Zircon, a mineral with the idealized composition ZrSiO4, is one of the chief sources of the element zirconium. Trace amounts of uranium and thorium are often present and the mineral may then be partly or entirely metamict. The name cyrtollite is applied to an altered type of zircon. Structurally, zircon is a nesosilicate, with isolated SiO4 groups.
Zircon is tetragonal in crystallization. It often occurs as well-formed crystals, which commonly are square prisms terminated by a low pyramid. The color is variable, usually brown to reddish brown, but also colorless, pale yellowish, green, or blue. The transparent colorless or tinted varieties are popular gemstones. Hardness is 71/2 on Mohs scale; specific gravity is 4.7, decreasing in metamict types.
Because of its chemical and physical stability, zircon resists weathering and accumulates in residual deposits and in beach and river sands.
Other properties include specific heat of 0.55 J/g/°C. It is chemically inert and stable to very high temperatures (liquidus >2205°C). Zircon has excellent thermal properties and its thermal conductivity is 14.5 Btu/ft2/hr/°F/in. and coefficient of thermal expansion is 1.4 x 10-6.
The extremely high thermal conductivity and chilling action of zircon makes it very useful in controlling directional solidification and shrinkage in heavy metal sections.
Uses
Zircon sand is used as refractory bedding material for heat-treating metal parts. It is used as a sealing medium for prevention of atmospheric leaks around doors and parts of heat-treating furnaces. Also, it is a high-qualiIty, uniform sandblasting medium for metal preparation prior to plating, enameling, or buffing. The heavy, rounded grains give consistent peening without stray digs or gouges to mar the finish. The tough, resilient grains resist breakdown and loss.
