In economic geology, limonitic ore is an important ore of which metal?

Introduction / Context:
This earth science question focuses on the relationship between specific ores and the metals extracted from them. Limonite (often written limonitic ore when describing deposits) is a common iron ore and appears frequently in geology and general science examinations. Knowing which metal is obtained from limonitic ore helps you connect ore names to the metals used in industry and daily life.

Given Data / Assumptions:

• The ore in question is limonitic ore, meaning ore composed mainly of limonite minerals.
• The options list iron, aluminium, zinc, and cobalt as candidate metals.
• We assume basic familiarity with common commercial ores for these metals.

Concept / Approach:
Limonite is not a single mineral with a precise chemical formula but a mixture of hydrated iron(III) oxides, typically represented as FeO(OH)·nH2O or similar forms. It forms in the weathering environment where iron bearing minerals oxidise and hydrate. Limonitic ores are therefore important sources of iron, particularly in some lateritic and bog iron deposits. Aluminium is mainly obtained from bauxite, which is a mixture of aluminium hydroxide minerals such as gibbsite and boehmite. Zinc is commonly obtained from sphalerite (zinc sulfide), and cobalt is extracted from minerals such as cobaltite and various nickel cobalt ores. Hence, limonitic ore is associated primarily with iron, not with aluminium, zinc, or cobalt.

Step-by-Step Solution:

Verification / Alternative check:
Standard geology and metallurgy textbooks list limonite along with hematite and magnetite as important iron ores, especially in regions where weathering has altered primary iron minerals. They give compositions indicating a high proportion of iron with water molecules, and they describe iron extraction processes applicable to limonitic ores. In contrast, when discussing aluminium, these books focus on bauxite; for zinc, they highlight sphalerite, and for cobalt, they mention cobaltite and other specialized ores. These consistent associations confirm that limonitic ore is indeed an ore of iron.

Why Other Options Are Wrong:

Aluminium, occurring mainly as bauxite, is wrong because aluminium is commonly extracted from bauxite, which is composed of aluminium hydroxide minerals, not from limonite.

Zinc, occurring in minerals like sphalerite, is incorrect since zinc ores are typically sulfides or carbonates and are not described as limonitic ores.

Cobalt, found in minerals such as cobaltite, is also wrong because cobalt is obtained from distinct cobalt bearing ores and not from limonitic iron ore deposits as the primary metal.

Common Pitfalls:
Students sometimes confuse ore names because they may encounter several lists of ores and metals at once and mix them up. Another common mistake is assuming that any brownish ore might contain multiple metals. To avoid these errors, it helps to memorise key metal ore pairs: limonite and hematite for iron, bauxite for aluminium, sphalerite for zinc, and cobaltite for cobalt. In this question, recognising limonitic ore as an iron ore leads directly to the correct answer.

Final Answer:
Therefore, limonitic ore is primarily an ore of iron, occurring as hydrated iron oxides.