Chapter 21

1. Chapter 21 Halides Evaporite Deposits

2. 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

3. 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

4. Fluorides Fluorite and Cryolite Groups Fluorite minerals Fluorite CaF2 Isometric Villiaumite NaF Isometric Fluocerite (Ce,La)F3 Hexagonal Cryolite minerals Cryolite Na3AlF6 Monoclinic Cryolithionite Na3Li3(AlF6)2 Isometric

5. 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

6. Fluorite Crystal structure Crystal form

7. Chlorides, Bromides, Iodide The Lawrencite Group Chloromagnesite MgCl2Trigonal Lawrencite FeCl2Trigonal The Halite Group HaliteNaClIsometric SylviteKClIsometric Carnalite Group CarnaliteKMgCl3.H2OOrthorhombic Chlorocalcite KCaCl3 Atacamite Group Eriochalcite Atacamite Cotunnite Group Cotunnite Matlockite Bismoclite Calomel Group Calomel Chloroargyrite Group Chloroargyrite Bromoargyrite Iodoargyrite Embolite

8. 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 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

9. Halite

10. Sylvite 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

11. 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

12. Physical and Optical Properties Study Table 21.1 Know chemical, crystal chemical, physical and optical properties of the most common halide minerals

13. 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

14. 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

15. Evaporites in marine basins

16. Marine basins on the continental shelf

17. 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

18. Continental salt lakes

19. 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  CaSO4 + 2H2O gypsum anhydrite water

20. Secondary salt deposits • Diapiric salt dome, mined for carnallite

21. 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