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Akrochordite, (Mn,Mg)S(OH)4(Hro)N(Ason)R:A Sheet Structure with Amphibole Walls

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Akrochordite, (Mn,Mg)S(OH)4(Hro)N(Ason)R:A Sheet Structure with Amphibole Walls American Mineralogist, Volume 74, pages256-262, 1989 Akrochordite, (Mn,Mg)s(OH)4(HrO)n(AsOn)r:A sheet structure with amphibole walls P,q,uL BnHN Moonn Department of the GeophysicalSciences, The University of Chicago,Chicago, Illinois 60637, U.S.A. Pruorp K. SrN Gur"r.l Department of Geology, Memphis State University, Memphis, Tennessee38152, U.S.A. Er.rvrnnO. Scrrr.nvrprn Department of Chemistry, University of Missouri, Columbia, Missouri 65211, U.S.A. Arsrrucr Akrochordite, (Mn,Mg)r(OH)o(HrO)o(AsOo)r,monoclinic holosymmetric, a: 5.682(2), b--17.627(5),c:6.832(DA,B:99.49(2),2:2,spacegroupP2,/c,D"o":3.19,3.26, D*. : 3.26 g'cm-3for (MnoroMgoro),has 12 non-H atoms in the asymmetricunit. R : 0.036for 1295independent ,F'.. Average distances are t6tMn(l)O 2.l74,r6tMn(2YO2.232, r6rMn(3FO2.188, and t4lA€ 1.686A. Bond-distanceaverages suggest that Mg2* substi- tutes for Mn2*, with a preferencefor the Mn(l) site followed by the Mn(3) site. The perfect {010} cleavageresults from a sheetlike structure with formula-unit com- position 1[M5(OH).(HrO)o(AsOo)r]parallel to this plane. Of the six unique hydrogenbonds O-H...O, four penetratethe c-axialglide planesat y: r1o,3Aand bond symmetry-equiv- alent sheetstogether. Two of thesebonds involve OW(2) - O(3) 2.79 and OH(6) - OW(2) 3.02 A, the arrows pointing to hydrogen-bondacceptors and the distancescorresponding to oxygen-oxygenseparatlons. Upon projection along [012], it is seenthat the sheetsare made of fundamental building blocks based o\ LlN4sAiADloctahedral walls of amphibole type. In amphiboles, the 4' : (OH ,F ), but in akrochordite it is apical O(l) of the [AsOo] tetrahedron. These walls are connectedin stepwisefashion bV O(2) and O(3) of [AsOo] to form the sheetlike modular unrts. The remaining O(4) of [AsOo] receivesthree hydrogen bonds, two of which penetrate the c-axial glide planes.These have oxygen-oxygenseparations OWlt;<" - O(4) 2.59 artd Ow(2) ' O(4) 2.62 A. The remaining bond, within the modular unit, is Ow(l) - O(4) 2.62 4,. Severalexamples of oxide terminal to a tetrahedral anionic group are known and include seamanite,metavauxite, and minyulite, all of which possesssimilar relatively short O-H...O oxygen-oxygendistances, ranging from 2.58 to 2.80 A. INtnonucrroN Akrochordite's name comes from the Greek akrochor- "a wart." And like a wart, its structure Determining the relationship betweencrystal structure don, meaning Single crystals, extremely and phaseparagenesis is a prime motivation for any sys- solution remained enigmatic. or tapered. Although approxi- tematic analysisof mineral-chemicalcrystallography, and rare, are usually splayed group could be obtained, members of the Mn;*(OH);-r,(HrO)"(AsO.)l- seriesare mate cell dimensions and space for structure analysis. no exception(Moore and Molin-Case, 1971).The species, the crystals were hardly suitable was to akrochordite because it usually althouglr rather exotic, occur in relatively close associa- Interest attached of other arsenates. tion with each other, and parageneseswere early studied crystallizes latest in the sequence (1885). by Sjiigren However, experimentally controlled ExpnnrlrnNTAl DETATLS conditions of temperature, pressure, pH, Eh, etc. are were largelyunexplored. It is unfortunately too early to reliably Several radial aggregatesof sharp akrochordite crystals repeatedlybroken until a crystal that yielded reasonablesingle- apply mineral structuresdirectly to mineral paragenesis, crystal precessionphotographs was obtained. The fragments came feasibility such a relationship is uncertain. One and the of from the type specimenof eveite,NRMS 390274,in the Swedish major problem is the lack of any strategyto obtain reli- Natural History Museum, which was also used in the original able thermochemical parametersdirectly from a crystal study of Moore ( I 967). The presentunit-cell parametersin Table structure; another is the difficulty in obtaining good I are in fair agteementwith the earlier study. The details of the temperatures for the gowth of successivecrystallizing experimental part of this investigation are summarized in phases. Table l. 0003-004x/89/0I 02-02 56$02.00 256 MOORE ET AL.: AKROCHORDITE 2s7 0 lfi-**roo 0(3) osin p tdfis)3ooi:: '- As 0(11. 32e '5 . zssi T,id'|,:0(2) l-lr2 623\ -51 r ggl.r*-i;# I u1!rl. -- ;f /a'' 0(3)39 Fig. l. Polyhedral representationof akrochordite crystal structure along [001]. The asymmetric unit is indicated on the left. Heights are given as fractional units. Hydrogen bond distances,angles, and arrows are included. Note the dotted c-glide plane. Conventional Lorentz, polarization, and absorption correc- A conventional polyhedral diagram projecteddown the tions were applied by empirical ry'-scan,and scaledtransmission [001] axis is presentedin Figure l. The asymmetricunit coefficientsranged from 0.80-1.00. Solution of the crystal struc- consists of labeled atoms in Table 2. The proposed O- ture was straightforward by means of the l'rurreN direct meth- H..'O hydrogenbonding schemeis shown by dashed procedure(Main The dis- ods et al., 1971). effect of anomalous arrows in the first quadrant along b in the diagram. The persion of Mo radiation by Mn, As, and O was included in { c-axial glides are also sketchedin, and only the hydrogen by using Af' and A/' (Ibers and Hamilton, 1974). All atoms, penetrate glides. which is closeto the exceptH atoms, were locatedin the asymmetric unit. The initial bonds these OW(2), hearry Mn and As afforded sufficiently precise phase anglesso glide plane, is coordinatedby Mn(3) and 3H. Thus, a that all remaining oxygen atoms in the asymmetric unit could sheet of formula unit of structure occurs parallel -t/q t/o be located. Refinementwas concludedafter five cyclesof atomic to {010}. The sheetis bounded dt < y < by the coordinate parameters and anisotropic thermal-vibration pa- c-axial glides and only hydrogen bonds penetrate the rametersshowed no significant shifts. It was hoped that H-atom centroids could be located by difference synthesis, but back- ground noise was prominent-a typical observation on crystals TraLe1, Experimentaldetails for akrochordite with elementsbeyond the third row. Therefore,hydrogen bonds (A) Crystalcell data were inferred by inspection ofbond lengthsand bond strengths. a (A) 5.682(21 However, of the six unique H-atom centroids, five appeared b (A) 17.627(5) among the maxima in difference synthesis,but they were not c (A) 6.832(1) 99.49(2) refined. Bf) Spacegroup nJc Least-squaresrefinements employed the full-matrix prog,ram z 2 sHELx-76.The conventional discrepancyindices are listed in Ta- Formula (Mn,M g)5(OH)1(H,O)4(AsO4), ble l. A total of 108 variable parametersand 1295 independent D, 3.194(Flink, 1922),3.26 (Moore, 1967) (g.cm 3.41 (for Mn, 3.26 (for MnoeMgo,o) reflectionsused in refinementgives a reflection: variable param- D" ") @), p (cm ') 91.0for MnoeMgo,o eter ratio of l2:1. The observedand calculatedstructure factors (A) Intensity measurements are available on request.r Crystal size (pm) 150x230x320 Diffractometer Enraf-NoniusCAD-4 DnscnrprroN oF THE sTRUCTURE Monochromator Graphite (sin 0.65 projections Max d/I) Conventional choices of axial disguise the Scan type 0-20 fact that akrochordite is constructed of sheets with for- Maximumscan time (s) 120 : : B: 0-35 mula-unit composition where Scan width (') 0 (A + I tan 0) where A 0.9, [M3*(OH)4@,O)o(AsOo)r], Background counts 4/6 time on peak scan, 1/6 time on each Mz* : principal Mn2* and subordinate Mg2*. Further- side more, these sheets are locally based on cubic close-packed Radiation Mor( (I : 0.71069A) 1545 strips of oxygen atoms and are knit to neighboring sym- Measuredintensities lndependentF" 1295, F"> 4o (F") used in refinement metry-equivalent sheets through four independent, rather Orientationmonitor After every 200 reflections,25 reflections refinement strong hydrogen bonds and the [O(l)]-As-[O(2),O(3)] used for cell fntensity monitor Every7200 s of X-ray exposure bridge of the arsenate tetrahedron. (G) Refinement of structure R 0.036 F:>(llF"l -lF.ll)l>F. - : ' A copy of the observedand calculatedstructure factors may R_ 0.037 R" : pv(lF.l lF.lYl>wF3l1P,w n " (O be ordered as Document AM-89-395 from the BusinessOfrce, Weights Each structure-factorassigned weight Mineralogical Society of America, 1625 I Street, N.W., Suite Scatteringfactors NeutralMn, As, and O (lbersand 414, Washington,D.C. 20006,U.S.A. Pleaseremit $5.00in ad- Hamilton.1974) vance for the microfiche. 258 MOORE ET AL.: AKROCHORDITE Treue2. Akrochordite:Atomic coordinateand anisotropicthermal-vibration parameters (x 10n) ur. U" uzt urs u' B* (41 M(1) 0 0 0 163(8) 195(8) 1s0(8) 64(6) -30(6) -3(6) 1.34(6) M(2) 0.4758(2) 0.0618(1 ) o7967(2) 120(5) 144(5) 171(5) 43(4) -36(4) 6(4) 1.15(4) M(3) -0 0s66(2) 0.131 s(l ) 0.6470(21 159(5) 163(5) 99(5) 15(4) -16(4) 8(4) 1.11(4) As 0.3683(1) 0.11 85(1 ) 0.3188(1) 118(3) 102(3) 4s(3) 4(21 -37(2) 2(31 0.8s(2) o(1) 0.3512(9) 0.0470(3) 0.1483(7) 168(24) 106(23) 116(23) -32(19) -63(18) -1(20) 1.03(18) o(21 0.5791(9) 0.096s(3) 0.s149(7) 142123) 211(261 86(22) 54(20) -70(18) 5(21) 1.16(19) o(3) 0.1033(10) 0.1335(3) 0.3884(7) 233(27) 254(291 91(23) 19(21) 34(20) 37(23) 1.52(19) o(4) 0.4380(9) 0.1973(3) 0.2037(7) 218(26) 101(23) 112(231 -3(19) 1q19) -22(2Ol 1.13(19) oH(s) 0.1252(9) 0.0235(3) 0.7272(7) 182(24) 115(221 62(21) -10(18) -12(18) 9(20) 0.95(18) oH(6) 0 8144(8) 0.1036(3) 0.9175(7) 153(23) 131(24) s6(20) 2(18) -22(17\ 1(19) 0.89(18) ow(l) 0.2964(9) 0.1734(3) 0.8232(7) 185(25) 128(24) e4e2) -9(18) -37(18) -28(20\ 1.0s(19) ow(2) 0 8588(9) 0.2483(3) 0.1s66(8) 176(2s) 163(25) 't74(25!, 6(21) -34(20) -7(21) 1.3s(20) Note: The U, are coefficientsin the expressionexp[ (41ff + Unk2 + U$12+ 2u,2hk + 2unhl + 2u2okl)l The equivalentisotropic thermal parameter is 84 : (8/3)l(U1j + U22+ U$)- boundaryalong [010].
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