Bytownite

The rarest of all plagioclase feldspars (albite-anorthite solid solution series). Bytownite has an anorthite content of 70-90%.

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Bytownit bytownite
Bytownite from Canada

Origin of name: firstly described in 1835, the mineral was named after an occurrence in Bytown (since 1855: Ottawa), Canada. Bytown owes its name to the English engineer John By, who played a major role in the construction of the Rideau Canal, connecting Ottawa to the city of Kingston at Lake Ontario.
Bytown is still used as a nickname for Ottawa.

Synonyms and trade names: Mexican bytownite has been marketed as "golden sunstone"

Can be confused with: other Feldspars and a plethora of colourless to golden yellow gems.

Localities:
as of Spring 2015, mindat.org lists 167 occurrences on all continents.
Probably the most important localities are in Mexico, the USA and in Canada

Bytownit bytowniteBytownit bytownite
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Bytownites from Ethiopia

Handling: due to the perfect cleavage bytownite should be set, handled and worn with care. Do not clean ultrasonically!

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Gemmological Properties of Bytownite

Formula:
(Ca,Na)[Al(Al,Si)Si2O8]
Crystal system:
triclinic
Mohs hardness:
6 - 6.5
Specific gravity:
2.72 – 2.74
Refractive index:
1.563 - 1.583
Max. Birefringence:
0.011
Dispersion:
0.012
Pleochroism:
none
Lustre:
sub-vitreous to vitreous
Cleavage:
perfect
Fracture:
uneven to conchoidal