Oxides and Hydroxides

Periodic Table 1

Periodic Table 2

Key Points: Oxides

• hard, dense and refractory

• accessory minerals in igneous and metamorphic rocks

• resistant detrital grains in sediments

• approximate closest packed oxygen structures

• strongly ionic bonding

• no cations with charge/coordination number >= 1 and therefore no strong anionic groups like we will see in the carbonates and silicates.

• chief ores of iron, chromium, manganese, tin, titanium, and uranium

Oxides:

The oxides are usually discussed in 4 structural groups based on the hematite, rutile, fluorite and spinel structures.

Hematite Group - X₂O₃:

• Based on hexagonal closest packing with cations in octahedral coordination.
• Each octahedra shares a face as the link between layers.
• Corundum and hematite are bar3 2/m while the ordered Fe-O and Ti-O layers in Ilmenite reduce the symmetry to bar3.
• Periclase MgO is very similar - all the octahedra are filled and in fact periclase is isometric.
• You should know hematite, corundum and ilmenite.
• Hematite structure movie (360K).

Rutile Group - XO₂:

• Octahedral coordination.
• Isostructural 4/m2/m2/m.
• Chains of octahedra parallel to the c-axis (4-fold).
• Different structure because +4 cations can't face-share octahedra because it puts them too close together. Here the octahedra edge share in chains.
• Gives rise to prismatic habit of the mineral.
• Radius ratio in the range 0.414 - 0.732 should have this structure.
• You should know rutile and cassiterite.

Fluorite structure Group- XO₂:

• Each oxygen is tetrahedrally coordinated by cations but each cation is large enough that it has 8 surrounding oxygens in the shape of a cube.
• UO₂ is the most important member of this group.
• Radius ratio in the range 0.732 - 1.0 should have this structure.

Spinel Group - XY₂O₄:

• Approximate cubic close packed planes along [111].
• Cations in both octahedral and tetrahedral coordination.
• A unit cell (X₈Y₁₆O₃₂) contains 32 possible octahedral sites and 64 possible tetrahedral sites; half the octahedral and 1/8 of the tetrahedral sites are occupied. (Z=8)
• {111} layers of octahedra linked to other layers by layers composed of both tetrahedra and octahedra.
• Note that there are both 'normal' and 'inverse' spinels.
• In 'normal' spinels the 8 X cations are in the tetrahedral holes while the 16 Y cations occupy the octahedral sites.
• In 'inverse' spinels the X's and half the Y's change places giving a formula of Y(YX)₄.
• Normal radius ratio concepts don't work here (larger lower valence cations in the tetrahedral holes) - crystal field stabilization energies take over.
• Importance as geothermometer - see Figure 11.21.
• You should know spinel, magnetite and chromite.

Key Points: Hydroxides

• low density and hardness

• secondary alteration and weathering products

• hand samples are commonly mixtures of minerals: bauxite and limonite are not minerals - they are mixtures

• structure is basically sheets of octahedra which we will see again in several of the silicate groups.

Hydroxides:

Brucite structure: Mg(OH)₂

• The brucite structure consists of Mg²⁺ octahedrally coordinated to (OH)^- with the octahedra all sharing edges.
• Because the Mg has a +2 charge, every octahedral site in the layer is occupied (analogous to periclase). This relationship is referred to as 'trioctahedral'.
• Because each (OH)^- is shared between 3 octahedra, all the charge is neutralized in a single sheet and so, unlike corundum or hematite, adjacent octahedral sheets are only weakly bonded to one another.

Gibbsite structure: Al(OH)₃

• The same structure as brucite except that because Al has a +3 charge, only 2/3 of the octahedral sites are filled (analogous to corundum). This structure is referred to as 'dioctahedral'.
• The sheet is still charge balanced and so again the intersheet bonds are very weak.

Dioctahedral and trioctahedral sheets are essential building blocks for the sheet silicates.

Diaspore AlO(OH) and Goethite FeO(OH)

These isostructural minerals have double chains of edge sharing octahedra which are cross linked to other double chains by their apical oxygens. These minerals are thus harder than brucite and gibbsite. Both minerals have polymorphs (boehmite and lepidocrocite respectively) which have apically linked chains of octahedra which edge share with adjacent chains to form corrugated sheets. The sheets are hydrogen bonded to adjacent sheets.