الفهرس 
The magnetic fabricOnly 14 pages are availabe for public view
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Abstract
Magnetization When a magnetic field is applied to any substance it induces a magnetization (M) which is directly related to the strength of the applied field (H), with a constant of proportionality being known the magnetic susceptibility (K). The magnetization (M) is thus defined as: M =KH The magnetization of a mineral consists of two types: remanent and induced magnetization. The latter is only present when the mineral is under the influence of an applied field (which includes the earth’s magnetic field). Diamagnetic minerals produce an induced magnetization opposite to the applied field i.e., (-ve) susceptibility. Paramagnetic minerals produce an induced magnetization in the same direction as the applied field i.e., (+ve) susceptibility. Certain few minerals have much stronger positive susceptibilities than the paramagnetic minerals and may also carry a strong remanent magnetization (ferromagnetic minerals). Such a spontaneous magnetization only arises if the ambient temperature is below the CuriC temperature. The direction of the remanence will be parallel (or occasionally anti-parallel) to the ambient field, it can therefore be used to infer the direction of the earth’s field at the time of acquisition. The spontaneous magnetization results fiom the ordering of atomic magnetic moments in the mineral, which can be of several types: ferromagnetic, antiferromagnetic and ferrimagnetic. Superparamagnetic minerals have a very short-lived remanence. At a given temperature, the magnetic ordering disappear. This is known as the N6el temperature for any given mineral. For a mineral that shows spontaneous magnetization, the remanence is destroyed and the NCel temperature is referred to as Curie temperature. Time has a direct effect on magnetization. NCel(1955) introduced the concept of relaxation time. If a grain has a direction of magnetization, then this direction will relax into the direction of an applied field in a specific time, the relaxation time. In super paramagnetic materials, the relaxation times are of the order of seconds or minutes, at the other extreme, the relaxation time of a single particle can be in millions or billions of years. NCel defined the relaxation time (t), for temperature below the Curie temperature as: where V is the volume of grain, B, is the coercivity, Js is the spontaneous magnetization, K is Boltzmann’s constant and T is the absolute temperature. The constant C is the frequency factor, which is about lo-’ S-’. This means that the relaxation time for a given magnetic grain is strongly dependent on the ambient temperature. As such a grain cools from its Curie temperature, it increases its relaxation time exponentially with the decrease in temperature. Close to the Curie temperature, the grain can act as a super paramagnetic substance, but by the time that the temperature has fallen to a level at which the relaxation time is of order of minutes, then any further temperature decrease means that the remanence will become blocked into the grain for a time longer than that of the laboratory experiment. This temperature is called blocking temperature (Tb). Any further decrease in temperature means that the remanence will be locked in the grain for increasingly long periods of time as cooling continues (Tarling and Hrouda, 1993). In a similar way, if the temperature is kept constant below that of its Curie temperature, a very small grain will have a very short relaxation time i.e, it will behave super paramagnetically. If it then grows, it will eventually reach a size at which its relaxation time is similar to that of the laboratory experiment and the ambient magnetic field direction become blocked in for the duration of the experiments. This volume is termed the blocking volume (vb). Further increase in size results in an exponential increase in the relaxation time range. Many magnetic properties are dependent on the size of the grains involved. If the grain is small (around 1 pm diameter) the magnetization within the grains is uniform in direction, usually along specific preferred crystallographic axes. However, if the grains are significantly greater than 10 pm in size, then each grain develops a number of volume elements. Each of these volume elements are termed a ”magnetic domain”. In the absence of external aligning forces, the domains arrange themselves to minimize their total magnetostatic energy. Two domains, for example, will be magnetized anti-parallel to each other, while four or more domains will try to form closure domains. The magnetic behaviours of a material containing only single domain sized particles is significantly different from that of one containing multidomain sized particles, even if the composition and the total quantity of the ferromagnetic materials are the same. In some multidomain grains, the presence of crystal lattice imperfections, due to impurities, vacancies etc., means that some of the domains cannot interact with neighboring domains as in more perfect multidomain particles, the magnetic behaviour is then close to that of single domain than multidomain particles and is termed pseudo-single domain behaviour. This behaviour tend to be commonest when the grain volume is larger than one but smaller than three domain volume (Tarling and Hrouda, 1993). 1.2. Ferromagnetic (s.0 Minerals Many authors such as Nagata (1961), Stacey & Banerjee (1974) and O’Reilly (1984) have discussed the magnetic properties and characteristics of minerals and rocks. Thompson and Oldfield (1986) have also described the magnetic properties of many relevant minerals. In igneous rocks the iron oxides are among the first phases to crystallize. In general, most iron oxides in basic melts have an initial titanomagnetite composition that tends to concentrate towards a solid solution of 5-6 % ~lvos~inel: 50-40 % magnetite as the magma cools and solidifies from > 1200’ C to about 800’ C. These solid solution grains have a composition that approaches that of the ilmenohaematite solid solution series which is characteristic of intermediate to acidic melts.The ~lvos~inel is one of the end-members of the titanomagnetite series, commonly oxidizes to ilmenite and magnetite. The ilmenite forms blebs within the magnetite or lamellae along the (1 11) planes of the magnetite. The theoretical magnetic mineralogy of igneous rocks is, simple as, neither ilmenite nor ~lvos~inel is ferromagnetic at room temperature, while the other two end-members, magnetite and haematite are strongly ferromagnetic. A small amount of impurities can affect the specific properties. If sulphur is present in the magmatic melt as both iron sulphides and oxides then form, the sulphur preferentially taking up the available iron. The magnetic properties of iron sulphides are particularly complex as only certain sulphides are ferromagnetic (s.1) and their magnetic properties are strongly dependent on the precise composition, crystal structure, grain size and rate of crystallization. Magnetite is usually stable in the moderately oxidizing conditions of unconsolidated sediments, but it becomes increasingly unstable as the redox conditions change as a result of later stage.Haematite occurs in monocrystalline and polycrystalline forms. Haematite may be associated with partially haematized grains of ilmenite, magnetite and other-ironbearing minerals. In addition haematite occurs as common early authigenic mineral, forming microcrystalline coatings, where authigenic haematite pseadomorphs magnetite (martite). Goethite is the main ferromagnetic (s.1) hydroxide. Haematite and magnetite commonly replace goethite. The magnetic iron oxides, hydroxides and sulphides in the unconsolidated sediments are, thus, a complex mixture that changes with pressure, temperature, time and redox conditions. Metamorphic minerals are clearly diverse reflecting a very wide range of pressure, temperature and redox condition. Many minerals are metastable or unstable at increased pressures, temperatures, therefore new metamorphic minerals develop, their composition reflect those of the original rocks, the pressure temperature conditions, the duration of burial of influence of circulating fluid of gases as in the serpentinization of oceanic crystal rocks. Generally, magnetite that is formed by disintegration of iron-bearing silicates at temperature below 550•‹C tends to be fairly pure. However in appropriate oxidation conditions at temperatures above 400•‹C magnetite is less stable and more common for ilmenite-haematite intergrowths. At temperatures > 600•‹C, magnetite can include more impurities with tectonic movements, especially the repeated uplift, burial influence the mineralogy as well as the physical properties. 1.3. Palaeomagnetic Assumption Palaeomagnetism or natural remanent magnetization (NRM) is acquired at the time the rock was formed. It can be used to determine both the direction and intensity of the geomagnetic field in the past. There are different mechanisms of which rocks can acquire NRM (Irving, 1964, Tarling, 1983, Collinson, 1983 and Piper, 1987). A. Types of NRM a Thermoremanent magnetization ( T w This is an NRM produced by cooling from above the Curie temperature (Tc) in the presence of a magnetic field. Just below the Curie temperature, the large single domain particles will have relaxation times of the order of few minutes, enabling their remanence to be measured, but the smaller grains will have shorter relaxation times of the order of many thousands or millions of years. In rocks, there is usually a whole range of grains with different volumes and compositions, reheating can result in overprinting by a partial thermal remanent magnetization (PTRM). Different components may, therefore, be investigated with stepwise thermal demagnetization.