Halides, Carbonates and other '...ates'

Key Points: Halides

• large -1 anions and large +1 or +2 cations

• high symmetry structures

• the halite structure was the first to be determined by X-ray diffraction techniques: FCC array of Na (or Cl) with the Cl (or Na) forming 6 bonds in the form of a regular octahedron around each

• slightly larger +1 cations have the CsCl-structure which has 8-fold coordination: both the Cs and the Cl are at the centers of a cube of neighboring atoms

• +2 cations commonly have the fluorite structure: FCC array of Ca with F in tetrahedral coordination - each Ca has 8 fluorine bonds at the corners of a cube

• nearly pure ionic bonding

• you should know halite, sylvite, fluorite

Key Points: Carbonates

• carbonates and the rest of the '..ates' are characterized by strongly bonded anionic groups or complexes in their structure

• radius ratio relationships correctly predict that carbon should have 3 nearest oxygen neighbors and this occur as a planar triangle

• the double bonds in CO₂ are stronger than the single bonds in the CO₃^-2 complex so in the presence of hydrogen ion, carbonates breakdown and 'fizz' in acid

• 3 structurally distinct groups: calcite, aragonite, dolomite

• two important hydrous copper carbonates: azurite, malachite

Calcite Group: CaCO₃

• Calcite, Magnesite (Mg), Siderite (Fe), Rhodochrosite (Mn), Smithsonite (Zn)
• All cations have ionic radii less than about 1 Angstrom.
• All have bar3 2/m symmetry which is basically the halite structure (again) but the parallel arrangement of the CO₃ triangles destroys 3 of the 4 3-fold axes and removes the symmetry along the 4-fold directions.
• Layers of cations in octahedral coordination with oxygens, each oxygen is bonded to two Ca and one C atom
• Ca:O radius ratio lies on the boundary between 6- and 8-fold coordination - calcite has an orthorhombic polymorph aragonite

Aragonite Group: CaCO₃

• Aragonite, Witherite (Ba), Strontianite (Sr), Cerussite (Pb)
• cation radii > 1.0 angstrom lead to irregular 9-fold coordination and orthorhombic structures
• pseudohexagonal symmetry reflected in crystal face angles and twinning

Dolomite Group: CaMg(CO₃)₂

• Dolomite and Ankerite (CaFe) are most important
• isostructural with bar3 symmetry
• The large size difference between Ca and either Mg or Fe causes cation ordering in the structure with the Ca atoms in one layer and the Mg or Fe atoms in another. This destroys the 2-fold axes and mirrors and reduces the symmetry.

Carbonate phase equilibria:

• Note the ranges of solid solution for each mineral in the triangular CaCO₃-MgCO₃-FeCO₃ diagram in Figure 12.5. Note also that this diagram is drawn for a specific temperature.
• Figure 12.11 shows the P-T stability of calcite vs aragonite - aragonite is found in some blueschist facies metamorphic rocks (high P at low T).
• Given Fig. 12.11, why is aragonite found as the iridescent 'mother-of-pearl' in clam shells?
• At low temperatures there is very little solid solution between the calcite and ordered dolomite structures. At higher temperatures solid solution increases - this relationship can be used as a geothermometer to deduce crystallization temperatures in carbonatites (igneous carbonate rocks) and temperatures of metamorphism.
• You should be able to explain the skewness of the calcite-dolomite solvus using crystal chemical arguments.

Key Points: Sulfates, Phosphates, ...

• nitrates are extremely soluble and thus extremely rare in nature

• borates are source for boron which is important in glass manufacturing

• BO₃ triangles can be polymerized in exactly the same ways that silica tetrahedra can - may return to this as an aside in couple of weeks.

• the covalently bonded SO₄^-2 tetrahedral anionic group is the fundamental structural unit in the sulfates

• various Ca, Fe, Mn tungstates are THE source of tungsten

• the covalently bonded PO₄^-3 tetrahedral anionic group is the fundamental structural unit in the phosphates

Anhydrous sulfates: Barite BaSO₄ and Anhydrite CaSO₄

• large Ba is in 12-fold coordination
• smaller Ca is in 8-fold coordination; very different structure

Hydrous sulfates: Gypsum CaSO₄^.2H₂O

• monoclinic structure consists of layers parallel to {010} - successive layers are hydrogen bonded which accounts for the excellent {010} cleavage, the softness, and the ease with which it is dehydrated
• heat gypsum to dehydrated it by 3/4ths, then add water to rehydrate it as plaster (or gypsum wallboard)

Phosphates: Apatite Ca₅(PO₄)₃(OH,F,Cl) is the most important

• significant substitution of As and V for P (all have +5 charge)
• phosphorous is one of the 3 main ingredients in fertilizer
• most of the rare-earth elements are recovered from phosphates like monazite
• Fluor-apatite is more stable than hydroxy-apatite - this is the main reason that water is fluoridated
• your bones and teeth are basically apatite - hence calcium supplements