Limonite Pseudomorph after Pyrite

Physical Properties

Hardness: 4-5.5 (variable); Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Crystal structure: Isometric/Cubic (inherited from parent pyrite); Cleavage: None; Specific gravity: 2.7-4.3.

Formation & Geological History

Formed through the chemical weathering and oxidation of pyrite crystals. As iron-rich sulfide minerals are exposed to oxygen and water, the sulfur is leached away and replaced by iron oxides/hydroxides, preserving the original cubic shape of the pyrite.

Uses & Applications

A major ore of iron historically; currently primarily used as mineral specimens for education and collecting.

Geological Facts

Commonly known as 'Devil's Dice' or 'Prophecy Stones' in certain desert regions, these are textbook examples of pseudomorphism, where one mineral takes the external shape of another.

Field Identification & Locations

Identify in the field by its distinct cubic geometry paired with an earthy, non-metallic brown streak. Commonly found in sedimentary clay beds or weathered hydrothermal deposits.

Identified on: 4/24/2026

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