Iridescent Hematite

Chi Ma and George R. Rossman

Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena CA  91125-2500, U.S.A.


Rainbow hematite

Abstract


Iridescent hematite, also called rainbow hematite, displays a variety of beautiful interference colors. Usually, a thin film comprised of aluminum and phosphate was found to coat such hematite, although one example of a coating with aluminum and arsenate was discovered. The thin films that range from tens to hundreds of nanometers thick are believed to cause the color. High resolution secondary electron imaging shows that the thin film consists of nano-crystals arranged in three directions (120º apart). The rod-shaped crystals have a width from 5 to 35 nm and display the same morphology on all samples irrespective if they are phosphate or arsenate. The composition of the bulk crystals coating the hematite have an Al to P in a ratio that varies from 2.2 to 3.8. The minute crystals have failed to produce either an X-ray powder diffraction pattern, an electron back-scatter diffraction pattern, or a Raman spectrum.


nanocrystals

SEM image of the Al- and P-containing nanocrystals on rainbow hematite from Minas Gerais, Brazil


Additional pictures of iridescent hematite

GRR 1462, Mina do Andrade, Minas Gerais, Brazil Picture SEM   SEM2
GRR 1960, El Savaldor Mine, Mexico Picture SEM
GRR 2377, Pico Mine, Minas Gerais, Brazil Picture SEM
GRR 2378, Graves Mountain., Georgia, USA Picture SEM
GRR 2380, Quartz Mountain, Oregon, USA Picture SEM
GRR 2522, Whittaker Mine, California, USA Picture SEM
CIT-11952, Alaska, USA Picture SEM
CIT-12889, Dequesne, Arizona, USA Picture SEM
CIT-14296, Ria Marina, Elba, Italy Picture SEM


Treatment for 30 minutes in 98°C 1.5 M HNO3

Before treatment SEM
After treatment SEM