ZIRCON
Zircon is a zirconium silicate mineral (ZrSiO4).
It is very hard (H=7.5), has an nonmetallic, adamantine luster, and varies in
color from brownish to grayish to greenish to reddish to clearish. Most
zircon crystals are very tiny. Well-developed crystals show that they
have four-sided prisms capped at both ends with pyramidal terminations.
The five zircon crystals shown below lack the prism portion of the crystal, and
only have the two pyramidal terminations, resulting in pseudo-octahedral crystals.
Other elements often take the place of Zr in this mineral. For example,
all zircons contain some Hf (hafnium). Iron (Fe), thorium (Th), and
uranium (U) also commonly substitute in place of zirconium. As a
consequence, zircons are radioactive. Zircons are hard and chemically
resistant, but many crystals are found destroyed by radiation from included Th
& U (metamict zircons).
Microscopic zircons are present in practically all
igneous rocks and metamorphic rocks. Many modern sands and ancient siliciclastic
sedimentary rocks contain detrital zircons. Zircon is used as a gemstone,
sometimes masquerading as diamond. Zircon is also used as a source of
zirconium metal (some Florida sands have up to 1% zircon, and are mined for
Zr). The most significant use for zircons in geology is isotopic
dating (radiometric dating).
Zircons
(each is ~5 to 7 mm across) - dark brown pseudo-octahedral zircon crystals.