The first mineral on our tour was quartz. Most weeks, we’ll only cover one mineral, but there was a very strong push to include magnetite & hematite when I polled the blogosphere, so I chose to add both.
Both magnetite (Fe3O4) & hematite (Fe2O3) are iron oxides. However, the oxidation state of iron (Fe) is not the same for both minerals. Hematite contains only the 3+ type of Fe. Magnetite in contrast contains both 2+ and 3+ Fe. Why is this important? Whether an element bonds in a higher or lower oxidation state will depend on how many free anions (usually oxygen) are available while the mineral is forming. More oxygen = higher oxidation state. Hematite forms in environments that are more oxygen-rich than magnetite, such as water-rich or geological environments in contact with the atmosphere.
There are several differing ways that oxidation state of an environment might be presented:
- oxygen fugacity = fO2 = frequently used in igneous petrology and is buffered by mineral reactions such as NNO (Nickel-Nickel-Oxide; lower oxygen) or QFM (Quartz-Fayalite-Magnetite; higher oxygen); metamorphic petrology will also use fO2, though not as common as in igneous fields
- activity of electrons = Eh = used in as vertical axis on Eh – pH diagrams to depict the relationship between oxidation state (high Eh = higher oxidation state) and the pH of a system; common in soils, sedimentary geology, hydrogeology type fields
- descriptive as either anoxic (oxygen-poor) or oxic (oxygen-rich) – this seems to occur frequently in paleotonology and coal & oil petrogenesis discussions
Where can we find magnetite? In order to have a combination of 2+ and 3+ Fe present, magnetite requires at least a moderate fO2 value. At depth, magmas are usually more mafic to ultramafic in composition tend towards lower fO2 values. Typically, these more primitive magmas are water-poor, which also influences their fO2 values. The first oxides in ultramafic or mafic magmas are frequently rutile (TiO2) or ilmenite (FeTiO3). [The iron in ilmenite is 2+] The iron in the system will preferentially go into olivine or another silicate in 2+ form. More evolved magmas (intermediate to felsic compositions) tend to be both more water-rich and have a higher fO2 value. In these rocks, magnetite will form as a minor component of the rock. Magnetite will also be found in metamorphic rocks in minor amounts. Magnetite can also be formed biologically (e.g. bird brains, magnetotactic bacteria). There are some cases where magnetite can be found in more abundant modes, such as in banded iron formations and layered mafic intrusions, serving as iron ores.
Before I move onto hematite, let’s quickly mention about an important property of magnetite–the fact that it will preserve a magnetic field. Chris Rowan of Highly Allochtonous has discussed the field of paleomagnetism several times in the past four years–I’m going to leave that explanation to him.
Where is hematite found? Magnetite alters easily to hematite in more oxygen-rich environments, so rocks that once contained magnetite may be altered to contain hematite instead. Hematite will also form in oxic sedimentary environments, mineral hot springs, as a weathering product within soils, and in some volcanic rocks. Hematite has also been reported from the surface from Mars, which has implications for the presence of water on the surface at some point in Mars’ past. Banded iron formations, if altered, may contain significant amounts of hematite and be another source of iron ore.
Ok, so how we recognize magnetite & hematite in the field? If you carry a Brunton Compass with you, you can use it to determine whether a rock is magnetic or not (just watch out for power lines–they also create magnetic fields that can screw up your readings). Magnetite is not quite as dense as hematite, but that’s a hard thing to judge in the field. If you carry a streak plate (I don’t), hematite will have a red-brown streak and magnetite a dark grey – black streak. Both minerals can either be metallic, submetallic, or non-metallic depending on how much their outer surfaces have weathered, but magnetite tends towards dark silvers to greys & black, which hematite will go from silver to black to reddish-browns. Magnetite tends to be massive in shape, though you can find octahedral crystals of it occasionally. Hematite also can be massive, but will occasionally be tabular in shape. Hematite is the harder of the two minerals (6.5 vs. 5.5-6), but not by much.