Genesis of Filamentary Pyrite Associated with Calcite Crystals

Eur. J. Mineral. 2005, 17, 905–913

Genesis of filamentary pyrite associated with calcite crystals

IVAN K. BONEV1*,JUAN MANUEL GARCIA-RUIZ2,RADOSTINA ATANASSOVA1, FERMIN OTALORA2 and SVETOSLAV PETRUSSENKO3

1Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria *Corresponding author, e-mail: [email protected] 2Instituto Andaluz de Ciencias de la Tierra, Universidad Granada, 18002 Granada, Spain 3National Museum of Natural History, 1 Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria

Abstract: The calcite of the hydrothermal Surneshko Kladenche copper vein deposit from the Rossen ore field, Bulgaria, sometimes encloses peculiar filamentary pyrite crystals. Three successive calcite generations were observed belonging to a low-temperature (<235°C) carbonate paragenesis formed in open cavities of the ore veins after the main chalcopyrite mineralisation. Three generations of pyrite crystals are associated with these calcite crystals: pyrite 1, with [001] elongated columnar crystals which crystallised in open space; pyrite 2, in groups of long (up to 10 mm) sub-parallel tortuous filaments of varying thickness (3-20 µm) which are oriented nearly perpendicular to the surfaces of a transient w{3145} scalenohedral calcite crystal zone; and pyrite 3,made of slightly elongated small crystals, located in the outermost v{2131} zone of the same scalenohedral calcite crystals. The columnar pyrite 1 is formed by subsequent thickening of thin straight whiskers rapidly grown under diffusional regime, whereas the filamentary pyrite 2 and 3 grew contemporaneously with the enclosing calcite crystal. Key-words: crystal morphology, filamentary crystals, calcite, filamentary pyrite, contemporaneous growth.

Introduction cinnabar inclusions in calcite. Examples of simultaneous growth of two minerals have been reviewed by Grigoriev & Pyrite, the most important and widespread sulphide mineral Zhabin (1975) (mostly from Russian literature), including on the Earth surface, usually occurs in the form of crystals inclusions of hematite in quartz (Laemmlein, 1973), cassit- displaying three main equilibrium faces, namely {100}, erite in fluorite (Lyakhov, 1966), etc. {111}, and {210} (Sunagawa, 1957, 1987). Nevertheless, We studied the formation of pyrite filamentary inclusions many other crystal forms and habits have been reported for in large calcite crystals collected from the Surneshko Kla- pyrite, including skeletal, dendritic and acicular crystals, denche vein copper deposit in the central part of the Rossen strongly deviating from the equilibrium morphology (Pabst, ore field, Bulgaria. The pyrite inclusions were isolated by 1971; Pshenichkin & Korobeinikov, 1974; Strunz, 1976; selective dissolution of the calcite matrix, and examined by Endo, 1978, etc.). White (1973) and Bideaux (1970) de- optical and scanning electron microscopy. The results of scribed whisker (hairlike), filiform, and even ring-like py- these observations are reported in the present paper, fol- rite formations. Some rare thin pyrite needles formed inside lowedbyadiscussionofthegrowthmechanisms. small vesicles have the characteristics of straight or kinked whiskers (Kamb & Oke, 1960; Roberts et al., 1974). A re- view on pyrite filiform crystals with right-angle bends from Geological setting different deposits was given by Henderson & Francis (1989). They show that the bending is a specific growth fea- The economically important Bourgas ore district is located ture of these crystals, not a result of (110) twinning. Bonev in the easternmost part of the Cretaceous Srednogorie met- et al. (1985) provided a systematic scanning electron micro- allogenic zone. It is comprised of three important ore fields scopic (SEM) and high voltage electron microscopic namely, Rossen, Vurly Bryag and Zidarovo. The Rossen ore (HVEM) study on pyrite whiskers and thin platelets from field is related to the Senonian Rossen volcanic-intrusive lead-zinc deposits in Bulgaria and discussed their growth structure of andesites and tuffs in which a central gabbro- mechanisms. monzonite-syenite plutonic core is intruded, followed by Syngenetic whisker inclusions of pyrite in quartz from porphyritic subvolcanic bodies and dykes (Bogdanov, Mangyshlak in Kazakhstan were investigated by Galuskin 1978). The numerous copper ore veins were intensively & Winiarski (1997). Zhabin (1986) described filamentary worked during 1945-1995 in several underground mines

0935-1221/05/0017-0905 $ 4.05 DOI: 10.1127/0935-1221/2005/0017-0905 ˇ 2005 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart 906 I. K. Bonev, J. M. Garc´ıa-Ruiz, R. Atanassova, F. Otalora, S. Petrussenko

and altered wall rocks cemented by coarse-grained calcite and fine-grained quartz. In the central part of the vein irregular open cavities up to several meters in size occur. Their walls are partly overgrown by calcite crystals forming druses, which are the subject of this work.

Methods Hand specimens of calcite druses, cleavage plates and dou- bly polished thin sections were examined with binocular microscope and optical transmission microscope. Calcite crystals were selectively dissolved to access to pyrite inclusions which were later studied by scanning electron microscopy. Low concentration acetic acid solutions (6% v/v) or hy- drochloric acid solutions (5% v/v) were used as solvents. The detached thin pyrite crystals were very delicate and fragile. Therefore etching of calcite was performed under continuous inspection to stop it before reaching the bases of pyrite filaments, in order to preserve them attached to the calcite substrate. Scanning electron microscopic studies were performed after carbon or gold coating of crystals. JEOL Superprobe-733 and Philips 515 SEM devices were used. Electron probe microanalysis and X-ray diffraction study in Gandolfi and rotation cameras were also applied for Fig. 1. Schematic geological map of the Rossen ore field, Bulgaria (adapted from Bogdanov, 1987). characterising the mineral phases and crystal orientation. Some filaments were measured by optical goniometry. such as Rossen, Koru Cheshma, Meden Rid, Surneshko Kla- denche, Propadnala Voda and Chiplaka (Fig. 1) and are now Sample description abandoned. The mineralisation formed mainly by open space filling includes an early pre-ore silicate stage (phlogopite, The filamentary pyrite crystals are associated to druses feldspars, pyroxenes, apatite, etc.), the main ore stage of sev- made of three successive generations of calcite crystals with eral parageneses (with main minerals chalcopyrite, pyrite, different morphology and position (Fig. 2). The earliest gen- quartz, chlorite and carbonates, together with magnetite, hematite, molybdenite, bismuthinite, etc.), and a final carbonate stage. According to Strashimirov & Kovachev (1992) the main ore stage was rather high-temperature (365-320°C), while for the late post-ore stage the T range is 300-100°C. Pyrite is an important mineral in the copper ores and occurs in the early parageneses mainly as dense masses. In the late carbonate paragenesis pyrite is only a minor but characteristic component. Calcite is the main mineral of the final carbonate paragenesis. It forms massive coarse-grained aggregates, well-formed crystals and crystal druses. In a detailed morphological study accompanied by thermometric fluid inclusion measurements, Naidenova (1966) distinguished several crystal habits of calcite in the Rossen deposits: earliest pinacoidal plates (270°C and lower), scalenohedral v{2131} (235- 175°C), prismatic (170-160°C), barrel-form, rhombohedral, short-prismatic, and flat rhombohedral (140-135°C). Minˇce- et al. va-Stefanova (1993) described giant, up to 30 cm wide, Fig. 2. Calcite (Ca) and its relationships with pyrite (Py): (a) Ideali- scalenohedral calcite crystals containing central thin tabular sed crystal morphology of calcite 2 with its habit evolution (r → w → {0001}seeds. The habit evolution of calcite crystals in these v) and with sceptre overgrowths of later prismatic calcite 3; (b) Rela- deposits mostly follows the general morphological depen- tionships between the different generations of calcite (Ca1, 2, and 3) dence of calcite with temperature, as known from various and pyrite (Py1, 2, and 3). Pyrite 1 is formed in open space on the fine mineral deposits (Kostov & Kostov, 1999). quartz crust (Qtz), overgrowing Ca1, as well as on the early rhombo- Our samples were collected at the level 280 of the ore hedral faces of calcite 2, whereas Py2 and Py3 starting from two later vein #185 of the Surneshko Kladenche mine. The vein con- internal zones of Ca2 continue their contemporaneous joint growth sists of brecciated pieces of the main chalcopyrite-pyrite ore to the surface. Schematic cross section. Genesis of filamentary pyrite associated with calcite 907

Table 1. Comparison of the filamentary crystals of the three pyrite generations (1, 2, and 3) associated with calcite 2 (Ca). Characteristics Pyrite 1 Pyrite 2 Pyrite 3 Position Free on crusts of quartz or included Always in distinct internal zone In the peripheral zone of Ca within Ca within Ca Habit Straight columnar or with 90°-kinks Single crystal tortuous filaments Cubo-octahedral {100} » {111}, elongated Crystal elongation // [001] Roughly // [001], with deviations Sligthly elongated // [111] or [001] Bounding: crystal faces Smooth {100} ( „ {210}, {111}) Striated {100} „ {210} and {111} Rough {111} „ {100} Crystal seeds Small cubic crystals Small complex crystals Small cubo-octahedral crystals Length 2-3 mm and more Up to 1-2 mm 0.1 mm Thickness Permanent: 20-50 µm Changeable: 3-20 µm Up to 10-20 µm Substrate Quartz or Ca: {1011} Ca: {3145} Ca: {2131} and {0332} Orientation on the Random Roughly Ќ „ 10-20° Nearly Ќ substrate Distribution and density Uneven dispersed with low density Even distribution with high density: High density: up to 100-120/cm2 of crystals up to 50/cm2 Growth mechanism Fast whisker growth and sub- se- Contemporaneous growth with the Contemporaneous growth with the quent thickening enclosing Ca enclosing Ca eration (calcite 1) consists of small, up to several mm wide The crystals of the next calcite 3 generation form auto- scalenohedral crystals v{2131}. They grow on the surfaces epitaxic sceptre overgrowths on the terminal parts of the of earlier formed minerals, mostly chalcopyrite. Quartz, as large scalenohedral crystals of calcite 2 (Fig. 2). They are fine-grained crusts of thickness up to 0.2-0.5 mm overlays bounded by the prismatic faces m{1010} and a{1120}, and the calcite 1 (Fig. 2b) and chalcopyrite crystals preventing the scalenohedron e{0112}. further calcite growth. The pyrite crystals belong to three successive generations Calcite 2 is the main calcite generation which includes the which are related to three growth zones observed within the pyrite filaments discussed in this paper (Fig. 3). The crystal crystals of calcite 2. Their characteristics and relationships morphology of these up to 5 cm calcite 2 crystals evolves with with calcite crystals are shown in Fig. 2b and Tables 1 and 2. time (Fig. 2a) as revealed by studying inner zonation by opti- According to microprobe analysis pyrite crystals have near- cal transmission microscopy. The morphological sequence in- ly stoichiometric composition, with some small (<0.5 cludes: (a) a central phantom core of the r{1011} rhombohe- wt.%) Co content: S 53.49, Fe 45.75, Co 0.46 (Ni, As <0.2), 7 dron; (b) an intermediate zone of a flat scalenohedron proba- 99.70 wt.%, and formula (Fe0.99Co0.01)S2.01. No measur- bly w{3145}; (c) an outer zone with steeper scalenohedral able compositional differences between crystals belonging habit bounded by the crystal forms v{2131} and with smaller to the three pyrite generations have been detected. acute rhombohedron h{0332}, sometimes with additional Pyrite 1 appears as small, up to 1 mm in size, isometric narrow w{3145} faces. From a chemical viewpoint, these cubic a{100} crystals with minor truncating octahedral crystals can be considered manganiferous calcite: Ca 39.34, o{111} faces. Often, these crystals serve as basis for straight Mn 0.56, Fe 0.13, Mg 0.09, C 11.98, O 47.89, 7 99.89 wt.%, filaments strongly following [001] directions (Fig. 4). The with formula (Ca0.982Mn0.012Mg0.003Fe0.003)CO3.

Fig. 4. Stereomicroscopic view of cubic crystals of pyrite 1 (black) Fig. 3. A druse of large scalenohedral crystals of calcite 2 containing with prismatic columnar [001] offshoots, grown on the fine-grained inclusions of pyrite 2 and 3. quartz crust in the open space between two large crystals of calcite 2. 908 I. K. Bonev, J. M. Garc´ıa-Ruiz, R. Atanassova, F. Otalora, S. Petrussenko

Fig. 5. SEM views of two prismatic T- shaped [001] elongated crystals of pyrite 1, extracted from calcite by etching: (a) and (d) general views, (b-c) details of their finely striated surface. length of these thin columnar pyrite formations ranges from tion of the crystals is random and evidently they formed un- 1to2mm,andtheirwidthrangesfrom50to100µm.They der similar crystallisation conditions. are bounded by flat cubic a faces and have square or rectan- Pyrite 2, the second generation of pyrite crystals, consists gular cross sections. In some cases, irregular stepped areas of numerous thin tortuous filaments embedded in crystals of with oscillating e{210} and o{111} faces form parts of their calcite 2 forming sub-parallel groups (Fig. 6a-b and 7). terminations or side faces (Fig. 5). Right angle bends are not These filaments nucleate and grow roughly perpendicular to uncommon. L-, T- and more complex frame-like skeletal the faces of an internal phantom scalenohedron w{3145} of shapes occur (Fig. 5a, d). The cubic surfaces have a fine-lay- calcite until they reach the outer crystal surface (Fig. 2b). ered structure with slightly rounded outlines nearly parallel The filaments are not perfectly straight and show serrated to the crystal edges. The thickness of the layers is smaller surfaces and variable thickness with changes of growth di- than 1 µm (Fig. 5b, c). Crystals of pyrite 1 can be found on rection. In some cases they look like a chain of small sub- two different locations either scattered over the fine-grained crystals with crystallographic coherence (Fig. 7 and 8). As quartz crusts (Fig. 4) or embedded as inclusions in calcite 2 established by morphological, goniometric and X-ray stud- crystals on top of an internal phantom rhombohedral ies the filaments are single crystals elongated along [001] {1011} surface (Figs. 2b and 6a). In both cases the orienta- direction. Their surfaces are strongly striated due either to

Fig. 6. Stereomicroscopic view (at two different magnifications) on a cleavage surface of a transparent calcite crystal containing pyrite inclusions in three internal zones: 1) central rhombohedral zone with columnar pyrite 1 followed by clean area without inclusions; 2) intermediate zone with long filaments of pyrite 2; 3) peripheral zone with abundant short crystals of pyrite 3. Genesis of filamentary pyrite associated with calcite 909

Fig. 7. SEM view of a group of filamentary crystals of pyrite 2, partly revealed after selective dissolution of the enclosing calcite (some broken and fallen parts of them lying on the calcite surface are not in their original position). Scale bar 100 µm. the oscillation of the thickness of the filament or to the Discussion change of the displayed crystal forms. Thus, cubic a{100} together with e{210}, o{111}, and sometimes d{110} and The three generations of pyrite obviously correspond to s{321} forms are present. The striation is usually uniform three nucleation events, which are associated with three (e.g. transverse on Fig. 8a-d and 9), but local changes in its growth zones of calcite 2 (Table 2). We did not find fluid in- orientation due to unequal development of additional faces clusions in the calcite crystals and therefore we do not have sometimes also occur. Some crystal segments have clearly direct evidence of the T at which the mineralisation took irregular or distorted octahedral o{111} habit (Fig. 7 and 9a- place. However, the fluid inclusions data of Naidenova b). Filaments formed by prevailing a faces have the shape of (1966) for scalenohedral calcite crystals from the same de- an [001] elongated prism (Fig. 9c). Complex branched posit (i.e. calcite 2) established that their crystallisation forms also occur (Fig. 8e). The length of the filaments from the hydrothermal fluids occur at a temperature be- reaches up to 1-2 mm, while their thickness varies from 2-3 tween 235 and 175°C. For the prismatic and the other fol- to 20 µm. The thinner section of the filaments corresponds lowing habit types the temperatures are still lower, to the connection between two sub-crystals. Interestingly, 170-140°C. This range of temperature discards a biological- there is a correlation between the thicknesses of neighbou- ly induced precipitation mechanism which has been sug- ring filaments (Fig. 7, 8a-d). The density of filaments in some places reaches up to 50 filaments per mm2. Neither epitaxic nor topotaxic relationships between calcite and py- Table 2. Mineral succession in the calcite paragenesis. rite exist. Minerals Habit Succession Pyrite 3 , the last generation of pyrite crystals, occurs Chalcopyrite bisphenoidal {112} ++ base within a narrow (<0.5 mm thick) peripheral growth zone be- – Calcite 1 scalenohedral {2131} — low the scalenohedral v{2131} surface of the main calcite 2 – – crystals (Fig. 2). This zone is rather enriched of pyrite inclu- Quartz, fine- prismatic– {1011} + {0111} + § crystalline {1010} sions in relation to all other internal calcite zones (Fig. 6a- – Calcite 2, rhombohedral {1011} — b). Pyrite crystal terminations projecting on the calcite sur- – ĺ zoned scalenohedral {3145} ĺ — – – face have cubic-octahedral habit and rather rough faces crystals scalenohedral {2131}+{0332} ———— (Fig. 10a-b). The elongated parts of these crystals inside cal- isometric {100}, and columnar Pyrite 1 ŷ cite, as revealed after slight surface dissolution (Fig. 10c) along (100) and [001] are heavily striated, mostly by growth rate oscillation. The Pyrite 2 filamentary ~ [001] elongated ŷŷŷŷ crystals of pyrite 3 are similar to the filaments of pyrite 2 but isometric {111}+{100}, elongated Pyrite 3 ŷ ~ [111] or [001] are shorter and more monolithic, often elongated also in – – [111] direction. The crystals are up to 10-15 µm wide and Calcite 3 prismatic– {1010} + {1120} + — {0112} three to four times larger in length but much shorter than ——ŷ free growth (calcite, pyrite) those of pyrite 2. ŷŷŷ contemporaneous growth 910 I. K. Bonev, J. M. Garc´ıa-Ruiz, R. Atanassova, F. Otalora, S. Petrussenko

Fig. 8. SEM view of the morphology of partly revealed crystals of pyrite 2: (a) a group of nearly straight and tortuous filaments; (b) enlarged details of the tips; (c) complex morphology of irregular crystals composed of parallel, more indi- vidualised segments. gested by Hofmann & Farmer (2000) for filamentous fab- grained hematite in the carbonate stage (Bogdanov, 1988) rics of some low-temperature minerals, including pyrite. points to oxidised conditions. As evidenced by experimental studies (Shoonen & Bar- The earliest straight prismatic [001] crystals of pyrite 1 nes, 1991; Kozerenko et al., 1995; etc.), nucleation of pyrite grew from small isometric crystals which are bounded in such hydrothermal solutions, is embarrassed and it can mainly by flat a faces. They grew from solution in an open only form initially through a monosulphide precursor. In the space with random orientation onto the crust of fine-grained presence of sulphur species (H2S, etc.) this is a fast process. quartz. They are also found inside calcite 2 crystals, on an The absence of marcasite (an indicator for acid conditions) internal rhombohedral phantom {1011} surface, having the in the samples studied is evidence of close to neutral or alka- same morphology and random orientation at the support. line conditions, representing the normal environment for These identical crystal characteristics are indicative of the carbonate precipitation. The deposition of scarce fine- same growth mechanism in an open space over the calcite

Fig. 9. SEM views of morphological features observed in highly striated pyrite filaments. These crystals are composed of (a) more or less in- dividualised distorted octahedra (arrows), or are bounded (b-c) by oscillating a{100} and e{120} faces. Genesis of filamentary pyrite associated with calcite 911

Fig. 10. The superficial cubic-octahedral (a+o)crystals of pyrite 3 on the outer scalenohedral v{2131} faces of calcite 2: (a) light microscopy view at low magnification; (b-c) SEM views at higher magnification. The elongated inwards pyrite crystals observed in (c) are revealed after slight dissolution of calcite. face in a period of cessation of its growth. Later, during the the screw-dislocation mechanism, but no direct evidence further growth of calcite, the embraced pyrite forms the first was given. It appears more realistic that the whiskers arose internal zone of solid pyrite inclusions. during the short cooling period by fast diffusion-controlled The screw dislocation mechanism of formation of growth. straight columnar crystals and whiskers from low-supersat- Bonev et al. (1985), in a detailed SEM and HVEM study urated solutions, as early proposed by several authors (e.g. on morphology and perfection of natural pyrite whiskers Amelinckx, 1958, for NaCl) is accepted by many authors and thin platelets from hydrothermal Pb-Zn deposits, did (Sunagawa, 1987). However, as Givargizov (1986) men- not find direct support for dislocation growth mechanism. tioned, our level of understanding of whisker growth from liq- They concluded that whiskers as highly non-equilibrium uid phase is poor in comparison with vapour growth, where a crystals were formed by rapid growth under a diffusion-con- proven VLS (vapour-liquid-solid) mechanism prevails. trolled regime from inhomogeneous highly supersaturated Henderson & Francis (1989) hypothesised that the solutions by two-dimensional nucleation on the tip. In fact, screw-dislocation mechanism is sufficient to explain both the whiskers represent highly anisometric acicular skeletal the acicular habit and the formation of right-angle bends ob- crystals. served in natural filiform pyrite. However, their conclusions Thus, in the case of pyrite 1 described here, considering are not supported by direct observation of dislocations. As the available experimental data and analysis of Bonev regards the right-angle bends, a migration of dislocations or (1990; Bonev et al.1985),weassumethatthegrowthpro- their bifurcation is even more hypothetical and unlikely. cess of this straight-linear pyrite is realised in two succes- The experimental works, known from the literature, es- sive stages: (1) initial rapid directed growth of thin [001] tablished rather different growth conditions for pyrite whis- whiskers at a diffusional regime and embarrassed nourish- kers. Yamada et al. (1979) studying the pyrite crystallisation ing, and (2) steady layer growth of the side faces of the whis- by chemical vapour transport in Cl2 gas have produced iso- kers and their uniform thickening, after overcoming the dif- metric polyhedral crystals under lower thermal gradients fusional resistance of the surroundings. Some isolated sin- (e.g.700→ 600°C), and blade-like crystals and whiskers gle whiskers form kinks following the heterogeneity of the under much higher gradients (700 → 400°C). These [001 fluid environment. pyrite whiskers, formed by two-dimensional growth at high Convincing natural cases of similar two-stage formation supersaturation, have a high growth rate of 1 mm/day. Tru- of columnar pyrite crystals by thickening of thin whisker- fanov et al. (1986) also produced dendritic and hair-like leaders have been reported for Eureca, Pennsylvania, USA, whiskers of pyrite and chalcocite from high-temperature by Henderson & Francis (1989, see their Fig. 19 and 20), chlorine solutions by abrupt, brief (in seconds) T and P de- and for Naica, Chihuahua, Mexico, by White (1973, his Fig. crease from 400-300°C down to 20°C. 4, 7 and 8). The apparent curvature of some pyrite columns, The diverse morphology of pyrite crystals, grown at dif- in the last case, is evidently inherited from the flexible ben- ferent hydrothermal conditions, was systematically studied ded primary thin whiskers. by Murowchick & Barnes (1987). A hydrothermal-trans- Pyrite 2, with its peculiar filamentary shapes consider- port method at controlled T (range 450-250°C) and T gradi- ably differs from known morphologies of this mineral. Sev- ents was used, followed by rapid (~30 min) cooling of the eral of its characteristics and the relations with calcite are glass-capsules before opening. As mentioned by these au- important for understanding its origin. thors, the degree of supersaturation rapidly increased during (1) All filamentary pyrite 2 crystals are entirely enclosed the cooling period. Thus, transition from surface-controlled in calcite and are not observed as free filaments out of it. In to diffusion-controlled growth was taking place, resulting in fact, only their terminal parts protrude on the calcite surface skeletal and dendritic forms and overgrowths. In one of the as small euhedral crystals (as seen on Fig. 10 for pyrite 3). runs (T 250°C and low gradient of 0.35°C/cm) needle-like (2) The pyrite filaments are located between an internal crystals were obtained. The authors presumed that their for- growth zone of calcite (a face of the scalenohedron mation occurred at a very low degree of supersaturation by w{3145}) and the outermost crystal faces (the scalenohe- 912 I. K. Bonev, J. M. Garc´ıa-Ruiz, R. Atanassova, F. Otalora, S. Petrussenko dron v{2131}), being oriented nearly perpendicularly to the ous growth at comparable quantitative relations of the min- base surface. These two limiting surfaces belong to different eral pairs. crystal forms and are not parallel. Thus, the lengths of the filaments in the different parts of the host crystal are rather different (up to twice and more times), and are clearly con- Conclusions trolled by calcite crystal growth. (3) The tortuous pyrite filaments have variable thickness, – Two main types of pyrite crystal formations, clearly dif- owing to the more autonomous growth of their individuali- ferent in their morphological characteristics and growth sed composing segments and the directions of the single fil- mechanisms, were found: (1) straight columnar prismatic aments not being emphatically straight-linear and parallel pyrite 1, formed by free growth in open space, and (2) (except when two adjacent filaments have a joint seed). chain-like and tortuous filaments (pyrite 2 and 3), formed However, a distinct correlation can be observed in neighbo- as inclusions in calcite during their contemporaneous uring filaments between their thinnest and thickest parts growth. No differences in the chemical composition of (Fig. 7, 8a-d) when removed from the base at equal length these pyrite crystals were established. from the base. – It is assumed that the prismatic crystals of pyrite 1 are Such morphological characteristics and the control of the formed by subsequent thickening of thin whiskers, previ- location, orientation and size of the pyrite filaments by the ouslyformedbyfast2Dgrowthonthetipfromhighlysu- enclosing calcite clearly suggest the contemporaneous persaturated solutions under diffusion-controlled regime. growth of calcite 2 and pyrite 2, and cannot be explained by – The chain-like single crystals of pyrite 2 show character- sequential growth. istic features of contemporaneous growth with enclosing The more favourable orientation of pyrite can be consid- calcite: their axes are approximately perpendicular ( „ ered to have occurred as a result of specific geometrical se- 10-20°)totheirstartinggrowthzoneincalcite;thereis lection in the competition between the two growing phases correlation in the variable thickness of neighbouring fila- in which the acicular crystals of pyrite 2 developed in their ments; some needles are composed of more individuali- direction of fast growth [001] survive. Due to the restrictive sed crystal (e.g. octahedral) parts. Similar, but much influence of spreading around calcite growth layers, the un- shorter, are the crystals of pyrite 3, located only in the pe- favourably oriented pyrite crystal-nuclei hindered their ripheral growth zone of calcite. growth (Fig. 8e). The changeable thickness of the needles is – The specific cross-orientation of the pyrite needles to the related to some fluctuations in the FeS2 supersaturation in calcite growth zones is the result of specific geometrical solutions, which slightly changed the ratio between calcite selection between these two simultaneously growing and pyrite growth rates. crystal phases. Pyrite 3 in principle is similar to pyrite 2, being enclosed in calcite 2 and crystallising contemporaneously with it. The Acknowledgements: The authors are indebted to CSIC of difference is that the new nucleation is accomplished only in Spain and the Bulgarian Academy of Sciences for the finan- a thin peripheral growth zone, closely below the outer crys- cial support and to D. Rickard, E. Libowitzky, E. Tillmanns tal surface (Fig. 6a, b and 10). This is why the crystals of py- and an anonymous referee for the useful and constructive rite 3, also growing roughly perpendicular to this zone, are comments. much shorter. These crystals have cubo-octahedral habit, and are elongated in [001] or in [111] direction. The large number of pyrite crystals in the peripheral calcite zone is in- References dicative for a period of considerably increased supersaturation of FeS2 phase. Of course, during this period the previ- Amelinckx, S. (1958): Dislocations in alkali halide whiskers.in ously arisen acicular crystals of pyrite 2 continue their “Growth and perfections of crystals”, R.H. Doremus, B.W. Ro- growth, although with some local thickening. berts, D. Turnbull, eds., Wiley, New York / Chapman & Hall, On the outer calcite surface the acicular inclusions of py- London, 139-153. rite 2 and 3 are revealed as normal isometric crystals (Fig. Bideaux, R.A. (1970): Mineral rings and cylinders. Mineral. Rec., 1, 10a, b). No pyrite filaments protruding beyond the calcite 105-112. faces occur. This is additional firm evidence for contempo- Bogdanov, B. (1987): Copper deposits in Bulgaria. Technica Publ. raneous running and ending of growth of these two pyrite House, Sofia, 388 p. (in Bulgarian). generations with calcite 2. The sceptre crystals of the later Bonev, I.K. (1990): Whisker growth of minerals. IMA 15th General calcite 3, overgrowing the terminations of crystals of calcite Meeting, Abstr. 1, 382-384. 2 after some interruption of mineralisation, do not contain Bonev, I.K., Reiche, M., Marinov, M. (1985): Morphology, perfec- any pyrite inclusions. tion and growth of natural pyrite whiskers and thin platelets. Phys. Chem. Minerals, 12, 223-232. The case of syngenetic thread-like pyrite inclusions in Endo, Y. (1978): Surface microtopographic study of pyrite crystals. quartz, studied by Galuskin & Winiarski (1997), shows sim- Bull. Geol. Survey Japan, 29, 701-764. ilar morphological features of the minerals. Other impres- Galuskin, E. & Winiarski, A. (1997): Syngenetic whisker inclusions sive cases are inclusions of cassiterite in fluorite (Lyakhov, of pyrite in quartz: morphology, structure and composition. N. Jb. 1966) and of cinnabar in calcite (Zhabin, 1986). Despite the Mineral. Mh., 1997, 229-240. differences in composition and structure, the similar charac- Givargizov, E.I. (1986): Highly anisotropic crystals. Reidel, Dord- teristics of these assemblages are due to the contemporane- recht / Terra, Tokyo, 393 p. Genesis of filamentary pyrite associated with calcite 913

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