Moqui Marbles The hand in this photo is holding a broken piece of one of an uncountable number of ir

Moqui MarblesThe hand in this photo is holding a broken piece of one of an uncountable number of iron-sand spheres found throughout Utah and Arizona, home of the Navajo Sandstone. These spheres, known as both the Moqui Marbles and as Shaman Stones, are concretions. The Navajo Sandstone is a permeable unit, meaning it allows groundwater to flow through it. Groundwater passing through other geologic formations can dissolve small amounts of many elements in it, and if that groundwater hits a small obstacle that makes it slow down or change direction it will cause those elements to precipitate, forming solids.The element that holds these together is iron. Groundwater carrying iron through the Navajo sandstone dumped that iron when it hit obstacles and as the concretion grew, the obstacle got bigger. The concretion gradually added layers as it grew outwards. Most of the concretion remains sand, but the pore spaces are filled in by iron oxides, making them resistant to erosion. Once the sandstone erodes away, the concretions are left covering the surface.These concretions have a long history. Recent work dated the inner and outer layers and found that in a single grain you could have an inner core as old as 25 million years old and an outer rim as young as 300,000 years old. The color difference between the inner and outer layers reflects some change in water chemistry and as a result a change in which iron mineral was forming; the inner layers are often Goethite or Hematite and where found the darker layers on the outside may be limonite.They also are considered an analog for the “blueberries” found on Mars by the rover Opportunity. Those “blueberries” were small pebbles littering the ground at the rover’s Meridiani Planum landing site, just as these concretions litter the ground in Utah. They received the nickname “blueberries” since they appeared blue in the multi-spectral images taken by the rover, although to human eyes they’d probably appear just as dark as these. Just as these concretions form by groundwater flowing through sandstone, the blueberries on Mars are taken as evidence of iron-rich groundwater flowing through a sandstone unit, indicating that Mars had flowing groundwater in its past.-JBBImage credit: https://flic.kr/p/5sQCMvReferences:http://www.mindat.org/article.php/1629/Moqui+Marble+Mysterieshttp://gsabulletin.gsapubs.org/content/early/2014/05/22/B30983.1.abstract -- source link

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