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Pyrite framboids and their development: a new conceptual mechanism
Article in Geologische Rundschau · April 1993
DOI: 10.1007/BF00563277
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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
It has been suggested that the two morphologies of sedimentary pyrite, framboids and euhedra, may
reflect two distinct pathways of pyrite formation. Framboids 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.
Abstract
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. Oleandry 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 (personal communication, 1990) has also observed arsenopyrite 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; Skripchenko, 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
149
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. Therefore, 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 functions (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 ferrimagnetic 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 appearance 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. According 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 measurements, 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 insignificant 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 understood.
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).
150
Relationship between framboidal and euhedral pyrite
The framboidal state is probably best described as
a metastable form. The possibillity of pyrite recrystallization 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 intimately 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 crystallographically 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; Sawlowicz, 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 Amstutz, 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 'MelnikovitePyrite') usually displays a lower reflectivity (sometimes
after etching) which results from chemical (As, Co, Ni
admixtures; Ramdohr, 1955) or physical (e.g. submicroscopical 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 'Sammelrekristallisation' 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 = reflectedlight, oil; K - P = scanning electron microscope;
single bar = 7 gin, double bar = 0.5 pro. (A) Pyrite framboidsand
euhedra of similar size in the Kupferschiefer; (B) framboids with
different stages of grain coalescenceand pyrite euhedra; (C) close
spatial relationshipbetweenpyriteframboid(left side), euhedra and
framboid with amalgamated outer ring grains (right side);
(D) different stages of development of pyrite framboidal texture;
(E) framboidrevealinggeometricalregularpattern and hipidiomorphic outline; (F) framboid with an idiomorphic outline built of
stronglyamalgamatedgrains; (G) hipidiomorphicpyrite grain with
holes in the core, resemblinga framboid; (H) Partly angular pyrite
spherulewithveryrough outline;(I) slightlyangularpyritespherule;
(J) two almost idiomorphicpyrite 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 buildinga polyframboid(note a hollowframboidin the lower
right corner); (O) framboidsbuilt of xenomorphicgrains of different
sizes (note framboidswithpartly hollowcores); (P) pyrite framboid
consisting of grains with minute depressions or holes
151
Fig. 1
152
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 interpenetrating 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 (Sunagawa 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. Hypotheticalpathways for the formation of euhedral pyrite
via framboids
a local source of iron. As might be expected in an
inhomogeneous sediment with specific microenvironments, 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 gmz
surface of a euhedral pyrite crystal
S
Fe
t
Si
Fe
K
,.,
"a
J
•
_
153
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.
NANOFRAMBOID? MICROFRAMBOID
.
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 occasionally 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
POLYFRAMBOiD
"-
.
,,,__,
../;W W
L2 @
50-100nm
From micro- to polyframboids
FRAMBOID
0.5-1um
5-10um
50-100um
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
154
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 gradational 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 central 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 properties 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. Similar 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). Nanoframboids 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 organicrich 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 b y the o r d e r e d a g g r e g a t i o n
o f smaller particles. T h e r e a c t i o n in an a m o r p h o u s p o l y m e r
( P M M A ) gave spherical crystalline particles. I n their study,
Bianconi et al. (1991) f o u n d that the particle or crystal size o f
CdS (2 n m to 2 gin) d e p e n d e d on the initial concentration o f
CdC12, a n d also t h a t the c r y s t a l h a b i t was a l t e r e d b y the
a d d i t i o n o f ligands. These results illustrate the i m p o r t a n c e o f
organic substances on different aspects o f f r a m b o i d a l pyrite
formation.
Acknowledgements I am grateful to M. J. Russell (University of
Glasgow) for discussion, correction of the English text and comments. 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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