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A New Fumarolic Sublimate from Izalco Volcano, El Salvador: Descriptive Mineralogy and Crystal Structure

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A New Fumarolic Sublimate from Izalco Volcano, El Salvador: Descriptive Mineralogy and Crystal Structure American Mineralogist, Volume 72, pages 1000-1005, 1987 Lyonsite, Cu3+Fel+(VOo)?-,a new fumarolic sublimate from Izalco volcano, El Salvador: Descriptive mineralogy and crystal structure JorrN M. Hucnns, Sln-nH J. Sr,q,mrv Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A. M.lnvANN L. Mlr,rNcoNrco. LawnnNcn L. Mar.rNcoNrco Department of Geology, Southern Illinois University, Carbondale,Illinois 62901, U.S.A. Ansrn-q.cr Lyonsite, Cu3*Fe1+(VOo)?, a new iron-copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral, which occurs with thenardite and a Na-Cu-Fe vanadate, formed as a sublimate product from the vol- canic gases.Lyonsite occurs as euhedral, black lathlike crystals up to 230 pm in greatest dimension. Lyonsiteis orthorhombic,Pmcn, a: 10.296(l),b: 17.207(2),c: 4.910(l)A. The atomic arrangementof the mineral has been determined and refined to R : 0.028, R* : 0.031. The structure is basedon a pseudohexagonalclose-packed oxygen array. The fun- damental building block consists of six isolated VOo tetrahedra that are linked, through corner sharing, to a central octahedron that is half-occupied by Cu2+ atoms. These "pin- wheel" building blocks are connectedon four sidesby FeOuoctahedra and on the remain- ing two sides by square-planarCuOo groups. The fundamental building block is isostruc- tural with the silicate pinwheels in dumortierite. Lyonsite is opaque and has a dark gray streak and metallic luster. Z : 2 and D*rc: 4.215 g/cm3.Lyonsite displaysa good {001} cleavage.The mineral is creamy white in reflected light in air. Reflectancevalues measured on (010) are (\, min., max.) 481 nm, 17.5,23.1o/o;547nm, 16.6,22.3o/o;591nm, 14.7,20.7o/o;and644nm,14.4, 18.80/0. The mineral is named after Dr. John B. Lyons of Dartmouth College.Milligram amounts of the phaseexist and are on deposit at the NMNH, Smithsonian Institution. INrnolucrroN tirement. The mineral and mineral name have been ap- The fumaroles of Izalco volcano, El Salvador,were first proved by the IMA. investigatedby n. P. Stoiber and colleaguesfrom Dart- mouth college. Early studies showed the presence of Gnolocrc SETTING severalhigh-temperature V minerals in the fumarole min- Izalco volcano is a basaltic composite cone that has eral suite (Stoiber and Duerr, 1963; Stoiber and Rose, been intermittently active since its birth in 1770. The 1974). Since that time, stoiberite (Cu,VrO,o), finger- volcanorises to an altitude of 1965m, with 650-m relief, ite [Cu,Or(VOo)6], mcbirneyite [Cur(VOo)r], ziesite and has an approximatevolume of 2 km3. The geology (B-CurVrO,), blossite (a-CurVrOr), bannermanite (Na"- of the volcano is describedby Meyer-Abich (1958), Rose VuO,,), and scherbinaite(VrOs) have been describedas and Stoiber(1969), and Stoiberet al. (1975). minerals from the site. To thesecompounds is now added Hughes and Stoiber (1985) describedthe conditions of lyonsite, Cu.Feo(VOo)u,the first iron-copper vanadate mineral genesisin the fumaroles of Izalco volcano. They mineral known. showed that the minerals formed as sublimates from a Lyonsite was collectedfrom the "Y" fumarole of Izalco vanadium halogen or oxyhalogen gas that was exsolved volcano and occurswith thenardite and a Na-Cu-Fe van- from a basaltic magma when the magma was 550 m be- adate currently under study. The mineral is found as eu- low the summit crater. As the gas rose to the fumaroles hedral lathlike crystals up to 230 pm in greatestdimen- and mixed with the atmosphere,the oxygen fugacity rose sion. Specimens have been deposited at the NMNH, from that of the magma to at least 10 35 bar. At the Smithsonian Institution. The authors are pleasedto name fumaroles,the vanadium compounds sublimated at tem- the mineral lyonsite in honor of Dr. John B. Lyons, Fred- peraturesup to ca. 800'C. erick Hall Professorof Mineralogy at Dartmouth College. Dr. Lyons has had a long and distinguished careerin the Cnnursrnv fields of igneous and metamorphic petrology, and it is Single crystals of lyonsite were mounted and polished fitting that he be so honored on the occasion of his re- for electron-microprobeanalysis. Qualitative energy-dis- 0003-o04x/87l0910-1000$02.00 1000 HUGHES ET AL.: LYONSITE r00l TneLe1. Chemicalanalyses of lyonsite TneLe2. X-raydiffraction pattern and unit-cellparameters of lyonsite ldeal Avg.' CuuFe"(VOo),, d* hkt V.ou 47.74(36) 49.44 8.79 8.835 20 <1.5 11 0 FerO" 2483 (12) 2894 4,423 4 418 40 40.7 22 0 MnrO. 3.41(6) 3.371 3.366 20 20.1 21 0 Tio, 2.27(21 3.279(b) 3.284 100 100.0 22 1 CuO 21.99(33) 21.62 3.264 35.2 15 0 Total 100.24 100.00 3.236 37.4 04 1 2.943 2.94s 20 b.u 33 0 Formula (O : 48): Cuu - or(Fe!;Ti6i3Mn335)>I 44V1150046 2872 2.868 20 10.4 UO 0 'Analysis representsaverage of three analysesof 2.779 z.t Io 40 24.8 31 typecrystal. One standard deviation of leastunits given 2.722 2.739 40 27.9 24 in parentheses. 2.718 17.2 15 2 526 2.526 60 56.7 33 2.477 2.476 10 17.5 0 6 2.259 2.260 10 8.9 4 1 2.215 2.216 10 2.O 2 0 2 persive analysesdemonstrated that Cu, Fe, and V were 2209 1.3 4 4 0 2.011 2.014 10 2.2 4 4 1 the major constituents, with minor Ti and Mn. Quanti- 2.008 3.836 1 tative electron-microprobe analyseswere performed on 1.887 1.886 10 15.8 3 3 2 the Southern Illinois University Etec Autoscan micro- 1.805 1.803 10 3.7 5 3 1 1.718 1.716 20 7.9 6 0 0 probe with a Kevex 8000 energy-dispersivesystem using 1.671 1.670 20 11.9 3 9 0 metal standardsfor Cu, Fe, V, Mn, and Ti. The results 1.591 1.592 40 10762 1 1.549 40 11.1 2 10 'I of these analyses are contained in Table l Based on 1 550 knowledge of the conditions of mineral genesisand as- +5 lines <1.500A sociatedminerals, all elementswere assumedto be fully . 114.6-mmGandolfi camera, in vacuo, Ni-filteredCu radiation,visual : oxidized. The subsequentcrystal-structure determination intensityestimates; (b) broad. .* Calculatedfrom single-crystalditfractometer parameters: a : 10.296(1), confirmed the existenceof Cu2*. Fe3+.and V5+. The anal- b : 17.207(21,c : 4.910(1)A. yses demonstrate minor substitution of Mn3+ and Ti4+ for Fe3+;however, in subsequentcrystal-structure calcu- Iations, the Fe3+octahedron was assumedto be occupied third of the total counting time was spent determining back- only by Fe3+. ground on both sides of the peak. Three intensity standardswere monitored every 4 h, and two X-ntY cRYSTALLoGRAPHY orientation standardswere monitored every 160 reflections.No A diffraction pattern of lyonsite was obtained using a significant deviation of intensity was noted during data collec- 114.6-mmGandolfi cameraand Ni-filtered Cu radiation, tion (-3 d). in vacuo. The indexed diffraction pattern is contained in Intensity data were collectedfor the octant ofreciprocal space Table 2. compromising +h, +k and +1 to 50 20. The 946 resulting re- polarization A single-crystal precessionstudy of lyonsite was un- flectionswere correctedfor Lorentz and effects,and absorption effectswere correctedby an empirical technique em- dertaken using Zr-filtered Mo radiation. The euhedral ploying intensity data obtained from 360'psi scansat 10'inter- form allowed rapid orientation of the crystals.Cone-axis vals for six reflections. Symmetry-extinct and "unobserved" data and precessionphotographs were taken about all three (I < 4o) were removed from the structure-factorlist. The final principal axes. The photographs indicated Laue group data set yielded 624 observedreflections. The zero-moment test mmm, and systematic extinctions h + k + 2n for hk} yielded statistics not incompatible with the centrosymmetric space reflections and I + 2n for hjl reflections. The absences group; thus subsequentcalculations were undertaken in space yield space group Pmcn, the bca setting of spacegroup group no. 62, Pmcn. no. 62, Pnma. The unit-cell parametersof lyonsite were A1l crystal-structure calculations were carried out using the refined from 25 diffraction anglesdetermined on a four- Enraf-Nonius SDP-Plus programs. The phaseproblem was solved circle diflractometer and are given in Table 2. by direct methods as implemented in uulrnN and associated programs (Main et al., 1980). The phase set with the highest Cnysrl.L srRUcruRE combined figure-of-merit ultimately revealedthe positions of all atoms. Experimentaldetails Final refinementwas undertakenby refining positional param- A euhedralsingle crystal measuring 0.23 x 0.06 x 0.025mm eters, scalefactor, anisotropic temperature factors, an isotropic wasselected for the crystal-structurestudy. A completesuite of secondaryextinction factor and the occupancyofthe Cu(l) site, precessionphotographs showed no evidenceof twinningor in- with a total of 96 parameters.The quantity lr( |f. I - l,F"| )'?was tergrownphases. The crystal was mounted on a cAD4difractom- minimized by full-matrix least-squaresrefinement, with w : eterutilizing graphite-monochromated MoKa radiation.Inten- loi + (0.007Ff,)21-'.Neutral-atom scattering factors, including sitieswere measured using a d-20scan technique, with thetascan terms for anomalous extinction, were taken from Tablesfor widthsdetermined by the relationshipd : 0.60"+ 0.34(tan d). X-ray Crystallography(197D.Upon assigningthe correct atomic A prescanofeach peak during data collection determined count- positions, the structure routinely refined to R : 0.028, R* : ing time, with a maximumtime of 4 min per reflection.One- 0.03 l, with a goodness-of-fitof 2.
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