Hematite-rich BIF ventifact from Wyoming, USA. (~5.9 cm across at its widest)
Ventifacts are lustrous, polished rocks with subplanar to curved facets. They form by natural abrasion and polish by winds that carry sand grains. Ventifacts are naturally sand-blasted rocks.
The rock shown above is a BIF - a banded iron formation. BIFs are unusual, dense sedimentary rocks consisting of alternating layers of iron-rich oxides and iron-rich silicates. Most BIFs are Proterozoic in age (although some are Late Archean), and do not form today - they're “extinct”! Many specific varieties of iron formation are known, and some are given special rock names. For example, jaspilite is an attractive reddish & silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite.
Because of their age, most BIFs have been around long enough to have been subjected to one or more orogenic (mountain-building) events. As such, most BIFs are folded and/or metamorphosed to varying degrees.
BIFs are known from around the world, but some of the most famous & extensive BIF deposits are found in the vicinity of North America’s Lake Superior Basin. Many BIFs have economic concentrations of iron and are mined. BIFs are the most important variety of iron ore on Earth.
This BIF specimen were originally eroded from outcrops in the western Seminoe Mountains of Wyoming, USA. It became a ventifact by being subjected to wind-blasting in Windy Gap between the Ferris Mountains and the Seminoe Mountains.
Stratigraphy: very likely derived from the "upper metasedimentary-metavolcanic unit" of Blackstone & Hausel (1991), Archean
Locality: ventifact recovered from sandy soil among low sand dunes in Windy Gap, northeastern Ferris Dune Field, north of Bradley Peak, between the Ferris Mountains & the Seminoe Mountains, northwestern Carbon County, south-central Wyoming, USA
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Reference cited:
Blackstone & Hausel (1991) - Field guide to the Seminoe Mountains. Wyoming Geological Association Guidebook, Forty-Second Field Conference: 201-210.
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