Galena – Mineral Properties, Photos And Occurrence

Galena, historically also named lead glance, is among the most abundant sulfides. Besides forming aesthetic mineral specimens, galena is the primary lead ore. And galena itself was a first semiconductor ever used.

Crystal Structure of Galena

The chemical formula of galena is PbS (lead sulfide). It belongs to the isometric crystal system, most common crystal shapes are cubes, octahedrons, and their combinations. The dodecahedron crystal shapes are much less common, but over 40 galena crystal shapes are known. Twinning is common on {111} (spinel law twins). Skeletal, hopper or etched crystals are quite rare. Galena also forms massive, grainy, fibrous or platy aggregates.

Various crystal shapes and combinations of galena crystals. Drawings by Crystal Shapes.

Lead and sulfur atoms in galena crystal lattice form cubic units similar to the halite structure. The galena structure can accommodate small amounts of other metals such as copper, arsenic, bismuth, antimony, selenium, and most importantly, silver. Galena can contain as much as 20 wt.% of silver, but content from 0.1 wt.% to 1 wt.% is most common in so called argentiferous galena. Galena crystals are natural semiconductors and were the first material used in diodes.

Galena belongs to the sulfide class, more precisely its octahedral sulfide. Galena is the only common member of the Galena group, other minerals in this group are uncommon or rare. Galena makes a solid solution with clausthalite (PbSe), which is also member of galena group.

Though the mineral contains lead and regularly hits viral nonsense most toxic minerals lists, it is generally safe to handle. The lead sulfide is highly insoluble and does not react with skin. We cannot recommend licking or eating though, and heating galena is a very stupid idea because of the production of toxic fumes. Also avoid inhaling galena dust.

Physical Properties of Galena

Galena is a very soft mineral, registering only 2.5 on the Moh's hardness scale. And even worse, galena has perfect cleavage and brittle tenacity. This means that specimens are very easily damaged by just a tiny chip or bruise. When dropped or smashed, galena easily splits into tiny cubes. The fracture is subconchoidal, but very difficult to observe because of the perfect cleavage.

Skeletal and etched galena crystal from Dalnegorsk, Russia. Size: 10 x 8 cm. Oliver Konczner collection, Albert Russ photo.

Fresh galena has shiny silver color, but it tarnishes quickly, turning lead grey and later dull grey. The galena streak is lead grey. The luster of fresh galena is brightly metallic, turning less bright to dull with tarnish.

The tarnish may be removed by simply cleaning with soap and water. However, the very low mechanical resistance and softness of galena make this always a risky operation.

The specific gravity of galena is about 7.6 g/cm3.

Origin

Deposits of galena are found in mid to lower temperature hydrothermal veins associated with igneous and metamorphic rocks. These veins may reach far into neighboring sedimentary layers. These veins are also known as polymetallic deposits because they usually contain also sphalerite (zinc ore), chalcopyrite (copper ore), pyrite, arsenopyrite and pyrrhotite.

Common associated gangue minerals include quartz, calcite, dolomite or fluorite.

Cluster of nice cubic galena crystals with pale baryte from Bolivia. Size: 12 x 10 cm. Alfred Schreilechner collection, Albert Russ photo.

Near-surface weathered parts of ore deposits (called gossan or iron cap) often contain significant amount of limonite with secondary minerals. Most of the lead bearing minerals originate by decomposition of galena, the most common being: cerussite, anglesite, pyromorphite, wulfenite, and less common like mimetite, vanadinite or crocoite.

Secondary galena can form in sediments with low oxygen content (reducing environment). Galena can also precipitate from hot fumes on the burning coal dumps or volcanic vents (fumaroles). Galena can form in some less common Pb-rich skarns or very rarely in pegmatites.

Industrial Applications

Galena is the primary ore of lead and may be found in a large variety of locations. Important even to early civilizations due to its relatively low melting point, it continues to be an often-mined mineral across the world.

Smelting galena can be as simple as throwing the mineral into a wood fire and collecting the cooled lead from beneath the ashes. Lead was used to create Roman plumbing networks, where the metal was known under its Latin name as plumbum, giving us the atomic symbol of Pb for lead.

Crystals of galena from classic hydrothermal deposit Banská Štiavnica in Slovakia, size 1.3 x 1 cm. Photo: Vítězslav Snášel

Though galena is the primary ore of lead, argentiferous (silver-bearing) galena is an important silver ore. Deposits of Ag-rich galena are much more important economically. Though silver usually makes up