 Halide compounds – halogen ions (Cl -, Br -, F -, I - )  Groups › Fluorides (fluorite - CaF 2 ) › Chlorides (halite - NaCl), bromides (bromargite - AgBr) and iodides (iodoargyrite - AgI) › Halogen salts (cryolite - Na 3 AlF 6 ) › Oxihalogenides (atacamite - Cu 2 Cl(OH) 3 )  Fluorides and chlorides most important  Occur as evaporites › minerals that crystallize during evaporation of water from a supersaturated solution

 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: Mg 2+ and Al 3+ › MgCl 2.6H 2 O

 Fluorite and Cryolite Groups  Fluorite minerals › FluoriteCaF 2 Isometric › VilliaumiteNaFIsometric › Fluocerite(Ce,La)F 3 Hexagonal  Cryolite minerals › CryoliteNa 3 AlF 6 Monoclinic › CryolithioniteNa 3 Li 3 (AlF 6 ) 2 Isometric

 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 CaF 2 › Octahedral; cube-octahedral or cubic habit  Chemical composition › CaF 2 ; 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

Crystal structure Crystal form

 The Lawrencite Group › ChloromagnesiteMgCl 2 Trigonal › LawrenciteFeCl 2 Trigonal  The Halite Group › HaliteNaCl Isometric › SylviteKClIsometric  Carnalite Group › CarnaliteKMgCl 3.H 2 OOrthorhombic › ChlorocalciteKCaCl 3  Atacamite Group › Eriochalcite › Atacamite  Cotunnite Group › Cotunnite › Matlockite › Bismoclite  Calomel Group › Calomel  Chloroargyrite Group › Chloroargyrite › Bromoargyrite › Iodoargyrite › Embolite

 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  Chemical composition › NaCl – also known as rock salt or table salt  Physical properties › White to yellow, blue or light purple › Salty taste › Soft  Optical Properties › Isotropic › Very low RI › Colorless › Not present in normal thin sections: soluble in water  Occurrence › Evaporite mineral: crystallise when high concentration of Na and Cl

 Structure › Each K surround by 6 Cl (and vice versa) › Cubic close packing of Cl with K in octahedral interstices › Cubic crystals  Chemical composition › KCl: 52% K; 48% Cl; some Na may be present at low T  Physical properties › White to yellow, red › Bitter-salty taste › Soft  Optical Properties › Isotropic › Very low RI › Colorless › Not present in normal thin sections: soluble in water  Occurrence › Evaporite mineral associated with halite, but scarcer because of greater sollubility and it crystallise after halite in the evaporation sequence  Use › Fertilizers, medicine, cosmetics

 Structure › Orthorhombic crystal structure  Chemical composition › KMgCl 3.H 2 O  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

 Study Table 21.1 › Know chemical, crystal chemical, physical and optical properties of the most common halide minerals

 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

 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

 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

 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  Gypsum bearing beds will be transformed in place by increasing T and P to form less hydrated or anhydrous minerals  CaSO 4.H 2 O  CaSO 4 + 2H 2 O gypsumanhydritewater

Secondary salt 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