How to identify Iron Ore

To correctly identify Iron Ore (Iron-rich sedimentary rock (e.g., Limonite, Goethite)), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable depending on the dominant iron mineral: Hematite is typically reddish-brown to steel-gray to black. Magnetite is black. Goethite and Limonite are yellowish-brown to dark brown. Siderite is yellowish-brown to grayish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable: Hematite can be dull, earthy, submetallic, or metallic. Magnetite is metallic to submetallic. Goethite and Limonite are earthy to dull. Siderite is vitreous to pearly.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable: Can be massive, earthy, oolitic, pisolitic, botryoidal, reniform, or banded (as in BIFs). Individual grains may be fine-grained to coarse-grained.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Iron ore is typically a rock composed of various iron minerals. The individual minerals have distinct crystal forms: Hematite (rhombohedral, tabular, massive), Magnetite (isometric, octahedral, massive), Goethite (orthorhombic, acicular, fibrous, massive), Siderite (rhombohedral, massive). Limonite is amorphous or cryptocrystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Variable depending on the constituent minerals: Hematite has no distinct cleavage but can show parting. Magnetite has no cleavage. Goethite has perfect {010} cleavage. Siderite has perfect rhombohedral cleavage {1011}. Limonite has no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Banded Iron Formations (BIFs) are found in ancient Precambrian shield areas. Oolitic ironstones are found in shallow marine sedimentary sequences. Lateritic iron ores are found in tropical and subtropical regions with extensive weathering profiles. Bog iron deposits are found in wetlands, swamps, and shallow lakes. Some deposits are associated with hydrothermal alteration zones.

Key Facts

Hardness

Variable, depending on the dominant mineral: Hematite (5-6), Magnetite (5.5-6.5), Goethite (5-5.5), Limonite (1-5.5, often soft and earthy), Siderite (3.5-4.5).

Specific Gravity

Variable: Hematite (4.9-5.3), Magnetite (5.18), Goethite (3.3-4.3), Limonite (2.7-4.3), Siderite (3.96). Generally higher than most common rocks.

Crystal System

Variable, depending on the dominant mineral: Hematite (Trigonal), Magnetite (Isometric), Goethite (Orthorhombic), Siderite (Trigonal). Limonite is amorphous or cryptocrystalline.

Color

Reddish-brown, steel-gray, black, yellowish-brown, dark brown.

Luster

Earthy, dull, submetallic, metallic, vitreous, pearly.

Transparency

Opaque to translucent (thin splinters of some goethite/siderite).

Fracture

Variable: Uneven, conchoidal, splintery, earthy.

Cleavage

Variable: Hematite (none distinct), Magnetite (none), Goethite (perfect {010}), Siderite (perfect rhombohedral {1011}), Limonite (none).

Composition

Primarily iron oxides (Fe2O3, Fe3O4), iron oxyhydroxides (FeO(OH)·nH2O), and iron carbonates (FeCO3), often mixed with silica (SiO2) and other impurities.

Physical characteristics

• Crystal Habit: Variable: Massive, earthy, botryoidal, reniform, oolitic, pisolitic, micaceous (specular hematite), octahedral (magnetite), acicular/fibrous (goethite), rhombohedral (siderite).
• Cleavage Type: Variable: None (hematite, magnetite, limonite), perfect {010} (goethite), perfect rhombohedral {1011} (siderite).
• Fracture Type: Uneven, conchoidal, splintery, earthy.
• Tenacity: Brittle (most crystalline forms), earthy (limonite).
• Luster Type: Metallic, submetallic, earthy, dull, vitreous, pearly.

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