instrumental techniques applied to mineralogy and geochemistry

X-ray Absorption Spectroscopy in Mineralogy and in the Earth and Environmental Sciences 51
two orders of magnitude. Consequently, S/N is enhanced and XANES spectra on even
very diluted heavy atoms can be measured proficiently.
v) Finally, it should be noted that XAS experiments can be performed on a highly
flexible sample environment. In this way, XAS experiments can be performed under
extreme conditions of temperature, different gas atmospheres, and even under highapplied
pressure on solid, liquid and gaseosous samples.
These characteristics make XAS an incomparable tool to face different problems in
Mineralogy and in the Earth and Environmental Sciences. Indeed, the flexible sample
environment allows approach the temperature and pressure conditions of the Earth’s
interior where phase transitions in minerals under high pressure are known to govern
some of the major geophysical properties. In a similar way, XAS provides an unique
insight into the accurate knowledge of trace-element behaviour in rock-forming and
accessory minerals that results fundamental to understand and model petrologic and
geochemical processes. These rock-forming minerals are always complex solid solutions
and, therefore, the only approach capable of characterising the local coordination of the
individual substituents is the use of element-selective spectroscopic methods like XAS.
This is for example the case of the characterization of trace Nd and Ce site
preference and coordination in natural melanites. The identification of the crystalchemical
parameters controlling trace-element partitioning between minerals and silicate
melts is a topic of considerable interest. Garnets represent an important group of rockforming
minerals in this regard, since they are stable over a wide range of physicochemical
conditions and are able to incorporate several trace elements commonly used in
geochemical modeling. For these reasons, aluminosilicate garnets (X 3 Al 2 Si 3 O 12 , X =
Fe 2+ , Mn 2 +, Mg, Ca) have received much attention, regarding their complex crystal
chemistry, their thermodynamic properties or trace-element behaviour. Despite the
importance of the geochemical behavior of REE in garnets, little work has been done to
decipher their site preference and local coordination in trace or minor amounts. Indeed,
there are no examples in the literature of direct structural investigations on the site
location and geometry of REE in natural garnets, possibly due to their low
concentrations. A recent work by Quartieri et al. has reported the XAS study of the site
location and geometry of trace amounts of neodymium (from 176 to 1074 ppm) and