Research
areas
At the interface between Earth and Environmental sciences, Physics and Chemistry, the structural organization of the materials, which constitute our environment, gives a background to understand their physicochemical properties and their formation conditions. At the same time, the possibility to determine the structural surrounding of elements in minerals, glasses/melts and aqueous solutions gives a better understanding of the molecular scale control of element transfer in natural systems. A molecular scale point of view is given by selective structural methods such as solid-state spectroscopy (UV-visible-near IR, EPR, EXAFS and XANES spectroscopy, Mössbauer effect) and diffraction/diffusion of x-ray and neutron radiations, and rationalized by numerical modeling. Major advances have been made in the last decades, largely owing to the wide spread of large user facilities, such as synchrotron radiation and neutron sources. This approach provides a transversal view of systems often investigated independently. By coupling the mineralogical reasoning with geochemical and environmental data or physical-chemical properties on technological materials, by comparing synthetic and natural systems, it is possible to get interesting predictions of properties in a large range of scientific fields: Earth sciences (formation conditions of minerals and glasses; molecular scale control of element transfer and concentration; source tracing of minerals and ore minerals), Environmental sciences (properties of low-temperature phases; contamination processes of soils; waste management) and Materials sciences (role of impurities and structural defects on the properties of glasses, ceramics and cosmetics).
-
Structure of silicate glasses and
melts
Silicate glasses and melts are an exciting research area as well in Materials sciences, owing to the difficulty of relating structure and physical-chemical properties, as in Earth sciences, due to the importance of magmatic systems in geological processes. The structure of these amorphous systems is still poorly understood. We have been mostly concerned by the structural organization of glasses at various scales: nature of cationic sites, relationships between cations and the glassy network or evidence of an extended structural order (at the nanometer scale). These data give insight on the processes, which govern the structural organization of glasses and melts, and their relationship with important physical-chemical properties, such as the coloration or chemical stability of glasses, or crystalline nucleation and element partitioning between minerals and magmatic liquids.
Molecular dynamics
modeling
of a multicomponent silicate glass (after Calas et al., 2003)..
It is now possible to
extend
these observations at high-temperature in order to investigate the
structural
modifications between glasses and melts, and at high-pressure in order
to
correlate structural modifications to the densification of magmas at
the
high pressures prevailing in the inner Earth.
Variation
of
the UV-visible absorption spectrum of Cr3+ in a silicate glass as a
function of temperature (in °C), at the origin of the modification of
glass, at high temperature (thermochromism) (after Calas et al.,
2006). These modifications
originate from the thermal expansion of Cr3+ sites. On the right:
thermochromism
of a green silicate glass turning yellow above 350°C (photo O. Villain,
IMPMC).
- Environnemental Mineralogy
The minerals formed at the Earth's surface are often poorly
crystallized
and sometimes amorphous and are used to constrain alteration processes.
The complexity due to the finely divided nature of soil components and
to the biological activity may now be overcome using synchrotron
radiation and other spectroscopic techniques. Provided the structural
information is set back
in its environmental context, it is possible to determine the formation
conditions of minerals and to investigate the crystal-chemical behavior
of elements
at the Earth's surface. In soils, minerals and organic components
directly control element speciation and mobility, with an important
role played by biological activity. This approach is applied to
toxic contaminants, which
may be found in soils (Pb, As, Zn, U…). Spatially-resolved
spectroscopic
analyses, coupled with geochemical and environmental data, give a link
between
local concentration, chemical speciation, mobility and element toxicity.
Paramagnetic
impurities in kaolinite from a lateritic soil: substituted Fe3+,
associated iron oxides and defect centers (after Muller et al., 1995).
- Radioactive waste management. Radiation-induced defects
Our
research on
waste materials
concerns the structure of nuclear glasses, their aging at
high-temperature
and under irradiation and the structural control of alteration.
The structure
of the gels formed during glass alteration is important to understand
the
long-term behavior of nuclear glasses. The gel structure may now
be determined
using the same structural tools and atomistic modeling is for glasses.
Natural irradiation of minerals may cause original colors and results
in radiation-induced defects, the limited thermal stability of which
gives a potential tool for absolutely dating or thermometry.
These defects are important
in clay minerals, owing to the high specific area of these minerals and
constitute
an important parameter in the near-field (engineered barriers) as well
as
in the far-field: in this last case, it is possible to use clay
minerals
as natural dosimeters able to track the transfer of radionuclides in
low-temperature
environments, as in the natural analogues of nuclear disposals in
geological
formations.
Relationship between the uranium concentration measured in the
present-day samples and that derived from kaolinite dosimetry, in the
Nopal (Mexico) natural
analogue. The insert represents a SEM photograph of the
association between
kaolinite (K) and secondary uranium minerals (weeksite, W,;carnotite,
C)
(after Muller et al..1995; Calas et al., 2003).
- Crystal chemistry of
minor
elements
The
crystal chemistry of transition elements and lanthanides
may be approached through the spectroscopic properties of
minerals. The
surrounding of these elements governs their partitioning between
minerals
and their formation media. However, the substitution processes of
major
components by trace elements in minerals are still poorly known: actual
nature
of the substituted site, charge compensation processes, extent of the
structural
relaxation show that the concept of solid solution relies on a
statistical
view of crystalline lattice. The inter-site partitioning and the
heterogeneous distribution of trace elements may be related to crystal
growth processes.
The location of impurities also explains the origin of the coloration
of minerals and industrial materials, and may be used to trace the
origin of heritage materials.