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Subclass: Arsenates

Arsenate mineral, any of a group of naturally occurring compounds of arsenic, oxygen, and various metals, most of which are rare, having crystallized under very restricted conditions. At the mineralogically famous Långban iron and manganese mines in central Sweden, more than 50 species of arsenate minerals have been described, many peculiar to the locality. Such compounds occur in open cavities and resulted from the reaction of arsenic acid (H3AsO4) upon pyrochroite [manganese hydroxide; Mn(OH)2] at moderate to low temperature. Arsenates at other localities are often oxidation products of arsenide ores and are deposited at low temperatures in late-stage veins and open cavities.

Only a few arsenate minerals have economic importance. Because the transition metals (e.g., cobalt, copper, nickel) give brilliant colour to some of the arsenates, these can be used to advantage in prospecting; such arsenate oxidation products, or blooms, as erythrite (bright pink) and annabergite (green) are indicators of nickel and cobalt arsenide ores. Many of the nickel and cobalt deposits at Sudbury and Cobalt, Ont., were located in this manner.

The arsenate minerals, which are salts of arsenic acid, contain arsenate groups (AsO4) in which four oxygen (O) atoms are arranged at the corners of a tetrahedron about a central arsenic (As) atom. Each arsenate tetrahedron has a net electric charge of -3, which is neutralized by large, positively charged metal ions (e.g., calcium, manganese, or ferrous iron) outside the tetrahedron. Unlike the similar silicate tetrahedra, which link to form chains, sheets, rings, or frameworks, arsenate tetrahedra are insular.

The crystal structure of the arsenate minerals is very similar to that of the phosphate and vanadate minerals; indeed, many arsenate minerals form solid solutions with both the phosphates and the vanadates.

 

Name Colour Lustre Image Crystal system
Adamite   Zn2(AsO4)(OH) Ideally white, colourless, frequently pale yellow Vitreous, Sub-Vitreous, Waxy, Greasy Orthorhombic
Adelite   CaMg(AsO4)(OH) Colourless white, grey, bluish grey, yellowish grey Vitreous, Greasy Orthorhombic
Agardite   CeCu6(AsO4)3(OH)6 3H2O light green, yellowish green Sub-Vitreous, Resinous, Silky Hexagonal
Annabergite   Ni3(AsO4)2 8H2O Green, light grey to light apple green, white; pale rose red when rich in cobalt Sub-Adamantine, Sub-Vitreous, Pearly, Earthy Monoclinic
Arsenosiderite   FeAs2 Steel gray to silver white Metallic Orthorhombic
Arsenoclasite   Mn52+(AsO4)2(OH)4 Red Vitreous Orthorhombic
Arsenocrandallite   CaAl3(AsO4)(AsO3OH)(OH)6 White, creamy yellowish-white, also blue to bluish-green Sub-Vitreous, Resinous, Waxy, Dull Trigonal
Arsenoflorencite   CeAl3(AsO4)2(OH)6 Colourless to white, light brown Sub-Vitreous, Resinous, Waxy, Dull Trigonal
Arsentsumebite   Pb2Cu(AsO4)(SO4)(OH) Emerald-green, grass-green, apple-green, pale bluish green Vitreous Monoclinic
Arthurite CuFe23+(AsO4)2(OH)2 4H2O Apple-green, emerald-green, pale olive-green Vitreous Monoclinic
Atelestite Bi2(AsO4)O(OH) Sulphur-yellow to yellowish-green, yellow-brown Adamantine, Resinous Monoclinic
Austinite CaZn(AsO4)(OH) Colorless to pale yellowish white or bright green; colourless in transmitted light. Sub-Adamantine, Sub-Vitreous, Greasy, Silky Orthorhombic
Bayldonite PbCu3(AsO4)2(OH)2 Green, apple-green, yellow-green; dark green to black (crystals) Resinous, Waxy, Greasy Monoclinic
Berzeliite (NaCa2)Mg2(AsO4)3 Yellow, Orange, colorless, brownish-orange Sub-Vitreous, ResinousGreasy Isometric
Carminite PbFe23+(AsO4)2(OH)2 Carmine-red, terra cotta-red, reddish brown; red in transmitted light Vitreous, Pearly Orthorhombic
Cafarsite Ca5.9Mn1.7Fe3Ti3(AsO3)12 4-5H2O Dark brown Vitreous Isometric
Caryinite (Na,Pb)(Ca,Na)CaMn22+(AsO4)3 Brown to yellowish brown; light yellow-brown in transmitted light ResinousGreasy Monoclinic
Duftite PbCu(AsO4)(OH) Olive-green, grey-green; light apple-green in transmitted light. Sub-Vitreous, ResinousWaxy, Dull Orthorhombic
Eveite Mn22+(AsO4)(OH) apple green, pale yellow ResinousGreasy Orthorhombic
Holdenite (Mn2+,Mg)6Zn3(AsO4)2(SiO4)(OH)8 Pink, yellowish red, deep red; pink in transmitted light Sub-Vitreous, ResinousWaxy Orthorhombic
Kottigite Zn3(AsO4)2 8H2O Should be colorless, but frequently is Carmine-red, red-orange, brown ResinouWaxy, Silky Monoclinic
Legrandite Zn2(AsO4)(OH) H2O Yellow to colourless; pale yellow to colourless in transmitted light Vitreous, Sub-Vitreous, Resinous Waxy Monoclinic
Leiteite Zn(As2O4) white to colorless Sub-Vitreous, Resinous, Waxy, Dull Monoclinic
Mixite BiCu6(AsO4)3(OH)6 3H2O pale green, blue-green, emerald-green, white; light green to colourless in transmitted light Vitreous Hexagonal
Nealite Pb4Fe2+(As3+O3)2Cl4 2H2O Yellow brown to orange brown Adamantine, Vitreous, Sub-Vitreous Triclinic
Olivenite Cu2(AsO4)(OH) Olive green to yellow or brown, gray-green, grayish white; light green in transmitted light Sub-Adamantine, Sub-Vitreous, Resinous, Silky, Pearly Monoclinic
Tyrolite Ca2Cu9(AsO4)4(CO3)(OH)8 11H2O Turquoise-blue, blue-green; light bluish green in transmitted light Vitreous, Silky, Pearly Monoclinic
Wendwilsonite Ca2Mg(AsO4)2 2H2O Pale pink to red Vitreous Monoclinic
Zincroselite Colourless, white Vitreous Monoclinic

Date: 2015-12-24; view: 368


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Subclass: Phosphates | Subclass: Vanadates
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