Intersite OH-F distribution in an Al-rich synthetic phlogopite
Papin, Arnaud; Sergent, José; Robert, Jean-Louis
European Journal of Mineralogy Volume 9 Number 3 (1997), p. 501 - 508
published: Jun 2, 1997
manuscript accepted: Jan 14, 1997
manuscript received: Aug 27, 1996
ArtNo. ESP147050903014, Price: 29.00 €
Abstract The investigation of OH-F substitution in synthetic Al-phlogopite K(Mg2.5Al0.5)(Si2.5Al1.5)O10 (OH,F)2 shows that the solid solution is complete in the range XF = F/(F+OH) <0.7. For higher fluorine contents in the system, the assemblage mica + sellaite + corundum is observed. XRD indicates a regular decrease of the cell parameter c, and of the cell volume V, and a minor increase of the ß angle, as a function of XF, whereas a and b remain constant within experimental uncertainties. The flattening of the cell along c* reflects the decrease of the repulsive interaction between H+ and K+ as F- replaces OH- in the mica. In the OH-stretching region, two main bands are observed: one at=3700 cm-1 is assigned to OH groups adjacent to Mg3 and one at =3650 cm-1 due to hydroxyl groups adjacent to Mg2Al. Each band is double and exhibits a minor low-wavenumber component which can be interpreted from the variable composition of 6(TO4) rings: Si4Al2 or Si3Al3. Along the (OH-F)-mica join, the ≈3770 cm-1 band vanishes much faster than the ≈3650 cm-1 one, which reflects the partition of F- in favour of the Mg3 sites with a partition coefficient DF = (F/OH)Mg/(F/OH)Mg2Al ≈ 3. The partition of F- in favour of the Mg3 sites can be interpreted from local charge balance considerations and explains why Al and F are negatively correlated in natural trioctahedral micas. The replacement of OH-by F- also modifies the silicate structure, in particular, causing a significant shortening of the T-O distances which can be detected by FTIR in the Si-O stretching region. Finally, we can conclude that the molar absorptivities are the same for hydroxyl groups adjacent to Mg3 and to Mg2Al.