If I turn off the ultraviolet light, this crystal will strangely glow.
In fact, this crystal will glow for a lengthy period of time, acting as a portable light
source that emits a color of cyan.
What you are observing is an extreme example of a fascinating mineralogical phenomena referred
to as phopshorescence.
In this case, the glowing material is composed of the compound known as strontium aluminate.
Strontium aluminate has the following chemical formula, with the element europium often substituting
in place of the strontium atom.
Strontium aluminate is so phopshorescent that simply bringing this specimen outside in sunlight
during a UV index of 6 for 30 seconds caused it to immensely glow thereafter, and continued
to glow at a lesser extent two hours after the initial exposure to sunlight.
While strontium aluminate is manmade, there are a number of natural compounds and minerals
which are phophorescent to a lesser extent, such as this piece of the caliche aka calcium
carbonate from Arizona.
Or, this piece of magnesite aka magnesium carbonate, which glows as a white color after
the UV light I used to trigger the reaction was turned off.
For each of these three compounds, a different ion triggered the phosphorescence.
In the caliche, while a combination of several compounds was the trigger, I can isolate dysprosium
3+ as one example.
In the magnesite, manganese 2+ was the trigger, while in strontium aluminate, europium 2+
was the trigger.
So, why did these compounds produce phosphorescence?
First, I want to remind my audience what the difference between fluorescence and phosphorescence
is.
From a visual perspective, fluorescence only occurs when the energy source, generally an
ultraviolet light is powered on.
Once the light is turned off, the glowing color immediately disappears in a fluorescing
compound.
In comparison, a phosphorescent compound will continue to glow for a period of time after
the energy source has been turned off.
Light has a variable wavelength with visible light representing an area in the middle of
the pack.
During both fluorescence and phosphorescence, a higher energy shorter wavelength of light
goes into a compound when then causes a reaction that releases a lower energy longer wavelength
of light.
In the examples I have provided so far, ultraviolet light is going in and visible light is coming
out.
In the case of a fluorescent compound such as the mineral fluorite, an ion is needed
that can trigger a fluorescent reaction which in the case of fluorite is europium 2+.
When ultraviolet light hits electrons orbiting the atom's nucleus, it can excite them, causing
them to temporarily jump to a higher orbital state.
However, the electrons are unstable there and immediately jump back down to their original
orbit, releasing energy via a photon of visible light in response to this return to normal.
The amount of energy released determines the wavelength of light created, causing a rock
to glow differing colors depending on what it is composed of.
In comparison, during phosphorescence something similar occurs as ultraviolet light hits electrons
orbiting the atom's nucleus, causing them to become excited.
These electrons then jump to a higher orbital state, but unlike during fluorescence, some
electrons have their electron spin direction completely flipped.
In this flipped spin state, they are stable for a period of time, meaning the ultraviolet
light could be turned off at this stage in the reaction and the subsequent steps would
not change.
At fairly random intervals individual electrons will suddenly flip back to their original
spin state, causing them to immediately jump to their original orbit, releasing a photon
of light.
This process can occur for seconds, minutes, hours, and in rare cases, days, causing a
decreasing amount of light to be released over time.
If you are wondering how europium 2+ is the cause of these reactions in both fluorite
and strontium aluminate despite not technically comprising their chemical formula, this is
how.
Atoms depending on the element and their oxidation state have a wide range of ionic radii.
Because of this, differing elements can sometimes substitute in place of another atom if their
charge and radii is about the same during the creation of a mineral or compound.
For this to occur, the ionic radii needs to be within 15% the ionic radius of the atom
it is replacing, along with being the same oxidation state or plus or minus 1 oxidation
state of the atom being replaced.
Since europium 2+ has the same oxidation state as both calcium 2+ and strontium 2+ and is
within 15% the radii of these two ions, it can sometimes replace them.
As a final note, I would like to thank my new patron Joaquin Durand for supporting this
channel!