Galena-calcite from Peru. (Carolyn Manchester collection)
<p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>The sulfide minerals contain one or more sulfide anions (S-2).  The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals.  Many sulfides are economically significant, as they occur commonly in ores.  The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc.  Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size.  These minerals will not form in the presence of free oxygen.  Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.
</p><p>Galena is a lead sulfide mineral (PbS).  It has a metallic luster, silvery-gray color, cubic crystals, cubic cleavage, is moderately soft, and has a very high specific gravity (it's very heavy for its size).
</p><p>Galena is the most important lead ore mineral.  It's principally found in hydrothermal vein systems and in Mississippi Valley-type deposits.  Silver (Ag) can fall into the lead position as an impurity, sometimes in relatively high proportions.  This results in argentiferous galena, (Pb,Ag)S.
</p><p>Historically, lead from galena has been used as a gasoline additive, a paint ingredient, and for making bullets.  Lead is also used to make various metal products, batteries, and radiation shielding.
</p><p>The well-formed crystals of galena shown above comes from the Huanzala Deposit in the Andes Mountains of Peru.  There, several limestone horizons of the Lower Cretaceous Santa Formation have polymetallic mineralization.  Origin interpretations vary - some consider this to be a contact metamorphic (metasomatized) deposit - a skarn, formed by the interaction between host limestones and Miocene dikes.  Others consider this to be a high-temperature, Cretaceous-aged, volcanic sedimentary ore deposit subjected to Miocene remobilization of minerals.
</p><p>Most galena specimens are cubic, but the crystals shown above are pseudohexagonal tabular contact twins.
</p><p>Locality: Huanzala Mine, western Peru
</p>
<hr><p>Photo gallery of galena:
www.mindat.org/gallery.php?min=1641
</p>
<hr><p>Some geologic info. synthesized from:
</p><p>Crowley et al. (1997) - The Raura-Huanzala Group (mining districts in Peru).  Mineralogical Record 28(4).
</p><p>Imai et al. (1985) - Mineralization and paragenesis of the Huanzala Mine, central Peru.  Economic Geology 80: 461-478.
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Soler et al. (1986) - Mineralization and paragenesis of the Huanzala Mine, central Peru - a discussion.  Economic Geology 81: 195-196.
James St. John
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