THECANADIAN A,TINERALOGIST Joumal of the Mineralogical A.ssociation of Canadb

Volume 12 August 1974 Part 6

Canadian Mineralogist Vol. 12, pp.365-369 (197a)

OBSERVATIO'NSON , AGUILARITE AND NAUMANNITE

W. PETRUK*, D. R.OWENS*E,J. M. STEWART'''!'exp E. J. MURRAY**

ABSTRACT tion, Geological Survey of Canada), and synthe- tic naumannite prepared by Canadian Copper The relationships between acanthite, aguilarite Refiners Limited (Rowland 1962). and naumannite were investigated by studying acan- thite and aguilarite in an ore suite from Guanajua- to, Mexico, naumannite, aguilarite, and acanthite Mops or OccunneNcE,oF THE MTNERATs in a sample from City, Idaho, and a synthetic - naumannite. Results obtained by electron micro- The Ag$ Ag:Se minerals in the Guanajuato probe and r-ray drffraction analyses, and by etch- orei occur as separategrains, 2 to 25O microns ing witb lOVo KCN confirm that naumannite, in diameter (mean 20 microns), disseminated aguilarito and acanthite are distinct mineral species. in dark grey in three different ore types Tho compositional limits suggested for these min- (Petruk & Owens 1974), The minerals could not erals are: acanthite AgzS to AggSo.agSeo.rs;aguilarite be differentiated optically, but they were defined AgrSo.gSer.os to AgaSr.roSeo.eo;and naumannite by etch tests combined with electron microprobe Ag2Se to AgnS6.12See..s. and x-ray diffraction analyses. The results show tlat different minerals of the Ag,S - Ag,Se sys- INrnopucrroN tem occur in samples from different ore types. Type 1 ore, which occurs at the lowest elevation While studying a silver ore from the Guana- above sea level, contains only acanthite (Sample juato area, Mexico (Petruk & Owens L974), Guanajuato H). Type 2 ore, which occurs at an some difficulf was encountered in defining intermediate elevation, contains separate acan- acanthits (AgzS), aguilarite (Ag.SeS), and nau- thite grains (Fig. 1A, Sample Guanajuato Cr), mannite (Ag'Se) on the basis of electron micro- acanthite with small aguilarite inclusions (Fig. probe and .r-ray diffraction data. A literature 18, Sample Guanajuato G), acanthite-aguilarite survey failed to reveal the compositional ranges intergrowths (Fig. 1C, Sample Guanajuato Ar), for these minerals; consequently an investigation and, in vugs, the acanthite with aguilarite in- of them was undertaken by studying acanthite clusions is bordered by naumannite (Fig. 1D). and aguilarite in an ore suite from Guanajuato, Type 3 ore, which occurs at higher elevations, Mexico, naumannite, aguilarite, and asanthite contains separate grains of of acanthite and in a naumannite sample from Silver Ci.ty, Idaho aguilarite (Samples Guanajuato B and Guana- (Sample No. X-40568, National Mineral collec- juato 1). Tho naumannite sample from Silver City, Idaho, consistsof massivenaumannite with agui- *Research Scientist, Mineral Sciences Division, larite lamellae which, in turn, contain acanthite Mines Branch, Department of Energy, Mines and Resources,Ottawa, Canada. inclusions (Fig. 1F). An analysis with image- **Technical Officers, Mineral Sciences Division, analysis equipment (Quantimet) shows that the Mines Branch, Department of Energy, Mines and naumannite encloses l5/o agailaite plus acan- Resour@s, Ottawa, Canada. thite. 365 366 TIIE CANADIAN MINERALOGIST

Pnopsnflns or Acexrurtp, AcurrenrrE, inverts at 776"C to argentite, the high-temper- Awp NeuvreNNrls ature cubic polymorph (Kracek 1946). Nau- mannite is the low-temperature Polymorphs orthorhombic polymorph of Ag,Se and it inverts to a high- Acanthite is the low-temperature monoclinic temperature cubic polymorph at 133'C (Conn polymorph of Ag,S which, on heating, reversibly & Taylor 1969). Aguilarite is the low-temper-

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,'i{S.?O i tr rs:!,f..i.qF* ffi tuEi3 Fro. 1A An acanthite grain embedded in araldite resin and etched I minute with lOVo KCN. The mineral etchesirregularly-.Thenlostinte,nsely.etchedareas are dark grey, the less intensely etched ones light grey, and the pits are black (Sample Guanajuato C1).

Fta. 1B Acanthite with two aguilarite inclusions and a galena grain. The white grain at the top of the field is galena and the weakly etched one dry"lly bllow itls aguilarite. The-acanthite etchel prefer- entially along, a crystallographic direction (NE-SW) so that sorie parts etched deeply (dark grey ro ' " ' black) and other parts appear as narrorv unetched zones (Sample Gianajuato q). Flc. lC,Acanthite-aguilarite intergrofih. The acanthite appears as dark grey irregular spots and the agui- larite is white (Sample Guanajuato Ar). Flc. lD Acanthite-aguilarite intergrofih (irregularly etched) bordered by naumannite (unetched). The unetched grain on the left is galena. Frc. 1E Aguilarite embedded in araldite resin and etched for 1 minute with lOVo KCN. The unetched grain on the left is pyrite (Sa.mple Guanajuato B). Frc. lF Naumannite from- Silver City, Idaho, etched for 15 seconds. The naumannite is unetched, agui- larito" lamellae are partly etched, and the, acanthite inclusions in aguilarite ui" a."prv Lt&.i. rn" acanthite inclusions are outlined la ink to enhance contrast. OBSERVATIONS ON ACANTHITE, AGUILARITE AND NAUMANNITE 367

ature orthorhombic polymorph of AgnSSe,and TABLEI. CHEI4ICILCOI'IPOSITIOII OFACANTIITE, AGUILARITE AND MTJ}IANNITE Main et al. (1972) have shown, by differential -+ 1oC, thermal analysis,that it inverts at l22oC to C1* Acanthite 86.4 12.0 1.6 100.0 2.030.95 0.05 presumably to a high-temperature cubic poly- 2.2 1.93 0.93 0.07 'fhsretlcalGuanajuato H Acanthite 85.4 12.3 99.9 morph. lgf 87,OG12.94 0.00 100.002.00 1.00 0.00 GuanaJuatoC2* Intergwti 83.7 10,6 5.1 99.4 I.97 0.84 0.16 GuanaluatoAr* Intergmth 82,9 9.0 8.5 100.5 1.98 0.72 0.28 Etch tests GuanajuatoG Interawth 80.9 7.4 ll.l 99.4 2.03 0.62 0,38 cuanajuafoB* Aguilarlte 80.7 7.1 13.4 101.2 1.920.53 0.47 15.2 101.2 1.890.51 0.49 'rhOreticalsllver Clty Agullarite 79.6 6.4 A lOVo solution of KCN, applied to the Gua- Aa4ses 79.50 5.91 14.59 100.002.00 0.50 0.50 'l0l.l najuato samples for L minute and to the Silver sllver ciw* Nammlte 74.a 1.3 25.0 I.94 0.ll 0.89 2.000.00 1.00 City sample for 15 seconds,clearly differentiated Ih@retical lSde 73.15 0.00 26.85'100.00 the mineral specie:. Acanthite etched strongly, *r0nly acanthite detsted by a-ray dlffmctlon, but etch sho{s agullarite lncluslo6 in acantltlte(Fig. lB). aguilarite moderately, and naumannite remained **Botll acanthlte and agullarite detsted by c-ray d'tffraction and unetched (Figs. 1A, lD, 1E and 1F). Micro- Dy elolng (r19. rLr. probe analyseson etched grains have significant- ly lower values for silver than were obtained on (Guanajuato A), and synthetic AgzSe prepared the same grains prior to etching, but the sulphur by Canadian Copper Refiners Limited (Row- and values remained approximately land 7962) were obtained with a 57.3 mm De- the same. This is probably due to differential bye-Scherrer camera using Fe-filtered Co radia- dissolution by KCN. tion. The acanthite pattern was indexed on the basisof a monoclinic cell (Swanson et al. L960), and the naumannite pattern using an orthorhom- Compositions ol the minerals bic cell (Weigers 197I) . The aguilarite pattern The mineral compositions were determined was indexed using a naumannite-type cell. A1l by microprobe analysesusing an expandedbeam, cell parameters were refined on the measured 15 to 80 microns in diameter, and a specimen d-values (Table 2) using the PARAM program current of 0.02 amps at 25 kv on a MAC (Ma- of Stewart et al. (1972). terials Analysis Company) microprobe. The ex- pandedbeam and low specimencurrent were used TABLE2. X-MY DIFFMCTTONPATTERNSA rcrc- Acamnr!e urilo- Aqultarlte Mmnntte besauseacanthite dissociatesunder a small beam cllnic (cuanajuatocl) rhonbtc (Guaiduato 82) (Sllver city) and high specimen current. On small grains, how- Indicc a"4.20,r"6.91 Indlcs a-4.33,b"7.09 a"4.31,b-7.02 o"7.88A. ever, the beam size had to be reduced and only 8-99.98' o-7,764 e-7.714 qualitative information could be obtained. Silver Tll 2 3.42 3.43 'l metal was used as a standard for silver, chalco- 'lll 11 i 3.33 3,34 2 3.29 3.31 3 3.06 3.06 stibite (CuSbS,) for sulphur, and synthetic I12 5 2.U 2.U ]02 5 2.88 2.89 3 2.87 2.87 CuSe for selenium. The data were processed 1m 2 2.66 2.65 112 2 2.67.2.68 t0 2.65 2.66 Tzt lo 2.59 2.60 121 2 2.59 2.59 10 2.56 2.57 with the computer program of Rucklidge & 112 7 2.45 2.45 0'13 10 2,43 2.43 4 2.42 2.41 Gaspanini (1969). I03 4 2.9 .2.39 031 3 2.21 2.21 031'l 3 2.23 2.24 2.23 2.22 Electron microprobe analyses on the Guana- l3 2.10 2.10 2.O9 2.09 ';l juato samples show that the acanthite contains r 2.oez.oe 2.08 2.07 211 .l1 2.00 2.00 'l2.00 2.@ l.Z ta 2.2 wt.% Se and 12.0 to 12.3 wl.Vo S, '132123 1.88 1.88 ,87 I .87 1.81 t.8l acanthite-aguilarite intergrowths contain 5.1 to 213 | .7? 1.72 ll.l wt.Vo Se and 7.4 to 70.6 wt.% S. and the 04t 3 I.73 r.73 '|1.74 1.76 ll4 ,71 I .71 aguilaritecontains l3.4wt.% Se and 7.1 wl.Vo 22 1.65 1.65 T41 1.58 1.58 S (Table 1). For the Silver City sample, the I .51 I .61 naumannite contains 25 wt.% Se and 1.3 vtt.Vo 2 1.60 t.60 To5 231 2 1.56 1.56 't.50 S, and aguilarite contains I5.2 w|% Se and 6.4 015 'r,46l.5t 1.52 223 1.50 T34 'I 'tI.46 232 4 1.48 1.48 wt.% S, Qualitative microprobe data confirmed 025 .41 .42 214 r.ir r.ir fi2 2 r.isr.is that the minute inclusions in the Silver City 111111 240 'I 'I aguilarite are acanthite. 044 .3',1 1 .31 1.29 1,4 I ',1''il' '1.231.26 1.26 I .23 243 '1.18t.m 1.20 'll.l8 X-ray diffraction t.l7 .17 160 't.09l.l3 l.l3 X-ray powder diffraction patterns of micro- 0t7 1 1.09 t.09 1.09 probed (Guanajuato ll7 t:9t 1:9t acanthite Cr), aguilarite '1642U I I.01 | .0r 'I 0.98 0.98 (Guanajuato B), naumannite (Silver City), 172 0.96 0.96 0.95 0.95 acanthite with minor aguilarite inclusions (Gua- 0.94 0.93 najuato Cz), acanthite-aguilarite intergrowths iobtalned rlur b/,J EDlroye-scnerer canera, m rao.r le nlet 368 TIIE CANADIAN MINERALOGIST

The r-ray diffraction powder pattern for the aguilarite, however, contains acanthite inclusions acanthite with minor aguilarite inclusions (Sam- which could indicate non-equilibrium conditions ple Guanajuato Cz) did not contain any lines for the aguilarite-acanthite, or possibly non- that could be identified as belonging to the pat- stoichiometry of one or more of the minerals. tern of aguilarite, and was therefore indexed as The presenceof acanthite, acanthite-aguilarite acanthite with a cell of a = 4.23, b : 6.90, c = intergrowths, and aguilarite in the Guanajuato 7.89 A and B = 99.47". The powder pattem for samples suggests that a two-phase field exists the acanthite-aguilarite intergrowths (Sample between acanthite and aguilarite; Our data indi- Guanajuato A) resembledthe aguilarite pattern cate that one-phase acanthite contains up to but could not be indexed until specific lines were 2.2 wt.Vo Se, and that the acanthite-aguilarite attributed to 1.hepresence of acanthite. mixtures contain more than 5,L wt.% Se. There- The x-ray diffraction pattern for synthetic fore, the boundary between acanthite and the AgeSe is the same as that quoted by Berry & two-phase field lies between these values. Kieft Thompson (1962) for naumannite, but Berry & Oen (1973) reported that acanthite from Sa- & Thompson include a line at d : 4.15A which lida, Indonesia, contains up to 4Vo Se, but they we believe may be a grease line. The nauman- do not report having etched it to determine nite pattern of Berry & Thompson gives a cal- whether it is one-phaseacanthite or an acanthite- culated cell of a = 4.33. b = 7.O4. and c : aguilarite intergrowth. Nevertheless, it is pos- 7.7sj-. sible that their material is one-phaseacanthite and that this acanthite has the maximum Se possible Dlscussrox content for the species.Therefore, it is suggestedthat the boundary between acanthite The occurrence of aguilarite lamellae in the and the two-phase field is AgrSo.eoSeo.rs(approx. Silver City naumannite shows that these two 4.7 wt.Vo Se). Similarly, our data indicate that minerals are s.eparatespecies, even though their one-phaseaguilarite contains at least 13.4 wt./o powder patterns can be indexed on essentially Se, and the acanthite-aguilarite intergrowth, a the same orthorhombic cell. It also suggeststhat, maximum of 11.1 w|% Se:,therefore, the boun- for the depositional conditions of this ore, the dary between aguilarite and the two-phase field naumannite has a maximum sulphur content for lies between these values and the composition this species (1.37o S, i.e. AgrSo.uSeo.ee).The AgrSo.s"Seo.n.(Se L3.2Vowt.%o) is suggested.

ocqnihilor Thb study a') . oguilorite,This study qconlhite. G€nth l89l n X oguilorito,Moin et ol 1972 ocorthite?Kisft ond Oen 1973 A + oguilorile,Genth l89l ond 1892 oguihrite-oconthfls intergrowth, This study o @ naumonnile,This study ;1e;$!e-ize:c CUBICAg2 (S,Se) (solid- solutionossumed )

6 6 o o e : nqumonnllo€o o E t g E E It ogullorlte6 t r a E (, I t a o g o t l** 3 o l'i o I aa

A9z$ ATOMIC PER CENT SULPHUR lN Ag2(S,Se) As2s I qnd 2 " @bicA94956 + oguiloritos Frc. 2 Speculative phase diagram for tle low temperature minerals of the AgrS - AgrS" system. OBSERVATIONS ON ACANTIIITE. AGUILARITE AND NAUMANNITE 369

On tle basis of this study, the acanthite com- KrsFr, C. & OrN, I. S. (1973): Ore minerals in the position varies from AgrS to AgzSo.esSeo.rr,the telluride-bearing gold-silver ores of Salida, Indo- aguilarilo composition from, perhaps, AgrS[.ss nesia, with special reference to the distribution Ser.osto about AgaSr.roSgo,goand the naumannite of selenium. Iulineralium Deposita, 8, 312-320. Knecnr, F. E. (1946)l Phase relations in the sys- composition from AguSe to about AgzSo.r"Seo.aa. tem -silver and the transition in silver sul- A speculative phase diagram for the ApS-Ag,Se fides. Trans. Amer, Geophysics Union 27' 36q-, system below 200"C, as interpreted from the 374. results of this study and from data reported in MArN, J. V., Roocnns, K. A., Koor, H. W. & Woop, the literature, is given in Figure 2. C. P. (1972): Aguilarite from Camoola reef, Maratoto Valley, New Zealand, Mineral, Iulag. 38, 96r-964. AcKNoWLEDGEMENTS Prrnur, W. & OwrNs, D, (1974)z Some mineral- ogical characteristics of the silver deposits in the The gratefully authors acknowledge H. R. Guanajuato mining district, Mexico. Econ, Geol. Steacy, Geological Survey of Canada for sup- (in press). plying the Silver City sample and W. H. Gross, Rowraxo, L F, (1962): X-ray diffraction investi- Puro Silver Mines l-imited, for the Guanajuato gation of samples of anode slimes and related samples. Thanks are extended to D. C. Harris materials submitted by Canadian Copper Refin- and L. J. Cabri (Mines Branch) and S. D. Kissin ers Limited, Montreal, Qnebec. Dept, Iulines (Post-Doctoral Fellow, Mines Branch) for dis- Tech. Surv. Mines Branch Investig. Rept. rR 62-95. cussionson phase relations, but the authors take Rucrrtlce, J. & GlsrennrNr, E. L. (1969): Elec- full responsibility for the contents and ideas tron microprobe data reduction (EMPADR VII). expressed in this paper. Dept. Geol. Univ. Toronto. Srnwenr, J. M., Knucr,n, G. J., Ovruox, II. L., (1972): RIFERENcES DIcKNsoN,C. & Her,L, S. R. The.r-ray systemof crystallographicprograms. Univ. Mary- Brnnl L. G. & THoMpso\ R. M. (1962)t X-ray land Computer ,Sci. Center, ..Tech. Rept. powder data for ore minerals. Geol, Soc, Amer, TR-192 Mem. 85,33-35. SwANsoN,H. E., Coor, M. I., EvANs,E. H. & ps CoNr.r, J. B. & Tevr-on, R. C. (1960): Thermoelec- Gnoot, J. (1960): Standard r-ray diffraction tric and crystallographic properties of Ag:Se. powder patterns: silver sulfide (argentite) AgrS lour. Electrochem. Soc. 107, 977-982. (monoclinic). N.B.S. Circ. 539, t0, 6L-62. GENTE, F. A. (1891): Contribution f6 mine13l6gy, Warcens,G. A. (1971): The crystal structuro of no. 51; Amer. J. Sci. 4L, 40L-402. the low-temperature form of silver selenide. (1892): Contribution to mineralogy, Amer. Mineral.56, 1882-1888. no. 54. Amer. l. Sci. 44. 38I-382. Manuscriptreceived April 1974