Excess protons in synthetic micas with tetrahedrally coordinated divalent cations

Micas of the K (sub 2) O-MgO-BeO-SiO (sub 2) -H (sub 2) O system, with a tetrahedrally coordinated divalent cation (Be (super 2+) or Mg (super 2+) ) and a partial dioctahedral character, exhibit an excess of hydroxyl water which increases with the (super [4]) M (super 2+) content. This is demonstrated by TGA analysis which shows an additional weight loss at nearly equal 900 degrees C, due to the excess of hydroxyl water, and by FTIR spectrometry in the OH-stretching region with the growth of a broad band at nearly equal 3700 cm (super -1) . The number of hydroxyl groups may reach up to (OH) (sub 2.5) p.f.u. instead of (OH) (sub 2) in conventional micas. The most likely proton acceptors are the strongly underbonded apical oxygens of (super [4]) M (super 2+) tetrahedra adjacent to two (super [6]) Mg (super 2+) and an octahedral vacancy.