304 0.615 0.385 M2 1.00 0. 00 l. 00 0.00 0.86 0. 14 0.746 0.254 0.687 0.313 M3 0.99 0.01 0.99 0.01 0.86 0.L4 0.634 0. 366 0 .572 0.428 M4 0.91 0.09 0.95 0.05 U. ]J 0.47 0.48r 0.5r9 0.350 0.650 M5 0. 14 0.86 0.09 0. 9l 0. 40 0.60 0.868 0.t32 0.876 0.L24

Total 4.05 0.95 4.04 0.91 0.05 3.54 0.60 0.86 3.425 l.)/) 3 .100 1. 900

e. s. d. 0. 007 0. 007 0. 004 0 . 004 0. 0I 0.0r 0.005 0.005

Reference This study Peacor et al. (I978b) Murakami and Finger and Tak6uchi (1979) Hazen (1978)

* Linear combination of Mn and Ca with a constraint of the bulk chemical composition. ** A1l Mg assigned to M4 t{ith occupancy fixecl at 0.95Mn+0'05M9' OHASHI AND FINGER: STRUCTI]RE OF SANTACI./IRAITE t6l

Ca'* (18 electrons),an occupancyof 0.9lMn * in M4 is most probably due to the presenceof a mi- 0.09Ca(22.5 electrons)was obtainedfor M4. A more nor amount of Mg in the site. probable interpretation of the M4 occupancy,how- ever, is to assignMg'* (10 electrons),which was not Hydrogenatoms includedin this stageof the refinement,to M4. When In other hydrous single-chainpyroxenoids for all availableMg, 0.05 cation on the basisof five cat- which structuresare known, there is only one hydro- ions, is assumedto occupythe M4 site in place of Ca, gen in the asymmetricunit. The hydrogen in these the occupancy would be 0.95Mn + 0.05Mg (22.4 pyroxenoidsforms a hydrogenbond betweentwo ox- electrons) in agreementwith the total number of ygens at the open side of the C-shapedtetrahedral electrons obtained in the refinement. Thus, occu- triplet, resulting in a slight shortening of the chain pancieshave been refined with that of M4 fixed at length in hydrous phasesas compared with an- 0.95Mn+ 0.05Mg(Model 2 in Table 4). hydrous counterparts (Prewitt and Buerger, 1963', Also comparedin Table 4 are the resultsof cation and Tak6uchi and Kudoh, 1978,for pectolite; Araki occupancyrefinements in synthetic rhodonites (Fin- andZolta|1972, for babingtonite;Narita et al., 1975, ger and Hazen, 1978; Murakami and Tak6uchi, atd Murakami et al., 1977,for nambulite). 1979)and a natural Mg-rich rhodonite (Peacoret al., From its chemical composition, four hydrogen 1978b).These studiessupport the preferenceof Mg atoms are expectedin the asymmetric unit of san- for the M4 site, though to different degrees.The de- taclaraite.One of the four is probably betweenOBI gree of ordering is less in the synthetic rhodonites, and OB4 as in other hydrous pyroxenoids,because probably becausethe crystalswere quenchedfrom a the c length of santaclaraiteis much shorterthan that ratherhigh temperature(- 1300'C). of rhodonite as a result of a shorter OBI-OB4 dis- The M5 coordination polyhedron in santaclaraite tance.All 17 atoms were found in the struc- is extremelydistorted and is almost seven-coordina- ture analysis,and thus the remaining three hydro- tion, the seventhM-O distancebeing 2.5994. A simi gensmust be bondedto someof theseoxygens. There lar kind of distortion is found for M5 in Ca-rich is no possibility, therefore,of the existenceof an in- rhodonite (e.9.Peacor and Niizeki, 1963).The cation dependentwater molecule as in inesite and . occupancyrefinement yielded 0.l4Mn + 0.86Ca for The anion chargedetermination method proposed M5 in santaclaraite.For both santaclaraiteand by Donnay and Allrnann (1970)has been applied to rhodonite a reasonableconclusion is that most of the predict forms of hydrogen in the structure. The re- Ca occursin M5 and that an apparentCa occupancy sults of the calculation, shown in Table 5, indicate

Table 5. Bond valences in valence units estimated bv the Donnay-Allmann method

MI M2 M3 M4 sit si2 si3 si4 si5 Tota I Inter- pretation

'I oA1 0.38 0.30 0.22 1.00 qn 'I oA2 0.34 0.32 0.32 1.01 OO oA3 0.32 0.24 0.33 r.04 1.93 oA4 0.38 r .02 2 .03 r0.3410.29 oA5 0.26 0.35 0.33 1.01 1.95

OBI 0.37 n oo a. Jo oH o oB2 0.43 U.JJ I.08 1.84 oB3 0.38 0.28 1.04 r.7 0 oB4 0.46 1.09 a: )f o Ho oB5 n ?? 0.38 1.08 r.83

oc1 1.03 1.00 2.03 oc2 u.zt 0.91 0.98 2.16 oc3 n ,1 0.94 0.95 z.Lo oc4 o.22 0.94 0.94 2.I0 oc5 0.96 0.98 r.94

oDt c0.34 0.38 1.08 OH to.go oD2 0.25 0.26 n ql H20

Total 2. 00 2.02 2.0I 2.02 2.02 3.98 4.00 4.00 4. 00 4 .0f 162 OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE

participatein the hydrogen bonding, as summarized in Table 6. Oxygenatoms ODI and OD2 The number of oxygens,including hydroxyls, in one formula unit is in general 3n for n-tetrahedJal-re- peat pyroxenoids;thus all oxygensare coordinatedto silicons in order to form an infinite (SiOr)- chain. Santaclaraitehas, in contrast, 17 oxygens in the asymmetricunit, two of which cannotbe coordinated to any of the five silicons.Atoms ODI and OD2 are oxygensthat are not bonded to silicons.These oxy- gen atomsare clusterednear the (01l) plane (shaded areain Fig. 8). Oxygen ODI on the upper layer in Figure 8, for example,is coordinatedto three metal cations, Ml, Fig. 7. Difference-Fouriermaps possible showing hydrogen which on the same positionsas positive residuals. Conrours are at an interval of 0.2 e/ Ml, and M4, are approximately A3 with the negativeareas shaded. level, and to hydrogen H2 directly above,forming a flattened coordination tetrahedron. A coordination around OD2 is an extremely distorted tetrahedron threetypes of hydrogen,forming (l) O-H ... O with with two longerbonds to M3 and M4 and two shorter OBI and OB4, (2) O-H with ODl, and (3) H-O-H bonds to H3 and H4 (Table 6 and Fig. 8). with OD2. Thermal parameters of these oxygens Modular crystallography have beenrefined, using an anisotropicmodel to im- prove the difference-Fouriermaps in the vicinity of Thompson's(1978) approach to modular crystal- hydrogen. Positive residuals in electron densities lography is here extendedto a comparison of san- were found on the diference-Fourier maps (Fig. 7) taclaraiteand rhodonite. from which coordinatesfor hydrogen were obtained (Table 6). Tetrahedralmodule As expected,hydrogen Hl occurs betweenOBI Santaclaraite,CaMn [Si,O,.' (OH)](OH)' H,O, is and OB4. Although the shapeof the peak is some- chemically a hydrated equivalent of rhodonite, what irregular,Hl is closerto OBI (0.884) than to OB4 (1.66A). Tak6uchi and Kudoh (1978) inter- Table 6. Hydrogen atoms in santaclaraite:positional parameters, preted the difference-Fouriermaps of pectolite as bond distancesand anglbs showing two statistically occupied hydrogen posi- Pos itional Patanetersf, tions,O(3)-H...O(4) and O(3)...H'-O(4).In san- Atom x taclaraile,however, no such secondresidual is found H1 0.101 0.655 0.702 H2 0.181 0.089 0.116 on the difference-Fouriermaps. This result is consis- H3 0.2I8 0.248 0.362 tent with the relatively large difference in bond H4 0.137 0.3s5 0.406 strength between the two oxygens in santaclaraite Hqdrogen Bonds (1.36 valenceunits for Donor Hydrogen Acceptor OBI and 1.55 for OB4), oxygen oxygen whereasthe bond strengthsestimated by the Don- OBI H1 oB4 2.491 0.88 r.55 156 nay-Allmann method are approximately equal for ODI H2 (oD2) 2.90I 0.1s (2.321 135 oD2 H3 oB3 2.645 0.96 I.12 163 the two oxygensin pectolite. oD2 H4 oc5 2.909 1.10 1.83 164 Hydrogen H2 is bonded to ODl, forming a hy- CoordinaXion atound ODl and OD2 droxyl ion. Two hydrogens,H3 and H4, areapproxi- Distance (A) Anqle (') Distance (A) Angle (') mately l.0A away from OD2 and make an angle of oDt H2 -ODl oD2 H3-OD2 -Ml I24 -H3 0.96 -H4 99 approximately100' at OD2.In addition to theseox- -Ml 2.I9'1 -Ml'111 -Il4 I .10 -M3 104 -r4t,2.t74 -M4 )-L7 -M3 2.3t2 -M4 L26 ygensto which hydrogensare bonded, oxygen OB3, -M4 2. r43 -M4 2.229 which showsa deficiencyin bond strength (1.70 va- H3 H4-OD2 -H4 r.57 -M3 93 lenceunits), is only 1.72Afrom H3. OxygenOC5 is -M4 130 1.83Afrom H4. Thus all hydrogen atoms except H2 * Obtained from difference Fourier ma OHASHI AND FINGER: STRUCTT]RE OF SANTACLARAITE

! I irl (D _ r.c o Upperoxygen r i\ o O Loweroxygen I ol

\.

I ! t I

Fig. 8. Arrangementof hydrogen and oxygen atoms in the santaclaraitestructure. Two layers of oxygen atoms, hydrogen atoms bondedto the upperoxygen atoms, and metal cationsbetween oxygen layers are shown. Solid lines repr€s€ntM-O (upper) bonds,and broken lines, M-O (lower) bonds. Three hydrogen-oxyg€ngroups, OBI-HI-OB4, ODI-H2, and H3-OD2-H4, are indicated at the bottom. The ODI and OD2 oxygenatoms, indicated by double circles,are arrangedin the shadedarea approximatelyparallel to the (0ll) layer.

CaMn SirO,r.As has beenshown in the previoussec- Octahedral-tetrahedralmodule linkages tion, both minerals are classified as five-repeat, single-chainsilicates, yet they differ in several re- Each octahedralband (Fig. a) can be taken as an spects.The most interestingdifference is the location infinite beam nodule. Santaclaraiteand rhodonite of the 2HrO in the santaclaraitestructure. A struc- have atmost identical octahedralmodules. The way tural module that containsfive tetrahedrais defined in which octahedral and tetrahedral modules are in Figure 9 to demonstratethe drechanismby which linked should be considered.Apical oxygensof five santaclaraiteaocommodates these additional atoms. tetrahedraform a flattened W shape(Fig. l0), each The tetrahedral modules so defined are packed "stroke" also representingO-O edgesof octahedra. tightly in the rhodonite structure (Fig. 9a). Because There are many waysto selectthe positionsof the W- the rnajor constraint of the module arrangementis shapedoxygen arrangementon the octahedralmod- the continuity of the chains, modules could glide ule. Among many possibilities are two modes of lengthwisewithout breakingthe chains.The santacla- stacking(Fig. l0) that are found in santaclaraiteand raite structurecan be obtainedwhen the glide opera- rhodonite. Another arrangement can be found in tion occurs every two modules (Fig. 9b). If every nambulite and babingtonite. module glides, the result is a hypothetical arrange- When only one tetrahedralchain and one octahe- ment (Fig. 9c) that is an even more open structure dral band are considered,their linkage seemsto be than that of santaclaraite.Two adjacent modules of more flexible at the apical oxygensof tetrahedrathan the samechain are slightly displacedat the junction at the basal oxygens of tetrahedra (Fig. I l). Two of the Sil and Si5 tetrahedra.This displacementhas structuralconstraints for the latter are the OBI-OB4 been called the horizontal offset (Burnham, 1966b) distanceat the open sideofthe tetrahedraltriplet and and is characteristicof pyroxenoids.There is no hori- the distorted M5 coordination polyhedron. [n san- zontal ofset in pyroxenes. taclaraite, OBI and OB4, bridged by the hydrogen OHASHI AND FINGER: STRUCTURE OF SANTACLAMITE

(o) Rhodonite

(b) Sontocloroite

Fig. 9. Module arrangements oftetrahedral chains in (a) rhodonite, (b) santaclaraite, and (c) a hypothetical case. In santaclataite every other modnle is displaced by half of the chain repeat length. atom, are too close to become an edge of the Mn oc- bondedto the sameoctahedral cation, M4 (Fig. I lb). tahedron. Thus, the octahedral module, relative to A similar relationshipwas found in the three-repeat the tetrahedral module, is arranged in such a way pyroxenoids(Ohashi and Finger, 1978,Fig. 5). that these two oxygens do not coordinate to the same octahedral cation (Fig. lla). On the other hand, the Dehydrationof santaclaraite OBI and OB4 oxygen atoms in rhodonite, which are The resultsof differential thermal analysis(DTA) far enough apart for an octahedral edge, are actually and thermogravimetricanalysis (TGA) of santacla- OHASHI AND FINGER: STRUCTURE OF SANTACLARAITE 165

structure to becomeunstable, so that the Hl atom can be removedat as low a temperatureas 550oC. The direct transformation from santaclaraiteto the anhydrous phase indicates that the existence of the intermediatecomposition, CaMno[Si'O'.(OH)] (OH), is unlikely as long as the basic structure remains a single-chain silicate. Such a monohydrated form, however,might be possibleif hydrogen atoms termi- natedsilicate chains at every five tetrahedra.This hy- potheticalstructure would be analogousto the struc- ture of rosenhahnite,Ca.SirOr(OH), (Wan et al., 1977;Jetrrey and Lindley, 1973),which has an inter- rupted wollastonite-liketrisilicate group: ooo tl ..' Hl-o-sil-o-si2-o-si3-o-H2'.. o o o A single crystal of santaclaraitetransforms when heatedto multiple crystalsthat give only a powder X-ray pattern of bustamite. Inesite, CarMnzSi,oOrr(OH),'5H2O, in contrast, was re- ported to transform to optically homogeneoussingle crystalsof rhodonite when heated to 800'C (Rich- sonlocloroite Rhodonile mond, 1942),although the exactdehydration temper- Fig. 10. Tetrahedral-octahedral linkage at the apical oxygen ature is unknown. Identification of the dehydrated atoms in santaclaraite and rhodonite. The main difference betweenthe two structuresis in the position of the Sil inesitewas, however,made with X-ray powder pho- tetrahedron. The other four tetrahedra, though different in tographs.What causesthe differencein thermal be- numbering,are similar overall. havior between santaclaraiteand inesite?One pos- sible explanation is the difference in structure modulesdiscussed in the previous section. raite are shownin Figure 12. Both show only one re- The structuresof santaclaraiteand bustamite are action at approximately550oC. The reactionis endo- different not only in silicate chains but also in oc- thermic, 102.3 cal/g (Gene C. Ulmer, personal tahedral modules.In santaclaraiterows of ten octa- communication).An X-ray powder pattern of heated hedra form bands two or three octahedrawide (Fig. santaclaraitematches that of bustamite but not that 4). The octahedralband in bustamite,on the other of rhodonite (Richard C. Erd, personal communica- hand. consistsof three rows of an infinite number of tion). An attempt to obtain single crystals of the octahedra (e.9., see Fig. 3 of Ohashi and Finger, high-temperaturephase was unsuccessful. 1978).A considerablerearrangement in the octahe- The aboveexperimental results may be interpreted dral modulesis required, therefore,to form the bus- as follows. Although the structural role of each of tamite structure from santaclaraite.As a result, the four hydrogen atoms in the asymmetric unit is not dehydratedcrystals do not have physical continuity the same,dehydration of santaclaraitedoes not occur controlledby an orientational relationshipto the hy- as multiple steps,probably becausedehydration of drated phase. one type of hydrogen atoms in the structure "trig- A major di-fferencein octahedralmodules between gers" dehydration of another. The dehydration tem- inesite and rhodonite is the length of the octahedral perature, 550oC, of santaclaraiteis considerably rows; ten-octahedralsequence of 5-4-3-2-l-l-2-3- lower than that of pectolite, 730"C (Skhirtladze, +5 n rhodonite vs. nine-octahedralsequence of 5- 1966).The hydrogenatom in pectolite(Takduchi and +3-2-l-2-3-4-5 in inesitewith Ml at the inversion Kudoh, 1978)is crystal-structurallyequivalent to the center (Wan and Ghose, 1978).It is then difficult to Hl atom in santaclaraite.Dehydration of the hydro- believe that the 5ingle crystalsof heated inesite ob- gen atomsother than Hl may causethe santaclaraite tained by Richmond (1942)were rhodonite, because OHASHI AND FINGER: STRUCTURE OF SANTACLAMITE

Sqnlocloroile

(o) (b) Fig. ll. Tetrahedral-octahedrallinkage at th€ basaloxyg€n atoms in (a) santaclaraiteand (b) rhodonite. The tetrahedraltriplet Sil- Si5-Si4has ditrerent orientations with respectto the octahedralbands. The oxygensOB I and OB4 are coordinatedto different octahedra in santaclaraitebut to the sameoctahedron. M4. in rhodonite.

the orientational continuity of crystalscould not be TGA DTA maintained if the octahedralmodules changedtheir fYligm lilt (%) Erxroilrtmic arrangement.Richmond's (1942) high-temperature phase is more likely CarMnrSi,oOrnthan CarMnrSi,oOro(rhodonite). Wan and Ghose (1978) mentionedboth possibilities.If the octahedral mod- ule remainsunchanged during the dehydration, the inesite transition may be regardedas gliding of the layer modulesparallel to the (230)plane. A more de- tailed discussionof the modular crystallography of pyroxenoidswith five-tetrahedralrepeats, including inesite,is in preparation. Conclusions The tetrahedralchain and the octahedralband in santaclaraiteresemble the corresponding parts in Fig. 12. ote and TGA santaclaraite. rhodonite.A summary comparison cures of Dehydration of the two struc- occursat around 550oC.Experiments were done at a rate of 10'C,/ turesis given in Table 7. The hydrogen atomsplay a min in a stream of helium with a flow rate of 3cm3/sec.The key role in determiningthe unique modular arrange- referencematerial usedfor DTA was Al,O.. OHASHI AND FINGER: STRUCTURE OF SANTACI'.ARAITE

Table 7. Comparisonof the structuremodules and modular relationship in santaclaraiteand rhodonitc

Bean nodule* Layer module**

Tetrahedral chain Similar. A single chain of five Every other chain is displaced bY tetrahedral repeat, (F'ig. 5) Lc in santaclaraite. (Fig. 9)

Octahedral band Similar. Rows of ten octahedra Every other band is displaced bY forming bands two or three Lc in santaclaraite. octahedra rriale. (Fig. 4)

Tetrahedral- Opposite. Antiparallel in santa- Opposite. Tetrahedral layers in octahedral claraite and parallel in rhodo- reversed orientation \di!h respect relationship nite. (Figs. 10 and I1) to octahedral layers in santa- claraite.

* ParaIIeI to c axis of II ceII for santaclaraite and C-l-cell for rhodonite. ** Formed by connectinq beam modules in the (I00) pIane.

ment in santaclaraite.The linkagesof octahedraland Universityof Pennsylvania.This study was partially supportedby tetrahedral modules as the basal oxygensof tetra- NSF grantEAR77-15703. hedra are controlled by the hydrogen Hl, which forms the hydrogen bond at the opening of the tet- References rahedral triplet Sil-Si5-Si4. Furthermore, the link- Araki, T. and T. Zoltai (1972)Crystzl structureof babingtonite.Z. age of octahedral and tetrahedral modules at the Kristallogr.,I 35,155-173. Brbnner,S. A. and P. H. Gum (1968)The tangent formula pro- apical oxy9ensof tetrahedrais primarily affectedby gram for the X-ray analysisof noncentro-symmetriccrystals. the existenceof the hydrogensH2, H3, and H4. U.S. Naval Res LaboratoryRep.6697. As a result, adjacent tetrahedral chains (and also Burnham, C. W. (1966a)Computation of absorption correction octahedralbands) in santaclaraiteare displacedby a and the significanceof end effect.Am. Mineral.,5l,159-16'1. - pyroxenoid-type poly- half c translation, when compared with rhodonite. (1966b) Ferrosilite III: a triclinic morph of ferrousmetasilicate. Science, 154, 513-516. This translation necessitatestwo more oxygensthat Donnay, G. and R. Allmann (1970)How to recognizeO2-, OH- are not connectedto silicatetetrahedra and resultsin and H2O in crystal structuresdetermined by X-rays. Am. Min- l7 oxygensper 5 silicons,whereas in all other pyro- eral.,55,1003-1015. xenoidsthe ratio Si:O: l:3 holdsas a consequence Finger, L. W. and R. M. Hazen (1978)Refined occupancyfactors pyroxmangite rhodonite. of the non-existenceof such oxygens. for synthetic Mn-Mg and Carnegie Inst. Wash.Year Book, 77,85U853. All the hydrogen atoms in santaclaraite,despite - and E. Prince (1975)A systemof FoRTRANIV computer their different structural roles, are dehydrated on programsfor crystal structure computations.Nall. Bur. Stand. heating in one continuous reaction with the maxi- (U.5.) Tech.Note 854. mum endotherm at 550"C. After the dehydration, Intemational Tablesfor X-ray Crystallography,Vol. 4 (1974) Ky- England. single crystalsare polycrystalline and have inverted noch Press,Birmingbam, Jeffrey,J. W. and P. F. Lindley (1973) Remarkablenew silicate to a phasethat probably hasthe bustamitestructure. structure.Nature, 241, 4243. Karle, J. and I. L. Karle (1966)The symbolic addition procedure Acknowledgmentb for phase determination for centrosymmetric and tron-c€ntro- We thank Dr. R. C. Erd for providing the santaclaraitespeci- symmetriccrystals. A cta Crystallogr., 21, 849-859. men and unpublishedresults of heating experiments;Dr. J. R. Main, P., M. M. Woolfsonand G. Germain (1971)MULTAN: A Clark for suggestingthe problem and discussion;Professor G. C. computer programme for the automatic solution of crystal Ulmer, Dr. R. Leonard, and Mr. E. Beeghefor DTA and TGA; structures(unpublished program manual). and Dr. J. H. Konnert and Dr. J. Karle for helpful suggestions Murakami, T. and Y. Tak€uchi (1979) Structure of synthetic concerningthe direct method. Ohashi thanks ProfessorF. Liebau rhodonite, Mna 565Mg6315SiO3, and compositionaltransforma- for severalhours ofconstructive discussionsofpyroxenoid crystal tions in pyroxenoids.Mineral. J. (Japan),9,286-3M. chemistry, which led to the correct assignmentsof hydrogen , T. Tagai and K. Koto (1977)Lithiumhydrorhodo- atoms.Critical commentsby Dr. J. R. Clark, ProfessorF. Liebau, ilte. Acta Crystallogr.,8 33, 919-921. Dr. H. S. Yoder, Jr., and ProfessorD. R. Peacoron an early draft Narita, H., K. Koto and N. Morimoto (1975)The crystal stnrcture weremost helpful in improving the manuscript.The experimental of nambulite (Li,Na)MnaSiOro(OH). Acta Crystallogr., B3I, part of this study was completedat the GeophysicalLaboratory, 2422-2426. and computationswere carried out at the GeophysicalLaboratory, Ohashi, Y. and R. C. Erd (1978) A new pyroxenoid Naval ResearchLaboratory, Lunar and Planetary Institute, and MnqCaSisOrs'2H2O: its structural relationship to rhodonite, 168 OHASHI AND FINGER: STRUCTURE OF SANTACI.IIRAITE

babingtonite,nambulite and marsturite(abstr.). Geol. Soc.Am. Tak6uchi, Y. (1976) Two structural seriesof pyroxenoids. Proc. Abstractswith Programs,10,465. JapanAcad., 52, 122-125. - and L. W. Finger (1978)The role of octahedralcations in - and Y. Kudoh (1978)Hydrogen bonding and cation order- pyroxenoidcrystal chemistry. L Bustamite,wollastonite and the ing in Magnet Cove pectolite.Z. Kristallogr., 146,281-292. pectolite-schizolite-seranditeseries. lrn. M ineral.,6 3, 27+288. Thompson,J. B. (1978)Biopyriboles and polysomaticseries. lnr. Peacor,D. R. and N. Niizeki (1963)The redeterminationand re- Mineral., 63,239-249. finementof the crystal stnrctureof rhodonite, (Mn,Ca)SiO3.Z. Wan, C. and S. Ghose (1975) Inesite, CarMn7Si,6O23(OH)2 Kristallogr.,I 19,98-116. ' 5H2O:a new chain silicatewith "Fiinferdoppelketten." Natw- P. Dunn and B. D. Sturman (1978a) Marsturite, wissenschaft en, 62, 96. Mn3CaNaHSi5O;51d.now mineral of the nambulite group from - and - (1978)Inesite, a hydrated calcium Franklin, New Jersey.Am. Mineral., 63, 1187-1189. silicate with five-repeatdouble chains.Am. Mineral., 63, 563- --, E. J. Essene,P. E. Brown and G. A. Winter (1978b)The 5'tl. crystal chemistry and petrogenesisof a magnesianrhodonite. _ and G. V. Gibbs (1977) Rosenhahnite, Am. Mineral., 63, 1137-1142. Ca3Si3Or(OH)2:crystal structure and the stereochemicalconfig- Prewitt,C. T. and M. J. Buerger(1963) Comparison of the crybtal uration of thc hydroxylatedtrisilicate group, [Si3Os(OH)21.Am. stnrcturesof wollastoniteand pectolite.Mineral. Soc.Am. Spec. Mineral., 62,503-512. Pap.,1,293-302. Richmond,W. E. (1942) Inesite,Mn7Ca2SiroO28(OH)2. 5H2O. Am. Mineral., 27, 563-569. Skhirtladze,N. I. (1966)Pectolite found for the first time in Geor- gia (English translation). Dokt. Acad. Scr'. USS& Earth Sci. Manuscript received, March 3, 1980; Section,169, 155-15'1. acceptedfor publication, May 8, 1980.