A merican M ineralogist, Volume62, pages 807-81i,,1977

A refinementof the structureof dioptase,Cur[Si6Or8].6H2O

Pnul H. Rtssn,G. V. Gtsss ANDMARTHI M. Henttl-l Department of Geological Sciences Virginia PolytechnicInstitute qnd State Uniuersity B lacks burg, Vi rginia 2 406 I

Abstract

The structure of dioptase,Cu.[Si6O,s] .6H"O (a : 14.566,c = 7.778 A, R3;, was refined to R : 0.039 using 969 non-zero reflections.The structure consistsof puckered trigonal rings of six water moleculeswith an ice-like configuration sandwiched between similarly puckered trigonal rings of six silicate tetrahedra bonded together laterally and verticallyby Cu atoms. The water moleculesare hydrogen-bondedto one another and to the bridging oxygensof adjacent [Si6O18]rings, resultingin O-H distancesin the water molecule of 1.1and0.9A.Theeffectiveaperturediameterofthe[001]channel throughtheringsisonly 2.0 A, preventingzeolitic transfer of the water molecules. The Si-O bond lengths[Si-O(br) : 1.645,1.646A, Si-O(nbr) : 1.617,1.600A] can be rationalized in terms of the valenceangles within and betweenthe silicatetetrahedra in the [Si6O18]ring and a Mulliken bond overlappopulation analysis.The Cu atom is coordinatedby four oxygensat 1.95-1.98A in nearly square-planararray, with two water moleculesat2.65 and 2.50 A forming a tetragonally-distortedoctahedron. Bond overlap populations suggest that the nonzeolitic water molecules are not bonded to the atom, although the empirical equation of Brown and Wu (1976)gives bond-strength sums around the Cu2+ion of l.8l for 4-coordinationand 1.99for 6-coordination.

lntroduction with a band-width at half-maximum of 6500 cm-'. re- The most recent structural investigationof diop- The characteristicgreenish-blue color of dioptase from transmissionnear 5000A" (Newnham and tase,Cuu[Siro,r].6HrO, was undertakenby Heideel sults Santoro, 1967,p. K88). Newnham and Santoroalso al. (1955).They found that trigonal [Si6O18]rings are becomesantiferromagnetic near bonded togetherboth verticallyand laterally by Cu2+ found that dioptase point, and proposed the magnetic in approximatesquare-planar coordination with the 70 K, the N6el a doubled unit cell with a : 14.61, nonbridging oxygensin adjacent rings. Their refine- structurebased on : 15.60A (Fig. l). ment representsa significant improvement on the c Heide et al. (1955) earlier structuredetermination by Belov et al. (1952). In their refinementof dioptase, suggestedthat they Heide et al. found that dehydration of dioptase be- located the water moleculesand puckered ring gins at - l00oC but that water lossis only completeat are hydrogen-bondedin a 6-membered are not sufficientlyprecise -700"C. They reported a concomitant change in of 3 symmetry.Their data atoms. They also reported color from emeraldgreen to grey black. The dioptase to locate the hydrogen structure was retained after dehydration, at least as Si-O bond lengthsto the bridging oxygensof 1.55 lengthsto the nonbridging nearly as could be determinedby powder-diffraction and 1.67A and Si-O bond values lack methods. Reabsorptionof the water did not occur at oxygensof 1.60and 1.63A. These clearly 200'C in an autoclave,arguing against the assertion the precision required for a meaningful analysisof of Belov et al. (1952) that the water in dioptase is bonding, and in order to remedythis situation and to zeolitic. find the hydrogen atoms, we undertook yet another The diffusereflectance spectrum of powdereddiop- refinementof dioptase(see Hamil and Ribbe,1971). peak tasehas "an intenseabsorption at 13,300cm-1 Experimental methods

I Present address: Geology Department, Rutgers University, A crystal of dioptase from Ren6ville,Congo, was New Brunswick, New Jersey08903. analyzedwith an electron microprobe, and no sub- 807 808 RIBBE ET AL: STRUCTURE OF DIOPTASE

Table 2. Anisotropictemperature factors (X lff) of dioptasewith standarddeviations given in parentheses.

-11R -22 R R-L2 -13 -23R

Cu 37(3) s9(3) 146(6) -7 (2) -28(4) 44(4) si s1(6) 4s(5) 118(14) 3o(5) -3(6) 4(7) o1 88(15) 129(1s) 273(4L) 68(13) 7(r9) -10(le) -4(19) -26(18) o2 55(14 ) 87(15) 144(39) L4(r2)

03 82(L4) 134(16) 142(34) 78(13) -s9(18) -84(20) 0w 304(22) 287(22) 469(56) 195(1e) s3(28) -e(2s) H1 L52x 752* 401* 76* 0 0 H2 208* 208* 548* 104* 0 0

* Isotropic equivalents Fig. l. An idealizationof theCu-Onetwork in dioptaseArrows in the smallcircles (Cu) indicaterelative spin directionsfor the proposedmagnetic structure. (After Newnham and Santoro, 1967, mal ellipsoids in Table 3. Interatomic distancesand p. K87). anglesare listed in Table 4. Table 5 containsobserved and calculatedstructure factors.2 stituentsfor either Cu or Si were detectedat the 0.02 Discussion weight percent level. Lattice parameterswere deter- mined on the Picker four-circle goniometer (a : In dioptase the bridging Ol oxygen is bonded to 14.566:c : 7.778 A). The 969 non-zero structure two silicon atoms in adjacent tetrahedra in the factors were collected using Zr-filtered Mo radiation, [Si6O,s]ring, whereas the nonbridging 02 and 03 corrected for Lorentz-polarization effects (not ab- oxygens are each bonded to one silicon and two 4- sorption), and weighted according to a sch€mepro- coordinatedcopper atoms.The water molecules,Ow, posed by Hanson (1965). The structure was refined are hydrogen-bondedto one another and to the Ol by conventional least-squaresmethods in space oxygens.We will discussthe structure in terms of the group R3. At the last stage of the isotropic refine- cooper coordination, the Si-O bond lengthsand an- ment, hydrogenswere introduced at positions L0 A gles, and the nonzeolitic water molecule. distant from the water oxygen, Ow, in the plane of 'To obtain a copy of Table 5, order Document AM-77-045 from the three water molecules parallel to (001) and re- the Mineralogical Societyof America BusinessOffice, 1909K St. fined. The final least-squarescycle on the anisotropic NW, Washington, D.C. 20006.Please remit S1.00in advancefor a model yielded an unweightedresidual of 0.040 with- copy of the microfiche. out the hydrogens, 0.039 with the hydrogens. The argument for the validity of the H positions is based Table 3. Orientation and magnitudeof principal axesof thermal on the reasonablewater-molecule configuration, as ellipsoids;estimated standard deviations in parentheses discussedbelow, and not on refinementstatistics. Aton Axis,r ms (A) Direction angle (') with respect to Atomic coordinates are given in Table l, an- acdlc isotropic temperaturefactors in Table 2, and orienta- 1 0.046(3) 74(11) 99(19) 19(1) tion and magnitude of the principal axesof the ther- 2 0.0s0(2) L2L(7) L49(7) 89(21) 3 0.10s (1) 36(1) 119(1) 109(1)

si I 0.0s2(4) 27(L4) 71(18) ro8(13) 2 .061(4) Log(22) 1e(r4) 87(34) Table I Atomic coordinatesand equivalentisotropic 0 3 0.06s(3) 72(L8) 87(34) 19(14) temperaturefactors B ofdioptase (standarddeviations given in parentheses) I 0.073(8) 124(10) 77(L6) 37(r2) 2 o.os2(7) 93(2s) 18(21) 108(19) 34(10) 78(26) se(ls) B(L2) 3 0.103(6) I o .060(10) 72(8) r34(50) 49(50) Cu 0.40646(4) 0.402s1(4) 0.06303(6) 0.41 2 0 .068( 9) 90( 20) 133(so) 137(so) si 0.17s63(8) o.21741(8) 0.04130(13) O.28 0.0e9( 5) 18(8) 77(9) 102(10) 01 0.07r47(2r) 0.18088(22) -0.08273(35) O.64 02 0.28070(20) o.29949(2r) -0.06410(34) 0.48 o.047 (11) 78(20) 152(11) 65(22) 0.062 ( 9) 46(10) 1oo(24) r3s(17) (5) (s) (6) 03 0.1s994(21) O.26776(2r) 0.21385(3s) 0.50 0.114(6) 46 64 ss Ow 0.142L7(29) 0.r.8201(29) 0.s7848(42) 1.s (7) L22(Lr) 38(12) 7L(6) H1 0.146(5) 0.106(s) o.ss9(9) 1.0 0.109 0.134(7) 140(1r) L27(L2) 77<9) H2 0.68(1) 1.3 0.109(6) 0.162(6) 0.r-63(6) 68(7) 23(6) e8(7) RIBBE ET AL: STRUCTURE OF DIOPTASE

Table 4. Interatomicdistances and angleswith standard A and two 03 oxygensat 1.983and 1.952A. Viewed deviationsgiven in parentheses with a stereoscope,the central region of Figure 2 shows that pairs of square-planar groups Distances (3,) Angles (') [CuOo] share O3-O3 edgesopposite a narrowed O3-Cu-O3 SiOO tetrahedron angle of 82.8o,forming nearly planar [CurOu]dimers si-ol r.646(1) 01-si-01, L06.4(2) that are in turn corner-linked to other [CurOu] ' (3 (r) si-01 r .645 ) o1-si-02 108.7 groups. si-o2 1.617(3) o1-si-03 108.7 (1) These edge-and corner-sharingdimers form s1-03 1.600(3) UZ-JA-UT r09. 1(1) a three-dimensionalnetwork in dioptase,as schemat- 02-si-o3 u2.6 (1) mean I.627 ically shown in Figure l. Becausethe 02 and 03 o3-si-o1' r11.2(1) o1-o1, 2.634(4) atoms are nonbridging oxygens,it is reasonableto 01-02 2.65r(4) si-01-si 130.0 ( 2) speak of the [Si.or.] rings as bonded together both (4) 0l-03 2.637 laterally and vertically 02...01' 2.657(4) by Cu atoms. 02...o3 2.677(4) At the opposite vertices of an elongated,tetrag- 03...01' 2.67s(4) onally-distorted octahedron around the Cu2+ ion in CUOO(HrO),polyhedron dioptaseare two water molecules,one at2.502 A and Cu-o2 1.952(3) 02-Cu-O2r 93.3 (1) the other at 2.648A (stericdetails of the Cu coordina- cu-02' r.959(3) 02-Cu-O31 96.8(r) tion are given in Table 4). This classicJahn-Teller | Cu-03r 1.983(3) 02 -cu-o3" 9o.s (1) distortion is also observedin nearly all of the more Cu-03" L.952(3) o3'-cu-03" 82.8(1) Cu-ow 2.648(4) than fifty Cu'+-(O,OH,H2O,CI) coordination poly- Cu-ow 2.502(4) 02-Cu-Ow e2.5(r) 02-Cu-Owl 84.4 (1) hedra we have examined in more than 25 02..-oz', 2.843(2) 02 | -Cu-ow 80.4(1) species.The Cu-O bond lengthsin dioptaseare very t -cu-ow' 02...03' 2.943(4) 02 u3.0(1) near mean for in 02'...03" 2.776(4) the all Cu2+-O distancesobserved I -Cu-Ow (L) 03'...03" 2.602(5) 03 79.9 ,and the Cu'..HrO distancesare on the 03'-cu-ow I 87.2(r) 03"-Cu-Ow e7.0(r) average0. I A longer than those observed in other 03"-cu-0w' 8s.6(1) minerals except for liroconite, CurAl(AsO4XOH)4. Ow-Cu-OwI L66.2(2) 4HrO (Kolesova and Fesenko, 1968), which has Others Cu' . .HrO : 2.55and 2.75A. The 4 oxygen-2water si-si '| 2.981(r) H1-Ow-H2 es.o(6.4) configuration around the copper atom in dioptase Cu-Cu 2.952(I) is unique among Cu-containing mineral species:in Cu-Cu 3.155(1) Ow-0w-Ow 101.2 others with water moleculesat the verticesof elon- H1-0w 1.15 (7) H2-0w 0.90(9) gated octahedra, hydroxyl groups replace some or Hl. . .H2 1.52(r0) all of the oxygens in square-planarcoordination. Ow...0wr 2.705(5) Extended Hiickel molecular orbital (EHMO) cal- culations of the bond overlap populations between The coordination of copper copper and the two water molecules at 2.648 and The Cu'z+ion in dioptaseis in nearly square-planar 2.502 A gave valuesof -0.02 and 0.00, respectively, coordinationwith two 02 oxygensatl.952 and 1.959 affirming the conclusion of Heide et al. (1955) that

Fig. 2. A stereoscopicdrawing of the structureof dioptaseviewed down [001],showing the [Si"O,.]rings (solid Iines)bonded together by copper atoms (dashed lines represent Cu-O bonds). Water molecules have been omitted (see Fig. 5). RIBBE ET AL: STRUCTURE OF DIOPTASE (o-si-o)ux - lo8" lo9" lloo t.650 tx Ol, bridging .E 6 U'

c I o =o a o2

r600 o03 - l,/cos(Si-O(br)-Si) o48 0.50 0.52 Fig.4. Themean of thetwo Si-O(br)bond lengths to a common ---- bridgingoxygen in selectedsingle-ring silicate structures versus the n(Si-O). Si-O(br)-Siangle and - l,/coslSi-O(br)-Sil.The data points (X's) Fig. 3. Plots of the observed Si-O bond lengths,d(Si-O), in from the followingstructures: high-pressure CaSiOs (Trojer, 1969), dioptaseversus n(Si-O) (solid dots, dashedline) and the mean of siloxanes(Oberhammer, 1972), benitoite (Fisher, 1969), and beryl the three O--Si-O angles,(O-Si-O)r, common to the bond (X's, (Gibbs et al., 1968\. solid line).

that Si-O bond lengths are dependent on the mean of waler nolecules are not bonded to the copper atoms. the three O-Si-O angles, (O-Si-O)s, common to the On the other hand, the empiricalequation of Brown bond. The bridging Si-O(br) bond lengthsof 1.645 and Wu (1976) gives bond-strength sums around and 1.646A are also consistentwith the narrow ob- CLr2r'crf l.8l and 1.99for 4- and6-fold coordination, servedSi-O(br)-Si angleof 130' (Brown et al., 1969), re$pectively,suggesting that the water moleculesmay and fit perfectly on the regressionline in Figure 4 l'rebonded to copper. which relatesmean Si-O(br) to -llcos[Si-O(br)-Si] for a number of preciselydetermined single-ring sili- 7 he Si-O bond lengths and angles cates. ETIMC)bond overlappopulations, n(Si-O), calcu- The nonzeolitic water molecule lated for the Si-O bondsin the [Si.O,,]ring of diop- tase correlate inversely with the observed bond Six water moleculesare sandwichedbetween pairs lengths(Fig. 3). Sincen(Si-O) valuesare believedto of [Si.O,.] rings, as illustratedin Figure 5. They form l:e a measureof the chargedensity between nuclei of crown-shapedclusters with 3 symmetry,as in ice,and Si and O, the correlationsbetween d(Si-O) and r(Si-O) adjacent molecules are hydrogen-bonded.Figure 6 are er.pected.The overlap calculations were per- showsthree adjacentmolecules and bridging Ol oxy- forrrred assumingconstant bond lengthsand using gen atoms projected onto the plane of one of the tlre r irr;ervedvalence angles. Thus Figure 3 shows water molecules.The H-Ow-H anele of 95 *6o and

Fig, 5 A stereoscopicdrawing of portions of the dioptasestructure viewed down [001], showing the orientation of the nonzedlite ,r1tr-"1 llrelgcrllqs in relation to the puckered [SLO,.] ring. RIBBE ET AL: STRUCTURE OF DIOPTASE 8l I

G tase is nonzeolitic is fully justified, both experrmen- +O4 tally and structurally.

Acknowledgments The authors are grateful to the Earth SciencesSection of the s, .A National ScienceFoundation for financial support under grant DES75-14912.M.M.H. was supportedas a researchassociate by an NSF Centersof Excellencegrant (GU-3192) to the Department of Geological Sciences,Virginia PolytechnicInstitute and State University. Dr Frank C Hawthorne of the University of Mani- toba offered constructivecriticism. We thank Mrs. R. Haycocks for typing the manuscript

.,r'/ G-.- __.i;Si" References [ool Belov, N. V., W. P. Butuso and N. I. Golovastiko (1952)Crystal structureof dioptase Dokl. Akad Naar&SSSR, 87,953-956. Fig 6. A projection of three adjacent water molecules Brown,G. 8., G. V. Gibbs and P. H. Ribbe(1969) The natureand (Hl-Ow-H2) and nearest-neighborOl atoms onto the planeof the variation in length of the Si-O and Al-O bonds in framework uppermostwater molecule.The largenumbers indicate interatomic silicates Am. Mineral , 54, 1044-1061. distances(solid lines indicateacceptor bonds, dashedlines donor Brown, I D and K. K. Wu (1976) Emprical parameters for bonds,and dotted linesOw . Ow separations).The smallernum- calculating cation-oxygen bond valences.Acta Crystallogr., bers with signsindicate heightsin Angstroms of the atoms above 832,1957-1959 (1969) von Benitoit (*) or below( ) the planeof the drawing.[010] in theplaneofthe Fischer,K Verfeinerungder Kristallstruktur drawing whereas[001] is inclined upward at -30o BaTi[Si.O"]. Z Kristallogr, 129, 222-243. Gibbs, G V., D W Breck and E P Meager (1968) Structural refinement of hydrous and anhydrous synthetic beryl, the H-Ow distancesof 0.9 and 1.15A are not statis- Alr(BerSiu)O,r and , Al, ,Cro'(Be.Si")O'.. Lithos, I, tically differentfrom thoseobserved in water. Each of 275-285 Hamil, M M and P H. Ribbe(1971) The locationof the hydro- the hydrogenatoms is distinct,however, in that Hl is gen atoms in the water molecule of dioptase, CuSiOg.HrO. bondedto two Ow's at l.l5 and 1.7A, whereas H2 is (abstr ) Geol Soc. Am Abstr Programs, J, 315. bonded to Ow at 0.9 A and to the bridging Ol oxygen Hanson, A. W. ( 1965) The of azulene, at2.0A. TheOw...Owdistanceof 2.7A isonly0.06 S-trinitro-benzene complex. Acta Crystallogr., i,9, 19-26 A shorter than that observed in ice, and the Heide,H. G., K. Boll-Dornberger,E Thilo and E. M. Thilo (1955) Die Struktur des Dioptas, Cuu(SiuO") 6H"O Acta Crystallogr., Ow-Ow-Ow angleis l0lo, closeto the 109.5oangle 8,425-430. observedin ice. Hirschflelder,J. O, C F Curtiss and R B. Bird (1954)Molecular Inasmuch as the trigonal [Si6O,8]rings are puck- Theory oJ Gasesand Liquids. John Wiley and Sons, New York. ered in dioptase,the (001) channelsare much more Kolesova, R V. and E. G Fesenko(1968) Determination of the constricted than those in cordierite, where, for ex- crystal structure of liroconite, Cu,Al(AsOr)(OH)n 4H"O. Souiet Phys Crystallogr., I 3, 324-328. ample, the fings are fully expanded and [AlrSirO,r] Newnham, R. E. and R. P. Santoro (1967) Magnetic and optical -2.8 have an effectiveaperture diameter of A (Smith properties of dioptase. Physica status solidi, 19, K87-90. and Schreyer,1962).The O1...Ol distancethrough Oberhammer, H. (1972) The molecular structures of cy- the centerof symmetry acrossthe channelin dioptase closiloxanes. (abstr ) Fourth Austin Symposium on Gas Phase is 4.'17A, and if one assumesthat the diameter of M olecular Structure, I l-12. Smith, J. V. and W Schreyer(1962) Location of argon and water oxygen is2.7 A, it becomesobvious that the effective in cordierite. Mineral. Mag., 33, 226-236 aperture diameter is much too small to permit HrO, Trojer, F J. (1969)The crystal structureof a high-pressurepoly- which has a kinetic diameter of 2.65 A (Hirschfelder morph of CaSiO.. Z Kristallogr., 130, 185-206. et a|.,1954,p. 215),to migrateback into the channels once it has been expelledby heating. Thus the con- Manuscripl receiued,December 27, 1976; accepted clusion by Heide et al. (1955) that the water in diop- for publication, February 28, 1977