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Formula: Zr(SiO4)
Nesosilicate (insular SiO4 groups) of zirconium
Varieties
Alvite is a metamict, often hafnium-rich variety of zircon from granite pegmatites
Cyrtolite is a hydrous thorium and uranium bearing variety of zircon
Properties
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+ (Mindat)
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Common impurities: Hf,Th,U,Rare Earth Elements,O,H,H2O,Fe,Al,P.
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,
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).
Localities
At the Mud Tank Zircon Field, Alcoota Station, Central Desert Region, Northern Territory, Australia, gem quality
zircon is found in weathered
carbonatite. This zircon
is unusual in that it contains almost no radioactive trace elements.
Mud Tank zircons are a mineralogical curiosity because they can be extremely large and they come from a
carbonatite. The Mud Tank deposit is unique as the only known
carbonatite containing gem-quality zircon. The general and
accessory mineralogy of the deposit includes biotite,
albite, amphiboles,
pyrite, chalcopyrite and
phlogopite — the last identified as
hydrobiotite. The majority of the
carbonatite at depth consists of small flakes of
biotite in a fine-grained matrix of
dolomite, calcite,
apatite and amphiboles with occasional
but scarce small zircon and
magnetite/martite crystals.
Zircon occurs as huge single crystals to 40 cm in size, but is more commonly found as small crystals under 2 or 3 cm,
broken crystal fragments, rounded water-polished pebbles and abraded fragments of crystals. Generally, zircon is found
loose in gravels and rarely in chunks of secondary and tertiary matrix; actual samples of unaltered
carbonatite are very rare and, like zircon, they have
usually been preserved by encapsulation in tertiary calcite. It seems that the
primary zircon matrix was a coeval crystallisation of fluorapatite and
magnetite, with the latter now partially altered to
martite.
Later-developed matrix includes compact limestone, silica replacing
carbonate, or a crumbly aggregate of
magnetite/martite,
goethite, limonite and secondary
calcite, with apatite. Even when found
isolated in gravels, zircon is usually covered by iron oxides and a tightly adhering film of
gypsum. All large Mud Tank zircon crystals show evidence of late-stage growth
forming “boiler-plate” or “paddy field” corrugations on pyramidal faces.
A large fractured, red-brown, single crystal of zircon weighing 32 kg has been found. It was originally doubly
terminated, and contained transparent gemmy material
(Minrec 51.6.815-824).
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).
Amity, Town of Warwick, Orange county, New York, USA, is an area of
granite intrusions into
marble and associated
gneiss. The marble is
mostly composed of white crystalline calcite that often has small flakes
or spheres of graphite and
phlogopite. Zircon occurs occasionally as individual dark brown
tetragonal prisms in massive scapolite. The prism is terminated by a
tetragonal pyramid and may be as large as 1.5 cm
(R&M 96.5.441).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Zircon crystals are relatively common as inclusions in scapolite, but
they require an SEM to observe them.
(R&M 96.6.552).
At Tigerville, Greenville 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.
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