Preface
Introduction
Chemistry & Science
Atoms, Lattice, and Color
How Can We See This?
Iron Pyrite, Fluorite & Color
Other Colors
Color Perception
Fluorescence and Color
What Is Ultraviolet Light?
Fluorescence
Two Museums of Note
Uses
Links
Members' Gallery
Article Contributors
Photo & Graphics Credits
Suggested Reading
Invitation to Members
Past Minerals of the Month
Photos by Ken Casey ©2006-7 |
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What makes Fluorite so Colorful?......The mysteries of Physics make it so! |
(Top, left): Pink Fluorite, Minas Navidad, Durango, Mexico
(Top, center): Yellow Fluorite, Cave-In-Rock, Illinois
(Top, right): Green Fluorite octahedra, Hunan Province, China
(Bottom, left): Green-Blue Fluorite, Rogerly Mine,
England
(Bottom, center):Purple Fluorite on Sphalerite, Elmwood Mine, Tennessee
(Bottom, right):Purple-Black Fluorite
w/ Hydrocarbons, Cave-In-Rock, Illinois
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Preface
This
month, we'll expand on our mineral color theory in: The
Colors of Fluorite.
This approach will differ a bit from our usual
treatment of Locations, Uses, Lapidary, and such,
as we will explore how light and the laws of physics govern nature's extraordinary color
palette.
Get ready for some enlightening science. Everyone, everywhere, please join us!
Let's go!
Introduction
Welcome back to
our newest installment of Mineral-of-the-Month!
Our journey this
time takes us into realm of Fluorite atoms, and how their structure reacts to
all kinds of light sources. Of course, there will be specimen photos and some
locality info; but,
mostly, we we'll concentrate on a brighter way in which we can enjoy our Fluorites!
Now, as the springtime sun warms our
temperate climes, we'll duck our heads outside for a
spell of our cheery sunlight, and how it affects the coloration of Fluorite. Later,
our journey will
also take us down to the sub-atomic level in our club's virtual physics lab. We
won't embark on
a coverage of nanotechnology per se, but a guided tour through 'missing' atoms is one of
our
stops.
There will be plenty of brightly-colored
pictures and diagrams, and a little conjecture on my
part. Please feel free to speculate on our topics. We can discuss them over
lunch at our virtual
lab. For this trip we'll allow lunches to be eaten right at our benches!
We are boarding our DMS tourbus now upon
exiting our club's March 3-4, 2007
Show. If
you missed the show, and want to catch up, please do visit our 2007 Show Page, as our show
theme was "Fluorite". Then, join us on the bus; we'll wait for you. Enjoy!
Today's advancements in Physics allow us a more specific view
into the subatomic realm
of Fluorite. We'll use basic tools, like X-ray Diffraction techniques (XRD), and
some newer
concepts in Physics, in tandem, to tour the colors of Fluorite.
You may remember such concepts as Bragg's Law and Planck's
constant from your
Geology, Chemistry, or Physics class. If not, I'll describe them briefly in our
treatment of
Fluorite. To assist you, I'll link to these terms, and
more, to websites with background
information.
We'll begin with a simple description of Fluorite formation
and its elemental structure.
Then, we'll delve into atomic particles and waves. Get ready for an innerspace
journey into
Fluorite. Let's go!
Calcium Fluoride (or Fluorite) is insoluble in water, though
on the other side of it's formation
equation, Calcium Carbonate and Fluorine gas are soluble. That is how calcite and
fluorite can
precipitate out of magma-heated groundwater. It is also is how Fluorite crystals
remain for us
to find in exposed vugs to collect.
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"This insoluble solid adopts a cubic structure wherein calcium is
coordinated to eight fluoride
anions and each F- ion is surrounded by four Ca2+ ions."
(In the drawing to our left, Yellow is
Fluorine, and Blue is Calcium.)(Source: wikipedia article: Calcium Fluoride)
We know that pure Fluorite is colorless and clear. It
has the most perfect of regular atomic structures that nature has to offer. When the
aspect of visible color is added in to the equation, reason suggests that either extra
elements,
missing electrons, and variations inhabit the
simple, revised crystalline structure. |
Ball and Stick drawing of
Fluorite atomic lattice
Courtesy of Licia Minervini, Imperial College of Science |
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Extra atoms might substitute
for one another at either some cation sites or interstitial sites,
such as a Magnesium cation for Calcium. Interstitial impurities exist when an ion
fills a hole in
the lattice. These occasional substitutional impurities, with regards to calcium
fluoride, change
the light absorption properties, thus giving off visible wavelengths of a novel color.
Electron variations also play a role. Evidence of
measured atomic-level patterns point towards
a resulting asymmetry of sorts, when variations are observed in the crystal lattice
structure. That
is, a 'missing' electron can alter the wavelength of light absorbed, thus rendering a new
and visible
color, usually purple. This defect type is called an 'F-Center', or 'color center'
from the German "Farbenzentrum".
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"An F-Center...or color center, is
the
anionic vacancy in a crystal filled by one or
more electrons
(depending on the charge of
the missing ion in the
crystal). It is a variety
of crystallographic defects.
The electron has a series of energy levels.
It can absorb light and jump to excited states. When it falls back, it emits energy in the form
of electromagnetic waves, e.g. light. This process is
responsible for the color of a
crystal.” In Fluorite, one electron takes the
place of a Fluorine atom (see: left).(Source: wikipedia article: F-Center) |
| A: Perfect Fluorite Lattice; B: Missing
Fluorine defect |
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Sometimes these interstitial atoms jump to join the
structure when another atom or ion leaves
its lattice, thus creating a vacancy. This is called a "Frenkel
defect". Or, interstitials occupy a
site in the lattice, where no atom usually resides. These high energy defect
configurations are
common to divalent metal halides with fluorite-type structure, and one could record this
structure
in CaF2 for comparison in Kroger-Vink Notation as:
FF > VF·+Fi'
(Source: Answers
to Problems of Solid State Course A: Properties and Reaction of Matter)
Therefore, an interstitial and its nearby vacancy pair
generate the defect in the crystal
sublattice structure. Out of place atoms fill in holes in the lattice, as nature
abhors a vacuum.
“The Calcium Fluoride (fluorite) Lattice.
This compound has formula CaF2, and exhibits
the lattice shown in [Views 1 and 2]. The Ca2+ ion is virtually the same size
as the F- ion, one of
those rare situations referred to above, and forms a face-centered cubic lattice. The F-
ions fill all
of the tetrahedral holes in the cation lattice. Since there are 8 such holes per 4 Ca2+
ions, the
stoichiometry is nicely accommodated. The coordination number of the Ca2+ ion
is 8, and that of
the F- ion is 4. (Note that the number of cations per formula unit multiplied
by the cation
coordination number is equal to the product of the number of anions per formula unit and
the
anion coordination number.) There are again 4 formula units per unit cell. The fluorite
structure is
very common for ionic compounds of 1:2 (or 2:1) stoichiometry.” This is
the stuff of crystallographers.
(Source: WPI Text
Concepts of Chemistry, Chapter 7: The Solid and Liquid Phases)
Bragg's equation is the basis of modern x-ray
diffraction measurement. By bombarding a
fluorite sample with X-radiation, a pictorial and measurable ray interference pattern
develops on
film. Hence, our micro-view becomes a macro-view. We can observe points and
patterns here.
As we calculate "d", from measurements of these, we
define the space between layers in the
crystal lattice structure.
Each mineral has its own unique 'd-spacing' measure. We
can then compare pure calcium
fluoride to samples from the field, thus giving us a spectrum of variations against which
we
might calibrate and chart our visible color interpretations. Lab-doped CaF2
can guide us, too.
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| nl
=2dsinq |
| Bragg's Equation & Diagram
(Refraction is key, as well) |
Understanding these data as color,
we can next apply the observations of other properties,
like melting point to color. (For example, fluorite's usefulness as a flux in
metallurgy suggests
it has synergistic properties to iron with its similar melting point.) Color may be
related on the
atomic and substitutional levels, with iron replacing calcium in fluorite's structure.
Pyrite Cube, York, PA
Photo by Ken Casey
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Melting Points
Iron: 1538 °C, 2800 °F, 1811 °K
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Pyrite, or iron-sulfide
(FeS), does occur naturally with fluorite. Associated fluorite is generally
green or purple. My field notes can correlate that pyrite and fluorite cubes of
similar size inhabit
many of vugs from which we collect in Pennsylvania, for example. Could properties,
like a
similar melting point, suggest concurrent formation? I've noted that pyrite and
fluorite cubes tend
to form after calcite and dolomite, as these crystals are perched upon complete dolomite
saddle
crystals in vugs and brecciations in the host limestone/dolostone.
Fluorite's chemical ability to dissolve oxides can aid us in
the study of the order of mineral
formation. Perhaps by catalyzing iron oxide into elemental iron, fluorite
facilitates iron's bonding
with sulfur to form pyrite. Geologists, please correct me, if I am wrong here.
As associated Calcite (CaCO3) is the most stable
form of Calcium, and the related Dolomite
(CaMgCO3)2 is too, these two minerals might be the bases upon which
the more reactive Fluorite
and Pyrite form simultaneously as cubic crystals.
 
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To encapsulate this theory, we need to familiarize ourselves
with the Laing Tetrahedron of Bonding and Material Type and ionic salts:
"Ionic
materials have crystal lattice with anions electrostatically attracted to adjacent cations
and cations electrostatically attracted to adjacent anions. Ionic materials are insulators
as solids, but are electrical conductors when molten and when dissolved in aqueous
solution. Ionic materials may dissolve in water (and sometimes in dipolar aprotic solvents
such as DMSO), but they are insoluble in non-polar solvents like hexane. Ionic materials
have moderately high melting points, usually 300-1000°C.”
(Source: The Chemogeneis web
book: The Laing Tetrahedron of Bonding and Material Type)
(Calcium, Fluorite Graphic Source: www.webelements.com) |
To properly forward this
theory of formation would require yet another special fieldtrip into
our virtual lab--perhaps at a future date. For now, we'll get closer to color
theory. So, let's
study the two major components of Fluorite: Calcium and Fluorine. We will add in
Iron later.
Naturally occurring Calcium, if present, is gray in color
under normal light. Fluorine gas
is pale yellow or brown. Both are highly reactive chemically, and larger quantities
of fluorine
gas that occur naturally in volcanism are poisonous to all known animal life on
earth. (So,
please, don't try to create Fluorite from scratch, unless you work in a well-equipped lab
and
possess the appropriate training, safety procedures, and other technical support.)
In Earth's
breathable atmosphere, the quantities are negligible--a small fraction of 1%.
That means that these two elements are in a ready state to
join, if the correct environmental
and geological conditions exist. Could the natural colors of these elements combine,
perhaps
as one might mix chemical pigments for paint? That is up for debate.
Generally, the final
color produced is dependent upon the new compound's optical properties, which in turn
derive
from its newfound physical characteristics. As we know, pure Calcium Fluoride is
colorless.
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What if we add Iron? Whether our iron component has its
source as magma, meteoric water, or pre-exists as metal
sulfide veins in limestone/dolostone brecciations, chemical
reactions can occur underground. Let's look at an example.
Calcium and Fluorine both react with water. In both nature
and the laboratory, fluorine and water combine to form
hydrofluoric acid (HF). Nature's waters may be so dilute that
the weak HF in near neutral in pH (man-made pollution
notwithstanding). |
www.webelements.com |
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Two other types of occurring fluoro-complexes are:
[FeF4]- and H2F+. Here is where I would
speculate that when iron is introduced into the process of concurrent Fluorite and Pyrite
formation.
Therefore, we can introduce another color variant. We get a green color, usually.
(Source: wikipedia article: Fluorine)
The most frequent mineral occurrence of fluorocompounds is as
Fluorite, which is among
the most stable of natural salts. It is amazing that two natural elements, once
combined, can
create a purely colorless crystal! If available iron enters the process, the forming
Fluorite might
"grab" dissolved iron, then add it to its cation lattice positions in place of
Calcium. Without
specific data, the current popular consensus says that a green-colored fluorite will
result. As
iron is abundant, and most world fluorite colors are either green or purple, this author
suggests
this hypothesis is plausible.
One common thread that I have garnered from from my studies is
that the type and distribution
of REEs in naturally occurring fluorites correlate as indicators for formation of Pyrite
under certain
geothermal pressure and temperature conditions. Both Iron and an REE may co-exist in
the
same crystal lattice. Also, Iron tends to cancel out any fluorescence that may
result in Fluorite
containing REEs, such as Europium and Samarium. So, we'll see green in daylight, and
no glow.
One could research a master's thesis or doctoral dissertation
on this subject. Perhaps you
will be inspired to do so in your academic and scientific careers!
As Fluorite enthusiasts, we all seem to know that our favorite
mineral appears in nature
in all colors of the visible spectrum. Compared to formal studies published upon
other mineral
colorizations, relatively few have been conducted on Fluorite itself, as it seems to have
no real
purpose in the uses of Fluorite, or in prospecting. Only pure science (for now) sees the need
to research color here. So, we'll cover at least one locale representing each color.
United Kingdom: Green Fluorite
England boasts of fine green (and fluorescing) Fluorite
cubes--they are world famous! And,
they are among my favorite specimens in my collection. I compliment Sir Stokes'
original
discovery of fluorescence, which began with the 'daylight fluorescing' material from his
native
United Kingdom. We can observe the same phenomenon in a new specimen from the
renowned
Rogerly Mine. Under normal lighting, it is a visible green; when exposed to bright
sunlight, it
fluoresces a brilliant blue-green! This daylight color stands alone; no UV lamp is
needed.
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| Ambient indoor light: Rogerly
Fluorite (Green) |
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Sunlight
(includes UV): Rogerly Fluorite (Blue) |
| Photos by Ken Casey
(from my collection) |
The additional excitable element in his (and my)
fluorite is Europium, a rare-earth element
(REE). Other REEs inhabit fluorites from many locales; however, these only affect
its visible
color when exposed to UV light. Since our sun emits a range of UV suitable to create
this
effect, daylight makes the Rogerly material "glow". Samarium (Sm3+)
is believed to be the
cause of its normal green color. We will cover the science behind this effect in our
colorful
and highly-detailed"Fluorescence and Color" segment below.
Some suggest Iron as a cation substitute for Calcium as a
cause of green daylight color,
especially in American fluorites.
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Europium from the Periodic Table and
as an atomic representation |
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Samarium from the Periodic Table and
as an atomic representationTable drawings by Ken Casey
Atomic renderings by www.webelement.com |
Mexico: Pink/Red Fluorite
In recent years, Minas Navidad in Durango, Mexico has produced
some of the finest
pink-red Fluorite octohedral clusters on matrix. I have two in my collection.
They are
becoming a bit rarer and pricey as high-grade specimens tend to do.
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That withstanding, not many world locales offer this color. The
hard-to-mine pink alpine fluorites of Europe are another story.
What makes these fluorites pink? Likely, it is a crystalline-level defect, much
like the purple and violet color-center. More study is needed on this fine fluorite. |
| Pink Fluorite from Minas
Navidad |
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Photo by Ken Casey |
United States: Yellow Fluorite
Over the past few decades, the State of Illinois has produced
fluorites of many colors,
the finest color in my estimation is yellow. The Cave-In-Rock mines of Hardin County
have offered up almost amber yellow cubes, some with chalcopyrite.
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Popular science has us noting that the yellow color derives from organic
compounds, like petroleum fractions within its fluorite's structure. Do you agree?
Some studies of lab-doped calcium fluoride propose that Y02 present guides the
light wavelengths to appear yellow to our eyes. |
| Yellow Fluorite, Cave-In-Rock,
IL |
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Photo by Ken Casey |
United States: Blue Fluorite
Yes, the American west has exposed mineralizations, many
famous, like the Kennecott
Copper Mine near Salt Lake City, Utah. It can be seen from space, while orbiting in
the
Space Shuttle. Fluorite mines are different in as much as they exist on a smaller
scale there.
A well-known locale for the blue fluorite is the Desert Rose
Mine, Bingham, Soccoro
County, New Mexico. These aqua to sky blue specimens command respect by collectors
all around the globe. They are still available from rock dealers and rock shows.
One can
even fee mine at this locale by visiting or contacting the Blanchard Rock Shop in Bingham.
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| Blue Fluorite,
Desert Rose Mine, Bingham, NM |
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Close-up of same Blue Fluorite |
| Photos by
Ken Casey |
Now, what causes us to see this
daunting blue color? I suspect a REE. And, as some
purple-blue color zoning is present in some recent specimens, that a lattice defect is one
cause. The greater the amount of defects, the bluer the specimen is the general
rule.
United States: Brown & Black Fluorite
From tan to rootbeer brown to black, Fluorite from the
midwestern American mines
vary in this deep color range. Some fluoresces cream-colored under UV light, most
does not. Clay Center, Ohio is a major source of such specimens.
What gives them their color range? Science has shown us
that hydrocarbons do
contribute to the darker colors, what of the lighter? I suspect that Lead and Zinc
might combine into Fluorite somehow. Chemists and Geologists out there, please tell
us if you think that this is plausible.
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| Rootbeer Brown Fluorite,
Clay Center, OH |
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Purple/Black Fluorite,
Cave-In-Rock, IL |
| Photos by
Ken Casey |
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First, let's ask ourselves, 'Is this an optical illusion?' As this
effect in not a simulated demonstration, it must be real. Next, we might pose, 'How
do we see this color?' The answer is simple, 'It is how the human eye perceives
color.' Much as a particular fluorite specimen reacts to any
light source of a specific absorption band, our brain interprets the dominant light
wavelength emitted as visible color. The excess wavelengths not absorbed in daylight
upon fluorite are what we see as normal, daylight color. Colorless Fluorite absorbs
no incident light, so we see right through it! |
Simple Color Wheel |
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The total amount of light
absorbed and transmitted through the crystal is the key to which
color we will see. Light play can vary from specimen to specimen. So, when you
are
collecting or shopping, bring your UV lamp, and ask a fellow rockhound. It can be
fun!
So, what gives Fluorite it's color? Three major factors
determine its color: range of chemical
purity, defects in crystal lattice structure, and type of light used to view the specimen.
Purity
refers to how much of the Fluorite is Calcium Fluoride, and how much are other elements
within
the structure. Defects are the missing or extra atomic components, such as Frenkel
defects,
or the presence of Calcium colloids, which scatter light. And, light type can vary
from daylight
and bright sunlight to specialized UV lamps. The resulting visible color relies on
the combined
effect of these properties and conditions.
In daylight, organic compounds, like petroleum and
hydrocarbons impart a yellow, brown, or
black color. Iron can render a visible purple or green. Other metallic cations
present can offer
us reds/pinks, blues, and yellows--and every color in between. For example, metallic
Calcium
absorbs light of 580nm, thus rendering a pinkish cast (as a variant of violet). The
Lanthanides
(or REEs) substitute best in nature for Calcium, giving us a range of colors in all light
types. Of
these, the best substitutes are Samarium, Europium, and Ytterbium.
Various associated minerals with the following elements could
impact Fluorite's final
formation and structure, such as: copper, cadmium, germanium, barite, nickel, iron,
sulfur,
arsenic, magnesium, aluminum, silicon, hydrogen, and oxygen, either in the process, or
as final constituents of the Fluorite crystal.
To better comprehend the promorphology and colors exhibited by
fluorite structures, we'll
need to quickly review some optical properties. As light passes through a solid
crystal (like a
prism), it slows down. Fluorite's unique atomic layering provides the frequency of
light which is
reflected, refracted, or transmitted, thus rendering the exact visible color that we will
see. This
light exposure and absorption excites electrons, which produce visible color photons.
Fluorite's cubic or octahedral crystal faces mimic its atomic
structure. Crystal growing
conditions and fluid state changes will affect its final optical color. Sometimes, a
bi-colored,
zoned or phantom crystal results, after which we may delight.
Next, we'll turn on our UV lamps. There is one at each
lab bench. So, hit the On button,
and let's have a look at the world of "Fluorescent Fluorite".
 
It seems fitting that pure calcium fluoride is nearly
chemically inert and optically favorable
to the transmission of light, that both infrared and ultraviolet radiation travel right
through with
minimal interference and birefringence. In fact, colorless fluorite is used
extensively in modern
optics, and was once employed as the second crystal laser, after the ruby type in the
1960s.
(Source: wikipedia article: Calcium Fluoride)
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| Colorless, pure Fluorite
Octahedron |
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Colorless Fluorite Cube
Cluster, Durham, England |
| Photos by
Ken Casey |
Only impure calcium fluoride
exhibits visible color and fluorescent color. This basic
characteristic lays as nature's foundation for its amazing fluorescent color properties,
which we
will delve into shortly. First, we need to understand more of the basics of optical
phenomena.
Newton stated in his landmark work "The Principia"
that light is both a wave and particulate
phenomenon. Today's quantum mechanics, physics, and string theory have all built
concepts
based upon this premise. So, we will talk in terms of wavelengths and atomic
particles.
Between these scientific developments occurred the discovery
of the first photoluminescent
property of fluorite, known as "fluorescence". In 1852, Sir George G.
Stokes wrote on this after
he had ascertained that the daylight color of British fluorite specimens changes (and
glows)
upon exposure to natural sunlight. His emerald green cubic crystals altered into an
aqua blue.
He named this phenomenon "fluorescence".
Though not all fluorite demonstrates this property, his local
specimens did. We will explore
why some fluorite "glows", and why some does not. Our examples will be
illuminated by certain
wavelengths of ultraviolet (UV) light.
"Ultraviolet light is invisible, high-energy light."
It's wavelengths are measured like visible
light, either in Angstrom units (Å), or more commonly nanometers (nm). A nanometer
is one
billionth of a meter.
For comparison, visible light ranges from approximately
700-400nm (red to violet). Ultraviolet
light ranges from 400-100nm. Three useful UV ranges of light are: UV-A (400-320nm),
UV-B
320-290nm), and UV-C (290-100nm).
(Source: http://www.steripen.com/faq.html)
The application of UV light onto a fluorite specimen can yield
some intense visible colors
for our eyes to see. This phenomenon is known as "fluorescence".
Generally, fluorescence occurs when fluorite molecules absorb
high-energy photons, and
emit related low-energy photons in response. In our case, exposure of fluorite to
certain UV
wavelengths (invisible to the human eye) creates longer wavelength visible light.
The molecules
vibrate, thus giving off light and heat. This is how naturally occurring fluorite
registers and reacts
to the difference between absorbed and emitted light. What we observe is an enhanced
or
different color.
(Source: http://www.spiritus-temporis.com/fluorescence/equation.html)
In the following equation,
S1 --> S2
+ hn
the presence of two excited singlet states demonstrate this phenomenon, where S1
is the
starting state, and S2 is the resultant state, when UV photonic energy (UV
light) is applied
with variables h = Planck's constant, and n = the fluorescent
photons' light frequency.
There are many other laws of physics that can guide us towards
understanding fluorescent
behavior. Two such concepts are the Kasha-Vavilov Rule and a Jablonski diagram.
The rule
states the quantum yield of luminescence is independent of the wavelength of exciting
radiation.
In our case, we'll use rays emitted from a UV lamp.
By viewing a Jablonski diagram, we can chart most of the
relaxation mechanism for excited
state molecules.
(Source: http://nanofm.com/terms/fluorescence.html)
Courtesy of George M. Coia,
Professor of Chemistry, Portland State University
The resultant emitted light stops immediately upon removal of
the UV light source. That is,
the electrons return to a normal state of excitation. Electrons are demoted to lower
orbitals. If
this sounds uninspiring, just turn on your UV lamp again to witness another
"promotion" to higher
orbitals. "D"-orbital split-field electrons jump shells in energized
states to deliver visible photons.
In the case of additional phosphorescence, the emitted light
continues over time, even after
the excitation source is removed. Yes, this second photoluminescent property means
that it can
really seem to "glow-in-the-dark"! This persistent visible light occurs
when fluorite is "excited to
a metastable state from which a transition to the initial state [S1] is forbidden.
Emission occurs
when thermal energy raises the electron to a state from which it can de-excite.
Therefore, phosphorescence is temperature-dependent."
(Source: http://scienceworld.wolfram.com/physics/Phosphorescence.html)
And, thermoluminescence might
occur when certain fluorites are exposed to heat, another
form of electromagnetic radiation, like light. (What if fluorite formation emits
light underground?)
In our physics, it is only the absorbed light that can render
a state change. Most light is
reflected; only wavelengths that affect energy transition levels are absorbed.
Geophysicists
who study absorption spectroscopy measure energy levels of fluorites in order to identify
their
particular makeup. They can measure by applying the Beer-Lambert Law.
(Source:http://www.chem.vt.edu/chem-ed/spec/absorption.html#)
The measurably brief interval of time between application of a
light source, and the time of
subsequent fluorescence, is called the "fluorescence lifetime". An example
of first order kinetics,
we can measure exponential decay rates to time this phenomenon. Phosphorescent
fluorite
has a relatively longer lifetime. The emission pathway is important to this process;
so, more
on that later.
In this process, three events occur, each with its own
timescale. They are separated by many
orders of magnitude. These steps are: excitation (in femtoseconds), relaxation (in
picoseconds),
and emission and return to ground state S0 (in nanoseconds). All three
steps occur in sequence
during a total of billionths of a second--imperceptible to us without measuring devices.
(Source: National
High Magnetic Field Laboratory, Florida State University)
Therefore, the fluorescent behavior of fluorite can be a
manifestation of the time-traveling of
light absorbed. Perhaps some future scientist (maybe you) will solve the mystery of
time-travel
of various fluorites with tools, such as string theory and fluorescence spectroscopy.
These molecular electronic states (S0,S1,S2)
determine molecular geometry and negative
charge distribution. Variance in the electron energy total and related symmetry of
electron spin
states govern which electronic state prevails in the fluorite. Atomically, each
electronic state is
comprised of vibrational and rotational energy levels, which affect bonding and atoms
present.
The ground state (S0) is the normal state for
fluorite at room temperature, and without being
illuminated by UV rays. When absorbed, the UV light advances the electronic state to
either
the first singlet (S1), or the second singlet (S2) state.
So, of course, fluorite will absorb some UV light, and react
to change it's electronic state.
By studying the Jablonski Diagram again (above) we can better understand the flow of
energy
and light through our specimens.
We can measure the quantum energy state change by light
absorbed into fluorite with the
application of Planck's Law, or E = hn = hc/l. Our quantum unit is the travel time of a UV
photon over one of its wavelengths, about one femtosecond).
As E = h\n = hc/l , E = energy, h = Planck's
constant, n = photon frequency, l= photon
wavelength, and c = the speed of light. I could go into more detail
here, but we want to stay
on course with fluorescence. So, suffice it to say that shorter UV wavelengths
produce a
greater quantum of energy. Excess energy than that required for a simple state
change or an
electron transition is converted into rotational and vibrational energy. That is why
our visible
fluorescent colors are so bright.
(Source: National
High Magnetic Field Laboratory, Florida State University)
The correct wavelength to make a particular fluorite specimen
fluoresce may vary per
specimen, or by collecting locale. That is why we rockhounds use a UV lamp source
that
emits three ranges of UV light: longwave (373nm), shortwave (254nm), and mid-range.
More
specific absorption band observations and data we leave to scientists for now.
So, turn off your UV lamps, tidy up your lunch pails, and
let's duck our heads outside
into the temperate air once more, before boarding our club bus for home.
Timescale Range for Fluorescence Processes
Transition |
Process |
Rate Constant |
Timescale
(Seconds) |
S(0) => S(1) or S(n) |
Absorption (Excitation) |
Instantaneous |
10-15 |
S(n) => S(1) |
Internal Conversion |
k(ic) |
10-14 to 10-10 |
S(1) => S(1) |
Vibrational Relaxation |
k(vr) |
10-12 to 10-10 |
S(1) => S(0) |
Fluorescence |
k(f) or G |
10-9 to 10-7 |
S(1) => T(1) |
Intersystem Crossing |
k(pT) |
10-10 to 10-8 |
S(1) => S(0) |
Non-Radiative Relaxation
Quenching |
k(nr), k(q) |
10-7 to 10-5 |
T(1) => S(0) |
Phosphorescence |
k(p) |
10-3 to 100 |
T(1) => S(0) |
Non-Radiative Relaxation
Quenching |
k(nr), k(qT) |
10-3 to 100 |
Courtesy of Michael W. Davidson, Mortimer
Abramowitz,
National High Magnetic Field Laboratory, Florida
State University |
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This month, our favored museums are: The American Fluorite Museum in
Rosiclaire, Illinois
and the Clement Mineral
Museum in Marion, Kentucky.
The American Fluorite Museum is also known as the Hardin
County Fluorspar Museum. It
boasts collections of Fluorite and associated minerals, mining artifacts and memorabilia,
and is
located on the former mine and mill site.
The Clement Mineral Museum features the lifelong collection of
Ben E. Clement. They host
an annual mineral show and dig the first weekend in June every year. The mainstay of
their
collection is Fluorite from the western Kentucky and Illinois fluospar mines! They
have mining
tools, plant fossils, gemstone carvings, and more!
As we fellow
MOTM-trekkers already know, we have visited Fluorite on three other occasion in:
January Mineral of the Month:
Fluorite, June Mineral of
the Month: Antarctic Fluorite (both in 2005),
and in our February 2007
Mineral of the Month: Pennsylvania Fluorite.
Also, check out our
November 13, 2006 club meeting Program: "The Colors of
Fluorite" for
more color pictures and insights.
In these articles, we
have covered fluorite's uses across the board. Hence, no purported uses
of colors of fluorite, save for lapidary work really apply here. I have weaved in
the occasional
mention of optical uses, such as lenses and lasers, into our discussion above.
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Colorless Calcium
Fluoride powder
Courtesy MTDC |
|
CaF2 Lens Blank
Courtesy of Eric B. Burgh |
Basic
Concepts of Fluorescence
"The Colors of Fluorite
Program" by Ken Casey
The Crystal Lattice
Gallery: Fluorite Structure
Fluorite at mindat.org
Penn State Earth & Mineral
Sciences Museum and Art Gallery
Virtual Mineral
Exhibit at the New York State Museum
Irénée DuPont Mineral
Museum, University of Delaware
Peabody
Museum of Natural History, Yale University
Here is where DMS Members can add their nice and
colorful Fluorite photos to share with us.
Until Next Time
We hope you have enjoyed our quaint visit to the colors of
Fluorite. Please join
us next month, for another article, and we shall journey together!
Until then, stay safe, and happy collecting. 
Article Contributors
I would like to gratefully acknowledge the generous contributions of our fellow
colorful Fluorite
enthusiasts, collectors, authors, curators, professionals, and club members who made this
work possible. Thanks.
©2007 All contributions to this article are covered under the copyright protection of
this article
and by separate and several copyright protection(s), and are to be used for the sole
purposes of
enjoying this scholarly article. They are used gratefully with express written
permission of the
authors, save for generally-accepted scholarly quotes, short in nature, deemed legal to
reference
with the appropriate citation and credit. Reproduction of this article must be obtained by
express
written permission of the author, Kenneth B. Casey, for his contributions, authoring,
photos, and
graphics. Use of all other credited materials requires permission of each
contributor separately.
Links and general contact information are included in the credits above, and throughout
this article.
The advice offered herein are only suggestions; it is the reader's charge to use the
information
contained herein responsibly. DMS is not responsible for misuse or accidents caused
from this
article. All opinions, theories, proofs, and views expressed within this article, and in
others on this
website, do not necessarily reflect the views of the Delaware Mineralogical Society.
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