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

Magnesiocarpholite, MgAl2Si2O6(OH)(4) is a high-pressure/low-temperature mi neral occurring in the metamorphic rocks of southern Tuscany (Monte Leoni a nd Monte Argentario). Chemical, Mossbauer, FTIR and single-crystal XRD char acterizations were carried out to understand the symmetry and crystal chemi stry of the mineral, since forbidden diffractions violating the space group Ccca have been reported for ferrocarpholite and oblique extinction for car pholite. 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(Mg0.65Fe0.322+Fe0.033+)(Sigma =1.00 )Al1.97Si2.00O5.90(OH)(3.95)F-0.15 and K-0.002(Mg0.65Fe0.342+Fe0.013+)(Sigm a =1.00)Al1.98Si2.00 O-5.92(OH)(3.96)F-0.12 for Monte Argentario and Monte Leoni, respectively. Single-crystal X-ray diffraction refinements (R-1 = 0. 020 and 0.024) largely confirm the apparent Ccca symmetry and indicate resi dual maxims corresponding to the K-site of non-stoichiometric potassium and fluorine-bearing carpholite. However, a few reflections violating the a gl ide 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; diffe rent spectral features in the Si-O stretching region, pointing to more dist orted bonding pattern in the Monte Argentario specimen). The contradictory results are explained in terms of different long-range (revealed by X-ray d iffraction) and short-range order (revealed by FTIR and Raman spectroscopie s), that involve local arrangements, like point defects.