Question

what are titanohematites? and in detail discuss their magnetism.

Answer 1

Titanohematites: In most igneous rocks, titanohematites and their oxidation products constitute a lesser portion of ferromagnetic minerals than do titanomagnetites (oxidation products). But for highly silicic or highly oxidized igneous rocks, hematite can be the dominant ferromagnetic mineral. In addition, hematite is always the dominant or exclusive ferromagnetic mineral in red sediments, a major source of paleomagnetic data.

The titanohematites are generally opaque minerals with a magnetic structure most easily described by using the hexagonal system.Layers of approximately hexagonal-close-packed O^–2 anions are parallel to the (0001) basal plane. For each 18 O^–2 anions, there are 18 potential cation sites in octahedral coordination with six surrounding O^–2 anions. In titanohematites, two thirds of these cation sites are occupied.

For hematite ), all cations are Fe³⁺ and occur in (0001) layers alternating with layers of O^–2 anions. Atomic magnetic moments of Fe³⁺ cations lie in the basal plane orthogonal to the [0001] axis. Atomic moments are parallel coupled within (0001) planes but approximately antiparallel coupled between adjacent layers of cations.

In addition, the magnetization from canting, some naturally occurring hematite has additional magnetization referred to as defect ferromagnetism, perhaps arising from lattice defects or nonmagnetic impurity cations. While the origins of the two contributions to net magnetization are complex and not fully understood, the effect is one of weak ferromagnetism with j _s ≈ 2–3 G (2–3 × 10³ A/m). The effective Néel temperature (temperature at which exchange coupling within an antiferromagnetic mineral disappears) of hematite is 680°C.

Magnetic moments of Fe²⁺ cations within a particular basal plane are parallel-coupled with magnetic moment oriented along the [0001] axis. Ionic substitution in the titanohematite series is exactly as in titanomagnetites, with Ti⁴⁺ substituting for Fe³⁺ and one remaining Fe cation changing valence from Fe³⁺ to Fe²⁺. The generalized formula is Fe_2–xTixO₃, where X ranges from 0.0 for hematite to 1.0 for ilmenite. The “Curie” temperature has a simple linear dependence on composition. But saturation magnetization, j _s, (adjusted to 0°K) varies in a complex fashion. The explanation lies in the distribution of cations in intermediate composition titanohematites. It should be noted that titanohematites with x > 0.8, like titanomagnetites with high Ti content, are paramagnetic at or above room temperature. For 0.0 < x < 0.45, titanohematites retain the canted antiferromagnetic arrangement of hematite, with Fe and Ti cations equally distributed amongst cation layers. Over this range of compositions, saturation magnetization is approximately constant and low (j _s ≈ 2 G). However, for x > 0.45, Fe and Ti cations are no longer equally distributed; Ti cations preferentially occupy alternate cation layers. Because Ti cations have no atomic magnetic moment, antiparallel coupling of two sublattices with unequal magnetic moment develops, and titanohematites with 0.45 < x < 1.0 are ferrimagnetic. Intermediate titanohematites also possess an additional (mercifully) uncommon magnetic property: selfreversal of thermoremanent magnetism. Depending on exact composition and cooling rate, intermediate composition titanohematites can acquire remanent magnetism antiparallel to the magnetic field in which they cool below the Curie temperature. This self-reversing property is now recognized as uncommon because titanohematites of this composition are rarely the dominant ferromagnetic mineral in a rock.