Geology Reference
In-Depth Information
of magnitude stronger susceptibilities than diamagnets and paramagnets
(1,000-10,000 × 10
-8
m
3
/kg), so a very small concentration of ferromagnetic
minerals will swamp the induced magnetizations of diamagnetic and para-
magnetic minerals in a sample.
Magnetite is ferrimagnetic and hence has a strong spontaneous magneti-
zation (92 Am
2
/kg) resulting from exchange interactions between the
unpaired spin moments of its 3D electrons. It has cubic crystal symmetry
and is an inverse spinel. Butler (1992) and Tauxe (2010) provide more
detailed information about the crystal structure of magnetite and the origin
of its spontaneous magnetization. Magnetite can be formed in igneous
rocks and it is magnetized as it cools down through its Curie temperature
(580°C) and the ferromagnetic exchange interactions are established.
Titanium can fit into magnetite's crystal structure and a solid solution series
exists between antiferromagnetic ulvospinel (Fe
2
TiO
4
) and magnetite. As
more Ti is added to the crystal structure, the Curie temperature drops to
low values of about -100°C for ulvospinel. An important titanomagnetite
is TM60 that is 60% ulvospinel and is the ferromagnetic mineral that crys-
tallizes in mid-ocean ridge basalts that make up the seafloor. Although
magnetite is typically considered to be a primary depositional magnetic
mineral, it can form long after deposition as a secondary magnetic mineral,
by tectonically driven fluid flow (McCabe & Elmore 1989) or burial diagen-
esis of clays (Woods et al. 2002). See Kodama (2012) or Van der Voo and
Torsvik (2012) for a summary of remagnetization processes. The grain size
of magnetite particles is important in paleomagnetic and environmental
magnetic studies. The magnetite particles important for carrying a stable
paleomagnetic signal are micron to submicron in size, and the principles of
fine particle magnetism describe their behavior. Magnetite has iron atoms
in both the 2+ and 3+ oxidation states, so magnetite can be oxidized either
to maghemite or hematite.
In rock magnetic cyclostratigraphy, it is important to know what magnetic
minerals are carrying cyclic behavior, particularly if it is identified as orbitally
forced cyclicity. Also, in designing the cyclostratigraphic study, it will be
important to know what laboratory-applied remanence will best measure
the concentration variations or grain size variations in the sedimentary
sequence. Magnetic minerals can be identified with two important pieces of
information, their coercivity or magnetic hardness, which will be explained
in Section 2.4 and their Curie temperature. Low temperature (<< room tem-
perature) behavior can also be diagnostic, but specialized equipment is
needed to make low temperature measurements. Table 2.2 summarizes
some diagnostic characteristics for the main magnetic minerals that are the
target of rock magnetic cyclostratigraphic studies, as well as the spontaneous
magnetization for these minerals.
Hematite is the second important ferromagnetic mineral (αFe
2
O
3
). Hematite
has hexagonal crystal symmetry with a canted anti-ferromagnetism. Sublattices
of its crystal have their 3D spin moments aligned antiparallel to each other, but
they are canted slightly from being exactly antiparallel, by <0.1°, so a net, but
