Germanium muscovites with excess hydroxyl water, KAl2[Ge3-xAl1+xO10-x(OH)x(OH)2] and the question of excess OH in natural muscovites
Ackermann, Lothar; Langer, Klaus; Rieder, Milan
published: Mar 4, 1993
manuscript accepted: Jul 9, 1992
manuscript received: Jul 8, 1991
ArtNo. ESP147050501016, Price: 29.00 €
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), ß = 101.51 (13)°. Microprobe analyses and single-crystal IR spectra prove an excess of Al and OH, thus yielding the formula KAl2[Ge3-xAl1+xO10-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 p near 10° with  in the ac plane. There is no evidence for the presence of oxonium, H3O+, 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 Si4+ + O2- ↔ Al 3+ + OH'excess. The existence of this substitution in natural dioctahedral micas is likely, 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.