Mineral of the Month

Mineral of the Month - Galena

Posted by Site Moderator Webmaster on Monday, April 25, 2011 Under: Mineral of the Month

Crystal system: Isometric Hardness: 2.5 – 3.0

Density: 7.5 average Streak: Lead-grey

Colour: Silver lead-grey

Cleavage: Perfect, parallel to the faces of the cubic crystals

Occurrence: In vein deposits and irregular pockets which, although present in many types of rock, occur often in limestones and dolomites which have been intruded by igneous rocks.

Habitat: Crystals usually isometric (cubic), octahedral, or cubo-octahedral, also tabular, skeletal, reticulated. Spinel twinning is also possible forming flattened crystals.

Composition: Sulfide PbS

The Roman naturalist, Pliny, used the Latin word galena to describe lead ore. Galena is the primary ore mineral of lead and it was mined for its lead content as early as 3000 BC. Galena is one of the most abundant and widely distributed sulfide minerals. It is often associated with the minerals dolomite, pyrite, sphalerite, calcite, quarts, gold and fluorite. Galena deposits often contain significant amounts of silver as included silver sulfide mineral phases or as limited solid solution within the galena structure. These argentiferous galenas have long been the most important ore of silver in mining. In addition zinc, cadmium, antimony, arsenic and bismuth also occur in variable amounts in lead ores. Within the weathering or oxidation zone galena alters to anglesite (lead sulfate) or cerussite (lead carbonate).

One of the earliest uses of galena was kohl, which in ancient Egypt, was applied around the eyes to reduce the glare of the desert sun and to repel flies, which were a potential source of disease. Galena is a semiconductor with a small band gap of about 0.4 eV, which found use in early wireless communication systems. For example, it was used as the crystal in crystal radio sets, in which it was used as a point-contact diode to detect the radio signals. The galena crystals were used with a safety pin or similar sharp wire, which was known as a “cat’s whisker”. Making such wireless sets was a popular home hobby in Britain and other European countries during the 1930s. Derbyshire was one of the main areas where galena was mined. Lead is also used as a petrol additive but the atmospheric pollution it caused has led to a shift to unleaded petrol. Lead is also used to make ammunition, brass and bronze alloys, and as an additive in pesticides.

Notable occurrences include USA, Germany, Peru, Mexico, Zambia, Bulgaria, Australia, Israel, Russia and England. Galena was of such economic importance to the early history of the Driftless area in Illinois, USA that one of the towns in the region was named Galena. Galena is the official state mineral of the states of Missouri and Wisconsin. The largest single documented crystal of galena, presumably from the USA, measured 25 cm x 25 cm x 25 cm. Can you believe it is not from South-Africa or Namibia?

In South Africa the most significant deposit is at the Pering Mine in North West Province, which ceased operations in 2003. In the Limpopo Province, the Leeuwbosch lead deposit, 16 km north of Thabazimbi, produced good galena specimens. Galena also occurred at the Bokkraal and Doornhoek lead mines near Zeerust. At the Nababeep West Mine at Nababeep, Northern Cape, rare, euhedral octahedral crystals 3 cm on edge are associated with chalcopyrite. In the Witwatersrand gold mines, secondary cubic galena crystals up to 5 cm on edge have been found in quartz veins. At the Broken Hill Mine at Aggeneys, large subhedral and euhedral galena crystals are found, up to 15 cm on edge. What is probably known as the oldest occurrence of lead in South Africa is situated a few miles upstream from the mouth of the Maitland River, due south of Uitenhage. Galena is rare at the N’Chwaning II mine in the Kalahari Manganese Fields where small, very attractive cubes up to 5 mm on edge were found with calcite in 1991.

Namibia has very small lead-zinc deposits that contain galena. At the Ai-Ais Mine in southern Namibia, veins of solid galena up to 30 cm thick were found. Good quality crystals have also been collected at the Rosh Pinah Mine as well as the Namib Lead Mine. Abaneb West had significant quantities of galena. Galena was one of the main ore minerals at the Tsumeb mine. Although not plentiful, crystals of galena were found at this mine, many with characteristic criss-cross etch marks (striated crystal faces) and some with peculiar flattened habit.

Galena specimens may tarnish when exposed to air, becoming dull in luster. The tarnish can be removed by scrubbing the specimen with water and mild soap. Galena specimens must be taken care of more than most other specimens. They are easily damaged and crystals may shatter into small fragments if put under slight pressure or dropped, so care should be taken when handling and transporting specimens - JDJ.