GALENA, lead sulphide (PbS), the chief ore of lead. The mineral was mentioned by Pliny under this name, and it is some times now known as lead-glance (Ger. Bleiglanz). It crystallizes in the cubic system, and well-developed crystals are of common occurrence; the usual form is the cube or the cubo-octahedron. An important character, and one by which the mineral may always be recognized, is the perfect cubical cleavage, on which the lustre is brilliant and metallic. The colour of the mineral and of its streak is lead-grey; it is opaque; the hardness is and the specific gravity 7.5. 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 numer ous bright cleavages.
The formula PbS corresponds with lead 86.6 and sulphur The mineral nearly always contains a small amount of silver, and sometimes antimony, arsenic, copper, gold, selenium, etc. Argentiferous galena is an important source of silver; this is present in amounts rarely exceeding o•1 %, equal to about 3o oz. per ton. Since argentite 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 metal liferous 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 pearispar 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 formation replacing organic matter, such as wood; and it is sometimes found in beds of coal. In the lead-mining districts of Derbyshire and the north. of England the ore occurs as veins and flats in the Carboniferous Limestone series, whilst in Corn wall the veins traverse clay-slates. In the Upper Mississippi lead region of Missouri, Illinois, Iowa and Wisconsin the ore fills large cavities or chambers in limestone.
Galena is met with at all places where lead is mined; of locali ties which have yielded finely crystallized specimens the follow ing may be selected for mention: Derbyshire, Alston in Cumber land, Laxey in the Isle of Man (where crystals measuring almost a foot across have been found), Neudorf in the Harz, Rossie, N.Y., and Joplin, Missouri. Good crystals have also been obtained as a furnace product. (L. J. S.)