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The low-temperature structural behaviour of natural cancrinite with a formula Na 6.59 Ca 0.93 [Si 6.12 Al 5.88 O 24 ](CO 3 ) 1.04 F 0.41 ·2H 2 O has been investigated by means of in situ singlecrystal X-ray diffraction and Raman spectroscopy. High quality structure refinements were obtained at 293, 250, 220, 180, 140, 100 and at 293 K again (at the end of the low-T experiments). The variation in the unit-cell volume as a function of temperature (T) exhibits a continuous trend, without any evident thermoelastic anomaly. The thermal expansion coefficient a V = (1/V)qV/qT is 3.8(7)610 À5 K À1 (between 100 and 293 K). The structure refinement based on intensity data collected at ambient conditions after the low-T experiment confirmed that the low-T induced deformation processes are completely reversible. The extraframework population does not show significant variations down to 100 K. The strong positional disorder of the carbonate groups along the c axis persists within the T range investigated. The structural behaviour of cancrinite at low-T is mainly governed by the continuous framework rearrangement through the ditrigonalization of the six-membered rings which lie in a plane perpendicular to [0001], the contraction of the four-membered ring joint units, the decrease of the ring corrugation in the (0001) plane, and the flattening of the cancrinite cages. A list of the principal Raman active modes in ambient conditions is provided and discussed.

Investigations of H 2 O-bearing synthetic well-ordered Mg-Fe-cordierites (X Fe = 0-1) with micro-Raman spectroscopy revealed a linear correlation between the Fe/Mg ratio and the position of certain Raman peaks. In the range between 100 and 1250 cm-1 , all peaks except for three peaks shift toward lower wavenumbers with increasing X Fe as a consequence of the substitution of the lighter Mg by the heavier Fe atom on the octahedral sites and the associated structural changes. Selected medium and strong peaks show a shift of 5 to 13 cm-1 , respectively. Based on recent quantum-mechanical calculations (Kaindl et al. 2011) these shifts can be attributed to specific vibrational modes in the cordierite structure, thus showing that the Mg-Fe exchange affects the vibrational modes of tetrahedral, octahedral, and mixed sites. The peaks (wavenumber Mg-/Fe-cordierite) at 122/111, 262/257, 430/418, 579/571, 974/967, and 1012/1007 cm-1 were selected for detailed deconvolution analysis. The shifts of these peaks were then plotted vs. X Fe and regression of the data lead to the formulation of a set of linear equations. Assuming ideal Fe-Mg mixing of the end-members and using linear peak shifts then allows the development of calibration diagrams for the semi-quantitative determination of the Fe-Mg contents of cordierite samples with Raman spectroscopy. In addition, the effect of different H 2 O contents and the degree of Al-Si ordering on the Fe/Mg determination were also investigated. Testing the calibration against data from six well-characterized natural cordierite samples yielded excellent agreement.

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