|
||
|
|
|
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) |