Mineral Galena

Posted: October 17th, 2013

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Mineral Galena

Introduction

Mineral Galena is the natural form of Lead (ii) sulfide and for this reason; it is considered the most important source of lead ore. It is also called galenite and lead glance. Galena is categorized as the most abundant sulfide mineral, and this is due to its widespread availability in several parts of the world. The ore where it is obtained from also contain other minerals like sphalarite, chalcopyrite and tetrahydrite. Galena has been well known over the centuries as the source of lead. Even as early as 3000B.C, it was known for its content of lead. The crystallization process of this mineral is unique as it results in the formation of several intriguing and distinct crystalline shapes. Galena is an important mineral with several uses in various sectors and industries worldwide.

Galena is classified as a primary mineral. A primary mineral is a mineral that is formed because of a combination of elements, as opposed to the more traditional method of the alteration of the individual components of the mineral in question. Since it is a primary mineral, several lead minerals, which are secondary minerals, are derived form it. Examples of the lead minerals obtained from galena include anglesite and cerrusite. As with most minerals, galena also contains some impurities like bismuth and silver. These impurities affect the structure of the mineral particularly the cleavage properties that it has. When galena is exposed to the sunlight, it is tarnished, and for this reason, it requires a lot more care than other minerals. This is because if it is tarnished the physical and chemical properties are also affected (Tarbuck and Lutgens, 78).

Tarnishing may be removed temporarily from galena if the ore is cleaned using water and mild soap. Based on the level of impurity in the mineral ore, two main varieties of galena are obtained. The varieties are acerilla, which is common in South America. This variety of galena is often granular. The other variety of galena is argentiferous galena. From its name, the ore in this case, has a lot of silver. This is because the Latin name for silver is ‘argentum’ and the name argentiferous is derived from the Latin name for silver. It is believed that galena, which is irregular and slightly tarnished, has a higher silver content as opposed to the plain regular ores. As mentioned before, it is one of the most common minerals in the world because it is situated in several areas. Notable amounts of the mineral are found in England, Germany, Mexico and the United States. Mining of the mineral takes place in the said locations.

This is an illustration of galena:

Mineral Crystallography

Crystallography refers to the study of the crystals a mineral has. A crystal is a polyhedral form, which is always regularly shaped. Chemically, a crystal is defined as a compound, which is in solid state because of the action of the inter-atomic forces, which hold it together. Six crystal systems exist, and they are based on basic geometry. For one to understand the crystallography of a mineral, it is imperative that they identify the crystallographic axes of the mineral in question. One of the crystal systems is the isometric or cubic system. In this system, the crystallographic axes are of the same lengths. In addition, three of the axes intersect with each other at an angle of 90 degrees. The isometric crystal system has the highest level of symmetry when compared to the other five crystal systems. Mineral galena occurs in the cubic crystal system, but may at times depict some octahedral forms. In rare occurrences, a combination of the cubic and octahedral forms may take place. The least common form of crystals of mineral galena is the dodecahedron. During the formation of these crystals of galena, several things may occur that affect the formation of a particular crystal shape. For instance, in order to for the octahedral crystals, some of the cubic crystals’ edges are cut off by the partial octahedral growth that takes place. Flattening of the octahedral crystals due to the cubic growth of some crystal also occurs. Simultaneous occurrence of the named processes leads to the formation of a crystal that is a mixture of the octahedral and cubic crystals (Pohl, 90). The following illustrations show the different crystal forms of galena that exist:

a)  An illustration of a cubic crystal that is modified with octahedral faces.

b) An illustration of an octahedral with no modification

c) An illustration of a cube that is unmodified

d) An illustration of a crystal with a mixture of the cubic, octahedral and dodecahedral crystal systems

Geological Significance

Galena is the most significant source of lead in the world. This is because; the ore of galena is composed of lead (ii) sulfide. Apart from the lead (ii) sulfide, galena ores also have other components. One such component is silver sulfide. The argentiferous variety of galena is rich with this component. It is therefore possible for silver to be acquired from the ores of galena. Galena occurs with several impurities like bismuth, zinc, antimony, cadmium and arsenic. Through special procedures, these impurities like zinc and arsenic may be obtained from the lead (ii) sulfide ore. The main geological significance of the mineral is the production of lead from the ore.

General uses of the mineral

Galena is used in the manufacture of several things. Galena was used in ancient Egypt to produce kohl. It was applied protectively around the eyes to reduce the sun’s glare in the desert areas, and to ward off flies, which were considered the cause of most of the diseases experienced then. Apart from this, galena is also the main source of lead, which is utilized in the manufacture of lead-acid batteries. Lead sheets are also made from the lead obtained from galena. Galena is a potent semiconductor, and for this reason, it was used in wireless communication initially. The bandgap of the mineral is about 0.4eV, which made it appropriate for the communication systems. For instance, galena was used in the manufacture of radio sets. Galena crystals were used in these devices as a point-contact diode whose function was the detection of the radio signals. This crystal was shapes as a sharp wire that was placed inside the radio set. The effectiveness and clarity of the radio made it one of the most popular brands in Britain in the late 1930’s. This effectiveness of the radio was due to the use of galena crystals, which were an effective conductor, and this enabled it to detect the radio signals appropriately. However, the modern wireless communication systems do not use galena any more. Instead, they have replaced the mineral with more reliable semiconductors with higher bandgap than that of galena, which is 0.4 eV. Since silver is also found in some of the galena ores, it may be acquired from galena. Silver has several uses such as the manufacture of jewellery, which is its most common function. Galena is used in the manufacture of alloys with low melting points. It is also used in the manufacture of solders though this use has declined considerably.

Conditions of formation of the mineral

Formation of a mineral is referred to as mineralization. It requires specific conditions of temperature and pressure. Alterations in the conditions ill result, in the formation of a different mineral altogether. Mineralization of galena takes place in several paragenetical stages in which layers of the mineral are formed gradually until they become an ore. Impurities can be found in the ore because of the mineralization process. During the formation of the layers, it is easy for other compounds to get into the forming ore thus, they are incorporated into the ore. Obtaining of the exact conditions required for the formation of any mineral is extremely difficult, and the values can only be estimated after research has been conducted on the mineral. In the case of galena, it was discovered that the mineral is made at a temperature of 292 degrees Celsius and at a pressure of 2.2 kbar (Piestrzynski, 250). The mineral is located mostly in the low temperate and medium temperature regions. Ores of galena are found in sedimentary and ingenious rocks, but they may be sometimes found in sediments. Galena is located in these regions because they are prone to open cavities. These cavities facilitate the proper formation of the crystals of galena.

Minerals associated with galena

Galena is associated with several other elements mainly because of their characteristics, but there are some distinguishing characteristics, which set them apart. Such minerals include the likes of tetrahydrite, jamesonite, fluorite, stibnite, chalcocite, calcite, acanthite and sphalarite. The distinguishing characteristic between galena and sphalarite is that the latter is lighter. Additionally, the latter does not have cubic cleavage and has a different streak while compared to galena. Acanthite differs from galena since it is softer and more sectile. Stibnite does not have fine cleavage and its crystals are of a different form not the isometric form of galena. Jamesonite crystals do not have fine cleavage and are lighter than galena crystals. In addition to this, the jamesonite crystals have a different crystal system. Chalcocite is not from the cubic crystal system like galena and the chalcocite crystals are dark colored and lighter than the galena crystals. Tetrahydrite crystals are harder, lighter and darker than their counterparts, the galena crystals, are. Sphalerite, calcite and fluorite crystals belong to different crystal systems and are of a different streak, as well (Guastoni and Appiani, 179).

Chemical composition of the mineral

Galena is the common name for lead (ii) sulfide. The chemical formula for the element is PbS whereby Pb is the chemical symbol for lead while S represents sulfur. The ore has been noted to have some impurities within it. Such impurities include the likes of arsenic (As), silver (Ag), bismuth (Bi), copper (Cu), and iron (Fe). Based on the impurities that the ore has, the chemical formula will vary. The variable formula that will arise will be (Pb Ag) S in the case of an ore containing silver as an impurity. Chemical composition of the two main components of the mineral is 13.4 % of sulfur and 86.6% lead (Bonewitz, 138). Crystallography is also part of the chemical composition of the mineral. Galena belongs to the cubic or isometric crystal system.

Physical properties of the mineral

Physical characteristic refer to the properties of the mineral that are determined experimentally or that one can see. Galena is a mineral that occurs in crystals, which are often dull or silver gray. It has a metallic luster and a gray streak, which may at times have a bluish tint. The cleavage of the mineral is perfect since it belongs to the isometric crystal system, which has axes of the same length. Crystals of galena are opaque and cannot allow any light to pass through them. The hardness of the mineral ranges between 2-2.5 in accordance to the Mohs scale, but according to the Vickers scale, it is between74 -104 kg/mm². Galena has a density of 7.58 g/cm³ when measured and 7.57g/cm³when calculated. The crystals are very brittle and easily destroyed if subjected to sudden pressure. The specific gravity of the mineral is 7.4-7.6.

The stability of the mineral

The mineral is stable under normal conditions of room temperature and pressure. However, if the mineral is exposed to a sudden change in these conditions, it becomes unstable. If galena is exposed to temperatures above 300 degrees Celsius, the mineral is expected to undergo re-crystallization. This is because the temperature is higher than the one required for its formation. If the temperatures are very high, precipitation of some of the impurities present in the ore may take place. Changes in pressure also have an effect on the crystals but the effects are not as evident as the ones caused by a change in temperature. However, it should be noted that galena is more sensitive to changes in pressure such that a change in about five atmospheres of pressure is likely to affect the chemical composition of the galena crystals (Lutgens and Tarbuck, 102).

Environmental hazards

Mining of the ore from the rocks causes environmental degradation. This is because mining leaves large bare holes that make the general out look of the area is displeasing. Wastes from these sites if not disposed off properly may affect the environment. This is because the chemicals used in the mining process may be toxic to the soil for example subsequently, these toxins encounter human beings and they may result in health complications. Since the ore is mostly comprised of lead, people who handle it should be very careful. Lead is a heavy metal, which causes several health risks the most common of them being lead poisoning, which occurs due to continued exposure to lead. People handling galena are required to avoid inhaling the dust or consuming it in any way, as it is dangerous to the body. Due to the existence of the large bare holes, the area is prone to soil erosion, which is part of environmental degradation. Mining causes pollution of the environment. Pollution in this case includes noise pollution due to the types of machines used. The mining process also results in the production of dust, which is another environmental effect. Mining of galena and other minerals, therefore, has significant effects on the environment.

Works Cited

Bonewitz, Ra. Rocks and Minerals. London: DK Publishing, 2012. Print.

Guastoni, Alessandro, and Appiani, Roberto Minerals. Richmond Hill, Ont: Firefly Books, 2005. Print.

Lutgens, Frederick K, and Tarbuck. J. Edward Essentials of Geology. Upper Saddle River, N.J: Pearson Prentice Hall, 2009. Print.

Piestrzyński, Adam. Mineral Deposits at the Beginning of the 21st Century: Proc. of the Joint Sixth Biennial SGA Meeting /Krakow/Poland. Taylor & Francis, 2001.Print.

Pohl, Walter. Economic Geology: Principles and Practice : Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits. Chichester, West Sussex: Wiley-Blackwell, 2011. Print.

Tarbuck, Edward J, and Lutgens K. Frederick, Earth: An Introduction to Physical Geology. Upper Saddle River, N.J: Pearson Prentice Hall, 2011. Print.