Iron Ore with Secondary Minerals

Sedimentary/Metamorphic (depending on origin and alteration)

Iron oxides (e.g., Hematite, Goethite) with secondary sulfates or carbonates

Also known as: Oxidized Iron Ore, Sulfidic Iron Ore, Carbonate-rich Iron Ore

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Description

Iron ore with secondary minerals refers to deposits where the primary iron-bearing minerals (typically hematite, Fe2O3; goethite, FeO(OH); magnetite, Fe3O4; or siderite, FeCO3) are intimately associated with or have been altered to include significant amounts of secondary sulfate or carbonate minerals. These secondary minerals are not part of the primary iron ore formation but rather result from later geological processes such as weathering, hydrothermal alteration, or diagenesis. The specific secondary minerals present depend on the geological environment, the composition of circulating fluids, and the original mineralogy of the iron ore and surrounding rocks.

How to Identify

Color: Highly variable. Iron oxides typically range from steel-gray to reddish-brown to black. Secondary sulfates can be white, yellow, brown (e.g., gypsum, jarosite). Secondary carbonates can be white, gray, brown, or pink (e.g., calcite, siderite, ankerite). The overall color will be a mix of these components.
Luster: Iron oxides can be metallic (hematite, magnetite) to dull/earthy (goethite). Sulfates and carbonates typically have a vitreous, pearly, or dull luster.
Texture: Can be massive, oolitic, pisolitic, banded, or earthy. The presence of secondary minerals might manifest as veins, coatings, disseminations, or distinct crystalline aggregates within the iron ore matrix.
Crystal Form: Iron oxides can be massive, botryoidal, reniform, micaceous (specular hematite), or granular. Secondary sulfates and carbonates can exhibit various crystal forms, from tabular to prismatic to rhombohedral, or be massive and granular.
Cleavage: Hematite typically has no distinct cleavage but can show parting. Goethite has perfect {010} cleavage. Magnetite has no cleavage. Sulfates (e.g., gypsum) often have perfect cleavage. Carbonates (e.g., calcite, siderite) exhibit perfect rhombohedral cleavage.
Geological Environment: Found in various environments where iron ores form and subsequently undergo alteration. This includes weathered zones of primary iron deposits, hydrothermal alteration zones, sedimentary basins with post-depositional fluid flow, and areas with sulfide oxidation.

Key Facts

Hardness: Variable. Hematite: 5-6.5. Goethite: 5-5.5. Magnetite: 5.5-6.5. Calcite: 3. Gypsum: 2. Jarosite: 2.5-3.5.
Specific Gravity: Variable. Hematite: 5.26. Goethite: 3.3-4.3. Magnetite: 5.18. Calcite: 2.71. Gypsum: 2.32. Jarosite: 3.15-3.26.
Crystal System: Variable, depending on the constituent minerals. Hematite: Trigonal. Goethite: Orthorhombic. Magnetite: Isometric. Calcite: Trigonal. Gypsum: Monoclinic. Jarosite: Trigonal.
Color: Highly variable, reflecting the mixture of iron oxides and secondary sulfates/carbonates.
Luster: Ranges from metallic to submetallic, earthy, vitreous, or pearly.
Transparency: Opaque for iron oxides. Translucent to transparent for many secondary sulfates and carbonates.
Fracture: Variable. Hematite: uneven to subconchoidal. Goethite: uneven. Magnetite: subconchoidal to uneven. Calcite: conchoidal. Gypsum: splintery.
Cleavage: Variable. Hematite: none to indistinct parting. Goethite: perfect {010}. Magnetite: none. Calcite: perfect rhombohedral {1011}. Gypsum: perfect {010}, good {100}, distinct {111}.
Composition: Primarily iron oxides (Fe2O3, FeO(OH), Fe3O4) with varying amounts of secondary sulfates (e.g., CaSO4·2H2O, KFe3(SO4)2(OH)6) or carbonates (e.g., FeCO3, CaCO3, CaMg(CO3)2).

Quick Check

Color: Variable, often a mix of reddish-brown, black, steel-gray with white, yellow, or pink patches/veins.
Luster: Can be metallic, submetallic, earthy, vitreous, or pearly, depending on the dominant minerals.
Streak: Hematite: reddish-brown. Goethite: yellowish-brown. Magnetite: black. Secondary sulfates/carbonates will have their characteristic streak (e.g., white for calcite, gypsum).

Physical Characteristics

Crystal Habit: Can be massive, granular, botryoidal, reniform, micaceous, or crystalline, depending on the specific minerals present. Secondary minerals may form distinct crystals, coatings, or veinlets.
Cleavage Type: Variable, from none to perfect, depending on the specific minerals. For example, perfect rhombohedral for calcite, perfect {010} for goethite and gypsum.
Fracture Type: Uneven, subconchoidal, or splintery, depending on the dominant minerals.
Tenacity: Variable. Iron oxides can be brittle. Gypsum is sectile. Calcite is brittle.
Luster Type: Metallic, submetallic, earthy, vitreous, pearly.

Formation

Iron ores primarily form through sedimentary processes (e.g., Banded Iron Formations, bog iron ores), magmatic segregation, or hydrothermal alteration. Secondary minerals like sulfates (e.g., gypsum, jarosite) and carbonates (e.g., siderite, ankerite, calcite) form through subsequent alteration processes. Sulfates often result from the oxidation of primary sulfide minerals (e.g., pyrite) associated with the iron ore, or from interaction with sulfate-rich groundwater. Carbonates can form from the interaction of iron-rich solutions with carbonate-rich fluids or rocks, or as a product of weathering in carbonate-rich environments.

Usage

The primary use is as a source of iron for steel production. The presence and type of secondary minerals can significantly impact the processing of the ore. Sulfates can be detrimental, requiring removal due to their corrosive nature and potential to introduce sulfur into steel. Carbonates can act as a flux in smelting but may also increase energy consumption if present in large quantities.

Age Distribution

Precambrian to Cenozoic, with major deposits in the Precambrian (Banded Iron Formations) and Phanerozoic (e.g., lateritic iron ores, bog iron ores). Secondary minerals can form at any geological age post-primary iron ore deposition.

Where to Find

Mesabi Range, Minnesota, USA

Known for its vast Banded Iron Formations, which have undergone significant weathering and oxidation, leading to enriched hematite and goethite ores. Secondary minerals like carbonates can be found in unoxidized or partially oxidized zones.

Carajás Mine, Pará, Brazil

One of the world's largest iron ore deposits, primarily high-grade hematite. Secondary alteration processes can introduce other minerals.

Pilbara Region, Western Australia

Extensive Banded Iron Formations and enriched hematite deposits. Weathering and supergene enrichment processes can lead to the formation of secondary minerals.

Krivoy Rog Basin, Ukraine

Significant iron ore deposits, including both primary and metamorphosed ores, where secondary alteration can introduce various minerals.

Iron Ore Deposits associated with sulfide mineralization

Globally, iron ore deposits that are spatially or genetically linked to sulfide mineralization (e.g., volcanogenic massive sulfide deposits, porphyry deposits) are prone to developing secondary sulfates due to the oxidation of sulfides.

Finding Tips

Look for color variations

The presence of secondary minerals often introduces distinct color variations (e.g., white, yellow, green, or pink patches) within the typical reddish-brown or black iron ore matrix.

Examine for crystalline growths or coatings

Secondary minerals may form as distinct crystals, coatings on fracture surfaces, or veinlets cutting through the iron ore.

Test for effervescence with acid

If carbonates are suspected, a drop of dilute hydrochloric acid (HCl) will cause effervescence (fizzing) due to the release of CO2. This is a key indicator for minerals like calcite or siderite.

Consider the geological context

In areas with known sulfide mineralization or significant weathering, the likelihood of finding secondary sulfates or carbonates is higher. Look for evidence of alteration zones.

Observe luster and hardness differences

Secondary minerals often have different lusters (e.g., vitreous, pearly) and hardnesses compared to the primary iron oxides, which can aid in their identification.

Similar Rocks

Banded Iron Formation (BIF)

Chert and iron oxides (hematite, magnetite)

Also known as: Taconite (when metamorphosed)

Lateritic Iron Ore

Goethite, hematite, clay minerals, gibbsite

Also known as: Limonite Ore

Bog Iron Ore

Amorphous hydrated iron oxides (goethite, ferrihydrite)

Also known as: Limonite

Siderite Ore

Siderite (FeCO3)

Also known as: Iron Carbonate Ore

Scientific Classification

Mineral Class: Oxides (for hematite, magnetite, goethite), Sulfates (for gypsum, jarosite), Carbonates (for calcite, siderite)
Group: Iron Oxides, Hydrous Iron Oxides, Sulfates, Carbonates
Crystal System: Variable (Trigonal, Orthorhombic, Isometric, Monoclinic)
Chemical Formula: Variable, e.g., Fe2O3 (hematite), FeO(OH) (goethite), Fe3O4 (magnetite), CaSO4·2H2O (gypsum), KFe3(SO4)2(OH)6 (jarosite), CaCO3 (calcite), FeCO3 (siderite)
Composition: A mixture of iron oxides and hydroxides with various secondary sulfate and/or carbonate minerals.

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