Data di Pubblicazione:
2008
Citazione:
The mechanism of coupling in the modulated structure of nephline / J.R. Angel, G.D. Gatta, T. Boffa Ballaran, M. Carpenter. - In: CANADIAN MINERALOGIST. - ISSN 0008-4476. - 46:6(2008 Dec), pp. 1465-1476.
Abstract:
The crystal structures of a nepheline, K0.54Na3.24Ca0.03□0.19Al3.84Si4.16O16, and that of the same sample annealed at high
temperature to induce K–vacancy disorder, have been determined at several temperatures down to 15 K by single-crystal
X-ray diffraction. The largest structural change in both crystals with decreasing temperature is the decrease of the T1–O1–T2
angle, corresponding to an increase in the tilt of the T1 and T2 tetrahedra within the framework. The tetrahedra in the annealed
sample have a smaller tilt than in the natural sample at any given temperature. The correlation of the tilts of the tetrahedra with
changes in the intensities of satellite reflections confirms that the satellites arise from a displacive modulation of the framework
of tetrahedra. Distance-least-squares simulations suggest that the modulation creates larger and smaller cavities within the
extra-framework channels that contain the K atoms. Analysis of the K–O bond lengths with both the state of K–□ order and temperature indicate that the coupling between K–□ order and the framework modulation occurs through the K–O2 bond. An increase in the average K–O2 bond length with decreasing temperature or increasing K–□ order supports the modulation of the framework. Shortening of the K–O2 bond leads to rotations of the tetrahedra that are opposite to those associated with the modulation, and thus suppresses it.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
nepheline ; low-temperature equilibration ; incommensurate structure ; X-ray diffraction ; structure refinement
Elenco autori:
J.R. Angel, G.D. Gatta, T. Boffa Ballaran, M. Carpenter
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