<p>Quartz-chalcopyrite from the Precambrian of Minnesota, USA. (public display, Soudan Underground Mine State Park visitor center, Soudan, Minnesota, USA)
</p><p>Whitish-gray = quartz
Brassy gold mass = chalcopyrite
</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 5400 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.
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<hr><p>Info. on quartz:
</p><p>The silicates are the most abundant and chemically complex group of minerals.  All silicates have silica as the basis for their chemistry.  "Silica" refers to SiO2 chemistry.  The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4.  Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon.  The resulting formula for silica is thus SiO2, not SiO4.
</p><p>The simplest &amp; most abundant silicate mineral in the Earth's crust is quartz (SiO2).  All other silicates have silica + impurities.  Many silicates have a significant percentage of aluminum (the aluminosilicates).
</p><p>Quartz (silicon dioxide/silica - SiO2) is the most common mineral in the Earth's crust.  It is composed of the two most abundant elements in the crust - oxygen and silicon.  It has a glassy, nonmetallic luster, is commonly clearish to whitish to grayish in color, has a white streak, is quite hard (H≡7), forms hexagonal crystals, has no cleavage, and has conchoidal fracture.  Quartz can be any color: clear, white, gray, black, brown, pink, red, purple, blue, green, orange, etc.
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<hr><p>Info. on chalcopyrite:
</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>Chalcopyrite is a copper iron sulfide mineral (CuFeS2).  Many pyrite-like minerals exist, such as pyrite, marcasite, arsenopyrite, pyrrhotite, and chalcopyrite.  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).  Chalcopyrite is an important copper ore mineral.
</p><p>Stratigraphy of host rocks: Soudan Iron-Formation, Neoarchean, ~2.722 Ga
</p><p>Locality: Soudan Mine, Soudan, northeastern Minnesota, USA
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<hr><p>Photo gallery of quartz:
&lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/gallery.php?min=3337">http://www.mindat.org/gallery.php?min=3337</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3337&lt;/a&gt;
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<hr><p>Photo gallery of chalcopyrite:
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&lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/gallery.php?min=955">http://www.mindat.org/gallery.php?min=955</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=955&lt;/a&gt;
<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/22004056073/">Quartz-chalcopyrite (Soudan Mine, Soudan, Minnesota, USA)</a>