birchite
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| birchite | |
|---|---|
| Name | birchite |
| Category | Phosphate mineral |
| Formula | Pb2Cu2(PO4)2(SO4)(OH)2·H2O |
| System | Orthorhombic |
| Class | Dipyramidal |
| Symmetry | Pmcn |
| Unit cell | a = 8.77 Å, b = 9.21 Å, c = 6.01 Å |
| Color | Blue-green |
| Habit | Prismatic crystals, radial aggregates |
| Cleavage | Good on {010} |
| Mohs | 3.5 |
| Luster | Vitreous |
| Streak | Pale green |
| Gravity | 5.6 |
| Opticalprop | Biaxial (-) |
| Birefringence | δ = 0.030 |
| 2V | 70° (measured) |
| Dispersion | r > v, strong |
| Fluorescence | None |
| Other | Soluble in acids |
birchite is a rare secondary lead-copper phosphate mineral containing sulfate and water. It forms in the oxidized zones of specific hydrothermal ore deposits where primary sulfide minerals have undergone weathering. First described from the Broken Hill ore deposit in New South Wales, its distinctive crystal structure and chemical composition make it of significant interest to mineralogists and geochemists studying supergene mineral formation.
Description and properties
birchite typically occurs as small, prismatic to acicular crystals exhibiting a vibrant blue-green color, though it can also form radial or spherulitic aggregates. It possesses a vitreous luster and leaves a pale green streak. With a Mohs hardness of approximately 3.5, it is relatively soft and shows good cleavage on one plane. Its high measured specific gravity, around 5.6, is attributable to its substantial lead content. Under polarized light microscopy, it is biaxial negative with a measured 2V angle of about 70 degrees and exhibits strong birefringence and dispersion. The mineral is soluble in common acids like hydrochloric acid and nitric acid but is not fluorescent under ultraviolet light.
Occurrence and formation
This mineral is a product of supergene alteration, forming in the oxidized caps of polymetallic sulfide deposits under arid to semi-arid climatic conditions. Its type locality is the world-class Broken Hill lead-zinc-silver deposit in New South Wales, Australia, where it is associated with other secondary minerals like pyromorphite, cerussite, and wulfenite. Similar occurrences have been identified in other oxidized ore deposits, such as those at Tsumeb in Namibia and Laurium in Greece. Formation requires the presence of primary copper and lead sulfides like galena and chalcopyrite, which are broken down by groundwater and oxygen, releasing ions that recombine with phosphate and sulfate anions, often derived from the alteration of apatite or baryte.
Crystal structure
The atomic arrangement of birchite is orthorhombic, with space group Pmcn. Its structure is characterized by chains of edge-sharing copper-centered octahedra that run parallel to the c-axis. These chains are linked by phosphate tetrahedra and lead atoms in irregular coordination, creating a three-dimensional framework. Channels within this framework accommodate the essential water molecules and the additional sulfate groups. This structure is related to, but distinct from, other hydrated lead-copper phosphates like tsumcorite and arsenate analogues such as duftite. The study of its structure aids in understanding the crystallography and stability fields of complex secondary minerals in the supergene environment.
History and etymology
The mineral was first discovered and characterized from specimens collected at the Broken Hill mine. It was officially described and validated as a new species by the International Mineralogical Association's Commission on New Minerals, Nomenclature and Classification in 1993. The name honors William Birch, a noted curator of mineralogy and petrology at the Museum Victoria in Melbourne, for his extensive contributions to Australian mineralogy and his work on the mineralogy of the Broken Hill region. The type material is preserved in the collections of Museum Victoria and the National Museum of Natural History in Washington, D.C..
Related minerals
birchite belongs to a broader group of secondary phosphate and arsenate minerals that form in oxidized zones. It is chemically and structurally related to minerals like tsumcorite (PbFe2(AsO4)2(OH,H2O)2) and duftite (PbCu(AsO4)(OH)), which feature different dominant anions and cations. Other associated secondary minerals from its typical paragenesis include the lead phosphate pyromorphite, the lead carbonate cerussite, and the copper arsenate olivenite. Its study contributes to the mineralogical family that includes vauquelinite and mimetite, all of which are important indicators of geochemical conditions during weathering processes in ore bodies.
Category:Phosphate minerals Category:Lead minerals Category:Copper minerals Category:Orthorhombic minerals Category:Minerals described in 1993