Periodic Minerals: Galena

    Another periodic mineral post! Since the last one was realgar, an arsenic bearing mineral, I thought I'd continue the theme of toxic minerals with galena. Galena is a very dense lead sulfide mineral (the chemical formula is PbS) and is the primary ore mineral that is mined for lead. Typically associated with zinc minerals such as sphalerite (ZnS), lead has long been mined in the US since the colonial period, but today there are only ten mines in the US that currently produce lead. Five of these are in Missouri (the Brushy Creek, Buick, Fletcher, Sweetwater, and Viburnum mines), two in Alaska (the Greens Creek and Red Dog mines), two in Idaho (the Galena and Lucky Friday mines), and one in Washington (the Pend Oreille mine). Although, there has been recent discussion in late 2018 to re-open the Bunker Hill Mine in Idaho. However, the Bunker Hill Mine is next door to the second-largest EPA Superfund site which houses the Bunker Hill smelter complex and tailing piles from the old mine and I suspect there will undoubtedly be many discussions regarding the re-opening of the mine among the locals and the various levels of governments, if it hasn't already.

Huge cubic galena from the Tri-State Mining District on display at the Tri-State Mineral Museum in the Joplin Museum Complex, Joplin, Missouri. 2.5-inch long pocket knife (left) for scale.

    Galena has also been mined in the Tri-State Mining District in Oklahoma, Missouri, and Kansas, which was one of the world's largest lead mining districts during its heyday around the turn of the 20th century. However, the mines eventually closed in the mid-20th century with the last mine closing in 1967, leaving behind the unfortunate legacy of being home to a major EPA Superfund site. Left behind are artificial white limestone hills on the low landscape called 'chat piles' that are made of waste rock material containing traces of lead that remain standing today as a reminder of the memories of yesteryear and the health problems of today. Lead dust leaching into the soil and blowing into the air from these chat piles have long been detrimental to the health of the local residents. Gradual flooding of the old underground mines by groundwater is also exacerbating the problem as the water is oxidizing the sulfide ores, including pyrite, creating acidic water which is mixing into the groundwater and surface water. In Google Maps, you can find the distinctly named Tar Creek appearing as if it has toxic sludge running through it. While it may be unsafe to some degree, it is these oxidized sulfides in the creek that are causing the discoloration. The Tar Creek Superfund site, as it is known, is located in the town of Pitcher, Oklahoma, where many of the residents' homes were bought out by the federal government in recent years due to severe health and safety concerns and the town officially became a ghost town in 2009, when the city dissolved its operations. Incorporated in 1918, the town went from boom to bust within a century.

    Crystals of galena are typically formed with a cubic habit (i.e. crystal shape) and more uncommonly with an octahedral habit (like eight-sided dice for the D&D players). Both the cubic and octahedral habits are both types of the isometric crystal system (each axis is of equal length and are 90° to each other) and if you envision the cubic and octahedral habits as being two end members of a spectrum, there would be a series of intermediate habits that are a combination of the two. The third figure on this article from Mindat shows the various combinations of the cubic and octahedral habits, called cubo-octahedral (undoubtedly someone very clever must have come up with that word). And galena exists all along this spectrum in the cubic, octohedral, and cubo-octahedral habits. In addition, galena can also exhibit a dodecahedral habit, which is just a ball-like shape with pentagonal faces. This isn't exclusive to galena either. Other isometric minerals, such as pyrite and fluorite, also exhibit these crystal habits as well. Interestingly, the largest known crystal of galena is a cubo-octahedral crystal measuring 10 inches x 10 inches x 10 inches (25 cm x 25 cm x 25 cm) weighing 260 lbs (118 kg) from the Great Laxey Mine, Isle of Man, UK (see image at end of page).

A cluster of cubo-octahedral galena from the Viburnum Trend Mining District, southeast Missouri.

    The galena found in several states, including Arkansas, as well as other countries, and are broadly grouped together with other lead and zinc minerals broadly classified together as Mississippi Valley-type (MVT) ore deposits. Named after the Mississippi River Valley, MVT ore deposits are a particular type of ore deposit different from gold, uranium, and other ores. They consist of lead and zinc minerals that form on cratons (continents) within carbonate rocks. These appear to form along orogenic belts (mountain ranges) at relatively low to moderate temperatures (less than 200° C, or about 400° F). It's thought that magmatic sources involved in the orogenic process (e.g. volcanic island arcs) contributes the metal ion component, but there is some question as to where the sulfur component originates. Ultimately, the metal components appear to be transported via hydrothermal fluids getting pushed away from the compression of the orogenic belt until they mix with sulfur and  a carbonate host rock. Carbonate rocks, such as limestone and dolostone, can be great hosts for ores, as hydrothermal fluids can move through fractures and faults within the rock, as well as dissolving parts of the rock, leaving void spaces to deposit the ores. Although they are a significant source of metal, there remains a lot to understand about MVT ores, such as galena, and how they form.

Largest known galena crystals from the Great Laxey Mine, Isle of Man, UK (year found/photographed unknown). I have not found another image of this sample. (credit: Rickwood, 1981, figure 3; GeologyIn)