Selenite ( #1)
|North America||Naica, Chihuahua, Mexico||12" x 7" x 7"|
Every mineral is defined by its chemical composition and its orderly, repetitive atomic arrangement (or crystal structure), and every mineral has an official name - one approved by the mineralogical community. Some minerals also occur in varieties that have "nicknames" of their own. Gypsum (of which this specimen is an example) has the chemical formula CaSo4 · 2H20, its crystal structure looks like this:
Look closely and you will see the repetitive pattern in the arrangement of atoms. All crystal structures have a repetitive pattern, by definition. Selenite is a variety of gypsum characterized by transparent blades or sheets. It is not a unique mineral, just a variety of the mineral gypsum - yes, the same gypsum found in drywall.