Galena - the Lead glance
Galena, historically also named lead glance, is among the most abundant sulfides. Nice crystals and clusters of galena are almost every mineral collection. But this common mineral has some interesting features and it source for many beautiful secondary minerals.
Structure of Galena
The chemical formula of galena is PbS (lead sulfide). The galena structure can also accomodate small amounts of other metals such as copper, arsenic, bismuth, antimony, and most importantly, silver. Structure of galena is capable to accommodate up to 20 % of silver instead of lead. Galena crystallizes into cubic forms often, though it can take on octahedral or tabular shapes as well. Combinations of cubes and octahedrons are common, some of these are labeled as galena variety steinmannite. Lead and sulfur atoms form cubic units similar to the halite structure.
Physical Properties of Galena
Galena is a relatively soft mineral, only registering around a 2.5 on the Moh's hardness scale. Fresh galena has shiny silver color but it tarnishes quickly, resulting in dull gray. The galena streak is dull gray as well. The luster of galena can be easily described as metallic, though this is more obvious on cleavage planes than anywhere else on the crystal. Galena displays perfect cleavage and breaks easily into tiny cubes. The luster of galena may fade over time as it tarnishes when exposed to air, however the tarnish may be removed by simply cleaning with soap and water. The specific gravity of galena is about 7.5 g/cm3. Galena crystals are natural semiconductors and were the first material used in diodes. Though the mineral contains lead, it is generally safe to handle.
Occurrence and Uses
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 fire, and collecting the cooled lead from beneath the ashes once the fire was out. Lead was used to create Roman plumbing networks, where the metal was known as plumbum, giving us the atomic symbol of Pb for lead.
Modern uses for galena include the use of galena crystals in earlier radio devices. The most frequent use for galena is as a lead ore, and the majority of lead is currently used to produce lead-acid batteries, a necessary component of automobiles. Galena is also very important silver ore.
Deposits of galena are found in mid to lower temperature hydrothermal veins associated with igneous and metamorphic rocks. These veins may reach far into neighbouring sedimentary layers. These veins are also know 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.
Near-surface weathered parts of ore deposits (called gossan or iron cap) often contain significant amount of limonite with secondary minerals: the main lead bearing being cerussite, anglesite, pyromorphite, wulfenite and less common like mimetite, vanadinite or crocoite. Secondary galena can form also in the sediments with low oxygen content (reducing environment) or on the burning coal dumps.
Though galena is the primary ore of lead, argentiferous (silver-bearing) galena deposits are much more important economically. Though silver usually makes up <1 % of galena deposits, many mines generate more revenue through silver production than lead. Silver-bearing galena can be identified through a change in cleavage for the mineral; the addition of silver to the galena crystal structure causes the mineral to form curved cleavage faces as opposed to the perfect cubic faces it is known for.
European localities include classic sites like Alston Moore and Weardale in England, Freiberg and Neudorf in Germany, Příbram and Stříbro in Czech Republic, Banská Štiavnica in Slovakia, Madan in Bulgaria, Herja and Turt in Romania, Trepča in Kosovo or ancient mines in Sardinia. Excellent banded aggregates of galena, sphalerite and marcasite (sometimes called schalenblende) occur in Olkusz in Poland, Segen Gottes Mine in Germany, and Schmalgraf Mine in Belgium.
Some of the most important galena deposits in North America are found in altered carbonate and cherts of the tri-state mining district of Upper Mississippi River Valley - the so called Lead Belt. Famous sites include Joplin and Reynolds in Missouri, Treece and Baxter Springs in Kansas and Pitcher in Oklahoma. There even exist towns called Galena in Missouri and Kansas. Other sites include Leadville, Empire and Silverton in Colorado, big lead mine is in Shullsburg, Wisconsin.
Other interesting localities include Sullivan mine in British Colombia, Broken Hill in Australia or Dalnegorsk in Russia. Many hydrothermal deposits are mined in Peru, Bolivia and Argentina. Tsumeb in Namibia and Oujda and Mibladen in Morocco are galena deposits with world class secondary minerals like cerussite, anglesite and many others.