<p>Arsenopyrite-dolomite from Mexico. (field of view 3.7 cm across)
</p><p>Silvery-colored crystals = arsenopyrite
Pale pink crystals = dolomite (CaMg(CO3)2 - calcium magnesium carbonate)
</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>The sulfide minerals contain one or more sulfide anions (S-2).  The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals.  Many sulfides are economically significant, as they occur commonly in ores.  The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc.  Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size.  These minerals will not form in the presence of free oxygen.  Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.
</p><p>Arsenopyrite has, as the name suggests, the chemistry of pyrite (iron sulfide) plus some arsenic.  Arsenopyrite is FeAsS - iron arsenic sulfide.  It is the most common arsenic-bearing mineral on Earth.  It has a metallic luster, silvery color, blackish streak, and a hardness of about 5.5 to 6.  When heated, it gives off a garlic smell (the smell of arsenic).
</p><p>Locality: Santa Eulalia Mining District, Chihuahua State, northern Mexico
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<hr><p>Photo gallery of arsenopyrite:
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<a rel="nofollow" class="external text" href="http://www.mindat.org/gallery.php?min=305">www.mindat.org/gallery.php?min=305</a>
<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/19221435745/">Arsenopyrite-dolomite (Santa Eulalia Mining District, Chihuahua, Mexico)</a>