Geology Reference
In-Depth Information
with pyrite as the final mineral in the sequence. Pyrite is paramagnetic,
but ferrimagnetic greigite is strongly magnetic and can carry a stable
paleomagnetic signal. If the sediment accumulation rate is fast enough,
sediments pass through the reductive diagenesis zone quickly enough so
the reaction doesn't go to completion, and greigite, rather than pyrite, is
left in the sediments. Also, it is likely that the magnetite may not be
completely dissolved, so the sediments are left with a mixture of primary
depositional magnetite and secondary greigite. Since magnetite and
greigite have similar coercivities; they can both contribute to a rock
magnetic cyclostratigraphy if the low coercivity magnetic minerals are
activated to measure concentration variations. The only way to separate
their contributions to the laboratory remanence applied for a rock
magnetic cyclostratigraphy is by thermal demagnetization since greigite
has a Curie temperature of about 300°C. However, heating organic-rich
sediments often leads to oxidation and formation of secondary magnetite.
Pyrrhotite (Fe
7
S
8
) is another important ferromagnetic iron sulfide min-
eral. It has been viewed as a product of reductive diagenesis, but Horng
and Roberts (2006) provide evidence that it forms very slowly at tempera-
tures lower than 180°C and that greigite is the only important magnetic
mineral formed by reductive diagenesis.
Goethite (FeOOH) is an anti-ferromagnetic mineral that can have a
spontaneous magnetization due to lattice vacancies. It is formed by
oxidation of Fe-rich precursor minerals such as Fe-rich clays or other
Fe-rich silicates. It has a very low Neel temperature at which it loses its
spontaneous magnetization (~125°C), but extremely high coercivities.
Both properties make it easy to identify. Most paleomagnetic evidence
suggests that goethite often forms by recent weathering, since it often
carries a paleomagnetic direction parallel to the present day geomagnetic
field. However, there is evidence that the ratio of ancient goethite to
hematite in a rock is a sensitive indicator of moisture in the sediment's
source area (Yapp 2001; Harris & Mix 2002). For this reason, magnetic
measurement of the goethite/hematite ratio could be an important
paleoclimate indicator for rock magnetic cyclostratigraphy; however, it
will be important to check, using paleomagnetism, whether the goethite
is ancient or a product of present day weathering. Maghemite (γFe
2
O
3
) is
another important secondary magnetic mineral. It is the product of low-
temperature oxidation of magnetite. It retains the crystal structure of
magnetite but has the chemical formula of hematite. Lattice vacancies
are left in the crystal as about one-third of the Fe
2+
ions migrate to the
surface for oxidation, so its magnetic properties can vary, like those of
goethite. The remaining Fe
2+
cations are oxidized but remain in the
crystal lattice. Maghemite is often found to form in modern soils, mak-
ing it potentially a depositional magnetic mineral for sedimentary rocks
derived from soil. A good way to identify maghemite is by heating.
Although its Neel temperature is high (~645°C), it inverts to hematite at
about 350˚C.
