Trinepheline

trinepheline

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Formula: NaAlSiO4
Tectosilicate (framework silicate), feldspathoid group, paramorph of nepheline
Crystal System: Hexagonal
Specific gravity: 2.642 calculated
Hardness: 5 to 5½
Streak: White
Colour: White to pale yellow
Luminescence: Not fluorescent in UV
Common impurities: Ca,K
Environments

Metamorphic environments

Localities

At the type locality, Tawmaw, Hpakant-Tawmaw Jade Tract, Hpakant Township, Mohnyin District, Kachin State, Myanmar, two new (in 2012) mineral species, trinepheline and fabrièsite, are described from serpentinised peridotite, replacing jadeite during late stage metamorphism along veins of the deposit. Trinepheline is known for the polymorphs of synthetic NaAlSiO4 with a value of the c parameter that is three times that of nepheline, hence the name "trinepheline". Fabrièsite and trinepheline occur intimately intergrown together with nepheline, more rarely with albite and other feldspar group phases such as banalsite and stronalsite; other associated minerals are jadeite and secondary products like natrolite and harmotome. Both fabrièsite and trinepheline are pseudomorphous after jadeite and occur as skeletal allotriomorphic (not having their own regular shape, as determined by their internal structure, but being shaped instead by adjacent minerals) crystals up to 15 to 20 μm long and 5 to 10 μm wide. They are white to yellowish in hand specimens and colourless in thin section; the streak is white and the lustre appears vitreous to greasy (EJM 26.257-263, HOM).

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