Which of the following is not an ore of iron? (a) Magnetite (b) Limonite (c) Cassiterite (d) Haematite

Magnetite is a highly magnetic mineral and one of the main iron ores. It is composed of iron oxide with the chemical formula (Fe_3O_4). With its dark, iron-black hue, this mineral is easily recognizable. Magnetite is often found in igneous and metamorphic rocks, along with mineral deposits formed through volcanic activity.

In magnetite ores, iron is present in the form of Fe^2+ and Fe^3+. These ores are significant due to their high iron content, typically between 70% to 72.4%. This makes them highly sought after for the steel-making process. Due to its magnetic properties, magnetite can be separated from other minerals using magnetic separation techniques, which adds to the efficiency of the extraction process.

Haematite, also spelled hematite, is another important ore of iron. Its chemical formula is (Fe_2O_3), and it can exhibit a metallic or earthy appearance. Its distinctive red-brown streak makes it distinguishable from other minerals.

It is less magnetic than magnetite, which means different extraction methods are needed, such as gravity or heavy media separation. Haematite is often found in sedimentary deposits and is a common iron ore, with around 60% to 70% iron content. The extraction of iron from haematite has been historically significant and continues to be a key part of iron production today.

Limonite is a generic term for hydroxide iron ore minerals and is most often recognized by its yellow-brown, rust-like appearance. It typically forms from the chemical weathering of iron-rich minerals, such as the decomposition of other iron ores. The general formula is often described as (FeO(OH)·nH_2O), indicating that it is composed of iron oxide and variable amounts of water.

Although limonite contains lower levels of iron compared to magnetite and haematite, with about 40% to 60% iron content, it is still an important iron ore. It is used in the production of iron and steel, as well as for pigment. Limonite's variable composition sometimes poses challenges for industrial extraction, but it remains a valuable source of iron.

Cassiterite stands out from the other minerals listed because it is not an iron ore. With its chemical formula (SnO_2), cassiterite is an ore of tin. Often found in hydrothermal veins and pegmatites, the mineral derives its name from the Greek word for tin, 'kassiteros'.

Cassiterite is a key ore in tin mining and has a high density, which aids in its extraction through gravity separation methods. The economic significance of cassiterite lies in its use in the production of tin, which is crucial in the manufacturing of alloys, solder, and electronic components. Unlike the iron ores, cassiterite does not contribute to iron production and therefore is the correct answer to the given exercise.