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.)

Ex: 4-7 goes to great lengthsto demonstrate that the amount of O2 contributed to groundwater fromThe following redox reaction:

2H+ + ½ O2 + 2e- H2O

is really really small

The limits of the natural Eh-pHEnvironment are determined byThe conditions which waterBreaks down to its gaesous components

H2

O2

H2O

Water unstable

Water unstable

Is Fe stableIn natural waters?

magnetite

Hematite

Now we have an Eh-pH diagram that shows the conditions at whichhematite and magnetite minerals are stable. We know that almost all mineralsshow at least some solubility…so we can add the Fe+3 and Fe+2 ions to theEh-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 greaterthan the minerals. In theory, the different minerals still exist within the zones originally designated by the Eh-pH equation. In practice, they have essentiallymostly dissolved into Fe2+ and/or Fe3+ ions

Aqueousphase

mineralphase

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 ironcarbonate mineral siderite.

Lastly, include iron-sulfide complexes

If this is groundwater,under what conditions is ironlikely to be mobile?

What happens when youpump gw from an iron-rich, but sulfur-pooraquifer, that is pH=5.8, Eh = -.050 to your bathtubexposed to the atmosphere?

Martianbasalt

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

Fig. 3.14

HydrothermHydrothermal ventsal vents

Black Smoker