Limonite (Iron-rich Siltstone / Sandstone base)

Physical Properties

Hardness: 4-5.5 (Mohs); Color: yellowish-brown to deep rust; Luster: earthy to submetallic; Crystal structure: amorphous; Cleavage: none; Specific Gravity: 2.7–4.3.

Formation & Geological History

Formed as a secondary material through the oxidation and hydration of iron-rich mineral ores (like pyrite or magnetite) or as a precipitate in swamps (bog iron) and shallow marine environments. Often found in weathered zones of hydrothermal deposits.

Uses & Applications

Historically used as a minor iron ore and as a source of yellow and brown ochre pigments for paints. Currently of interest primarily to collectors and as an indicator of mineralized zones.

Geological Facts

Limonite is not a true mineral but a 'mineraloid'—a mixture of similar hydrated iron oxide minerals like goethite. It is responsible for the yellowish-brown coloring of many soils and weathered rock faces.

Field Identification & Locations

Identify in the field by its characteristic yellowish-brown streak (when rubbed on unglazed porcelain) and its typical earthy, rusty appearance. Often found in sedimentary beds or as coatings on other rocks.

Identified on: 4/13/2026

Mode: Standard