Framboidal Pyrite Nucleation and Crystal Growth :sx a progress report .sx by David Rickard and Ian Butler .sx Framboidal pyrite represents a simple two component inorganic system that readily reaches a remarkable degree of organizational sophistication .sx Apart from their aesthetic attributes , framboids are interesting because they may tell us something about the fundamental processes involved in the development of organized systems and about the physico-chemical conditions of ancient environments .sx Framboids also have a close relationship with biological materials and processes :sx ( a ) they are intimately involved in biomineralization , especially in pyritization of plants ( b ) microbial sulphate reduction is the major source of framboidal sulphide and ( c ) organic matter forms the matrix between microcrysts in some framboids and enclosing outer sacs in others .sx Framboid characteristics .sx Any model for the formation of framboids must account for the following properties :sx ( 1 ) the dominant mineral form in framboids is pyrite ( 2 ) up to 10 7 pyrite mycrocrysts ( usually < 1 mu m in size ) occur in spheroids usually less than 100 mu m in diameter ( 3 ) the even size of the microcrysts within any framboid ( 4 ) the varying habits of the microcrysts in different framboids ( 5 ) the occurrence of ordered patterns involving either ccp or hcp organization ( 6 ) the occurrence in some framboids of organic material between the microcrysts and/or around the framboids .sx Pyrite formation .sx The formation of pyrite from aqueous solutions at low temperatures proceeds via three known pathways :sx ( 1 ) Reaction of precursor metastable iron sulphides ( MIS ) with polysulphides ( Rickard 1975) .sx Schoonen ( 1989 ) examined the reaction in some detail and Luther ( in prep ) has confirmed Rickard's kinetics .sx ( 2 ) Reaction of Fe 3 S 4 ( greigite ) with sulphur species to form pyrite ( Sweeney and Kaplan 1973) .sx This reaction can lead to the formation of framboids .sx ( 3 ) Reaction of Fe ( II ) sulphide with H 2 S to form pyrite with the production of H 2 gas ( Taylor et al 1979 ; Dobner et al. , 1990) .sx We have reproduced this reaction and detected H 2 gas .sx We are at present running a programme to examine the kinetics and mechanism of the process .sx In each case , pyrite formation at low temperatures proceeds through a MIS precursor .sx It appears that this precursor pathway for pyrite formation is related to its propensity to form the framboidal texture .sx Direct precipitation of pyrite from solution .sx The direct precipitation of pyrite has not been achieved experimentally .sx One of the problems of experimental work in these systems is the enormous supersaturations necessary in most experimental work .sx If the kinetics of nucleation of MIS was far faster than that for pyrite , then these supersaturations , within the MIS stability zones , might precipitate the metastable phase before pyrite :sx i.e. the pyrite requirement for a precursor phase could be merely kinetic and pyrite would precipitate directly from solution if the solutions were undersaturated with respect to the MIS phases and oversaturated with respect to pyrite .sx Schoonen ( 1989 ) arranged experiments below saturation for the MIS but saturated with respect to pyrite , but was unable to precipitate pyrite directly .sx Dales and Rickard ( unpublished ) have arranged such an experimentation through the diffusion of aqueous iron ( II ) species and sulphur-bearing species from opposite ends of a tube filled with a porous medium .sx Pyrite did not form and this implies that pyrite does not precipitate directly from solution .sx This conclusion has been supported by Luther's ( in prep ) experimentation .sx Precursor Metastable Iron Sulphides ( MIS ) .sx The nature of the precursor material is important for the formation of the framboidal microarchitecture .sx Rickard ( 1990 ) showed that the first formed phase was probably a sic !sx iron ( II ) bisulphide cluster colloid ( Silvester et al 1991 ) with a size between the conventional 0.1 mu m colloidal range and the 1nm cluster range .sx This undergoes rapid re-reaction with the expulsion of H 2 O and H 2 S to form amorphous Fe(II ) sulphide .sx A broad x-ray peak at around 0.5nm is observed after 2 days of ageing at 25 degree C coincident with the [001] reflection of the tetragonal iron ( II ) sulphide mackinawite .sx This appears to be evidence for the beginning of long-range ordering in the Fe(II ) sulphide .sx Further mackinawite reflections appear over the next two years of ageing , indicating the increase of short-range ordering in the material .sx The pathway for the formation of the cubic Fe 3 S 4 , greigite , is not well documented .sx As far as we are aware , greigite has not been synthesised directly from solution , but is thought to be produced as an oxidation product of iron ( II ) sulphide .sx Pyrrhotite-group iron ( II ) sulphides , with basic hexagonal symmetries , have been produced from aqueous solutions at low temperatures ( Rickard 1969 ; Sweeney and Kaplan 1974) .sx However , the detailed conditions for pyrrhotite development at low temperatures are unknown .sx Pyrite nucleation .sx The occurrence of precursor MIS before the more stable pyrite has been cited as an example of Ostwald's Rule of Successive Reactions .sx This is only partly true , since pyrite is more oxidised than the precursor MIS and its formation requires oxidation reactions as well as equilibration .sx For pyrite to nucleate , the solution must be supersaturated with respect to pyrite .sx Rickard ( 1975 ) assumed that the apparent reluctance of pyrite to nucleate directly from solution was purely mechanistic in origin .sx The reaction between polysulphide and iron ( II ) salts was investigated in detail by Schoonen ( 1989) .sx His results show that reactions like .sx formula .sx are faster than reactions like .sx formula .sx Once the reaction involves one or more precursor solid phases it becomes potentially kinetically slower .sx On the other hand , this also means that substantial supersaturations can build up and exist metastably for some time .sx This would be consistent with the framboidal property that the pyrite microcrysts within any given framboid are of similar size :sx this property requires that the pyrite microcrysts nucleated throughout the framboid .sx If this were not so then the pyrite would tend to grow by simple crystal growth mechanisms , which Schoonen has shown is relatively rapid for pyrite , and form crystal aggregates with diverse sizes .sx However , it does not explain the organisation of microcrysts in some framboids .sx Mann ( 1988 ) showed that , with respect to organic matrices , two conditions must be satisfied :sx ( 1 ) molecular preorganisation and ( 2 ) molecular complementarity between inorganic ions and local matrix binding sites .sx Applying this to framboids , microscopic and molecular organization within the precursor MIS must be completed before pyrite nucleation starts .sx Otherwise , nucleation will be at random sites throughout the iron sulphide matrix .sx This is the situation observed in the experiments of Rickard ( 1969 ) and Luther ( in prep ) where continuous reaction between MIS and oxidised sulphur species gave precipitates of random pyrite microcrysts :sx the MIS reactant was relatively young and still disorganized .sx Some degree of order must be present in the precursor MIS either as specific microdomains within the material or as a regular aggregate of particles for the regular arrays of pyrite microcrysts to be developed .sx Such order develops through ageing and/or re-reaction of the MIS .sx In this interpretation the observed variation in degree of ordering in pyrite framboids reflects the preorganization in the precursor MIS matrix .sx The development of microdomains or regular particle aggregates will produce active sites within the precursor iron sulphides , which in themselves may catalyse the nucleation of pyrite .sx Indeed , Taylor ( 1980 ) suggested that the S 2 moiety may only be produced at active sites on MIS surfaces or within MIS aggregates .sx Both mackinawite and greigite are possible precursors to pyrite , although only greigite has definitely been implicated in framboid formation to date .sx There is thus at least a theoretical possibility for different organizational geometries in the precursor material , giving rise to the differently ordered arrays of pyrite microcrysts observed in framboids .sx Microcryst form .sx At high temperatures , Murowchik and Barnes ( 1987 ) were able to show that pyrite crystal morphology varied in response to increasing saturation in the order cube-octahedron-pyritohedron .sx The variation in microcryst form ( usually cubes or octahedrons ) in framboids may reflect this .sx Pyrite may nucleate under varying degrees of supersaturation dependent on the local free energy of the surface .sx Interestingly we have also observed microcrysts with holes in them , suggesting that crystal growth in these cases proceeded from the outside inwards .sx We have also observed regular framboid microcrysts consisting of aggregates of sub-spherical pyrite particles in the cluster colloid size range .sx Organic matter .sx The origin of the organic matter in some framboids has been of interest since Schneiderhohn ( 1923 ) originally thought the textures to be fossilized bacteria and Love ( 1957 ) isolated apparent microorganic remains from framboids .sx Rickard ( 1970 ) suggested that the organic matter derived from a coercevate that had been sulphidised , in process three-dimensionally equivalent to the synthetic production of CdS in a polymer film ( Bianconi et al 1991) .sx The role of organics is interesting since as Mann ( 1988 ) pointed out , they could act to reduce the surface free energy of the pyrite nucleating system , thereby catalysing microcryst nucleation .sx At present , it is unknown whether the organic matter within some framboids is a natural abiologic equivalent of nucleation within a crystalline polymer or merely organic substances absorbed onto pyrite during diagenesis ( Kribek 1975 ) or forced into the spheres during burial .sx ( Elverhoi 1977) .sx Conclusions :sx Hypothesis and test .sx In this model , different degrees of ordering of pyrite framboids reflect different degrees of organization of the precursor MIS .sx The crystallinity of the precursor MIS is a function of the structural state of the precursor material prior to pyritisation .sx This in turn is dependent on the age and composition of the MIS .sx The nucleation energy barrier for pyrite may be overcome by a variety of factors , including supersaturation ( which may determine microcryst growth habit ) , active sites in the MIS and organic catalysis .sx The model appears to be testable and we are at present attempting to synthesise framboids under controlled conditions using well-defined starting materials .sx ASPECTS OF BIOLOGICAL SILICIFICATION ; MODEL SILICA PRECIPITATION STUDIES IN THE PRESENCE OF CARBOHYDRATE POLYMERS .sx Carole C. Perry .sx Silica is an important industrial chemical manufactured worldwide with applications including glasses , catalyst supports , adsorbates , pigments and fillers .sx The structure of the silica determines the properties of the material and yet the basic mechanisms of particle formation and aggregation in aqueous solution remain imperfectly understood .sx Biology has mastered the art of producing well controlled macroscopic silica structures eg .sx sponges , diatoms , plant components etc. although all biogenic silicas are complex materials comprising both organic and inorganic phases .sx Biogenic silicas are built up from essentially monodispersed particles ranging from 2-15nm in diameter depending upon the structure .sx At a higher level , these particles are organised into structural motifs ( fibrils , tubules , sheets etc. ) which indicates a substantial degree of control over both polymerisation and particle aggregation .sx The formation of all inorganic solids including silica from aqueous solution is achieved by a combination of three physico-chemical steps ; supersaturation , nucleation and crystal growth or maturation .sx A further essential element for controlled biological mineralisation is spatial localisation , which may occur either through the use of membrane bounded compartments or specific cell wall regions and allows for the local regulation and control over physico-chemical factors through selectivity in biochemical processes such as ion and molecular transport .sx Studies of the formation of biological silica in relation to changes in mechanical stresses , ionic composition and polymer matrices have shown that all these factors may be relevant in the production of specific aggregate structures , often with varying surface chemistry .sx It is important to note that local ionic environment effects can have little or no effect on the regulation of morphological features at the micron level and other factors must be important .sx Macromolecular assemblages are thought to be extremely important in the regulation of both nucleation and growth processes for crystalline and amorphous minerals .sx In many instances of crystal deposition , for both in vivo and in vitro systems , the nature of the interaction involves a specific charge/ stereochemical matching but for amorphous materials including silica the precise nature of the interaction between the organic and inorganic phases is not known .sx At present our biological studies are directed towards this end .sx A direct result of our biological studies has been the development of model chemical systems for the investigation of crystal or aggregate formation under carefully controlled experimental conditions .sx This lecture will give information on ( 1 ) the structure and form of biological silicas , ( 2 ) the role of polymers , the ionic environment and physical factors in the control of biosilicification , and ( 3 ) the precipitation of silicas from silicon containing complexes in the presence of carbohydrate polymers .sx