Zircon

nesosilicate

thorium

Formula: Zr(SiO4)
Nesosilicate (insular SiO4 groups)
Specific gravity: 4.6 to 4.7
Hardness: 7½
Streak: White
Colour: Brown, reddish-brown, colourless, grey, green. The blue colour of some zircon is caused by quadrivalent uranium U4+.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments (common)
Pegmatites (common)
Carbonatites
Placer deposits
Sedimentary environments
Metamorphic environments (less common)

Zircon is a common and widely distributed primary mineral in granite and granite pegmatites, and placers. It is less common in metamorphic rocks, but may be found in gneiss after surviving regional metamorphism (Lauf p62). Because it is hard and chemically stable, zircon is a common accessory mineral in sediments.

Zircon is common in nepheline syenite, and it also may be found in granite, granodiorite, diorite, monzonite, rhyolite, andesite, limestone, gneiss, schist, andesite and phyllite.

Because zircon is one of the few minerals that can accommodate large tetravalent ions in its structure, it often contains the radioactive elements uranium and thorium (Lauf p61-62).

At Zircon hill, Harts Range, Northern Territories, Australia, gem quality zircon is found in weathered carbonatite. This zircon is unusual in that it contains almost no radioactive trace elements (Lauf p68).

At the John Dole quarry, near Bancroft, Ontario, Canada, zircon is associated with fergusonite (Lauf p68).

At Girardville, Quebec, Canada, zircon is enriched in hafnium (R&M 88.5.432).

At the placer gold mines at Dawson, the Yukon, Canada, zircon is associated with gold (R&M 85.1.41).

At Bo Loei in Cambodia, zircon is enriched with uranium and thorium.

At Mt Malosa, Zomba district, Malawi, zircon occurs in alkaline rocks associated with aegirine, arfvedsonite and feldspar (Lauf p66).

At Palabora, South Africa, zircon is associated with phlogopite and calcite (R&M 92.5.449-450).

In Swaziland, zircon is associated with beryl in granite pegmatites (R&M 85.5.451).

At Tigerville, Greenvile county, South Carolina, USA, zircon occurs in a pegmatite in a matrix of partly altered microcline, riebeckite and anatase (Lauf p66).

At the Nine Mile granite pegmatite, Marathon County, Wisconsin, USA, zircon is a common accessory mineral, exhibiting late-stage enrichment with hafnium. Zircon with closely associated albite is probably one of the last minerals to form in the pegmatite. Other associated minerals include cassiterite, columbite and pyrochlore.

Common impurities: Hf,Th,U,Rare Earth Elements,O,H,H2O,Fe,Al,P.

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