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Property Overview

The key properties of magnetite are compared and contrasted to those of some common mineral fillers. A cursory look at some chemical and physical properties reveals this to be a rather special material. Next we will look at some of the properties in more detail and relate the attributes to extant or emergent applications (Table 1).

Table 1 Properties of magnetite in contrast to common fillers such as calcium carbonate and talc

Property

Typical mineral filler

Magnetite

Color

White

Black

Density

2.5-3.0 g cm—3

5.2 g cm—3

Mohs hardness

2-3

5.5-6

Attraction to a magnet

No

Yes

Electrical conductivity

Insulator

Semiconductor

Chemical composition

Carbonates and silicates

Fe3O4 (an oxide)

Volumetric heat capacity

2.1 kJL—1 K—1

3.8 kJ L—1 K—1

Microwave heatable

No

Yes

Radiation blocking

No

Yes

Chemistry

Magnetite is often written as Fe3O4 but is more correctly expressed as FenFe2 mO4 showing that it contains iron in the 2+ and 3+ oxidation states making it the only “stable” iron oxide containing Fe2+. It is chemically inert but is dissolved slowly by acids (Tremaine and LeBlanc 1980) or bases at high temperatures (Ziemniak et al. 1995). The dissolution rate depends on the surface area, pH, complexing ligands, presence of oxygen, and dissolved salts such as Fe2+, UV light, and temperature.

When heated above 300 °C, synthetic nano-magnetite experiences conversion to maghemite (y-Fe2O3) and then to hematite (a-Fe2O3) (Sidhu et al. 1981). Over 500 °C, it converts directly to hematite. However, natural magnetite is found to be much more stable. In fact, it is metastable under atmospheric conditions, which is how massive natural deposits have survived. Mechanical energy can also convert magnetite to hematite. Synthetic one-micron magnetite was ball milled and thereby completely converted to a-Fe2O3 after 70 h (Sorescu 1998). This supports the above findings that finer particles are less stable. It might be expected that the high surface area of finer particles facilitates oxidation to hematite.

Interestingly, it has been found that encapsulation of magnetite within thermoplastics can alter the surface oxidation state, but in some cases, the polymer helps preserve the Fe2+:Fe3+ ratio and the half-metal nature of the surface (Parkinson etal. 2012; Fig. 1).

Surface Chemistry

Magnetite is amphoteric, i.e., it can behave as an acid or a base. The reactive sites at the surface are hydroxyl groups (—OH) and the density of groups has been measured as 5 per square nanometer (Parkinson et al. 2012). Freshly ground natural magnetite has a pH of 6.5 (Milonjic et al. 1983). Organosilanes, titanates, sulfonates, and carboxylic acid groups (Korolev et al. 2002) have all been reported as surface treatments that bind effectively.

Safety

Pure magnetite is considered safe for food contact applications. In fact, it has been shown to exist within every living organism from bacteria to plants and humans.

Stability of natural and nano-magnetite

Fig. 1 Stability of natural and nano-magnetite

Such biogenic magnetite is crucial to the guidance systems of bats, birds, and bacteria. All naturally occurring unmodified minerals are automatically TSCA listed and REACH exempt.

 
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