Intersite OH-F distribution in an Al-rich synthetic phlogopite

In Al-phlogopite, K(Mg (sub 2.5) Al (sub 0.5) )(Si (sub 2.5) Al (sub 1.5) )O (sub 10) (OH,F) (sub 2) , solid solution was found to be complete in the range X (sub F) = F/(F + OH) up to 0.7. For higher F contents, the assemblage mica + sellaite was found. XRD indicates a regular decrease in c and cell volume V, and a minor increase in the beta angle, as a function of X (sub F) , whereas a and b remain constant. The flattening of the cell along c* reflects the decrease in repulsive interaction between H (super +) and K (super +) as F (super -) replaces OH (super -) in the mica. In the OH-stretching region, two main bands are seen: one at approximately 3700 cm (super -1) is assigned to OH groups adjacent to Mg (sub 3) and one at approximately 3650 cm (super -1) is due to hydroxyl groups adjacent to Mg (sub 2) Al. Along the (OH-F) mica join, the approximately 3770 band vanishes much faster than the 3650 cm (super -1) one, reflecting the partition of F (super -) in favour of the Mg (sub 3) with a partition coefficient D (sub F) = (F/OH) (sub roman Mg sub 3) /(F/OH) (sub Mg sub 2 Al) approximately 3. The partition of F (super -) in favour of the Mg (sub 3) sites can be explained from local charge-balance considerations and explains why (super [6]) Al and F are negatively correlated in natural trioctahedral micas. The replacement of OH (super -) by F (super -) also modifies the structure, causing a significant shortening of the T-O distances, which can be detected by FTIR in the Si-O stretching region.