Balkanite, Gurag,Hgsr, a New Lvlineral from the Sedmochlslehitsi

American Mineralogist, Volume 58,pages 1I-15, 1973

Balkanite,GurAg,HgSr, A NewlVlineral from the SedmochlslehitsiMine, Bulgaria

Vasn A. AraNessov.eNo GponcrN. Krnov Higher Institute ol Mining and Geology,Sofin, Bulgaria; Depattmentol M ineralogy and Crystallography, IJ niu er sity' KI. Ohrids ki', Sofia, Bulgaria

Abstract

79.4-91.6 kg/m:,u* (abottt 31/zon Mohs' scale).

Introduction a) A tennnntite-chalcopyriteassociati'on, which is middle levels of the deposit' Balkanite was found in 196l-1962 in six samples characteristic of the are the main ore min- ranging in size from 1 to 4 cm, originating from the Tennantite and chalcopyrite Sedmochislenitsimine, Vratsa district, in the western part of the Stara Planina, Bulgaria. Most of these samplesare, in fact, polished sections.As an un- known silver mineral it was reported by Stoinov, is parageneticallyconnected with this as- Atanassov and Lilov (1964), while some of the sulfosalts, are pyragyrite' pyrostilpnite, stepha- morerecent papers by Atanassov(1'970, l97l) con- sociation;these pearseite, acanthite, jalpaite and tain data on its chemical composition.To date, bal- nite, polybasite, of native silver. kanite is the only known sulfideof copper, silver and minor amounts association,wherein the mercury, either as a mineral or as a syntheticprod- b) A tennantite-bornite are tennantite,bornite, chalcopy- uct. It was approved as a new mineral and mineral main ore minerals primary chalcocite,is characteristicof the name by the Commissionof New Minerals and Min- rite and It is accompaniedby another well- eral Names, IMA, in April I97l' The suggested lowest levels. mineral association:stromeyerite, mac- name balkanite stemsfrom the medieval and popu- definedsilver comparativelylarge amountsof native lar name of the Stara Planina, the main mountain kinstryite and is usually mercury-bearing.The mer- range giving its name to the Balkan Peninsula. silver which cury content in mercurian silver varies over a wide Description of the TYPeLocalitY range evenwithin a singlemineral grain (Atanassov, The Sedmochislenitsistratiform deposit results 1969). The silver from samplescontaining cinna- from the hydrothermal metasomatism developed barite and balkanite shows a higher and more con- no around a systemof tectonic faults cutting limestones stant mercury content. It should be noted that and dolomitesof Middle Triassic age.Two types of silver sulfosalts are found in the tennantite-bornite industrial ores are mined in the deposit, lead-zinc association. para- ores in the upper levels and copper ores at greater Balkanite is a rare mineral in the mineral depths. Consid-erabledifferences in mineral *-po- genesis of the deposit and occurs only in the sition exist among the copper mineralizationsfrom tennantite-borniteassociation in samplestaken from the middle and lower levels. Two clear-cut mineral ore body 2 at the 890 meter level. The associated associationsare observed: minerals are: tennantite' bornite' chalcocite' mer-

ll 12 ATANASSOV AND KIROV

Frc. 3. Grained balkanite with polysynthetic twins. Frc. 1. Lamellar balkanite crystals. The elongation of the Crosded nicols. Magnification 330 X. lamellae is parallel to the c-axis. Crossed nicols. Magnifica. tion 340 X. synthesiswas attemptedon the basisof the chemical curian silver,chalcopyrite, cinnabarite, stromeyerite, analysisof the natural material.The synthesiswas mackinstryite,wittichenite, native bismuth, rammels- achievedin evacuatedquartz tubes from a stoichio- bergite,djurleite, digenite, pyrite, calcite,barite and metric mix of Cu2S,HgS, Ag and S, heatedfor 48 aragonite. hoursat 550'C and furnacecooled. A comparatively coarse crystalline material was obtained while very Type Material small crystalswere formed at the top of the tube. The samplesin which balkanite was found are The physical properties and X-ray powder patterns kept in the Mineralogical Museum of the Higher of the naturalmaterial and the syntheticproduct are Institute of Mining and Geology,Sofia (Specimen identical. Under the microscopeminor amounts of No. 619) and in the Museumof the Univeriity'KI, chalcocite,djurleite and covellite are observedin the Ohridski',Sofia (Specimen No. 1351). syntheticmaterial.

Synthesis Morphology and Physical Properties The very small amount of available natural ma- Balkaniteoccurs as grainsup to 2-3 mm, in or terial was insufficientfor a detailed studv. Therefore around small fissures and rock cavities. Rod-like prismatic balkanite crystalsfrom 0.01 to 0.2 mm were found in druseJike cavities overlying barite, chalcocite, bornite and mercurian silver, and cov- ered by calcite and aragonite. The balkanite crystals are elongated along the c-axis and have an almost isometric cross-section. They are boundedby prismaticfaces striated parallel to the elongationand by a flat bipyramid with bright faces. Goniometer 'owing measurementswere impossible, to the small size of the crystals and the striation of the prismaticfaces. In polished sections the balkanite crystals are euhedral to subhedral and are almost always twinned (Figs. 1, 2), consistingof two or three lamella systems which are visible both with and FrG. 2, Balkanite lamellae along several directions. The without analyzer,owing to the strong anisotropy of larger lamellae are internally laminated. Crossed nicols. the mineral.The largerlamellae are sometimesmade Magnification 330 X. up of smaller ones (Fig. 2). Balkanite aggregates BALKANITE 13

TASLE 1. REPLECTIVITIES OF BALIONITE AND ASSOCIATED MINERALS

l.lavelengths (B) 620 Mlneral 435 500 520 540 33.4 32.8 32.4 32.L 3r.8 31.5 Natural Rs' 35.2 Jf.4 35.6 35.4 35.0 34.0 34.0 26.5 16.0 25.6 25.L 24.9 Balkanlte Rp' 30.4 30.6 31.0 31.0 30.2 38.9 28.0 27.2 31.8 3L.4 3r.0 30.7 30' 5 Synthetic K8' 34,0 34.2 34.9 34.4 33.6 33.0 32.4 25,4 25,O 24.7 24.5 24.4 24.3 Balkanite Rp' 27.5 27.7 28.4 28.0 27.4 26.7 25.9 31.9 31.4 30.7' 30.2 29'A Mackin- Kg' 39.2 38.9 37.7 36.1 5).4 34.2 33.3 32.6 27.4 27.O 26.5 26,2 26.0 25.7 st ry lte Rp' 31.1 3L,2 31.1 30.5 29.6 28.8 28.O 2g.5 29,O 28.6 28'4 28.2 28'0 S troBey- Rs' 39,5 39.3 31.6 35.2 33.1 31,4 30.3 26.8 26.5 26.4 26.2 26.2 26'L erLte Rp' 30.3 30.0 29.3 2E.7 28.2 27.7 27,2 32,2 31.3 30.8 30.2 29.9 29.6 Chalco- Rs' 36.8 36.7 36.3 35.E 35.I 34.1 33.1 3L.7 31.0 30.2 29.4 28.8 28'4 clte Rp' 36.4 36.2 36.0 35.3 34.8 33.8 32.5 36.4 36.0 35.8 35.5 35'r 34.7 34'4 lJlttlche- Rg' J). ) 3s.5 35.9 36.4 36.5 36.5 32.7 32.6 32.4 32,L 31.8 3L'4 nite Rp' 32.4 32.3 3r.5 31.7 32,O 32.4 32.7 3o.o 29.7 29.2 28.8 28.5 28'2 Tennant 1 te R 30.4 30.5 30.E 30.9 3r.o 30.5 30.2 l,letalltc St 35.8 35.0 34,7 34.4 R 43.5 43.O 4L.2 39.9 38.9 38.9 37,1 36,4 (standard)

consistingof graphically and subgraphicallytwinned Diagnostic etch reactions are also useful in dif- grainsare also observed(Fig. 3). Isolatedlamellae ferentiating balkanite from the associatedminerals. and systemsof lenticular balkanite exsolutionswere Balkanite reacts with 20 percent KCN, 5 percent occasionally observed in chalcocite grains (Figs. HgCl2 and 2O percent FeCl2 and does not react 4,5). with 1:1 l{NOr 1:1 HCI and 40 percentKOH. Balkanite is steel-gray with metallic luster. In color, luster, striation and crystal habit it is almost X-Ray and Thermal Data indistinguishablefrom chalcocite and stromeyerite. powder patterns of the synthetic and No cleavagewas observed.Density (6.318 g/cm") The X-ray (Table 2) differ substantiallyfrom was measuredonly on synthetic balkanite because natural material mineral speciesand cannot of the small amount of natural mineral available. those of other known copper, silver and The Vickers microhardness,determined from 20 be interpreted as a mixture of measurementson natural material and from 10 on mercury minerals. oscillation photographsof a syntheticmaterial was 79.4-9 I.6 kg/mm" and 82.4- Laue, rotation and showedthat the mineral is 93.4 kg/mm'? respectively(about 3.5 on Mohso 0.15 mm long crystal scale). The measurernentswere carried out with a PMT-3 equipmentusing a load of 10 g. Under the sameconditions a value of 20.5 kg/mm' was found for NaCl. Under the microscopebalkanite is white-gray. It differs from chalcocite and djurleite by its strong anisotropy, but its polishing hardnessis about the same.From stromeyeriteit difiers by its higher relief and the lack of a violet hue; frorn tennantite by its lower relief and anisotropy; and from wittichenite by its bluish hue and stronger anisotropy.Balkanite can be distinguishedfrom mackinstryite only after instrumental measurementof the dispersion of re- flectivity over a sufficiently large interval of the visible light spectrum. In Table 1, the reflectivitiesof natural and synthetic balkanite are comparedwith optically-similar Frc. 4. Balkanite lamellae (light gray and dark gray) in mineralsfrom the samedeposit. chalcocite (gray). Crossed nicols. Magnification 300 y'. l4 ATANASSOV AND KIROV

idealizedformula is CueAgHgSs.As comparedwith the atomic ratios in Table 3, such a formula exagger- ates both the copper and silver content. The calcu- lated density for this formula would be 6.68, which is considerablylarger than the observedvalue. On the other hand, a composition CueAgHgSs has a calculateddensity of 6.41, which is closer to the experimentalvalue, but it would hardly be realistic to assumesuch atomic ratios in view of the analy- tical results.

d."1". Synthetlc Natural hkl I balksnite balkanlte d-oeae.8 d-ueas.R r

L20 4.30 4.29L 4.30 I Fro. 5. Micrograph in backscattered HgLa radiation. 00r 3.9r 3.900 3.88 2 Balkanite lamellae in chalcocite. r01 3.675 3.654 5 3.66 4

220 l. )Lz 3.50 5 111 3.440 3.44 4 orthorhombic with cell dimensions4 = 10.62 ! 310 3.310 3.302 4 3.32 2 = t = *.0.04 20L 3.L52 3.160 6 O.O2A, b 9.42 A.O2A, c 3.92 A. 3.15 6 The spacegroup is P222,Pmm2, or Pmnwn.Density, 030 3.140 3.L43 7 as calculatedfrom the two chemical analysesand a o2L 3,0r2 3.008 8 3.09 7 130 3.0r0 cell volumeof 392.14", is 6.421.and 6.497 g/cm". 2.984 10 2,98 10 2LL 2,989 The density (6.32 glcms) of synthetic balkanite is 320 2,829 2.826 7 2.81 5 slightly lower, perhaps becauseof chalcocite, djur- 230 2,704 2.702 8 ,co7 leite, covellite possibly, 301 2.627 2,625 and, of air bubbles. 2.6L 8 The DTA curve of synthetic balkanite displays 22L 2.620 2.674 9 endothermicpeaks at 90, I4O, 45O and 69OoC, 4r0 2.555 2. 550 10 2.55 10 the first and the third being relatively weaker. All 3r1 2.530 2,529 5 2.53 4 031 2.452 2.464 8 2.45 8 effectsare reversibleon the cooling curve. peak The 330 2.349 2.350 2.35 3 at 690'C should be attributed to the melting of the 420 2.3r2 2,309 4o 2,3L 2b sample,while the remaining ones are most probably 240 2.L53 2,L55 I 2.r5 I due to polymorphic transitions. 4LL 2.740 2.r40 2.r4 1 The DTA curve recorded in air also shows the 510 2.072 2.069 7 2.O5 5 endothermic peaks at 90 and 1,40"C, but after 430 2,O21 2.027 2.02 1 42r 1.992 1.991 8 1.985 8 350'C a very strong and broad exothermic efiect 340 I. 96I L.962 9 1.95s 9 appears,connected with the oxidation of the sample. 002 1.960 511 1,832 1. 831 r.825 2 43L 1. 801 r.802 L.799 2 Chemical Composition and Fonnula 600 r.770 L.765 5 t.764 5 The chemicalcomposition of balkanite was deter- 440 r.762 mined with a Cameca MS 46 electron microprobe 24L 1.620 L.62L 0.5 L.62T I 232 1.589 r.586 I 1.581 I in the Laboratory for Experimental Mineralogy of 630 I.542 t.542 L.537 3 the SiberianDepartment of the Acadernyof Sciences 242 L.449 L.449 I 1.450 I of the USSR, Novosibirsk. The results of two full 161 L.444 L.444 0.5 1.440 0.5 analysesare summarizedin Table 3. 640 I. CIJ I.4I5 I L.420 I

Table 3 includesalso the resultsof the calculation gpef4tlng condltlong - Synthetlc balkanlte: Dlffractoaeter tlace, filtered copper radlatioir. Natural balkanlte: powder photograph, of the analyseson the basisof eight S atoms,which 57.3 o Debye Scherrer ceera. flltered copper radlatLon. correspondsalmost exactly to Z = 1. The suggested Expoaure 5 hourg, BALKANITE 15

TABLE 3. CHEUICAL COMPOSITIONOF BALKANI?E FROM SEDIiOCHISLENITSI as yet concerningthe participation of mercury and 81@ent6 ttt.Z I Atoolc ratio ttt.Z II Atonic ret{o its influenceon the phaserelationships in theAg2S- Cu 8.78 37.L 8,76 Cu2Ssystems. At 4,58 4.69 The lamellar structure of most balkanite grains Rg 14.0 1,04 13.8 1.04 the s 17.2 8.00 16,9 8.00 indicatestransformation twinning. Furthermore, Total 101. 7 IOI.I internal lamellaewithin the larger lamellaemay well

Operatiog condltlono atrd standarde mark a second phase transition upon temperature CuKo, 30 kV, l0 EA - netural chalcopyrlte decrease.The DTA curvesupon heatingand cooling A8La, 15 kV, 30 il - sFthetlc oiargyrlte and el@eotary silver aswell asthe parageneticrelationships in the deposits E8Ko, 30 kV, I0 oA - synthetic cinnabarlte show that thesephase transitions occur at relatively SKd, 15 kV, 30 oA - synthetlc stlbnlte low temperaturesbetween 90 and 150'C. Absorptlon correctioaa to sprl (L966) Acknowledgments Origin The writers wish to expresstheir appreciationto Yu. G. Balkanite is a result of low-temperaturehydro- Lavrentiev of the Geological Institute of the Siberian De- thermal activity. Together with chalcocite,djurleite, partment of the Academy of Sciencesof the USSR for the The assistanceof Professor digenite, stromeyerite,wittichenite, silver and mer- electron microprobe analyses. M. S. Bessmertnayaof IMGRE, Moscow, in determining curian silver. it is one of the last-formedminerals of the optical properties is gratefully acknowledged. The the deposit.Balkanite and chalcocitevery frequently synthesiswas performed by I. Vesselinov;V. Pavluchenko replace tennantite and bornite, thus forming zonal helped in the single crystal X-ray measurements.Thanks aggegates with an inner zone consisting of inti- are also due to Professor I. Kostov for fruitful discussion N. Poulieft for the English mately mixed balkanite and chalcocite grains to of the manuscript, and to C. up translation. 2-3 nm in size and an outer zone which is relatively coarser grained. The replacementof bornite by bal- References kanite and chalcocite is connected also with the (1969) formation of chalcopyrite.FlameJike exsolutionsof AuNlssov, V. A. On the natrlral amalgams of silver. C. R. Acad. Bulg. Sci. 22, 577-580 lin English]. chalcopyrite are observedaround balkanite veinlets (1970) Cinnabar in the SedmochislenitsiMine, dis- cutting bornite grains.Such exsolutionsof chalcopy- trict of Vratsa. Annu. Ecole Sup. Mines GeoL 14, 7-13 rite in bornite also occur around chalcociteand strG. [in Bulgarian, English summary]. meyerite veinlets (Stoinov, Atanassov, and Lilov, (1971) Silver-amalgam from the Sedmochislenitsi 1964). ore deposit, district of Vratsa. Reu. Bulg. Geol. Soc. 32, 105-1.13[in Bulgarian, English summary]. The balkanite lamellae in chalcocite are also of STINNEn,B. J. (1966) The system Cu-Ag-S. Econ. Geol. interest (,Figs.4 and 5). Evidently,they constitute 61, l-26. exsolutions of balkanite formed upon temperature SeRrNcen,S. (1966) Die Korrektur des Ordungszahlef- decrease from a high-temperature polymorph of fekts bei der Elektronenstrahl-Mikroanalyse. Neues I. Mineral. Mh. ll3. chalcocitein which mercury and silver may be pres- SrorNov, S. C., V. A. Ar.rNlssov, exo T. I. Lrov (1964) ent in larger amounts.Very possiblythis is the high- On the silver minerals from the Sedmochislenitsi ore de- temperaturehexagonal phase of chalcocite(Skinner, posit, Vratsa region. Recueil en I'honneur de I'Acad- 1966) which, at temperaturesabove 103.5oC,is emicien L S. Ioutcheo.Sofia. p. 643. stable for all compositions along the chalcocite- stromeyerite join. The Cu/ Ag ratio in balkanite Manuscript receiued April 10, 1972; accepted for favors such an explanation. No data are available publication, September 7, 1972.