Cyclosilicate, benitoite group
Specific gravity: 2.724
Colour: Pale to dark brown, white, yellow, colourless, flesh-pink, rose-red
Solubility: Slightly soluble in hydrochloric and nitric acids, but not by sulphuric acid
Common impurities: Ti,Al,Fe,Mn,Mg,Ca,K
Hilairite is a rare zirconium-bearing mineral.
At the type locality, the Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada, hilairite occurs as 0.1 to 1.0 mm crystals and more commonly as aggregates in miarolitic cavities and altered pegmatite dikes in nepheline syenite. The crystals are found on analcime, microcline, or natrolite, usually associated with calcite, aegirine, albite, catapleiite and gaidonnayite. They are often coated with a veneer of chlorite. Other associated minerals include elpidite, epididymite, fluorite, pyrite, sphalerite, aegirine and galena (CM 12.237-240, HOM, Dana).
At Langesundfjord, Norway, hilairite occurs in hydrothermally altered pegmatites (Dana).
On heating hilairite loses all of its water at 220oC and at least 95% of the original water is recovered on cooling from 855oC to room temperature (CM 12.237-240).
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