Goethite

specimen number:
8311086

location:
Milpillas Mine, Sonora, Mexico

description:
Botryoidal goethite covering most of this matrix. This is attractively iridescent and in great condition.

The Milpillas Mine in Sonora, Mexico, is a copper mine located near the town of Santa Cruz close to the United States border. The deposit is part of a porphyry copper system with a large oxidized zone that produced many secondary copper minerals. Commercial mining began in 2006 and the mine quickly became famous among mineral collectors for producing exceptional crystals of azurite, along with malachite, cuprite, and other copper minerals.

Iridescence in goethite is caused by the interference of light reflecting from very thin surface layers on the mineral. These layers may form through oxidation, hydration, or slight structural changes on the surface of the crystal. When light strikes the thin film, different wavelengths interfere with each other and produce shifting colors such as blue, purple, green, and gold. This effect is similar to the colors seen in soap bubbles or thin oil films and is sometimes called thin film interference.

Questions about this specimen?
Send an email or call us: 314-341-1811.