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The Halide Minerals Halide compounds – halogen ions (Cl-, Br-, F-, I-) Groups › Fluorides (fluorite - CaF2) › Chlorides (halite - NaCl), bromides (bromargite - AgBr) and iodides (iodoargyrite - AgI) › Halogen salts (cryolite - Na3AlF6) › Oxihalogenides (atacamite - Cu2Cl(OH)3) Fluorides and chlorides most important Occur as evaporites › minerals that crystallize during evaporation of water from a supersaturated solution Composition of halides Cl- and F- chemically active – easily ionized Large anions, therefore bonds with metallic cations › Na, K, Ca, Mg, Sr Commonly contains molecular water in crystal structure - compensate for smaller cations such as: Mg2+and Al3+ › MgCl2.6H2O Fluorides Fluorite and Cryolite Groups Fluorite minerals › Fluorite › Villiaumite › Fluocerite CaF2 NaF (Ce,La)F3 Cryolite minerals Isometric Isometric Hexagonal › Cryolite Na3AlF6 Monoclinic › Cryolithionite Na3Li3(AlF6)2 Isometric Fluorite Most common of fluorides Structure › › › › Primitive cubic lattice of F with Ca in alternate interstices Each Ca linked to 8 F; or each F linked to 4 Ca Unit cell contains 4 groups of CaF2 Octahedral; cube-octahedral or cubic habit Chemical composition › CaF2; may contain rare earth elements in isomorphic subsititurions Physical properties › Colorless to deep purple (Sr), green (Sm) or yellow › Soft › Strong fluorescence and phosphorescence Optical Properties › › › › Isotropic Low Refractive Index (RI) Colorless to light purple Perfect cleavage Occurrence › Orthomagmatic, pegmatitic, hydrothermal Fluorite Crystal structure Crystal form Chlorides, Bromides, Iodide The Lawrencite Group › › Chloromagnesite Lawrencite MgCl2 FeCl2 Trigonal Trigonal › › Halite Sylvite NaCl KCl Isometric Isometric › › Carnalite Chlorocalcite KMgCl3.H2O KCaCl3 Orthorhombic › › Eriochalcite Atacamite › › › Cotunnite Matlockite Bismoclite › Calomel › › › › Chloroargyrite Bromoargyrite Iodoargyrite Embolite The Halite Group Carnalite Group Atacamite Group Cotunnite Group Calomel Group Chloroargyrite Group Halite Structure › › › Each Na surround by 6 Cl (and vice versa) Cubic close packing of Cl with Na in octahedral interstices Isometric crystals; holosimmetric etch figures and hopper growth forms › NaCl – also known as rock salt or table salt › › › White to yellow, blue or light purple Salty taste Soft › › › › Isotropic Very low RI Colorless Not present in normal thin sections: soluble in water › Evaporite mineral: crystallise when high concentration of Na and Cl Chemical composition Physical properties Optical Properties Occurrence Halite Sylvite Structure › › › Each K surround by 6 Cl (and vice versa) Cubic close packing of Cl with K in octahedral interstices Cubic crystals › KCl: 52% K; 48% Cl; some Na may be present at low T › › › White to yellow, red Bitter-salty taste Soft › › › › Isotropic Very low RI Colorless Not present in normal thin sections: soluble in water › Evaporite mineral associated with halite, but scarcer because of greater sollubility and it crystallise after halite in the evaporation sequence Chemical composition Physical properties Optical Properties Occurrence Use › Fertilizers, medicine, cosmetics Carnallite Structure › Orthorhombic crystal structure Chemical composition › KMgCl3.H2O Physical properties › White to pink mineral › Bitter-salty taste › Soft Optical Properties › Colorless Occurrence › Forms during evaporation of sea water and found in saline sedimentary rocks › Usually forms with sylvite › Most important K bearing chloride mineral Use › K and Mg source; fertilizer Physical and Optical Properties Study Table 21.1 › Know chemical, crystal chemical, physical and optical properties of the most common halide minerals Origin of Halides 2 factors influence formation and stability › Chemical properties of compounds › Abundance of halogen atoms F, Cl, Br, I Linear relation of abundance and electron affinities, melting point Inverse relation of above with ionic radius, volatility and solubility › Fluoride minerals: high T endogenic processes › Chloride minerals: endogenic and exogenic processes 3 Common environments for formation of halides (except fluoride): › Evaporites in marine basins › Continental salt lakes › Secondary salt deposits Evaporites in marine basins Most halides present in evaporite rocks (except fluoride) › Result of evaporation of water › Chemical precipitates crystallizing from supersaturated solutions; concentrating at bottom of a basin Arid and hot climate Closed or partially closed basin Basin forms when continental shelf closed off when water supply goes down and evaporation increases Sequence of crystallization › Calcite › Sulfates, gypsum, anhydrite › Halite › Sylvite › Carnallite and bischofite Evaporites in marine basins Marine basins on the continental shelf Continental salt lakes Deserts Variable amounts of water present Composition varies according to: › Chemical weathering of the surface › Penetration of groundwater into local rocks Can be soda, sulfate, boron, nitrate lakes › Often enriched in rare minerals: lithium, boron Continental salt lakes Secondary salt deposits Ideal crystallization sequence rarely seen in nature Repetition and alternation of layers indicate drying under changing geological conditions › New fresh water influx will dissolve precipitated primary minerals and start a new sequence to precipitate the same minerals as secondary minerals Deeply buried salts can become buoyant and intrude upwards to form anticlinal salt domes: major salt deposits Fig 21.11 Gypsum bearing beds will be transformed in place by increasing T and P to form less hydrated or anhydrous minerals CaSO4.H2O gypsum CaSO4 anhydrite + 2H2O water Secondary salt deposits Diapiric salt dome, mined for carnallite Commercial deposits Halite, sylvite, carnallite, kainite, bischofite, mirabilite K-salt deposits: sylvite, carnallite › Canada, Gulf of Mexico Halite deposits: › SA: Port Elizabeth; western Free State › International: Namibia; Egypt; Poland; Russia