Which of the following is an iron ore ?

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Correct — D, Limonite. Limonite is a hydrated iron oxide, written FeO(OH)·nH₂O, and it is one of the three principal ores from which iron has actually been smelted — the other two being haematite and magnetite — a use documented from at least 400 BC. Strictly, modern mineralogy no longer treats limonite as a single mineral species at all: it is a field name for an intergrown mixture of hydrated iron oxides — goethite, lepidocrocite, akaganeite and jarosite — and telling the components apart needs X-ray diffraction. Its field identifiers are all iron identifiers. It is medium to dark yellowish-brown, it occurs in earthy, concretionary, botryoidal or stalactitic masses rather than in crystals, and it leaves a yellowish-brown streak on unglazed porcelain — which is precisely how a geologist separates it from haematite, whose streak is red, and magnetite, whose streak is black. The name itself records how it forms: 'limonite' is from the Greek leimṓn, 'wet meadow', because it precipitates as bog iron ore in marshes where iron-rich groundwater meets air. It is also the everyday product of iron weathering — the rust on a gate, the brown stain down a rock face and the brown iron cap of a laterite profile are all limonite. In the four-fold classification of iron ores used across Indian geography teaching — magnetite, haematite, limonite, siderite — limonite is squarely on the list, and none of the other three options is.

Why the other options are wrong — (a) Bauxite is the world's main source of aluminium and gallium — a sedimentary rock built mostly from the aluminium minerals gibbsite Al(OH)₃, boehmite and diaspore. It is a real trap because bauxite is dull reddish-brown, and that colour comes from the iron oxides goethite and haematite mixed into it as impurities. But the metal it is worked for is aluminium. The French geologist Pierre Berthier described it at Les Baux in Provence in 1821, and the name bauxite was established from that locality in 1861.  (b) Cinnabar is mercury(II) sulphide, HgS, and is the principal ore of mercury. It is unmistakable in a specimen tray — cochineal-red, adamantine, a scarlet streak, Mohs hardness only 2 to 2.5, and an extraordinary specific gravity of about 8.2 — and it is the mineral source of the vermilion pigment. It contains no iron whatsoever, so it cannot be an iron ore under any reading.  (c) The one option that can talk a strong candidate out of the right answer. Pyrite genuinely is an iron mineral — FeS₂, iron(II) disulphide, the most abundant sulphide mineral on Earth. But it is not worked for its iron: the sulphur is the point, and pyrite has been weathered, leached and roasted since the 15th century to make copperas and sulphuric acid, becoming the dominant sulphuric-acid feedstock by the 19th century. Its brass-yellow metallic lustre earned it the name fool's gold. Fittingly, weathering converts pyrite crystals into limonite pseudomorphs — the two minerals are chemically linked, but only one goes to the blast furnace.

Concept: An ore is not simply a mineral that contains a metal. A mineral is called an ore only when the metal can be won from it at a profit and in a form industry can actually use, which turns on grade, ease of reduction and what impurities ride along. Iron makes the distinction sharply. The four minerals taught as iron ores are magnetite (Fe₃O₄, which works out to about 72% iron by formula weight), haematite (Fe₂O₃, about 70%), limonite (hydrated iron oxide, variable and lower) and siderite (FeCO₃, about 48%). Three are oxides and one a carbonate, and every one of them reduces cleanly with carbon in a blast furnace. Pyrite carries more iron per tonne than siderite does, yet it is not on the list, because it is a sulphide: sulphur carried into steel makes the metal crack when worked hot, and driving it off costs more than the iron recovered is worth. So pyrite is mined for sulphur, and any iron is a by-product.

State PCS 2024 Q1 — Geography PYQ with Answer

State PCS 2024 Q1 — Geography PYQ with Answer