Part Two, The Mighty Atom Revisited:

Last month, Part One stated that atoms could share electrons, bond together into compounds, and form a crystal lattice (geometric pattern).  Atoms can also gain or lose one or more electrons, creating an ion with an unstable electrical charge.  Ions attract other ions that have an opposite (positive or negative) electrical charge.  Two or more different ions come together and form an atomic bond (stuck together). 5   Imagine the ions as spheres of different sizes for different atoms (elements), larger spheres for elemental Fluorine (F) and smaller spheres for elemental Calcium (Ca).

F wants another electron, so it can get a negative one (-) charge.  Ca easily loses two electrons, getting a plus two (2+) charge.  The two ions (Ca and F) attract each other and form the chemical compound calcium fluoride or CaF2 (one calcium ion bonds to two fluorine ions).  In the crystal growth process, a square (cubic) three-dimensional structure of alternating Ca and F spheres grows into the mineral fluorite.  The cubic structure is the geometric pattern for the crystal growth for the mineral fluorite. 4

Definitions:

A mineral is defined as a naturally occurring (no artificial help from man), homogeneous (all the same) solid (not a liquid or a gas) with a definite (but generally not fixed, or just one type) chemical composition (CaF₂) and a highly ordered atomic arrangement (cubic structure).  Minerals usually (but not always) are formed by inorganic (not organic) processes₁.  An organic process involves formerly living things, like the formation of petroleum hydrocarbons by heat and pressure working on buried plant (carbon) matter.  An inorganic process could be a hydrothermal (hot water) fluid that keeps the ions moving too fast to combine (grow), then cools enough to allow the crystal growth of a mineral.  

Crystals are a polyhedral form (geometric solid), with a specific set of faces, edges, and corners (cubes have six faces or sides, eight edges, and eight corners), which is consistent with the geometric packing of the atoms within the crystal (the geometric form of a cube, which the atoms create as they crystallize into a mineral). ₂  

Sometimes a crystal may not visually appear to have a geometric form that is typical for that mineral.  The crystal form may be present at the microscopic level, but is invisible to the human eye.  The crystal represents a pseudomorph, a mineral whose outward crystal form is that of another mineral₃.  Crystals of one mineral can melt or dissolve by geologic or geochemical processes, and leave their crystal pattern as an open space (void) without removing different minerals or rocks surrounding the missing mineral.  Liquid fluorine and calcium ions later enter the void left by the missing mineral, and re-crystallize in the missing crystal shape, while maintaining a cubic structure invisible to the eye.   

Other times the crystal form can be the result of several related basic crystal forms combining (cubic is one of seven simple crystalline forms which can combine in the isometric system).

 Jim Barton

Thanks again to my professors at the University of Nebraska at Omaha, my college mineralogy text (Manual of Mineralogy, Klein and Hurlbut, 20^th Edition)₁; Simon & Schuster's Guide to Rocks and Minerals₂; Dictionary of Geological Terms, 3^rd Edition, Bates and Jackson, AGI₃; Mineralogy, Sinkankas; and my college chemistry text (Chemistry, the Central Science, 4^th Edition, Brown & LeMay) for providing the basis for this article.

Continuation of this Series of Articles:
The Mighty Atom, Part Three
The Mighty Atom, Part Four
The Mighty Atom, Part Five