Pyrite-chalcopyrite mass from Mexico.
<p>Brassy gold = pyrite (FeS2 - iron sulfide)
Bluish-purplish = artificially tarnished chalcopyrite (CuFeS2 - copper iron sulfide)
</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 about 5500 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>Pyrite is a common iron sulfide mineral (FeS2).  It’s nickname is “fool's gold”.  Pyrite has a metallic luster, brassy gold color (in contrast to the deep rich yellow gold color of true gold - www.flickr.com/photos/jsjgeology/sets/72157651325153769/), dark gray to black streak, is hard (H=6 to 6.5), has no cleavage, and is moderately heavy for its size.  It often forms cubic crystals or pyritohedrons (crystals having pentagonal faces).
</p><p>Pyrite is common in many hydrothermal veins, shales, coals, various metamorphic rocks, and massive sulfide deposits.
</p><p>Chalcopyrite is a copper iron sulfide mineral (CuFeS2).  Chalcopyrite has a metallic luster, a deep yellowish-brassy color, a dark gray streak, a hardness of about 3.5 to 4, and no cleavage.  Many specimens have a multicolored iridescent tarnish, which can be artificially produced by exposure to certain chemicals (often acid) - the above specimen is an example of this.  Chalcopyrite is an important copper ore mineral.
</p><p>Locality: undisclosed/unrecorded site in Zacatecas State, central Mexico
</p>
<hr><p>Photo gallery of chalcopyrite:
</p>
www.mindat.org/gallery.php?min=955
James St. John
CC BY 2.0
<a rel="nofollow" class="external free" href="https://www.flickr.com/photos/47445767@N05/34167532156/">https://www.flickr.com/photos/47445767@N05/34167532156/</a>