High pressure behavior and structural transition of beryl-type johnkoivulaite, Cs(Be₂B)Mg₂Si₆O₁₈

Author(s)

G. Diego Gatta, Martin Ende, Sofija Miloš, Nicola Rotiroti, Aaron Palke, Ronald Miletich

Abstract

The beryl-group mineral johnkoivulaite, Cs(Be₂B)Mg₂Si₆O₁₈,

was compressed hydrostatically in a diamond-anvil cell up to 10.2 GPa.

In situ Raman spectroscopy and X-ray crystallography revealed a P6/mcc-to-P3c1 (second-order) phase transition on isothermal compression at the critical transition pressure P_c

= 4.13 ± 0.07 GPa. The elastic parameters determined for the volume

elasticity of the two polymorphs correspond to a Birch-Murnaghan

equation of state with K₀ = 148 ± 2 GPa and K′ = 0 for P < P_c and K₀ = 75.5 ± 0.9 GPa with K′ = 4 for P > P_c. The low-P

polymorph shows anomalously linear compression behavior, as reported

for several other beryl-derived framework structures. The high-P polymorph, which was found to follow a a′ = a·√3, c′ = c superstructure according to P3c1, is almost twice as compressible as its low-P

form. This is unique for any beryl-derived structure and can be

attributed to the high degree of freedom for atomic displacements in the

superstructure. The reduced symmetry can also be understood as the

effect of the driving mechanism of the transformation. The

extra-framework Cs channel components counteract any lateral deformation

of the channels parallel to [0001] within the microporous framework

and, similar to pezzottaite, are responsible for maintaining the

trigonal/hexagonal lattice metrics.

Organisation(s)

Department of Mineralogy and Crystallography

External organisation(s)

Università degli Studi di Milano-Bicocca, Gemological Institute of America

Journal

American Mineralogist

Volume

109

Pages

15-23

No. of pages

9

ISSN

0003-004X

Publication date

01-2024

Peer reviewed

Yes

Austrian Fields of Science 2012

105116 Mineralogy, 104026 Spectroscopy

Portal url

https://ucrisportal.univie.ac.at/en/publications/be4c9398-c5a3-4f0a-aa7a-b623ef590682