How to identify Iron Ore with Secondary Minerals

To correctly identify Iron Ore with Secondary Minerals (Iron oxides (e.g., Hematite, Goethite) with secondary sulfates or carbonates), 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. 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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Iron oxides can be metallic (hematite, magnetite) to dull/earthy (goethite). Sulfates and carbonates typically have a vitreous, pearly, or dull luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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