Geochemical classification of elements

1
Geochemical classification of elements

• Three major types of geochemical affinities
• Lithophile rock-loving oxides and silicates.
  Lithophiles preferentially combine/mix/dissolve
  with oxides and silicates.
• Siderophiles metal-loving free metals.
  Siderophiles preferentially combine/mix/dissolve
  with metals
• Sulfides sulfur-loving sulfides. Sulfides
  preferentially combine/mix/dissolve with sulfides
• This classical scheme has shortcomings, but is
  widely used
• Iron can be any of these simultaneously
• Type may change with temperature, pressure or
  oxidation state

2
Mineral classes

• Native elements Cu, Au, Ag
• Sulfides (S2-) pyrite FeS2
• Oxides and hydroxides (O2-, OH-) hematite Fe2O3,
  brucite Mg(OH)2
• Halides (Cl-, F-, Br-, I-) halite NaCl (salt)
• Carbonates (CO32-) calcite CaCO3
• Sulfates (SO42-) barite BaSO4
• Phosphates (PO43-) apatite Ca5F(PO4)3
• Silicates (SiO44-) quartz SiO2, pyroxene
  (Mg,Fe)SiO4, olivine (Mg,Fe)2SiO4, Na-feldspar
  NaAlSi3O8

3
Important silicate groups
Olivines (dark and/or green)
Feldspars (Fe-free, light)
Pyroxenes
4
Rocks

• Composed of assemblages of different minerals
• Four major groups
• Primitive
• Igneous
• Metamorphic
• Sedimentary

5
Primitive rocks

• Formed directly from material condensed in the
  solar nebula
• Have not undergone subsequent transformations
• Common on asteroids and in meteorites
• Chondrites have the same refractory element
  abundances as the Sun

6
Igneous rocks

• The most common rock type on objects that have
  undergone melting
• Formed from cooling magma
• Intrusive or plutonic crystallized slowly at
  depth, coarse grained (granite)
• Extrusive or volcanic crystallized rapidly on or
  near the surface, fine grained (basalt)
• Classification according to decreasing silica
  content
• Acidic, intermediate, basic, ultra-basic

7
Metamorphic rocks

• Altered by chemistry, pressure or temperature
  following initial crystallization (gneiss)
• Formed near the surface
• May be created by impact-induced shocks

8
Sedimentary rocks

• Formed under moderate pressure by accumulation of
  igneous, metamorphic or sedimentary mineral
  grains or organics
• Three major types
• Clastic (deposited fragments of older, weathered
  rock)
• Organic
• Chemical

9
Classification based on origin
Classification based on texture and acidity