© Author(s) 2022. CC Atribution 4.0 License Pyrite with lower cubic symmetry from Lavrion, Greece Pirit z nižjo kubično simetrijo iz Lavriona, Grčija Mirjan ŽORŽ1, Panagiotis VOUDOURIS2 & Branko RIECK3 1Prešernova 53, SI-1290 Grosuplje, Slovenia; e-mail: zorz@siol.net 2National and Kapodistrian University of Athens, Faculty of Geology & Geoenvironment, Department of Mineralogy and Petrology, University Campus-Zografou, 15784, Athens, Greece; e-mail: voudouris@geol.uoa.gr 3Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Wien, Austria; e-mail: rieckb49@univie.ac.at Prejeto / Received 4. 10. 2021; Sprejeto / Accepted 15. 7. 2022; Objavljeno na spletu / Published online 22. 7. 2022 Key words: Lavrion, pyrite, morphology, tetrahedral crystals, twins, point group 23, crystal structure Ključne besede: Lavrion, pirit, morfologija, tetraedrski kristali, dvojčki, točkovna skupina 23, kristalna struktura Abstract In this study, we examined the morphological, chemical, and structural details of tetrahedral pyrite crystals from the Jean Baptiste mine in Lavrion, Greece. Pyrite occurs in three generations. Tetrahedral crystals of the first generation are left- or right-handed with the lowest cubic 23 symmetry. In this generation, there are twins with higher cubic m _ 3 and hexagonal 6 symmetry. All crystals of the second generation are primarily interpenetrated into twins with a cubic _ 4m3 symmetry. Some, however, continue to twin up to crystals with the highest cubic m _ 3m and hexagonal 6mm symmetry. Third-generation crystals overgrow second-generation crystals in a non-oriented manner. Chemical analysis confirms chemically pure pyrite, and single-crystal X-ray analysis of the first- and the second-generation crystals confirms the pyrite-specific m _ 3 symmetry. The morphology of the single crystals and twins indicates that first generation of single pyrite crystals should have the lowest cubic 23 symmetry, which is not confirmed by the structural analysis. This discrepancy may be due to changed pT conditions and the consequent transformation of the original pyrite structure with symmetry 23 into a secondary structure with m _ 3 symmetry, or to suboptimal conditions in determining the structure by X-ray diffraction. Izvleček V tej študiji smo preučili morfološke, kemijske in strukturne podrobnosti tetraedrsko oblikovanih kristalov pirita iz rudnika Jean Baptiste v Lavrionu. Pirit se pojavlja v treh generacijah. Prvo predstavljajo tetraedrski levo oziroma desno sučni kristali z najnižjo kubično 23 simetrijo. V tej generaciji so dvojčki z višjo kubično m _ 3 in heksagonalno 6 simetrijo. Vsi kristali druge generacije so že primarno zdvojčeni do kubične _ 4m3 simetrije. Nekateri pa se dvojčijo še naprej dokler ne dosežejo najvišje kubične m _ 3m ali heksagonalne 6mm simetrije. Kristali tretje generacije neorientirano prekrivajo kristale druge generacije. Kemijska analiza potrjuje kemijsko čist pirit, monokristalna rentgenska analiza kristalov prve in druge generacije pa za pirit značilno m _ 3 simetrijo. Očitno je torej, da morfološke oblike posameznih kristalov in dvojčkov kažejo na to, da imajo najnižjo kubično simetrijo, česar pa strukturna analiza ne potrjuje. Ta diskrepanca je lahko posledica spremenjenih pT pogojev in posledične transformacije prvotne strukture pirita s simetrijo 23 v sekundarno strukturo z m _ 3 simetrijo ali pa neoptimalnih pogojev pri določitvi strukture z rentgensko difrakcijo. GEOLOGIJA 65/1, 5-19, Ljubljana 2022 https://doi.org/10.5474/geologija.2022.001 ering an outcrop area of about 150 km2 (Fig. 1a). The Lavrion mines were operated almost contin- uously from the 4th millennium BC until the Late Roman period, and then until late 20th century (Conophagos, 1980; Morin & Photiades, 2012). The Lavrion ore district hosts a carbonate-re- placement Pb-Zn-Ag-Au deposit (Marinos & Introduction Lavrion ore district, located about 50 km southeast of Athens (Greece), is famous for ex- ploitation of silver-rich lead ore during ancient times (Marinos & Petrascheck, 1956). The district includes two extensive mining centers, Plaka and Kamariza, as well as several smaller ones, cov- 6 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK Fig. 1. (a) Simplified geological map of the Lavrion ore district (from Marinos and Petraschek 1956, Scheffer et al. 2016 and modified after Voudouris et al. 2021); (b) Cross-section A-A’ of the Kamariza deposit (see geological map, figure 1a) (from Marinos and Petraschek 1956, modified after Voudouris et al. 2008). Petraschek, 1956). It is well established that in addition to the carbonate-replacement style ores, four types of mineralization occur at Lavrion district: porphyry-type Mo-W ores in granodio- rite, Cu-Fe-skarns, and Pb-Zn-Ag bearing brec- cias, and Pb-Zn-Ag-Au rich veins (Skarpelis, 2007; Voudouris et al., 2008; Bonsall et al., 2011; Scheffer et al., 2017, 2019). The ore deposits in Lavrion are structurally and lithologically con- trolled, and ore formation occurred under ex- tensional kinematic conditions (Skarpelis, 2007; Berger et al., 2013; Scheffer et al., 2017, 2019). 7Pyrite with lower cubic symmetry from Lavrion, Greece Carbonate-replacement Pb-Zn-Ag±Au deposits at Kamariza are located in the central part of the district (Fig. 1a; Voudouris et al., 2008). The carbonate-replacement mineralization occurs in the form of stratabound massive sulfide replace- ment bodies (mantos) and chimneys, crosscut- ting with respect to layering in the host marble (Skarpelis, 2007; Voudouris et al., 2008; Bonsall et al., 2011, Scheffer et al., 2017 and 2019; Fig. 1b). Stratabound massive sulfide bodies (mantos) at Kamariza occur within the marbles as well as along the contacts of rocks with different per- meabilities (marbles and schists). Ore deposition took place mainly from high-T magmatic fluids during the transitional ductile/brittle and brittle deformation stage of the host rocks (Bonsall et al., 2011; Scheffer et al., 2017, 2019). Beneath the individual manto orebodies, the rocks are cut by N-S to NE-SW, and NW-SE to E-W trending and steeply dipping veins that follow faults and fis- sures. The veins are usually zoned and brecciat- ed, and in the breccias, the fragments are cement- ed by a sulfide-rich matrix. Pb-Zn-Ag veins are generally found below but also above the Lavrion detachment in the marbles of the Lavrion unit, in the Upper marble, in the Kamariza schists and at the interface between the Kamariza schists and the Lower marble. N-S trending veins crosscut the “Subordonnés” formation in the Jean Baptiste deposit (Fig. 1b). The veins are thought to be flu- id pathways and feeder zones for the stratabound mineralization. The vein-style deposits were formed as the rock entered the brittle regime, by mixed seawater and meteoric fluids (Bonsall et al., 2011; Scheffer et al., 2017, 2019). The Jean Baptiste mine is located in the northwest area of the Kamariza district (Figs. 1a and b). Its min- eralization consists of several vertical veins that have been the target of modern mining on the first level (and above) of the mine in the begin- ning of 20th century. The veins show ore zoning, whereby the crystallized minerals are present in the veins and in horizontal fissures. Tetrahedral pyrite crystals were found by chance in 2001 in a detached chamber within the Jean Baptiste mine that used to be a small ore prospection. It was obviously of no significant commercial value and consequently abandoned. Euhedral tetrahedral pyrite crystals grew within the banded marble and in the open ore veins hosting crystallized ar- senopyrite, sphalerite, galena, calcite, dolomite, aragonite, and quartz. Pyrite is the most widespread sulfide miner- al that occurs in a wide range of morphological shapes. It comes in the form of single crystals belonging to cubic m _ 3 point group (symmetry further in text). The most frequent crystallo- graphic forms are cube {100}, octahedron {111}, and pentagon dodecahedron, i.e. pyritohedron {210} that dictate the basic crystal morphology, which is further modified by many accessorial crystallographic forms. Distorted, i.e. elongated single pyrite crystals are common appearances. Their symmetry depends on elongation direction. If they are elongated in (111)- and (100)-direc- tion they develop 3-fold form with morphologic _ 3m symmetry and 2-fold form with morphologic mmm symmetry, respectively. All other orienta- tions yield elongated crystals with 1-fold mor- phologic symmetry. Situation changes if the crys- tals are attached to a matrix. If they are attached with their [111]- and [110]-axis they develop chi- ral morphologic 3 and 2 symmetry, respectively. All other attachment possibilities result in chi- ral morphologic 1 symmetry, with a single ex- ception of [001]-axis attachment that results in a non-chiral mm2 morphology (Žorž, 2019). Pyrite basic m _ 3 symmetry does not allow the existence of tetrahedral crystals. If this is the case then the octahedron {111} transforms either to posi- tive t{111} or to negative _ t{ _ 111} tetrahedrons with the resulting lower cubic _ 4m3 or 23 symmetries. Morphologic distortion of octahedral crystal morphology that would lead to a predominance of four octahedron faces and consequently to a tetrahedral crystal shape is not known. Pyrite is often associated with other miner- als in epitactic, i.e. in oriented growth relation- ships. Of them, the most frequent is epitaxy of pyrite on marcasite and vice versa. Richards et al. (1995) reported on oriented growth of single pyrite crystals with their {001} face on {010} face of a single marcasite crystal. In this case, pyrite crystals have two orientations with respect to the marcasite {010} face. The reason for that lies in non-alignment of 2-fold axes between pyrite in marcasite. Brock and Slater (1978) described another epitactic relationship, where marcasite (101)-twins grow on {001} face of a single py- rite crystal, whereby their (101)-twin planes are oriented perpendicularly with respect to pyrite {001} face. This time the 2-fold axes of both min- erals coincide, which results in a single orienta- tion of twinned marcasite on pyrite. In these two cases, the epitactic relationship was ascribed to alignment of Fe-S chains in the structures. Gait and Dumka (1986), and Gait et al. (1990) pub- lished the case of single pyrite crystals growth on a cyclic (101)-twinned marcasite. Miklavič et al. (2006) reported on oriented pyrite growth along 8 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK the twinning planes of the (101)-twinned marca- site. Orientation of pyrite with respect to marca- site and vice versa is in these two cases the same and results in a single pyrite orientation. Orient- ed growths of pyrite on arsenopyrite and pyrite on pyrrhotite were described by Zebec (2012). Py- rite is attached with its {001} face to {001} face of arsenopyrite. Non-alignment of 2-fold axes of both minerals again requires two different ori- entations of pyrite, which is observed on speci- mens from Trepča. Pyrite is attached with its {001} face to {001} face of pyrrhotite. Octahedral {111} face of pyrite has a chiral trifold symmetry without any mirror plane, whereas the pyrrho- tite {001} face has a 6-fold symmetry with 6 mir- ror planes. For that reason pyrite can have six different orientations on the pyrrhotite that are rotated by 60°, of which four are spatially equiv- alent. The remaining two are rotated by 180°with respect to each other. Chiral symmetry of the oc- tahedron face requires another two orientations of which one is left- and the other right-handed. In the end, four different orientations remain, of which two are left- and two right-handed, and at the same time rotated by 180°. The outcome is in- tergrown pyrite with the same surface symmetry as pyrrhotite. Zebec did not specified individual orientations of pyrite on arsenopyrite and pyr- rhotite. Twinned pyrite crystals are rare appearances. The most common are “iron-cross” twins formed by interpenetration of two crystals, rotated by 90° about [110]-axis with respect to each other. The twin acquires the highest cubic m _ 3m sym- metry. The study of Donnay et al. (1977) found no evidence of impurity metals at the (110)-twin boundaries and that the twinning planes were actually irregular surfaces. The study of Rečnik et al. (2016), on the contrary, showed that a mono- layer of Cu atoms was necessary to stabilize the {110} twin structure. Goldschmidt (1922) published figures (No. 134 and No. 135) of pyrite crystals from Bösingsfelde near Lippe in Germany that are twinned in ac- cordance with two other laws. The first twin type forms by 60°-rotation about the [111]-axis and acquires a hexagonal 6/m symmetry. The second type is generated by mirroring in (110) plane with simultaneous 60°-rotation about [111]-axis and yields a twin with the highest trigonal _ 3m sym- metry. Pabst (1971) reported on pyrite crystals with the unusual form that consist of a central pyrite crystal to which six other are presumably {001}-twinned. The composite crystal resembles a cruciform steacyite twin. In absence of morpho- logical details, an X-ray determination revealed that they were single crystals. Here, we report on five new twinning laws that were determined on the pyrite crystals from Jean Baptiste section in Lavrion mines. Methods In situ sampling of pyrite specimens took place in the period between 2001 and 2020. Specimens for morphological determination with Olympus SZ-11 stereomicroscope were cleaned in an ul- trasonic bath filled with a demineralized water. Morphology of single and twinned pyrite crystals was reconstructed using a program SHAPE 7.1. Camera Sony Alpha III, equipped with LAOWA 25 mm, F 2.8 ultra macro lens was used to photograph specimens, using the focus stack- ing method. Three pyrite crystals were chosen for quanti- tative chemical analysis. All samples were dis- solved in hot aqua regia. Varian Spectra AA 110 instrument was used to determine Fe, Cu and Zn, and Agilent 7900 ICP-MS instrument was uti- lized to determine Bi. Antimony content was analysed by means of electron micro probe analysis (EMPA). The work- ing conditions were set at 20 kV, 10-nA beam cur- rent, 2-μm beam size, and peak counting time of 20 s. Nine different crystals within the same pol- ished section were analyzed. A single crystal of the first generation and a twinned crystal of the second generation, free of visible sfalerite or quartz inclusions, were chosen for a single-crystal X-ray analysis that was con- ducted on Oxford Xcalibur3 single-crystal X-ray diffractometer equipped with a CCD detector (MoKalpha). Results Quantitative chemical analysis Table 1 summarizes results of iron, copper, zinc, bismuth, and antimony content obtained on samples of tetrahedral pyrite crystals from Jean Baptiste mine section. Iron content in the single crystal of the first generation is close to a theo- retical pyrite composition. Contents of other met- als are below 0,06 wt%. (001)-twins of the second generation are less pure, which is ascribed to me- chanical inclusions of other minerals, especial- ly sfalerite and quartz. Percentage of bismuth is low in all cases and antimony was determined below 0.1 wt % on the (001)-twin of the second generation. 9Pyrite with lower cubic symmetry from Lavrion, Greece Morphological analysis First generation Pyrite crystals of the first generation are im- bedded in marble concordantly with respect to its banding and cover the walls of open veinlets that are perpendicular to the banding (Fig. 2). They are attached to thin crusts of quartz crystals. The smallest pyrite crystals (less than 1 mm) are combinations of strongly striated cube a{100} and pentagon dodecahedron d{210} faces (Fig. 3A). Sizes of cube and dodecahedron faces on each single crystal vary greatly, giving the crystals the appearance of deformation. Those imbedded in the marble are fresh and have a strong luster on crystal faces, whereas those from the veins are golden brown due to oxidation. An increase in crystal dimensions is reflected in onset of tetra- hedron t{110} faces. The larger are the crystals the more developed are the tetrahedrons and the nar- rower are cube a{100} and dodecahedron d{210} Table 1. Percentage content of metals in pyrite from Jean Baptiste mine section. Sample 1 was a first generation crystal, pre- pared from the marble, free of microscopically visible sphalerite and quartz inclusions. Samples 2 and 3 were of the second generation and detached from the matrix covered with sphalerite and quartz crystals. Fig. 3. Crystal morphology of the primary pyrite crystals. Smaller crystals are combinations of cube a{100} and pentagon do- decahedron d{210} faces. Their alternations are responsible for characteristic striations on crystal faces (A). Larger primary crystals are tetrahedral, and exhibit left- (B) or right-oriented striations on a and d faces (C). This is a reflection of the pre- sence of positive t{110} or negative tetrahedron _ t{ _ 111} faces. Symmetry-defining chiral class-specific forms of left ‘k{11.10.14} (D) and right k’{11.10.14} (E) tetrahedral pentagon dodecahedron are frequently present on the crystals. Fig. 2. In situ photograph of marble with mineralized ore vein in Jean Baptiste mine section. Pyrite crys- tals of the first generation are imbedded concordantly with respect to the mar- ble banding and along the veinlets that are oriented more or less perpendicu- larly to the banding. Pyrite crystals of the second gen- eration with a characteris- tic rugged pattern grow on quartz and sphalerite crys- tals in the ore vein. Field of view: 17 cm × 11 cm. 10 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK faces. Striations on cube faces that are the result of alternations of cube and dodecahedrons have a quality of enantiomorphism, which is reflected in crystals with left- or right-handedness. Crystals with positive tetrahedrons t{111} are left-handed and vice versa for those with negative _ t{ _ 111} tet- rahedrons (Figs. 3B, 3C and Fig. 4). Consequent- ly, the pentagon dodecahedron d gains on speci- ficity by its transformation to class-specific left ’d{210} or right d’{120} tetrahedral pentagon do- decahedron. Handedness of crystals is enhanced by a presence of highly reflective faces of a tetra- hedral pentagon dodecahedron k{11.10.14}. This class-specific form clearly defines handedness of crystals by its left ’k{11.10.14} or right k’{ _ 1.10.14} analogue (Figs. 3D and E, and Figs. 4A and B). Fig. 5. (001)-interpenetration twins of the first pyrite generation. Photograph A shows the twin (one mm in diameter) with a typical reentrant between tetrahedron t faces and characteristic striations on cube a faces. Better-developed twin on photo- graph B is 2-fold in all (100) planes. Note the orientations of striations on its cube a faces. Twin dimensions: 1.3 mm × 1.3 mm. Faces of dyakis-dodecahedron k are present on both crystals in mirror-symmetric positions with respect to (100) planes of symmetry. Fig. 4. Single pyrite crystals of the first generation on quartz matrix. Calcite crystallized later. Pyrite morphology is defined by tetrahedron t and striated cube a faces. Photographs A and B show left-handed and right-handed crystal, respectively. Note the class-specific faces of the corresponding left ‘k{11.10.14} and right k’{11.10.14} tetrahedral pentagon dodecahedron. Both crystals measure one mm on their edges. 11Pyrite with lower cubic symmetry from Lavrion, Greece Fig. 6. (001)-interpenetration of single left- (A) and right-handed (B) tetrahedral crystals yields a twin (C). Chiral faces of tetrahedral pentagon dodecahedron forms ‘k and k’ transform to non-chiral dyakis-dodecahedron k (C). Fig. 7. 6-fold (111)-interpenetration twin (C) is a combination of two left- (shown in the drawing) or two right-handed tetrahe- dral single crystals, whereby one of them is simultaneously rotated by 60° (B) with respect to the other one (A). Some of the first pyrite generation crystals are twinned. Two twinning possibilities are noted. The first is twinning by (001)-interpenetration of two tetrahedral crystals of which one is left- and the other right-handed. The resulting twin has deep re-entrants and characteristic orientation of striationson cube a faces. Tetrahedral penta- gon dodecahedron faces ’k and k’ are present in twinning positions, whereby they lose their chi- ral character and consequently transform to dy- akis-dodecahedron k (diploid). These twins are rare (Figs. 5 and 6). The second possibility is interpenetration of two left- or two right-handed tetrahedral crys- tals about [111]-axis by simultaneous 60°-rota- tion. The resulting twinned crystal is 6-fold and has a common face (pedion), composed of two tet- rahedrons t{111} and deep re-entrants between all other tetrahedron faces. The twin retains the Fig. 8. (111)-interpenetration twins are never theoretically developed. Real crystals exhibit lower 3-fold symmetry, be- cause of the incomplete development of the crystal that sur- rounds the central one. Drawings above and below show the projection of the right-handed twin along its [111]-twin axis and its projection perpendicularly to the same axis, respecti- vely. Cube a faces are present only on the central crystal. Note the characteristic chiral-specific patterns. 12 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK Fig. 10. Pyrite crystals of the second generation have a sim- ple tetrahedral morphology and exhibit characteristic 3-fold mirror symmetric pattern on their faces. Fig. 9. A group of (111)-in- terpenetrated twins of the first pyrite generation. Twins in the middle-left and at the top are right-handed and that in the middle-right is left-handed. Heavily stria- ted cube a faces are present on the central crystals only. Surrounding crystals are without them - note their sharp tetrahedral edges. The largest twin measures three mm on its edge. handedness of the interpenetrated single crys- tals. Faces of tetrahedral pentagon dodecahedron k are not present on (111)-twinned crystals. Typ- ical chiral-specific striations appear on tetrahe- drons as the result of alternations between tetra- hedron t and tetrahedral pentagon dodecahedron d faces (Figs. 7, 8 and 9). This twinning type may take place on any tetrahedron face, which leads to the formation of more common multiple twins. Second generation Pyrite crystals of the second generation have a dull luster and grow only on the walls of the open veins (Fig. 2). If the crystals of the first generation are present, then those of the second generation may overgrow them. They also have a tetrahedral morphology and reach up to 20 mm on their edges. Crystals are sharp-edged due to absence of cube a, and dodecahedron d faces and lack any traces of handedness. Instead of that, 13Pyrite with lower cubic symmetry from Lavrion, Greece they exhibit a characteristic mirror-symmetric 3-fold rugged pattern on their tetrahedral faces (Figs. 10 and 11). Twinned crystals occur in the second gener- ation as well. Again, two twinning possibilities are present. The first is an interpenetration along [001]-axis. Twin is composed of two character- istically patterned tetrahedral crystals and has deep reentrants between them. All patterns are mirror-symmetric in all (100) planes of the twin (Figs. 12 and 13). The second possibility is an interpenetration along [111]-axis that yields a 6-fold twin that has a common pedion face composed of two tetra- Fig. 12. (001)-interpenetration of two tetrahedral crystals of the second pyrite generation. Crystal B simultaneously rotates by 90° with respect to crystal A. The resulting twin has deep reentrants between tetrahedrons and a mirror-symmetric pattern along all symmetry planes (C). Fig. 11. Pyrite crystals of the second generation with characte- ristic 3-fold mirror-symmetric rugged pattern on their faces. The largest crystal measures six mm on its edge. 14 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK Fig. 13. (001)-interpenetration twins of the second pyrite generation. Photograph A shows the twin, measuring three mm on its edge, with a typical reentrant between tetrahedron faces. Mirror-symmetric pattern on its faces is present but less-pronoun- ced. Twin photographed along its 4-fold [100]-axis, is presented on photograph B. Its diameter is two mm. Fig. 14. 6-fold (111)-interpenetration twin (C) is a combination of two tetrahedral crystals, whereby one of them (B) is rotated by 60° with respect to the other one (A). Twin has deep reentrants between tetrahedrons. Note the mirror-symmetric pattern in all symmetry planes of the twin. hedron faces and with deep reentrants between all other faces. Characteristic mirror-symmet- ric rugged pattern is present on all faces as well (Figs. 14, 15 and 16). Multiple (111)-interpenetra- tion twinning is very frequent with crystals of this generation (Fig. 16). Third generation Pyrite crystals of the third generation do not exceed three mm on their edges. They are oxi- dized, striated, and composed of cube a, penta- gon dodecahedron d, and exceptionally octahe- dron o{111} faces (Fig. 17). They cover partially or completely pyrite crystals of the second gen- eration. Crystals of the third generation are preferentially attached with their cube faces in parallel with respect to tetrahedron faces of the second-generation crystals, yet in a completely random manner (Fig. 18). Single-crystal X-ray analysis All the collected reflections obtained with this structure-discerning method belonged to the common pyrite structure defined by an m _ 3 point group. No evidence of tetrahedrite inclusions and twin-related reflections were observed neither on the crystal of the first nor on the (001)-twinned crystal of the second generation, after the refine- ment of the structure to R = 2 %. 15Pyrite with lower cubic symmetry from Lavrion, Greece Fig. 16. (111)-interpenetration twins of the second pyrite generation. Photograph A shows a typical twin (lower right) with the central crystal and less-developed surrounding crystal. It measures six mm on its lower edge. Twin at the top left shows its pe- dion face. Crystals are partially covered with thin gypsum crusts. Photograph B shows a multiple (111)-interpenetration twin attached to quartz and sphalerite crystals. A sharp tetrahedral edge of the central crystal protrudes out of the surrounding crystals. Height of the twin is five mm. Characteristic mirror-symmetric rugged pattern is present on all crystals. Fig. 15. (111)-interpenetration twins of the second generation are never theoretically de- veloped. Real crystals exhibit lower 3-fold symmetry, because of the incomplete de- velopment of the crystal that surrounds the central one. Drawings above and below show the projection of the twin along its [111]-twin axis and its projection perpendicularly to the same axis, respectively. Note the characteris- tic mirror-symmetric patterns. 16 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK Fig. 19. Schematic presentation of the observed pyrite crystals from Jean Baptiste section with respect to starting symmetry, generation, and interpenetration mode. Pyrite crystals of the first generation appear as single tetrahedral crystals with left- or right-handed 23 symmetry, as twins with isometric m _ 3 symmetry and as simple or multiple twins with left- or right-handed hexagonal 6 symmetry. The second pyrite generation is primarily twinned and has the corresponding _ 4m3 symmetry. Crystals of this generation appear as tetrahedrons, as twins with the highest isometric m _ 3m symmetry and as simple or multiple twins with hexagonal 6mm symmetry. Fig. 17. Pyrite crystals of the third genera- tion have a cubic morphology that is slightly modified by pentagon dodecahedron d and octahedron o{111} faces. Fig. 18. Oxidized pyrite crystals of the third generation cover pyrite crystals of the second generation. Note their random orientations on the tetrahedron faces of the secondary generation. The largest crystal measures 11 mm on its tetrahedral edge. Bluish-white coatings are aragonite crystals. 17Pyrite with lower cubic symmetry from Lavrion, Greece Discussion The first to report on pyrite crystals with tet- rahedral morphology from Lavrion mine was Mügge in 1895 and 1903. His drawings show left- and right-handed tetrahedral pyrite crystals of the first generation. Crystallographic forms of cube a{100}, tetrahedral pentagon dodecahedron d{210}, and tetrahedron t{111} define the crystal morphology, with chiral-specific striations on a faces. The third drawing shows a crystal of the second generation. The apparent low symmetry of the crystals is, in accordance with his opinion, the result of a pyrite pseudomorph after tetra- hedrite, whereby he could not detect any traces of tetrahedrite within the crystals, and chemical analysis was negative for copper. Evident tetar- tohedral symmetry of the pyrite would require its twinning in accordance with “iron cross” in order to adapt to a higher point group of tetra- hedrite, which is _ 4m3, but no signs of such twin- ning on the primary crystals existed. He con- cluded therefore, that the evident morphologic tetartohedry could lead to a wrong opinion that the pyrite crystals from this location should have such a low symmetry and stressed that the ex- act pyrite symmetry was already known at that time beyond any doubt. Mügge (1895) also add- ed that the pyrite crystals appear together with quartz and prismatic arsenopyrite in single and (101)-twinned crystals. The proposed oriented growth of pyrite on tetrahedrite can be under- stood as a contact of pyrite octahedron {111} face with a tetrahedron {111} face of tetrahedrite. Symmetry of the tetrahedron face is 3-fold with three mirror planes. This symmetry requires left- and right-handed orientation of pyrite. In this way, pyrite acquires the same planar sym- metry on its {111} face, i.e. 3-fold with three sym- metry planes. Tetrahedrite {111} face, completely overgrown with pyrite in the discussed orienta- tion, could indeed exhibit twinned structure as observed on the second generation of pyrite from Jean Baptiste section. However, it is very like- ly that the scattered pyrite crystals would only partially cover the tetrahedral faces of real tet- rahedrite crystals in two distinctly recognizable chiral orientations, which is not the case here. Oriented growths of pyrite on tetrahedrite or vice versa are not known. Since then, no new works on the subject, and no new material had appeared until 2001, when a group of researchers from Athens rediscov- ered the location and collected new specimens (Voudouris et al., 2004). A preliminary SEM anal- ysis was performed to determine which mineral the tetrahedral crystals belonged to. It turned out that they were pure pyrite without indication for replacement of another mineral. It is clear that Mügge described the primary crystals from exactly the same location, because the tetrahedral pyrite crystals from the new find have the same morphology and come together with arsenopyrite in single and (101)-twinned crystals. Besides that, no other location with tet- rahedral pyrite crystals has ever been encoun- tered in Lavrion mines. It has to be stressed that the pyrite crystals of the first generation exhibit the evident morphological lowermost cubic sym- metry 23. ICP-MS analysis of primary crystals confirmed the purity of the first-generation py- rite crystals with respect to their Cu, Zn, and Bi content. EMPA analysis of the second-generation pyrite crystals showed the comparably low anti- mony content (see Table 1). Single-crystal X-ray analysis excluded tetrahedrite inclusions in sin- gle and in twinned crystals. Discovery of different interpenetration twin types on the new material sheds a completely new light on the trivial symmetry of the tetrahedral pyrite crystals at Jean Baptiste location. Forma- tion of (001)-interpenetration twins, exhibiting a cubic m _ 3 symmetry as presented in Figure 5, is not possible unless the starting symmetry is the lowermost isometric 23 (tetartohedry). Figure 6 shows the formation of such a twin. (111)-in- terpenetration twins are another proof of lower primary symmetry, because they are chiral and retain the handedness of the twinned single crys- tals, which is only possible if the starting sym- metry is 23. The resulting twins with symmetry 6 are for that reason either left- or right-handed (see Fig. 7). Real twinned crystals of this type ex- hibit lower, but evidently chiral 3 symmetry as the outcome of the incomplete interpenetration (Figs. 8 and 9). All tetrahedral pyrite crystals of the second generation have a characteristic 3-fold mir- ror-symmetric pattern on tetrahedron faces. This fact reflects a higher symmetry, which is the result of (110)-interpenetration in which two tetrahedral crystals of the opposite handedness interpenetrate (Fig. 19). All pyrite crystals of the second generation are therefore twins with _ 4m3 symmetry, which enables further twinning by (001)-interpenetration yield- ing a twin with the highest cubic m _ 3m symmetry. Twins of this type exhibit sharp tetrahedral edges and characteristic rugged pattern on their faces that is mirror-symmetric along all (100) and (110) planes of symmetry (Figs. 12 and 13). 18 Mirjan ŽORŽ, Panagiotis VOUDOURIS & Branko RIECK The highest twin symmetry appears when two crystals of the second pyrite generation are (111)-interpenetrated by simultaneous 60°-rota- tion about the [111]-axis. The resulting twin has a hemimorph 6mm symmetry with a characteris- tic pattern on all tetrahedral faces that is mirror symmetric along six mirror planes that are par- allel to the twin’s main 6-fold axis (Fig. 14). Real crystals exhibit lower symmetry due to incom- plete interpenetration (Figs. 15 and 16A). Multi- ple twinning of this type is frequent (Fig. 16B). Figure 19 shows a schematic presentation of all described twinning possibilities. Pyrite crystals of the third generation are cubes that are slightly moderated by pentagon dodecahedron d and octahedron o{111} faces on cube corners. There are no twinning signs on them, which confirms their trivial m _ 3 symmetry. The cubic m _ 3 symmetry of the first-generation single crystals as well as of the (110)-twinned crystals of the second generation derives from the findings of the single-crystal X-ray analysis conducted on them. It is therefore evident, that the results of morphological and structural anal- ysis do not match. It is not the aim of this study to specify the environmental and structural de- tails leading to the formation of the observed twins. To explain this discrepancy, we may only propose that the pyrite crystals of the first gen- eration crystallized under conditions that fa- vored their initial low isometric 23 symmetry, and consequently the formation of tetrahedral crystals and twins. Crystallization conditions might change in the course of time and the initial symmetries (crystal structures) of all single and twinned crystals of the first and the second gen- eration transformed to structures with higher m _ 3 symmetry, whereas their morphologies remained unchanged. The other explanation might consider a pos- sible insufficient specificity of the single-crystal X-ray analysis. Namely, pyrite trivial m _ 3 point group ranks to a group of centrosymmetric Laue classes. Two crystals measured in this study, on the contrary, belong to morphologically evident non-centrosymmetric 23 and _ 4m3 point groups that may mimic the centrosymmetric m _ 3 one if a resonant scattering (anomalous dispersion) was negligible or not detected. Future work on this topic should therefore go into direction of determining conditions during the formation and growth of three pyrite genera- tions, which would include the determination of physico-chemical growth parameters, fluid in- clusions in crystal paragenesis and into the op- timization and refinement of the single-crystal X-ray analysis. Acknowledgements We truly thank to Luca Bindi from Dipartimento di Scienze della Terra Università degli Studi di Firenze, who conducted the single-crystal X-ray analysis, and to Radmila Milačič from Department of Environmental Sciences, Jožef Stefan Institute, Ljubljana, for the chemical analysis. 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