Galena (“lead”) is a heavy, soft, somewhat brittle ore of lead. It has a
brilliant metallic luster, and silvery gray color on a freshly broken
surface. Where weathered it appears dull gray. It can be scratched with
a knife, and breaks with surfaces at 90°, forming cubes. The unbroken,
original crystal form of galena which has grown unobstructed in a vein
opening is commonly cubical in habit or a modification thereof. It
leaves a dark, lead-gray to black mark or streak when rubbed across
unglazed porcelain or chert.
[Illustration: A “cube” of galena.]
[Illustration: Cluster of galena crystals from Joplin region.]
Galena is lead sulphide, PbS, and when pure contains 86.6 per cent lead
and 13.4 per cent sulphur. Small amounts of silver may also be present.
Galena commonly occurs in Missouri as a cavity filling in crushed
limestone or chert, or as a replacement in limestone or dolomite, or in
shale, so that a large quantity of practically worthless enclosing rock
(gangue) must be taken out in order to obtain the desired galena. If a
person desires to estimate the value of his galena (lead) prospect by
having an analysis or assay made of his ore, he must include in his
sample the gangue rock that would of necessity have to be taken out when
mining his ore. Too often persons carefully select for analysis a choice
galena specimen which may run over 80% lead, only to find that as a
practical mine product it would be reduced to less than 5% lead in all
the rock which also would have to be taken out.
After a galena ore is mined it is customarily crushed and the galena
removed from the gangue by a gravity-separation process which takes
advantage of the difference in “heaviness” (specific gravity) between
galena (7.5) and limestone (calcite) or chert (2.7-2.6), or by a froth
flotation process in which the galena is preferentially wetted and
carried off by an oily froth or foam. The galena concentrate is roasted
to burn out the sulphur, reduced by carbon, and smelted to metallic
lead. The origin of some Missouri lead deposits is debatable, but the
writer believes the most reasonable explanation to be that warm,
chemically active waters arose from an igneous body below and carried to
the place of deposition the lead which they held in solution.
Missouri is one of the leading producers of lead in the world from its
Flat River, Fredericktown, Joplin, and central Missouri districts, from
which in 1941 lead concentrates having a value of over $15,000,000 were
produced.