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                           Mineral of the Month--March

                              The Colors of Fluorite

                                              Calcium Fluoride

                                      CaF2 (more, and less!)

                                        "The Colors of Fluorite"

                                              By Ken Casey

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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IMGP3284.JPG (312222 bytes) IMGP3281.JPG (432641 bytes) IMGP3268.JPG (242107 bytes)


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

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!

fluorite_colors.gif (4589 bytes)

 

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!

 

Chemistry & Science

     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!

 

Atoms, Lattice, and Color

     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.

CaF2_atom.gif (8898 bytes)

     "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

     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".

missing_fluorine.gif (7978 bytes)       "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

     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)  

CaF2_lattice.gif (6955 bytes) fig7-10c.gif (5357 bytes)
View 1: Fluorite Lattice structure diagram, color
Courtesy of Jaap Bax
View 2: Fluorite Lattice structure diagram
Courtesy of Worcester Polytechnic Institute

 

CaF2_lattice2.gif (8251 bytes) IMGP3270.JPG (288788 bytes)
3-D Representation of the multi-unit cubic structure
Courtesy of the University of Arizona
Cubic Fluorite Crystal
Photo by Ken Casey

  

 

How Can We See This?

      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.

braggslaw.gif (5517 bytes)
nl =2dsinq
Bragg's Equation & Diagram (Refraction is key, as well)

 

 

Iron Pyrite, Fluorite & Color

     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.  


PYRITE2.JPG (133849 bytes)
Pyrite Cube, York, PA
Photo by Ken Casey

Melting Points

Calcium Fluoride: 1402 C, 2555 F, 1675 K  
 

                         Iron: 1538 C, 2800 F, 1811 K

     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.

Ca_atom.jpg (13622 bytes)

Fluorine_atom.jpg (11673 bytes)

     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 300-1000C.”

(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.

CaF2CaF2.gif (5554 bytes)

     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.

Fe_atom.jpg (13937 bytes)

     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

      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!

 

Other Colors

 

     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.

IMGP3297.JPG (370986 bytes) IMGP3296.JPG (332780 bytes)
Ambient indoor light: Rogerly Fluorite (Green) 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.

  europium.gif (1702 bytes) Eu.jpg (16975 bytes) Europium from the Periodic Table and
as an atomic representation

 

samarium.gif (1724 bytes) samarium_atom.gif (4756 bytes) Samarium from the Periodic Table and
as an atomic representation

Table 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. 

IMGP3262.JPG (332660 bytes)

     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 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.

IMGP3260.JPG (167948 bytes)

     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 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.

IMGP3266.JPG (338878 bytes) IMGP3267.JPG (346306 bytes)
Blue Fluorite, Desert Rose Mine, Bingham, NM 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.

IMGP3265.JPG (328697 bytes) IMGP3268b.JPG (242107 bytes)
Rootbeer Brown Fluorite, Clay Center, OH Purple/Black Fluorite, Cave-In-Rock, IL
Photos by Ken Casey

 

Color Perception

colorwheel.gif (3661 bytes)

     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

     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!

colorperception.gif (36630 bytes)

     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.

lathanides.gif (4267 bytes)

     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.

periodic_table.gif (19508 bytes)

     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".

aniuvlamp.gif (5887 bytes)anifluorite2.gif (3273 bytes)

 

Fluorescence and Color

 

    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)

IMGP3275.JPG (92949 bytes) IMGP3283.JPG (284859 bytes)
Colorless, pure Fluorite Octahedron 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.

 

What is Ultraviolet light?

     "Ultraviolet light is invisible, high-energy light."  It's wavelengths are measured like visible
light, either in Angstrom units (), 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)

 

kens_spectrum2.gif (13647 bytes)

 

Fluorescence

     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)

Jablonski_energy_diagram.jpg (45783 bytes)
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

  

Two Museums of Note

     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!

 

Uses

     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.

CaF2_colorless_powder.jpg (33605 bytes) caf2_6.jpg (579354 bytes)
Colorless Calcium Fluoride powder
Courtesy MTDC
CaF2 Lens Blank
Courtesy of Eric B. Burgh

 

    

 

Links

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

Irne DuPont Mineral Museum, University of Delaware

Peabody Museum of Natural History, Yale University

 

Members' Gallery

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. hardhat2a.gif (5709 bytes)

 

Article Contributors

 

 

 

Photo & Graphics Credits

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.

George M. Coia, Professor of Chemistry, Portland State University

Licia Minervini, Research Student, Dept. of Materials, Imperial College of Science, London, United Kingdom

Jaap Bax, Philosophy of Pattern Website

Worcester Polytechnic Institute, Chemistry & Biochemistry Department

Michael W. Davidson, Mortimer Abramowitz, National High Magnetic Field Laboratory, Florida State University

Mountain Technical Development Center for Non-ferrous Metals

Eric B. Burgh, SALT/PFIS Instrument Scientist, SAL, University of Wisconsin, Madison

University of Arizona

 

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. 


Suggested Reading:

Spectroscopic Characterization of Fluorite: Relationships Between Trace Element Zoning, Defects and Color by Carrie Wright (Master's Thesis)

 

KEN.JPG (31503 bytes)

   About the Author:  Ken is current webmaster of the Delaware Mineralogical Society.  He has a diploma in Jewelry Repair, Fabrication & Stonesetting from the Bowman Technical School, Lancaster, PA, and worked as jeweler.  He has also studied geology at the University of Delaware.  And, he is currently a member of the Delaware Mineralogical Society and the Franklin-Ogdensburg Mineralogical Society.  E-mail: kencasey98@yahoo.com.


Invitation to Members

Members,

Want to see your name in print?  Want to co-author, contribute, or author a whole Mineral of the Month article?  Well, this the forum for you!

And Members, if you have pictures, or a story you would like to share, please feel free to offer.  We'd like to post them for our mutual enjoyment.   Of course, you get full photo and author credit, and a chance to reach other collectors, hobbyists, and scientists.  We only ask that you check your facts, give credit where it is due, keep it wholesome for our Junior Members watching, and keep on topic regarding rockhounding.

You don't even have to be experienced in making a webpage.  We can work together to publish your story.  A handwritten short story with a Polaroid will do.  If you do fancier, a text document with a digital photo will suit, as well.   Sharing is the groundwork from which we can get your story out there.

Our club's webpages can reach any person surfing the net in the world, and even on the International Space Station, if they have a mind to view our website!

We are hoping for a possible tie-in to other informative programs upon which our fellow members might want to collaborate.  Contact any officer or board member with your suggestions.

Our next MOTM will be a surprise.  For 2007, we are waiting for your suggestions.  What minerals do you want to know more about?

aniagate.gif (1920 bytes)

____________________________________

Most of the Mineral of the Month selections have come from most recent club fieldtrips and March Show Themes, and from inspriring world locales. thus far.  If you have a suggestion for a future Mineral of the Month, please e-mail me at: kencasey98@yahoo.com, or tell me at our next meeting.

 

 

Past Minerals of the Month
February 2007 Mineral of the Month: Pennsylvania Fluorite
January 2007 Mineral of the Month: Sillimanite
December 2006 Mineral of the Month: Hedenbergite by Karissa Hendershot
November 2006 Mineral of the Month: Brandywine Blue Gneiss
October 2006 Mineral of the Month: Spessartite by Karissa Hendershot
September 2006 Mineral of the Month: Native Silver
August 2006 Mineral of the Month: Kryptonite
July 2006 Mineral of the Month: Azurite
June 2006 Mineral of the Month: Pyromorphite
May 2006 Mineral of the Month: Tsavorite by Karissa Hendershot
April 2006 Mineral of the Month: Variscite
March 2006 Mineral of the Month: Petrified Wood, Part II
February 2006 Mineral of the Month: Petrified Wood, Part I
January 2006 Mineral of the Month: Strontianite by Karissa Hendershot
December Mineral of the Month: Clinozoisite
November Mineral of the Month: Bismuth
October Mineral of the Month: Wulfenite by Karissa Hendershot
September Mineral of the Month: Turquoise
August Mineral of the Month: Peridot
July Mineral of the Month: Ruby
June Mineral of the Month: Antarctic Fluorite
May Mineral of the Month: Dolomite, Part 2
April Mineral of the Month: Dolomite, Part 1
March Mineral of the Month: Calcite
February Mineral of the Month: Agate
January Mineral of the Month: Fluorite
December Mineral of the Month: Pyrite
November Mineral of the Month: Stilbite  
October Mineral of the Month: Celestite   
 

This page last updated:  February 19, 2011 10:14:47 AM

 

       

  


Next Meeting
 

April Program, Monday, April 8, 2013:

"Destruction of the Fossil Exposures in the Chesapeake Bay Area" presented by Dr. Lauck Ward

General Club Meeting:
April 8, 2013
(Monday)

We are meeting at
Greenbank Mill


Special Meetings:
 

*Show Committee Meeting, April or May, 2013

*New Home/Lapidary Committee, 2013

*Board Meeting,  April, 2013

Next Field Trips
 

Fieldtrips!

Past Fieldtrips
 

Next Show
DMS March Show
March 1-2, 2014 at DelTech Stanton

 


Our 2013 Show Theme was:
"All That Glitters is as Good as Gold!"

March Show 2013 Report

Updates!

 

 

 
Articles

 

Fossil Forum


"Dinny, the Dino"

"Belemnites are coming"

 

MOTM June also commemorates our 50th Show!

It's shiny, yellow, and is a symbol of 50 Years!Can you guess?

Past MOTM

Collecting Adventure Stories:

"Sunny Brook Crick Goethite" by Joe Dunleavy