When the rock was originally going through the rock forming process a thin patina of hematite coated the quartz grains creating a cement holding them together. Later, water (with certain reducing agents) moving through the sandstone picked up the iron. When this dissolved iron combines with oxygen the dissolved iron loses an electron and its solubility is reduced. The hematite is immediately precipitated out forming the iron concretion.
Monday, April 20, 2009
A portion of last weekend was spent walking through Ute canyon in the Colorado National Monument. In some areas we were walking across an eroded Wingate surface. The Wingate sandstone is early Jurassic in age and is a windblown quartz dominant sandstone with obvious crossbedding through out the formation. While walking on the Wingate (or the Navajo ss) there are sometimes collections of small marble like objects on the ground. These marbles are iron concretions.