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 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 ; it displays a conchoidal . 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 , 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. the crystalscommonly are deformed and curved during For reflectance measurements, we used a Leitz samplepreparation. Therefore, it is rather difficult to microscope with a Zeiss-SiC reference standard.Data investigatethe morphology by opticalgoniometer or to measured in air only are listed in Table 3. A compari- determinethe crystalstructure ofgraeserite by single- son of the data with values for derbylite and tomichite crystal X-ray methods.Graeserite exhibits a moderate reveals a close similarity between graeserite and cleavagealong { 100} and a conchoidalfracture. The derbylite (Frg. 3).Pleochroism, bireflectance and inter- densitycould not be measureddirectly by immersionin nal reflections were not observed. The optical data are heavyliquids, as it is greaterthan that of Clerici solu- given in Table2. tion, but was calculatedon the basisof the empirical Raman spectra were obtained on a Renishaw Raman formula; Dx 4.56 g/cm3.The Vickers hardnesswas system 1000 (Swiss Gemmological Institute SSEF,

Hc. 2. SEM photomicrograph of an aggregate of graeserite crystals. Scale bar: I mm. r086

-fl- 11 graeserite --t- 12 graeserite s --o- rl derbvlite --.- C) r2derbylite (J -{- r1 tomichite d -a- 12 tomichite I(.) +ig

15.0 400 500 600 700 wavelength I [nm]

FIc. 3. Comparison of reflectance data for the derbylite-group minqal5. The reflectance vafies RVo versus wavelengtl )t [nm] diagram (measured in air) reveals a close similar- ity between gaeserite and derbylite.

Basel) with an argon laser (25 mW, omnichrome). A thetic Sb2O3).Vanadium and barinm, which are impor- typical spectrum for graeserite is shown in Figure 4, as tant constituents of tomichite and barian tomichite, were it may aid in identification. found to be below the limit of detection. All data were corected for matrix effects by aZ$-type online pro- X-nay Cnysrar-r-ocRApr{y cedure. The average composition determined from aNo Crnpncer Covtposmoll results of 39 analyses(6 samples)is shown in Table 5. The chemical composition of all the samples is rather X-ray-diffraction analyses by precession and uniform, although the concenhations of Fe, Ti, Pb and Weissenbergtechniques showed graeseriteto be mono- Sb show some variation from sample to sample. Back- clintc A2lm, The cell parameters were refined from scattered-electronimages @SE) revealed no compo- powder data and are compared with those of the other sitional zoning within the analyzed samples. members of the derbylite group (Table Z\The indexed The normalization of the graeserite formula X-ray powder-diffraction data for graeserite are lis[ed (Table 5) is based on a total of 27 negative charges in Table 4. There are strong similafi1ies between the (i.e., O2*e + OH- ) and a sum of eight cations at the powder pattern of graeserite and those of other derbylite- n43*,lul+*,and Z sites, as frst proposed by Moore & group minerals, which point to a close structural rela- Araki (1976) for derbylite. The H2O content was calcu- tionship among this group of minerals (Nickel & Grey lated assuming one hydroxyl group per formula unit 1979,Mellini et aI. l983,Grey et al. 1987,Berlepsch & (Moore & Araki 1976). According to Grey et al. (1987) Armbruster 1998). and Mellini et al. (1.983), charge balance is usually All samples of graeserite were first analyzed quali- maintained by variation in the ratio of Fe2* to Fe3*. In tatively by energy-dispersion spectroscopy (ED-)GF) graeserite, this ratio (- 0.130) is comparable with the for element identification. These analyses and photo- one found in derbylite (E;e2+6;e3+= 0.155: Mellirtt et al. graphic documentation were carried out with a Philips 1983). The similarity in the values may be interpreted 515 scanning electron microscopeoequipped with an as a further indication of a close structural relationship ED-XRF spectrometer (SEM Laboratory, Univenity of between these fwo minerals. Basel). The empirical formula for graeserite, derived from Six samples were analyzed quantitatively using a electron-microprobe data (based on the average Cameca SX-50 electron microprobe with an accelera- results, Table 5), is (Fe3+2.91Fe2+o.gsTio.saPbo.rs)>3.e8Ti3 tion voltage of 20 kV and a beam current of 10 nA. The (As3+q.eaSb3*o.or)>r.orOrr(OH), which is in good following elements were measured (standards in paren- agreementwith the composition Pbq.1a(Fe,Ti)7AsO12", theses):Fe (hematite), Ti (rutile), Mn (spessartine),Pb (OH)z-, obtained from the structure determination (galena),Si (quartz), As (synthetic GaAs), and Sb (syn- @erlepsch & Armbruster 1998) on material from the GRAFSERITE. MONTE LEONE NAPPE. SWTIZERLAND 1087

Renishaw Raman Spectrometer 743.3 1600 591.5 421.6 297.5 1200 t63.4 1451.8

800

camemgain hieh exposuretime[sec] 90 400 zero level 0 light source[cm-l] 19436.0 numberofPoints 1134 I ! ll 400 800 1200 1600 deltawavenumbers [cm-1] Frc. 4. Ramal spectrum of graeserite,as obt?ined with a Renishaw Raman spectrometer.

same locality. The idealized formula of graeseritemay Dlscusslox be written as Fe+TigAsOrs(OH).The analyzedsamples all contain some Pb2* and Fe2* (calculated on the basis Derbytite, tomichite, barian tomichite, hemloite, and of charge balance) and a small excess in Ti# with re- graeserite are rare accessory phases known to date from spect to the idealized stoichiometry. The incorportion only a few deposits worldwide: Tripuhy (Brazil), of these substituting elements is probably due to the Kalgoorlie (Australia), Hemlo (Ontario, Canada),Buca heterovalent substitution [i] (Fe, Pb)z*Ti4* at the della Vena (Italy), and Liircheltini (Swizerland). The trrl3*site.A plot (Fig. 5) of amout of Fe3* versus complex mineralizations of these deposits are com- >(Fe,Pb)2*+ Tie reveals a distinct negative correlation monly attributed to metasomatic and hydrothermal flu- (stope: -1.043, R2 = 0.924), which fits well with this ids (Mellini et al. 1983, Grey et aI. 1987, Graeser & msghanism of substitution. A bivariate plot for As3* Roggiani 1976). Graeserite, as well as the other versus Sb3+exhibits only a weak negative correlation, although the incorporation of minor Sb in graeserite is easily explained by a simple exchange substitution be- TABLE 5. CHEMICAL COMPOSITIONOF GRAESERITE tween Sb3+and As3* at the arsenic site.

lrru s9o mge lo qlu

TABIT 4, X-RAY-DIFFRACTION DATA FOR GRAESERITE Tio2 40.E9 (39.75- 4r.52) 0.46 FqQ 33.s (3r.65- 35.55) 0.85 Fs$ 2.911 F€O 3.94 (3.20- 4.t5) o.43 Fea 0.38. L d* t" d* d* hH Pbo 5.00 (2.22-6.3s) 1.05 Pts' 0.15 MrO 0.06 (rd - 0.13) 0.03 Mn? o.ol A&r03 13.sl (13.12-14.3D 0.30 AgF 0.94 30 3.117 3.119 220 20 r.72i4 r.7L3s 222 sqq r.43 (O.7-2.33) 0.45 sb} o.o7 EO 2.U6 2.U7 r31 20 l.62lE t.6n4 213 sio, 0.02 (rd -0.08) 0.o2 si* o.@ t@ 2.68t 2.683 tll t.5834 Hp 1.30 (1.?a-"1.34) 0.01 oH- l.@* 20 2.495 2.493 tv2 50 1.5825 t.s8u 351 Tot8l 99.79 20 2.225 2.226 l5l 1.5816 162 2.148 2.145 2s1 20 1.5150 1.5151451. 30 2.029 2.029 1.4405 153 ElEtr@-Bidoprcbe dtu reportd m wf,/o lhe mgp of 39 ualys io r€'puted' 20 1.8093 1.8088 ?5r l0 tAM t.w 431 The nomdiztiq is b8s€d on 8 @tios ps formil8 mit ud a toal of 27 rcgcivo 2T Ln65 r.n60 202 1.3661 4t3 chtrgs. The prcportion of FeF ud Fd" m elorlaal bamd on c'hrgebalrc 20 1,.3&7 1.3648 t8z onstairrs. fne neO aa Fe2O3 @ddts v@ r@l@l8te4 on the bffi! of tho Fe*/Fd. rcio. The proportim of HrO ru detmined on the MPtion of w qI Ivlellid Mru

4.1 Gnansan,S. (1966): Asbecasitund Cafarsit,zwei neue O9 Mineralien aus dem Binnatal (Kt. Wallis). Schweiz Min- eral. Petrogr. Mitt. 46, 367-37 5. ! +.0 \JX .H x^ & Roc,cIANI,A.G. (1976): Occurrence and genesis

f UU^m' of rare arsenate and phosphate minerals around Pizzo N Cervandone, Italy/Switzerland,. Rend. Soc. ItaL Mineral. L 9d" n -r 3.9 c(CX;-ac Petrogr.32,n9- 288, + Y''/\/ G4 '?a -o 3.8 C.M., Pllnrr, T. & REUsSER,E. (1994): Fetiasire (Fe2+,Fe3*,Ti)3O2[As2O5],a new arsenite mineral: its 2.7 2.8 2.9 3.0 3.1 description and structure determination. Am- Mineral.79, per* [p.f.u.J 996-1002.

Ftc. 5. A plot ofthe concenfation ofFe3*yersus l(Fe,pb)2* + GREv, I.E., Mnosrx, I.C. & HARRrs,D.C. (1987): Barian Ti+ reveals a distinct negative correlation, which fis very tomichite, Ba6.5(As2)6.5Ti2(V,Fe)5O13(OH), its crystal well the proposed mechanism of heterovalent substitution structure and relationship to derbylite and tomichite. Arn. [i] (Fe,Pb)2*TieFe]_2. [pfu]: per formula unit. Mineral.72,2Ol-208.

HARRrs,D.C., Hosmts, B.F., Gnnv,I.E., Cnnolr, A.J. & Srar- rcy, C.J. (1989): Hemloite (As,Sb)2(Ii,V,Fe,Al)rzOzOH: a new mineral from the gold membersof the derbylite group, seemto be related Hemlo deposit, Hemlo, to Ontario, and its . Can. Mtneral 2T, the presenceof Au mineralization(e.g., tomichite: 4n440. Kalgoorliegold deposit;barian tomichite: Hemlo gold deposit,hemloite: Hemlo gold deposit).At Liirchelrini, HUcI, M. (1988): Mineralogie und Petrographie der theoccurrence ofnative gold (traces)close to graeserite Ltircheltinizone (Monte Leone Decke, Binntal, VS). has recentlybeen confirmed (T. Mumenthaler,pers. Diploma thesis, Mineral. Petrograph. lnstitute, Univ. of conrmun.). Bem, Bem, Switzerland. Finally, it seemslikely that an isomorphicseries (partial or evencomplete) exists between graeserite and HussAK, E. & Pnron, G.T. (1897): On derbylite, a new both derbyliteand tomichite. This antimono-titanate of iron, from Tripuhy, Brazi. Mtneral conclusionis based Mas. lI, 176-179. on the proposedmechanism of substitutionon various sites in graeserite,and in addition on strucftlraland KnzsLfl\'IcKI, M.S. ( 1996): Mineralogical Investigations on crystallographicanalogies between these minerals (e.s., Hydrothermal As- and REE-Bearing Minerals within Tables2, 3). thz Gneisses of thc Monte leotrc Nappe (Binntal Region, SwitzerbnA. Ph.D. thesis, Mineral. Perograph. Institute, Acrnqowuocewxrs Univ. of Basel, Basel, Switzerland.

The authorsthank R. Guggenheimand D. Mathys Mullrm, M., OnraNol, P. & Pnncmaz, N. (1983): Derbylite (SEM Laboratory,Basel) for their valuable from Buca della Vena mine, Apuan Alps, Italy. Can Min- contribution eral. 21,513-516. to this study.Raman spectra investigations were kindly providedby H.A. Hiinni (SwissGemmological Insritute Moonn, P.B. & Anan, T. (1976): Derbylire, Fe3+4Ti43Sb3*O13 SSEF,Basel); the assistance of M. Doppler(University (OH), a novel close-packed oxide structure. Neues Jahrb. of Geneva,Switzerland) for the ore-microscopymeas- M ine ral., AblL IXi, 292-303. urementsis appreciated.We acknowledgethe fruitful discussionswith R. Gier6,J. Bruggerand P. Berlepsch. Nrcreq E.H. & GREv, I.E. (1979): Tomichite, a new oxide Thanks also to T. Mumenthalerand R. Liissi, who mineral from Western Australia. Mineral. Mag. 43, kindly provided5s1rs samples of graeserite.This study 469471. wasfinancially supportedby the SwissNational Science Srtcr, A. (1987): - Foundation(grants Le massif du Simplon rdflexions sur la No. 21-3383092and20-40603.94). cin6matique des nappes de gneiss. Schweiz. Mineral. Reviewsby P.B. Leavensand an anonjrmousreferee are Petrogr. Mitt. 67, 27 45. gatefuly acknowledged. SrnEclcrsnN, A., WrNrc, E. & Fnry, M. (1974): On steep RsrBnnNcrs isogradic surfaces in the Simplon uea. Connib. Mineral. Perrol. 47,8l-95. BERLEpscH,P. & ARMBRUSTR,T. (1998): The crystal struc- ture of Pb2+-bearing graeserite, Pbo.rqfe,Ti)rAsO12*^ (OFI)z-", a mineral of the derbylite group. Schweiz. Min- Received October 16, 1996, revised manuscript accepted eral. Petrogr. Mitt.78, l-9. JuIv 9. 1998.