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Lead oxide constitutes a family of compounds with varying numbers of oxygen atoms per molecule, depending on the valence state (+2 or +4) of the lead, and varying crystal morphs, even with the same chemical formulas. These compounds are familiar to geologists as ores and crystal minerals, to inorganic chemists, and to manufacturers as pigments.
Massicot, an orthorhombic crystal of lead monoxide (PbO), and the mineral litharge, the tetragonal morph, are found on the edges of lead ore bodies, and to a petrologist, are very indicative of the degree of oxidation. A minor ore of lead, massicot is used as a pigment and is collected by rock hounds as a mineral specimen. Litharge figures prominently in the manufacturing of ceramic products, including capacitors, Vidicon tubes, and electrophotographic plates. Lead from litharge supplies the lead salts used as stabilizers for polyvinyl chloride and other plastics, soaps (lead stearate), oil refining, and driers for paints. Lead monoxide, also known as lead protoxide, has a molecular weight of 233.19, a boiling point of 1,472 degrees Celsius, and a melting point of 888 degrees Celsius. Adding carbon to lead monoxide will yield elemental lead and carbon monoxide.
Another lead oxide mineral, minium, is also known as "red lead" and is an oxidation product of other lead minerals. Minium is more widely used as a pigment in paints and dyes and less important to rock collectors. Commercially produced minium has become less popular in the dye and paint industries since the health concerns for lead has reduced consumer demands, but it has played a stellar role in ferrous metal protective paints. To the novice chemist or geologist, it might appear that lead takes on a third valance state based on the chemical formula for red lead. However, these compounds are actually composed of lead having the two valence states (Pb+2 and Pb+4), and are typical members of the spinel group of oxide minerals. Known as lead tetraoxide, minium red lead, or Saturn red, minium has a molecular weight of 695.60, appears as a heavy, odorless red powder, and features a melting point of about 500 degrees Celsius.
Crystals of minium are red, ranging from scarlet to brick red, tetragonal, opaque, and generally well striated. Under ultraviolet light (UV), some specimens of this variety of lead oxide fluoresce an orange color. In contrast, massicot crystals are yellow to reddish yellow, opaque, and orthorhombic. Both feature specific gravities well above average for any mineral (8.9-9.2 for minium; 9.6-9.7 for massicot). As with other lead compounds, lead monoxide and lead tetraoxide are considered toxic if ingested or released into the environment.
A mixture of 70 percent lead monoxide and 30 percent metallic lead, sometimes referred to as black or gray oxide, is used to manufacture lead acid batteries. Because of the size of the lead acid battery industry due to the large number of automobiles and airplanes worldwide, lead monoxide is the most important commercial compound of lead, based on volume. Annual lead oxide production exceeds 64,000 tons.
Lead dioxide, known also as lead peroxide, lead brown, lead oxide brown, and lead (IV) oxide (PbO(2)), is a very strong oxidizer and may react violently with reducing agents. It has a molecular weight of 239.20 and is a known carcinogen. As a solid, lead dioxide forms a dark brown powder that is odorless and insoluble in water, with a melting point of about 290 degrees Celsius. Lead (IV) oxide is used as an industrial oxidizing agent, in storage batteries, and in the textile industry.
Omar Alvarado, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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