American Mineralogist, Volume 71, pages206-209, 1986
Lammerite, Cur(AsOo)r,a modulatedclose-packed structure
F. C. HnwrHoRNE Department of Earth Sciences, University of Manitoba Winnipeg, Manitoba R3T 2N2, Canada
Abshact The crystal structureof lammerite, Cur(AsOo)r,monoclinic, a : 5.079(l), b : ll.6ll(2), c : 5.394(l)A, P : tll.72(2f, V:295.5Q) k,Z:2,space groupP2,/a,hasbeensolved by direct methods and refined by a least-squaresmethod of an R index of 3. l0lousing 822 observed(I > 2.5o) reflections.Lammerite is a close-packedstructure. There are a-PbO, type chains ofoctahedrally coordinated Cu running parallel to [00]. The chains are ar- ranged in layers parallel to (001), and these layers are linked by arsenate tetrahedra and by additional Cu octahedra.The a-PbO, chains have beendisplaced from their ideal close- packed position by differential shear along [00], and the close-packedlayers show a prominent modulation along [010]. Such a modulation is common in copper minerals containing edge-sharingoctahedra; it would seem to be a cooperative effect ofthe local Jahn-Teller distortions around each Cu octahedron in the structure.
Introduction (1985a).A total of 869 reflections were measuredout to Lammerite is a copper ortho-arsenate,Cur(AsOo)r, first a maximum 20 of 6C'.An empirical absorption correction characterizedby Keller et al. (198l) from a hand specimen was applied (p-scan method), approximating the crystal ofcorroded quartzoserock from Laurani, Bolivia. It was shape by a triaxial ellipsoid; R(symmetric) for the azi- later reported from the Tsumeb copper deposit (Keller, muthal data was reduced from 10.60/oto 3.2Vo.Lorentz. I 98 l), where it is associatedwith chalcanthite,anhydrite, polarization and background corrections were also done, leightonite and various silicate minerals (Keller and Bar- and the data were reducedto structure factors; ofthe 869 telke, 1982).As part of a generalstudy of copper minerals unique reflections,822 wereclassed as observed (1 > 2.5a). (Hawhorne 1985a,b;Hawthorne and Groat, l985a,b; Hawthorne and Eby, 1985),the structure of lammerite is Solution and refinement examined here. Poulsen and Calvo (1968) reported the Scattering curves for neutral atoms together with anom- structure of a monoclinic form of Cur(AsOo)r,differing alous dispersion coefficients were taken from Cromer and from that of lamrnerite. Keyite @mbrey et al., 1977) is a Mann (1968)and CromerandLiberman(1970). R indices mineral from Tsumeb with the formula (Cu, Zn, are of the form given in Table I and are expressed as Cd)r(AsOo)r,different from lammerite or Cur(AsOo)r;it percentages. is not clear whether or not Cd is an essentialcomponent The systematicabsences hll, h : 2n + l; 0k0, k: in keyite, this must await further structure work. Thus 2n + I uniquely define the space group as P2r/a. The lammerite has at least one polymorph, adding further structure was solved by direct methods. The phase set complexity to that already complex gxoup,the M3(TO4), with the higbest combined figure of merilgave a solution minerals. that was compatible with the known stoichiometry. This refined rapidly to an R index of 4.60/ofor an isotropic Experimental thermal model. Conversion to anisotropic temperature The crystal used in this work is from the Tsumeb de- factors for all atoms resulted in convergenceat an R index posit, Namibia, and was obtained from the Department of 3.lolofor the 822 observedreflections. Final parameters of Mineralogy and Geology,Royal Ontario Museum, col- are given in Table 2, observed and calculated structure lection number M38818. The crystal was mounted on a factors and anisotropic temperature factor coefficients in Nicolet R3m automated four-circle diffractometer. and Table 3' and interatomic distancesand anglesin Table 4. the cell dimensions were refined from the setting angles An empirical bond-valencecalculation, using the param- of twenty-five automatically aligned intense reflections. etersof Brown (1981)is given in Table 5. The (monoclinically constrained)cell dimensions are list- ed in Table l; they are statistically identical with those t To receive a copy of Table 3, order Document AM-86-291 given by Keller et al. (1981). from the BusinessOfrce, MineralogicalSociety ofAmerica, 1625 Intensity data were collected according to the experi- I Street,N.W., Suite 414, Washin4on, D.C. 20006. Pleaseremit mental procedure described by Hawthorne and Groat $5.00 in advancefor the microfiche. 0003404x/86/0l 024206$02.00 206 HA WTH O RNE : LAMME R ITE STRUCT URE 207
Table l. Miscellaneousinformation: lammerite. Table 4. Selected interatomic distances (A) and angles (') for lammerite.
a 5.079(l)fi Crystal size (m) 0.18x0.20x0.35 Cu(1)-0(l)a,b1.974(4)x2 cu(2)-0(1)f 2.282(51 't (4) b ll.6ll(2) Rad/Mono Mo/Gr cu('1)-0(2)c,d 2.923(5)'l.933(5)x2 cu(2)-0(2) .947 Cu(l)-0(4)a,b x2 cu(2)-0(2)d 2.782(sJ't.972(5) c 5.394(l) Total unique lFol 869
0(l )a-0(2)c 3.587(']0)'10 x2 0(l)a-Cu(l)-0(2)c92.'l(l)x2 0(l)a-0(2)d 3.467 ( ) x2 0(l)a-cu( l )-0(2)d 87.9(l) x2 0(l)a-0(4)a 2.758(9\x2 0(l)a-cu(1)-0(4)a89.8(2)x2 Shuctural description 0(l)a-0(4)b 2.768(9\ x2 0(1)a-cu(l )-0(4)b 90.2(2\xz 0(2)c-0(4)a 4.173('l0)x2 0(2)c-cu(1)-0(4)a 117.0(2\x2 There is one unique arsenicposition in lammerite; the 0(2)c-0(4)b 2.674(9\x2 0(2)c-cu(l)-0(4)b 63.0(2)x2 As is coordinated by four O'z- anions, the resulting tet- <0-0>Cu( I ) 3.238 <0-Cu(l)-0> 90.0 rahedron showing a typical range of bond-lengths and 0(r)f-0(2) 3.6140 0) 0(1)f-cu(2)-0(21 117.2Q\ angles.The variation in As-O bond-lengthscorrelates well 00 )f-0(3) 2.934(9) 0(l)f-cu(2)-0(3) 86.9(2) 0(r)f-0(3)d 3.344(9) 0(l)f-cu(2)-0(3)d l0l.6(2) with the local bond-valence requirements of the anions 0(r)f-0(4) 2.970(9) 0(l)f-cu(2)-0(4) 89.0(2) (Table 5). There are two unique copper positions, 0(2)-0(2)d 2.963(9) 0(2)-Cu(2)-0(2)d 75.3(l) each 0(2)-0(3) ?.747(9) 0(2)-cu(2)-0(3) 89.0(2) Cu being coordinated by six Oz- anions in distorted oc- 0(2)-0(4) 2.737(9) 0(2)-cu(2)-0(a) 89.5(2) 0(2)d-0(3) 3.079(9 ) 0(2)d-Cu(2)-0(3\ 78.7(21 tahedral arrangements.Cu(l) is on special positiorl.2d, a 0(2)d-0(3)d 2.748(8) 0(2)d-cu(2)-0(3)d 67.6(l) centerof symmetry. It showstypical well-developedJahn- 0(2)d-0(4) 3.808(r0 ) 0(2)d-Cu(2)-0(4) 106.1(2) 0(3)-0(3)d 2.930(9) 0(3)-cu(2)-0(3)d 94.2(l) Teller distortion, with four short meridional bonds ((Cu- 0(3)d-0(4) 2.8',t7(9) 0(3)d-cu(2)-0(4) 90.4(2) O) : 1.954A) and two long axialbonds ((Cu-O) : 2.923 <0-0>Cu(2 ) 3.058 <0-Cu(2)-0> 90.5 A). Cu(Z) is on the generalposition 4e, andhence has no a2 l/2-x, -1/?+y,1-z; b: -\/?+x, symmetry constraintson its bond geometry.It also shows -l/2+y, l-z; di 1/2+x,l/2-y+1, z; f: x, y, z+1. Jahn-Teller distortion, but it is not as strongly developed as for Cu(l), with four short meridonal bonds ((Cu-O) : 1.972A) and two long axial bonds ((Cu-O) :2.$2A} The structure of lammerite is illustrated in Figures I Figure I at first sight. However, Figure 2 provides the key and 2. A prominent feature of the structure is the sheet to the rationalization of the structure. The staggeredoc- ofa-PbO, type staggerededge-sharing octahedral chains tahedralchains are seenend-on, and adjacentchains form running parallel to [00] (Fig. l); theseconsist ofthe Cu(2) layers orthogonal to [001]; between these layers are in- octahedrathat show the lesseramount of Jahn-Teller dis- tercalatedthe highly distorted Cu(l) octahedraand the As tortion of the two types of Cu octahedra in the structure. tetrahedra. However, as is apparent in Figure 2, these These chains are crossJinked in the two directions or- layers are not planar but have a prominent modulation t/, thogonal to their lengths by the Cu(l) octahedraand As in the [0 I 0] direction, with an amplitude of waveJength. tetrahedra. It is difficult to see any further regularity in An idealized version of two of these layers is shown in Figure 3. In a single close-packeddouble-layer ofoxygen Table 2. Atomic parametersfor lammerite. atoms, chains of edge-sharingoctahedra are alternately
- "equiv Table 5. Empiricalbond-valence (v.u.) table for lammerite. Cu(l) 0 1/2 1/2 r.25(3) Cu(2) 0.2462(?l 0.8367(r) 0.8563(2) r.48{3) Cu\21
AS -0.r'ilrfi) 0.885r(r) 0.262s(r) 0.96(2 ) 0il) 0.43512 0.t82 1.?63 1.880 0(l) 0.'1921(8) 0.9r73(4) 0.2203(8) 1 .3{r) 0.472 1.877 0(2) o.04l 12 0.055 1.309 0(2) -0.0946(9 ) 0.8r57(4) 0.5390(8) r.5il) 0.431 1.196 2.003 0(3) 0.r7r7(9) 0.687](4) 0.9904(8) r.3('r) 0(3) 0.370 0(4) 0.2893(9 ) 0.9902(4) 0.7332(8) r.zil) 0(4) 0.4e312 0.481 1.122 2.096
uequiv= u"orruxlo2 1.997 4.890 208 HA WTHORNE : LAMME RITE S TRU CT URE
T a t I
l<__ b ______r{ b ------> Fig. l. The crystalstructure of lammeriteviewed down [00I ]; Fig. 3. The ideal close-packedlammerite arrangement.The the Cu(l) octahedraare shaded in largedots, the Cu(2)octahedra chains of octahedra in the lower layer are alternately occupied are dashedand the As tetrahedraare dotted. Note the a-PbO" (shaded) and vacant (not shaded), except for the rightmost oc- edge-sharingchains ofoctahedra parallel to a. cupied chain which is not shaded. The tetrahedra occupy the higher layer. The Cu of the higher layer does not occupy the regular octahedral site (such as A), but occupies the irregular octahedral site B (the large dotted octahedron). occupied OV Cu(2) atoms) and empty. A third layer of oxygen atoms overlies the first two to produce a cubic close-packedsequence. The As atoms occupy tetrahedral intersticesin this secondlayer. The Cu(l) atoms occupy octahedralinterstices in this secondlayer; however, they do not occupy completely regular octahedral sites (such as A. at which each octahedron shares 2 faces with As tetrahedra), but occupy axially compressed octahedra which share only 2 edgeswith adjacent As tetrahedra. In (a) (b) the real lammerite structure,adjacent edge-sharing chains Fig. 4. This shows the efect of ditrerential shear of the a- are slightly displaced along [00]; as shown in Figure 4, PbO, type chains, producing an axial elongation ofthe Cu(l) type (site this has the effectofshortening one pair ofCu(l)-O bonds octahedra. Note that this type of octahedron B in Fig. 3) is considerably shortened along the vertical direction; differential and lengtheningthe other pair to produce an octahedron shearshortens one ofthe remaining pair ofbonds and lengthens with a strong axial elongation.Thus lammerite is actually the other to produce elongation in a diagonal direction. a cubic close-packed structure that has undergone slight differential shear together with a periodic layer modula- tion orthogonal to the shear direction. The reason for the layer modulation can also be found in the spontaneous electronic relaxation of the structure. This is illustrated in Figure 5. If a planar layer of a-PbO, (a) (b) type octahedral chains is modulated such that the octa- Fig. 5. The efect of layer modulation, producing an axial hedral chains correspondto the maximum amplitude po- elongation of the Cu octahedra of the a-PbO, type chains. Of modulation actually occurs because of the sponta- sitions, one pair course, the trans ofbonds in each ofthe octahedra neous distortion of the octahedra rather than vice versa. is elongatedrelative to the other four bonds. Thus mod- ulation of the structure arises from the Jahn-Teller type distortion observedin the Cu(2) octahedra.Such modu- lations may be quite widespreadin copperminerals; Haw- I t a I t I
F1- b --_--r{ Fig. 2. The crystal structureoflammerite viewed down [100]; the shadingis as in Fig. l. Note the close-packedlayers, moduled Fig. 6. The crystal structure of synthetic Cur(AsOo)r, pro- along b. jected dom [001]; note the close-packedlayers parallel to (l l0). HAWTHORNE : LAMMERITE STRUCTURE 209 thorne and Eby (1985) showed that lingrenite is a mod- Cromer, D. T. and Liberman, David (1970) Relativistic cal- ulated close-packed structure, and wroewolfeite culation of anomalousscattering factors for X-rays. Joumal of l89l-1898. (Hawhorne and Groat, 1985a), shattuckite and plan- ChemicalPhysics, 53, Cromer, D. T. and Mann, J. B. (1968) X-ray scatteringfactors ch6ite (Evans and Mrose, 1977) also show strong layer computed from numerical Hartree-Fock wave functions. Acta modulation. Crystallographica,424, 321-324. A [001] projection of the syntheticCur(AsOo), structure Embrey, P. G., Fejer, E. E. and Clark, A. M. (1977) Keyite: A is shown in Figure 6. Two ofthe three unique Cu positions new mineral from Tsumeb. Mineralogical Record, 8, 87-90. Evans, H. T., Jr. and Mrose, M. E. (1977) The crystal chemistry are [6]-coordinate, and the other is [5]-coordinate. The of the hydrous copper silicates, shattuckite and planch€ite. structure is close-packedwith the rather corrugated anion American Mineralogist, 62, 49 l-502. layersrunning parallel to (l l0). The octahedraledge-shar- Groat, L. A. and Hawhorne, F. C. (1985) Refinement of the papagoite. ing dimers prominent in Figure 5 link back along [001] crystal structure of Tschermaks Mineralogische und Mitteilungen, in press. to share corners with arsenatetetrahedra and the Petrographische Cu(l) Hawthorne, F. C. (1985a) Towards a structural classification trigonal bipyramid; thus the layers have identical com- of minerals. The uMrYTr@minerals. American Mineralogist, position, and are characterizedby a very open packing 70,455-4'73. with few sharededges. This is apparent in the calculated Hawthorne, F. C. (1985b) Refinement of the crystal structure densities of Cur(AsOo),and lammerite, which are 5.032 of botallackite. Mineralogical MagazinLe,49, 85-89. Hawthorne, F. C. and Eby, R. (1985) Refinement of the crystal and,5.263 g/cm3respectively. structureoflindgrenite. NeuesJahrbuch liir Mineralogie, I 985, 234-240. Acknowledgments Hawthorne, F. C. and Groat, L. A. (1985) The crystal structure I am pleasedto acknowledgethe cooperationof ProfessorsJ. of wroewolfeite,a mineral with [Cu.(OH).(SO.XHTO)]sheets. Mandarinoand F. J. Wicks,curators at the Departmentof Min- American Mineralogist, 70, I 050-1055. Keller, P. (1981) Iammerit, Cur(AsOo)r,und seine Paragenses eralogyand Geology, Royal Ontario Museum, Hogtown.Finan- in Tsumeb. Der Aufschluss, 32, 437-441. provided cial supportwas by the NationalSciences and Engi- Keller, P. and Bartelke, W. (1982) Tsumeb! New minerals and neeringResearch Council ofCanada, in theform ofa fellowship, their associations.Mineralogical Record, 13, 137-147. an operatinggrant, and a majorequipment grant. Poulsen, S. J. and Calvo, C. (1968) Crystal structure of Cur(AsOo)r.Canadian Journal of Chemistry, 46,917-927. References Brown, I. D. (1981)The bond-valencemethod: an empirical approachto chemicalstructure and bonding.In M. O'Keefe and A. Nawotsky(Eds.), Structure and Bondingin Crystals, Manuscript received,April 23, 1985; Vol. II, p. 1-30.Academic Press, New York. acceptedfor publication, September4, 1985.