Native lead is known, but
is of exceedingly rare occurrence. Lead is a bluish-grey metal, whose
freshly cut surface shows a bright metallic luster which, however, quickly
oxidizes on exposure to the air. It is soft, may be scratched with the
finger-nail, and makes a black streak on paper. The specific gravity of the
metal is 11.40. It fuses at 325 C., and crystallizes when cooled slowly. It
has little tenacity, and cannot be drawn into wire, but is, however, readily
rolled or pressed into thin sheets, or exuded when in a semi-molten state
through dies to form piping. Its properties are materially affected by the
presence of small quantities of impurities. Lead is very easily reduced from
its compounds, and is readily soluble in dilute nitric acid; hydrochloric
and sulphuric acids have but little action. It forms several compounds of
commercial importance; for example, litharge and red lead are oxides, white
lead is a basic carbonate. Metallic lead is used in the form of sheet, pipe,
etc. It is used to make weights, bullets and shot. It is a constituent of
various alloys such as solder (lead and tin), type metal (lead and
antimony), and low fusing alloys (lead, bismuth and tin). A small amount of
lead is used in the form of the basic carbonate, which is known as white
lead, and is very valuable as a paint pigment. The oxides of lead, litharge,
and minium, are used in making fine grades of glass, in glazing earthenware
and as pigments. Lead chromates are used as yellow and red paints. Lead
acetate, known as sugar of lead, has important uses in various industries.
For the production of lead, the ore containing
galena or
cerussite is first partly roasted or calcined, and then
smelted in reverberatory or blast furnaces. Most lead ores contain silver,
and this metal is obtained, by cupellation or other means, from the lead.
Sphalerite is frequently associated with galena, and as the
presence of zinc causes difficulties in smelting, lead ores containing more
than 10 per cent, of zinc are heavily penalized by the smelters. Mechanical
separation (dressing) of the two minerals is largely resorted to, but
sometimes with considerable losses of both lead and zinc. When antimony is
associated with galena, the ore is frequently smelted direct for the
production of antimonial lead.
The chief ore of lead is galena; deposits of galena oxidize in their upper parts into numerous oxy-salts. The chief minerals of lead are therefore Galena, Cerussite, Anglesite, Pyromorphite, Mimetite, Vanadinite, Crocoite and wulfenite. Lead compounds color the blowpipe flame a pale sky blue, a poor color, and of little value. When lead minerals are heated alone on charcoal, they give a sulphur yellow encrustation; when heated with potassium iodide and sulphur, they give a brilliant yellow encrustation (a good test). Roasted with sodium carbonate and charcoal on charcoal, lead minerals are reduced to metallic lead, which shows as a lead grey bead, bright while hot, but dull when cold, which is malleable, and marks paper.
Galena is the usual primary ore of lead and furnishes by far the greater part of the metal. Cerussite and anglesite are secondary minerals which occur in smaller amounts in the oxidized zone of lead deposits. Galena occurs most commonly associated with zinc ores, especially sphalerite, or in connection with silver ores. Lead, which is derived from ores that are free from silver, is known as "soft lead," while "desilverized" lead, which is obtained from silver ores, is known as "hard lead." Lead ores are most commonly found as replacement deposits in limestone rocks, either in the form of beds or irregular bodies, or as small masses disseminated through a stratum of the rock.
The most, important of these lead-ore minerals is galena, which is often argentiferous. It is invariably as galena that lead ore occurs in deep-seated veins which have not suffered oxidation. Where the veins have been oxidized and weathered, cerussite and anglesite arise as alteration products, cerussite being more common. Anglesite is unstable compared with cerussite, and changes into the latter when acted upon by carbonated waters. Where the solutions in the oxidation zone traverse phosphatic rocks, pyromorphite develops. Where the galena is argentiferous, silver minerals arise as the result of oxidation, and some of the most important silver ores in the world have been formed in this way. Numerous minerals occur in association with galena in veins of lead ore, one of the commonest of these being sphalerite. The more important of the other associated minerals are calcite, dolomite, siderite, pyrite, chalcopyrite, barite, and fluorite.
The deposits of lead are treated in three different classes, according as they produce or have produced lead alone, lead and zinc, or lead and silver. Of late years the lead silver ores have been the greatest source of the metal. Only the southeast Missouri region is of much importance among the others, although considerable lead is also obtained in association with zinc. There are two general methods of extracting silver from its ores, the one indirectly, by smelting with and for lead; the other by amalgamation, cyanide, or some such process. Hence under silver there are two classes of mines lead-silver and high-grade silver ores. Both have almost always varying amounts of gold. The lead -silver mines furnish also, as noted above, by far the greater portion of the lead produced in the United States. Ores adapted to lead-silver metallurgical treatment form, in general, the oxidized alteration products of the upper parts (above permanent water level) of deposits of galena and pyrites. They may be well-marked fissure veins, chimneys, chambers, or contact deposits. Ores which of themselves are adapted to other processes are often worked in with the lead ores, and unchanged sulfides are artificially oxidized by roasting preparatory to smelting.
Galena occurs in both beds and vein deposits. Metasomatic lead and zinc veins occur in contact metamorphic deposits in limestone, as at Derbyshire, Flintshire, Cumberland in England, at Sala (Sweden), Raibl, and Bleiberg (Carinthia), Leadville (Colorado), Utah, Wisconsin, etc. Hydrothermal primary veins are another important mode of occurrence of galena; it is there associated with sphalerite, pyrites, quartz, and barite, as at Cardigan, Minera, Isle of Man, Cornwall, Derbyshire, Aspen and Rico (Colorado), Broken Hill (N.S.W.), where the galena is associated with silver, and at Freiberg (Saxony). About 90 percent of the world's output of lead ore was once produced by the United States of America. Much smaller amounts have been produced recently. The countries of Spain, Australia, Germany and Poland, and Mexico have also produced significant amounts of lead. Smaller amounts have been produced by Great Britain, France, Canada, Greece, Italy, and Austria.
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Precious and Base Metal Ores
This is Cerussite, an ore of lead found in the zone of surface oxidation. It
a lead carbonate, and
is often formed from the oxidation of Galena.
Galena - lead sulfide, the most common ore of lead. When well formed, the crystals are cubic. This specimen is from Inyo County, California. Galena normally contains a small amount of silver.
