Oxides and hydroxides
Review of ionic crystals

200 minerals; 10% of all mineral species
– Oxides, A n
(B p
)O m
:
Quartz – 12 vol% of earths crust; discussed with the silicates
Ice – Seasonal phase, major mineral of Earth and Mars polar caps; also large part of other planets
CO
2
– present as a mineral on Mars, other planets and meteorites
Fe-oxides only 0.2 vol% of crust - Magnetite, Hematite major iron ores
More than 40 elements found in oxide form
– Hydroxides, A n
(B p
) (OH) m
: gibbsite, goethite, diaspore
25 elements occur in hydroxide form

Oxides and hydroxides occur in two types:
– Simple
Single element as cation
Oxides: Periclase (MgO); Corundum (Al
2
O
3
)
Hydroxides: Gibbsite (Al(OH)
3
); Brucite (Mg(OH)
2
); Manganite
(Mn 2+ Mn 4+ O
2
(OH)
2
)
– Complex
Two or more main cations
Oxides: Spinel (MgAl
2
O
4
); Perovskite (CaTiO
3
)
Less important hydroxides: Romanechite (BaMn 2+ Mn
9
4+ O
2
0.3H
2
O)

Structure names named after first mineral described for that structure i.e.: NaCl crystallizes in the ‘halite structure’
Ionic bonds rules:
– Close packing (or almost closed packing)
– Anions forms regular coordination polyhedra
– Cations generally smaller than anions
Four simplest ionic structures for A-X compounds:
– Halite, Nickeline, Sphalerite, Wurtzite
– CsCl structure for large cations
Common ionic structure for A n
B m
X p
: spinel structure; perovskite structure; rutile structure; brucite and gibbsite structure

Table 25.3
Minerals such as:
– Corundum (Al
2
O
3
)
– Hematite (Fe
2
O
3
)
– Ilmenite (FeTiO
3
)

Table 25.4
Minerals: Magnetite, Spinel, Chromite, etc.
Tetrahedral and octahedral polyhedra forms that are deformed due to non-ideal close packing of oxygen

Table 25.5
Minerals: Perovskite, Loparite, Silicate perovskite
Cubic close packing of oxygen; one oxygen missing in every second layer – filled by Ca 2+ - coordinated by 12 oxygens
Can accommodate large cations such as REE in this large cavity
Transforms under high pressure – distorting structure if large cation is smaller than oxygen

Body centered tetragonal unit cell
Ribbons of edge sharing TiO
6
8octahedra that link at free corners
Cassiterite (SnO
2
)
Pyrolusite (MnO
2
)
Stishovite (SiO
2
)

Stacked layers of octahedral sheets
Brucite: all octahedra occupied
– Trioctahedral
Gibbsite: one out of three vacant
– Dioctahedral

Cuprite
Corundum
Hematite
Ilmenite
Periclase
Magnetite
Chromite
Ringwoodite
Rutile
Anatase
Pyrolusite
Uraninite
Perovskite
Cu
2
O
Al
2
O
3
Fe
2
3+ O 3
FeTiO
3
MgO
Fe 2+ Fe
2
3+ O 4
FeCr
2
O
4
Mg
2
SiO
4
TiO
2
TiO
2
MnO
2
UO
2
CaTiO
3

Brucite
Gibbsite
Mg(OH)
2
Al(OH)
3
Diaspore AlOOH
Boehmite AlOOH
Manganite MnOOH