Geoscience Reference
In-Depth Information
Minerals and phases of the mantle
each other without much effect on the crystal
structure.
The olivine structure is based on a nearly
hexagonal closest-packing of oxygen ions:
VI
Mg
2
IV
SiO
4
The four (IV) oxygen atoms surrounding each sil-
icon atom are not linked to any other silicon
atom. Mg is surrounded by 6 (VI) oxygen atoms.
Olivine is a very anisotropic mineral and is easily
aligned or recrystallized by stress and flow, mak-
ing peridotites anisotropic as well. Peridotites are
named after peridot, another name for olivine --
which is named after olives because of its dis-
tinctive color. No one knows the origin of the
word
peridot
. Olivine crystals may contain CO
2
and helium-rich fluid inclusions but are hosts for
few other of the incompatible elements that form
the basis for trace element and isotope models of
the mantle.
As far as physical properties and major ele-
ments are concerned, the most important upper-
mantle minerals are olivine, orthopyroxene,
clinopyroxene and aluminum-rich phases such
as plagioclase, spinel and garnet. Olivine and
orthopyroxene are the most refractory phases
andtendtooccurtogether,withonlyminor
amounts of other phases, in peridotites. Clino-
pyroxene and garnet are the most fusible compo-
nents and also tend to occur together as major
phases in rocks such as
eclogites
and
garnet clino-
pyroxenites
.
All of the above minerals are unstable at
high pressure and therefore only occur in the
upper part of the mantle. Olivine and ortho-
pyroxenestarttocollapseatdepthsnear400km.
Clinopyroxene -- diopside plus jadeite -- may be
stable to depths as great as 500 km. Garnet
is stable to much greater depths. Olivine trans-
forms successively to
(Mg
,
Fe)
2
SiO
4
−
spinel
Natural olivines (ol) and orthopyroxenes (opx)
transform to higher pressure phases at about
the right temperature and pressure to explain
the 410-km discontinuity (Figure 22.1). Fayalite
transforms directly to the
spinel structure
; olivines
having high MgO contents occur in two modifi-
cations related to the spinel structure,
β
-spinel, a distorted spinel-
like structure, near 400 km and to
-spinel, a true
cubic spinel, near 500 km. At high pressure it dis-
proportionates to (Mg,Fe)SiO
3
in the perovskite
structure plus (Mg,Fe)O, magnesiowustite, which
has the rocksalt structure. FeO is strongly parti-
tioned into the (Mg,Fe)O phase. One useful rule
of thumb is that adding Fe generally decreases
transition pressures, and adding Al generally
increases the stability of an assemblage.
A few examples of high-pressure phases will
be discussed. More detail is found in the first edi-
tion of Theory of the Earth; (http://caltechbook.
library.caltech.edu/14/17/TOE16.pdf) and websites,
journal articles and topics on
mantle min-
eralogy and high-pressure phase dia-
grams.
See also http://www.uni-wuerzburg.de/
mineralogie/links/teach/diagramteach.html,
http://www.agu.org/reference/minphys.html
γ
β
-spinel
and
-spinel. Both of these have much higher
elastic-wave velocities than appropriate for the
mantle just below 400 km, so there must be other
components in the mantle that dilute the effect
of the
γ
phase change and phase changes in
opx. Although
normal
,or
real
, spinel is a struc-
tural analog to silicate
spinel
, it should be noted
that Mg occurs in 4-coordination in MgAl
2
O
4
and
in 6-coordination in
α
−
β
-
spinel
with a con-
sequent change in the Mg--O distance and the
elastic properties.
IV
Mg is an unusual coordina-
tion for Mg, and the elastic properties and their
derivatives cannot be assumed to be similar for
IV
MgO and
VI
MgO compounds. In particular, nor-
mal spinel has an unusually low pressure deriva-
tive of the rigidity, a property shared by other
4-coordinated compounds but not
β
-and
γ
(Mg
,
Fe)
2
SiO
4
−
olivine
Olivine is the name for a series between two
end-members, fayalite and forsterite. Fayalite
is the iron-rich member with a formula of
Fe
2
SiO
4
. Forsterite is the magnesium-rich mem-
ber.
VI
MgO-bearing
compounds.
The spinel structure consists of an approxi-
mate cubic close packing of oxygen anions. Spinel
The
two
minerals
form
a
series
where
the
iron
and
magnesium
are
substituted
for
