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Lammerite, Cur(Asoo)R, a Modulated Close-Packed Structure

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Lammerite, Cur(Asoo)R, a Modulated Close-Packed Structure 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 <Cu(l)-0> 2.277 cu(2)-0(3) '111.72(2)0 cu(2)-0(3)d 2.0?8(4) I No. of lFol>3o 822 As-0(I ) r.680(5) cu(2)-0(4) r.94r(4) As-0(2) 1.670(5) <Cu(2)-0> 2.159 V 295.5(2)83 R(observed) 3.1% As-0(3) 1.695(4) As-0(4)b r.7r3(4) SpaceGroup Pzlla R"(observed) 3.5% <As-0> I .690 'l'r8.9(2) Unit cell contents: 2[Cu,Asr0UJ 00)-0(2) 2.885(9 ) 0('1)-As-0(2) 0(r ) -0(3) 2.751l8) 0(l)-As-0(3) 109.2(2) R = rIFol-lFol)/rlFol 00 )-0(4)b ?.766t81 0(l)-As-0(4)b 109.2(2) 0(2)-0(3) 2.751(81 0(2)-As-0(3) 109.7t2) (2)-0(4)b 2.673(8) 0(2)-As-0(4)b r04.4(2) = 0 R* [xw(lFol-lFc]\?/LwFozlI, w = 1 0(3)-0(4)b 2.695(8) 0(3)-As-0(4)b r04.5(2) <0-0>As 2.754 <0-As-0> 109.3 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.
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