New High‐Pressure Forms of Al 2 SiO 5

• Published in: Geophysical research letters Vol. 45; no. 16; pp. 8167 - 8172
• Main Authors: Zhou, Y., Irifune, T., Ohfuji, H., Kuribayashi, T.
• Format: Journal Article
• Language: English
• Published: 28-08-2018
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078960

Abstract
Phase relations of Al
2
SiO
5
have been studied by multianvil experiments at pressures of 13–23 GPa and temperatures of 2000–2900 K. Al
2
SiO
5
kyanite was found to transform into two new high‐pressure forms of Al
2
SiO
5
(kyanite II and III) at temperatures exceeding 2300–2500 K and pressures of 14–23 GPa: The first phase transition occurs near 14 GPa, and the second occurs near 17 GPa. The new Al
2
SiO
5
phases have triclinic and monoclinic crystal symmetries with zero‐pressure densities of 3.876(2) and 3.982(1) g/cm
3
, respectively, which are significantly denser than kyanite (
ρ
0
 = 3.666 g/cm
3
) but less dense than the isochemical mixture of Al
2
O
3
corundum and SiO
2
stishovite (
ρ
0
 = 4.036 g/cm
3
). The exceptionally high stability temperatures of the new Al
2
SiO
5
phases suggest that they are unlikely to form in the present mantle but may be found in some impact craters and shocked meteorites and act as important indicators of pressure and temperature for the shock events.
Plain Language Summary
Aluminosilicate in the mantle is an important issue for Earth science. Previous studies argued the existence of a high‐pressure form of Al
2
SiO
5
in the lower mantle, in addition to those of the well‐known polymorphs, andalusite, sillimanite, and kyanite, found in shallow metamorphic rocks. However, the existence of such a high‐pressure form of Al
2
SiO
5
has been controversial among previous studies, due to the large discrepancies in its stability region and crystal structure. Here we figure out the phase relations of Al
2
SiO
5
at mantle‐transition‐region pressures (~13–23 GPa, corresponding to ~410–660 km in depth) and temperatures of 2000–2900 K via multianvil high‐pressure experiments and show that kyanite transforms into two new high‐pressure forms of Al
2
SiO
5
at temperatures exceeding 2300–2500 K in the pressure range of 14–23 GPa: The first phase transition occurs near 14 GPa, and the second occurs near 17 GPa. Although the extremely high stability temperatures of the new Al
2
SiO
5
phases suggest their absence in the present mantle, they may be found in some impact craters and shocked meteorites and act as important indicators of pressure and temperature for the shock events.
Key Points
We investigated the phase relations of Al
2
SiO
5
by multianvil experiments at pressures of 13‐23 GPa and temperatures of 2000‐2900 K
We confirmed the existence of two new high-pressure forms of Al
2
SiO
5
at pressures of 14‐23 GPa and temperatures exceeding 2300‐2500 K
Meteoritic impact may cause potential natural formation of the new Al
2
SiO
5
phases