24 research outputs found
High temperature structural and thermoelastic behaviour of mantle orthopyroxene: an in situ neutron powder diffraction study
The temperature induced structural evolution
and thermoelastic behaviour of a natural (Pbca)
orthopyroxene (Opx), with chemical formula
M2(Mg0.856Ca0.025Fe2+
0.119) M1(Mg0.957Fe2+
0.011Fe3+
0.016
Cr0.011Al0.005)Al0.032Si1.968O6, from a suite of high
pressure ultramafic nodules of mantle origin, have
been investigated by in-situ neutron powder diffraction
at several temperatures starting from 1,200C down
to 150C. Unit-cell parameter variations as a function
of T show no phase transition within this temperature
range. The volume thermal expansion coefficient,
a = V\u20131(dV/dT)P0, varies linearly with T. The axial
thermal expansion coefficients, aj = lj
\u20131(dlj/dT)P0, increase
non-linearly with T. The principal Lagrangian
unit-strain coefficients (e//a, e//b, e//c), increase
continuously with T. However, the orientation of the
unit-strain ellipsoid appears to change with T. With
decreasing T, the values of the unit-strain coefficients
along the b and c axes tend to converge. The orientation
at DeltaT = 1,080 C is maintained down to the lowest
temperature (150 C). The two non-equivalent tetrahedral
chains, TAnOA3n and TBnOB3n, are kinked
differently. At room-T, the TBnOB3n chain is more
strongly kinked by about 23\ub0 than the TAnOA3n chain.
With increasing T, the difference decreases by 3\ub0 for
the TBnOB3n chain. The intersite cation exchange
reaction between M1 and M2 (Mg2+ and Fe2+) shows a
slight residual order at 1,200 C followed by reordering
with decreasing temperature although seemingly not
with a definite progressive trend. At the lowest temperature
reached (150 C), reordering has occurred
with the same value of partitioning coefficient KD as
that before heating. The absence of the expected phase
transition is most likely due to the presence of minor
amounts of Fe3+, Al, Ca and Cr which must play a
crucial role on the thermoelastic behaviour and phase
stability fields in natural Opx, with consequent important
petrologic and geological implications