Pegmatites are intrusive igneous rock formations characterized by having large crystals of 5 cms up to several meters in size and are often composed predominately of quartz, feldspar and muscovite mica.
Frequently pegmatites are elongated tabular dykes near vertically inclined and range from 1 to 10 meters in thickness. Overall composition is close to granite with which they are often associated. Their origin is considered to be the last stage of granite magma crystallization from hydrothermal solutions rich in water and other volatile components at temperatures of the order of 500 to 600 degrees C. What makes pegmatites interesting and valuable are that concentrated in these residual fluids are many exotic elements that go to form rare and valuable minerals.
Pegmatites are mined for two main reasons. Firstly, on a small scale they are mined for gemstones and mineral specimens such as in New England region and California USA, Brazil and Zimbabwe. The low viscosity hydothermal solutions allow for rapid growth of large crystals in an open environment thus producing crystals having many natural faces and often doubly terminated. Pegmatite deposits produce our best specimens of many gemstones, such as topaz, aquamarine, quartz varieties (rock crystal, smoky, citrine, amethyst etc), tourmaline, garnet, beryl, chrysoberyl, spodumene and many other rare gemstones.
Mining of gemstone pegmatites is often done underground by small syndicates in a fresh hard rock situation, or by washing the weathered remains of an ancient deposit. Being a geologist and gemologist I have always taken the opportunity to examine any pegmatite mining operations that come my way in traveling around the world.
An amazing gemstone pegmatite is that of St Annes, near Kariba in Zimbabwe, which produces superb blue gem topaz crystals with some aquamarine and tourmaline. The pegmatite is more like a gently dipping coal seam up to 3 meters thick. Drives were made down the “seam” with tunnels off to follow any favorable crystallization. My visit was 35 years ago and I don’t know whether the present dodgy political situation is favorable to mining. Further south around Bulawayo are many pegmatites mined for feldspar and the lithium mineral spodumene, both of which are used in the ceramic industry. Close by is the famous Sandawana emerald mine.
Brazil is our greatest source of semiprecious stones which will be obvious to any tourist who has visited Rio de Janeiro and drooled over the H. Stern or Amsterdam Sauer jewelry exhibits of aquamarines, topaz , tourmaline and quartz gemstones mined in the hinterland of Minas Gerais state. I have visited the mining operations around the village of Cristalina near Brazilia. Here the lapidary families use sluicing operations to wash away the deeply weathered soils formed from pegmatites to reveal any gemstones which here are predominantly smoky quartz, citrine and amethyst. Whole families are engaged in the industry of gem cutting and will sell you beautiful faceted stones for a trifle.
At the other extreme are the large scale usually open pit mining operations of pegmatite zones for recovery of beryl, lithium minerals (spodumene & petalite), cassiterite (tin), tantalum and niobium minerals. These minerals contain the metal ions Be, Li, Cs, Sn, Ta, Nb and Rare Earths which are of an oddball size or valence and so are unable to be incorporated in the common rock forming minerals but concentrate in the residual hydrothermal solutions from which pegmatite crystallizes.
Beryllium goes to form beryl and rarely chrysoberyl. A beryl crystal measuring 18 feet long and 4 feet in diameter, weighing 18 tons is recorded from a pegamatite at Albany, Maine. Emerald is rarely found in pegmatites because they are pretty much devoid of the coloring elements chromium and vanadium. It is when beryllium-bearing pegmatite solutions intrude into more basic rocks rich in chromium do you get the happy result of emerald formation. Beryllium metal is used in special alloys and for making beryllium chemicals.
A good example of large scale mining of pegmatites are the Talison Minerals Ltd (a private equity consortium) mines for lithium, tantalite and columbite in Western Australia, at Greenbushes and Wodgina.
At Greenbushes, 250 kms south-east of Perth, spodumene and tantalite are mined from open pits in unweathered pegmatite zones. Five or ten meter benches are blasted and the broken ore is removed in 90 tonne dump trucks. The nearby processing plant uses gravity, heavy media and flotation methods to give a range of lithium concentrates with a capacity of 260,000 tpa, which are exported to China and Japan. The Greenbushes operation consists of an open pit, underground mine, primary and secondary tantalum processing plants, tin smelter and a lithium plant.
What do we want lithium for? Your watch and digital camera run on lithium batteries. The latest important use is to make lithium ion batteries used to power hybrid electric vehicles. Spodumene goes to make special ceramics. You probably have some in your kitchen if you use Corning Ware casseroles and dinnerware. Lithium chemicals are important of which lithium carbonate is the commonest. The flux-grown synthetic emeralds such as Gilson and Chatham are crystallized from a molten salt composed of lithium molybdate.
The Talison mine at Wodgina, near Port Hedland, Western Australia, is the world’s largest resource for tantalum. Open pit and underground operations produce tantalite and some beryl and cassiterite. Production capacity is 1.3 million pounds of Ta2O5 per year. The primary concentrate is shipped to Greenbushes for further processing.
Tantalum metal and oxide are extracted from the mineral tantalite, of composition (Fe, Mn)Ta2O6 ideally, but in nature there is complete “solid solution” with the mineral columbite of composition (Fe,Mn) Nb2O6. Nb is the metal niobium which was once called columbium. The mineral is a heavy, dull black, metallic ore conveniently referred to as “coltan” . Concentrates dominated by tantalum are called tantalite.
What do we want tantalum and niobium for? Refined coltan is used to make capacitors for electronic devices in computers and mobile phones. Tantalum similarly has many electronic uses as well as both metals being a component of special heat resistant alloys for jet engine and rocket components. Tantalum carbide is an abrasive used in high temperature cutting tools.
The selling price of tantalite and coltan is variable depending on the % of contained Ta and Nb oxides. Spot market concentrates sell in the range $40 to $100 per kilo. The Australian Talison mines production (2007) of tantalite was ca. 435 tonnes of an estimated world production of 985 tonnes. Other major producers were Brazil (235 tonnes), Mozambique, Canada and Ethiopia each producing 70 tonnes.
Pegmatite mining is exciting stuff. Beautiful gemstones are produced often by small syndicates using simple procedures such as fresh rock tunneling or water sluicing of weathered pegmatites. On the other hand large scale open pit mining is done by sophisticated mining companies that produce a wide range of ceramic materials such as feldspar, mica and spodumene plus the more valuable exotic mineral ores of Ta, Nb, Th, U, Cs, Ce and Rare Earth elements. Next time you use your mobile phone or travel by jet plane give thanks to all those tantalum miners!
