Pyrite framboids and their development: a new conceptual mechanism

Geol Rundsch (1993) 82: 148-156 © Springer-Verlag 1993

Z. Sawlowicz

Pyrite framboids and their development: a new conceptual mechanism

Received: 21 August 1990/Accepted: 28 September 1992

Dedicated to Professors G. C. Arnstutz and L. G. Love for their unrivalled contribution to the knowledge of pyrite framboids

Abstract It has been suggested that the two morphologies of sedimentary pyrite, framboids and euhedra, may reflect two distinct pathways of pyrite formation. Fram- boids form indirectly via iron monosulphides, whereas euhedra form from direct precipitation from solution. A third pathway which is bridging these two forms is proposed here, namely the continuous growth from a monosulphide globule through framboids to a euhedral single crystal. It is also suggested that framboids probably occur over a range of three orders of magnitude, from the least complex microframboids through framboids to polyframboids.

Key words Pyrite - pyrite formation - framboids

Introduction

The spherical aggregates of minute pyritic grains resembling raspberries, which were first described as framboids by Rust (1935) from French word 'framboise', have fascinated many scientists for decades. The recognition of this texture and its relationship with other minerals has been important in determining the time of formation of many ore deposits, the biogenic or inorganic origin of mineralization, and it has also gained attention as a possible culture medium for the origin of life (Russell et al. 1989; 1990).

The occurrence of pyrite in framboidal aggregates is common in Recent marine sediments and in black shales,

Z. Sawlowicz Institute of Geological Sciences, Jagiellonian University, ul. Olean- dry 2A, 30-063 Cracow, Poland

but it has also been reported from many other environments, e.g. magmatic rocks (Steinike, 1963; Kanehira and Bachinski, 1967; Love and Amstutz, 1969), carbonate rocks (Menon, 1967), coals (Wiese and Fyfe, 1986), tills (Stene, 1979) and even beach sands (Hossain, 1975). Framboidal textures are usually associated with pyrite, although other minerals occur in the same form, e.g. magnetite (Suk et al., 1990), hematite (Lougheed and Mancuso, 1973), limonite, magnesioferrite (Taylor, 1982), chalcocite/digenite (Sawlowicz, 1990). Urban (per- sonal communication, 1990) has also observed arsenopy- rite framboids. Some of the non-pyrite framboids could be oxidation products of pyrite (magnetite, limonite), or precursors of pyrite (greigite).

In sedimentary rocks, pyrite fralnboids are generally considered as syngenetic (formed in the water column, e.g. Degens et al., 1972; Skei, 1988) or early diagenetic components (e.g. Love and Amstutz, 1966; Berner, 1970). In some instances, however, they can also form during late diagenesis, e.g. by the pyritization of biotite (Menon, 1967) or magnetite (Canfield and Berner, 1987). Different genetic origins have been proposed for framboids, ranging from a purely inorganic origin, based on laboratory synthesis (e.g. Berner, 1969; Farrand, 1970; Sunagawa et al., 1971 ; Sweeney and Kaplan, 1973) and occurrences in magmatic rocks, through indirect biogenic formation (e.g. Vallentyne, 1963; Kalliokoski and Cathles, 1969; Lougheed and Mancuso, 1973), to a direct biogenic origin (e.g. Schneiderhoehn, 1923; Fabricius, 1961; Skripchen- ko, 1968; Locquin and Weber, 1978). Reviews of the various hypotheses can be found in Love and Amstutz (1966), Kalliokoski and Cathles (1969), Rickard (1970), Taylor (1982) and Schallreuter (1984).

The formation and stability of pyrite framboids have been explained in terms of formation processes (via intermediate monosulphides) and the surface and crystal chemistry of the pyrite microcrystals (e.g. surface tension, electrostatic and magnetic field effects). In the experiments of Sweeney and Kaplan (1973), spherulization of the framboids took place when the initial iron sulphide precipitate was transformed to greigite. Observations in

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nature suggest that spherulization might result from pseudomorphism of organic globules (Papunen, 1966; Kalliokoski and Cathles, 1969; Rickard, 1970) or of gas vacuoles (Rickard, 1970). The formation of grains in framboidal spheres can be explained by the 'particulation' and the 'agglomeration' hypotheses (Kalliokoski and Cathles, 1969). Most of the workers studying framboids prefer the particulation of primary gel droplets. As Kalliokoski (in Kalliokoski and Cathles, 1969) suggested, it is difficult to imagine the formation of polyframboids, comprising subunits of the same size, through the process of aglomeration. The presence of closely packed framboids of the same size infilling microfossils (e.g. McNeil, 1990) also strongly supports this theory. However, there are also reports showing the systematic build-up of pyrite microcrystallites around earlier formed nuclei (Menon, 1967; Morrissey, 1972). It therefore seems reasonable to expect that in some instances the agglomeration may lead to simple framboids, but that the formation of several sizes of pyrite framboids (e.g. microframboids - framboids - polyframboids) requires particulation of the original form. Organic matter is not necessary, but it probably plays an important part in framboid formation. In experiments where organic compounds were excluded, framboids mostly formed by agglomeration, i.e. the clumping of granules (Sunagawa et al., 1971) and had to be isolated from the solution shortly after precipitation (Farrand, 1970; Kribek, 1975). The limited stability of framboids in the absence of organic matter may explain the relative rarity in nature of framboids which crystallized from an inorganic iron sulphide gel (Rust, 1935; Love and Amstutz, 1966; 1969; Love and Brockley, 1973; Ostwald and England, 1977). In experiments where gel droplets were stabilized by organic substances (Kizilshtein and Minaeva, 1972; Kribek, 1975), framboids formed by particulation and were relatively durable.

There is no consensus on the origin of pyrite framboids, probably because there is no universal genesis of the framboids and most of the suggested hypotheses are valid under specific environmental conditions. Two conditions must be fulfilled for the formation of pyrite framboids: the availability of iron and of sulphur (of organic or inorganic origin). These conditions are met in different environments, especially if they are organic-rich. There- fore, it may be assumed that the formation of pyrite in biological systems is mainly a chemical process between metals and biologically produced hydrogen sulphide. Dark carbonaceous sediments, which are the typical hosts of pyrite framboids, are ideal environments for their growth. They contain abundant reactive iron (Canfield, 1989) and abundant organic matter with different func- tions (e.g. a source of energy for sulphate-reducing bacteria, a source of spherule moulds for framboid pseudomorphs, stabilization of the gel and fossil tests in which framboids are protected). Framboids are often found within confined spaces such as foraminifera tests, diatom frustules, polychaetes tubes and plant cells (Love, 1967; Honjo et al., 1965; Thomsen and Vorren, 1984;

McNeil, 1990). The suggestion of Taylor (1983) of the possibility ofmagnetotactic bacteria containing ferrimag- netic sulphides as an alternative to magnetite has been confirmed in nature. The discovery of various forms of iron sulphides (pyrrhotite?, greigite, pyrite) in living, primitive magnetotactic bacteria (Farina et al., 1990; Mann et al., 1990) suggests that the formation of iron sulphides might sometimes be more directly dependent on metabolism than is commonly believed. A colony of magnetotactic bacteria is similar to framboids in appear- ance and size (5 - 10 ~tm, Farina et al., 1990; Fig. 1 A), the cells containing discrete and organized iron sulphide particles (Mann et al., 1990; Fig. 1A). When a large number of magnetic particles occur in one cell, they may form iron monosulphide, and eventually pyrite (micro)framboids. The significance of magnetic attraction in framboid formation has been stressed by Taylor (1982).

Goldhaber and Kaplan (1974) and Raiswell (1982) suggested that the two morphologies of sedimentary pyrite, framboids and single crystals (euhedra), may reflect two distinct pathways of pyrite formation. Accord- ing to Goldhaber and Kaplan (1974) iron monosulphides typically form from solutions supersaturated with respect to iron monosulphides and pyrite, whereas pyrite forms from solutions undersaturated with respect to iron monosulphides. The relative under-supersaturation is thought to depend on several factors, e.g. pH (Howarth, 1979; Berner et al., 1979), sulphide concentration (Howarth, 1979), iron concentration (Raiswell, 1982) and the presence of soluble polysulphide sulphur (Rickard, 1975). The direct formation of euhedral pyrite, without precursor iron sulphide phases, is probably rapid (Rickard, 1975; Howarth, 1979; Luther et al., 1982), although Raiswell (1982) argued, on the basis of sulphur isotopic measure- ments, that in iron-poor environments euhedral pyrite may not form rapidly. However, the experiments of Schoonen and Barnes (1991) suggest that the strict differentiation between direct and indirect pyrite formation is not needed. These workers pointed out that the rate of FeS 2 formation initiated by FeS 2 nucleation is insigni- ficant compared with FeS2 formation via an FeS precursor. However, once pyrite nuclei are formed, they can continue to grow directly from solution. It is also interesting to note that crystalline iron monosulphide formation is preceded by the formation of initial 'Fe(SH)2', which is in turn transformed to amorphous FeS (Rickard, 1989). The role of these first precipitates in iron sulphide reaction pathways is still not under- stood.

The aim of this paper is to show that a third pathway exists for framboidal pyrite formation, which integrates the two mechanisms just described, and which is a continuous growth from globules through framboids to euhedral single crystals. Another aim is to show that framboids probably exist over a range of three orders of magnitude in size and complexity, from the least complicated microframboids through typical framboids to polyframboids (and even dumbels of polyframboids; Fabricius, 1961).

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Relationship between framboidal and euhedral pyrite

The framboidal state is probably best described as a metastable form. The possibillity of pyrite recrystalliza- tion from framboidal crystals to single grains has been reported by several works (e.g. Love, 1965; Kalliokoski, 1965; Love and Amstutz, 1966; Ostwald and England, 1977; Sawlowicz, 1987). However, as a result of dispersed and scanty information, this problem has not been widely discussed.

Disseminated euhedral pyrite crystals are often inti- mately mixed within a few micrometres of sediments with perfectly formed framboids (with seParate crystals) of similar size (Fig. 1A); the intermediate textures between these two types are also often observed (Fig. 1 B - J ) (Love and Amstutz, 1966; Chen, 1978; Sawlowicz, 1987). Dispersed pyrite crystals with similar sizes to those comprising framboids are also often observed together with the framboids; they are considered to originate from disaggregated framboids. Among the typical intermediate textures are polygonal framboids (Fig. 1 E, F), closely packed framboids (with hardly recognizable boundaries between the microcrystals), rounded and nearly homogeneous masses with only small holes leaving an impression of a framboidal texture (Fig. 1 B, D, G), framboids with larger outermost crystals, and massive sperules (Fig. 1 H, I). The arrangement of the microcrysts within framboids varies from random, linear or concentric to polygonal (Lougheed and Mancuso, 1973; Chen, 1978). It seems that the regular arrangement of microcrystals in framboids can be as early as for a primary colloidal iron hydroxide. Love and Amstutz (1966) gave detailed descriptions of ordering patterns in the framboids. However, it is not obvious if the ordering of microcrysts in a framboid is a uniform feature partly or completely lost during diagenesis, or if some of the framboids are primarily disordered. It is worth noting that both the ordered framboids (Amstutz et al., 1967) and the single euhedra of pyrite (Ramberg and Ekstrom, 1964) may show a preferred orientation of a crystallo- graphically orientated pattern of ordering, or crystal faces against the plane of bedding or schistosity of the rock. A number of imperfectly rounded framboids show tendencies towards polygonal outlines (Fig. I E, F) (hexagonal, pentagonal), sometimes with a distinct affinity to the internal ordering pattern (Love and Amstutz, 1966; Morrissey, 1972; Elverhoi, 1977; Sawlo- wicz, 1987). The habit (cubes, octahedra and pyritohedra) of single pyrite euhedra is often the same as the habit of microcrystals which constitute the coexisting pyrite framboids in sediments (Wiese and Fyfe, 1986). Pyrite framboids are often found inside sacs built of organic matter (Love, 1958; Love and Amstutz, 1966). Sometimes square or polygonal outlines of organic sacs (obtained after the artificial dissolution of pyrite) mimic the pyrite crystal habit, as if they were moulded from within during the transformation of the framboids into euhedra (Love, 1965; Neves and Sullivan, 1964). Additional pyrite often

precipitates on the organic sac surrounding the framboids (Sawlowicz, 1987).

Different mechanisms may lead to the homogenization of framboids and the formation of euhedral crystals: infilling, 'Sammelrekristallisation' (german term for 'collection or fusion-recrystallisation'; Love and Am- stutz, 1966) and the transformation of framboids due to a continuous supply of the constituting materials. In the process of infilling, new pyrite fills the spaces between the granules or microcrystals in the framboids (Love and Amstutz, 1966; Love and Brockley, 1973; Raybould, 1973). This new pyrite (often referred as 'Melnikovite- Pyrite') usually displays a lower reflectivity (sometimes after etching) which results from chemical (As, Co, Ni admixtures; Ramdohr, 1955) or physical (e.g. submicro- scopical porosity; Chauchan, 1974) differences. The interstices in pyrite framboids of ore deposits are often infilled by other sulphides such as galena or sphalerite (Love and Zimmerman, 1961; Ncube et al., 1978), copper sulphides and copper- iron sulphides (Love, 1962; Sawlowicz, 1992). These examples suggest that the infilling processes take place later in the history of pyrite framboids and are not directly connected with the framboid formation, although the final result may also be an euhedral crystal. In the process of 'Sammelrekri- stallisation' the existing pyrite crystals weld together without the addition of new material (Love and Amstutz, 1966). Although in some instances this process can probably produce framboids, it cannot lead to the development of euhedral crystal from a framboid because, as Love (1965) pointed out, this simple reorganization should bring about a reduction in volume, which is never observed. It is suggested here that a continuous supply of constituting material to the microenvironment of framboid formation represents the simplest way of leading to a euhedral pyrite crystal, with framboidal pyrite as an intermediate stage. The process of transformation of the framboids into euhedra might be the result of a trend towards minimization of the surface energy.

Fig. 1. Different development stages of pyrite framboids: A- J = reflected light, oil; K-P = scanning electron microscope; single bar = 7 gin, double bar = 0.5 pro. (A) Pyrite framboids and euhedra of similar size in the Kupferschiefer; (B) framboids with different stages of grain coalescence and pyrite euhedra; (C) close spatial relationship between pyrite framboid (left side), euhedra and framboid with amalgamated outer ring grains (right side); (D) different stages of development of pyrite framboidal texture; (E) framboid revealing geometrical regular pattern and hipidiomorphic outline; (F) framboid with an idiomorphic outline built of strongly amalgamated grains; (G) hipidiomorphic pyrite grain with holes in the core, resembling a framboid; (H) Partly angular pyrite spherule with very rough outline; (I) slightly angular pyrite spherule; (J) two almost idiomorphic pyrite crystals; (K) polyframboid; (L) aggregations of minute particles forming spherical grains (microframboids) in framboid; (M) aggregations of minute particles forming subhedral grains in framboid; (N) framboids of different sizes building a polyframboid (note a hollow framboid in the lower right corner); (O) framboids built of xenomorphic grains of different sizes (note framboids with partly hollow cores); (P) pyrite framboid consisting of grains with minute depressions or holes

Fig. 1

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The hypothetical pathways of the evolution from framboids to euhedra presented in the following are of three types (Fig. 2)

(1) A continuous growth of microcrystals (granules) in the framboids producing amalgamation, as interpe- netrating cubes and/or octahedra are common in framboids, which fill in the remaining spaces and induce the formation of massive pyrite spherules. Such spherules might further evolve to pyrite euhedra (Fig. 2 B).

(2) When the microcrysts in the framboid are closely packed, a layer of elongate grains is sometimes observed. Typically this layer appears to be one grain thick (Love and Brockley, 1973), although Kosacz and Sawlowicz (1983) observed layers two grains thick. Love and Brockley (1973) believe that such a layer has to be formed during the main stage of crystallization of pyrite framboids. In some instances further growth of the outermost pyrite grains can transform this texture into an idiomorphic crystal (Fig. 2A).

(3) When the internal geometric pattern is stable and/or the surrounding material is plastic enough to be displaced, the development of a framboid can lead straight to the growth of regular faces of euhedra via a polygonal framboid (Figs 1 F and 2C). Such growth may also lead to the development of zoned crystals (Sawlowicz, 1987).

Homogenization of framboids usually spreads from the centre to the surface of the spherules (Love, 1965; Sawlowicz, 1987). During homogenization, substances such as organic matter (Love, 1965; Love and Amstutz, 1966) or clays (Scheihing et al., 1978; Love et al., 1984), which are common in the interstices of framboids, can be expelled or caught as inclusions. The latter probably explains the presence of Si, A1 and K peaks in energy dispersive spectra taken from the surface of about 20 gm 2 of a euhedral pyrite grain (Fig. 3), which is evidence for the formation of euhedra via a framboidal stage.

Transformation of pyrite framboids into pyrite euhedra has been obtained in the laboratory. The experiments of Farrand (1970) showed that framboids synthesized in an aqueous medium must be isolated from the solution within one week otherwise they turn into large cubic crystals. The preservation of the framboids probably depends on two factors: the isolation of framboids and their constituents from the influence of the external diagenetic environment and the interruption of the supply of at least one essential compound in the fluid (iron and/or H2S). Isolation might be produced by organic matter as organic skins or inclusion within organic laminae (Farrand, 1970; Kribek, 1975), carbonates (Su- nagawa et al., 1971), silica (Massaad, 1974), amorphous carbon (Laufer et al., 1985), other metal sulphides such as Cu sulphides (Sawlowicz, 1992) or intimate overgroycths of external grains in the framboid (Farrand, 1970). The interruption of supply might be due to stopping the activity of bacterial sulphate reduction or exhaustion of

o o' • • o •

Fig. 2. Hypothetical pathways for the formation of euhedral pyrite via framboids

a local source of iron. As might be expected in an inhomogeneous sediment with specific microenviron- ments, both the pyrite framboids and euhedra, together with their intermediate forms, occur side by side. Even in relatively homogeneous environments such as laboratory flasks different stages of framboid growth can be observed at the same time (Kizilshtein and Minaeva, 1972).

The habit of pyrite crystals can change during growth (Amstutz, t963). Sunagawa (1957), for instance, found that the habit of pyrite varied with grain size in Japanese hydrothermal vein deposits. Murowchick and Barnes (1987) showed that the degree of supersaturation governs the mechanism of crystal growth, resulting in different habits of hydrothermally grown pyrite. A decrease in supersaturation can generally be expected during the growth of framboids, as it is realized by precipitation and declining iron availability, decreasing sulphur activity and decreasing pH. The latter is sometimes visible by the formation of radial marcasite crystals around the pyrite framboid. The experiments of Schoonen and Barnes (1991) suggest that once the first crystallization centers of iron monosulphides in the framboids are formed and transformed to pyrite (crystal seeds), they grow further by direct precipitation of pyrite. The degree of supersaturation required for growth is much lower than that required for nucleation (Murowchick and Barnes, 1987). The change from indirect (via iron monosulphides) to direct precipitation of pyrite would give a relatively rapid

Fig. 3. Energy dispersive spectrum (KEVEX) taken from a 20 gm z surface of a euhedral pyrite crystal

S Fe

Si

K

t Fe

, . , • _

" a J

growth of microcrysts in a framboid leading to homogenization and the formation of well crystallized euhedra.

In summary, the following support the direct transformation of framboidal pyrite into euhedral pyrite: (1) the similar size and coexistence of framboids and euhedra; (2) the continuous range of intermediate forms between framboids and euhedra; (3) the habit of euhedra is often similar to the geometric pattern in framboids; (4) the habit of framboid-sized euhedra is often similar to the habit of the microcrystals comprising the framboid; (5) preferred orientation of ordered framboids and euhedra in shale; (6) the presence of isolated clays in pyrite euhedra; and (7) 'euhedral' organic moulds resembling pyrite euhedra.

From micro- to polyframboids

Mandelbrot's The Fractal Geometry of Nature (1982) and the transformation of framboids into euhedra have encouraged thought about the surprising similarities between the granules building framboids and polyframboids. As a consequence, it is proposed here that the form of microframboids should be considered, in addition to the well known framboid and polyframboid forms. The very small size (usually less than 1 gin), the rare use of high magnifications (even in a scanning electron microscope) during mineralogical observations, and the high tendency for this form to change into euhedra have meant that microframboids have been overlooked for a long time. It seems that the different scales of size and complexity of framboidal forms are related to a continuous growth and rearrangement of framboids into euhedra.

The microframboid, which is a spheroid form consisting of discrete equant nanocrysts, is structurally similar to the 'normal' framboid but is one order of magnitude smaller (Fig. 4). The size of microframboids covers the range from 0.1 to 1 ~tm, which is the size typically given to granules or microcrysts building framboids. Framboidal microspheres of > 0.2 gm (Berner, 1969; Farrand, 1970) were obtained from laboratory experiments and microspheres of 0.7 pm from deep sea sediments (Schallreuter, 1984) or 0.6 ~tm from pyritized diatoms have been described. Microframboids comprise irregular particles of about 50 - 100 nm (Fig. 1 L). Sometimes such aggregations tend to be facetted (Fig. 1 M), similar to observations of 'normal' framboids. It is worth noting that the mean size of iron sulphide particles, aggregates of which have been found in magnetotactic bacteria (Mann et al., 1990; Farina et al., 1990), range from 75 to 150 nm.

Most of the reported framboids range from 5 to 20 gm in size, but framboids as large as 250 pm have occasional- ly been found (Sweeney and Kaplan, 1973). Framboids as small as 1 gm (Love and Zimmerman, 1961; Solomon, 1967) were also seen. For contrast, the size of polyframboids ranges from 35 to 900 (average 5 0 - 200) gm (Love, 1971). These data show that the size ranges of the distinct

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NANOFRAMBOID? MICROFRAMBOID FRAMBOID POLYFRAMBOiD

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../;W W L2 @

5 0 - 1 0 0 n m 0 .5 -1um 5-10um 5 0 - 1 0 0 u m

Fig. 4. Idealized diagram showing various orders of size and complexity of pyrite framboids and their tendency towards the formation of euhedra

types of framboidal forms are overlaping. Framboidal aggregates about 60 gm in diameter can consist of both framboids and crystallites, the former being polyframboids and the latter framboids. Analogies can be drawn for framboids and microframboids.

Love (1971) reported a number of similar forms of pyrite framboids and polyframboids considered as aggregates of higher complexity, and suggested similar modes of formation. Individual pyrite grains or framboids are generally of similar size (Fig. 1 K), although exceptions do exist. There is a tendency for larger aggregates to consist of larger constituent structures, the ratio of microcryst (or framboid) to framboid (or polyframboid) is typically 1 : 10. This proportionality of size might be explained by an inverse relationship between the size of primary droplets and their iron concentration, as reported from the experiments of Kizilshtein and Minaeva (1972). It seems that saturation might stimulate not only the habit ofeuhedra formed from framboids, as described earlier, but also the size and complexity of framboidal forms. The inhomogeneous distribution of iron can probably induce the formation of polyframboids consisting of framboids of various sizes (Stene, 1979) (Fig. 1N). Although polyframboids typically consist of framboids, some may have been changed into euhedra. Polyframboids can also give evidence for homogenization and overgrowth. It therefore seems possible that some of the extra large framboids built of euhedra could be recrystallized polyframboids. For instance, Spiro and Rozenson (1980) described pyrites consisting of framboids ranging from 60 to 200 gm in diameter, whereas their individual components measured 5 - 1 0 gm. These ranges are typical for polyframboids and framboids, respectively.

Some of these phenomena, which were observed in framboids, suggest that a similar relationship exists between polyframboids and framboids as exists between microframboids and framboids. Framboids can be built ofmicroframboids, especially at the earliest stages of their

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development, microeuhedra or irregular grains (Fig. 1 O). Kalliokoski (1974) emphasized that the smallest granules of framboids are spherical, that granules of intermediate size may be spherical or faceted, and that the largest are faceted, although the exceptions are not rare. In the opinion of Kalliokoski and Cathles (1969), such a grada- tional sequence has to represent the diagenetic growth of individual pyrite granules and can be compared with the growth from framboids to euhedra, as described earlier. The nature of the smallest grains of framboids remains uncertain. The surfaces of particles of pyrite and greigite, which have a mean size of 75 nm and were found inside magnetotactic bacteria, range from oval (nanoframboid?) to rhombohedral and hexagonal (Mann et al., 1990).

Further evidence that spherules building framboids can themselves be framboids is the occurrence of so-called annular framboids (Fig. 1; Kosacz and Sawlowicz, 1983) of different scale size. Such 'hollow' framboids consist of a sphere (a discontinuous ring in section) built of minute pyrite grains (Figs 1N and 5). The experiments of Kizilshtein and Minaeva (1972) have shown that they can be the first step in the development of 'complete' framboids.

Annular framboids are observed in nature, probably preserved because of the low availability of iron and/or sulphide (Papunen, 1966; Love, 1967), early sealing of the empty interior by phosphates or metal sulphides, or replacement of metastable iron sulphide intermediates by some metal sulphides (Sawlowicz, 1992). Schallreuter (1984) observed a framboid from deep sea sediments with some hollow crystallites and suggested that these structures may be identical to the 'unexplained minute depressions ... on the faces of individual crystals' observed by Kalliokoski (in Love and Amstutz, 1966). Framboids with similar grains and holes in their cores were also observed in black shales from the Polish Carpathians (Fig. 1 P). These hollow crystallites may represent 'annular microframboids'. It is worth noting that Ahn and Buseck (1990) described spheres, 120-200 nm in diameter, consisting of haematite platelets surrounding a cen- tral void. These workers suggest that the spheres formed by structural ordering and dehydration of colloidal iron hydroxite particles. The empty interiors of annular framboids may result not only from a scarcity of building material, but also from the differential dissolution pro- perties of a core. If the formation of pyrite framboids begins from the surface of a gel globule, it is reasonable to suppose that the outer grains of primary iron monosul-

Fig. 5. Hypothetical pathway for the formation of framboids via annular framboids

monosu[pides

oO pyrite

phides recrystallize more rapidly and earlier to more resistant pyrite than the metastable monosulphide grains in the core of a developing framboid (Fig. 5).

In sediments, the ratio of pyrite euhedra to pyrite framboids increases with the decreasing size of these forms (Love and Amstutz, 1966). Although microframboids commonly form euhedra, the large hipidiomorphic grains formed from polyframboids are rare (Kosacz and Sawlowicz, 1983). The comparison between microframboids, framboids and polyframboids suggests that the trend towards the formation of idiomorphic grains via a framboidal stage is inversely dependent on the size and complexity of the aggregate (Fig. 4), i.e. the euhedra are much more easier formed from microframboids than from polyframboids. Simi- lar to the mechanism described by Kalliokoski and Cathles (1969), it is suggested here that the original gel droplet undergoes several stages of particulation into immiscible smaller droplets. The number of subsequent divisions probably depends on the initial size of the droplet, the concentration of iron (see Kizilshtein and Minaeva, 1972) and of other compounds (organic matter, silica, carbonates, clay minerals) stabilizing the gel and determining its viscosity, and the activity of sulphur species determining the rate of iron monosulphide and pyrite formation. It seems reasonable to expect that the microcrysts in framboids do not always form via microframboids. In some instances the smallest droplets probably do not undergo further particulation and then the framboid will initially consist of single granules, which will turn into microcrystals, and not of microframboids. A similar relation may also be observed among framboids and polyframboids. On the basis of size and association Kalliokoski (1974) reported the existence of spherical pyrite structures, which might be described as 'granuleless framboids' and as 'framboidless polyframboids' for some of them. It is not impossible that in some instances the process of particulation, which induces the formation of microframboids and/or framboids, can be followed by the process of agglomeration, which induces the formation of framboids or polyframboids.

In summary, the following evidence for microframboids has been considered: (1) the occurrence of framboids of the size of granules building 'normal' framboids; (2) the occurrence of particles less than 0.1 pm in size, forming aggregates in bacteria; (3) hollow microgranules (annular microframboids ?) similar to annular framboids; (4) the transformation of spherules into crystals observed in framboids of all size ranges; (5) similiarities between the development of forms of grains building framboids and polyframboids.

By analogy with the described three-scale range of framboids, it is speculated that microframboids can be built from even smaller nanoframboids(?) (Fig. 4). Na- noframboids might consist of aggregations of particles of microcluster size. Microclusters are tiny aggregates consisting of two to several hundred atoms (Duncan and Rouvray, 1989).

155

Possible influence of organic matter on framboid formation

The common occurrence of pyrite framboids in organic- rich sediments and in microfossils, and the presence of organic matter in framboid interstices suggests the possible influence of organic materials on framboid formation. According to Mann (1988), the influence of organic macromolecules is important in the regulation of growth of the mineral and in the resulting specificity of crystal morphology and particle aggregation. The growth of pyrite framboids in organic materials has not been studied so far, but experiments on other sulphides seem to show a number of features observed in framboids as described earlier. Dameron et al. (1989) described the biosynthesis of CdS in yeasts. In this experiment the nucleation and growth of CdS crystallites into spherical intracellular particles, 20 ~ in diameter, was controlled by peptides. The experiments of Bianconi et al. (1991) on the synthesis of CdS in an organic polymer matrix showed that the latter can mediate the inorganic reaction and determine the morphology of the product. When CdC12 reacted with S[Si(CH3)312 in a crystalline polymer (PEO), regular cubic crystals of CdS (1 g m in size) formed, p r o b a b l y by the o rdered aggrega t ion o f smaller part icles. The reac t ion in an a m o r p h o u s po lymer ( P M M A ) gave spherical crystal l ine part icles. In their study, Bianconi et al. (1991) found that the part icle or crystal size of CdS (2 n m to 2 gin) depended on the initial concentra t ion of CdC12, and also tha t the crys ta l hab i t was a l te red by the addi t ion of ligands. These results i l lustrate the impor tance o f organic substances on different aspects o f f r amboida l pyri te fo rma t ion .

Acknowledgements I am grateful to M. J. Russell (University of Glasgow) for discussion, correction of the English text and com- ments. My thanks also go to M. A. A. Schoonen (SUNY-Stony Brook) for reading the early draft, to N. Clauer (Strasbourg) for improving the final version, and to J. Faber for technical assistance with the scanning electron microscope. S. Philippe and U. Wiechert kindly translated the abstracts. Financial support of the Alexander von Humboldt Foundation (Bonn) is gratefully acknowledged.

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Pyrite framboids and their development: a new conceptual mechanism

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