Structural Evolution of Metal Sulfide Precipitates

David Vaughan (david.vaughan@man.ac.uk)¹, Lesley Moyes, Richard Pattrick, Francis Livens², John Charnock & David Garner

¹ Dept of Earth Sciences, Manchester University, Manchester M13 9PL, England

² Dept of Chemistry, Manchester University, Manchester M13 9PL, England

The structures of the first formed solids rapidly precipitated from aqueous solution on reaction in systems such as those involving metals and sulfur are poorly understood, being very fine particle "X ray amorphous" materials. Furthermore, the structural changes that take place when these precipitates age and transform to more stable phases, the rates at which these changes occur, and the surface reactivities of all of these phases are also little known. Such materials may be of considerable importance in natural environments such as Recent sediments and in manmade systems such as waste dumps. We have been pioneering studies of these systems using X ray absorption spectroscopy. For example, investigating the Cu-S system in which a CuS phase with a primitive wurtzite-derived structure forms initially and evolves to a covellite structure (Pattrick et al, 1998), or the Fe-S system in which either a hexagonally close packed or a cubic close packed FeS forms initially depending on solution conditions and develops via tetragonal FeS (mackinawite) to greigite (Lennie & Vaughan, 1996). Further work using Energy Dispersive X ray Absorption Spectroscopy and Diffraction methods for the rapid acquisition of data has enabled rates of transformations to be determined. Thus, at 25°C an initial CuS precipitate transforms to one of covellite character on a short range after ~1 hour; at above 40°C a highly ordered covellite rapidly forms. Preliminary work on the Hg-S system shows a cinnabar-like structure after a few seconds with Hg bonded to two S atoms at 2.37Å, whilst after 2 hours the Hg is surrounded by four S atoms at 2.53Å as in metacinnabar. Structural development of cubic MnS (alabandite) involves initial precipitation of a species with S in a low coordination which transforms to crystalline cubic MnS in a few minutes.

Pattrick RAD, Mosselmans JFW, Charnock JM, England KER, Helz GR, Garner CD & Vaughan DJ, Geochim Cosmochim Acta, 61, 2023-2036, (1998).

Lennie AR & Vaughan DJ, Geochem Soc Spec Pub, 5, 171-206, (1996).

Melting Phase Relationships in Hydrous Systems Revisited

Daniel Vielzeuf (vielzeuf@opgc.univ-bpclermont.fr)

Observatoire de Physique du Globe de Clermont, Laboratoire Magmas et Volcans, 5, rue Kessler, 63038 Clermont-Ferrand, France

The presence of a fluid phase or a hydrous mineral in an assemblage depends on pressure, temperature, composition, and affects the solidus. Theoretical and experimental studies on melting processes in hydrous systems usually take into consideration only one hydrous mineral. However, petrological studies and a critical reevaluation of experiments show that, at least two hydrous phases must be considered to decipher partial melting relationships over a wide P-T range. These two predominant hydrous minerals are (i) muscovite and biotite in metapelites and metagreywackes, (ii) epidote/zoisite and amphibole in metabasalts. Rocks that underwent high pressure metamorphism are excellent illustrations of the progressive replacement of a hydrous phase (e.g. biotite)by others (phengite + zoisite).

Theoretical phase relationships in a simple binary model system involving two hydrous phases are depicted to clarify some of the principles of melting in hydrous systems. This model is used to show how the shape and the location of the solidus in pressure - temperature diagrams change as a function of the H₂O content of the bulk composition.

In agreement with this model, a scheme for partial melting of metasediments is proposed first in the KN(FM)ASH system with a combination of consistent pressure - temperature and liquidus diagrams, and isobaric-isothermal projections. Further complexities arise from taking into consideration calcium as an additional component. The anorthite component in the plagioclase plays an important role in the subsolidus reactions leading to the breakdown of biotite or amphibole with increasing pressure. These reactions involving anorthite have a large V, a low dP/dT slope and generate liquidi with complex S-shaped curves.

This study emphasizes the remarkable analogies in melting behaviour of metapelites, metagreywackes and metabasalts.

Germanium Coordination and the Germanate Anomaly

Halan Wang (halan@afm1.geology.utoronto.ca) & Grant S. Henderson (henders@afm1.geology.utoronto.ca)

Geology Department, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada

Alkali germanate glasses exhibit maxima or minima in their physical properties with increasing alkali oxide content (Ivanov and Estropiev 1962). This characteristic is called the 'germanate anomaly'. It has been attributed to a change in the coordination of ^IVGe to ^VIGe and has been widely used as a model for pressure induced coordination changes. However, Henderson and Fleet (1991) proposed an alternative explanation based on the formation of 3-membered GeO₄ rings. Here we reinvestigate the anomaly and extend the composition range of our earlier work to include the Li₂O-, Rb₂O- and Cs₂O-GeO₂joins. Density measurements of all five glass series exhibit the germanate anomaly (a maximum in density). The lithium, sodium and potassium glasses exhibit a decreasing trend in the density maximum from Li₂O to K₂O consistent with previous studies (Murthy and Ip, 1964). However, the rubidium and cesium glasses show anomalous trends within the 40 mol% compositional range. The rubidium glasses exhibit a maximum at ~15 mol% and a minimum at ~32.5 mol%. The cesium glasses have a density plateau at ~17.5 mol%, but exhibit a further density increase at ~32.5 mol%. The density behaviour of the Cs-bearing glasses clearly differs from that of the other alkali germanate compositions. The behaviour of Raman vibrational bands is systematic with increasing alkali content and becomes more pronounced as one moves from Li to Cs. Three major trends are observed: 1) a decrease in the 4-membered ring vibrational band (420 cm^-1) coupled with an increase in the 3-membered ring band (520 cm^-1); 2) a shift of the 840 cm^-1 and 950 cm^-1 bands to 750 cm^-1 and 860 cm^-1 and, 3) a shift in 520 cm^-1 band to higher frequency and the disappearance of shoulder peaks. Analysis of these trends indicates that the germanate anomaly is not due to a coordination change of Ge and further strengthens the model proposed by Henderson and Fleet [3].

Ivanov AO & Estropiev KS, Dokl. Akad. Nauk SSSR, 145, 797, (1962).

Murthy MK & Ip J, Nature, 201, 285, (1964).

Henderson GS & Fleet ME, J. Non-crystalline Solids, 134, 259, (1991).