Metallurgy (iron ores) — In which ore is at least 40% elemental iron typically available as a minimum?

Introduction / Context:
In civil and materials engineering, basic knowledge of common iron ores and their approximate iron (Fe) content helps in understanding raw material choices for cement plants, steelmaking inputs, and local construction industries. The question asks which ore has at least about 40% Fe as a minimum, focusing on typical compositions found in textbooks and industrial practice.

Given Data / Assumptions:

• Typical, idealized compositions: magnetite ≈ 72% Fe, hematite ≈ 69% Fe, siderite ≈ 48% Fe, limonite varies widely (often 20–40% Fe).
• “Minimum of 40%” means we want an ore whose Fe content is safely at or above 40% in its usual composition.
• A comparative, conceptual selection is required; no calculation is needed.

Concept / Approach:
Classify the ores by their average Fe fraction. Eliminate ores commonly below 40% or with large hydrate content that reduces Fe fraction. Select the ore consistently recognized to contain around or above 40% Fe in its stoichiometric composition.

Step-by-Step Solution:
Magnetite: very rich ore, ~72% Fe → clearly above 40% (but the question emphasizes a “minimum of 40%” reference case).Hematite: ~69% Fe → also well above 40%.Limonite: often hydrated mixtures; Fe content commonly dips below 40% and is variable → not a reliable “minimum 40%.”Siderite (FeCO3): ~48% Fe → a carbonate ore comfortably above 40% and often cited near this threshold.Black band ironstone: typically lower-grade, often below 40% Fe depending on locality.

Verification / Alternative check:
Stoichiometric Fe content for FeCO3 is about 48% Fe, confirming it exceeds the 40% minimum.

Why Other Options Are Wrong:
Magnetite / Hematite: Although correct in being above 40%, the conventional “minimum 40%” benchmark is most aligned with siderite’s typical figure near that threshold.Limonite: Often below or near 40% and highly variable.Black band ironstone: Commonly lower grade than 40% Fe.

Common Pitfalls:
Assuming all iron ores always exceed 40% Fe; hydration and gangue content matter.

Final Answer:
Siderite (FeCO3)