 |
| Titel |
Crystallization Experiments in the MgO-CO2-H2O system: Role of Amorphous
Magnesium Carbonate Precursors in Magnesium Carbonate Hydrated Phases and
Morphologies in Low Temperature Hydrothermal Fluids |
| VerfasserIn |
Manolis Giampouras, Juan Manuel Garcia-Ruiz, Carlos J. Garrido |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149819
|
| Publikation (Nr.) |
 EGU/EGU2017-14211.pdf |
|
|
|
|
|
| Zusammenfassung |
| Numerous forms of hydrated or basic magnesium carbonates occur in the complex
MgO-CO2-H2O system. Mineral saturation states from low temperature hydrothermal fluids
in Semail Ophiolite (Oman), Prony Bay (New Caledonia) and Lost City hydrothermal field
(mid-Atlantic ridge) strongly indicate the presence of magnesium hydroxy-carbonate
hydrates (e.g. hydromagnesite) and magnesium hydroxides (brucite). Study of formation
mechanisms and morphological features of minerals forming in the MgO-CO2-H2O system
could give insights into serpentinization-driven, hydrothermal, alkaline environments,
which are related to early Earth conditions. Temperature, hydration degree, pH
and fluid composition are crucial factors regarding the formation, coexistence and
transformation of such mineral phases. The rate of supersaturation, on the other hand, is
a fundamental parameter to understand nucleation and crystal growth processes.
All these parameters can be examined in a solution using different crystallization
techniques.
In the present study, we applied different crystallization techniques to synthesize and
monitor the crystallization of Mg-bearing carbonates and hydroxides under abiotic
conditions. Various crystallization techniques (counter-diffusion, vapor diffusion and
unseeded solution mixing) were used to screen the formation conditions of each phase,
transformation processes and structural development. Mineral and textural characterization of
the different synthesized phases were carried out by X-ray diffraction (XRD), Raman
spectroscopy and scanning electron microscopy coupled to dispersive energy spectroscopy
(FE-SEM-EDS). Experimental investigation of the effect of pH level and silica content under
variable reactant concentrations revealed the importance of Amorphous Magnesium
Carbonate (AMC) in the formation of hydroxy-carbonate phases (hydromagnesite and
dypingite). Micro-structural resemblance between AMC precursors and later stage
crystalline phases highlights the critical role of internal molecule re-organization to
form crystalline structures. Aggregation of AMC spherulites triggers biomimetic
morphologies forming curling laminar structures and rings. The size and number of
nesquehonite (MgCO3.3H2O) crystals are controlled by pH and Mg2+ ions at pH < 9. As
pH increases, nesquehonite transforms to spherical, rosette-like dypingite and/or
hydromagnesite. Crystallization experiments within silica gel impedes the normal growth of
prismatic nesquehonite crystals and generates peculiar dendritic crystalline structures.
Finally, vapor diffusion techniques resulted in synthesis of NH4+-bearing hydrated
compounds after ammonium incorporation when [NH4+]/[Mg2+] ≥ 1 and ≥ 0.5M
[NH4+].
Funding: We acknowledge funding from the People programme (Marie Curie
Actions - ITN) of the European Union FP7 under REA Grant Agreement n˚ 608001. |
|
|
|
|
|
|