Silicates · Sourced PROFILE
Zircon
ZrSiO₄
A durable zirconium silicate found in small amounts in many rocks; its uranium–lead ages help date how the Pamir's deep crust rose to the surface.
AT A GLANCE
- Formula
- ZrSiO₄
- Crystal system
- Tetragonal
- Space group
- I4₁/amd
- Mohs hardness
- 7.5
- Density
- 4.6–4.7 g/cm³
- Optical character
- Uniaxial (+); isotropic when metamict
CRYSTALLOGRAPHY
Crystal form & structure
Tabular to prismatic crystals with square cross-sections, terminated by {111}, up to 30 cm; also irregular grains or massive. Geniculate twins on {101}.
HAND-SAMPLE PROPERTIES
Physical properties
- Cleavage
- Indistinct on {110} and {111}
- Fracture
- Conchoidal
- Tenacity
- Brittle
- Streak
- White
- Luster
- Vitreous to adamantine; greasy when metamict
- Transparency
- Transparent to opaque
- Observed colours
- Reddish brown, yellow, green, blue, grey or colourless
LIGHT & OPTICS
Optical properties
- Optical class
- Uniaxial (+); isotropic when metamict
- Refractive index
- nω 1.925–1.961; nε 1.980–2.015
- Fluorescence & electrical behaviour
- May fluoresce under ultraviolet light; also thermoluminescent and cathodoluminescent. Radioactive when metamict.
GEOLOGICAL SETTING
Formation
An accessory mineral in igneous and metamorphic rocks that reaches large crystals in mafic pegmatites and carbonatites, and survives erosion into heavy-mineral sands.
MINERAL COMPANY
Common associations
Feldspars, amphiboles, mica and quartz
FIELD NOTES
How to identify it
Very high lustre and refractive index, hardness 7.5, high density and square prismatic crystals are good clues; radiation-damaged (metamict) grains look greasy and may be weakly radioactive.
NAME & HISTORY
Where the name comes from
Thought to derive from the Persian zar (gold) and gun (colour), through the Arabic zarqun.
SCIENTIFIC SOURCE
Handbook of Mineralogy — Zircon
External reference; factual profile paraphrased and attributed. Copyright remains with the publisher.
PROVENANCE
Documented occurrences
Shakhdara–Alichur gneiss dome, South-West Pamir · Zircon from the dome is dated by U/Th–Pb, fission-track and (U–Th)/He methods, which together with other minerals show that doming began about 21–20 million years ago. Region-level context only.
Sourced · Region precisionThe giant Shakhdara migmatitic gneiss dome, Pamir, India–Asia collision zone: 2. Timing of dome formation