Magnetite, Fe3O4 crystallizes with the spinel structure. The large
oxygen ions are close packed in a cubic arrangement and the smaller
Fe ions fill in the gaps. The gaps come in two flavors:
tetrahedral site: Fe ion is surrounded by four oxygens
octahedral site: Fe ion is surrounded by six oxygens
The tetrahedral and octahedral sites form the two magnetic sublattices,
A and B respectively. The spins on the A sublattice are antiparallel to
those on the B sublattice. The two crystal sites are very different and
result in complex forms of exchange interactions of the iron ions
between and within the two types of sites.
Oxidation – Reduction diagrams…Eh vs pH diagrams
Figure 4-2. Stability limits for natural
waters at the earth’s surface in terms of Eh
and pH at 25oC. The limits are based on
partial pressures of oxygen of 1 and 10-83.1
atm. Also shown is the emf in pe units. The
range of Eh and pH conditions for various
natural environments is modified from
Garrels and Christ (1965.)
The limits of the natural Eh-pH
Environment are determined by
The conditions which water
Breaks down to its gaesous
components
Ex: 4-7 goes to great lengths
to demonstrate that the amount of
O2 contributed to groundwater from
The following redox reaction:
2H+ + ½ O2 + 2e-  H2O
is really really small
O2
H2O
H2
Water unstable
Hematite
Water unstable
Is Fe stable
In natural waters?
Now we have an Eh-pH diagram that shows the conditions at which
hematite and magnetite minerals are stable. We know that almost all minerals
show at least some solubility…so we can add the Fe+3 and Fe+2 ions to the
Eh-pH plot.
The approach for doing this is Eby p. 104-106. Here are the results for Fe3+ and
Fe2+. Contour lines for the activity of Fe3+ or Fe2+
All the previous can be combined into a composite Eh-pH diagram. In this case,
the ions are shown to occupy regions where they are either in equilibrium or greater
than the minerals. In theory, the different minerals still exist within the zones
originally designated by the Eh-pH equation. In practice, they have essentially
mostly dissolved into Fe2+ and/or Fe3+ ions
Aqueous
phase
mineral
phase
Since we can assume that CO2 has dissolved into solution at a conc. in
equilibrium, we now have some carbonate chemistry to add. Specifically, the iron
carbonate mineral siderite.
Lastly, include iron-sulfide complexes
If this is groundwater,
under what conditions is iron
likely to be mobile?
What happens when you
pump gw from an iron-rich,
but sulfur-poor
aquifer, that is pH=5.8,
Eh = -.050 to your bathtub
exposed to the atmosphere?
Martian
basalt
Martian blueberries
Jarrahdale bauxite, Australia
Arkansas bauxite
Martian blueberries
Continental margins
Fig. 3.6
Trailing-Edge Margin
Anatomy of a passive margin
Continental margin
Fig. 3.7
Continental slope and submarine canyons
Fig. 3.8a
Volcanic features of mid-ocean ridge
• Hydrothermal vents
– Heated subsurface seawater migrates through cracks
in ocean crust
• Warm-water vents <30oC or 86oF
• White smokers >30oC <350oC or 662oF (white
because of barium sulfide)
• Black smokers > 350oC
– (black because of metal sulfides; Fe, Ni, Zn)
• Important for maintaining the supply of metals to
the ocean
Hydrothermal vents
• Dissolved metals precipitate to form
metal sulfide deposits
• Unusual biological communities
– Able to survive without sunlight
– Archaeons and bacteria oxidize
hydrogen sulfide gas to provide food
Hydrothermal
vents
Fig. 3.14
Black Smoker