Transcript Chapter 21 Hoofstuk 21

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
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
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
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
Isotropic
Low Refractive Index (RI)
Colorless to light purple
Perfect cleavage
Occurrence
› Orthomagmatic, pegmatitic, hydrothermal
Fluorite
Crystal structure
Crystal form
Chlorides, Bromides, Iodide

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



The Lawrencite Group
›
›
Chloromagnesite
Lawrencite
MgCl2
FeCl2
Trigonal
Trigonal
›
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Halite
Sylvite
NaCl
KCl
Isometric
Isometric
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Carnalite
Chlorocalcite
KMgCl3.H2O
KCaCl3
Orthorhombic
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Eriochalcite
Atacamite
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Cotunnite
Matlockite
Bismoclite
›
Calomel
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Chloroargyrite
Bromoargyrite
Iodoargyrite
Embolite
The Halite Group
Carnalite Group
Atacamite Group
Cotunnite Group
Calomel Group
Chloroargyrite Group
Halite

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

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Structure
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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
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NaCl – also known as rock salt or table salt
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White to yellow, blue or light purple
Salty taste
Soft
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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
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Structure
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Each K surround by 6 Cl (and vice versa)
Cubic close packing of Cl with K in octahedral interstices
Cubic crystals
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KCl: 52% K; 48% Cl; some Na may be present at low T
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White to yellow, red
Bitter-salty taste
Soft
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Isotropic
Very low RI
Colorless
Not present in normal thin sections: soluble in water
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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
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Halite deposits:
› SA: Port Elizabeth; western Free State
› International: Namibia; Egypt; Poland; Russia