MSCC3 3rd MINERAL SCIENCES IN THE CARPATHIANS ...

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Acta Mineralogica-Petrographica, Abstract Series 5, Szeged, 2006Important families of sulphosalt minerals in RomanianoccurrencesThough rare, Bi-based sulphosalt minerals identified inRomanian occurrences are so diverse that to describe them intheir entire complexity would be far beyond the reach of thislecture. Therefore, only the most remarkable examples willbe mentioned and illustrated here.The pavonite homologous series is represented by a familyof crystal structures with two types of slabs parallel to(001): thinner slabs with single metal octahedra alternatingwith pairs of square pyramids [Bi(Pb)S 5 ], and thicker slabscomposed of parallel chains of little deformed metal octahedralin a galena-like pattern. Without significant amounts ofCu, the general formula of the series is Ag (N–1)/2–x Pb 2x Bi (N+7)/2–xS N+5 , where N is the number of octahedra in one octahedralchain across thicker, ‘galena-like’ slabs in the structure, and xis the degree of substitution for Pb (KARUP-MØLLER &MAKOVICKY, 1979).The co-ordination requirements of Ag and Cu, as well asnear-octahedral co-ordination of Bi play a decisive role in thelayout of the series, and control – by still incompletely knownmechanisms – the exsolution phenomena that are typical forpavonite homologues (Fig. 3).Fig. 3: Typical aspects of makovickyite–cupromakovickyite exsolutions from Băiţa Bihor; krupkaite,friedrichite and hodrushite are associated with the pavonite homologues. BSE image.Several occurrences such as those at Băiţa Bihor, ValeaSeacă, Ocna de Fier and Oraviţa are unique in holding anentire range of exsolved or homogeneous Cu-free and Curichpavonite homologues, among which the presence ofhitherto completely unknown 6 th and 9 th homologues and thepresence of mummeite-cupromummeite pair (the 8 th homologue,KARUP-MØLLER & MAKOVICKY, 1979) is assumed.Furthermore, new investigations are expected to clarifythe background of the doubling of the c unit-cell dimensionof cupropavonite (KARUP-MØLLER &MAKOVICKY, 1979), a copper-rich variety of the 5 th homologue,in respect to the c axis of pavonite, and to observe asimilar mechanism for Cu-rich 4 th homologues, which arefrequent in the above mentioned occurrences.Due to their modular structure, the members of thepavonite series are also prone to developing intermediatehomologues with mixed layers of N and N + 1 octahedrawide, already found in synthetic products such asCd 2.8 Bi 8.1 S 15 (CHOE et al., 1997).The junoite-felbertalite-(neyite) series may be interpretedas an accretional homologous series, with the general formulaCu 2 Pb 3N Bi 8 S 3N+13 , where junoite – Cu 2 Pb 6 Bi 8 (S,Se) 16 (a =26.4, b = 4.01, c = 16.6 Å, β = 126.5º, C2/m, Z = 2) is the N =1 and felbertalite – Cu 2 Pb 3 Bi 8 S 19 (a = 27.6, b = 4.05, c =20.7 Å, β = 131.3º, C2/m, Z = 2) is the N = 2 homologue(MAKOVICKY et al., 2001). Both structures posess identical(100) PbS layers, which are two atomic layers thick and whichare periodically sheared, giving rise to tetrahedral Cu sites.The accretion takes place in the kinked and periodicallysheared (111) PbS layers, which are single-octahedron in junoiteand double-octahedron in felbertalite.The crystal structure of neyite – Ag 2 Cu 6 Pb 25 Bi 26 (S,Se) 68(a = 37.5, b = 4.07, c = 43.7 Å, β = 108.8º, C2/m, Z = 2;MAKOVICKY et al., 2001) can be described as an alternationof triple-octahedra (111) PbS -like layers with (100) PbS -likelayers, two atomic planes thick. Both sets of layers aresheared, giving rise to tetrahedral Cu sites, and truncated bycomplex PbS-like layers (001) ney , three atom planes thick.The resulting structure can be described as a supercomplexstructure, built up from three different elements. Neyite doesnot belong to the junoite homologous series, but it relates to itby containing elements of what should correspond to the hypothetical3 th homologue of the stepped layer structure of thejunoite–felbertalite homologous series. A new occurrence ofneyite in association with cosalite, has recently been identifiedat Dognecea (IL<strong>IN</strong>CA & TOPA, 2005, unpublisheddata).Recent investigations revealed the presence of junoite inthe skarn deposits of Băiţa Bihor and of felbertalite at Ocnade Fier and Oraviţa (Fig. 4). Junoite from Băiţa Bihor isamong the very few occurrences departing from the Sebearingarchetype described at Tennant Creek, Australia(MUMME, 1975) and requires new crystal structure determinations.Wider knowledge on the detailed crystal chemistry44www.sci.u-szeged.hu/asvanytan/acta.htm
Acta Mineralogica-Petrographica, Abstract Series 5, Szeged, 2006of the extremely rare felbertalite and neyite is also likely tooccur from these new findings.The same stands for proudite – which has been recentlyidentified in a Se-free form – CuPb 7.5 Bi 9.3 (S) 22 at Ocna deFier, Southwestern Banat, where it associates with cosalite,felbertalite and bismuthinite derivatives (Fig. 4).Fig. 4: Complex sulphosalt assemblage with bismuthinite (bm), gladite (gl), krupkaite (kr), proudite (pr), cosalite(cos) and felbertalite (fb) – Ocna de Fier, Southwestern Banat. White laths are Bi (sulpho-) tellurides(BSE image).Some rare occurrences such as the one at Ciclova, potentiallycontain at least two new sulphosalt minerals withchemical compositions plotting in the felbertalitenordströmiteregion: phase A: ~Cu 0.6 Ag 0.3 Pb 3.1 Bi 5.7 S 12.3 andphase B: ~Cu 0.4 Ag 0.8 Pb 3.4 Bi 5.3 S 11.9 (Fig. 5) and for which,crystal structure determinations are ongoing. Phase B displaysa relic character due to widespread replacement by wittite andbismuthinite derivatives. Due to elevated Ag contents, phaseB might be a false felbertalite plot in a combined Cu + Agcompositional diagram such as the one in Fig. 2.Fig. 5: BSE imaging of Bi-sulphosalt assemblage from Ciclova, Southwestern Banat: unknown phases A andB, wittite (W) and bismuthinite derivatives (bd). White needles are Bi tellurides.www.sci.u-szeged.hu/asvanytan/acta.htm 45
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