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Formula: Mg2SiO4
Nesosilicate (insular SiO4 groups), orthorhombic high pressure
paramorph of orthorhombic
forsterite, isometric
ringwoodite and orthorhombic
wadsleyite
Crystal system: Orthorhombic
Specific gravity: 3.326 calculated
Environments
Poirierite occurs as a dense metastable paramorph of magnesium
silicate in shocked meteorites, associated with ringwoodite and
wadsleyite
(Mindat, HOM).
Localities
There are two co-type localities, the Tenham meteorite (S6 L chondrite), Tenham Station, Windorah, Barcoo Shire, Queensland,
Australia, and the Suizhou meteorite (Suizhou L6 chondrite), Xihe, Zengdu District, Suizhou, Hubei, China.
At the type localities nanoscale lamellar crystals of poirierite occur intergrown within
wadsleyite and within
ringwoodite in the shocked meteorites. Associated minerals include
akimotoite, asimowite,
hemleyite, hiroseite,
lingunite, majorite and
ringwoodite
(Mindat).
At the Miami meteorite (H5 chondrite), Roberts County, Texas, USA, and at the Tenham meteorite (see above), it is
suggested that that the olivine grains in the host rock were initially
transformed into stable high-pressure phases by nucleation and growth mechanisms in prograde shock metamorphism
without melting. Wadsleyite and
ringwoodite occur as monomineralic aggregates entrapped in
shock-induced melt veins. The nanoscale lamellar poirierite is intergrown within the
wadsleyite and ringwoodite
grains. Rapid cooling and decompression during retrograde shock metamorphism would have hindered the direct
back-transformations from
wadsleyite/ringwoodite to
olivine but would have promoted the metastable formation of
poirierite by a shear mechanism
(Japan Association of Mineralogical Sciences https://doi.org/10.2138/gselements.17.3.195).
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