Excess protons in synthetic micas with tetrahedrally coordinated divalent cations
Robert, Jean-Lοuis; Hardy, Michel; Sanz, Jesus
European Journal of Mineralogy Volume 7 Number 2 (1995), p. 457 - 462
published: Mar 29, 1995
manuscript accepted: Dec 15, 1994
manuscript received: Nov 7, 1994
ArtNo. ESP147050702020, Price: 29.00 €
Abstract Micas of the K2O-MgO-BeO-SiO2-H2O system, with a tetrahedrally coordinated divalent cation (Be2+ or Mg2+) and a partial dioctahedral character, exhibit an excess of hydroxyl water which increases with the M2+ content. This is demonstrated by TGA analysis which shows an additional weight loss at ~ 900°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 ≈ 3700 cm-1. The number of hydroxyl groups may reach up to (OH)2.5 p.f.u. instead of (OH)2 in conventional micas. The most likely proton acceptors are the strongly underbonded apical oxygens of M2+ tetrahedra adjacent to two Mg2+ and an octahedral vacancy.