t493

The Canadian M ineralo gi st vol. 37. pp 1493-1498(1999)

ORLANDIITE,Pb3Gla(SeO3)'H2O, A NEW MINERALSPECIES, AND AN ASSOCIATEDLEAD-COPPER SELENITE CHLORIDE FROMTHE BACCU LOCCIMINE, SARDINIA, ITALY

ITALO CAMPOSTRINI ANOCARLO MARIA GRAMACCIOLI$

Dipartimentodi Scienzedella Terra, Universitddeeli Studi, Via Botticelli23' I-20133Milano, Italy

FRANCESCODEMARTIN

Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universitd degli Studi, Via Venezian21, I-20133 Milano, Italy

AssrRAcr

Orlandiite, ideally Pb:Cla(SeOr).HzO, is a new mineral species ftom a selenium-rich pan of the oxidation zone at the old lead-arsenic mine ofBaccu Locci, near Villaputzu, Sardinia, Italy. The mineral is colorless to white, translucent, brittle, has a vitreous to silky luster, and is not fluorescent in ultraviolet light. The calculated density is 5.66 g/cm3 for the ideal formula, with Z = 2. The mineral is birefringent, with nreas= ncarc.= 1.96 (+0.05). An X-ray study carried out on a single crystal indicates a triclinicsymmetry,spacegroupPl(no.2),a 8.136(3),b8.430(6),c9.233(7)L,s62.58(7),971.8+(4),"y75.13(4)'.The strongestsix lines oi the X-ray-diffraction pattern ld in A!)(hkt)l are: 4.000(100)(002),3.258(75Xl2l). 3.188(75X201), 3.818(55)(201), 3.731(44)(122), and 2.103(40)(142). Orlandiite occurs as very small, elongate, tabular crystals up to 0.1 mm in length, colorless to white, in association with chalcomenite, pseudoboldite, anglesite, quarlz and other selenites.The chemical formula has been determined ftom electron-microprobe data and a crysta1-structue determination [final R index = 0 .042 on 1226 reflections with I > 3o(I)1. The structure contains two independent Pb atoms belonging to layers with idealized composition PbClz; a third disordered lead atom is located between theselayers, together with a disordered SeO3group and the H2O molecule. In most of the sites occupied by Cl atoms, there is a partial replacement by OH groups. Associated with orlandiite and chalcomenite, and presently under study, is another new selenite, Pb+CuCl:(SeO:):(OH), in the form of minute lemon-yellow aggregatesof tiny platy crystals. The strongest six lines of the X-ray diffraction pattem for this_phaseld irt A(l)(hkl)l are: 8.279(100X010),6.674(80X1 l0), 11.100(76X001 ),7 .344(70)(100),5.788(65X101), and 6.036(60)(112), which leadsto the fol- lowing unit-cell data:a 8.290(8),b 10.588(13),c 13.587(15)A, u 124.a7$), p 110 60 (9),I 63.26(9)', arldZ = 2.

Keywords: orlandiite, new mrneral species,selenites, Baccu Locci, Sardinia, Italy

SowleInB

Nous prdsentons ici les caractdristiques de la orlandiite, nouvelle espdce min6rale dont la composition id6ale serait Pb:CL(SeO:).HzO; elle provient de la partie riche en s616niumde la zone d'oxydation de l'ancienne mine de plomb-arsenic de Baccu Locci, situde prds de Villaputzu, en Sardaigne,Italie. Le min6ral est incolore ou blanc, translucide, cassant,ayant un 6clat vitreux h soyeux; il n'6met aucunefluorescence en lumidre ultraviolette. La densitdcalculde est 5.66 g/cm3pour la formule iddale, avecZ=2.Lemin6ralestbir6fringent,avecnmes=flcarc=1.96(t0.05).Unedtude-endifftactionXsurcristaluniqueindiqueune sym6trietriclinique, groupe spatial P1 (no. 21,a 8.136(3),b 8.430(6),c 9.233(7)A, c 62 58(7), B 71.84(4),1 75.13(4)".Les six riies les ptus inlenses du spectre de diffraction ta en A(f)(aOj sont: 4.000(100)(002),3.258(75X121), 3.188(75)(201), 3.8 I 8(55X20 1), 3 731(44)(122) er 2.103(40)(142). La orlandiite se prdsenteen trds petits cristaux incolores h blancs, tabulaires, allongds,jusqu'd 0.1 mm en iongueur, en association avec chalcom6nite, pseudoboldite,angl6site, quartz et autres sdl6nites.La formule chimique a 6t6 ltablie d partir de donn€es obtenues h la microsonde dlectronique et des r6sultats d'une dbauche de la structue cristalline [R final = 0.042, 1226 rdflexions ayant I > 3o(I)]. La structure contient deux atomes ind6pendantsde Pb faisant partie de couchesde composition iddale PbClz; un troisibme atome est ddsordonndentre ces couches,en relation avec un groupe SeO3ddsordonnd et ra moldcule de H2O; dans la plupart des sites occupdspar le Cl, il y a un remplacementpartiel par des groupes OH. Une deuxiBme nouvelle espdce,dtroitement associded la orlandiite et la chalcomdrute,fait pr6sentementI'objet jaune- d'une 6tude; il s'agit d'un autre sdldnite, Pb+CuCI:(SeO:):(OH), qui se prdsente sous forme Q'aggrdgatsde plaquettes citron trds petites. Les six raies les plus intenses du spectrede diffraction de cette phase ld en A(I)(hkI)l sonf 8.279(100X010),

$ E-mail ad.dress:[email protected] 1494 THE CANADIAN MINERALOGIST

6.674(80)(110),11.100(76)(001),1.344('70)(100), s.788(65X101) er 6.036(60X112), ce qur mdne d la maille 6ldmentaire: a 8.290(8),b 10.588(13), c 13.587(15) 4,o124.47(8), B 110.60(9),t 63.26(9)",erZ=2.

(Traduit par la Rddaction)

Mots-clds'.orlandiite, nouvelle espbce mindrale, sdldnites, Baccu Locci, Sardaigne,Italie

INrnooucrroN SSE direction. Such discontinuities were formed as a result of tectonic readjustmentsafter the last episode of The old lead and arsenic mine at Baccu Locci near intrusive activity, and were subsequentlymineralized by Villaputzu, in southeasternSardinia, is one of several hydrothermal solutions. Veins are more or lessfrequent hydrothermal ore deposits hosted in the Paleozoic within fractures, since the distribution of the ore miner- "black shales" occurring in the well-known silver-rich als depositedby the mineralizing fluids was controlled mining zone of Sarrabus-Gerrei. The mine is situated by the mechanical behavior of the host rocks. Such an at the northern border of this region and was exploited origin explains why stockwork-type structures are ob- for about a century until the 1960s,when it was finally served within the porphyries, whereas elongate abandoned.Chalcomenite constitutes an interestins find orebodies can be observed within the shale unit. in the dumps; the quality of the crystals permitte-dthe The tectonic breccia filling the fractures was prob- structure refinement to be carried out on natural samoles ably impregnated by the ores, thereby resulting in an for the first time (Pasero& Perchtazzi 1989;. Our iirst unusual mineralization mainly consisting of galenaand visit to the old underground workings in1996 resulted arsenopyrite in almost equal amounts. Accessory min- in the discovery of exceptional specimens of chalco- erals are sphalerite, chalcopyrite, pyrite and pyrrhotite. menite in some abundance,with crystals up to 2 cm in The deposit is typically zoned,as can easily be observed length, together with other secondary copper minerals in the upper part of the mine, where a surface zone, an (seebelow), in some caseseven in fine specimens,and oxidation zone and a secondaryenrichment zone can be a number of other selenites.In a Se-rich areain the mine, identified. and in close connection with chalcomenite, two new In the oxidation zone of the deposit (upper Santa lead selenitescontaining essential amounts of chlorine Barbara level), there is a notable concentration of sec- have also beendiscovered, one ofthem being orlandiite, ondary minerals, mainly anglesite, amite, brochantite and the other PbaCuCl3(SeO3)3(OH), which is presently and gypsum, with minor linarite, together with subordi- under study. nate amountsof severalother species,such as atacamite, Orlandiite is named in honor of Paolo Orlandi (b. bayldonite, beaverite, carbonate-cyanotrichite,chalco- 1946), AssociateProfessor of Mineralogy at the Univer- alumite, chalcophyllite, chlorargyrite, devilline, oliven- sity of Pisa, who has been very active in determining new ite, osarizawaite, pseudobol6ite, pyromorphite, minerals and mineral occurrences in Italy. The new min- serpierite, smithsonite, spangolite, native sulfur, vana- eral and name have been approved by the IMA Commis- dinite and wulfenite. In addition, there is a local con- sion on New Minerals and Mineral Names (No. 98-O38): centration of selenites, especially chalcomenite; a Type specimens, including the holotype, are deposited rn detailed illustration of these minerals is given in the mineral collection of the Department of Earth Scr- Gramaccioli et al. (1997). ences at the Universitd degli Studi, Milan, Italy. Orlandiite occursin thesechalcomenite-rich concen- trations as very small elongate tabular crystals (up to Occunr.sNce 0.1 mm in length), which are colorless to white, and in places associatedwith lemon-yellow aggregates(up to A description of the Baccu Locci mineral deposit is 0.1 mm in diameter) of platy crystals of the other new given in Zucchetti (1958a,b) and Bakos et at. 1Seg. phase (hereafter referred to as the "second phase"). The ore mainly consistsof galena and arsenopyrite,and These are generally found together with chlorides, such occurs within sedimentary and igneous formations of as green atacamite or very small small dark blue cubes lower Silurian age; these formations are covered by a of pseudobol6ite or, much more rarely, with chlorar- transgressiveformation of Eocene age (Monte Cardiga gyrite of an unusual pale pink color. In the same asso- transgression). The Paleozoic sedimentary rocks are ciation, there are several other selenium oxysalts, all of typically representedby black shales,whereas the igne- them in very minor amounts with respect to chalco- ous rocks are typically representedby porphyries. The menite, which is by far the most abundantselenium-rich unconformity between the Silurian and Eocene sedi- mineral. ments is quite distinct, and can be easily noticed in the Among these oxidized selenium-bearing minerals, landscapeof the nearby mountains. there are elongatebundles of olsacheriteor, much more The Paleozoic terranes are crossed by a series of rarely, mandarinoite, in small spherical aggregatesof subparallel fractures and faults oriented along a NNW- prismatic yellowish greencrystals. At the outermostpart ORLANDIITE, A NEW MINERAL SPECIES, SARDINIA, ITALY 1495 ofthe selenium-rich concentrations,we found attractive HzO and the OH group (3410-3160 and 1586 cm-t), blue crystals of schmiederite, in cases closely resem- and absorptionsat 788 and724 cm-r due to the selenite bling linarite, or francisite in earthy green masses.A group (Ross 1974).For the other new phase,the IR spec- detailed study of all these secondary minerals is in trum recorded from a very small grain using the same progress. In some aspects,the assemblageof selenium instrument and conditions shows sharp absorptions in minerals at Baccu Locci appearsto be rather selective, the OH region (3700-2900 cm-l) and absorptionsat 810 since some of the most widespread selenium oxysalts and 738 cm-1 due to the selenite group. are altogether absent. For instance, notwithstanding a persistent and systematic search,no clinochalcomenite Cupt'rtcal CovrposrrloN has ever been found at the locality, and in spite of the presenceof lead and the seleniteion, no molybdomenite Electron-microprobe analyses (wavelength disper- or kerstenite has yet been identified in this association. sion) of the two phases were performed on polished Similarly, with the exception of a single nodule (1 grain-mounts using an ARL-SEMQ instrument of the cm in diameter) consisting of a solid solution of Italian National ResearchCouncil (C.N.R.) at Centro di clausthalite (PbSe) and galena in almost equal propor- studio per la Geodinamica Alpina e Quatemaria,Milan. tions (PbS = 57 mol7o), no pure selenides have been The standardsand operating conditions are reported in discovered yet in the locality; most of the unoxidized the footnote of Table 1. Because of the very small ore in the assemblageconsists of galena, covellite, and amount of material available, no direct determination arsenopyrite. of the HzO content has been carried out. For orlandiite, the amount of H2O present in the mineral has been de- Pnvsrcar- A\D OPTTcALPnopennss duced from the results of crystal-structure refinement, and for the other new phase,the correspondingamount Orlandiite forms elongatetabular crystals (Figs. lA, has been deducedfrom the OH content basedon charge B), invariably twinned on {010}; they are brittle, white balance. In both cases,the total is very reasonable.In to colorless, with a vitreous to silky luster and a perfect order to overcome volatilization of water and possibly cleavageon {010}. The secondphase appears as very oflead chloride, the electron beam was set at a diameter small aggregates(100 pm in diameter) of lemon-yel- of 15 pm during the analysis ofboth minerals. The re- low platy crystals, up to 10 pm long and 2 pm thick. sulting chemical formula for orlandiite is in good agree- Neither mineral is fluorescent in either short-wave ment with that obtained from crystal-structuredata. The (254 nm) or long-wave (366 nm) ultraviolet light. They only possible check for the other phase, for which the can be easily distinguished from each other using the crystal structure is unknown, was to consider mass and stereo-microscope,and with someexperience, also from charge balance (which needs some OH); the total ls olsacherite, and from much more common minerals closeto 1007o. such as pyromorphite, gypsum, or vanadinite. The measurementof the optical properties of these lead selenitesis very difficult, owing not only to the very TABLE I CIIEMICAL COMPOSMON OF THE NEW LEAD SELEMTES small size of the crystals, but also to their strong reac- FROM BACCU LOCCI. SARDINIA. ITALY tions with all available liquids of high index of refrac- tion. Thesereadily attack the minerals, turning them into W€/o Orlmdiite Ruge Tho- S@ond Raoge Tho- retical nwpmw retical a black opaque residue, thereby preventing any useful observation with a petrographic microscope. The only '1328-15 33 1373 841 8 29-{ 49 8 35 possibility in this respect was to measurethe Brewster cl 1435 Pbo 741E 1173-7549 15 50 6126 5997 12 11 60 80 angle with the optical goniometer; in this way, in spite sd" 1231 tt 9t-12 52 1167 2484 23 90_259t U 67 of the extremely small size of the grains in polished so, o2o 012-037 o14 0 12-016 0 14 mounts, we obtained approximate values of the mean ZtO 0 35 0 15--080 t75 1 69-1 9l 1 74 index of refractionn = 1.96 (t0.05) and 1.83 (10.05), cdo 0 1l 007417 144 1 24-r 55 1 43 424 respectively, for orlandiite and the other new phase.If CuO 0 39 0 20-4 8l 429 40t441 F€O 0.42 0 40-4,43 040 0 38-041 0 40 the values of the calculated densities from the unit cell HrO- 234 220 0 ll 011 and the chemical composition are consideredfor the two LessO=Cl 3 24 310 18E 188 minerals(D"ul" = 5.66 and5.25g/cm3, respectively), the corresponding estimates of the mean indices of refrac- Total 10141 10000 10076 10000 tion using the Gladstone-Dale rule and Mandarino's (1981) constantsand criteria are 1.96 and 1.87,respec- * From the structure deemimtioo ud chuge balmce (relative to OII) s@nd good Numbq ofusllss at the elstrotr microprobe: eiglrt for orlodiite, six for the tively. These are in excellent to agreement with nw phase. Adoating rcltage 20 kV, smple cE@t on brass I 5 nA observedvalues. stmduds: phosgmite (Cl), dglesite (Pb, S), pue m€tals (Se, zq C4 Cq Fe) For orlandiite, the infrared spectrum recorded in Theoretical composition. for orlmdiite md for the second new phe' from tbe mpirical chmicat fomulae Pb,(C1..*OIt,Js(Seq)'EO ud transmission on a very small crystal using an FT-IR (Pb3 ,aCd. rr)x3 sr(Cuo rrZno r"Feo.or)", 'oCl.[(Ser orSo o,)"r.o.Or]3(OH0 ?7Cl0r3), infrared microprobe shows sharp absorptions due to rcspectiwly t496 THE CANADIAN MINERALOGIST

FIG. 1 Orlandiite, tabular elongate crystals on matrix. Photos taken with a Hitachi scan- ning electron microscope. The maximum length of the crystal aggregatesis 150 pm.

Whereas in orlandiite lead is the only cation present X-Rev Dare as an essentialconstituent, other cations, such as copper as well as lead, seemto be essentialin the secondphase. The determination of accurate X-ray crystallographic On crystal-chemical grounds, zinc and cadmium are data of orlandiite was particularly difficult, owing to the very likely substituentsfor copper, and probably also scarcity of suitable material. Single-crystal data show iron. For this reason, all these elements have been the mineral to be triclinic Pl (no. 2), with a8.136(3), b grouped in the proposed chemical formula. For 8.430(6),c 9.233(7)A, a 62.58(7)', B 71.84(4)', 1 orlandiite, minor amounts of these sameelements have 75.13(4)",andZ= 2. Powder-diffractiondata obtained also beenfound; with the possible exception of Cd, they using a Gandolfi camera are reported in Table 2. From are not considered to be likely replacementsfor Pb in thesedata,-a unit cell with a 8.146(12),b 8.428(22),c the sfiucture and have been omitted from the chemical 9.241(22) A, a 62.32(2r), g 7 r.64(17), 1 75.22(19)" can formula. be deduced,in good agreementwith the resultsobtained ORLANDIITE, A NEW MINERAL SPECIES, SARDINIA, ITAIY 149'l

TABLE 2 X-RAYPO\I'DER-DIFFRACTION DATA 8.290(8),b 10.588(13), c 13.587(15)A, a 124.47(8),9 FOR ORT^ANDIITE* I 10.60 (9), 1 63.26(9)", and Z = 2 canbe deduced,and the reflections are indexed on this basis. hkt d*" dd rlI" hkl de, dd vI" (A) (47 A) (A) (A) The single-crystal data for orlandiite were obtained using a Enraf-Nonius CAD4 diffractometer; the crystal '7 001 8 090 965 36 t23 2 365 2368 structure was solved by direct methods and refined us- 101 5 010 4 982 71 124 2298 2298 23 ing 1226 reflections with I > 3o(I); the final R index is 002 4 000 3 983 100 024 2 r73 7 0.042. Full details of the crystal structure will be pub- 201 3 818 3 E13 fi 142 2 to3 2 107 40 122 3 731 44 320 I 989 I 986 u lished in a separatepaper (Demarttn et al., in prep.). The 022 3 548 3 552 1t 223 I 953 1 957 3 structure contains layers with idealized composition t2l 3 258 3 261 75 t44 I 875 I 871 ll 201 3 188 3 lEl 75 l4l | 84'.1 I 845 6 2PbC12and PbSeO:.H2O, respectively, alternatively 113 3 046 3 051 4 322 I 812 1 8li 10 stacked along [100]. Two independent Pb sites belong T22 2489 2,881 6 143 | 760 1 764 9 2728 2728 38 205 I 606 1 608 6 to the PbClz layers and display tricapped-trigonal pris- 003 2 666 2 655 501 | 444 1 445 4 '7 matic coordination, similarly to that observed in a num- 300 2 s64 2 5s5 28 1 441 r 440 ber of other minerals containing lead and chlorine J'1 2 484 2 447 t2 laurionite group, barstowite PbaCl6(CO:).HzO: Merlino et al. (1991,1993,1996)1. * Gmdolf mqa (dimets 1146 m), Ni-filts€d Cu/(s radistim Intmsities oalolated on the bsis ofthe crystalstructre The PbSeO3'!l2O layer displays extensive disorder due to the splitting of the Pb site into two different posi- tions, having a population of about0.75 and 0.25, respec- tively; a corresponding splitting also is observed for the TABI.E 3 X-RAY POWDER-DIFFRACTIONDATA FOR TIIE SECONDPIIASE* selenite groups. Such a statistical disorder can be consid- ered as a random stacking of the PbSeO3.H2O layers in the crystal, with the prevalence of one configuration. doa ddw hhl de dd vL (A) (A) (A) (A) DrscussroN 001 ll 100 lt 104 76 22s 2545 2548 20 010 8 279 I 255 100 5Zt 2 s20 2 519 20 The presence of a variety of secondary selenium 100 1 344 1 338 70 301 2462 2458 20 minerals at Baccu Locci is notable becauseof the rarity ll0 6674 6665 80 524 2440 2445 20 of these minerals in nature. This locality seems to be 112 6 036 6 064 60 031 24tt 24ll 20 l0l 5 7EE 5'182 65 035 2388 2389 20 the richest in selenium oxysalts in the world, after the 022 456E 4537 45 222 2361 2365 20 celebrated El Dragon mine in Bolivia (Grundmann er 4 335 4 309 40 T4s 2331 2330 20 al. 1990, Mandarino 1994). A particular characteristic 102 4 142 4 171 40 343 2297 2298 25 of the Baccu Locci selenium-rich assemblage is the Tl1 4 105 4 tOO 44 T44 2214 2212 20 presenceof a significant number of chloride minerals. 023 3 840 3 839 40 35) ) a<1 a a

AcrNow-goceueN'rs Int. Conf. on Polytypes, Modulated Structures and Quasicrystals,Balatonszeplak, Hungary. We are grateful to the courteous assistance of (1993):Crystal structure of Messrs. Antonio and Raimondo Manca from Lanusei, & - paralaurionite and its OD relationships with laurionite who led us to the Baccu Locci mine; essentialhelp in M ineral. Mag. 57, 323-328. freld collecting was provided by Dr. Paolo Biffi and Mr. Luigi Saibene. Assistance in performing the chemical & _ (1996): Struttura della analysis with the electron microprobe was provided by barstowite PbaClo(CO:).HzO. Plinius 16, 151-152. Dr. Valeria Diella and Mr. Danilo Biondelli of the Centro Studi per la Geodinamica Alpina e Quaternaria PASERo,M. & PERcHIAzzt,N. (1989): Chalcomenite from of the Italian National ResearchCouncil (C.N.R.). Dr. Baccu Locci, Sardinia, Italy: mineral data and structure Lucia Carlucci helped in the acquisition of the IR spec- refinement. Neues Jahrb. Mineral.. Monatsh.. 55 1-556 tra. Drs. R.F. Martin and W.D. Birch provided consid- Ross, S.D. (19'14): The infrared spectra of minerals (V.C erable help in revising the manuscript. We are also Farmer, ed ). Mineral. Soc., Monogr. 4,423-444. particularly indebted to C.N.R. for financial help in the proJect. SsMENovn,T.F., RoznossrvENsKAyA, I.V., Fnerov, S.K. & VERGAsovA,LP. (1992): Crystal structure and physical RBreneNces properties of sophiite, a new mineral. Mineral. Mag. 56, ut-245. Beros, F., CmcnNcru,G., Fnoo.l,S- &M,tizznrrr. A. (1991): The Au-Ag-As-Fe-Cu-Pb-Zn-Sb-W paragenesisof VERGAsovA,L.P., Frl.qrov, S.K., SevpNove, T.F. & BaccuLocci: an exampleof multistageevolution from a FnosoFova,T.M. (1989): Sofiite, Zn2(SeO:)Clz,a new protore of volcanic-sedimentaryoigin. Proc. Zuffar' days mineral of volcanic exhalations. Zap. Vses. Mineral. '88, 9 (abstr.). Obshchest.118,65-69 (in Russ.).

Gnevnccrorr,C.M., Celrposrnnr,I. & Sren-l,P. (1997):Die KRrvovrcHEv. S.V.. Snrr,rBNove.T F.. Ftr-ATov. BaccuLocci Mine (Villaputzu,Cagliari) auf Sardinien. S.K. & Au,q.NIBv,V.V. (1999): Chloromenite, CuqOz ktpis 22(9),13-26. (SeO:)+Clo,a new mineral. Eur. J. Mineral ll, 119-123.

GmrNorureNx,G., LeHRBBncBn,G. & ScuNonruR-Korfl-ER,G. SnNENove. T.F.. Ssuvr,lov. R.R., Fruq.rov, S.K. (1990):The E1 Drag6n mine, Potosi, Bolivia. Mizeral. Rec. & ANemBv, V.V . (1997): Ilinskite, NaCu5O2(SeO3)2C13,a 21,t33-t46. new mineral of volcanic exhalations. Doklady Russ.Acad. Science s. 353. 641- 644. KRrvovrcHEv,S.V., Frlerov, S.K., SerasNov,q,T.F. & RozrnesrvsNsKAve,I.V. (1998):Crystal chemistry of in- Zarr.rsoNlNr,F. & CoNrcr-Io, L. (1925): Sulla presenza di organiccompounds based on chainsof oxocenteredtetra- composti solubi[ de1 selenio e del tellurio tra i prodotti hedra.I. Crystalstructure of chloromenite,CuqOz(SeO:)+ dell'attuale attivitd del Vesuvio. A nn. R. Osserv. Vesuviano C16.Z. Kri stallo g r. 2I3, 645-649. )1

MeNo,qnrNo,J.A. ( 1981 ): The Gladstone-Dalerelationship. ZuccHETrr, S. (1958a): The lead-arsenic-sulfide deposit of IV. The compatibilityconcept and its application.Can. Baccu Locci (Sardinia-Italy). E co n. Ge o l. 53, 867 -876. Mineral. 19,441-450. _ (1958b): Il caratteristico deposito a solfurati di -(1994): Naturaland synthetic selenites and selenates piombo e arsenico di Baccu Locci (Sardegna).Boll. Sent. and their Gladstone-Dalecomoatibilitv. Eur. J. Mineral. GeoI.It. 53,275-3Ol 6,337-349.

Mnnrnvo,S., P.Aseno, M. & PERcHrAzzr,N. (1991):OD-struc- tures amonglead hydroxychlorides:fiedlerite-lA, fied- Received March 26, 1999, revised manuscript accepted No- leite-2M and laurionite-paralaurionite 1n MOSPOQ'9I vember 17, 1999.