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Encyclopedia Britannica - Main :: GAG-GEO |
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GALENA , an important ore of lead, consisting of lead sulphide (PbS). The mineral
octahedron
mineral
presence of polysynthetic twinning is sometimes shown by fine striations running diagonally or obliquely across the cleavage surfaces. Large masses with a coarse or fine granular structure are of common occurrence; the fractured surfaces of such masses present a spangled appearance owing to the numerous bright cleavages.The formula PbS corresponds with lead 86.6 and sulphur 13.4%. The mineral nearly always contains a small amount of silver, and sometimes antimony, arsenic, copper, gold, selenium, &c. Argentiferous galena is an important source of silver; this metal is present in amounts rarely exceeding 1 %, and often less than 0.03 % (equivalent to ro4 ounces per ton). Since argentite (Ag2S) is isomorphous with galena, it is probable that the silver isomorphously replaces lead, but it is to be noted that native silver has been detected as an enclosure in galena. Galena is of wide distribution, and occurs usually in metalliferous veins traversing crystalline rocks, clay-slates and lime-stones, and also as pockets in limestones. It is often associated with blende and pyrites, and with calcite, fluorspar, quartz, barytes, chalybite and pearlspar as gangue minerals; in the upper oxidized parts of the deposits, cerussite and anglesite occur as alteration products. The mineral has occasionally been observed as a recent
series , whilst in Cornwall
Galena is met with at all places where lead is mined; of localities which have yielded finely crystallized specimens the following may be selected for mention: Derbyshire, Alston in Cumberland, Laxey in the Isle of Man (where crystals measuring almost a foot across have been found), Neudorf in the Harz, Rossie in New York
Coarsely grained galena is used for glazing pottery, and is then known as " potters' ore " or alquifoux. The galena group includes several other cubic minerals, such as argentite (q.v.). Mention may also be made here of clausthalite (lead selenide, PbSe) and altaite (lead telluride, PbTe), which, with their lead-grey colour and perfect cubic cleavage, closely resemble galena in appearance; these species are named after the localities at which they were originally found, namely, Klausthal in the Harz and the Altai mountains in Asiatic Russia. Altaite is of interest
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