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Formula: Na☐KZrSi8O19.5H2O
Phyllosilicate (sheet silicate), rhodesite group,
zirconium-bearing mineral
Crystal System: Orthorhombic
Specific gravity: 2.352 calculated
Hardness: 3 to 4
Streak: White
Colour: Colourless to pale yellow
Luminescence: Green fluorescence under short wave UV
Environments
Melansonite is a relatively new mineral, approved in 2018 and to date (August 2023) reported only from the
type locality.
Localities
At the type locality, the Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec,
Canada, samples containing melansonite were found in a zone at the contact between a
marble xenolith and the host
nepheline syenite. The melansonite was observed in
vesicles within the marble xenolith. The xenolith is predominantly
white to light brown in colour and contains numerous, small, randomly dispersed vugs less than 1 cm across. Samples
consist primarily of albite, calcite
and aegirine, with an assemblage of a sodic
clinoamphibole (arfvedsonite?),
quartz, an apophyllite group
mineral, gaidonnayite,
pyrrhotite (?), and melansonite comprising the vugs. While most
aegirine at this locality is typically green to very dark green in colour,
that associated with melansonite is distinctly brown; the cause of the unique colour of the
aegirine is not known.
Melansonite is paragenetically a late-stage mineral that is considered to have formed as a result of the
interaction of late-stage alkaline fluids, enriched in SiO2 and ZrO2, that interacted with a
marble xenolith under conditions of low pressure and temperature, less
than 200oC
(CJMP 61.2.387–400).
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