Spheres of allophane intergrown with blue chrysocolla in iron oxide gossan. Field of view is about 8 cm, left to right. From the Reward Mine, Inyo County, California. (Photo by W. Cordua.)

Formula: (Al2O3)(SiO2)1.3-2.2.5-3H2O Amorphous


Allophane is a clay mineral with the general chemical composition of kaolinite but lacking a crystalline structure. It is formed by the breakdown of other alumino-silicates, such as feldspar, due to weathering or hydrothermal alteration.

DODGE COUNTY: A colorless, banded isotropic mineral found in the matrix of the Neda iron ore was tentatively identified as allophane by Hawley and Beavan (1934) based on optical and microchemical tests.