Formula: NaCaAlF6.H2O
Halide, dimorph of pachnolite and epitaxial on it.
Specific gravity: 2.891
Hardness: 2
Streak: White
Colour: Colourless, white, pale lilac; brownish or reddish tinted due to staining; colourless in transmitted light.
Solubility: Readily soluble in sulphuric acid

Volcanic igneous environments

Thomsenolite occurs as an alteration product of cryolite and other alkali aluminum fluorides, most commonly in granite pegmatites, associated with cryolite, pachnolite, hydrokenoralstonite, chiolite, cryolithionite, elpasolite, sellaite and fluorite (HOM).

At the type locality, the Ivigtut Cryolite deposit, Sermersooq, Greenland, Denmark, thomsenolite is associated with pachnolite, fluorite and hydrokenoralstonite (Mindat, Dana).

At the Spider claim, Juab county, Utah, USA, thomsenolite occurs in rhyolite associated with tridymite and sodium boltwoodite (Dana).

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