Goethite with Limonite

Description

Goethite is an iron oxyhydroxide mineral, α-FeO(OH), crystallizing in the orthorhombic system. Limonite is not a true mineral but a field term for a mixture of hydrous iron oxides, primarily goethite, often amorphous or cryptocrystalline, with variable water content (FeO(OH)·nH2O). It typically forms as a weathering product and is characterized by its earthy, often porous, and concretionary habits. The distinction between goethite and limonite is primarily based on crystallinity and water content, with goethite being crystalline and limonite being a more general term for amorphous or poorly crystalline hydrous iron oxides.

How to Identify

Color

Yellowish-brown to dark brown, blackish-brown, or reddish-brown. Limonite is typically more yellowish-brown.

Luster

Goethite can be adamantine, submetallic, silky (fibrous varieties), or dull/earthy. Limonite is typically dull and earthy.

Texture

Goethite can be fibrous, botryoidal, stalactitic, reniform, or massive. Limonite is typically earthy, porous, concretionary, mammillary, or stalactitic.

Crystal Form

Goethite forms prismatic, acicular, or tabular crystals, often radiating or fibrous. Limonite is typically amorphous or cryptocrystalline, lacking distinct crystal forms.

Cleavage

Goethite exhibits perfect cleavage on {010} and good cleavage on {100}. Limonite, being an aggregate, does not exhibit true cleavage.

Geological Environment

Found in the oxidized zones of iron ore deposits, gossans, bogs, swamps, and as a weathering product of iron-rich rocks and minerals. Common in sedimentary environments where iron-rich waters precipitate.

Key Facts

• Hardness: Goethite: 5-5.5 (Mohs); Limonite: 1-5.5 (variable, often softer and earthy)
• Specific Gravity: Goethite: 3.3-4.3; Limonite: 2.7-4.3 (variable)
• Crystal System: Goethite: Orthorhombic; Limonite: Amorphous to cryptocrystalline (no defined crystal system)
• Color: Yellowish-brown, brown, blackish-brown, reddish-brown
• Luster: Adamantine, submetallic, silky (fibrous), dull, earthy
• Transparency: Opaque
• Fracture: Goethite: Uneven to subconchoidal; Limonite: Earthy, conchoidal
• Cleavage: Goethite: Perfect on {010}, good on {100}; Limonite: None (aggregate)
• Composition: Goethite: Iron oxyhydroxide (FeO(OH)); Limonite: Hydrous iron oxides, predominantly goethite, with variable water content (FeO(OH)·nH2O)

Quick Check

• Color: Yellowish-brown to dark brown, blackish-brown
• Luster: Dull, earthy, submetallic, silky
• Streak: Yellowish-brown

Physical Characteristics

• Crystal Habit: Goethite: Prismatic, acicular, tabular, fibrous, radiating, botryoidal, stalactitic, reniform, massive. Limonite: Earthy, porous, concretionary, mammillary, stalactitic, massive.
• Cleavage Type: Goethite: Perfect {010}, good {100}; Limonite: None
• Fracture Type: Goethite: Uneven to subconchoidal; Limonite: Earthy, conchoidal
• Tenacity: Goethite: Brittle; Limonite: Friable to brittle
• Luster Type: Adamantine, submetallic, silky, dull, earthy

Formation

Goethite and limonite are secondary iron oxyhydroxide minerals that form through the weathering and oxidation of primary iron-bearing minerals (e.g., pyrite, siderite, magnetite, hematite) in the presence of water and oxygen. They are common products of hydrothermal alteration, supergene enrichment, and bog iron formation. Limonite is specifically a mixture of hydrous iron oxides, predominantly goethite, but also lepidocrocite and ferrihydrite, often amorphous or cryptocrystalline.

Usage

Historically, limonite was a significant iron ore, particularly for early iron smelting. Today, it is still used as a pigment (ochre) in paints, ceramics, and cosmetics. Goethite is also a minor iron ore and a source of pigment. Both are important in soil science and environmental studies due to their role in heavy metal adsorption and water purification.

Age Distribution

Forms continuously in surficial environments; therefore, its age varies widely from geologically recent to ancient, depending on the specific deposit.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Oxides and Hydroxides

Group

Goethite Group (for Goethite)

Crystal System

Goethite: Orthorhombic; Limonite: Amorphous/Cryptocrystalline

Chemical Formula

Goethite: FeO(OH); Limonite: FeO(OH)·nH2O (general formula)

Composition

Hydrous iron oxides

Explore Goethite with Limonite