<p>Riebeckite and/or richterite in magnetite-quartz banded iron formation from the Precambrian of Wyoming, USA. (button magnets for scale)
</p><p>Banded iron formations, or 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 &amp; silvery gray banded rock consisting of hematite, red chert (“jasper”), and specular hematite or magnetite.
</p><p>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. 
</p><p>BIFs are known from around the world, but some of the most famous &amp; 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.
</p><p>Some iron mines in west-central Wyoming exploit BIFs in the Goldman Meadows Formation, a Mesoarchean unit exposed in the Wind River Range.  These rocks have been multiply metamorphosed during the Precambrian.  The result of this metamorphism is highly contorted folding and fracturing.
</p><p>The rock shown above is a magnetite-quartz meta-BIF (see broken edge at the upper left of the sample).  The bluish-colored surface facing the viewer is a thin layer of sodium amphibole.  X-ray analysis shows it to be from the riebeckite series or the richterite series or a mixture of both.  This blue sodium amphibole formed during a post-lithification metamorphic event.
More info. on riebeckite: &lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/min-3418.html">http://www.mindat.org/min-3418.html</a>" rel="nofollow"&gt;www.mindat.org/min-3418.html&lt;/a&gt;
More info. on richterite: &lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/min-3416.html">http://www.mindat.org/min-3416.html</a>" rel="nofollow"&gt;www.mindat.org/min-3416.html&lt;/a&gt;
</p><p>Stratigraphy: Goldman Meadows Formation, upper Mesoarchean, 2.87 Ga (metamorphosed in the Archean at 2.8 Ga and in the Mesoproterozoic at 1.4 Ga)
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Locality: Atlantic City Iron Mine, northwestern side of Rt. 28, southwestern side of South Pass, Wind River Range, west-central Wyoming, USA
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/20579434403/">Riebeckite and/or richterite in magnetite-quartz meta-BIF (Goldman Meadows Formation, Mesoarchean, 2.87 Ga; Atlantic City Iron Mine, Wind River Range, Wyoming, USA)</a>