Smith College - Geology 222b - Petrology

Petrographic Data File

Formula ZrSiO4 Zircon always contains hafnium (Hf); solid solution may extend to hafnon (HfSiO4). Zircon is also so known to contain rare earth metals and uranium and thorium. Decomposition of these radioactive elements may cause the mineral to become metamict.
Crystal System Tetragonal  (4/m2/m2/m) a = 6.61 A,  c = 5.98 A,   Z = 4
Crystal Habit Microscopic crystals  Tetragonal prismatic, with dipyramidal terminations. Euhedral growths may be present over subhedral or rounded cores.
Cleavage Poor {110} prismatic,  Poor {111} pyramidal Conchoidal fracture
Hardness 7.5 brittle
Streak White
Specific Gravity 4.68
Color/Pleochroism Grayish, yellowish, or reddish brown. Also may be colorless, yellow, gray, pink, or blue-green. Colorless to pale brown in thin section.


In thin section, samples may be cloudy, show concentric zoning or patchy color. Due to small size of crystals, color in thin section may not be very helpful.

Weak pleochroism (w < e) in colored varieties

Fluorescence Thermoluminesecent, cathodoluminescent, may fluoresce under UV light  
Optic Sign Uniaxial (+)  
2V not usually observed due to small crystal size

Metamict zircon is usually isotropic. but may show biaxial figures of low 2V.

Optic Orientation length slow  
Refractive Indices
omega =
epsilon = 
1.920 - 1.960 (1.924 - 1.934, natural non-metamict)

1.967 - 2.015 (1.970 - 1.977)

High Hf Zircon has higher indices. Metamict Zircon has lower indices/birefringence and may be isotropic with an index  of about 1.80.
Max Birefringence 0.036 - 0.065

0.036 - 0.053 (natural non-metamict)

Elongation along C-axis  
Extinction parallel in longitudinal or in grain mount  
Dispersion **  
Distinguishing Features Zircon is not usuall seen in hand sample; when it is, it may be distinguished by crystal habit and hardness.

In thin section, Zircon is found in the form of small crystals included in other minerals, showing very high relief and high-order interference colors. Distinctive dark halos formed by radioactive decay may  be present when Zircon is included in minerals such as biotite, hornblende, or cordierite.

Occurrence Zircon is a fairly common accessory mineral, found in the form of microscopic crystals in igneous, sedimentary, and metamorphic rocks. It is often found with heavy minerals in sediments, as aside from radioactive decay does not easily break down. Detrital grains may be found in igneous rocks formed from heavy sediment melts. Zircons which have undergone igneous processes may display overgrowths of newer zircon grown over older cores.

Zircon is a useful mineral in that the radioactive elements contained in it are key to uranium-lead dating. Zircon has been of particular use in the dating of Pre-Cambrian plutons.

Editors Cassie Stearns (06), Sarah Brisson ('14)

Photomicrograph of zircon in plane light. The Zircon is barely visible in this shot, included in the biotite (brown), and circled by black halos. The halos are a result of interaction between radioactively decaying elements in the zircon and the surrounding biotite. The biotite is part of a granite.

Mag. 10x. From Sample W-12, slide (w-12), Smith College

Photomicrograph of zircon in plane light. This is a close-up picture of the above sample (note the string-like feature in both photos). The semi-square zircon crystals can be seen, as can more detail of the halos.

Mag. 50x  From Sample W-12, slide (w-12), Smith College

Photomicrograph of zircon in plane light. The zircon in this picture is also included in biotite. Note the presence of the dark halos around the zircon.

Mag. 50x From Sample W-12, slide (w-12), Smith College

Photomicrograph of zircon in plane light. Note the difference in orientation of this grain, as well as a the less pronounced halo. This is also biotite from a granite.

Mag. 50x From Sample From Sample W-11, Smith College

To the left is a photomicrograph of an individual zircon crystal. Zircon does not form large crystals in this granite, but appears as small cube-like crystals embedded within other minerals. The zircon crystal is distinguishable by the dark halo around the mineral caused by reactions between decaying elements within the zircon and the surrounding mineral. Click on the image to view the zircon in cross polarized light.

Mag. 10x From sample SN-9, Smith College

To the left is a photomicrograph of a zircon crystal. The halo surrounding the zircon crystal disappears as the stage is turned. Click on the image to view the halo disappearing.

Mag. 10x From sample SN-9, Smith College

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Sources: Nesse, W.D. (2000) Introduction to Mineralogy, Oxford University Press, 442p.