Moqui Marbles

The iron oxide concretions found in the Navajo Sandstone exhibit a wide variety of sizes and shapes. Their shape ranges from spheres to discs; buttons; spiked balls; cylindrical hollow pipe-like forms; and other odd shapes. Although many of these concretions are fused together like soap bubbles, many more also occur as isolated concretions, which range in diameter from the size of peas to baseballs. The surface of these spherical concretions can range from being very rough to quite smooth. Some of the concretions are grooved spheres with ridges around their circumference.^[4][5]

The abundant concretions found in the Navajo Sandstone consist of sandstone cemented together by hematite (Fe₂O₃), and goethite (FeOOH). The iron forming these concretions came from the breakdown of iron-bearing silicate mineralsby weathering to form iron oxide coatings on other grains. During later diagenesis of the Navajo Sandstone while deeply buried,reducing fluids, likely hydrocarbons, dissolved these coatings. When the reducing fluids containing dissolved iron mixed with oxidizing groundwater, they and the dissolved iron were oxidized. This caused the iron to precipitate out as hematite and goethite to form the innumerable concretions found in the Navajo Sandstone. Evidence suggests that microbial metabolism may have contributed to the formation of some of these concretions.^[12] These concretions are regarded as terrestrial analogues of the hematite spherules, called alternately Martian "blueberries" or more technicallyMartian spherules, which the Opportunity rover found at Meridiani Planum onMars.^[4][5]