Crystal-chemistry of magnesiocarpholite : controversial X-ray diffraction, Mössbauer, FTIR and Raman results

Magnesiocarpholite, MgAl₂Si₂O₆(OH)₄, is a high-pressure/low-temperature mineral occurring in the metamorphic rocks of southern Tuscany (Monte Leoni and Monte Argentario). Chemical, Mössbauer, FTIR and single-crystal XRD characterizations were carried out to understand the symmetry and crystal chemistry of the mineral, since forbidden diffractions violating the space group Ccca have been reported for ferrocarpholite and oblique extinction for carpholite. Tuscany magnesiocarpholite contains significant fluorine and minor potassium. Iron occurs mostly as ferrous iron, within one octahedral site. Crystal chemical formulae are: K_0.002(Mg_0.65Fe²⁺_0.32Fe³⁺_0.03)_∑=1.00 Al_1.97Si_2.00O_5.90(OH)_3.95F_0.15 and K_0.002(Mg_0.65Fe²⁺_0.34Fe³⁺_0.01)_∑=1.00 Al_1.98Si_2.00 O_5.92(OH)_3.96F_0.12, for Monte Argentario and Monte Leoni, respectively. Single-crystal X-ray diffraction refinements (R₁ = 0.020 and 0.024) largely confirm the apparent Ccca symmetry and indicate residual maxima corresponding to the K-site of non-stoichiometric potassium and fluorine-bearing carpholite. However, a few reflections violating the a glide plane were observed in the Monte Argentario specimen, which also shows anomalous values of the unit-cell parameters.

The Monte Argentario magnesiocarpholite also shows differences in the Raman and FTIR spectra, with respect to both Monte Leoni magnesiocarpholite and ferrocarpholite (four to five O-H stretching bands rather than three; different spectral features in the Si-O stretching region, pointing to more distorted bonding pattern in the Monte Argentario specimen). The contradictory results are explained in terms of different long-range (revealed by X-ray diffraction) and short-range order (revealed by FTIR and Raman spectroscopies), that involve local arrangements, like point defects.