mineral

Limonite after Marcasite (Propeller Conchoid)

Goethite/Limonite pseudomorph after Marcasite (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5; Color: Dark brown to blackish-tan; Luster: Submetallic to dull/earthy; Crystal structure: Orthorhombic (inherited from marcasite); Specific gravity: 3.6-4.0

Hardness: 4
Color: Dark brown to blackish-tan
Luster: Submetallic to dull/earthy

Identified May 24, 2026More mineral →

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Physical properties

Hardness: 4.0-5.5; Color: Dark brown to blackish-tan; Luster: Submetallic to dull/earthy; Crystal structure: Orthorhombic (inherited from marcasite); Specific gravity: 3.6-4.0

Formation & geological history

Forms through the chemical weathering (oxidation) and hydration of original marcasite or pyrite crystals. This usually occurs in sedimentary environments like limestone or chalk deposits over thousands of years.

Uses & applications

Primarily collected as mineral specimens; historically used as a low-grade iron ore and as a pigment for ochre/brown dyes.

Geological facts

These are known as 'pseudomorphs' (meaning 'false form') because the iron minerals have perfectly replaced the original crystal structure while retaining the physical shape of the marcasite.

Field identification & locations

Identify by looking for 'propeller' or 'cogwheel' shapes in weathered sedimentary rock or loose soil. Common in Egypt (White Desert) and parts of the UK and USA. They are non-magnetic and often have a yellow-brown streak.