American Mineralogist, Volume 68, pages 1220-1222, 1983
The crystal chemistry of shcherbinaite,naturally occurring VzOs
JosN M. HucHns' Department of Earth Sciences,Dartmouth College Hanover, New Hampshire 03755
eNo Lennv W. FrNcr,n GeophysicalLaboratory, Carnegie Institution of Washington Washington,D.C. 20008
Abstract
Shcherbinaite,naturally occurring V2O5,has been discoveredin the high-temperature fumaroles of Izalco volcano, El Salvador, and Bezymyanny volcano, Kamchatka, U.S.S.R. The Izalco mineraloccurs as euhedralgolden-yellow needles and laths up to I cm in length.The crystals,which formed as sublimatesfrom volcanicgases, are found coating basalticbreccia fragments in the summit crater fumaroles.The mineralis associatedwith bannermanite. An electron microprobestudy of shcherbinaitedemonstrated that the mineral is pure V2O5and containsno detectableimpurities or alkalimetals such as occur in solidsolution in oxide bronzecompounds. A comparisonofthe lattice parametersand crystal structuresof shcherbinaiteand syntheticV2O5 showed that the compoundsare essentiallyidentical. All lattice parameters are within I standard deviation and all atomic positional and thermal parametersare within 2.3 standarddeviations of thosefound in the syntheticcompound.
Introduction system(Hughes and Finger, 1983),a detailedstudy ofthe shcherbinaitefrom Izalco volcano was undertaken. Stoiber and Duerr (1963) reported the first natural Locality and occurrence occuffenceof crystalline V2O5.The compound, which Izalco volcano is a basaltic was the first high-temperaturevanadium mineral known, compositecone that has been intermittently active since its birth in 1770. was discovered as fine-grainedeuhedral crystals coating The volcano rises to an altitude 1965m, with basalticbreccia fragmentsin the summit crater fumaroles of 650m relief. It hasan approximatevolume of 2 km3.The geology oflzalco volcano,El Salvador.Subsequent to its discov- of the volcano is describedby Meyer-Abich (1958),Rose and ery at Izalco volcano the mineral was found in the Stoiber(1969), and Stoiber et al. (1975). fumarolesof the andesiticNovvy dome at Bezymyanny Since 1963 the fumarolic gases volcano, Kamchatka,U.S.S.R. On the basis of this and minerals of the summitcrater have received extensive study (Stoiber discovery, the mineral was named shcherbinaite(Bori- and Duerr, 1963;Stoiber and Rose, 1970,1974; senkoet al., l97O).The descriptionof the Bezymyanny Stoiberer a/., 1975;Birnie and Hughes, 1979;Hughes and Birnie, 1980; phasewas, however, only qualitativein many respects. Hughes and Finger, 1983).Vanadium was found to Becauseofthe poor quality ofthe crystals,single-crystal be one of the distinctive elements in the fumarole X-ray photographswere unobtainable.Furthermore, ow- mineral suite. The suite includes stoiberite (Cu5V2O16),ziesite ing to incrustations coating the crystals and limitations of (F-CuzYzO), bannermanite wet chemicalanalytical methods, a singlechemical analy- [(Na,K),VX+V;l"Or5], Cu3V2Os,and CuaV2Oe. sis totaling lessthan 40 wt.% V2O5was obtained. Shcherbinaiteis found in the "MR" and "L" fuma, In view of the complete lack of detailed data on the roles at Izalco (Stoiberet al., 1975,Fig. l). The mineral phaseand the discovery of bannermanite,a coexisting occurs as a sublimate from the fumarolic gases, and is mineral in the natural alkali metal vanadium oxide bronze found as golden-yellow laths up to I cm long. The euhedral crystals are found coating basaltic breccia frag- mentsin the oxide zoneof the fumaroles,approximately I I Presentaddress: Department of Geology,Miami University, m below the surface. Shcherbinaite is often intimatelv Oxford, Ohio 45056. intergrown with bannermanite. 0003-004x/83/lI I 2-1220$02.00 1220 HUGHES AND FINGER: SHCHERBINAITE r22l
(r^ methods. +ro belowthe level detectableby energy-dispersive d{J |!'{ This analysis did suggestthat heavier ions were not cc ..t (t present !E in the V2O5from Izalco. kl. oo Quantitative analysesfor V, Na, and K were performed ,cc oc .cd on a single-channel wavelength dispersive rcol 35 o.Q seu/Microprobe (15 kV acceleratingvoltage and l9l picoampsbeam referencecurrent). Vanadiummetal, di- opside-jadeite(DIJD35), and orthoclase were used as standardsfor V, Na, and K, respectively. Analyses of 15 crystals showed that only V was pres- ent; analysestotaling as high as 97.55wt.VoY2O5 (Oz by stoichiometry) were obtained. The difference from l00Vo is assumedto be due to unpolishedsurfaces and finitely purity 0 1.0 2.0 3.0 4.0 s.o 5.0 thick grains. To verify further the of the com- Fig. l. Rangeof solid solutionsin the oxide bronze system pound, multichannelwavelength scans were made using Na,V2O5.Striped areas represent one-phase regions, with phase LiF, PET, and TAP analyzing crystals. These scans designation above region. Phase regions determined at 600'C coveredthe entire rangeof elementsdetectable (Z = 9). (after Hagenmuller,1973). Stripchart recordingsshowed that no elementsother than V were present. The first quantitativechemical analyses of shcherbin- aite thus demonstratethat it is the pure pentavalentoxide vanadium.The absenceof any detectableamounts of Chemistry of cationsother than V is probablya result ofcrystallization Pure V2O5occurs as an end-memberoxide in the alkali through the natural sublimationprocess. The results of metal vanadium oxide bronze system (Fig. l). In this the electron microprobe study suggest that the solid system,solid solutionoccurs with the additionof mono- solutionfield ofthe vanadiumoxide bronzeis nonexistent valent ions to the structure (Hughesand Finger, 1983). in the natural system,and the compositionof shcherbin- The electronsdonated by the monovalentelectropositive aite is limited to the pure V2O5end member. ions (Na,K) are contributed to the covalent V-O array, and chargebalance is maintainedby reduction of one V5* X-ray crystallography to Va+ for each monovalent ion present in the structure. Borisenko et al. (1970)were unable to obtain single- The monovalentions are locatedin crystallographicsites crystal X-ray photographsfrom the Bezymyanny shcher- that are created by tunnels in the V2O5structure. The a- binaite becauseof the poor quality of the crystals. For phase vanadiumoxide bronze,which is isostructuralwith this reason, a precession-study was undertaken with pure V2O5,can accommodateminor amountsof Na, up to MoK" radiation (0.70930A, Zr filter). The high-quality (Hagenmuller, Naa.s2V2O5at 600"C 1973). Because the singlecrystals from Izalco provided excellent material for Izalco shcherbinaiteinvariably coexists with bannerman- single-crystalfilm studies. ite, the pphase vanadium oxide bronze (Hughes and Finger, 1983), it is suggestedthat the two minerals (bannermaniteand shcherbinaite)actually crystallized in the two-phaseregion of coexistinga- and pphase vanadi- um oxide bronzes (Fig. l). In order to determine the extent of solid solution in shcherbinaite,a complete electron microprobe study was undertaken. Numerous single crystals were prepared for electron microprobeanalysis by mountingthe laths with the short dimensionperpendicular to the microscopestage, with the largest surface area for analysis. Becauseof the thin dimension perpendicular to the beam (often less than 5 pm) the sampleswere not polished;many crystalshad, however, an exceptionally smooth natural surface, suit- able for microprobe analysis. The best of the samples were initially analyzed on a reol sr,u/Microprobewith energy-dispersivetechniques. No elementsother than V were noted;however, the small Fig. 2. SEM micrograph of a twinned crystal of concentration of Na2O present in the a-phase vanadium shcherbinaite(photo courtesyof M. L. Malinconico).Scale bar oxide bronze(0.34 wt.% in Na6.s2V2O5)would be nearor representsl0 pm. 1222 HUGHES AND FINGER: SHCHERBINAITE
The single-crystalprecession photos taken precessing Acknowledgments about the a and b axes showedthe diffraction geometry to The crystal structure portion of this research was undertaken be consistent with the orthorhombic spacegroup Pnmm while Hughes was a Predoctoral Fellow at the Geophysical (No. 59).This resultagrees with the work of Bachmanner Laboratory of the Carnegie Institution of Washington. H. S. al. (1961\on syntheticsingle crystals. The axesof shcher- Yoder, Jr., Director, is gratefully acknowledgedfor that oppor- binaite are related to those of the Bachmann study by the tunity. The manuscriptwas greatlyimproved by reviews by or transformation matrix [010/100/001]. discussionswith R. W. Birnie, H. R. Naslundand R. E. Stoiber, The unit cell parameters of the Izalco shcherbinaite DartmouthCollege; R. M. Hazen,R. L. Ralphand H. S. Yoder, were determined by a least-squaresmethod with 12 Jr., GeophysicalLaboratory; F. R. Voner, Miami University. This work was supported in part by NSF grant EAR79-19768. reflections measuredon a four-circle diffractometer. The parametersare a:4.364(8), b:11 .50(2'),and c = References 3.561(a);all valuesdiffer by