American Mineralogist, Volume 83, pages 390-399, 1998

Georgeericksenite,NauCaMg(IO.)u(CrO o)r(Hro)r' a newmineral from OficinaChacabuco, Chile:Description and crystalstructure

Manr A. Coopnnrr FnaNK C. HnwrnoRNErr ANnnnw C. Ronnnrsr2 Jonr- D. Gnrcnr3 JonN A.R. Srmr,rNG,2 ANDEr,rztnnrn A. Morrlrro

rDepartmentof Geological Sciences,University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada '?GeologicalSurvey of Canada,601 Booth Street,Ottawa, Ontario, KIA 0E8, Canada 'ResearchDivision, CanadianMuseum of Nature, PO. Box 3443, Station D, Ottawa, Ontario KlP 6P4, Canada aCanadianConservation Institute. 1030 Innes Road. Ottawa. Ontario KIA 0C8. Canada

Ansrnncr Georgeericksenite,NauCaMg(IO.)u[(Croroso,6)Oo]r(HrO),r, space grotp C2lc, a : 23.645(2),b:10.918(1),c:15.768(1)A,g:114.42(6)",V:3707.3(6)A..2=4,is a new on a museum specimen labeled as originating from Oficina Chacabuco, Chile. It occurs both as isolated and groupings of 0.2 mm sized bright lemon-yellow micronodules of crystals on a host rock principally composedof , , and . Associatedminerals include plagioclase,clinopyroxene, and an undefined hydrated Ca-K- Ti-iodate-chromate-chloride.Georgeericksenite crystals average 30 x 5 x 5 pm in size and are prismatic to acicular, elongate along [001] and somewhatflattened on {110}, and tLey have a length-to-widthratio of 6:1. Forms observedare {100}, {110} majoa and {233 } minor. Crystals are pale yellow, possessa pale-yellow ,are transparent,brittle, and vitreous, and do not fluoresceunder ultraviolet light. The estimatedMohs hardnessis between3 and4, the calculateddensity is 3.035 g/cm., and the mineral is extremely soluble in cold H,O. The optical propertiesof georgeerickseniteare biaxial (+) with u : 1.647(2), 9:1.674(2),1:1.704(2),2V.",": +88.4'andtheorientationisZ-c.Pleochroismis slight with X = very pale yellow and Z = distinct yellow-green. The crystal structureof georgeericksenitehas been solved by direct methodsand refined to an R index of 3.57ousing 2019 observedreflections measuredwith MoKct X-radiation. There is one unique Cr site occupied by 0.84 Cr6* + 0.16 S and tetrahedrallycoordinated by four O atoms, one unique Mg site octahedrally coordinated by O atoms, three unique I sites octahedrally coordinated by O atoms and HrO groups, three unique Na sites with octahedral, augmentedoctahedral and triangular dodecahedralcoordinations, one unique Ca site with square antiprismatic coordination, and six unique (HrO) groups. The cation polyhedra link by corner-, edge-, and face-sharingto form dense heteropolyhedralslabs parallel to (100); these slabs are linked by bonding. The formula derived from the crystal-structurerefinement is NauCaMg(IO,)u[(Crorrs',u)Oo]r(Hro),r.Crystals of geor- geericksenite are extremely unstable under the electron beam during electron microprobe analysis,and the analyzedamounts of all elementsfluctuate strongly as a function of time and crystallographic orientation relative to the electron beam. However, exffapolation of the chemical composition to zero time yields values that are in reasonableaccord with the chemical formula derived from crvstal structure analvsis.

InrnonucrroN SEM energy-dispersionspectra contained major peaksfor peaks Georgeericksenitewas first encounteredas a potential I' ca' and^Cr and minor for Mg, Na, and S' The X-ray diffraction pattern and corresponding powder new mi-neral speciesin 19g4, when one of us iR.C.n.; was engagedin the acquisition of X-ray powder siandardi mount were set aside until the study was reopened in at the Pinch Mineralbgical Museum, then situated in 1994. By this time, the mineral collections belonging to Rochester,New York. One particular specimen, labeled William W. Pinch had been purchasedon behalf of the dietzeitefrom Oficina Chacabuco,Chile, had somebright CanadianMuseum of Nature and were located in Ottawa, lemon-yellow micronodules of crystals that gave an X- Ontario, Canada.The specimenwas retrieved and the en- ray powder-diffraction pattern matching no data listed in suing mineralogical study was successful;the results are the powder diffraction file for inorganic compounds, reported herein. clearly indicating that the unknown could not be dietzeite. The new mineral is named for George E. Ericksen 0003-o04x/98/0304-0390$05.00 390 COOPERET AL.: STRUCTUREOF GEORGEERICKSENITE 391

o m a

Frcunn1. Idealizedprojection ofa georgeericksenitecrystal. a: {100},m: {rr0},o: {233}.

(1920-1996) of Reston, Virginia, who was a noted re- searcheconomic geologist with the U.S. Geological Sur- vey for fifty years. Among his many accomplishments, we wish to highlight his studiesof the nitrate depositsof Chile that provided significant new information on the origin of saline accumulationsin deserts.An outgrowth of this particular work led him to discover and help name six new from Chile; fuenzalidaite,, and hectorfloresiteare recently describedl-bearing phases Frcune 2. SEM photomicrographof part of a georgeerick- that bear his imprint" The mineral and mineral name have seniteaggregate: scale bar : 2O pm, note the crackingof the been approvedby the Commission on New Minerals and crystalsdue to H,O lossunder the SEM vacuum. Mineral Names (MA). Holotype material, consisting of the specimen,polished thin section, and the crystal used for the structure determination are preserved in the Dis- A dearth of suitable crystals and the extreme solubility play Seriesof the National Mineral Collection of Canada of the mineral combined to make the acquisition of com- at the CanadianMuseum of Nature. Ottawa. Ontario. un- plete optical propertiesvirtually impossible.We could not der catalog number CMNMI 82914. mount or remove grains for spindle-stagework; all mea- surementsare basedon one grain. We could not measure DnscnrprroN 2 V for two reasons:(1) AB is the elongate axis (which Occurrenceand paragenesis unfortunately was not the orientation of the one grain The locality label is Oficina Chacabuco,Chile. The co- suitable for measurement);(2) no extinction curves were ordinates of this locality are latitude 239' S, longitude usable as the crystal was attached to the side of a thin 69"37' W. Nothing else about the pedigree of the speci- glass rod. men is known. It originally measured5 x 4.5 x 3 cm and contains no more than 10 mg of georgeericksenite. Electron microprobe examination The host rock is dirty lighrgray-brown in color and is Two acicular crystals were attachedto the surface of a composedprincipally of halite, nitratine, and niter. Clasts plexiglas disk with epoxy. The disk was coatedwith car- of brown fine-grained plagioclase and black crystalline bon and carbon paint was then applied to the ends of the clinopyroxene are found embeddedboth in the matrix and crystal. One crystal had the (100) surface uppermost;the in cavities. A secondnew mineral, an undefinedhydrated other crystal had a growth face of the form {110} up- Ca-K-Ti-iodate-chromate-chloride,occurs as small 0.1 permost; we designatethis as (110). Before analysis,the mm sized fibrous tufts that protrude from small cavities crystals were translucent. After analysis, the remnants and are also randomly scatteredon the matrix. Individual were opaque and pitted and the crystal surface was crystals of this secondphase are lemon yellow and acic- stained brown (presumablyby Ir). ular with a length-to-width ratio approaching 100:1. Fur- The crystals were examined with a CAMECA SX-50 ther study of this phaseis currently underway. electron microprobe operating in wavelength-dispersive Georgeerickseniteoccurs as isolated or, in some cases, mode at 15 kV and various currents from 20 nA to 0.5 as groupings of bright lemon-yellow micronodules of nA. The instrument has three spectrometers and the con- crystals that are concentrated on the surface of one part stituents were examined in the sequence(Na, Cr, I), then of the mineral specimen. These micronodules average (Mg, S, Ca). For at least the first 200 s when the crystal about 0.2 mm in size and each is composedof numerous was exposedto the electron beam, the counts per second individual crystals in random orientation. We can only on each element varied with time by up to an order of speculateon the provenance,but it certainly seemsto be magnitude, indicating that the crystals are extremely un- the last mineral to form in this assemblase. stable in the electron beam. In addition, the counts per secondfor each element at a given time are very depen- Mineral data dent on the surface [(100) or (110)] analyzed. This be- An idealized projection of a georgeericksenitecrystal havior is summarizedin Figure 3. is shown in Figure 1; SEM photomicrographof a part of Over very short counting times (<10 s) at 15 kV and an aggregateis given in Figure 2. The characteristicsof 5 nA, there is a drop in NarO and a gain in IrO, relative the mineral are given in Table 1 and the fully indexed X- to the ideal values. Other components approximate the ray powder-diffraction pattern is given in Table 2. ideal values, although a significant orientation effect ex- 392 COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE

Trele 1. Descriptivecharacteristics of georgeericksenite

Physical properties Color pale yellow (crystals)to bright lemon yellow (aggregates) Streak pale yellow Luster vitreous Diaphaneity transparent(crystals) to translucent(aggregates) Fluorescence none observed Fracture unKnown Hardness(Mohs) 3-4 (estimated). Density 3 035."bg/cm3t Tenacity brittle none observed Solubility extreme in cold HrO Morphology Habit prismaticto acicularalong [001] and somewhatflattened on {110} General appearance subhedralto euhedral Forms {100},[110] major,and [233] minor Crystal size 30 x 5 x 5 pm (average);up to 0.1 mm in longestdimension Length-to-widthratio 6:1 Twinning none observed megascopicallynor during single-crystalstudy Optical properties Indicatrix biaxial oositive c 1.647(2) p 1.674(2) 1 1.704(2) 2Vuo +88 4" Orientation Z- c Pleochroism slight;X : very pale yellow,Z: distinctyellow-green Gladstone-Dalecompatibility (1 - KlK.) 0.021(excellent)+ Unit-celldatas Monoclinic,C2lq Na"CaMg(lO")u[(Cro.oSo,u)Oo],(H"O),, a (4) 23645(2) b (+) 10 e18(1) c (A) 15 768(1) B (") 114.42(6) v (A1 3707.3(6) z 4 a:b:c 2j657:1:1.4442 . Nodules are much sofler than individualcrystals. f Derived from crystaFstructureformula and unit-cellparameters. f Crystal-structureformula and calculateddensity; k : 0.200 for MgO $ Derived from cryslal-structureanalysis. ists for SO, and CaO, values for the (100) and (110) sur- value of 8.15 wt%o.Figure 3 indicates that the (100) sur- faces straddling the ideal values. With increasing expo- face respondsmore rapidly to the electron beam than the sure to the electron beam, NarO increasesand all other (110) surface. We conjecture that the (100) surface has oxides decrease,and this behavior is much more rapid on lost Na during the first secondor so in the beam and then the (100) surface than on the (110) surface. After about continues to evolve as indicated in Figures 3 and 4. 100 s, the oxide values show little further change on Georgeerickseniteis extremely unstableunder the elec- (100); the same occurs for the (110) surface after 200 s. ffon beam, even at low current and short counting times. (100) The surface seemsto be more reactive to the elec- The brown staining of the crystal is consistent with re- (110) tron beam than the surface.but both surfacesseem action with I, and the decreasein analyzedIrO. with in- to equilibrate with the beam and give comparatively sim- creasingtime. Moreover, the analytical values at any giv- ilar oxide weight percent values after 200 s. en time are strongly dependenton the crystallographic The variation in NarO as a function of time was further orientation of the material analyzed. The overall quanti- investigatedat 15 kV and 1 nA. Counts were taken every tative behavior of georgeericksenitein the electron beam 5 s without moving the beam and are shown in Figure 4. (as and 4) is consistent with the The lines fitted to the data points were extrapolated to summarizedin Figs. 3 zero time and the resulting values were converted to chemical composition derived from crystal-structurere- weight percentNa. For the (110) surface,the extrapolated finement(ideally I,Os 59.13,CrO,9.92, SO. 1.51,MgO value is 8 wtVo Na, in close agreement with the ideal 2.38, CaO 3.31,NarO 10.98,H,O 12.77wt%o). However, value of 8.15 wt%o(in addition, there was a complemen- it does not seem reasonableto use any compositionsde- tary rapid increasein counts for I in the first three count- rived from electron-microprobeanalysis considering the ing periods). However, for the (100) surface,the extrap- reactive nature of the crvstals over even extremely short olated value of L6 wt%oNa falls well below the ideal trmes. COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE 393

TreLe 2. X-ray powderdiffraction data for georgeericksenite

/"0" 40" 4n" Iuo hkl /"* 4* duo [," hkl 100 1069 10754 100 200 3 2287 2.290 1 626 20 974 9,705 10 110 3 2237 2-233 2 244 z 20 8.82 1834 7 111 5 2210 2.209 3 444' 7 575 I 202 3 2 195 2.189 3 643 30 748 7 416 13 111 2.166 3 640

3 714 7 157 2 002 2.165 1 1023 50 6.36 6 358 24 311 2 165 2 044 20 2 159 o 10 5 99 5 986 4 310 2.163 1 135 50 565 5 650 29 312 2160 2 533 5.377 5 400 00 30 533 2 153 4 1022 5,307 I 112 2128 2129 2 1024- 5 5.09 5 083 3 021 2 096 2 096 2 351 4948 1 311 10 2078 2.083 3 1112- 3 4.91 4 881 1 221 5 2 050 2 050 2 627 15 4.64 4 638 5 113. 2024 6 227 25 2.O21 40 4.36 4 355 19 221 2023 3 1115 E 25 3.909 3 915 16 204. 1999 1 842 3

10 1.996 ( 100) 25 3.790 3 788 12 404. 1 995 1 930 3 586 32 023 1 993 1 515 70 3.590 3 585 15 600 3 1.971 1 969 1 335 0 20 40 60 r00 200 100 0 20 40 60 r00 200 J00 1 968 1 552 Time (s) Tihe (s) 30 3.524 3 533 12 423 3 518 12 511 1 958 3 827 Frcunr 3. Variation in constituentsin weight percent as a 30 3.453 3450 13 421', 3 1 956 function of time (increasingexposrue of the crystal to the elec- 1 954 2 534 3 3.389 3393 2 514 5 1936 1937 2 352" fton beam); note that the data points do not necessarilycore- 3290 2 132 40 1 913 1 912 15 840" spond to the same times as not all elementswere acquired si- 10 3.296 multaneously The dashed horizontal lines correspond to the 3290 3 331 40 1 887 1 886 12 446. weight percentvalues indicatedby the chemical composition de- 10 3.181 3179 6 332. 30 1 848 1 845 7 1206" 80 3.121 3123 54 223" 20 1.819 1822 3 1027 rived from crystal-structue analysis. 80 3.051 3 054 58 623- 1 821 2 118 10 2.966 2 960 7 133. 1.818 2 918 2919 4 515 1790 1 1135 5 2.912 15 1789 2 896 2 115 1 787 1 355 5 2.853 2.853 3 532" 1 767 1 754 500 20 1 765 2AO 5 2.765 2761 3 332. 1 763 2 262

Frcunr 4. Variation in counts for Na in georgeerickseniteas a function oftime for the (100) and (110) surfaces;the number of counts correspondingto 8 wtTo Na is indicated. 394 COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE

groups are characterizedby two bandsat 909 (strong) and 947 (medium) cm ', which are both stretchingvibrations (Nakamoto1978).

Cnvsr,q.L srRUcruRE Data collection E A 0.046 x 0.059 x 0.060 mm3 crystal was mounted F on a SiemensP4 automatedfour-circle diffractometer and aligned using 42 reflections automatically centered fol- lowing measurementfrom a rotation photograph.MoKa radiation was used. The orientation matrix and unit-cell dimensions (Table 1) were determined from the setting anglesby least-squaresrefinement. A total of3812 unique

1) reflections was collected out to 60' 20 with a fixed scan Wavenumber (cm speed of L33" 2llmin We corrected for absorption by Frcunn 5. Infrared-transmissionspectrum for georgeericksenite.Gaussian quadrature integration; corrected for Lorentz, polarization, and background effects; and reducedthe in- tensities to structure factors. Of the 3872 unique reflec- Infrared study tions, 2019 were classedas observed(l{l>5ol{l). The equipment and proceduresfor acquiring the infra- red spectrumfor powdered georgeericksenitedispersed in Structure solution and refinement KBr are identical to those used to obtain the spectrumfor All calculationswere done with the SHELXTL PC Plus mcalpineite (Roberts et al. 1994). A very strong absorp- system of programs; R and Rw indices are of the con- tion band (Fig. 5), which stretchesfrom 3442 to 3294 ventionalform: R : >(lF"l-lr'"l)Dl{l and Rw : Ew(lF.l- cm 1and is centereda13367 is due to O-H stretch- lF.D'Dnl. The value for the absorption coefficient ing, and a medium band, centered"--t, at 1653 cm ', is due p(mm ') is 5.86; min. transmissionis 0.134; and max. to H-O-H bending in the H,O groups. The iodate groups transmissionis 0.827. The structure was solved by direct are characterizedbystrong bands at74O vr) and776 (vr) methods. The E statistics indicate that the structure is cm ' which are stretchine vibrations. The chromate probably centrosymmetric, and systematic absencesin-

Teale 3. Finalatomic parameters for georgeericksenite

u", U,, U,, U"" U"" U," uP

Ca0 o 1223(3) % 16(1) 16(2) 14(2) 14(2) 0 2(1) 0 Mg th 0 1056(s) 1/+ 15(2) 23(3) 12(3) 13(3) 0 11(2) 0 Na1 0.1379(2) 0 1783(s) 0 1780(3) 27(2) 2e(3) 28(3) 25(3) -1(2) 13(2) 3(3) Na2 0.1228(2\ 0 3922(5) 0 3304(3) 25(2) 26(3) 22(3) )2(r\ -3t2\ s(2) 1(2) Na3 0.3777(2) 0 1291(5) 0.0189(4) 34(2) 30(3) 33(3) 37(3) 0(3) 12(2) 4(3) 11 0.12628(3) 0 11 870(7) 0.4e2s8(5) 162(3) 164(4) 153(4) 143\4) 18(4) 38(3) 7(3) -e(3) Re/a\ _a/e\ 12 0.00440(3) o 374s1(7) 0 08827(5) 133(3) 14e(41 137(4) 111(3) vv\v, v\v/ 13 0.36796(3) 0.15586(7) 0 295s5(5) 17s(3) 177(4\ 181(4) 175(4\ 3(4) 6s(3) 41(4) Cr 0.4843(1) 0.3465(2) 0 0852(1) 14(1) 16(1) 12(1) 13(1) 1(1) 4(1) 1(1) o1 o 3272(4\ 0 2589(7) 0.4317(5) 23(3) 22(5\ 24\5) 18(4) -3(4) 4(4) -5(4) 02 0.178e(4) 0.0616(8) 0 4456(6) 30(4) 27(5\ 35(5) 31(5) 1(4) 14(4) 11(5) 03 0.0786(4) 0.2143(7) 0 3950(s) 1e(3) 1e(5) 16(4) 15(4) 5(4) 0(4) 3(4) 04 0.0511(3) 0.2896(8) 0 1e28(5) 18(3) 14(4) 27(5) 11(4) 4(4) 2(3) 4(4) 05 0 0336(3) o 4799(7) 0.3624(s) 20(3) 15(4) 27(51 1s(4) 6(4) 4(4) -4(4\ 06 0 0630(4) 0.4704(7) 0.0767(5) 22(3) 13(4) 24(5\ 26(5) -1(4) 5(4) -7(4) 07 0.3272(4\ 0 2846(8) 0.2247(61 32(4) 43(6) 22(5) 24(5) 3(4) 7(41 23(4) 08 0 3081(4) 0 0427(9) 0.2601(7) 4e(5) 23(5) 53(7) ss(7) 26(6) 1(5) -e(5) o9 0 407s(4) 0 0e91(8) 0.2278(51 28(3) 32(5) 41(6) 17(4) 3(4) 16(4) 18(4) 01 0 0 0614(4) 0 1818(9) 0.0206(6) 40(4) 40(6) 4s(7) 20(4) -11(5) 1(4) e(5) 01 1 O4772(6\ o 2344(9) 0.14s0(7) 56(6) so(e) 32(6) 52(7) 15(5) 36(7) -4(6) o12 0 4668(4) 0 47s9(8) 0.1204(6) 30(4) 30(5) 25(5) 26(5) -8(4) 3(4) - 1(4) o13 O 4446(4) 0 3s03(9) 0.4038(7) 4e(5) 25(s) 56(7) 6s(7) 14(6) 21(5) -1(s) ow1 0 0975(4) 0 0281(8) o.2522(61 32(4) 38(6) 21(5) 38(5) -4(41 18(5) 5(4) ow2 o 1947(4) 0 3s70(s) 0.1437(5) s2(4) 44(6) 28(5) 24(5) 7(41 14(41 2(5) ow3 01982(4) 0 2442(81 0.3359(6) 27(4) 20(5) 31(6) 33(5) -7(41 13(4) 4(41 ow4A 0 1895(7) -0.003(1) 0 167(1) 27(3)I ow4B 0 1979(6) 0 063(2) 0.11 8(1 ) 27(3\I ows 0 1865(4) 0.4556(8) 0 4851(6) 27(41 32(5) 26(5) 24(5) 7(4) 12(4) -1(4) ow6 0 3130(5) 0.2809(9) o o42o(7') 42(4\ s0(7) 43(6) 41(6) 11(5) 27(5) 4(6) Note: U valuesare x 103 except tor lodine,which are 104. t lsotropicand constrainedto be equal COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE 395

Trele 5. Selectedinteratomic distances (A) and angles(') in DnscnrprroN oF THE srRUcruRE georgeericksenite Cation coordination Ca polyhedron Mg octahedron The one symrnetrically distinct Cr site is tetrahedrally Ca-O3,b 2 485(7) x2 Mg-Osc,d 2.119(8)x2 Ca-O4,b 2.552(9)x2 Mg-Og,h 2.068(9)x2 c-oordinatedby four O atoms at a mean distance of 1.61 Ca-O12d,i 2 454(9)x2 Mg-O11,h 2.07(1) x2 A, indicating that the Cr cation is hexavalent. The ob- Ca-OW1,b 251(1) x2 (MS-O) 2.0e (Ca-O) 25O served site-scatteringand the mean bond-length at the Cr site are less than that expected for complete occupancy Nal polyhedron Cr tetrahedron Na1-O4 2.48(1\ Cr-Olof 1.597(8) by Cf*. This can be accountedfor by some substitution Na1-O10 2 393(8) Cr-O11 1.60(1) of S. Hence the occupancyof the Cr site was considered Na1-OW1 2 43(11 Cr-O12 1.633(9) variable (Cr, S) and site-scatteringrefinement converged Na1-OW2 2 s9(1) Cr-O13h 1.62(1) Na1-OW3 2 411(9) (Cr-O) 1.61 to an occupancyof 0.84(2)Cr + 0.16(2)S. This value is Na1-OW4A 237(2\ (Cr-O) l1 pyramid in accord with the observed distanceand detection Na1-OW4B 237(2) of S with an electron microprobe. (Na1-O) 2 41 l1-O1a 1.824(7\ t1-o2 1.81(1) Each of the three I sites is coordinatedby three O at- Na2polyhedron r1-o3 1.819(7) oms arrangedin a triangle to one side of the cation and Na2-O3 2 60(1) (11-o) 1.82 Na2-O4 2407(€) at distancesof -1.81 A, such that the resulting IO. group 12pyramid Na2-Os 2.55(1) constitutesa triangular pyramid with the I site at the apex t2-o4 1.814(71 Na2-O8g 3 06(1) (Fig. Na2-O9g 243(11 r2-o5b 1.820(9) of the pyramid 6). For each site, there are three Na2-OW3 2.38(1) 12-06 1.806(9) additional ligands (Table 5, Fig. 6) such that the I atoms Na2-OW5 2378(8) (r2-o) 1.81 have very distorted octahedral coordination and occupy (Na2-O) 2 54 off-centeredpositions within each octahedron.The I1 site polyhedron Na3 13pyramid is coordinated by one HrO group, OW4, that shows po- Na3-O5d 2704(8) r3-o7 1.806(8) Na3-O6f 2.68(1) r3-o8 1.786(9) sitional disorder (Fig. 6) but the coordination is similar Na3-O9 3 09(1) r3-os 1.80(1) to that of the other I polyhedra. This type of eccentric Na3-O10f 273(1) (r3-o) 1.80 coordination is typical for Is* with a stereoactivelone- Na3-O11 2 64(1\ Na3-OW2l 2 4s3(8) pair of electrons.Note that the long bonds still contribute Na3-OW5d 2.41(1\ significant bond-valence (-0.20 v.u. per bond) to the Na3-OWG 2 38(1) (Na3-O) 2.64 bonded anions. There are three unique Na sites (Fig. 6, Table 5), each Long lodine Bonds of coordination. Nal is surrounded t1-o12d 2 686(8) r2-O3b 2.722(9\ t3-O1 2.92(1) with a different type l1-O13a 2 80(1) l2-OGe 2.957(7) r3-O6d 2846(7\ by two O atoms and four HrO groups in a distorted oc- l1-ow4Aj 2.82(21 r2-O10 2 93(1) l3-O13 2 854(9) tahedral arrangementwith a (Na1-0) distance (0 : un- l1-ow4Bj 281(2\ (r2-O) 2.87 fl3-O) 2.87 (r1-o) 277 specified ligand) of 2.4I A. Na2 is surroundedby five O ow4A-ow4B 1.13(3) atoms and two HrO groups in an augmented o-ctahedral

Note. Symmetryoperators: a: X + th,, + tt, Z + 1;b: t, y,2 + 1/2; arrangementwith a (Na2-S) distance of 2.54 A. Na3 is C : x + 1/2,y - Lh, zi d : X + th, y - 1/2,2 * Yzie : r,, +'1,, 2; | - * surroundedby five O atoms and three HrO groups in a 1/z, lb, : 1/z, |h, 1h; * l'1 2; g i I f + 2 + Yz;h : l + 1, y, 2 + i : x triangular dodecahedralarrangement with a (Na3-0) dis- th, - 1/2,z; : x, z + 1,4. Y i Y, tance of 2.64 A. The variation in (Na-0) distance as a function of coordination number is in accord with the radii of Na given by Shannon(1916). dicate the presenceof a c glide for the C-centeredcell. There is one Mg site coordinatedby six O atoms in an The solution with the highest combined figure-of-merit in octahedral anangement with a (Mg-O) distance of 2.09 the space group C2lc proved to be correct, and the struc- A; the site-scatteringand observedmean bond-length are ture was refined by a combination of least-squaresrefine- in accord with this site being occupied by Mg. There is ment and difference-Fourier synthesisto an R index of one Ca site coordinated by six O atoms and two HrO 3.5Voand Rw : 3.5Vo.Site occupancieswere assignedon groups in a square-antiprismaticarrangement (Fig. 7) the basis of site-scatteringrefinement and crystal-chemi- with a (Ca-d) distanceof 2.50 A. The observedsite-scat- cal criteria. Final atomic parametersare listed in Table 3, tering and mean bond-lengthindicate that this site is com- observedand calculated structure-factorsare listed in Ta- pletely occupied by Ca. ble 4' and selectedinteratomic distancesand angles are given in Table 5. A bond-valence analysis is shown in Structure topolog;r Table 6. The structural arrangement in georgeericksenite con- sists of thick slabs of polyhedra orthogonal to (Fig. 'For [100] a copy of Table4, DocumentAM-98-006, contact the 8). These slabs have the composition of the mineral and BusinessOffice of the MineralogicalSociety of America(see link- insidefront coverof recentissue) for priceinformation. Deposit are one-half a unit cell thick in the [00] direction; itemsmay alsobe availableon the AmericanMineralogist web age with adjacent slabs is solely by hydrogen bonding. site (seeinside back cover of a currentfor web address). Although the slab appears complicated when viewed 396 COOPERET AL.: STRUCTUREOF GEORGEERICKSENITE

Tre|-e 6. Bond-valence*table lor georgeericksenite,omitting the H atoms bondedto O4A and O4B

Mg Na1 Na2 Na3 t1 H9 H10 H11 H12 o1 140 o 17 0.20 020 197 o2 145 0.20 0 15 o3 O 25tz! 012 1.42 0.23 202 o4 O 21 t2! 016 018 1.44 199 o5 O 32\2t 0.13 0.09 142 196 o6 0.10 147 019 192 1.47 015 0.20 o8 004 1 54 0.20 o9 0 36!'1 017 0.04 1.49 zvo 010 160 019 0.09 0.17 205 011 0 36xrr 1.59 0.11 2.06 012 027'2t 1 45 0.25 197 1.51 0.20 019 190 ow1 0.23"2t o17 ow2 019 0.17 0 85 0.80 201 ow3 0 18 0.20 0.80 0.80 198 OW4A 0.10 0.10 OW4B 010 0.10 ow5 0.20 0.18 080 085 203 ow6 020 0.80 0 80 2.oo 1.92 204 615 109 104 0.98 4.92 4.89 5.05 1 00 1 00 1 00 1.00 1 00 1.00 1.00 100 - Bond-valencecurves from Brownand Altermatt(1985) for Ca, Cr, S, and from Brown(1981) for Mg, Na, and I

down the c axis (Fig. 8), one seesthat the edges of the tends approximately in the [011] direction. This trimer slab are bounded by near-planarlayers of anions.Further links by edge-sharingbetween the Na3 and Nal polyhe- inspection shows that there are three planar layers of cat- dra to another trimer extending in the [011] direction. ions within the slab and parallel to its edges, indicating This motif continues to form an [Na30,4] zigzag chain that the slab consists of three layers of polyhedra. The top and bottom of the slab are related by c-glide sym- metry, and hence the slab may be factored into two dis- tinct sheetsof polyhedra, illustrated in Figure 9. The outer layer of the slab (Fig. 9a) containsprominent zigzag chuns of Na polyhedra extending along the c di- rection. The Nal octahedronshares an edge with the Na2 augmentedoctahedron, which sharesa face with the Na3 triangular dodecahedronto form a linear trimer that ex- O12l

o12d t2 I3

Nal Na2 Na3 Frcunr 6. The coordination environmentsof the I and Na sites in georgeericksenite;I - large black circles; Na - cross- Ga hatchedcircles; O : highlighted circles; H,O : unshadedcircles Frcuns 7. The coordination environment of the Ca site in One of the bonds to the disorderedOW4 HrO group is shown georgeericksenite;Ca - diagonalJined circle; legend as in Fig- as a broken line. ure 6. COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE 397

TOP,/BOTTOM

O ta't T T a Na,Na a/2 c MIDDLE I I al OCa o lrtg

"Cr ------l b F-b---i Frcuns 8. The crystal structureof georgeericksenitepro- FrcunnL0. The cationarray of the slabin georgeericksenite jectedalong [001];legend as in Figures6 and7. projecteddown [100]. The cations occupy the vertices of 4' plane nets,and the occupiedvertices are joined by edgesof the nets. The top andbottom nets superimpose (the heavyblack lines) and extending in the c direction (Fig. 9a). In each embayment the middlenet (the dottedlines) is out of registrywith the top of this chain, the polyhedra are decoratedby two (IO.*,) andbottom nets. groups. This outer layer of the slab consists of transla- tionally equivalent decoratedchains parallel to c that link only through one (unique) weak I-O bond. (Fig. 9a). The 10, which shows only the cation positions in each layer inner layer of the slab (Fig. 9b) consists of one unique for clarity. All cations occupy the vertices of a 4a net, Mg octahedronthat sharescorners with two Cr tetrahedra and occupied vertices are joined by the edgesof each net to form an [MTrg,r] cluster (Hawthorne 1985). The other in Figure 10. In the middle layer, all verticesare occupied two meridional anions of the Mg octahedronlink by cor- by cations (I, Ca, Mg, and Cr), and the edgesjoining the ner-sharingto two (IO.*.) groups (both involving the 12 occupied vertices are shown as dotted lines. In the top site). These [Mg(CrO"),(IO.*.),O,] clusters link together and bottom layers, only some of the vertices are occupied two (Cagr) polyhedra to form chainsparallel to the b axis by cations, and the edgesjoining the occupied vertices (Fig. 9b). These chains link by weak I-O bonds to form are shown as heavy full lines. The top and bottom pat- the central layer of the slab. terns of occupied vertices are almost exactly in registry The relations between the outer layers (Fig. 9a) and but the chemical identities of occupied vertices that over- inner layer (Fig. 9b) of the slab is illustrated in Figure lap (in projection) are different: vertices that contain Na

c c I I F- b ----r k- b --+l Frcunr 9. The of georgeerickseniteprojected down [00]; legend as in Figures 6 and'|, plus Mg octahedron dash-shadedand Cr tetrahedronrandom-dot-shaded; (left) the outer layer(s) of the slab shown in Figure 8; (right) the inner layer of the slab shown in Fieue 8. 398 COOPERET AL: STRUCTUREOF GEORGEERICKSENITE

Teele 7. Bond-valencetable for georgeericksenite:the disorderedH"O groups

OW4B H1 H2 H1 o1 197 02 a)e 202 o4 1.99 1.96 o6 1.92 'l 07 015 197 92 o8 198 1.98 o9 2.06 010 205 011 2.06 o12 197 013 1.90 ow1 0.85 2.05 210 ow2 201 0 05 206 ow3 1.98 OW44 0.15 080 200 OW4B 0.80 0 90 r.90 ow5 2.03 ow6 2.OO :, 100 100 1.00 100 1.00 100 /Vote.Bond-valence curves from Brownand Altermatt(1985) for Ca, Cr, S, and from Brown (1981)for Mg, Na, and I

in one layer and I in the other layer are shown as large the final stagesof refinement, the displacementparame- black circles, and vertices that contain Na in both top and ters of the OW4 group were very anisotropic. and it was bottom layers are shown as small black circles. Note that modeled as a positionally disordered (split) site, labeled the middle net is out of registry with the top and bottom OW4A and OW4B. These split positions refined to a sep- nets by one half of the diagonal of the unit cell of the aration of 1.13 A. fn" reasonfor this disorder is not clear, net. Thus, despite the complexity of each of the layers although the two sites do show a different hydrogen- (Figs. 9a and 9b) of the slab, the layers mesh in an orderly bonding irrrangement(Table 8). geometric manner. Chemical composition HrO groups The observed site-scatteringand local stereochemistry The H atoms were not located in the final stagesof the at each site in georgeerickseniteenable us to write an refinement. However, HrO groups were identified by the accurate chemical formula for this mineral: bond-valencedistributions in the structure (Table 6), and NauCaMg(IO,)u[(Cro*So,u)Oo],(H,O),,. This is particular- a reasonablehydrogen-bonding scheme was derived from ly important for georgeerickseniteas the crystals are ex- bond valence and stereochemicalcriteria (Tables 7 and tremely unstable under an electron beam. However, pro- 8). There are six unique HrO groups, labeled OW; during jecting the evolution of analyzedcomponent as a function of time back to zero time for each element detected by electron microprobe analysis (Figs. 3 and 4) does show Taele 8. Proposedhydrogen-bonding for georgeericksenite reasonableagreement with the chemical formula derived by crystal-structureanalysis. o-s (A) 0-0-o f) 2.88(1) Hl 02-OW1-OW4A 1ol 3(4) Relation to other minerals 3.01(2) H2 02-OW1-O7d Dietzeite, Ca(IO.),(CrO.XH,O), (Burns and Hawthorne 3 14(1) H2' OW4A-OW1-O7d :: i'?l 2.e1(1) H3 O7-OW2-O8g r66.iiji 1993) is the only other chromate-iodatemineral. How- 2.73(1) H4 ever, dietzeite has a heteropolyhedral framework structure 2.7e(1) H5 01-OW3-O2 974(3) no re- 2.80(1) H6 of very different connectivity and shows structural 2.62(21 H7 08-OW4A-O7d 1010(5) lation to georgeericksenite.Fuenzalidaite and carlosrui- 3.00(2) H8 zite (Konnert et al. 1994) are sulphate-iodatesfrom Ofi- 2 65(2) H7', O8-OW4B-O2 tJ!.?l|l 2 eo(2) H8', O8-OW4B-OW2 cina SantaLuisa in the Chilean nitrate fields, and contain 3 03(2) H8', O2-OW4B-OW2 tzs.gizi small amounts of Cr (-l wt%oCrO.) substituting for S. 2 77(1) Hg Ola-OW5-O2a e52(3) They are also sheet structures,but the sheetsare signifi- 2 91(1) H10 276(1) Hl1 07-OW6-OW6f 983(6) cantly different in connectivity from georgeericksenite. 2 80(2) Hl2 - AcxNowr.nocMENTS Hydrogen bonding across structural units; symmetry operators as in Table5 We thank William M Pinch and GeorgeRobinson for their cooperation in supplying material for this work, Ron Chapmanfor his assistancewith COOPER ET AL.: STRUCTURE OF GEORGEERICKSENITE 399 the electron microprobe work, and Peter C. Burns and an anonymous Hawthorne, FC. (1985) Towards a structural classificationof minerals: reviewer for their commentson the manuscript.FC H. was supportedby The VIMIVT,O"minerals American Mineralogist, 7O, 455-47 3 Natural Sciencesand EngineeringResearch Council of CanadaOperating Konne4 JA, Evans,H.T, Jr.,McGee, J.J, and Ericken, GE. (1994)Min- and Major Equipment Grants eralogical studies of the nitrate deposits of Chile: Vtr. Two new saline minerals with the composition Ku(Na,K)nN4Mg,u(XOo),,(IO.),,'l2H,O: Rrrnnrncns crrnD Fuenzalidaite(X : S) and carlosruizite (X - Se). American Mineralogist, 79,1003-1008. Brown, I.D. (1981) The bond-valencemethod: an empirical approachto Nakamoto, K (1978) Infrared and Ramar spectraof inorganic and coordina- chemical structure and bonding In M. O'Keeffe and A. Navrotsky, tion compounds Wiley, New York Eds., Structureand Bonding in Crystals:II Academic Press,New York, Roberts,A.C., Ercit, T.S , Criddle, A.J., Jones,G C, Williams, R.S., Cur- l-30 eton, EF, and Jensen,M.C. (1994) Mcalpineite, Cu.TeOu'H.O,a new Brown, I.D. and Altermatt, D (1985) Bond-valenceparameters obtained mineral from the McAlpine mine, Tuolumne County, California, and from a systematicanalysis of the inorganic crystal structuredatabase from the Centennial Eureka mine, Juab County, Utah. Mineralogical Acta Crystallographica,B41, 244-24'7 Magazine,58, 417-424 Burns, PC and Hawthome, FC. (1993) The crystal structureof diezeite, Ca,H,O(IO.),(CrO.), a heteropolyhedralframework mineral Canadian Mmuscnrpr RECETvEDMw 2'7, 1997 Mineralogist, 31, 313-319 MeNuscnrn AccEprEDNoveNrsen 6, 1997