GRAESERITE, Featisasol3(OH), a NEW MINERAL SPECIES of the DERBYLITE GROUP from the MONTE LEONE NAPPE, BINNTAL REGION, WESTERN AL
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1083 The Canadian Mineralogist Vol.36, pp. 1083-1088(1998) GRAESERITE,FeaTisAsOl3(OH), A NEWMINERAL SPECIES OF THE DERBYLITE GROUP FROM THE MONTELEONE NAPPE, BINNTAL REGION, WESTERN ALPS, SWITZERLAND MICHAEL S. KRZEMNICKII Mineralagisch-Petrographischzs Instint, Universitiit Ba$eI, Bemoullistr. 30, CH-4056 Basel, Switzerl.and ERIC REUSSERI Iwtitut fiir Mincralogie und Petrographie, ETH-7zntram, Sonneggstr. 5, CH-8092 Zurich SwitzerLanl AssrRAcr Graeserite, ideally Fe4t3AsOl3(OH), is a new mineral species of the derbylite gloup, which includes derbylite, tomichite, a"ndhemloite. It is found in needle-shapedcrystals, elongate along the c axis. Graeserils i5 6sn6alinig, spacegtoup AXm, with the cell paramelersa 7 .184(2), b 14.289(6),c 5.006(2) A, p 105.17(2)",V 495.9(2) At, z=2, D,a.. =!.56 g/cm3.The VHNxgis_52L (Mohs hardness -595). The strongest five lines of the X-ray powder-diffraction pattern [d in An)(hkD] are: 2.681(100)@31), 2.846(80X131), 1.583(50X351), 3.117 (30)(220), nd 2.029(30)(122). Graeserite is black and metallic, with a black streak; it displays a conchoidal fracture. Pleochroism, bireflectance a"ndinternal reflections were not observed. The measured values of reflecrance in air are compared with those of other members of the derbylite group. Electron-microprobe analyses gave TiO2 40.89, Fe2O333.64, FeO"6" 3.94, PbO 5.00, As2O3 13.51,Sb2O3 1.43, and H2O"a" 1.30, loral 99.80 wt.7o. The empirical formula, based on 13 atoms of oxygen and one hydroxyl group, is (Fe3*z.qrFe2*o.gsTlos+Pbo.rs)>r.grTi3(As3*s.9aSb3*e.m)>r.orOrg(OH). Graeserite o@urs as a hydrothermal mineral in Alpine-qpe fissures in gneisses of the Monte l,eone nappe at the locality l,2ircheltini, in the Binntal region of Switzerland, in association with other rare arsenic oxides. With the name, we honor Stephan Graeser, Universiry of Basel, Switzerland, for his fundamental studies of arsenic-bearing mins6ls. Keywords: graeserite, new mineral species, derbylite group, Binntal fegion, Western Alps, Switzerland. Somraenr La graeserite, de composition id6ale FeaTi3AsOl3(OH), est une nouvelle espdce min6rale du groupe de la derbylite, qui inclut derbylite, tomichite, et hemloi'te. Elle se pr6sente en cristaux aciculaires allong6s selon I'axe c. La graeserite .sno€linique, "r1 groupe spatial A2l^m,paramdresr6ticulaires a7.184(2),b 14.289(6),c 5.006(2) A, p 105.17(2)",V 495.9(2) A3,Z=2, Dcarr.= 4.56 glc^a.Ia duretd VHN25" est de 521 (-5% sur l'6chelle de Mohs). Les cinq raies les plus intenses du spectre de diffracrion X (m6thode des poudres) t7 en A0(arrll sonr 2.681(100)831),2.u6(80)(131), 1.583(50)(351),3.1r7 (30)(220), et 2.029(30)(122). La graeserite est noir m6tallique, avec une rayure noire; elle possbde une fracture conchoidale. Nous avons d6ce16 ni pl6ochroisme, ni birdflectance, ni rdflexions internes. Les valeurs mesur6es de r6flectance dans I'air sont compar6es avec celles d'autres membres du groupe de la derbylite. Les analyses obtenues par misosonde 6lecronique onr donne TiO2 40.89, FezOz 33.64, FeO"a" 3.94, PbO 5.00, As2O3 13.51, SbzO: 1.43, er H2Od. I.3O, totzl,99.80Vo (par poids). La formule empirique, calculde sur une base de 13 atomes d'oxygbne et un groupe d'hydroxyle, est (Fe3*2.e1Fe2+6.3sTi9.5aPbs.15)23.9sTi3(As3*s.9aSbh9.67br.orOrr(OH).La graeserite est le produit d'une cristallisation hydrothermale dans des fentes alpines dans des gneiss de la nappede Monte Leone d Liircheltini, dans la r6gion de Bimtal, en Suisse;elle y est associ6etr d'autres oxydes d'arsenic rares. Par le choix du nom, nous honorons Stefan Graeser,de I'Universit6 de Bdle, en Suisse, qui a fait des dtudes fondamentalesdes mingl4rx d'arsenic. (Iraduit par Ia R6daction) Mots-cl4s: graeserite, nouvelle espbce min6rale, groupe de la derbylite, r6gion de Binntal, Alpes occidentales, Suisse. I E-mail addre sses:[email protected]. [email protected] 1084 TTIE CANADIAN MINERALOGIST IlrrnopuquoN Graeserite,ideally FeaTi3AsOl3(OH),is a new mineral speciesfound in Alpine-type fissuresin paragneissesof theMonte Leone nappe, Binntal region, WesternAlps, Switzerland.It belongsto the derbylite group,which consists of thespecies derbylite, tomichite (includingbarian tomichite), and hemloite(Table l). Except for hemloite,the mineralsof this group exhibit the generalformula lttf*;tl+n}rrlO,H). The cation site M3* is occupiedby Fe3*and V3+ (with occasionally someFe2+, Pb2+, and Ti4), and the M4* siteby Tia. Both thelld+ andlla+ sitesare octahedrally coordinated. The tetrahedrallycoordinated Tsite is occupiedby Sb3*, As3+,and occasionallyBa2* in bariantomichite. The valuesfor the stoichiometriccoefficients r:y are com- monly 4:3 (derbylite,tomichite, and graeserite), but may alsoamount to 5:2 (bariantomichite). Ftc. 1. Generaltectonic map of the Binntal region, Western Derbylitewas discovereda c€nhtryago (Hussak & Alps, Switzerland.Symbols: /. locality l-ftcheltrnl, MIN Prior 1897) in cinnabar-bearinggravels at Tripuhy, MonteLeone nappe, PN Penninicnappes, MMS Mesozoic Minas Gerais,Brazil. Its close-packedoxide crystal metasediments,AM Aar massif (granites),sdl Simplon structurewas determinedby Moore & Araki (1976) and detachmentfault. refined by Mellini et al. (1983).Tomichite was first reportedby Nickel & Grey(1979) from the Kalgoorlie gold depositin Australia.The crystal structureof orogeny, which affected the whole Alpine region by tomichite was determinedby Grey et al. (1987), who extensive ductile-brittle deformations and regional further reportedbarian tomichite at the Hemlo gold metamorphism reaching conditions of the lower deposit,Ontario. From the 5smedeposit, Harns et aI. amphibolite facies in the Monte Leone nappe. The (1989)described hemloite as a furthermember of the regional geology is discussed in more detail by derbylite group. Streckeisenet a/. (1974), Steck (1987), Hiigi (1988), and In this study,wepresentmineralogical, chemical and Krzemnicki (1996). Several Prealpine ore concentra- preliminary structuraldata for graeserite,Graeserite is tions (As-Fetu-Pb sulfides) within this nappe were namedin honorofProfessor Stefan Graeser (b. 1935), locally remobilized, thus generating some unique of the Mineralogical-PetrographicInstitute, University hydrothermal mineralizations (e.g., Lengenbach, Ytzzo of Basel,Switzerland, who hasbeen involved for years Cervandoneo\{annigletscher, Liircheltini; cf Graeser ir researchon oxides and sulfosaltsof arsenicin the 1966, Graeser& Roggiani l9T6,Krzemnicki 1996). Binntal regionof theMonte t eonenappe (Graeser 1966, Graeser& Roggiani 1976,Graeser et al. 1994, and Pnvslcer AND OmcAL Pnoppnrns referencestherein). Both the mineraland the namehave beenapproved by the IMA Commissionon New Miner- Table 2 presents a summary of the physical proper- alsand Mineral Names (proposal96-{10). Specimens are ties of graeserite compared with the other members of heldat theNailral HistoryMuseum in Basel Switzerland. the derbylite group. Graeserite, monoclinic A2lm, is generally found in well-developed, euhedral crystals OccunneNcE that are elongate along the c axis. Commonly, they are needle-shaped(<10 pm thick, up to 5 mm long, Fig.Z). Graeseritewas discovered1995 in hydrothermal Lineations parallel to the c axis indicate exlensive twin- veinsthat occur in a two-micaparagneiss of theMonte ning, Occasionally, graeserite occurs in radial aggre- Leonenappeo Binntal region,Westem Alps, Switzer- land, or in small cavitiesin proximity to suchveins (wallrock alteration).Numerous specimens have been TABLE I. MINERAIJ OF ITIEDERBYLITE GROT]P found at the locality L?ircheltini(Frg. l), accompanied by a wide rangeof otherhydrothermal minerals, includ- Chmi@lFoml8 Spam Gro'p ing anatase,arsenopyrite, asbecasite, bournoniteo cafarsite,cervandonite-(Ce), chernovite, fetiasite, gold Dqbylile FeofbS@g(OID nln (traces),hematite, magnetite, monazite-(Ce), rutile and Tmiche (V,Fe)aTi3AsOts(OID F4tn tennantite. Bria Tonicbne (V,re!fipaiiAq)orOr(OH) AZm Gmffiit! Fe.Tr"ArO"(OH) AAn The formation of graeseriteis attributedto the hy- Ilmloite CTLYTqAI)IA!, Sb)rotsoH PT drothermalactivity associatedwith the Tertiary Alpine GRAESERITE. MONTE LEONE NAPPE, SWTIZERLAND 1085 TABLE2. COMPARITIONOFTHEDERBYLTTE4ROUPMINERALS TABLE 3. @ABSERITB: REFLECTEI>IJGHT MICROSCOPY Gffiite TmichitE B{iu tonic.bite Mylits rcdeotane vals olqvahq Cifl,miffi Rl M, RI Sydm mon@litric m@lilic rclidc mon@li[ic Sp@goup A2lm P)rln AAn P2Jn a (A) 1.ru 7.tr9 7.105 7.16 4@m 22.6Yo 23.5o/o x 0.2997 0.298 ,(A) r42tg 14.t76 14.21 t4.347 420 21.7 2.5 c (A) s.oo6 4.92 5.M3 4.97 44 20.9 2t.6 y 0.3M9 0.304E p c) 105.17 105.05 1M.97 104.61 460 20.t 20.8 v (A) 4e5.e 480 19.6 20.3 Y% 18.6 r1.s DdG/d) 4.56 4.42 4.76 500 t9.3 19.9 nimbrdnw 52.O 19.0 19.6 vrlNd 521 800 5.10 18.7 19.3 @lor opaque,black opaqu8,blsc& opque,black mhoug black 560 18.4 19.0 I d 476.4 475.9 coldvalB Rl M Rl p, Rl M, 580 ft.2 lE.9 x 0.300 0.3m 0.3110.312 0.2ffi 0.298 600 l8.t 18.6 P eo/o 5.2 5.2 y 0.30s 0.305 0.316 0.3t7 0.303 0.302 620 t'1.9 18.5 Y/o 18.6 17.5 t1.t l'1.8 18 l9.t ffi 17.7 18.3 x. 476 476 504 589 476 475 6@ 17.5 l8.l P 4/o 5.2 5.2 0.3 0.'l 5.9 6.2 580 1,7.3 17.9 7@ 17.3 t',1.9 typ€ lmliry l&cheltiri Kstgmli€ Holo Tripuly Birdal Wdm deposit MilB G@is Switzslmd Arstralir Odrio Bna:l gates,which are attachedto fetiasiteand in somecases measured with a load of 25 g on a polished section of a intergrownwith feldsparcrystals. graeserite aggle9ate. The mean value of VHN25, 521, Graeseritedisplays a distinct ductile behavior;thus correspondsto a Mohs hardnessof about5Vz.