Galena

Galena is the most abundant lead mineral, has been mined for millennia, and remains our major source of that metal. It is a metallic, lead-gray mineral with cubic cleavage and a distinctly high density that makes galena samples feel much ‘heavier’ than expected for their size. Because it melts at a relatively low temperature and is so easily worked, lead was one of the first metals to be extensively used by many societies. Over centuries, lead use has led to the historic development of coins, bullets, batteries, televisions, and computers. Its use pervades past and present societies providing many benefits, but also raising some serious health issues.

Description and Identifying Characteristics

Its perfect cubic cleavage, metallic luster, lead-gray color, and relative softness readily distinguish galena from most other metallic minerals. Perhaps its most distinctive feature though, is its very high density (high specific gravity). Samples of galena feel significantly heavier than similar-sized samples of most minerals, including other metallic minerals. The crystal structures of galena (PbS) and halite (salt or NaCl) are identical, so it is not surprising that the two exhibit similar perfect cubic cleavage. Both minerals will break along three directions of weakness that meet at right angles. Galena’s hardness is similar to that of a fingernail, so it is easily scratched by a nail or metal implement.

The most common and widespread lead mineral, galena occurs in low-temperature to medium-temperature hydrothermal veins in igneous rocks, pegmatites, and contact-metamorphosed sedimentary rock. As a replacement mineral, galena can be found in sedimentary rocks, either disseminated throughout the rock or filling pore spaces and dissolved cavities in carbonate rocks. Often found with other metallic ore minerals, such as pyrite, chalcopyrite, sphalerite, and marcasite, galena also occurs with more common vein minerals like quartz, calcite, fluorite, chert and dolomite. In North America, some of the most economically important galena deposits occur in altered carbonate and chert rocks of the Upper Mississippi River Valley area.

The Roman naturalist Pliny may have been the first to record the use of the name ‘galena’ for lead ore. Galena (lead sulfide) is easily the most important source of lead, and often contains enough silver impurities that it is also a valuable source of that metal. Historically, its ready availability, high density, low melting point, and ease of working made lead one of the first metals to be extensively used by humans, with its known use dating back nearly 8,500 years. Lead was used as pipes in both the Egyptian and Roman worlds and for centuries has been used in glazes and pigments for ceramics and glass. In the more recent past, lead has been extensively used in the manufacture of bullets and was a common trade item on the American frontier. As odd as it may seem though, lead has never been used in the manufacture of lead pencils. The ‘lead’ in pencils is actually a mixture of graphite and clay.

Lead continues to be one of the most widespread, but least visible, metals in modern industrial societies. It is an important part of television screens and computer monitors, allowing people to safely watch the electronic pulses that form the backbone of modern communication. In the medical fields, lead also plays an important role as radiation shielding to minimize patients and staff exposure to X-rays. By far though, batteries account for most of the lead presently used in western society, comprising nearly four-fifths of the United States’ current domestic consumption.

In the past, lead was also extensively used in paints, as the solder for food cans, and as an additive to gasoline, but many of these uses have been reduced or eliminated in western society because they can lead to dangerous levels of lead poisoning. Regrettably, this has not stopped western companies from continuing to promote these practices in less wealthy nations.

The past and present widespread use of lead in western society has had some serious environmental consequences. Even ingesting small amounts of lead can cause brain damage or death, especially in very young children. Some researchers have argued that the Roman’s widespread use of lead for water pipes and eating utensils may have in part contributed to their civilization’s decline.

More recently, the pervasive use of lead in gasoline, paint, and as solder for canned food resulted in widespread lead contamination throughout the northern hemisphere. In the 1950’s Claire Patterson, a geologist interested in determining the Earth’s age, first realized the extent of lead contamination in urban areas when he tried to develop a clean environment in which to measure the very small quantities of lead isotopes present in meteorite samples. Patterson later became a leader in the movement to ban the use of lead in gasoline, paint, and food packaging. The tangible results of this movement began with the Clean Air Act of 1970, which has since significantly reduced the levels of lead contamination American and European children are exposed to. Although leaded gasoline is no longer sold in Europe and North America, it is still extensively used in other areas and poses an increasing health threat. Even within the United States, there are still many highly urban areas where residual lead contamination from past leaded gasoline use continues to pose a significant environmental risk for young children.

Rich galena deposits in southwestern Wisconsin were responsible for the first significant wave of Euro-American immigration to that area in the early 1800’s. Early on in this Lead Rush, many miners lacked the time and materials to construct houses. They found it cheaper and faster to convert abandoned mines into crude dwellings. Visitors to the region began to call these miners ‘badgers’because they lived in underground ‘dens’. Later the name was extended to cover the entire ‘Badger State’. In Minnesota, galena deposits are only found in the southeastern counties of Fillmore and Olmstead, which together with southwestern Wisconsin form the northern extent of the Upper Mississippi lead-mining district.

Usually, galena’s distinctive lead gray color, high specific gravity and cubic cleavage are enough to identify it, but at first glance there are a couple of minerals that might be mistaken for galena.

Hematite:

Hematite is a metallic mineral that can occur in many varieties. Some of the metallic varieties share a superficial appearance with galena, but the two can be distinguished by their streak and galena’s greater specific gravity. When a sample of hematite is rubbed across a rough hard surface, it leaves behind a characteristic red-brown streak, while samples of galena will leave a lead gray streak. Galena also feels much heavier than similar sized pieces of hematite.

Sphalerite:

Galena and sphalerite commonly occur together and darker varieties of sphalerite may initially be confused with galena. Pure sphalerite samples can be distinguished from galena because they are harder (3.5 to 4) and have a lower specific gravity (3.9-4.1). The color of their streak provides an even simpler way to distinguish the two. A sample of galena rubbed across a rough, hard surface leaves a lead gray streak, while sphalerite’s streak is white to cream colored.