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bytownite

Bytownite is a plagioclase feldspar mineral, a calcium-rich member of the plagioclase solid solution series. It lies between labradorite and anorthite in composition, with higher calcium and lower sodium relative to more sodium-rich varieties. The anorthite content is typically appreciable, commonly described as roughly 50–70% anorthite (An). Its general formula is (Ca,Na)Al1-2Si3-2O8, reflecting ongoing substitution between Ca and Na in the crystal structure.

Bytownite forms in igneous rocks of granitic to dioritic composition and can occur in metamorphic rocks that

Physically, bytownite is typically colorless to white, gray, or pale; it has a vitreous to pearly luster

Bytownite was named after Bytown, the former name of Ottawa, Ontario, Canada, where the mineral was first

have
undergone
granulitic
conditions.
In
plutonic
rocks,
it
may
appear
as
euhedral
crystals
and
as
constituents
of
coarse-grained
granitoids;
in
rocks
of
calc-alkaline
and
high-temperature
facies
it
can
occur
as
well
as
accessory
grains.
and
a
Mohs
hardness
of
about
6–6.5.
Like
other
plagioclases,
it
exhibits
two
directions
of
cleavage
at
about
90
degrees
and
often
shows
characteristic
polysynthetic
twinning
and
striations
on
cleavage
surfaces.
Specific
gravity
is
approximately
2.65–2.75.
described.