<p>Fluorite with color zonation from the Cretaceous of China.
</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.
</p><p>The halides are the "salt minerals", and have one or more of the following anions: Cl-, F-, I-, Br-.
</p><p>Fluorite is a calcium fluoride mineral (CaF2).  The most diagnostic physical property of fluorite is its hardness (H≡4).  Fluorite typically forms cubic crystals and, when broken, displays four cleavage planes (also quite diagnostic).  When broken under controlled conditions, the broken pieces of fluorite form double pyramids.  Fluorite is a good example of a mineral that can be any color.  Common fluorite colors include clear, purple, blue, green, yellow, orange, and brown.  The stereotypical color for fluorite is purple.  Purple is the color fluorite "should be".  A mineral collector doesn't have fluorite unless it's a purple fluorite (!).
</p><p>Fluorite occurs in association with some active volcanoes.  HF emitted from volcanoes can react with Ca-bearing rocks to form fluorite crystals.  Many hydrothermal veins contain fluorite.  Much fluorite occurs in the vicinity of southern Illinois (Mississippi Valley-type deposits).
</p><p>The Chinese color-zoned fluorite shown above is from a mineralized (carbonatite?) vein in Early Cretaceous syenite at the Weishan Mine.  The mine targets rare earth elements in northwest-trending, ~1 meter-wide veins that extend for several hundred meters.  The principal ore mineral is bastnasite (CeCO3F - cerium fluoro-carbonate).  Fluorite is a gangue mineral at the site.  Vein mineralization occurred during the late Early Cretaceous (~110 Ma).
</p><p>Locality: Weishan Mine, near Weishan Lake &amp; near the town of Weishan, Weishan County, Jining Prefecture, southern Shandong Province, eastern China
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<hr><p>See info. on locality:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/loc-144544.html">https://www.mindat.org/loc-144544.html</a>" rel="nofollow"&gt;www.mindat.org/loc-144544.html&lt;/a&gt;
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<hr><p>More info. on the geology of the Weishan Mine:
See pages 771, 773, 777-778 of:
&lt;a href="<a rel="nofollow" class="external free" href="http://www.fieldexexploration.com/images/property/1_RareEarths_FLX_02.pdf">http://www.fieldexexploration.com/images/property/1_RareEarths_FLX_02.pdf</a>" rel="nofollow"&gt;www.fieldexexploration.com/images/property/1_RareEarths_F...&lt;/a&gt;
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<hr><p>Photo gallery of fluorite:
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&lt;a href="<a rel="nofollow" class="external free" href="http://www.mindat.org/gallery.php?min=1576">http://www.mindat.org/gallery.php?min=1576</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=1576&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/33719235184/">Fluorite (Early Cretaceous; Weishan Mine, Shandong, China) 2</a>