<p>Selenium in sandstone from New Mexico, USA. (SDSMT 3909, South Dakota School of Mines and Technology Museum of Geology, Rapid City, South Dakota, USA)
</p><p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>Elements are fundamental substances of matter - matter that is composed of the same types of atoms.  At present, 118 elements are known (four of them are still unnamed).  Of these, 98 occur naturally on Earth (hydrogen to californium).  Most of these occur in rocks &amp; minerals, although some occur in very small, trace amounts.  Only some elements occur in their native elemental state as minerals.
</p><p>To find a native element in nature, it must be relatively non-reactive and there must be some concentration process.  Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state.
</p><p>The rock shown above is from a "roll front deposit", in which native selenium occurs along a redox front in fluvial sandstone (see Granger &amp; Santos, 1982).
</p><p>Stratigraphy &amp; age of host rock: fluvial sandstone, Westwater Canyon Member, Morrison Formation, Upper Jurassic
</p><p>Locality; Section 23 Mine, Ambrosia Lake Mining District, north of Grants, New Mexico, USA
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<hr><p>Reference cited:
</p>
Granger &amp; Santos (1982) - Geology and ore deposits of the Section 23 Mine, Ambrosia Lake District, New Mexico.  United States Geological Survey Open-File Report 82-207.  70 pp.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/17332622570/">Selenium in sandstone (Westwater Canyon Member, Morrison Formation, Upper Jurassic; Section 23 Mine, north of Grants, New Mexico, USA) 1</a>