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Formula: Mg2SiO4
Although this is the IMA formula, wadsleyite is actually a sorosilicate (Si2O7 groups) and
not a member of the
olivine group. The formula Mg4O(Si2O7) better
illustrates the structure (Mindat). It is the orthorhombic, high-pressure
paramorph of orthorhombic
forsterite, orthorhombic
poirierite and isometric
ringwoodite
Specific gravity: 3.84 calculated
Colour: Light greyish brown
Common impurities: Cr,Mn,Ni,Ca,Zn
Environments
Wadsleyite was found initially in the Peace River meteorite from Alberta, Canada; it is thought to be formed from
the transformation of olivine during an extraterrestrial shock event, such as
a meteorite impact. It is known to be a stable and probably the most abundant phase in the transition zone of the Earth's
upper mantle, between 400 and 525km in depth
(Mindat).
Localities
A the type locality, the Peace River meteorite, Peace River, Alberta, Canada, wadsleyite has been found in the
enstatite-ferrosilite -
olivine
chondritic meteorite. The
meteorite is largely an assemblage of
olivine, orthopyroxene,
plagioclase,
iron-nickel alloys and
troilite. The plagioglase
has been extensively converted to its amorphous equivalent, "maskelynite",
which indicates that the meteorite has been subjected to a
high-pressure shock event. In addition to this shock-produced phase, the Peace River
meteorite contains thin, black, sulphide-rich veins, which
pervade the body of the specimen. Typically, within rounded fragments found in these veins, the constituent grains of
olivine and orthopyroxene have been
transformed into their high-density paramorphs, the
spinel-structure ringwoodite and
the garnet-structure majorite,
respectively. The vein studied in the Peace River meteorite differs significantly from those described in other
meteorites, however, in that it contains significant quantities
of wadsleyite.
The wadsleyite occurs as microcrystalline aggregates that pseudomorph
previously existing olivine fragments within the vein. The
wadsleyite-bearing fragments rarely exceed 0.5 mm in diameter, and the actual grain-size of the crystals within
the polycrystalline fragments is between 0.5 and 5 microns. The pale fawn, polycrystalline aggregates of wadsleyite
are invariablv fractured.
In addition to wadsleyite, the fragments may contain small amounts of
majorite or microcrystalline aggregates of
pyroxene pseudomorphing
majorite and highly faulted
ringwoodite
(CM 21.29-35).
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