American Mineralogist, Volume 59, pages 896-899, 1974

Metavivianite,Fe.(POu),.8HrO, a New Mineral'

Cnlnrps Rrrz Department of Chemistry,

Entc J. EssnNn,,c,Nu DoNlr,D R. PEAcoR Department of Geology and Mineralogy, The Uniaersity of Michigan, Ann Arbor, Michigan 48104

Abstract

Metavivianite, Fe*(POn),'8H,O, is a new mineral occurring with kryzhanovskitein solution cavities in triphylite at the Big Chief pegmatite near Glendale, South Dakota. It is triclinic, - - - with a 7.814, b 9.084., c 4.65A, a - 94.77",F - 97.15",r - 107.37",and is isostructural with symplesite, Fe"(AsO.),.EH,O. Electron probe analysis indicates that it is a polymorph of vivianite. Vivianite and parasymplesite are therefore isostructural, monoclinic dimorphs cif the isoetructural and triclinic minerals metavivianite and symplesite.

fntroduction the two phosphatesand the two arsenates.Type The pegmatitesof the Black Hills, South Dakota, material of metavivianite is present in the collec- are well known as sourcesof a variety of primary tions of the U. S. National Museum (Smithsonian and secondaryphosphates. In 1969, Dr. David Institution) and the Department of Geology and Garske of the South Dakota School of Mines kindly Mineralogy, The University of Michigan. The min- donated to us specimens of phosphate minerals eral and the name have been approved by the which are products of the alteration of triphylite. I.M.A. Commissionon New Minerals and Mineral Crystals from one of these specimenswere used Names. in a single-crystal study, with preliminary results Occurrence indicating that they were triclinic, and similar to symplesite,Fes(AsOa)2.8II2O. However, chemical The mineral occurs on one specimen collected analysisindicated the presenceof P but no As. at the Big Chief pegmatitemine, one half mile south Symplesiteis the triclinic dimorph of monoclinic of Glendale, Pennington County, South Dakota parasymplesite.The mineral vivianite, Fes(PO+)2. (43"52.3' N, 103'22.1' W). Phosphatesfrom this SH2O, is isostructuralwith parasymplesite(Mori locality have been describedby Roberts and Rapp ( and Ito, 1950).Our preliminary results thus indi- 1965). Crystalsof primarytriphylite, some measur- catedthat our specimenswere a dimorph of vivian- ing up to 6 feet in diameter, have been altered by ite and related to that mineral as symplesiteis to late-stagepegmatitic solutions. Secondaryminerals parasymplesite.Detailed examinationhas verified are abundant and include ludlamite. hureaulite. this relationship.The name metavivianitehas been beraunite, rockbridgeite, laueite, heterosite-purpu- chosento emphasizethe dimorphousrelationship rite. and strunzite. to vivianite,as with the phosphates(meta)vauxite The metavivianiteoccurs on a specimenof mas- and (meta)strengite.The nature of the existing sive triphylite containing veinlets of spessartine-rich names for the two related arsenatesunfortunately garnet, pyrrhotite, sphalerite, and quartz. The sur- precludesa choice of names which would more face of the triphylite is pitted with solution cavities clearly illustrate the isostructural relations between up to I mm in diameter which generallyfollow the in -;Co"triUution traces of cleavages.The metavivianiteoccurs No. 318 from the Mineralogical Labora- the solution cavities, intimately intergrown with tory, Department of Geology and Mineralogy, The Uni- dark red kryzhanovskite.The identity of the latter versity of Michigan, Ann Arbor, Michigan 48104. mineral was establishedusing single-crystaldiftrac- 896 METAVIVIANITE, A NEW MINERAL E97 tion (a = 8.26A, b : IO.I2 A, c : 9.37A, space TrsrB 1. X-Ray Powder Data for Metavivianite* group Pcnb) and qualitative microprobe analysis, d..1(A) dol"(A) d""t (A) dou" (A) I with comparisonwith the data of Moore (1971). hkl Kryzhanovskiteis the only mineral observedto be 8.60 8.59 40 2tr z.7d 010 t I 30 in an apparent equilibrium relationship with meta- 110 6.73 6.71 100 22), . 11f---2. present 110 4.89 4.46 40 vivianite, although hureaulite is in some 011 4.27 4.27 IO 2OI 2.O I Z.O I 5 ^- other cavities. ^- rl1 z.ovl lll J.OOI 1 af in - -^-z.ol, L0 210 r.ei 310 z. ]91 Physical Properties ll1 3.7r 3.72 2 320 2.49 2.50 10 010 3.64 3.63 2 l2l 2.48 2.48 2 Metavivianite occurs as flat, prismatic crystals, 300 2.45 2.46 2 e ocl r.743 5 elongated,alongc, and with principal form {110}. III t0 2to 3.Q61 1. 631 l0 Somecrystals having more regular cross-sectionsare 1.503 2 (110) r3g 3.01 3.00 5 r.456 l0 striated parallel to c. There is a perfect 021 2.95 2.95 5 1. 341 2 color is leek-green.Most crystals are 12I 2.91 2.90 20 cleavage.The 1. 309 2 opaque to translucent, but small grains are trans- *FeK Mn-filtered radiation. CaDEra radlus 57.3 m. Inlen- parent. The specific gravity was not measureddue sities visully estiMted' Thele are a nutbslr of addirional, to the small size of the crystals and the presence very weak lines qith d-values less than 1.3 A. of impurities.Thb calculateddensity is 2.69 gmfcml. Optical properties bf metavivianite are generally similar to those of Vivianite.By direct measurement, the single-crystal study. One other line not listed 2V" is +. 85 :L 5o. Indices of refraction ore a = in Table 1 with a spacing of 3.47 A appeared on 1.579! 0.006,B = 1.603:L 0.002 andy = 1.629 the photographs. This was not indexable using the i 0.002. The relation between the opticdl ,direc- metavivianite unit cell, but since it varied in in- tionsand cleavage is X I (110), YZ ll (110). The tensity from 2 to 30 on different photographs, and pleochroic formula is X > Z > Y, with X = light- since some impurity was present in all X-rayed blue to blue-green,Y - yellow-greento light green, samples, it is assumed to be due to an impurity. Z - yellow to light green. The pleochroiGcolors As expected for two isostructural compounds, are presumablyintensified by oxidation of the iron, the patterns of symplesite and metavivianite are but the physical properties were measuredon light- similar. The larger interplanar spacings for symple- colored grains. site, relative to corresponding lines for metavivian- ite, result in clear differences between the patterns. CrystallograPhy The patterns for vivianite and metavivianite are also Singlecrystals were studiedusing Weissenbergand similar, as expected from their structural similarity' precessionmethods. Unit-cell parameters are a : The patterns of each have unique features with t.st(z)A, b : 9.08(2)A, . : 4.6s(l)A, o : respect to the first peak at approximately 8 A. 94.77(10)",p : 97.15(10)o,y : 107.37(10)'.The Chemical Analysis spacegroup is Pl or Pi. X-ray photographsshowed the presenceof two twinned lattices. The crystals A specimen of metavivianite and a vivianite from are twinned on {110}. Wolfe (1940) showed that an unknown locality were analyzed using the electron the lattices of the monoclinic members of the vi- microprobe. The results are listed in Table 2. The vianite group (parasymplesitewas not recognizedat metavivianite is very similar to the vivianite, both that time) are closely related to the lattices of the phaseshaving essentiallythe same wt/percent ratios triclinic phases (e.g., symplesite).The twin iilane of Fe/Mn, approximately 10:1. Only traces of metavivianite' {l l0} of the triclinic phasesis structurally equivalent other elements are present in the to the true mirror plane (010) in the monoilinic There is less than 0. I percent As since it was not phases.We have verified this relation by comparing detected using repeated, slow spectrometer scans. photographs of untwinned vivianite and twinned The slightly high totals of wt/percent oxides and metavivianite.Twinning was not observedoptically. cations sums are probably due to oxidation of part Data from an FeKa powder photographare listed of the iron and concomitant loss of some water' in Table 1. Calculated interplanar spacingsand We conclude that metavivianite and the vivianite are indiceswere obtainedusing lattice parametersfrom nearly identical in composition for the elements 898 RITZ, ESSENE, AND PEACOR

Tesrn 2. Chemical Analyses The sampleswere imbeddedin KBr pellets.A spec- trum of KBr blank showed no absorption above Wt Percent 400 cm-'. and noticeablelines to absorbed oxides Metavivianlte Vivianlte Fe3(pob)2,gH2O no due HzO. The spectra of vivianite and metavivianite Fe0 38.9 37,1 43,0 MnO 3.5 show broad absorptionsaround the regionsof 3400, Nto 0.13 0.6 Cuo <0,05 0, 14 1600, 90O,and 50Ocm-1. The first two broad peaks Z\O <0.05 indicate the presenceof water, and the last two Ca0 0.5 <0,06 peaks are characteristic of POas-. The qualitative Na2o <0.07 0.8 28.4 28.0 28.3 spectra indicate the similarity in composition and tu.os <0.1 <0.1 Hzo (28.7) (28.7) 24.7 crystal structure of vivianite and metavivianite.

suh 100,8 100.9 t00.0 Discussion Cations for 8 orygens (atonic basis)

Fe 2,7L 2,63 3.00 Metavivianite and vivianite are members of a Mn 0. 30 0.25 series of hydrated iron phosphates:Fes(PO+)z' Ni 0. 01 0. 04 Cu 0,01 nH2O, which includes phosphoferrite (n : 3), Zt 0.09 ludlamite (n = 4), and anhydrous sarcopside Ca 0.05 Na 0.13 (Moore, l97l).'z All theseminerals show substan- 2.OO L97 2,00 o 8.00 8.00 8.00 tial solid solution with Mn, and are easily oxidized H-0 (8.00) (8.0o) 8,00 to Fest with loss of H-. su The membersof this seriesare related by simple cations 5,06 S .f2 5. 00 *Mlcroplobe reactionswith increasingtemperature: standards used: Fe-Blnns ilnenite, Mn_ Broken HI11 rhodonite, Ni-synthetic NlAl2Or, Cu_Cu'necal, zn-zlncian calclte, Ca-Calclte, Na_AneIia albiCe, p_Black vivianite or Hllls -ludlamite HrO+ phosphoferrite HzO triphylire. ,"tuuiuiunit" * * Operating conditions: 0.006p6 sabple currenr on calcite. + scarcopside+ HrO using Rucklidge's EMPADRvII program (J.,- _corrections: The kryzhan- Rucklldge and E.L. casparritri, Dep. of Geoloiy,-Univ. coexistenceof metavivianite and Toronto, writEen. conn., 1969) ovskite (the oxidized equivalent of phosphoferrite) may be due to the solid solution of Mn and the potential for oxidation, or it may represent a dis- with Z ( 1O and therefore by analogy with vivian_ equilibrium assemblage.If metavivianite-kryzhan- ite, the formula of metavivianiteis (Fe,Mn)g(pO+)z ovskite is an equilibrium assemblage,them some '8Hro' oxidized Mn-bearing ludlamite of equivalent com- position cannot be stable at the same conditions. Infrared Spectrum Moore (197l) has also argued for the persistent The infrared absorpion spectra,shown in Figure stability of the seriesphosphoferrite-kryzhanovskite I for vivianite and metavivianite, were obtained and the relative instability of crystalline ferric with a BeckmannIR-12 using the fast scan mode. equivalents based on crystal-chemical and struc- tural considerations. The P-T relationships of vivianite and meta- vivianite are unclear. Insufficient data are available on vivianite and metavivianite solid solutions to infer which has the larger cell volume for the same composition. Since these polymorphs have sub structural elements in common, the free energy differences are probably small. The formation of a specific polymorph may be controlled by non- equilibrium factors rather than P-T-X variables.

'Moore (written communication, 1973) informs us of Frc. Infrared absorption spectra of metavivianite (a) the existence of still another iron phosphate hydrate with and vivianite (b). n=1, METAVIYIANITE, A NEW MINERAL E99

References Blacks HiIIs. Espe Printing Co., Rapid City, South Dakota. Moone, P. B. (1971) The Fe':+s(HgO)"(PO*),homologous Worrr, C. W. (1940) Classification of series: crystal-chemical relationships and oxidized equiva- minerals of the type A"(XQ,) nH"O, A m. Mineral. 25, 787 -809, lents. Am. Mineral 56. l-11 . "' Monr, H., eNo T. Iro (1950) The structure of vivianite and symplesite. Acta Crystallogr, 3, 106. Manuscript receiued, December 6, 1973; accepted Rouenrs, W. L., euo G. RApp (1965) Mineralogy of the lor publication, April 19, 1974.