Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1688

506 MINERALS/Arsenates end-members; they are generally associated with aegirine in peralkaline igneous rocks and have a bluish green to yellow-brown pleochroism See Also Igneous Rocks: Granite Minerals: Definition and Classification; Other Silicates; Pyroxenes Rocks and Their Classification Further Reading Deer WA, Howie RA, and Zussman J (1997) Rock Forming Minerals: Vol 2B Double Chain Silicates, 2nd edn xii ỵ 764 pp London: Geological Society Leake BE, Woolley AR, Arps CES, et al (1997) Nomenclat ure of Amphiboles: Report of the Subcommittee on Amphiboles of the International Mineralogical Associ ation Commission on New Minerals and Mineral Names Mineral Magazine 61: 295 321 Arsenates K Hudson-Edwards, University of London, London, UK ß 2005, Elsevier Ltd All Rights Reserved Crystal Structure The arsenates are a subclass of the phosphate mineral class, which has a basic chemical unit of [AO4] with a negative three charge (-3) and a tetrahedral symmetry In the case of the arsenates, the ‘A’ in the tetrahedron is the element arsenic (As) These tetrahedra are generally linked to one or more metal–oxygen, –hydroxide, and/or –water octahedra, which in turn are linked by edge- and corner-sharing arrangements Other weaker bonds exist in the arsenate structures, such as H bonds between the octahedra and arsenate tetrahedra atoms Some of the arsenates are sheet-like, with intersheet spaces occupied with, and bonded by, cations, halogens, or H2O molecules A good example of an arsenate structure is that of the mineral scorodite [FeAsO4 Á 2H2O], which is composed of Fe(III)–O octahedra that share the oxygen atoms of arsenate tetrahedra The arsenate tetrahedra are also weakly linked to the Fe(III)–O octahedra through H bonding Several dimorphous (same chemical formula, different structure) arsenates exist (e.g., roselite, monoclinic–beta-roselite, triclinic; symplesite, triclinic–parasymplesite, monoclinic) Chemistry and Nomenclature There are well over 180 arsenates They form a diverse subclass of minerals, exhibiting widely varying chemistries and structures, and comprising a number of groups and solid-solution series Table lists examples of the most common arsenate groups, and their crystal system(s) and point group(s) A common feature of the arsenates is their incorporation of water or hydroxide into their structure The amount of water incorporated depends on temperature, vapor pressure, and crystal structure Some of the more common hydrous and complex arsenates are summarized in Table Those arsenates that have a phyllosilicate-like, layered structure, with interlayer water molecules, are often incorrectly termed ‘micas’ (e.g., the autunite arsenates are often referred to as ‘uranyl micas’) Many of the arsenates form solid-solution series, with the As in the arsenate tetrahedron being replaced by P or V In fact, many of the groups listed in Table also include phosphates and vanadates (only the arsenate members of the groups are shown in Table 1) For example, the apatite group contains arsenates (e.g., mimetite Pb5(AsO4)3Cl), phosphates (e.g., pyromorphite Pb5(PO4)3Cl), and vanadates (e.g vanadinite Pb5(VO4)3Cl), and the mixite group contains arsenates (e.g., mixite, BiCu6(AsO4)3(OH)6 Á 3H2O) and phosphate minerals (e.g., petersite (Ca,Fe,Y,Ce)Cu6 (PO4)3(OH)6 Á 3H2O) Solid solutions also exist between members of different arsenate groups, where metal cations substitute for one another (the ‘X’ positions in Table 1) In some cases, the solid solutions are complete, with no miscibility gaps (e.g., annabergite– erythrite), but in others the series are incomplete (e.g., annabergite-koă ttigite) At least 75 solid solution series involving arsenates have been described Physical Properties and Stability Probably because of their wide-ranging chemistries, the arsenates exhibit a wide range of physical properties Many of the arsenates are green or a variety of green (bluish, bright, bright apple, emerald, grass, grey, olive, pale, yellowish), but a wide number of colours have been reported, including blue, brown, grey, pink, orange, purple, red, yellow, white, and shades in-between The colour is often dictated by the incorporation of transition metals such as Ni, Cu, U, and Co into the mineral structure Similarly,