Anglesite

Anglesite forms in the oxidation zone of galena-bearing lead ore deposits, where primary lead sulfide (PbS) reacts with oxygenated groundwater and dissolved sulfate ions. The transformation begins when galena surfaces are exposed to weathering. Sulfuric acid generated by the oxidation of associated pyrite attacks the galena, converting lead sulfide directly into lead sulfate. This process occurs at relatively low temperatures, typically between 15 and 50 degrees Celsius, under near-surface conditions where oxygen and water are abundant.

The crystal growth of anglesite is heavily influenced by the chemistry of the surrounding solutions. In cavities where evaporation concentrates lead and sulfate ions slowly, spectacular prismatic or tabular crystals can develop over thousands of years. The adamantine luster that makes fine anglesite specimens so striking comes from the high refractive index caused by the heavy lead atoms in the crystal lattice. Crystals often form as pseudomorphs after galena, preserving the cubic shape of the original mineral while completely replacing its internal structure.

Anglesite commonly occurs alongside other secondary lead minerals including cerussite (lead carbonate), pyromorphite (lead phosphate), and wulfenite (lead molybdate). The specific secondary mineral assemblage depends on the availability of carbonate, phosphate, and other anions in the groundwater. In carbonate-rich environments, cerussite tends to dominate over anglesite, while sulfate-rich conditions favor anglesite formation.

Anglesite is identified in hand specimen by its exceptionally high specific gravity of 6.3 to 6.4, which makes even small crystals feel surprisingly heavy. The adamantine to vitreous luster on crystal faces is distinctive and gives well-formed specimens a diamond-like brilliance. Crystals are typically prismatic or tabular in the orthorhombic system, often with complex terminations featuring multiple faces.

Distinguish anglesite from cerussite (PbCO₃) by the effervescence test. Cerussite fizzes vigorously in dilute hydrochloric acid, while anglesite is nearly insoluble and shows no reaction. Both minerals have similar high densities, but cerussite tends to form pseudo-hexagonal twins while anglesite produces simpler prismatic forms. Distinguish from barite (BaSO₄), which shares the same crystal structure but has a lower specific gravity of 4.3 to 4.6. The heavier weight of anglesite compared to a similar-sized barite specimen is immediately noticeable. Under shortwave UV light, some anglesite specimens fluoresce yellow, which can help confirm identification.