Zircon

Introduction

Zircon is a zirconium silicate mineral found in small quantities in many rocks. It can be a variety of colours and some crystals are of gemstone quality. Zircon is sometimes confused with the metal zirconium, the zirconium oxide called zirconia, and cubic zirconia, a synthetic gemstone with an appearance that is very similar to diamond.

Properties

Two images of the same uncut zircon crystal. One image is the zircon appears glassy white with brown and orange sections and in the other the zircon appears green and yeloow. The second image has been taken under UV light.

Zircon from Strangways Range Northern Territory. The zircon has been photographed in plain light (left) and appears fluorescent-yellow in ultraviolet light (right). R30458. Source: Geoscience Australia

Zircon is usually reddish brown, but can be white, orange, yellow, yellowish brown, purplish red, light red, blue, green and sometimes colourless. It is usually translucent but can be transparent. In fluorescent light zircons appear yellow (see Google Arts and  Culture: Fluorescent Minerals: Rocks that get excited under ultraviolet light). Their average size in granite is 0.1 to 0.3 mm, but they can grow  larger in pegmatites and carbonatites. A crystal has been described from Canada that was 30 cm long and weighing 7 kg.

The Properties of Zircon
Chemical symbolZrSiO4
Relative density4.6-4.7 g/cm3
Hardness7.5 on Mohs scale
NameNamed after the Arabic word ‘Zargun', (derived from the Persian words:
zar meaning gold and gun meaning colour).

Uses

A long thin cyliner of copper coloured metal

Zirconium rod. Source: Wikimedia Commons, Dschwen. Source: Wikimedia Commons

Zircon's hardness makes it useful as an abrasive and it has a high melting point (over 2500°C), so it is used in the steel industry, to line furnaces. Zircon contains the metals zirconium and hafnium. Zirconium is extremely hard and resistant to  corrosion so is used to make pipes for harsh chemicals, nuclear reactor cladding, heat exchangers and speciality alloys. Zirconium is found in computer disc drives, in lightweight clothing and in many domestic products such as ballpoint pens and wear-resistant knives. Hafnium is used in nuclear control rods and super alloys. Zircon products are also used in engines, electronics, spacecraft and the ceramics industry.

Zircons can survive processes like erosion, transport and metamorphism, so they preserve a record of past geological  processes. They are one of the key minerals used by geologists to determine the age of rocks. Zircons contain trace amounts of uranium and thorium, which decay radioactively over time. Because the rate of radioactive decay is known, scientists can measure the tiny amounts of these elements in a zircon to work out how old a host rock is. Zircons extracted from rocks from Jack Hills in Western Australia are up to 4.4 billion years old.

Further  information on how zircons are used to work out the age of rocks.

Mineral specimens housed in the collection archive.

Sensitive High Resolution Ion Microprobe (SHRIMP) used for analysing the age of zircons. Source: Geoscience Australia.

Three images of zircon crystals under different lighting conditions. The first they crystals appear white, the second they appear translucnt and glassy and the third black, grey  and white zoning is visible in the crystals.

Reflected light, transmitted light and cathodoluminescence images of tiny zircons from the Walleroobie Volcanic rocks in New South Wales. In the last image the darker areas of the crystals have more uranium. Scientists use images like these to choose where to analyse zircons to determine their age. Source: Geoscience Australia.

Four square cut zircon crystals. They appear orange to red in colour.

Zircon gems from Queensland. Each gem is about 2.5 mm wide. Source: Geoscience Australia.

Zircon is a popular gemstone because  it is available in a variety of colours and colourless, faceted zircons have been  used as a substitute for diamonds. Hyacinth is the name of gem-quality zircon that is yellow-red to red brown and starlite the name given to blue, heat-treated  zircon. Zircon is sometimes confused with cubic  zirconia, the synthetic crystalline form of zirconium dioxide (ZrO4),  that can be cut and used as a gemstone.

History

Zircon is a popular gemstone that  has been used for nearly 2000 years. Gem-grade zircon has been produced from river  deposits in Sri Lanka, Cambodia, Myanmar, and Vietnam for hundreds of years. Zircons  are mentioned in the Bible and a Hindu legend describes green zircon leaves given  to a god as a gift.

In 1789, while analysing the  composition of zircon Martin Heinrich Klaproth, a German chemist discovered zirconium.  This metal was then isolated from zircon by Jöns Jacob Berzelius, a Swedish chemist,  in 1824.

Blue zircon is found in many  pieces of English estate jewellery produced in the 1880s, because it was  fashionable at the time. However, during the early 1900s zircon was sometimes used  fraudulently as a substitute for diamond so often the gem is thought of as an ‘imitation'  gemstone.

Mineral sands that included  zircons were first mined in Australia at Byron Bay, New South Wales in 1934. The  sand mining industry then expanded along the coast as far north as Fraser  Island in Queensland. Further deposits of mineral sands were discovered in Western  Australia in the 1970s and 1980s. Mineral sand deposits are now known to exist  in every Australian state and the Northern Territory.

Formation

Zircon is very common and widely distributed  in the Earth's crust. It is found in most igneous and metamorphic rocks; however,  it may not be noticed because of its very small particle size. Due to its hardness  and durability it does not weather easily, so is also found in sedimentary deposits  and is a common constituent of most sands. However, zircon is rarely found in  mafic rocks and very rarely in ultramafic igneous rocks.

A brown rock.

A radioactive, thorium and uranium rich zircon called cyrtolite. R26527. Source: Geoscience Australia

Grains of zircon are usually only  a few millimetres in size, but some of the largest crystals of are formed in pegmatites,  carbonate-rich igneous rocks, and in limestones that have been altered by hot fluids  (hydrothermal metamorphism).

Zircon deposits may take millions  of years to form. When rocks containing zircons are weathered and eroded, tiny grains  of zircon are released. They may remain in soils, sediments or sedimentary rocks  for millions of years surviving multiple cycles of weathering, erosion, deposition  and uplift. Alternatively the grains may be transported, along with other harder  minerals, by streams to the sea. The heavier minerals are then washed back up onto  the beach by waves. These processes eventually create a large deposit of zircon sands on the beach.

Resources

Australia has the world's largest  resources of zircon (about 35% of the world's zircon). Australia also produces  more zircon than any other country; in 2016 alone, 560 kt of zircon concentrate  was exported from Australia. Most of this material was extracted from heavy  mineral sand deposits in Victoria and Western Australia. These alluvial deposits  contain mainly sub-millimetre grains of zircon derived from the weathering of rocks.

A pile of fine white sand sits in front of a glass jar of sand. The jar is labelled zircon.

Zircon sand. Source: Geoscience Australia

Mineral sand deposits are  wide-spread in Australia; they occur along the coast of eastern Australia from  central New South Wales to Cape York in Queensland. Large old beach deposits  are found in Victoria, south-western New South Wales and South Australia in the  Murray Basin and in the Eucla Basin in South Australia and Western Australia. Elsewhere  in Western Australia, deposits are found along the coast line from the southern  tip up to Derby. About 10% of zircon resources are not able to be mined because  they are located near the coast, the land is used for agriculture, national  parks, or urban development.

The major source of Australian gem quality zircons is alluvial deposits from alkaline basalts in Tasmania, New South Wales and Queensland. The most well-known fossicking location  for gem-quality zircon is the Mud Tank field in the Strangways, Northern Territory. In addition,  zircon is often found alongside sapphires, so can be found in the Queensland  and New South Wales gem fields.

A map of Australia showing the location of gem deposits and mines including zircons

Gemstone occurrences and mines in Australia. Source: Geoscience Australia

Further resource and production information.

Mining

Zircon has been mined from stream  gravels for over 2000 years. This early mining of zircon was mainly to obtain attractive  crystals for use as gemstones. Today, most zircon is produced from mining sediments  from beach or alluvial deposits. Zircon is often a co-product at mining or processing  operations where ilmenite and rutile are being mined for their titanium content.

Zircon is extracted by dry mining  methods if the deposit is shallow or contains harder materials. Excavators and  scrapers are used to dig out the ore, which is then transported by trucks or  bulldozers to the initial processing plant. The ore is then conveyed to a wet  concentrator plant by overland conveyer or via a slurry pipeline.

Dredge (wet) mining methods are  used if the ore is below the water table or in high dune areas. This process  involves a dredge floating in an artificial pond. It scrapes up sand which is  then sent to a wet concentrator attached to the dredge for the first stage of  processing. The dredge continually moves leaving behind the tailings sand. After  extracting the mineral sands, mining companies rehabilitate the land and replace  the vegetation.

Some of the best gem-grade zircon  crystals are mined directly from cavities in pegmatite. The zircons are then  weighed and graded before being sent to be cut or treated.

Processing

Zircon's density allows the grains  to be recovered from the other sand minerals by mechanical separation. In the  wet concentration plant, the mined mineral sand or ore is washed through a  series of spiral separators to separate the heavy minerals from the lighter  quartz and clay materials. Then the heavy minerals are separated into the  component minerals in a dry processing circuit using magnetic and electrostatic  plate separators under various temperatures. The zircon is dried, graded and  sized, and sometimes ground in mills before it is transported.

Most natural gem-quality zircons are  yellow, red, or brown. Heating and irradiation can be used to produce colourless,  blue, green, and many other zircon colours. Blue is the most popular zircon gem  colour. Zircon that is faceted for use in jewellery, is mostly without  inclusions but they can contain long parallel inclusions that create the  cat's-eye effect when the stone is cut as a cabochon. Zircons are typically cut  using the brilliant style, but there  is also the zircon cut, which has eight  extra facets around the lower part of the gem.

More  information about gemstone cuts

Further information

Further resource and production information
More information about gemstones