Vivianite (#20)

A single lustrous gem to gemmy parallel growth crystal group grading from purple to green from one end to the other.
North America Blackbird mine, Lemhi County, Idaho, USA 2 ¾" x ¾" x ¼"

 

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The chemical formula of vivianite is Fe32+(PO4)2 · 8H20. The "dot-8H20" means that there are 8 water molecules that are bound within the crystal structure. When the mineral is fresh it is colorless to pale green. On exposure to light, it quickly oxidizes as follows: A photon knocks a proton (H atom) off a water molecule, converting it to (OH)-. An Fe2+ is then oxidized to Fe3+ to maintain charge balance. This leads to Fe2+ - Fe3+ charge transfer, and a dark blue to dark blue green color. (If you don't know what that means, read the "Learn More" for specimen #15. Basically, photons bump electrons between the two oxidation states of Fe atoms, and the energy required to do that is lost from the spectrum - what's left is what you see.)

Generally, those who discover a new mineral have the privilege of naming it. Some name '1heir" mineral after the locality where it was found, after another scientist, after some physical property such as distinctive color, or perhaps after a prominent element in its chemical formula. Often, particularly in the past, the name comes from the Greek words for some property. Vivianite was named in 1817 by a German geologist/mineralogist; it was named for John Henry Vivian, a British mineralogist.

Many minerals have had names for centuries, and they don't need approval. Today the names of new minerals are approved by an international scientific commission, the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. A few dozen new minerals are discovered each year, on average.