mineral

Limonite Pseudomorph after Pyrite

Goethite/Limonite FeO(OH)·nH2O replacing Pyrite FeS2

Hardness: 4.0-5.5 (Mohs); Color: Dark brown to rusty ochre; Luster: Dull/Earthy; Crystal Structure: Isometric (cubic habit preserved from original pyrite); Specific Gravity: 2.7-4.3

Hardness: 4
Color: Dark brown to rusty ochre
Luster: Dull/Earthy

Identified March 18, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5 (Mohs); Color: Dark brown to rusty ochre; Luster: Dull/Earthy; Crystal Structure: Isometric (cubic habit preserved from original pyrite); Specific Gravity: 2.7-4.3

Formation & geological history

Formed through chemical weathering and oxidation (pseudomorphism) where groundwater causes iron oxide minerals to replace a pre-existing pyrite crystal while maintaining the original cubic shape. These occur in various geological ages where sedimentary or metamorphic rocks are exposed to oxygen.

Uses & applications

Primarily for mineral collecting and education; historically used as a minor iron ore or as a pigment (brown ochre).

Geological facts

Commonly known as 'Propeller Stones' or 'Devil's Dice' when found in clusters. A 'pseudomorph' means 'false form' because the chemistry is limonite/goethite but the shape is a crystal system it cannot naturally form on its own.

Field identification & locations

Identify in the field by looking for distinct cubic shapes in dark brown rocky matrixes, often in weathered shales or limestones. Common in the eastern USA (Virginia, Pennsylvania) and desert regions worldwide.

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