Iron Ore with Malachite and Jarosite

Description

This describes a mineral assemblage rather than a single mineral or rock type. It consists of iron oxide minerals (commonly Goethite, FeO(OH), or Hematite, Fe2O3) as the primary component, associated with the secondary copper carbonate Malachite (Cu2(CO3)(OH)2) and the potassium iron sulfate Jarosite (KFe3(SO4)2(OH)6). The iron oxides provide the bulk of the material, while malachite imparts green coloration and jarosite typically appears as yellow to yellowish-brown coatings or masses. This combination is indicative of a complex weathering environment, often in the oxidized zone of a polymetallic sulfide deposit.

How to Identify

Color

Iron oxides will be reddish-brown (Hematite) to yellowish-brown (Goethite). Malachite will be bright green to dark green. Jarosite will be yellow, yellowish-brown, or ochre-yellow.

Luster

Iron oxides can range from dull earthy to submetallic (Hematite). Malachite is typically silky, dull, or earthy. Jarosite is dull to vitreous.

Texture

Can be massive, botryoidal, reniform, or earthy for iron oxides and malachite. Jarosite often forms powdery coatings, crusts, or fine-grained masses.

Crystal Form

Iron oxides are often massive, botryoidal, or stalactitic. Malachite can be botryoidal, stalactitic, fibrous, or massive. Jarosite typically forms minute tabular or pseudo-cubic crystals, often as crusts or earthy aggregates.

Cleavage

Iron oxides (Goethite) have one perfect cleavage, but often not observed in massive forms. Hematite has no cleavage. Malachite has one perfect cleavage, but rarely seen. Jarosite has one perfect basal cleavage.

Geological Environment

Oxidized zones of sulfide ore deposits, particularly those rich in iron and copper. Often found in gossans, which are weathered caps over sulfide bodies.

Key Facts

• Hardness: Goethite: 5-5.5; Hematite: 5-6.5; Malachite: 3.5-4; Jarosite: 2.5-3.5 (Mohs scale).
• Specific Gravity: Goethite: 3.3-4.3; Hematite: 5.26; Malachite: 3.9-4.03; Jarosite: 3.15-3.26.
• Crystal System: Goethite: Orthorhombic; Hematite: Trigonal; Malachite: Monoclinic; Jarosite: Trigonal.
• Color: Reddish-brown, yellowish-brown, bright green, yellow, ochre.
• Luster: Earthy, dull, submetallic, silky, vitreous.
• Transparency: Opaque (iron oxides, malachite, jarosite in bulk), translucent (thin malachite crystals).
• Fracture: Conchoidal to uneven (iron oxides, malachite), splintery (jarosite).
• Cleavage: Goethite: One perfect; Hematite: None; Malachite: One perfect; Jarosite: One perfect basal.
• Composition: Iron oxides (FeO(OH), Fe2O3), copper carbonate hydroxide (Cu2(CO3)(OH)2), potassium iron sulfate hydroxide (KFe3(SO4)2(OH)6).

Quick Check

• Color: Reddish-brown, yellowish-brown, and bright green, often with yellow or ochre patches.
• Luster: Dull, earthy, submetallic, or silky.
• Streak: Iron oxides: Reddish-brown (Hematite) or yellowish-brown (Goethite). Malachite: Light green. Jarosite: Pale yellow.

Physical Characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, fibrous, earthy, powdery, crusts, minute tabular crystals.
• Cleavage Type: Perfect in Goethite, Malachite, and Jarosite; absent in Hematite.
• Fracture Type: Conchoidal, uneven, splintery.
• Tenacity: Brittle.
• Luster Type: Earthy, dull, submetallic, silky, vitreous.

Formation

This assemblage forms through the supergene alteration of primary iron-rich sulfide deposits (e.g., pyrite, chalcopyrite) or pre-existing iron oxide deposits that have been subsequently exposed to copper-bearing solutions and oxidizing conditions. Goethite and Hematite are common iron oxides. Malachite forms from the oxidation of copper sulfides (like chalcopyrite) in the presence of carbonate-rich waters. Jarosite forms in acidic, oxidizing environments, often associated with the weathering of iron sulfides, where potassium is also available.

Usage

Primarily as an iron ore, though the presence of malachite indicates potential for copper extraction. Jarosite is generally an undesirable gangue mineral in iron and copper processing due to its potassium and sulfate content, which can complicate metallurgical processes. Malachite, if abundant, can be a minor copper ore or used as an ornamental stone.

Age Distribution

Variable, depending on the primary iron ore deposit and subsequent alteration events. Malachite and Jarosite are typically secondary minerals formed much later than the primary iron oxides.

Where to Find

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Scientific Classification

Mineral Class

Oxides (Goethite, Hematite), Carbonates (Malachite), Sulfates (Jarosite)

Group

Iron Oxides, Carbonates, Sulfates

Crystal System

Orthorhombic (Goethite), Trigonal (Hematite, Jarosite), Monoclinic (Malachite)

Chemical Formula

FeO(OH) (Goethite), Fe2O3 (Hematite), Cu2(CO3)(OH)2 (Malachite), KFe3(SO4)2(OH)6 (Jarosite)

Composition

Hydrated iron oxide, anhydrous iron oxide, basic copper carbonate, basic potassium iron sulfate.

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