Germanium muscovites with excess hydroxyl water, KAl2[Ge3-xAl1+xO10-x(OH)x(OH)2] and the question of excess OH in natural muscovites

Abstract

Single crystals of Ge-muscovite, up to 500 μm in diameter and up to 60 μm in thickness, were grown hydrothermally at 1.5 to 2.0 kbar/425-450°C and at 4.0 to 6.0 kbar/600°C. Refined cell parameters, based on 1 M indexing, are a = 5.283 (6), b = 9.129 (7), c = 10.656 (11), β = 101.51 (13)°. Microprobe analyses and single-crystal IR spectra prove an excess of Al and OH, thus yielding the formula KAl 2 [Ge 3−x Al 1+x O 10−x (OH) x (OH) 2 ] with x = 0.13 and 0.16 for micas from the two runs. The «normal» OH valence vibration of Si-muscovite (3628 cm −1 ) is found at 3640 cm −1 in single-crystal spectra of the Ge-muscovite. Additional bands occur at 3480 and 1165 cm −1 . In all cases, the dipole moment vector makes an angle ρ near 10° with [100] in the ac plane. There is no evidence for the presence of oxonium, H 3 O + , replacing K + in the Ge-muscovites. The results suggest that water exceeding two hydroxyls per formula unit in muscovite may be caused by the substitution Si 4+[4] +O 2− ↔Al 3+[4] +OH − excess . The existence of this substitution in natural dioctahedral micas is ikely, but the present data (37 published analyses) show that it is rather restricted and does not exceed about one-third of the variation encountered in the set examined