Tag Archives: iron

Geo-pic of the week: Iridescent Iron Oxide on Novaculite

iridescent iron edited(photo courtesy of John David McFarland)

The photo above illustrates surface iridescence.  Iridescence is a play of colors caused by the interference of light waves.  This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens.  Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors.  Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves.  A color is seen when the waves interfere constructively.  The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.

Chalybeate spring along Cedar Creek Trail, Petit Jean State Park


Chalybeate (pronounced ka-lib-e-it), or mineral springs, is a term used when water is highly saturated with iron that precipitates out of solution forming an iron-rich stain along the surface of an outcrop. These iron-rich waters are in solution until reaching the surface, where the iron becomes oxidized, forming an iron coating along the water’s path. The different colors are a result of the different oxidation states of iron. At one time, these mineral springs were believed to have healing powers with many health benefits. Unfortunately, these springs simply leave you with a lingering iron taste with no health benefits, but do provide an aesthetically pleasing photo opportunity.