 |
| Titel |
Studying the pseudomorphic replacement of olivine by silica at acidic conditions |
| VerfasserIn |
Helen E. King, Thorsten Geisler, Andrew Putnis |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038607
|
|
|
|
|
|
| Zusammenfassung |
| Silicification is an important reaction occurring in volcanic environments that aids the
neutralisation of the extremely acidic fluids produced. Replacement of the original minerals
by silica results in the reacting fluid becoming enriched in cations that can be subsequently
transported through the hydrothermal system (Varekamp et al., 2000). In some examples
olivine and pyroxene phenocrysts within the reacted layers are pseudomorphically replaced
by silica (Africano and Bernard, 2000). We have performed unstirred batch experiments using
sulphuric acid and whole olivine crystals to explore the parameters that control
phase precipitation and the pseudomorphic replacement of forsteritic olivine. The
experiments were conducted in Teflon-lined steel autoclaves at temperatures between 80
and 120 Ë C. In lower temperature experiments the reaction of olivine with 0.1
M sulphuric acid produced a red precipitate coating the olivine surface. The red
phase was too low in concentration to be identified but is expected to be hematite
(Fe2O3). In contrast, experiments conducted in sulphuric acid between 1 and 3.6 M
produced an amorphous silica pseudomorph of olivine. The dependence of phase
precipitation on pH was tested using a sulphuric acid solution neutralised to pH 2
with a resulting ionic strength equivalent to 1 M. At low temperatures the olivine
reacted to produce amorphous silica and a yellow fluid indicating that Fe remained in
solution, possibly as a complex with sulphate. Higher temperatures allowed the
iron to be liberated from the fluid and precipitated as hematite in association with
amorphous silica. Elemental content microprobe data for the olivine and silica rim
from pseudomorphic replacement reactions suggests that the majority of cations
from olivine dissolution remained in solution. This observation was confirmed by
inductively coupled plasma-mass spectroscopy, which showed a high content of
Mg and Fe in the fluid. The variation of replacement products at different reaction
conditions highlights the sensitivity of olivine replacement in acidic conditions to
factors such as pH. We used an 18O-enriched fluid to trace the mechanism of the
pseudomorphic replacement. Raman spectroscopy of the silica layer showed that the
18O was taken into the silica rim in enough concentration to produce a shift in
the Raman peaks resulting from 18O-Si bonding within the silica network. The
products from experiments with different sulphuric acid concentrations showed that as
concentration was reduced the silica layer became increasingly fragile until it no longer
precipitated as a pseudomorph. This observation and the incorporation of 18O into the
amorphous silica structure indicate that the formation of a pseudomorph occurs via
an interface-coupled dissolution-reprecipitation mechanism (Putnis and Putnis,
2007).
References
Africano F. and Bernard A. (2000), J. Volcanol. Geoth. Res., 97 475-495
Putnis A. and Putnis C. V. (2007), J. Solid State Chem., 180, 1783-1786
Varekamp J. C., Pasternack G. B. and Rowe Jr. G. L. (2000), J. Volcanol. Geoth. Res., 97
161-179 |
|
|
|
|
|
|