Formula: Na2Be2Si6O15.H2O
Inosilicate (chain silicate), amphibole group, the monoclinic paramorph of epididymite, which is orthorhombic
Specific gravity: 2.55
Hardness: 6
Streak: White
Colour: White, colourless
Solubility: Insoluble in common acids
Common impurities: Al,Fe,Ca


Eudidymite is a late-stage mineral in alkalic nepheline syenite pegmatites, associated with albite, neptunite, aegirine, elpidite, natrolite, analcime, fluorite and quartz (HOM).


At Mont Saint-Hilaire, Montérégie, Quebec, Canada, eudidymite occurs in pegmatite veins and rarely in breccia cavities (Dana).

At the type locality, the Eudidymite locality, Lille Arøya, Langesundsfjorden, Larvik, Vestfold, Norway, eudidymite and epididymite grow in cavities or in dense masses in the central portion of the zoned alkalic pegmatites and are associated with albite, neptunite, schizolite and narsarsukite. Both paramorphs are products of late stage metamorphism (AM 57.1345-1354). Other associated minerals include nepheline, natrolite, molybdenite, K-feldspar, fluorite, biotite, astrophyllite, apophyllite and analcime (Mindat).

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