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Formula: Ca3Si3O9
Anhydrous cyclosilicate (ring silicate), margarosanite group,
high-pressure, triclinic paramorph of high-pressure, isometric
davemaoite, high-temperature, monoclinic
pseudowollastonite and triclinic
wollastonite
Crystal System: Triclinic
Specific gravity: 3.072 calculated
Colour: Colourless
Luminescence: Not fluorescent under UV
Environments
Breyite is a relatively new mineral, approved in 2018. It is the second most abundant mineral inclusion in
super-deep diamonds; ferropericlase is the most abundant.
Localities
The type locality is the São Luis river alluvials, Juína, Mato Grosso, Brazil. Earth’s lower mantle most likely mainly
consists of ferropericlase,
bridgmanite and a CaSiO3 phase in the
perovskite structure. If separately trapped in
diamonds, these phases can be transported to Earth’s surface without reacting
with the surrounding mantle. Although all inclusions will remain chemically pristine, only
ferropericlase will stay in its original crystal structure, whereas in
almost all cases bridgmanite and
CaSiO3-perovskite will transform to their lower-pressure
paramorphs. In the case of
perovskite structured CaSiO3, the new structure that is formed
is closely related to that of walstromite. This mineral is now approved by
the IMA and named breyite; it is the second most abundant mineral inclusion after
ferropericlase in diamonds of
super-deep origin. The occurrence of breyite has been widely presumed to be a strong indication of
lower mantle (>670 km depth) or at least lower transition zone
(>520 km depth) origin of both the host diamond and the inclusion suite.
The finding of breyite alone in a diamond is not a reliable indicator
of the formation depth in the transition zone or in the
lower mantle and paragenetic phases such as
ferropericlase together with MgSiO3 are needed.
(AM 106.1.38-43).
Other minerals associated with breyite include
β-Ca2SiO4 larnite,
titanite-structured CaSi2O5 and
ringwoodite
(HOM).
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