The phengite case

The 0-H bond tends to be parallel to (001) and in the direction of the vacant M site and does not hinder the compressibility of the interlayer. The 0-H pointing towards the interlayer cation is instead an obstacle to this compressibility in the trioctahedral micas. 

A higher structural flexibility In 37 following from the presence of more than one 

A spiral disposition in 37 of the 0-H directions across the three M layers of a cell 

In particular, the stability of 37 phengite at high P relative to the 2M polytype occurs by 37phengite possessing the characteristics 2. and 3. 

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