The quantitative analysis of H2O and CO2 in cordierite using single-crystal polarized-light and microspectroscopy.

We report a FTIR (Fourier transform infrared)
study of a set of cordierite samples from different occurrence
and with different H2O/CO2 content. The specimens
were fully characterized by a combination of techniques
including optical microscopy, single-crystal X-ray diffraction,
EMPA (electron microprobe analysis), SIMS
(secondary ion mass spectrometry), and FTIR spectroscopy.
All cordierites are orthorhombic Ccmm. According to
the EMPA data, the Si/Al ratio is always close to 5:4; XMg
ranges from 76.31 to 96.63, and additional octahedral
constituents occur in very small amounts. Extraframework
K and Ca are negligible, while Na reaches the values up to
0.84 apfu. SIMS shows H2O up to 1.52 and CO2 up to 1.11
wt%. Optically transparent single crystals were oriented
using the spindle stage and examined by FTIR microspectroscopy
under polarized light. On the basis of the
polarizing behaviour, the observed bands were assigned to
water molecules in two different orientations and to CO2
molecules in the structural channels. The IR spectra also
show the presence of small amounts of CO in the samples.
Refined integrated molar absorption coefficients were calibrated
for the quantitative microanalysis of both H2O and
CO2 in cordierite based on single-crystal polarized-light
FTIR spectroscopy. For H2O the integrated molar coefficients
for type I and type II water molecules (m3 modes)
were calculated separately and are [I]e = 5,200 ± 700 l
mol-1 cm-2 and [II]e = 13,000 ± 3,000 l mol-1 cm-2,
respectively. For CO2 the integrated coefficient is
eCO2 = 19,000 ± 2,000 l mol-1 cm-2.