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Comparative high-pressure crystal chemistry of karrooite, MgTi (sub 2) O (sub 5) , with different ordering states

Hexiong Yang and Robert M. Hazen
Comparative high-pressure crystal chemistry of karrooite, MgTi (sub 2) O (sub 5) , with different ordering states
American Mineralogist (February 1999) 84 (1-2): 130-137

Abstract

Two karrooite crystals, one with a disorder parameter (X = Ti content in the M1 site) of 0.070(5) and the other with X = 0.485(5), were mounted together in one diamond-anvil cell and studied by single-crystal X-ray diffraction at several pressures up to 7.51 GPa. The most noticeable effect of increasing cation disorder on the high-pressure behavior of the structure is to increase the compressibilities of the mean "M2-O" bond length from 0.00148(2) GPa (super -1) in the ordered sample to 0.00163(7) GPa (super -1) in the disordered one and decrease those of the mean "M1-O" bond length from 0.00243(5) to 0.00193(12) GPa (super -1) . These changes are responsible for the compressibility difference between the two phases observed by Hazen and Yang (1997). Both compressibilities of the mean "M-O" bond lengths and the octahedral volumes in two phases decrease linearly with increasing the Ti contents in the octahedral sites. All octahedra in two samples become less distorted as pressure increases, but those in the more disordered structure exhibit larger decreases in terms of the octahedral angle variance than the corresponding ones in the more ordered structure. The influence of pressure on the interatomic angles is small compared to the interatomic distances, suggesting that compression of the karrooite structure is controlled primarily by the bond-length shortening, rather than by bond-angle bending. The strong compressional anisotropy of the structure is a consequence of the differential compressibilities of the weaker Mg (super 2+) -O and stronger Ti (super 4+) -O bonds and the complex edge-sharing linkage involving the M1 and M2 octahedra.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 84
Serial Issue: 1-2
Title: Comparative high-pressure crystal chemistry of karrooite, MgTi (sub 2) O (sub 5) , with different ordering states
Affiliation: Carnegie Institution of Washington, Geophysical Laboratory, Washington, DC, United States
Pages: 130-137
Published: 199902
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 29
Accession Number: 1999-022428
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 199908
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