Abstract Spectroscopic Characterization of Fluorite

ABSTRACT

SPECTROSCOPIC CHARACTERIZATION OF FLUORITE: RELATIONSHIPS BETWEEN TRACE ELEMENT ZONING, DEFECTS AND COLOR

By Carrie Wright

This thesis consists of two separate papers on color in fluorite. In the first paper,

synthetic fluorites doped with various REEs (10-300 ppm) were analyzed using direct

current plasma spectrometry, optical absorption spectroscopy, fluorescence

spectrophotometry, and electron paramagnetic resonance spectroscopy before and after

receiving 10-25 Mrad of 60Co gamma irradiation. The combined results of these techniques indicate that the irradiation-induced color of the Y-, Gd-, La- and Ce-doped

samples are the result of a REE-associated fluorine vacancy that traps two electrons.

Divalent samarium may be the cause of the irradiation-induced green color of the Sm-

doped sample.

In the second paper, fluorite crystals from Bingham, NM, Long Lake, NY, and

Westmoreland, NH were similarly investigated to determine the relationship between

sectorally zoned trace elements, defects, and color. The results indicate causes of color

similar to those in the synthetic samples with the addition of simple F-centers.

SPECTROSCOPIC CHARACTERIZATION OF FLUORITE: RELATIONSHIPS BETWEEN TRACE ELEMENT ZONING, DEFECTS AND COLOR

A Thesis

Submitted to the

Faculty of Miami University

In partial fulfillment of

The requirements for the degree of

Master of Science

Department of Geology

Carrie Wright

Miami University

Oxford, OH

2002

Advisor______Dr. John Rakovan

Reader______Dr. Hailiang Dong

TABLE OF CONTENTS

Chapter 1: Introduction to the cause of color in fluorite 1 Manuscript 1-Chapter 2 29 “Spectroscopic investigation of lanthanide doped CaF2 crystals: implications for the cause of color” Manuscript 2-Chapter 3 95 “Spectroscopic characterization of fluorite from Bingham, NM, Long Lake, NY and Westmoreland, NH: relationships between trace element zoning, defects and color

ii TABLE OF FIGURES

Chapter 1 Figures 21 Figure 1a. Ball and stick model of fluorite in the [100] direction 21 Figure 1b. Ball and stick model of fluorite in the [110] direction 22 Figure 1c. Ball and stick model of fluorite in the [100] direction 23 Figure 2. Schematic of Frenkel and Schottky defects in fluorite 24 Figure 3. Schematic of energy levels of F centers 25 Figure 4. Schematic of types of F centers 26 Figure 5. Color center model by Staeber and Schnatterly (1971) 27 Figure 6. Image of fluorite from Bingham, NM with color zoning 28

Chapter 2 Figures 55 Figure 1a-m. Optical absorption spectra for each synthetic sample before and after irradiation 55 Figure 2a-m. Fluorescence spectra for each synthetic sample before and after irradiation 68 Figure 3a-m. EPR spectra for each synthetic sample before and after Irradiation 81 Figure 4. Color center model by Staeber and Schnatterly (1971) 94

Chapter 3 Figures 133 Figure 1. Color center model by Staeber and Schnatterly (1971) 133 Figure 2. Image of slices of naturally colorless Long Lake fluorite before and after irradiation 134 Figure 3. Image of slices of naturally colored Long Lake fluorite before and after irradiation 135 Figure 4. Fluorite crystal from the Tex Mex Mine in Bingham, NM 136 Figure 5a-c. Chondrite normalized REE patterns for each locality 137 Figure 6a-m. Optical absorption spectra for each natural sample before and after irradiation 140

iii Figure 7a-m. Fluorescence spectra for each natural sample before and after irradiation 153 Figure 8a-q. EPR spectra for each natural sample before and after irradiation 166

iv TABLES

Chapter 2 Tables 53 Table 1. REE concentrations (ppm) and irradiation induced color of synthetic samples 53 Table 2. Luminescence peaks due to scattering 54 Chapter 3 Tables 131 Table 1. Sample names, localities and color before and after Irradiation 131 Table 2. REE concentrations (ppm) for natural samples 132

v ACKNOWLEDGEMENTS

I would like to thank several people for helping me through the process of finishing this thesis. First and foremost, the completion of this project would not have been possible without the support, patience, encouragement and constant availability of my advisor John Rakovan. Thank you for all of your help! To Dr. Hailiang Dong, thank you for agreeing to review this thesis. There are many people in the Chemistry Department at Miami University who helped me with experiments and data analyses. Thanks to Dr. Andy Sommer, Dr. Gil Pacey and Brian Patterson, who helped me with my optical absorption experiments. Thanks go to Dr. Mike Crowder and two of his graduate students, Patrick Crawford and Nathan Wenzel for helping me with EPR experiments. In the Geology Department at Miami University, many people helped me with experiments. To John Morton, thank you for all your patience and guidance with my DCP experiments. The first stages of sample preparation could not have begun without the knowledge and support of Stephanie Bosze, and thanks go to Art Losey for his help and encouragement. A special thanks to Joseph Talnagi at the OSU Nuclear Reactor Laboratory for irradiating my many batches of samples, and another to Craig Hemann in the OSU EPR lab for helping me with my last EPR experiments. Thanks to all the graduate students in the department who were understanding and encouraging especially during the rough patches of this journey, including Stephanie Bosze, Art Losey, Tatia Taylor, Allison Crowley, Darin Snyder, Nicki Richmond, Jen Wingate, and many others. Finally, thanks to my family and Glen for their constant support and understanding, and especially to my Dad and Grandma Murrell, who constantly asked me “Is it done yet?” and helped me push towards the finish!

CHAPTER 1.

INTRODUCTION TO COLOR IN FLUORITE

Introduction

Mineralogists, physicists, and chemists have studied the colors of fluorite for almost a century. The reasons are diverse, including gemological concerns about color,

the desire to understand the nature of defects within crystals (Berman, 1957), and the use

of fluorite as lasers (Dantes, et al. 1996) to name a few. A great deal of spectroscopic data

has been collected on both natural fluorite and synthetic fluorite doped with various

impurities (Smakula, 1950; Scouler and Smakula, 1960; Staebler and Schnatterly, 1971;

Anderson and Sabisky, 1971; Gaft et al., 2001 and many others). A few theories on the

cause of specific colors have been accepted, such as the F center and purple color. More

complex color centers involving impurities and structural defects are difficult to

characterize. This study examines the color of fluorite samples from Bingham, NM, Long

Lake, NY, and Westmoreland, NH, which have not been previously investigated.

Pure fluorite with few or no defects detectable by present technology is invariably

colorless. Add some impurities and/or structural defects and the color possibilities of

fluorite are extremely varied in hue and intensity. The complex relationships between

impurities and structural defects have led to great difficulty in pinpointing the exact cause

of color in many fluorites, both synthetic and natural (Bill and Calas, 1978).

The goals of this study include the determination of the cause of color in natural

fluorites from three locations that have not been thoroughly examined (Bingham, NM;

Long Lake, NY; Westmoreland, NH), and investigate the relationship between sectoral

zoning of REEs, defects and color. This chapter is an overview of the causes of color in minerals, and a review of previous studies of color and structural defects in fluorite.

Color in Minerals

There are many different causes of color in minerals, and most involve the interaction of light with electrons in the mineral structure. Nassau (1978) provides a good overview of the various causes that will be discussed only briefly here in the context of fluorite.

Crystal field theory explains many of the accepted causes of color in fluorite. This formalism is predominantly associated with ionic crystals containing ions with unpaired electrons, like transition elements (with partially filled d orbitals), actinides, and lanthanides (with partially filled f shells). Of these, the lanthanides are by far the most prevalent in fluorite, in which they substitute for Ca2+ within the crystal structure. In such ions, the unpaired electrons may interact with visible light, absorbing certain wavelengths, and producing color. The energy levels (ground and excited states) at which the unpaired electrons can exist depend on the valence state of the ion, the symmetry of the ion in the crystal, the strength of the crystal field, and the strength of the bonding.

Many authors have suggested, as well as presented evidence for, that lanthanides in certain oxidation states can play a role in the color of fluorite by crystal field transitions on them.

Unpaired electrons do not exist exclusively on ions in minerals. They can also exist on structural defects such as vacancies. In alkali halides, extensive research has resulted in the confident characterization of various types of structural defects, most of

which have been found in fluorite, and some have been implicated as causes of color in fluorite. These will be discussed in more detail below.

Several authors have also found evidence of defect complexes, some involving both impurities and structural defects, which are linked to color in fluorite. Crystal field theory may not be adequate to describe some of these, as they may involve molecular orbitals. This requires molecular orbital theory (MOT), which can describe situations where electrons are not located on single ions or structural defects, but in orbitals with multiple centers. MOT is similar to crystal field theory in that they both describe a set of possible energy levels for the electron(s), as well as the probability that a transition between levels will occur.

A third mechanism for the cause of color in fluorite is the optical effect of scattering of light. It has been suggested that this occurs in fluorite, which has large

(micron-sized) aggregates of defects such as calcium colloids (Bill and Calas, 1978;

Braithwaite et al, 1973).

Defect Centers

The nature of the fluorite structure makes it accessible to a wide variety of defects, including many that have the potential to cause color (Nassau, 2001). These include certain ion impurities of specific oxidation states, structural vacancies, and combinations of the two. The fluorite structure can be described as a cubic arrangement of F- ions with a Ca2+ ion at every other body-center site. Figures 1a-c are ball and stick diagrams of the fluorite structure in three different crystallographic directions. Some cations, primarily REE, can substitute easily for Ca2+ within the fluorite structure. Anion

impurities may enter the fluorite structure as charge compensation for cations such as O2-

(Merz and Pershan, 1967) and excess fluorine (Nassau, 2001). Other types of defects do not involve impurities, and these are purely structural.

Structural Defects

Two very basic types of structural defects that do not, by themselves, absorb light in the visible are Frenkel and Schottky defects. Frenkel defects consist of an anion

(fluorine in the case of fluorite) displaced from its normal position in the crystal lattice to an interstitial position (Nassau, 2001). Schottky defects involve the displacement of an anion and a nearby cation to the surface of a crystal. Both of these types of defects are pictured schematically in Figure 2. The most well-defined defect center in fluorite that gives rise to color is the F center, which is an electron trapped in a fluorine ion vacancy.

The F center is a common cause of purple color in fluorite (Nassau, 2001). Upon irradiation, an electron can be excited from its valence band into a higher energy state. If the irradiation is energetic enough, the electron can be excited into the conduction band of the crystal. Upon relaxation, in the case of an F center, the electron is trapped in a fluorine vacancy before it can return to the ion from which it originated. Subsequent irradiation by visible light can excite the electron giving rise to color. If the light is energetic enough, it may release the electron and produce bleaching (loss of color by destruction of the color center). Figure 3 is an energy level diagram describing the relative energies required to produce an F center, produce color (as an interaction between light and the electron of the F center) and destroy the F center (bleach the color).

There are several, more complicated variations of F center type defects that are

+ also thought to cause color in fluorite, including F’, F2, R and F2 centers. An F’ center is

an F center where two electrons have been trapped in a single fluorine vacancy. An F2 center consists of two adjacent F centers, and an R center is a group of three adjacent F

+ centers along the [111] (Bill and Calas, 1978). The F2 center consists of two adjacent

fluorine vacancies that “share” an electron. All of these are classified as electron color

centers, and they are all related to the simple F center (Nassau, 2001). A schematic

diagram of these centers is given in Figure 4. Electron hole centers, such as the Vk center,

where an electron is displaced from its normal position, can also cause absorption of visible light. For the Vk center, this situation occurs where two adjacent fluorine atoms

have a –1 charge between them (Sierro, 1965). Each of these defect centers must have

nearby charge compensation to maintain charge neutrality.

Impurity Ions

The fluorite structure readily incorporates many impurity ions, some of which

have been purported to cause color. REEs are very common impurities in fluorite, along

with certain transition elements, because they are close in size and charge to calcium.

Electronic transitions involving electrons from the d and f orbitals of these elements are a

major cause of color in minerals including fluorite (Bill and Calas, 1978). Under the Eh

conditions found in most natural environments, the majority of the REEs will exist in the

trivalent oxidation state (Merz and Pershan, 1967). Thus, if they are incorporated into

fluorite it will most likely be as REE3+, necessitating a coupled substitution to retain

charge neutrality. Only Sm, Eu, and Yb are easily incorporated into fluorite in the

divalent state (Merz and Pershan, 1967). The charge difference of the trivalent REEs can

be compensated for in one of several ways: by an interstitial F- ion, a nearby Ca2+ replaced with a sodium atom, an O2- replacing one F- ion, or by more complex substitution schemes (Naldrett et al, 1987). Interstitial fluorine ions in the body-centered position adjacent to the REE3+ and O2- ions substituting for F- are the two most

commonly found forms of compensation (Merz and Pershan, 1967). Both of these change

the symmetry of the Ca site, where the substituent REE3+ resides, from cubic to tetragonal

and trigonal, respectively.

Trivalent REE either do not absorb in the visible, or are not strong enough

absorbers to have an effect on the color (Naldrett et al, 1987). However, several divalent

REE, including Sm2+ and Eu2+, are purported to absorb strongly in the visible, and cause

color in fluorite (Bill and Calas, 1978; Morozov et al., 1996; Bill et al., 1967; Marfunin,

1979). It is thought that, upon irradiation from the decay of radioactive elements within

the structure, or from nearby sources, some of the trivalent REE in natural fluorites can

be reduced to the divalent state (Naldrett et al, 1987). This has been confirmed in

spectroscopic studies of irradiated synthetic samples doped with each REE (Merz and

Pershan, 1967; McClure and Kiss, 1963 and references therein]. Both groups, and the

work they summarized, found that, in general, only those trivalent REE in cubic sites

(therefore, with non-local charge compensators) could be reduced. This is most likely

because they are less stable within the structure than REE3+ with local compensation

forming a stable complex. Also, the ease of reduction varied from REE to REE. The

second half of the lanthanide series, whose ionic radii are closer in size to calcium, are

slightly easier to reduce (Merz and Pershan, 1967). The other possibility for the presence

of divalent REEs is that the fluorite grows from a sufficiently reduced fluid, where

certain REE are divalent upon incorporation into the fluorite structure (Naldrett et al,

1987).

Divalent samarium has been implicated in the cause of color in light to emerald

green fluorite (Bill and Calas, 1978; Bill et al., 1967; Morozov et al, 1996). The

uncertainty in assigning a particular REE to specific colors in fluorite arises from the fact that usually all or most of the lanthanides are present in natural samples, and there is usually more than one divalent REE that could cause the color. Several different electron transitions from various impurities could be occurring simultaneously to produce a color.

Or, impurities and nearby structural defects could be interacting to produce absorption in

the visible.

REE-associated defect centers

A great deal of research has been conducted on synthetic fluorite doped with various lanthanides over the past century by solid-state physicists and chemists in the interest of the material as a laser and as host to very complex defect structures that affect the physical properties of fluorite. Lanthanide ions are prime candidates for lasers, for instance, because they have many discrete energy levels in the infrared and visible

regions of the electromagnetic spectrum, and they can easily be incorporated minerals

such as fluorite that can be used in optical applications (Merz and Pershan, 1967). Out of this research came the recognition of a 4-band absorption spectrum (bands around 225 nm, 335 nm, 400 nm and 580 nm) common to many REE-doped synthetic samples exposed to a Ca vapor at low temperatures (additive coloration), x-ray irradiation at room

temperature, or gamma irradiation at room temperature. Smakula (1950) was the first to

observe this spectrum when he presented the first optical absorption spectra of color

centers in synthetic and natural CaF2 produced by x-ray irradiation. Subsequent studies revealed that the spectrum is intensified by the presence of yttrium (Scouler and Smakula,

1960 and others), and that structure within the spectrum is anisotropic (Gorlich et al,

1967). The latter information negated the idea by some (O’Connor and Chen, 1963) that the four-band absorption is due to divalent REEs that were reduced during the irradiation or additive coloration. This hypothesis was based on previous studies (Merz and Pershan,

1967) that found no evidence of the reduction of REE3+ to REE2+ in sites with symmetry

other than cubic. The spectrum was also found in a control sample of “pure” CaF2, but

they acknowledged that their samples might have contained some small amounts of

impurities that could be responsible for the absorption.

Staebler and Schnatterly (1971) developed a model for the complex responsible

for the four-band spectrum through extensive spectroscopic investigation of synthetic

fluorite doped with REEs, and the EPR work of Anderson and Sabisky (1971), as well as

the theoretical work of Alig (1971). They concluded that the structure responsible was a

REE3+ associated with an adjacent F- vacancy that had trapped two electrons, and was

thus electrostatically neutral. Figure 5 is a schematic diagram of the model for this center.

Staebler and Schnatterly used synthetic fluorites from Optovac that were doped

with individual REEs (Staebler, 1970), exposed them to a Ca vapor at low temperatures to induce additive coloration, and found the four-band spectrum most strongly in those samples doped with Y, La, Lu, Ce, Gd, and Tb. Each of these rare earths has a single d electron in the divalent crystal field ground state. The spectra were only slightly different

from one sample to another, indicating that each dopant had its own effect, but that they

were similar enough to be considered the same center.

Linear dichroism results of Staebler and Schnatterly (1971) confirmed the

anisotropy of the center, its three-fold symmetry similar to that of a REE next to an F

center and that the four-band spectrum is entirely associated with a single center by

revealing how the spectra changed with the direction in which the light was polarized.

The linear dichroism experiments also allowed for the characterization of the symmetry

and orbital degeneracy of each individual band (either pi or sigma). The lowest energy band (around 580 nm) was found to have pi symmetry, and thus circular dichroism experiments, which involve a magnetic field, could be performed. Only Gd-, Ce-, and Tb- doped samples had circular dichroism because they are paramagnetic in the trivalent state. These results confirmed the model of an electrostatically neutral center consisting of a REE3+ and a F- vacancy with two trapped electrons oriented in the [111] direction.

The EPR studies on these same samples by Anderson and Sabisky (1971)

confirmed the model presented by Staebler and Schnatterly, although their EPR spectra

were primarily of the centers after they were ionized with a specific wavelength of visible light. When exposed to blue light (400 nm), the centers described by Staebler and

Schnatterly (1971) lose an electron, leaving a trivalent REE next to an F center. Alig

(1971) confirmed, in theory, the plausibility of the Staebler and Schnatterly model through discussion primarily of the ionized center. The discussion was in terms of the

molecular orbital theory because the two trapped electrons are shared by the REE3+ as

well as the F- vacancy.

Subsequent authors have not rejected this model, but have proposed much more

complex models such as hole centers associated with monovalent and trivalent impurities

(Dantas et al, 1996), and have primarily studied the effects of various growth and

coloration methods on the four-band spectrum, as well as other bands. There are

contradicting data and theories to explain many of the optical absorption bands found in fluorite, but the Staebler and Schnatterly model remains the most simple and has the most corroborating evidence. Most studies have been concerned with the physical properties of fluorite with REE impurities as a laser material, and do not mention the color produced during their individual experiments. A few mineralogical studies do mention the colors associated with a few REE-associated F centers, and will be discussed below.

Color in Fluorite

For each color in which fluorite occurs, various causes have been proposed with varying degrees of evidence. The fluorites investigated in this study are various shades of green, blue and purple. Therefore, these colors will be the focus of this synopsis of previous work on the cause of color in fluorite. Bill and Calas (1978) provides a summary of the work on a broader range of fluorite colors.

Green Fluorite

In discussing the role of REE in luminescence and the color of fluorite, A. S.

Marfunin (1979), without providing references or the mechanism(s), states that Sm2+,

Dy2+, and Tm2+ are the cause of green color in fluorite. Sm2+ has been suggested in other

studies, but Dy2+ and Tm2+ have not, and it is uncertain whether Marfunin was referring

to the luminescence of these ions in CaF2, or absorption of visible light by these ions.

Luminescence of Yb3+ is also mentioned as the cause of the yellow-green glow of some fluorites.

Sm2+ is implicated as a cause of green color in fluorite in various other studies.

Bill et al. (1967) presents optical absorption data of Sm3+-doped fluorite that had been

irradiated by X-rays. The colorless sample became green upon irradiation. The absorption

peaks at 680, 610, 440, 422, 396, 355, 305, 281, 255, 240 and 218 nm were attributed to

divalent samarium, which is stable in the fluorite matrix at room temperatures. No EPR

signal was associated with the impurity, as it is diamagnetic. The authors report bleaching

the green color at 300°C, and explain this as the divalent samarium reverting back to

trivalent.

George Rossman’s spectroscopy website, http://minerals.gps.caltech.edu, contains

two optical absorption spectra of green fluorite. The bright green color of the fluorite

from Brazil is attributed to divalent samarium ions, with OA bands near 690, 611, 440, and 422 nm. The other spectrum of green fluorite contains bands at 580 and 400 nm, which are attributed to F centers in the presence of trivalent yttrium, and a band at 714 nm caused by F centers near Ce ions. Bill and Calas, 1978, present their own data on these coexisting centers, which they implicated as the cause of yellowish-green color in fluorite. Bands at 230, 335, and 400 nm are similar to those in other fluorite studies like those mentioned above, with the addition of a band at 306 nm, probably due to the

4f→5d transition of trivalent cerium (Alig et al, 1969). Merz and Pershan (1967) attributed this band to hole centers produced upon irradiation. Also, bands at 590 nm and

712 nm were attributed to yttrium and cerium-associated defect centers, respectively. The

Ce- and Y-associated F centers can be bleached upon heating (Staebler and Schnatterly,

1971).

Blue Fluorite

The review of color in fluorite by Bill and Calas (1978) mentions the Y-

associated F center characterized by Staebler and Schnatterly (1971) as the cause of blue

color in some fluorites, with an optical absorption spectrum of blue fluorite showing the

characteristic four-band spectrum first observed by Smakula (1950).

Murr (1979) made direct observations of defect centers in fluorite from Bingham,

NM with a transmission electron microscope, and found that the blue coloration

intensified as the defect concentration increased. A variety of structural defects were

observed, but the majority appeared to be defect aggregates, or many F centers, in the

(111) planes of the fluorite samples.

Blue-John-type fluorites are purported to get their deep blue color from the

absorption produced by calcium colloids, which are the result of the coagulation of color

centers that involve fluorine vacancies, like F centers (Braithwaite et al., 1973; Bill and

Calas, 1978). These produce a very intense absorption between 560 and 580 nm, which

usually swamps whatever other absorption is present. Bill and Calas (1978) suggest that

the color is due to metallic calcium particles, but it could also be due to the plethora of F

centers present to maintain crystal neutrality. The absorption at shorter wavelengths

(closer to 560 nm) causes more of a purple color.

The blue-violet luminescence of Eu2+ at 413 nm has been implicated as the cause of bluish-purple color in some fluorite (Marfunin, 1979; Recker et al., 1968; Bill and

Calas, 1978). Europium is fairly stable in fluorite in the divalent oxidation state,

compared to many of the other rare earth elements (Merz and Pershan, 1967).

Purple Fluorite

F centers are the most popular theory for purple color in fluorite (Berman, 1956;

Nassau, 1978) primarily because F centers produce similar color in other alkali halides,

with a broadband absorption around 560 nm. The color is also relatively easy to produce

by irradiation or additive coloration, and is also relatively easily bleached by heat in most

samples. The location of the absorption band of an F center in fluorite is not agreed upon

in the literature. Mollwo (1934) [cited in Scouler and Smakula, 1960] suggested that the

F center band falls at 375 nm, while Bontinck, (1958) contends that it falls at 525 nm.

Both of these bands are found in a few natural fluorites, as well as additively colored

(baked in a Ca vapor so that Ca2+ was added to the system, creating fluorine vacancies)

synthetic fluorite. Staebler and Schnatterly (1971) concur with Mollwo (1934), and

suggested that the 400 nm band of the four-band Smakula spectrum is the F center band

perturbed by a nearby REE impurity because it is usually the most intense of the four.

Others (including Nassau, 2001) assume, for reasons mentioned above, that the F center

band is at 560 nm.

Calcium colloids, discussed above, also exhibit an absorption band at 560 nm. If both F centers and calcium colloids produce purple color and have similar OA spectra, the idea that it is not actually the Ca colloids that produce the color, but the F centers present, is reinforced.

Naldrett et al. (1987) suggest that divalent REEs, reduced initially by radioactive materials present in the natural crystals or nearby, are responsible for purple color. The thermal history and degree of bleaching produces the other colors observed in fluorite.

These ideas are based on the problem of correlating the enrichment or depletion of any one REE to a given color, which are present most commonly in fluorite in the trivalent oxidation state. However, only one divalent REE has been associated with pale bluish- purple color, Eu, even when it is not the most abundant REE in a sample. Complex absorptions by multiple REE could explain certain colors, but simpler theories exist and are more widely accepted.

Color zoning in fluorite

Fluorite color in natural samples is commonly concentrically zoned, indicating variations in growth conditions. The degree of zoning varies, but emphasizes the fact that color is very dependent on conditions of growth and slight differences in the fluid from which the fluorite crystallizes can lead to color change. Some authors (Dickson, 1980) have observed concentric zoning and blotchiness of color in irradiated natural samples, as well as differences in color intensity among samples exposed to the same amount of irradiation, indicating a variable susceptibility to irradiation-induced color (usually blue or purple) among fluorites (Berman, 1956). The blotchiness observed by some authors may actually be sectoral zoning of irradiation-induced color or color intensity. Recent studies outlined below have established the sectoral zonation of REE, as well as irradiation-induced color intensity, in fluorites from Long Lake, NY and Bingham, NM.

Sectoral zonation of REE in fluorite

Geochemical studies on the fluorites from Long Lake, NY and Bingham, NM by

Bosze and Rakovan (2001) revealed compositional heterogeneities with respect to the lanthanides between symmetrically nonequivalent sectors within single crystals (sectoral zoning). The use of synchrotron X-ray fluorescence microanalysis (SXRFMA) and cathodoluminescence (CL) as two of their analytical techniques produced structural defects, or color centers, in the fluorite samples. These defects produced a purple color that was more intense in some sectors than others, and is thus correlated with REE distributions.

The sectoral zoning of REE within single fluorite crystals documented by Bosze and Rakovan (2001), coupled with the apparent sectoral zoning of color intensity produced as a result of analytical irradiation, indicates that the REE composition of fluorites has some control over the color produced by F centers, or possibly the production of F centers. Dickson (1980) irradiated several fluorite samples of different colors and localities, and found that all of the samples became purple to blue in color.

These samples were not analyzed for REE content. However, Dickson found that the intensity of color varied by concentric growth zone, and by “region” (possibly sectors).

The preferential incorporation of REE between sectors studied by Bosze and Rakovan is controlled by surface properties during growth of symmetrically nonequivalent crystal faces of a single fluorite crystal. As a crystal grows, certain faces preferentially incorporated all the REE over others. The irradiation-induced color occurs after growth, and therefore variation in intensity of color between the sectors cannot be a direct result of surface properties during growth. If the sectors varied in their affinity to produce F

centers upon irradiation, this could perhaps be tested if a variation in the number of F

centers (intensity of the electron peak in an EPR spectrum or OA band) is noticeably different between sectors. The sectors with more intense peaks should exhibit more

intense color. However, Bosze and Rakovan (in review) used a constant radiation source,

which theoretically should produce a constant number of defects throughout the exposed

fluorite. The established post-growth difference between the sectors is their relative

concentration of REE. A more likely hypothesis is that the REE exhibit some control over the color produced by the irradiation-induced defect centers. A few, but not all of the samples analyzed by Bosze and Rakovan (2001) show a qualitatively positive relationship between concentration of REE and intensity of irradiation-induced color.

Figure 6 is a plane-light image of irradiated fluorite. The purple color is post-irradiation, and SXFRMA analyses reveal that REE concentration is >(110)>(100)>(111). The exact relationship between the purple color and REE content is not known. A few authors have suggested that some REE can prevent the formation of color centers (Bill and Calas,

1978) with no explanation. They may be referring to the Staebler and Schnatterly model, in which pure F centers are of lesser importance (and perhaps in lesser abundance) than the REE-associated 2-electron F center with respect to color. Different studies indicate that color is enhanced or intensified by the presence of specific impurity elements, such as yttrium (Staebler and Schnatterly, 1971; O’Connor and Chen, 1963; Scouler and

Smakula, 1960; Gorlich et al, 1963). In the case of the Bingham fluorite in Figure 6, the most intensely colored sector is the one with the higher concentration of REE, indicative

of the second aforementioned relationship.

Recent irradiation experiments

Irradiation of fluorites from the same locales used by Bosze and Rakovan (2001),

as well as Westmoreland, NH, show mixed results in the degree of zonation of color. The

samples received between 10 and 20 Mrads by the gamma source at the Ohio State

University Nuclear Reactor Laboratory. The irradiation experiments were conducted by

Joseph Talnagi.

Naturally blue and purple Bingham samples intensified in color to such a degree,

that only slight concentric zoning and some blotchiness of color could be discerned.

Naturally pale green and colorless samples became pale blue or teal, with some green regions that could be (111) and (110) sectors.

The Westmoreland samples remained uniform in color except for the late stage very pale green sample, which showed some sectoral zoning of pale green and pale blue.

The Long Lake samples were discretely sectorally as well as concentrically zoned by color, not just color intensity. Five distinct colors were produced by the irradiation:

maroon or purplish-brown, blue, green, and purple, as well as the naturally gray center,

which did not change upon irradiation. These color zones match the CL data by Richards

and Robinson (2000), indicating the relationship between the color and the variations in

the REE contents of the sectors.

Summary

The past century of research into the cause of color in fluorite has revealed its

complexity compared to more simple systems, specifically the alkali halides, for which

color centers have been very well defined. It is apparent that each color may have more

than one possible cause, and a spectroscopic investigation is necessary to characterize the color of a previously unstudied deposit. One goal of this study is to determine the cause of color in fluorites from three deposits, which have been well examined, but not in terms of color. The second research goal is to characterize the relationship that exists between

REE and color/color intensity in terms of the sectoral zonation of both in the natural fluorites.

Synthetic samples were obtained for the purpose of controls in this study. The irradiation-induced color of synthetic fluorite doped with various lanthanides has also been investigated, but primarily in the interest of solid-state physics and materials science, which are not concerned with the actual color produced. From a mineralogical/gemological perspective, the color induced by irradiation is very applicable to understanding the cause of color in the more complex natural crystals. The third research objective, therefore, is to examine the relationship between color and REE in terms of commonly used synthetic samples, and explore the implications for explaining the color observed in natural samples.

References

Alig, R. (1971) Theory of photochromic centers in CaF2. Physical Review B, 3, (536- 545).

Alig, R., Kiss, Z., Brown, J., McClure, D. (1969) Energy levels of Ce2+ in CaF2. Physical Review, 186, (276-284).

Anderson, C. and Sabisky, E. (1971) EPR studies of photochromic CaF2. Physical Review B, 3, (527-536).

Berman, R. (1957) Some physical properties of naturally irradiated fluorite. American Mineralogist, 42, (191-203).

Bill, H. and Calas, G. (1978) Color centers, associated rare-earth ions and the origin of coloration in natural fluorites. Physics and Chemistry of Minerals, 3, (117-131).

Bill, H., Sierro, J., and Lacroix, R. (1967) Origin of coloration in some fluorites. The American Mineralogist, 52, (1003-1009).

Botinck, W. (1958) The hydrolysis of solid CaF2. Physica, 24, (650-658).

Braithwaite, R.S.W., Flowers, W.T., Hazeldine, R.N., and Russell, M. (1973) The cause of colour of Blue John and other purple fluorites. Mineralogical Magazine and Journal of the Mineralogical Society, 39, (401-411).

Bosze, S. and Rakovan, J. (2001) Surface-structure-controlled sectoral zoning of the rare earth elements in fluorite from Long Lake, New York, and Bingahm, New Mexico, USA. Geochimica et Cosmochimica Acta, 66, (997-1009).

Dantas, N.O., Watanabe, S., Chubaci, J.F.D. (1996) Optical absorption (OA) bands in fluorites by heavy gamma irradiation. Nuclear Instruments and Methods in Physics Research B, 116, (269-273).

Dickson, J. (1980) Artificial colouration of fluorite by electron bombardment. Mineralogical Magazine, 43, (820-822).

Gaft, M., Panczer, G., Reisfield, R., Uspensky, E. (2001) Laser-induced time-resolved luminescence as a tool for rare-earth element identification in minerals. Physics and Chemistry of Minerals, 28, (347-363).

Gorlich, P., Karras, H., Symanowski, C., and Ullmann, P. (1968) The colour center absorption of x-ray coloured alkaline earth fluoride crystals. Phys. Stat. Sol., 25, (93- 101).

Merz, J. and Pershan, P. (1967) Charge conversion of irradiated rare-earth ions in calcium fluoride. Physical Review, 162, (217-235).

Morozov, M., Trinkler, M., Plotze, M., and Kempe, U. (1996) Spectroscopic studies on fluorites from Li-F and alkaline granitic systems in central Kazakhstan. Granite-Related Ore Deposits of Central Kazakhstan and Adjacent Areas, (359-369).

Murr, L.E. (1973) Ordered lattice defects in colored fluorite: direct observations. Science, 183, (206-208).

Spectroscopy, Luminescence and Radiation Centers in Minerals, A. S. Marfunin, Springer-Verlag, New York, 1979.

Naldrett, D.L., Lachaine, A., and Naldrett, S.N. (1987) Rare-earth elements, thermal history, and the colour of natural fluorites. Canadian Journal of Earth Science, 24, (2082-2088).

Nassau, K. (1978) The origin of color in minerals. American Mineralogist, 63, (219-229).

The Physics and Chemistry of Color, K. Nassau, John Wiley & Sons, Inc, 2001.

O’Conner, J. and Chen, J. (1963) Color centers in alkaline earth fluorides. Physical Review, 130, (1791-1795).

Richards, R. and Robinson, G. (2000) Mineralogy of the calcite-fluorite veins near Long Lake, New York. The Mineralogical Record, 31, (413-422).

Scouler, W. and Smakula, A. (1960) Coloration of pure and doped calcium fluoride crystals at 20oC and –190oC. Physical Review, 120, (1154-1161).

Sierro, J. (1965) Paramagnetic resonance of the Vf center in CaF2. Physical Review, 138, (648-650).

Smakula, A. (1950) Color centers in calcium fluoride and barium fluoride crystals. Physical Review, 77, (408-409).

Staebler, D. (1969) Optical studies of a rare-earth F center complex in rare-earth doped calcium fluoride. Ph. D. thesis, Princeton University (unpublished).

Staebler, D. and Schnatterly, S. (1971) Optical studies of a photochromic color center in rare-earth-doped CaF2. Physical Review B, 3, (516-526).

Figure 1a. Ball and stick model of the fluorite structure, looking down the [100] direction. The rust brown balls represent calcium atoms, and the golden balls represent fluorine atoms. The black box represents the boundaries of one unit cell, and the green sticks represent bonds between the atoms.

Figure 1b. Ball and stick model of the fluorite structure, looking down the [110] direction. The rust brown balls represent calcium atoms, and the golden balls represent fluorine atoms. The black box represents the boundaries of one unit cell, and the green sticks represent bonds between the atoms.

Figure 1c. Ball and stick model of the fluorite structure, looking approximately down the [111] direction. The rust brown balls represent calcium atoms, and the golden balls represent fluorine atoms. The black box represents the boundaries of one unit cell, and the green sticks represent bonds between the atoms.

Figure 2. Schematic of Frenkel and Schottky defects. Looking down the [100] direction.

Figure 3. Schematic energy diagram of the different energy levels involved in creating and destroying an F center, as well as in producing color.

Figure 4. Schematic of the different types of F centers. Looking down the [100] direction.

Figure 5. Schematic of photochromic center in fluorite involving a molecular orbital with two electrons shared by a trivalent REE and a fluorine vacancy. Modified from Staebler and Schnatterly, 1971.

Figure 6. Plane light image of polished fluorite from Bingham, NM. The purple color is post-cathodoluminescence imaging, which created color centers with energized electrons. The intensity of the color is different among sectors.

CHAPTER 2

Spectroscopic investigation of lanthanide doped CaF2 crystals: implications for the cause of color

Abstract

REE-doped synthetic fluorite (CaF2) samples were obtained (synthesized by

Optovac, Inc. c. 1960’s) and analyzed before and after 10-25 Mrad of gamma irradiation

by various spectroscopic methods to further clarify the relationship between lanthanides,

defects and color. Samples used were doped with 10-300 ppm of La, Y, Ce, Pr, Nd, Sm,

Eu, Gd, Dy, Ho, Er, Yb and Lu. All samples, except the Eu- and Sm-doped, were

colorless before irradiation. Irradiation produced color in all of the samples except those doped with Dy and Yb. DCP analyses indicate that the samples contain significant

concentrations of REE or Y besides the dopant, with the exception of those samples

doped with Ce, La, Gd and Y. Optical absorption experiments confirmed the presence of

Sm2+-induced absorption in the Sm-doped sample, which became green upon irradiation.

The “Smakula” optical absorption spectrum (Smakula, 1950) is present in the samples

doped with Ce, Gd, Y, La, and others, which corresponds to the color center model put

forth by Staebler and Schnatterly (1971). Luminescence is not likely strong enough to

produce the color, and is diminished by irradiation. Electron paramagnetic resonance

(EPR) data indicate the presence of various REE in non-cubic symmetry, which also

corresponds to the model (Staebler and Schnatterly, 1971) of color centers in some

fluorites. In samples with impurities other than the main dopant (Pr, Nd, Sm, Eu, Dy, Ho,

Er, Yb, Lu-doped) it is not possible to unequivocally assign the cause of color to the

presence of a color center involving the main dopant. For the samples doped with Ce, La,

Gd and Y the combined spectroscopic data indicate that a dopant-associated color center

is responsible for the irradiation-induced color. The “Smakula” optical absorption data

suggest a lanthanide-associated F-center with two trapped electrons as the cause of color

in these samples. Slight differences in the absorption peak position give blue color in the

Gd and Y samples and green in the La and Ce samples.

Introduction

Fluorite is of great interest to mineralogists for a variety of reasons. The colors of fluorite and their causes have been the subject of a many studies, of which Bill and Calas

(1978) is a good synopsis. Geochemical applications (Constantopoulos, 1988; Hill et al.,

2000) of REEs in fluorite are important in the context of ore exploration and understanding mineral deposit formation. Recent studies investigating sectoral zoning of trace elements in fluorite, have implications for crystal growth mechanisms (Bosze and

Rakovan, 2001). Solid-state physicists and chemists study the changes in the physical and spectroscopic properties of fluorite with different dopants as a laser material (Dantas et al, 1996) and charge conversion efficiency (Merz and Pershan, 1967). Mineralogists have studied the cause of color in natural fluorite extensively. The result is several theories for each color, from the simple F center which can produce purple color (Nassau, 2001) to complex structural defects involving impurities like the YO2 center responsible for yellow color in some fluorite (Bill and Calas, 1978). Studies of REE-doped and irradiated fluorites are primarily focused on the spectroscopic properties of the color centers rather than the color of the samples. Nonetheless, they have helped to clarify the complex relationship between color and impurity ions, specifically REEs, even if the actual color produced in the fluorite samples is rarely mentioned. Smakula (1950) was the first to

observe a series of four absorption bands found in many synthetic and natural fluorites

colored by irradiation. Further experiments and discussion of these absorption bands

(around 225 nm, 335 nm, 400 nm, and 580 nm) led to a study by Staebler and Schnatterly

(1971) in which they characterized the center responsible for the bands and color as a

trivalent REE coupled with two electrons and a nearest-neighbor fluorine vacancy. This

model was based on optical absorption, linear and circular dichroism, as well as EPR

experiments and theoretical work by Anderson and Sabisky (1971) and Alig (1971),

respectively. These authors studied synthetic fluorite doped with La, Ce, Gd, Tb, Lu and

Y, and presented no chemical data. In this study, irradiation experiments comparable to those in previous studies are performed with a focus on the colors that arise and their

relationship with REE(s) present. In addition to the REE(s) mentioned above, data from

samples doped with Pr, Er, Nd, Sm, Dy, Yb and Ho will also be presented, and chemical

data will be presented for each. Optical absorption (OA), Electron Paramagnetic

Resonance spectroscopy (EPR), Direct Current Plasma spectroscopy (DCP), and luminescence data are included.

Materials and Methods Samples

Single crystal synthetic fluorites doped individually with La, Ce, Pr, Nd, Sm, Eu,

Gd, Dy, Ho, Er plus Pr, Yb, Lu and Y were used in this study. The samples were given

to the authors by Rob Sparrow of Corning, and were synthesized by Optovac in the

1960’s using the Stockbarger growth technique (Staebler and Schnatterly. 1971). In their synthesis, naturally occurring fluorite from Mexico was mixed with a measured dopant

(XF3), as well as a small percentage of lead fluoride to prevent hydrolysis. The starting material was powdered and loaded in a graphite crucible into a vacuum furnace where any gases from the powder were evacuated while the material melted. The melt crystallized in a boule, and went through annealing and cooling to room temperature. The concentration of trace elements in the resultant crystal depends on the initial purity of the raw material and any loss of dopant during synthesis. Samples were chosen for this study with REE dopant concentrations between 8 and 300 ppm to approximate the concentrations found in natural samples studied by the authors (Wright and Rakovan, in review). Table 1 lists the samples and the weight percent of the dopants used in synthesis by the manufacturer.

Irradiation

The synthetic crystals were irradiated at the Ohio State University Nuclear

Reactor Laboratory by Joseph Talnagi. The crystals received between 10 and 26 Mrad of irradiation from their 60Co gamma source. Table 1 lists the samples and their color before and after irradiation.

Direct Current Plasma Spectrometry (DCP)

The use of a natural starting material creates the possibility of contamination by

REE and Y, and the lack of chemical data from the manufacturer necessitated chemical analyses. Slices of the synthetic fluorite boules were washed in methanol, followed by a deionized water rinse. The samples were powdered using a mortar and pestle, placed in

uncapped glass vials in an oven overnight at 110˚C to eliminate any moisture. After heating, the vials were capped and placed in a dessicator until cool.

Approximately one gram of powdered sample was well mixed with 1.5 grams of lithium metaborate flux, placed in a graphite crucible, heated at 950˚C for 30 minutes, then poured into a 250 mL polyethylene bottle containing 100 mL of 5% HNO3. The

bottles were placed on a shaking table for at least an hour to improve dissolution of the

sample.

To remove any graphite from the crucibles, the samples were filtered through

quartz wool before being loaded onto 20 x 1 cm columns of AG50W-X8 cation exchange

resin. After the samples passed into the resin, 150 mL of a mixed acid (2.6 N HNO3 + 2.5

N HCl) was used to elute the sample matrix that did not contain any REE. The REE fraction was eluted by 250 mL of 6 N HCl, which was collected in a 250 mL Teflon beaker and dried on a hot plate. Upon drying, the REE fraction was taken up in 1-2 mL of the mixed acid, and loaded back onto their respective cleaned columns. The above procedure was repeated (mixed acid then HCl acid) to remove any lingering matrix elements. The resulting sample was taken up in 4 mL of 5% HNO3 spiked with 3000 ppm

K+ which acted as an ionization buffer. The sample solutions were measured against external REE standard solutions using a Beckman Spectra Span V direct current plasma

(DCP) spectrometer at Miami University.

Optical Absorption Spectroscopy

All of the fluorite samples were cut using a Buehler Isomet Saw. The cuts were of various thicknesses, depending on the intensity of the fluorite color. Samples were cut up

to 5 mm thick, and samples that became darkly colored after irradiation were re-polished

to as thin as 1 mm. All of the samples were initially polished with a HI-TECH

DIAMOND electric lap wheel using 1200 mesh and 3000 mesh lap disks. Many samples were subsequently hand-polished with ALLIED water-based polycrystalline diamond

suspension sprays in 6 and 1 micron grits on glass plates.

Optical absorption spectra were obtained using a Hewlett Packard 8453 UV-

Visible Spectrophotometer. The spectrophotometer contains a 1024 element photodiode

array spectrometer. Spectra of the fluorites were taken in absorption mode with the

maximum integration time of 25.5 s and with a maximum number of 2 spectra added.

Data were collected in the range of 200 to 800 nm. The photodiode array and wavelength

range allow a sampling interval of approximately 1 nm. The spectrophotometer includes a concave holographic grating, the slit width is 1 nm, and the wavelength resolution is 2

nm. Fluorite samples were masked with a 1mm metal aperture for spatially resolved

analyses.

Luminescence

Luminescence experiments were conducted on a Perkin Elmer LS55

Luminescence Spectrometer with entrance and exit slit widths at the minimum of 2.5 nm.

Ten scans were added for each spectrum with a scan rate of 500 nm/min in the

wavelength range of 350 nm to 800 nm. An excitation wavelength of 300 nm was used

for each sample. UV-Vis absorption spectra were used to identify excitation additional

wavelengths for each sample.

Electron Paramagnetic Resonance Spectroscopy

Rectangular sections, approximately 10 mm by 1 mm by 1 mm in size, were cut

from the lanthanide-doped synthetic fluorite samples. The samples doped with Ce, Pr and

Er, Dy, Sm, Pr, and Gd were not oriented in a crystallographic direction. Those that were include the samples doped with Nd [110], Yb [100], Ho [110], Lu [111], La [100], and

Eu [100], and were oriented with this direction perpendicular to the long axis of the rectangle. One end of each of the fluorite sections was then glued with Epo-Tek 301 epoxy to separate quartz glass rods with an outer diameter no greater than 2.5 mm. The rods minimized tilt of the samples when placed in the EPR sample tubes.

Single-crystal EPR spectra of each of the fluorite samples (before and after irradiation) were collected using a Bruker EMX-6 X-band CW-EPR spectrometer containing an ER041XG microwave bridge and a TE102 cavity coupled with an Oxford liquid helium controller at the Department of Chemistry, Miami University. The experiments were run with a field modulation frequency of 100 KHz, an average microwave frequency of 9.4 GHz, and temperatures ranging from 4-5 K. The spectral resolution was 1024 data points over 2000 G, or 2.0 G. The field-swept EPR experiments were performed at 45-degree intervals within one plane of each crystal. This was accomplished by rotating the sample tube within the cavity approximately 45 degrees, with a maximum rotational error of 15 degrees.

Results

Irradiation

The colors of the synthetic samples before and after irradiation are listed in Table

1. All of the samples were colorless before irradiation except the Eu-doped sample and

the Sm-doped sample, which were pale purple and pale green, respectively.

Luminescence emission of Eu2+ in fluorite is known to produce purple color (Marfunin,

1979), and Sm2+ has been established as a cause of green color in fluorite. Divalent REEs may have existed in the natural starting material before doping and synthesis, or perhaps

a small portion of the dopant was reduced during synthesis. Both REEs are more stable in

the fluorite structure in the divalent oxidation state (Merz and Pershan, 1967). Upon

irradiation, both samples became different shades of bright green. Three other samples

also became green upon irradiation, those doped with Ce, Pr plus Er, and Pr. The samples

doped with Y, Ce, Gd and Ho all became different shades of blue upon irradiation. The

samples doped with Nd and Lu both turned golden-yellow while the samples doped with

Dy and Yb remained colorless.

DCP The results of the DCP analyses for each of the samples are listed in Table 1. The

concentrations of the REE analyzed (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb, Lu plus

Y) are in ppm. Four of the samples contain very high concentrations of the dopant cation

and very low concentrations of all the other REE analyzed, these include the samples

doped with La, Ce, Gd and Y. Seven samples had high concentrations of the dopant

cation, as well as one or more other REE. The Pr and Er doped sample is also high in Nd and Dy; the Pr-doped sample is high in La; the Nd-doped sample is high in La and Y; the

Eu-doped sample is high in La; the Sm-doped sample is high in La; the Ho-doped sample

is high in La and Y; and the Dy-doped sample is high in Y. The Yb-doped and Lu-doped

samples contain concentrations of the dopant cation much lower than reported by the manufacturer (< 10 ppm) and were not much higher than the concentration of lanthanum in the sample. The Yb-doped sample was very high in yttrium as well. The concentration of yttrium in the Sm and Lu samples was not analyzed due to inadequate amounts of sample. It is evident from the DCP data that most of the synthetic samples have relatively high concentrations of REE besides that of the dopant.

Optical absorption The optical absorption spectra of all the samples before and after irradiation are in figures 1a-m. The spectra of each sample changed with irradiation. Seven spectra of irradiated samples contain peaks around the values of the four-band (225 nm, 335 nm,

400 nm, and 580 nm) Smakula spectrum (Smakula, 1950), including the samples doped with Gd, Y, Pr, Lu, La, Ho, and Ce. The Sm-doped sample (Fig.1f) gained peaks at 243 nm, 256 nm, 308 nm, 422 nm, 445 nm, and 622 nm. The peaks at 256 nm, 308 nm, 422 nm, 445 nm and 622 nm correspond closely to those found in fluorite with irradiation- induced green color, which has been attributed to Sm2+ (Bill et al, 1967; Bill and Calas,

1978; Recker et al, 1968; Loh, 1968). The optical absorption spectra of the Dy- (Fig.1i) and Yb-doped (Fig.1k) synthetic samples did not change significantly after irradiation, and they remained colorless. The Dy-doped sample gained a peak at 283 nm, while the complex spectrum of the non-irradiated Yb-doped sample became slightly more intense upon irradiation. The spectrum of the irradiated Pr and Er-doped sample (Fig.1d) gained broad peaks at 390 nm, 490 nm, and 600 nm, along with high intensity noise in the UV

region, obscuring any peaks. The holmium-doped sample (Fig.1j) gained peaks at 233

nm, 262 nm, 317 nm, 390 nm, 560 nm, and 679 nm after irradiation. The Nd-doped sample (Fig.1e) gained peaks at about 260 nm and 560 nm, and the Eu-doped sample

(Fig.1g) gained peaks at about 339 nm and 414 nm upon irradiation.

Luminescence The results of the luminescence experiments are shown in Figure 2a-m. Each graph contains the spectra for a single sample at various excitation wavelengths before and after irradiation.

The background spectra noted in Table 2 for each excitation wavelength are instrumental artifacts arising from reflection of stray light off of the solid sample holder.

Additional filters would have obscured the spectra and so were not used to correct the problem.

EPR The spectra collected for each sample before and after irradiation are shown in

Figure 3a-m. Each graph contains up to eight spectra, one for each of the four positions in which spectra were collected before and after irradiation.

Several broadband peaks found in every spectrum can be attributed to the Epo-

Tek glue used to stabilize the samples within the EPR tubes. These peaks are indicated

with asterisks in Figure 3e. Spectra were collected of the empty cavity, an empty EPR

tube, and an EPR tube with a quartz glass rod with Epo-Tek glue. The sharper peak at a

g-factor of approximately 2 is found in all of the spectra, and is due to contamination of

the EPR cavity and perhaps paramagnetic entities in the fluorite with a g-factor of 2 as

well. The larger broadband peaks are only found in the EPR tube with a quartz glass rod with Epo-Tek glue.

Five samples displayed no discernable structure before or after irradiation, other

than these extraneous signals, including the Sm-, La-, Eu-,Y- and Pr-doped samples. Four

samples have low-intensity structure, including the non-irradiated Ce-, Dy-, Ho-, and Nd-

doped samples. The Er+Pr-, Yb-, and Lu-doped samples have very complex, high-

intensity structure, both before and after irradiation.

Many paramagnetic lanthanides and other defects of interest have g-factors close to 2 (Anderson and Sabisky, 1971), and their structure may be present but obscured by the contamination signal of the empty cavity. The sharper peaks resulting from the Epo-

Tek glue around the broadband peak at lower Gauss could also obscure structure from

paramagnetic entities within the fluorite.

Discussion

DCP

Only the samples doped with La, Ce, Gd and Y contain no other REE in

concentrations in the same order of magnitude as the dopant. The irradiation-induced

colors of these samples (all shades of blue and green) are likely due to the dopant. It is

more difficult to attribute any irradiation-induced color to the dopant in the samples with

significant concentrations of other REE known or thought to produce color (notably

yttrium). For instance, the La-doped sample became green upon irradiation. If this green

is due to a defect center involving La, we may find green in other samples with high

concentrations of this REE. The Eu-doped sample also became green upon irradiation.

However, the concentration of La is higher than that of Eu in the sample, and so perhaps the green color here is also due to La.

Optical absorption

REE-associated F center

The four-band “Smakula” spectrum found in several of the irradiated samples

(bands around 225 nm, 335 nm, 400 nm, and 580 nm) is common to many irradiated fluorites, both natural and synthetic. A detailed study of this spectrum was conducted on synthetic samples made by the same manufacturer (Optovac) and around the same as the samples in this study by Staebler and Schnatterly (1971). Through analysis of optical absorption and linear and circular dichroism experiments, the authors devised a model of the center responsible for the spectrum with concurrent EPR work by Anderson and

Sabisky (1971) and theoretical work by Alig (1971). They found the center to be a trivalent REE (Ce, Gd, La, Lu, Tb, or Y) next to a nearest neighbor fluorine vacancy that trapped two electrons upon irradiation of the sample (Fig.3). The authors intimated that color was produced, but did not specify which color(s). The same spectra are seen in several of the samples in the present study with the same dopants, and the irradiation- induced color is reported. In addition to the focus on the color, this study differs also in that more samples with a wider range of dopants are investigated (Pr, Er, Ho, Yb, Dy, Nd and Sm, in addition to those mentioned above), and chemical and luminescence data are presented.

From the DCP data, it is evident that the absorption spectra of some of the samples may be complicated by REE other than the dopant present in high

concentrations. This is not the case for the La-, Ce-, Gd-, and Y-doped samples. The OA

spectra of the irradiated samples of these four dopants contain the four-band spectrum,

and so we can attribute the color produced by irradiation to centers involving these

dopants. The Ce- and La-doped samples became different shades of green upon

irradiation, while the Gd-doped and Y-doped samples became different shades of blue. It

is to be expected that the colors are similar based on the fact that the positions of the

absorption peaks are also very similar. The Lu-doped sample also has the Smakula

absorption spectrum, and became pale brown upon irradiation.

Divalent samarium The irradiation-induced color of the Sm-doped sample is most likely due to

divalent samarium based on the DCP and OA data. The concentration of samarium in the

sample is approximately 87 ppm, while the only other REE in a concentration of the same

magnitude is La (17 ppm). The color could be due to the La-associated F center discussed

above, however, the optical absorption data more closely matches that of divalent

samarium reported by Bill and Calas (1978); Morozov et al. (1996); Bill et al. (1967).

Dysprosium and Ytterbium Neither the Yb- nor the Dy-doped samples changed color upon irradiation, and

their OA spectra changed very little. Neither element is known to cause color, however, both samples contain concentrations of yttrium (which does cause color) on the same order of magnitude as the dopant concentrations. Perhaps this contamination is not high enough to produce absorption, or the Y3+ in the sample has local charge compensation.

This would perhaps prevent the formation of the Staebler and Schnatterly (1971)

impurity-associated defect center. Merz and Pershan (1967) in a study of the change in oxidation state of trivalent impurity ions to divalent in fluorite with irradiation. They

found that those REE3+ in non-cubic symmetry with local charge compensation (usually an interstitial fluorine ion) are less likely to be affected by irradiation.

Neodymium, Holmium, Praseodymium and Erbium The spectrum of the irradiated Pr-doped sample contains the four-band Smakula spectrum. Pr was not analyzed for by DCP, however, this sample contains a high concentration of lanthanum, which could be the cause of the spectrum as well as the green color.

The UV region of the absorption spectrum of the irradiated sample doped with both Pr and Er is obscured by noise, however, the spectrum does contain peaks near 400 nm and 580 nm. This sample does not contain any REE that could be responsible for the

Smakula spectrum at the same concentration level as Er (147 ppm), however it does contain relatively high concentrations of Dy and Nd, neither of which are known to produce the green color induced by irradiation in this sample.

The Nd-doped sample became golden-yellow upon irradiation, and contains concentrations of no other REE on the same order of magnitude as Nd (204 ppm). No previous studies were found that indicate Nd is a cause of color in fluorite.

The Ho-doped sample contains the four-band spectrum, along with several other peaks. Ho was not analyzed for on the DCP, but the sample contains relatively high concentrations of Y (13 ppm) and La (14 ppm), which could contribute both to the absorption peaks as well as the blue irradiation-induced color of the sample.

Luminescence

For the majority of the samples, the irradiated fluorite luminescence spectra have

most of the same peaks as their non-irradiated counterparts, but are diminished in

intensity. All of the non-irradiated synthetic fluorite samples (with the exception of the

Sm- and Eu-doped samples) are colorless while most of the irradiated synthetic samples

(with the exception of the Dy- and Yb-doped samples) are intensely colored. The

dampening of the emission of the irradiated samples is most likely due to the increased

absorption of the luminescence emission.

The highest emission intensity occurs in samples (non-irradiated) that are

colorless to the human eye in daylight and under ordinary fluorescent light. Thus

indicating that whatever emission is occurring under those conditions does not account

for the color we see in the irradiated samples which have far less intense emission.

The non-irradiated synthetic fluorite that exhibited some color, the Eu- and Sm-

doped samples, may have luminescence emission intense enough to cause that color. It is

well established that the luminescence of Eu2+ in fluorite produces a blue-violet color

(Marfunin, 1979; Bill and Calas, 1978) as well as a result of luminescence emission

within the range of 430 and 450 nm (Gaft, et al, 2001). We see both in the Eu-doped

sample. Luminescence activated by Yb2+ is known to produce yellow-green color in fluorite, but the concentration of Yb in the Sm-doped sample is below 1 ppm, and the luminescence spectra of the non-irradiated and irradiated samples reveal no resolvable emission peaks. The green color could be the result of Sm2+ absorption, although there is

little resolvable structure in the non-irradiated Sm absorption spectrum. Emissions arising

from d-f transitions of divalent Sm and Eu typically produce broadband peaks. Both Eu

and Sm commonly enter fluorite in the divalent state, in which they are very stable (Merz

and Pershan, 1967). Therefore, it is possible that the XF3 dopant used by Optovac

contained some divalent lanthanides, which entered the fluorite structure and produce the

color we see in these samples before irradiation.

The broadband emission peaks around 450 nm found in several of the samples are

most likely due to divalent europium. A few non-irradiated samples (those doped with

La, Nd, Pr and Er, and Gd) contain a set of sharp peaks at approximately 444 nm, 458

nm, 472 nm, 481 nm, 492 nm, and 508 nm. These are most likely the result of one or

more lanthanides (Gaft et al., 2001), but have no effect on the irradiation-induced color.

Marfunin (1979) asserts that the luminescence of only Eu2+ and Yb2+ have any effect on

the color of fluorite.

EPR

Er and Pr-doped sample

The high-intensity, multiple line structures centered on a g-factor (g) of

approximately 6.8 in the non-irradiated spectra for this sample is most likely at least

partially the result of Er3+ (Morozov et al, 1996). The Er3+ structure is clearer in the

irradiated spectra. The shifting of the peaks with change in crystal position indicates non-

cubic site symmetry and thus local charge compensation. In fluorite, an interstitial

fluorine ion in an adjacent body-center position and an O2- ion substituting for one

nearest-neighbor fluorine are the most common types of trivalent REE charge

compensation mechanisms (Merz and Pershan, 1967). Local compensation decreases the cubic symmetry of the Ca site (now occupied by a trivalent impurity) to tetragonal

(interstitial fluorine) or trigonal (O2- substitution), and produces anisotropic EPR signals, or those that shift with changes in sample orientation, when the center is paramagnetic.

Non-local compensation, the rarer case, occurs when the charge-compensator(s) are distant enough from the trivalent impurity to preserve the cubic site symmetry while still maintaining overall crystal neutrality. This situation would result in an isotropic EPR signal, one that does not shift upon change of sample orientation, when the center is paramagnetic. The two-line structure of Er3+ coupled with an interstitial F- at 6.8 g is most likely also present. Erbium is not a known cause of color in fluorite.

Yb-doped sample

Yb3+ is known to produce a three-line structure at about 3.65 g (Morozov, et al,

1996), which can be seen in the data for this sample. Again, the peaks shift with change

in sample orientation, indicating non-cubic site symmetry and local charge compensation.

All of the structure found in the non-irradiated spectra is diminished in intensity or gone

in the irradiated spectra.

Lu-doped sample

The structure present in the non-irradiated spectra for this sample could be due at

least in part to a coupling of Lu3+ and an interstitial F- ion present as charge

compensation. The shifting of the peaks with change in sample position confirms the

anisotropy of this site symmetry. The peaks are not present in the irradiated spectra. Lu3+

coupled with an F center is know to produce structure with a g-factor of 2.0 +/- 0.05

(Anderson and Sabisky, 1971), however, no structure in either set of spectra can be

distinguished from the cavity signal at that g-factor. Lu3+ in cubic site symmetry has a non-paramagnetic ground state, and thus has no signal.

Nd-doped sample

The three small peaks of the non-irradiated spectra for this sample are at g-factors close to 2.85, 2.65, and 2.5 respectively. They shift only slightly with changes in sample orientation. The irradiated spectra also contain three peaks, although they are very high in intensity and shift a great deal with rotation of the crystal and have different g-factors.

The larger peaks obscure any small features that could be related to those in the non- irradiated spectra. We have not found this EPR signal previously reported.

Ce-doped sample

Anderson and Sabisky (1971) list several paramagnetic entities involving cerium in fluorite. The first is Ce3+ coupled with an F center in trigonal symmetry, which has a g- factor of about 2.38. There is a peak at this g in the 90˚ irradiated spectrum for this sample. It shifts with change in sample orientation, confirming non-cubic site symmetry.

Another is Ce3+ with F- charge compensation in trigonal symmetry with a g of 3.67. Very low intensity peaks in a few of the irradiated spectra could be the result of this paramagnetic entity. The absence of these before irradiation would indicate that the irradiation produced F centers. The non-cubic site symmetry of the Ce3+ in the fluorite corresponds with the Staebler and Schnatterly (1971) model.

Gd-doped sample

Several authors have observed EPR resonance of Gd3+ at a g-factor of 1.99 (Bill and Calas, 1978; Anderson and Sabisky, 1971; Low, 1957; and Morozov et al, 1996).

The g-factor is only slightly different for Gd3+ in cubic or tetragonal symmetry. Along with other structures, the complex seven-line structure of Gd3+ can be seen in the spectra of this fluorite sample both before and after irradiation. The peaks shift, for the most part, with changes in sample orientation, indicating that tetragonal symmetry is present. The non-cubic symmetry corresponds to the model of the trivalent lanthanide coupled with a fluorine vacancy and two electrons put forth by Staebler and Schnatterly (1971).

Conclusions

Sm2+ is responsible for the green, irradiation-induced color of the Sm-doped synthetic fluorite sample. The DCP data indicate that La is the only other REE present in high enough concentration to have an effect on the color. La is known to produce color in irradiated fluorite (Staebler and Schnatterly, 1971), however the optical absorption spectrum for the irradiated sample contains peaks more closely related to those observed by previous authors (Bill and Calas, 1978; Bill et al., 1967; Morozov et al., 1996; Recker et al., 1968; and Loh, 1968) that have been associated with divalent samarium.

The samples that contain no other REE in significant concentrations besides the dopant are green (those doped with Y and Gd) and blue (those doped with La and Ce) in color upon irradiation. They each contain the Smakula spectrum that is associated with a trivalent REE sharing two electrons with a fluorine vacancy (Staebler and Schnatterly,

1971). The luminescence data do not reveal any possible luminescence emissions that could be responsible for the color, and the EPR data indicate that Ce3+ and Gd3+ are

present in non-cubic symmetry in their respective samples. These data correspond to the

Staebler and Schnatterly model, and indicate that Gd and Y are responsible for blue color, while Ce and La can produce green color in some fluorite. These ions may be responsible for the color in a few of the other samples, including those doped with Pr and Er (mint green and contains La and Y), Pr (yellow-green and contains significant La), Ho (blue and contains significant Y) and Eu (yellow-green and contains significant La), although the optical absorption and EPR data are inconclusive.

It is unclear whether Nd and Lu are responsible for the irradiation-induced yellow-gold color of their respective samples. The OA spectrum of the Lu-doped sample after irradiation was in close agreement with the Smakula spectrum, and Staebler and

Schatterly (1971), among others, assert that the Lu-doped synthetic fluorites used in their studies gain color upon irradiation. Neither sample has the 434 nm OA peak responsible for yellow color associated with yttrium and oxygen (Bill and Calas, 1978).

Yb and Dy appear to prevent the production of color centers or the absorption of visible light when in significant concentrations. Despite the fact that both the Dy- and

Yb-doped samples contain significant amounts of REE previously established as color- producers in fluorite (La and Y), neither sample became colored upon irradiation. Further investigation is necessary to confirm and characterize this relationship.

There are relationships between impurity ions, structural defects and color in fluorite. However, they are often so complex that the exact characterization of these relationships remains difficult and obscure. The association of a specific lanthanide with a color in this system is rarely conclusive, especially in terms of natural samples. A following paper relates the results of the present research to natural samples to help

characterize the color and the sectoral zonation of both REE and irradiation-induced color.

References

Alig, R. (1971) Theory of photochromic centers in CaF2. Physical Review B, 3, (536- 545).

Alig, R., Kiss, Z., Brown, J., McClure, D. (1969) Energy levels of Ce2+ in CaF2. Physical Review, 186, (276-284).

Anderson, C. and Sabisky, E. (1971) EPR studies of photochromic CaF2. Physical Review B, 3, (527-536).

Berman, R. (1957) Some physical properties of naturally irradiated fluorite. American Mineralogist, 42, (191-203).

Bill, H. and Calas, G. (1978) Color centers, associated rare-earth ions and the origin of coloration in natural fluorites. Physics and Chemistry of Minerals, 3, (117-131).

Bill, H., Sierro, J., and Lacroix, R. (1967) Origin of coloration in some fluorites. The American Mineralogist, 52, (1003-1009).

Botinck, W. (1958) The hydrolysis of solid CaF2. Physica, 24, (650-658).

Constantopoulos, J. (1988) Fluid inclusions and rare earth element geochemistry of fluorite from South-Central Idaho. Economic Geology, 83, (626-636).

Dantas, N.O., Watanabe, S., Chubaci, J.F.D. (1996) Optical absorption (OA) bands in fluorites by heavy gamma irradiation. Nuclear Instruments and Methods in Physics Research B, 116, (269-273).

Dickson, J. (1980) Artificial colouration of fluorite by electron bombardment. Mineralogical Magazine, 43, (820-822).

Gorlich, P., Karras, H., Symanowski, C., and Ullmann, P. (1968) The colour center absorption of x-ray coloured alkaline earth fluoride crystals. Phys. Stat. Sol., 25, (93- 101).

Loh, E. (1968) 4f n-1 5d spectra of rare-earths in crystals. Physical Review, 175, (533- 536).

Merz, J. and Pershan, P. (1967) Charge conversion of irradiated rare-earth ions in calcium fluoride. Physical Review, 162, (217-235).

Murr, L.E. (1973) Ordered lattice defects in colored fluorite: direct observations. Science, 183, (206-208).

Spectroscopy, Luminescence and Radiation Centers in Minerals, A. S. Marfunin, Springer-Verlag, New York, 1979.

Naldrett, D.L., Lachaine, A., and Naldrett, S.N. (1987) Rare-earth elements, thermal history, and the colour of natural fluorites. Canadian Journal of Earth Science, 24, (2082-2088).

Nassau, K. (1978) The origin of color in minerals. American Mineralogist, 63, (219-229).

The Physics and Chemistry of Color, K. Nassau, John Wiley & Sons, Inc, 2001.

O’Conner, J. and Chen, J. (1963) Color centers in alkaline earth fluorides. Physical Review, 130, (1791-1795).

Recker, K., Neuhaus, A., Leckebusch, R. (1968). Fluorite. Proc. I.M.A. Cambridge. (145- 152).

Richards, R. and Robinson, G. (2000) Mineralogy of the calcite-fluorite veins near Long Lake, New York. The Mineralogical Record, 31, (413-422).

Scouler, W. and Smakula, A. (1960) Coloration of pure and doped calcium fluoride crystals at 20oC and –190oC. Physical Review, 120, (1154-1161).

Sierro, J. (1965) Paramagnetic resonance of the Vf center in CaF2. Physical Review, 138, (648-650).

Smakula, A. (1950) Color centers in calcium fluoride and barium fluoride crystals. Physical Review, 77, (408-409).

Staebler, D. (1969) Optical studies of a rare-earth F center complex in rare-earth doped calcium fluoride. Ph. D. thesis, Princeton University (unpublished).

Staebler, D. and Schnatterly, S. (1971) Optical studies of a photochromic color center in rare-earth-doped CaF2. Physical Review B, 3, (516-526). Alig, R. (1971). Physical Review B, 3, p. 536-545.

Wright and Rakovan (in review) Spectroscopic characterization of fluorite from Bingham, NM, Long Lake, NY, and Westmoreland, NH: relationships between trace element zoning, defects and color

Table 1: Color before and after irradiation,sample thickness for OA and REE concentrations (ppm) of synthetic fluorite samples Sample 0.01 0.05 PrF3 + 0.1 YF3 0.003 0.01 YbF3 0.01 0.01 NdF3 dopant LaF3 0.1 ErF3 PrF3 DyF3 Color Colorless Colorless Colorless Colorless Colorless Colorless Colorless Irradiated Pale Mint green Blue Yellow- Colorless Colorless Pale color green green yellow Thickness 2 2 2 2 3 2.6 2.6 (mm) La 120.9 1.15 2.34 38.17 3.46 2.06 11.04 Ce 0.72 3.29 bdl 1.62 0.34 3.30 1.146 Nd 0.44 38.55 0.47 2.87 0.39 1.69 204.87 Sm 0.05 5.64 0.23 0.24 0.26 0.44 0.69 Eu 0.17 0.18 0.34 0.09 0.12 0.97 0.60 Gd 0.37 0.85 1.03 0.30 0.75 0.73 0.64 Dy 0.09 19.92 0.96 0.82 0.35 60.38 1.65 Er 0.34 147.33 0.52 0.32 0.45 bdl 0.68 Tm 0.02 bdl 0.07 0.02 0.07 0.15 0.07 Yb 0.18 1.02 0.58 0.26 8.38 0.58 0.28 Lu 0.02 0.09 0.05 0.07 0.21 0.02 0.91 Y 5.79 2.98 314.36 5.30 29.24 11.76 12.75 Sample 0.01 0.05 LuF3 0.01 0.001 EuF3 0.15 SmF3 0.05 dopant CeF3 GdF3 HoF3 Color Colorless Colorless Colorless Pale purple Pale green Colorless Irradiated Bright Golden Blue Yellow- Bright Grey- color green yellow green green blue Thickness 1.5 2 1.5 2 3 2 (mm) La 4.89 2.03 2.28 35.57 17.61 14.78 Ce 224.64 0.99 Bdl 2.96 Bdl Bdl Nd 0.73 0.79 0.19 1.98 Bdl 0.88 Sm Bdl 0.29 0.47 0.44 87.17 0.74 Eu 0.07 0.09 0.11 26.23 Bdl 0.18 Gd 0.29 0.42 170.79 0.45 0.99 2.31 Dy 0.62 1.21 Bdl 1.23 Bdl 1.32 Er 0.19 0.17 0.24 0.25 Bdl Bdl Tm Bdl 0.03 0.03 Bdl Bdl Bdl Yb 0.18 0.27 1.24 0.23 0.10 0.29 Lu 0.04 8.66 0.25 0.41 0.18 0.03 Y 5.11 N/A 6.59 4.49 N/A 13.32

For each synthetic fluorite sample, the wt. % dopant used by the manufacturer (Optovac) is given, as well as the color before and after irradiation. The REE concentrations obtained by DCP are also included. Bdl = “below detection limits”. N/A = “not available” due to lack of sample.

Table 2. Luminescence peaks due to Raleigh scattering of excitation wavelengths Excitation 200 210 220 225 230 260 270 300 310 320 330 370 λ (nm) Reflection 386, 420, 440, 435, 460, 370, 505 540 586 617 625 658 λ (nm) 586 630 660 660 690 740 The peaks at the reflection wavelengths for each specific excitation wavelength in the luminescence data are due to Raleigh scattering and reflection of the excitation light off the solid sample holder for the Perkin Elmer LS 55.

1.4

1.2

1.0

0.8

0.6 La-doped fluorite, irradiated La-doped fluorite

0.4 Absorbance Units Absorbance

0.2

0.0

200 300 400 500 600 700 800 nm Figure 1a. Optical absorption of La-doped synthetic fluorite before and after irradiation.

4.5 Ce-doped fluorite, irradiated Ce-doped fluorite 4.0

3.5

3.0

2.5 Absorbance Units

2.0

1.5 200 300 400 500 600 700 800 nm Figure 1b. Optical absorption of Ce-doped synthetic fluorite before and after irradiation.

4.5

4.0

3.5

3.0

2.5 Absorbance Units 2.0

1.5 Pr-doped fluorite, irradiated Pr-doped fluorite

1.0 200 300 400 500 600 700 800 nm Figure 1c. Optical absorption of Pr-doped synthetic fluorite before and after irradiation.

8 Pr and Er-doped fluorite, irradiated Pr and Er-doped fluorite

4 Absorbance Units

0 200 300 400 500 600 700 800 nm Figure 1d. Optical absorption of Pr and Er-doped synthetic fluorite before and after irradiation.

Nd-doped fluorite, irradiated 5 Nd-doped fluorite

3 Absorbance Units

1 200 300 400 500 600 700 800 nm

Figure 1e. Optical absorption of Nd-doped synthetic fluorite before and after irradiation.

2.4

2.2

2.0

1.8

1.6

1.4 Absorbance Units

1.2

Sm-doped fluorite, irradiated 1.0 Sm-doped fluorite

0.8 200 300 400 500 600 700 800 nm Figure 1f. Optical absorption of Sm-doped synthetic fluorite before and after irradiation.

3 Eu-doped fluorite, irradiated Eu-doped fluorite 2 Absorbance Units 1

200 300 400 500 600 700 800 nm Figure 1g. Optical absorption of Eu-doped synthetic fluorite before and after irradiation.

2 Gd-doped fluorite Gd-doped fluorite, irradiated Absorbance Units 1

200 300 400 500 600 700 800 nm Figure 1h. Optical absorption of Gd-doped synthetic fluorite before and after irradiation.

5.0

4.5

4.0 Dy-doped fluorite, irradiated Dy-doped fluorite 3.5

3.0

2.5 Absorbance Units

2.0

1.5

1.0 200 300 400 500 600 700 800 nm Figure 1i. Optical absorption of Dy-doped synthetic fluorite before and after irradiation.

3.5

Ho-doped fluorite, irradiated 3.0 Ho-doped fluorite

2.5

2.0 Absorbance Units

1.5

1.0 200 300 400 500 600 700 800 nm Figure 1j. Optical absorption of Ho-doped synthetic fluorite before and after irradiation.

6 Yb-doped fluorite, irradiated Yb-doped fluorite

3 Absorbance Units

1 200 300 400 500 600 700 800 nm Figure 1k. Optical absorption of Yb-doped synthetic fluorite before and after irradiation.

6 Lu-doped fluorite, irradiated Lu-doped fluorite

3 Absorbance Units

1 200 300 400 500 600 700 800 nm Figure 1l. Optical absorption of Lu-doped synthetic fluorite before and after irradiation.

6 Y-doped fluorite, irradiated Y-doped fluorite

3 Absorbance Units

1 200 300 400 500 600 700 800 nm Figure 1m. Optical absorption of Y-doped synthetic fluorite before and after irradiation.

La-doped fluorite Relative Intensity Relative

400 500 600 700

220 nm nm Irradiated, 220 nm Irradiated, 300 nm 300 nm 330 nm Irradiated, 330 nm Figure 2a. Luminescence of La-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Ce-doped fluorite 300 nm Irradiated, 300 nm Irradiated, 225 nm Irradiated, 335 nm Relative Intensity Relative

400 500 600 700 nm Figure 2b. Luminescence of Ce-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Pr-doped fluorite Relative Intensity Relative

400 500 600 700

220 nm nm 260 nm Irradiated, 260 nm 300 nm Irradiated, 300 nm 320 nm Irradiated, 320 nm Figure 2c. Luminescence of Pr-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Pr and Er-doped fluorite

210 nm

Relative Intensity Relative Irradiated, 210 nm 300 nm Irradiated, 300 nm

400 500 600 700 nm Figure 2d. Luminescence of Pr and Er-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Nd-doped fluorite

Irradiated, 260 nm

Relative Intensity Relative 260 nm 300 nm

400 500 600 700 nm Figure 2e. Luminescence of Nd-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Sm-doped fluorite

Relative Intensity Relative 300 nm Irradiated, 300 nm Irradiated, 260 nm

400 500 600 700 nm Figure 2f. Luminescence of Sm-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Eu-doped fluorite Relative Intensity Relative

400 500 600 700 nm 225 nm Irradiated, 225 nm 330 nm Irradiated, 330 nm 300 nm Irradiated, 300 nm Figure 2g. Luminescence of Eu-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Gd-doped fluorite Relative Intensity Relative

400 500 600 700 nm 220 nm Irradiated, 220 nm Irradiated, 300 nm 300 nm 330 nm Irradiated, 330 nm Figure 2h. Luminescence of Gd-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

300 nm Irradiated, 300 nm Dy-doped fluorite 290 nm Irradiated, 290 nm Relative Intensity Relative

400 500 600 700 nm Figure 2i. Luminescence of Dy-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Ho-doped fluorite

210 nm Irradiated, 210 nm 240 nm Irradiated, 240 nm 260 nm Irradiated, 260 nm Relative Intensity

400 500 600 700

Ho-doped fluorite

300 nm Irradiated, 300 nm 310 nm Irradiated, 310 nm 360 nm

Relative Intensity Relative Irradiated, 360 nm

400 500 600 700

nm Figure 2j. Luminescence of Ho-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Yb-doped fluorite Relative Intensity Relative

400 500 600 700 nm 230 nm Irradiated, 230 nm 270 nm Irradiated, 270 nm 300 nm Irradiated, 300 nm 370 nm Irradiated, 370 nm Figure 2k. Luminescence of Yb-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Lu-doped fluorite Relative Intensity Relative

400 500 600 700 nm 220 nm Irradiated, 220 nm 300 nm Irradiated, 300 nm 320 nm Irradiated, 320 nm Figure 2l. Luminescence of Lu-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

Y-doped fluorite

300 nm Irradiated, 300 nm Irradiated, 220 nm Irradiated, 330 nm Relative Intensity Relative

400 500 600 700 nm Figure 2m. Luminescence of Y-doped synthetic fluorite before and after irradiation at various excitation wavelengths.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

La, 225o

La, 135o

La, 90o

La, 0o

La, irradiated, 225o

La, irradiated,135o

La, irradiated, 90o

La, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3a. EPR of La-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Ce, 225o

Ce, 135o

Ce, 90o

Ce, 0o

Ce, irradiated, 225o

Ce, irradiated, 135o

Ce, irradiated, 90o

Ce, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3b. EPR of Ce-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Pr, 225o

Pr, 135o

Pr, 90o

Pr, 0o

Pr, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3c. EPR of Pr-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Er and Pr, 225o

Er and Pr, 135o

Er and Pr, 90o

Er and Pr, 0o

Er and Pr, irradiated, 225o

Er and Pr, irradiated, 135o

Er and Pr, irradiated, 90o

Er and Pr, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3d. EPR of Pr and Er-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

* * * * *

Nd,* 225o

Nd, 135o

Nd, 90o

Nd, 0o

Nd, irradiated, 225o

Nd, irradiated, 135o

Nd, irradiated, 90o

Nd, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3e. EPR of Nd-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Sm, 225o

Sm, 135o

Sm, 90o

Sm, 0o

Sm, irradiated, 225o

Sm, irradiated, 135o

Sm, irradiated, 90o

Sm, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss

Figure 3f. EPR of Sm-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Eu, 225o

Eu, 135o

Eu, 90o

Eu, 0o

Eu, irradiated, 225o

Eu, irradiated, 135o

Eu, irradiated, 90o

Eu, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3g. EPR of Eu-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Gd, 225o

Gd, 135o

Gd, 90o

Gd, 0o

Gd, irradiated, 225o

Gd, irradiated, 135o

Gd, irradiated, 90o

Gd, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3h. EPR of Gd-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Dy, 225o

Dy, 135o

Dy, 90o

Dy, 0o

Dy, irradiated, 225o

Dy, irradiated, 135o

Dy, irradiated, 90o

Dy, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3i. EPR of Dy-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Ho, 225o

Ho, 135o

Ho, 90o

Ho, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3j. EPR of Ho-doped synthetic fluorite single crystal before irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Yb, 225o

Yb, 135o

Yb, 90o

Yb, 0o

Yb, irradiated, 225o

Yb, irradiated, 135o

Yb, irradiated, 90o

Yb, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3k. EPR of Yb-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Lu, 225o

Lu, 135o

Lu, 90o

Lu, 0o

Lu, irradiated, 225o

Lu, irradiated, 135o

Lu, irradiated, 90o

Lu, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3l. EPR of Lu-doped synthetic fluorite single crystal before and after irradiation at various angles.

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

Y, 225o

Y, 135o

Y, 90o

Y, 0o

Y, irradiated, 225o

Y, irradiated, 135o

Y, irradiated, 90o

Y, irradiated, 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 3m. EPR of Y-doped synthetic fluorite single crystal before and after irradiation at various angles.

Figure 4. Schematic of photochromic center in fluorite involving a molecular orbital with two electrons shared by a trivalent REE and a fluorine vacancy. Modified from Staebler and Schnatterly, 1971.

CHAPTER 3 SPECTROSCOPIC CHARACTERIZATION OF FLUORITE FROM BINGHAM, NM, LONG LAKE, NY AND WESTMORELAND, NH: RELATIONSHIPS BETWEEN TRACE ELEMENT ZONING, DEFECTS AND COLOR

Abstract

Fluorites from Bingham, NM, Long Lake, NY, and Westmoreland, NH were

investigated using optical absorption, direct current plasma spectrometry, electron paramagnetic resonance spectroscopy, and fluorescence spectroscopy before and after 10-

20 Mrad of 60Co gamma irradiation to determine the cause of color and the relationship

between defect centers and sectorally zoned trace elements. Samples from Bingham and

Long Lake exhibit color zonation (both concentric and sectoral) before irradiation, and

samples from all three locations exhibit color zonation upon irradiation. The irradiation-

induced color zones of the Long Lake fluorites correspond well with

cathodoluminescence images of previous studies, indicating a relationship between trace

element zoning and color. Chemical (59 ppm yttrium) and optical absorption data

(Smakula (1950) spectrum) indicate that the irradiation-induced blue color of a naturally

pale green Wise Mine (NH) sample is caused by a yttrium-associated color center first

modeled by Staebler and Schnatterly (1971) consisting of a Y3+ next to a fluorine

vacancy that has trapped two electrons. The color of the dark green Wise Mine sample,

which intensified upon irradiation, is most likely due to the same type of center involving

Ce3+, La3+ and Y3+. The Y3+-associated center may also be responsible for the irradiation-

induced blue color zones of the irradiated Long Lake fluorite, as well as the irradiation- induced blue color of several Bingham, NM samples that were naturally colorless to pale

green in color. Fluorescence emission, if present, is diminished or destroyed upon

irradiation because of the increased absorption of optical wavelengths in all of the

samples due to their darker, irradiation-induced color. The dark purple and blue color of

some of the Bingham samples is most likely the result of F centers, although the OA spectra also contain the Smakula spectrum. EPR data suggest a possible increase in F centers in some Bingham samples.

Introduction

The cause of color in fluorite has been studied for over a century: by mineralogists in the interest of aesthetics and links to geochemistry of mineral deposits

(Bill and Calas, 1978; Morozov et al, 1996; Hill et al., 2000; Bosze and Rakovan, 2001), and as spectroscopic entities by materials scientists in the interest of the use of CaF2 doped with various luminescence activators for lasers (Smakula, 1950; Staebler and

Schnatterly, 1971, Merz and Pershan, 1964; Loh, 1968; Sierro, 1965; Dantas et al. 1996; and many others). Three types of color centers, or variations in the normal structure of a mineral that produce color, have been established in fluorite (Bill and Calas, 1978).

Impurity ions such as divalent samarium and structural defects like F centers (fluorine vacancies occupied by a single electron) are relatively well established color centers in

fluorite. Impurity-associated structural defects, such as those modeled by Staebler and

Schnatterly (1971), are more complicated.

Wright and Rakovan (in review) obtained REE-doped synthetic fluorite (CaF2) samples (synthesized by Optovac, Inc. c. 1960’s) and analyzed them before and after 10-

25 Mrad of gamma irradiation by DCP, OA, EPR and fluorescence spectroscopy to

further clarify the relationship between lanthanides, defects and color. Samples used were

doped with 10-300 ppm of La, Y, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb and Lu. All

samples, except the Eu- and Sm-doped, were colorless before irradiation. Irradiation

produced color in all of the samples except those doped with Dy and Yb. DCP analyses

indicate that the samples contain significant concentrations of REE or Y besides the

dopant, with the exception of those samples doped with Ce, La, Gd and Y. Optical

absorption experiments confirmed the presence of Sm2+-induced absorption in the Sm-

doped sample, which became green upon irradiation. The “Smakula” optical absorption

spectrum (Smakula, 1950) is present in the samples doped with Ce, Gd, Y, La, and

others, which corresponds to the color center model (Fig.1) put forth by Staebler and

Schnatterly (1971). Luminescence is not likely strong enough to produce the color, and is

diminished by irradiation. Electron paramagnetic resonance (EPR) data indicate the presence of various REE in non-cubic symmetry, which also corresponds to the model

(Staebler and Schnatterly, 1971) of color centers in some fluorites. In samples with impurities other than the main dopant (Pr, Nd, Sm, Eu, Dy, Ho, Er, Yb, Lu-doped) it is not possible to unequivocally assign the cause of color to the presence of a color center involving the main dopant. For the samples doped with Ce, La, Gd and Y the combined spectroscopic data indicate that a dopant-associated color center is responsible for the irradiation-induced color. The “Smakula” optical absorption data suggest a lanthanide-

associated F-center with two trapped electrons as the cause of color in these samples.

Slight differences in the absorption peak position give blue color in the Gd and Y samples and green in the La and Ce samples.

Recent work by Bosze and Rakovan (2001) confirmed the differential incorporation of all the REEs among the growth sectors (sector zoning) of fluorite crystals from Bingham, NM and long Lake, NY using Synchrotron X-ray fluorescence microanalysis (SXRFMA) and cathodoluminescence (CL). These techniques, which involve irradiation of the sample by X-rays and energetic electrons respectively, produced color centers that were also zoned with respect to the different growth sectors.

Thus there exists a relationship between the production of color centers and REE. In order to investigate this relationship, samples from the same locations as well as the Wise

Mine in Westmoreland, NH were investigated spectroscopically using a variety of techniques before and after gamma irradiation in the present study. Information garnered from similar spectroscopic investigation of synthetic samples (Wright and Rakovan, in review) is used to interpret the resulting spectra.

Materials and Methods

Natural fluorites from three locations were used in this study. Fluorite samples from the Hansonburg Mining District in Bingham, NM made up the bulk of the natural samples, with a few fluorite samples each from the William Wise Mine in Westmoreland,

New Hampshire and a deposit in Long Lake, New York.

The Bingham samples exhibit different shades and intensities of purple, blue, green, as well as being colorless. These samples also show a wide range of morphologies, including cube, hexoctahedron, and others (Bosze and Rakovan, 2001). The hydrothermal fluorite deposits at Bingham are considered to be of the Mississippi-Valley Type

(MVT)(Chapin, 1979; Putnam, et al, 1993). Distinct sectoral zonation of color,

particularly purple, is evident in a few samples while concentric zoning is very common.

The samples used in this study are from four of the Hansonburg mines, the Sunshine 2,

Royal Flush, Desert Rose, and Mex-Tex. For a detailed description of the deposit see

Bosze and Rakovan (in review). The Bingham fluorites are divided into two distinct

groups based on paragenesis, REE chemistry and color (pale green and colorless versus

dark purple and blue), and debate continues over the source or sources of fluid for each.

Morozov et al (1996) suggested that varying irradiation-induced color in concentrically

zoned natural fluorite is indicative of changing fluid sources over time, with an increase

or decrease in radiogenic isotopes available to produce color centers, resulting in

generations of different colors.

Fluorite from the Wise Mine is either deep green or pale green, mostly in the form

of octahedrons. For a detailed description of the deposit, see Young (1990). The fluorite

from the Wise mine is of such great clarity it is used as a gem material. Irradiation

produces a very intense green in the naturally dark green samples, and an intense blue in

the naturally light green samples.

The samples from calcite-fluorite veins located in near Long Lake, New York are

primarily translucent and colorless to bluish-purple, with an average size of 1 to 2 cm.

Many of them contain a dark gray cubic center, which shows each crystal’s initial

morphology (Richards and Robinson, 2000). It is evident from this internal morphology,

as well as the external morphology, that the {100} and {111} forms were dominant

throughout crystal growth. The Long Lake fluorite grew in hydrothermal veins of

unknown age that intruded into granitic gneiss (Richards and Robinson, 2000). The

mineral assemblage of these veins is predominately calcite and fluorite, along with

chamosite, hematite, pyrite, quartz, cerian epidote, and kainosite-(Y) in a complex

paragenetic sequence involving two generations of calcite, and three of chamosite

(Richards and Robinson, 2000). The samples that are primarily colorless are from veins which contain few minerals other than the fluorite and calcite. The samples described in

Richards and Robinson (2000) as concentrically zoned or radially zoned (sectorally zoned) with respect to color and trace elements and were used in this study were located in veins in close proximity to those of the colorless samples; however, the veins from which the colored samples originated from contain a greater variety of minerals. The fluorites from NY have distinct growth zones, both concentric and sectoral, based on cathodoluminescence work. These zones are also apparent upon irradiation, with each being a distinct color.

Irradiation

The samples were irradiated at the Ohio State University Nuclear Reactor

Laboratory by Joseph Talnagi. The crystals received between 10 and 26 Mrad of irradiation from their 60Co gamma source.

Direct Current Plasma Spectrometry (DCP)

Samples were chosen for DCP analyses based on color and clarity, the latter to avoid inclusions. The fluorite samples were washed in methanol, followed by a deionized water rinse. The irradiated samples from Long Lake were cut into thin slices, then individual sectors and concentric zones (based on color) were separated cutting perpendicular to those original cuts. To accumulate enough material for analysis, sectors

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or zones of the same color from up to 15 different crystals were combined. The samples were crushed to an average size of about 2 mm, checked again for inclusions, then powdered using a mortar and pestle, placed in uncapped glass vials in an oven overnight at 110˚C to eliminate any moisture. After heating, the vials were capped and placed in a dessicator until cool.

To remove any graphite from the crucibles, the samples were filtered through quartz wool before being loaded onto columns of AG50W-X8 cation exchange resin.

After the samples passed into the resin, 150 mL of a mixed acid (2.6 N HNO3 + 2.5 N

HCl) was used to elute the sample matrix that did not contain any REE. The REE fraction was eluted by 250 mL of 6 N HCl, which was collected in a 250 mL Teflon beaker and dried on a hot plate. Upon drying, the REE fraction was taken up in 1-2 mL of the mixed acid, and loaded back onto their respective cleaned columns. The above procedure was repeated (mixed acid then HCl acid) to remove any lingering matrix elements. The

+ resulting sample was taken up in 4 mL of 5% HNO3 spiked with 3000 ppm K which acted as an ionization buffer. The sample solutions were measured against external REE standard solutions using a Beckman Spectra Span V direct current plasma (DCP) spectrometer at Miami University. Trace element (REE) geochemical analyses were performed on the samples from Bingham, NM by Bosze and Rakovan (in press).

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Optical Absorption Spectroscopy

All of the fluorite samples were cut using a Buehler Isomet Saw. The cuts were of

various thicknesses, depending on the intensity of the fluorite color. Pale samples were

cut up to 2 cm thick, while the darker samples were as thin as 1 mm. All of the samples

were initially polished with a HI-TECH DIAMOND electric lap wheel using 1200 mesh

and 3000 mesh lap disks. Many samples were subsequently hand-polished with ALLIED water-based polycrystalline diamond suspension sprays in 6 and 1 microns and glass plates.

Optical absorption spectra were obtained using a Hewlett Packard 8453 UV-

Visible Spectrophotometer. The spectrophotometer contains a 1024 element photodiode array spectrometer. Spectra of the fluorites were taken in absorption mode with the maximum integration time of 25.5 s and with a maximum number of two spectra added.

The photodiode array and wavelength range allow a sampling interval of approximately 1 nm. The spectrophotometer includes a concave holographic grating, the slit width is 1 nm, and the wavelength resolution is 2 nm.

Fluorite samples were masked with a 1 mm metal aperture for spatially resolved analyses. This allowed for the collection from specific sectors or regions of different color within single crystals, as well as the avoidance of cracks, inclusions, and poorly polished surfaces.

Fluorescence

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Luminescence experiments were conducted on a Perkin Elmer LS55

Luminescence Spectrometer with entrance and exit slit widths at the minimum of 2.5 nm.

Ten scans were added for each spectrum with a scan rate of 500 nm/min in the wavelength range of 350 nm to 800 nm. An excitation wavelength of 300 nm was used for each sample. UV-Vis absorption spectra were used to identify additional excitation wavelengths for each sample.

Electron Paramagnetic Resonance Spectroscopy

Rectangular sections, approximately 10 mm by 1 mm by 1 mm in size, were cut from Bingham, Wise Mine, and Long Lake fluorite samples. The cuts of the Bingham samples were cut parallel to {100}, the Long Lake samples were cut parallel to {111}, and the Wise Mine samples were cut parallel to {111}. One end of each of the cut fluorite samples was then glued with Epo-Tek 301 epoxy to separate quartz glass rods with an outer diameter no greater than 2.5 mm. This was done to minimize tilt of the samples when placed in the EPR sample tubes.

Single-crystal EPR spectra of each of the fluorite samples were collected using a

Bruker EMX-6 X-band CW-EPR spectrometer containing an ER041XG microwave bridge and a TE102 cavity coupled with an Oxford liquid helium controller at the

Department of Chemistry, Miami University. The experiments were run with a field modulation frequency of 100 KHz, an average microwave frequency of 9.4 GHz, and temperatures ranging from 4-5 K. The spectral resolution was 2.0 G. The field-swept

EPR experiments were performed at 45-degree intervals within one plane of each crystal.

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This was accomplished by rotating the sample tube within the cavity approximately 45 degrees, with a maximum error of 15 degrees.

Results

Irradiation

Table 1 lists all samples and their color before and after irradiation. Many of the samples exhibit zoning of irradiation-induced color, among growth zones and between symmetrically different sectors. In color zoned crystals, both or all of the colors are mentioned in Table 1. The most extreme and observable color zoning is exhibited in samples from Long Lake, NY. Images of two Long Lake crystals taken before and after irradiation are shown in Figures 2 and 3. The sample in Figure 2 was colorless except for the dark gray center; the other crystal (Fig.2) exhibited very pale coloration other than the dark gray center. Both samples were cut parallel to the (111) face and received 17.6 Mrad of gamma irradiation from a 60Co source. The irradiation-induced colors are segregated into sectoral and concentric zones comparable to those found in cathodoluminescence

(CL) images in Richards and Robinson (2000). In the Richards and Robinson (2000) images, the CL activators, particularly REEs, are segregated into sectoral and concentric zones. The irradiation-induced color zones match the CL image zones, and thus indicate the dependence of the color on the presence of REE(s) or Y. The initial morphology of the crystals was cuboctahedral. The core of the crystals, where this morphology is evidenced, is deeply colored before and after irradiation. Before irradiation, the core is a homogenous deep gray. Following irradiation, the color of the core was not altered except for the addition of a few maroon and purple areas. The purple areas are the

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octahedral sectors of the core. After the growth of the core, the relative size of the octahedral face increases throughout the rest of the growth history so that the external morphology is dominantly octahedral. After irradiation, the outer concentric zones

(relative to the core) show concentric and sectoral zoning of color. The contrast in color between the core and outer portion of the crystal allows one to see the shape of the crystal at that interface. The irradiation-induced maroon area, seen as a triangle or multiple triangles due to the cut, is found in all the samples with slight variations. The maroon area is the next stage of cubic growth following the gray core. The purple triangles radiating out from the core are octahedral growth sectors. The yellow and green areas are concentric growth zones within the cube sectors that grew on top of the cube faces of the maroon areas. In the very last stages of growth, numerous octahedral overgrowths cover the cube surfaces of the fluorite crystals. As growth continues, these coalesce to leave an octahedral volume of crystal on the cube faces. This volume, which becomes blue after irradiation, appears serrated, with the apices of the multiple octahedral overgrowths pointing away from the center of the crystal. The serrations are small (111) faces overgrown on the cube, and hence the blue volumes of the crystal are actually (111) sectors. The bluish-purple serrated faces on top of the blue zones are evidence that this morphological transition was not completed. This stage is not found in the crystals that are colorless before irradiation. The two different types of fluorite crystals (colorless except for the gray center and colored before irradiation) were found along the same road cut in two separate veins. The naturally colored samples with the extra generation were found with a variety of other minerals, including calcite, chamosite, hematite, epidote, kainosite, pyrite and quartz. The colorless samples were found in a separate vein along

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the road cut with primarily calcite. The additional growth zones of the colored fluorite suggest that the vein in which it grew received more fluids than that of the colorless sample. The irradiation-induced color of both samples is only an intensified version of that found in the colored sample before irradiation, and there is little variation in the colors or the zones between the samples after irradiation. These points suggest that the two types are very similar (in growth history and geochemically) up until the additional growth zones of the colored sample formed. They also suggest that the mineral assemblage found with the colored samples may contain sufficient radiogenic elements

(i.e., the REE in kainosite) to produce the color in the fluorite of one vein, while the fluorite in the other vein lacks both.

Several of the fluorite samples from Bingham, NM exhibit natural sectoral zoning of color. In Figure 4 the crystal exhibits purple {321} sectors and colorless {100} sectors.

A few of the samples from New Mexico and New Hampshire exhibit discernable color zoning after irradiation as well. Concentric growth zones are the most apparent, while the majority of sectoral zonation of color or color intensity in the samples is difficult to see because of the subtle contrast between the colors. An example of sectoral color zoning is a pale green Bingham sample, B7, which became intense teal and green upon irradiation.

The teal color appears to be primarily in the {100} sectors, while the green is in the

{110} sectors. Bosze and Rakovan (2000) found sectoral zoning of both trace elements and irradiation-induced color in Bingham fluorites. A cubo-octahedral Wise Mine sample that was pale green before irradiation became blue and pale green after irradiation. The color is extremely pale, but when held over a strong light source, one can tell that the cube sector is blue while the octahedral sectors are green. It is apparent that irradiation of

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fluorite can reveal the growth sectors and zones, enabling one to characterize the internal

morphology and the growth history.

DCP

The results of the DCP analyses are given in Table 2. Figures 5a-c show the chondrite-normalized (Nakamura, 1974) REE patterns for the individual crystals from the

Wise Mine, individual concentric and sectoral growth zones from Long Lake fluorites,

and individual crystals from Bingham, NM (taken from Bosze and Rakovan, in review).

Wise Mine

The two Wise Mine samples, one dark green and one light green, exhibit similar

patterns (Fig.5a) of REEs + Y with one order of magnitude difference in concentration;

the dark green sample contains the higher concentrations. Focusing on the geochemical

data and its possible relationship with the color of the fluorite, the important REEs to

consider are those that may affect color, such as yttrium, gadolinium, cerium, lanthanum, samarium, europium, and possibly lutetium (Bill and Calas, 1978; Staebler and

Schnatterly, 1971; Wright and Rakovan, in review). The dark green sample is green before and after irradiation, and contains relatively high concentrations of REEs associated with green color such as La (21 ppm) and Ce (46 ppm) (Wright and Rakovan, in review), and Sm (4 ppm) (Bill and Calas, 1978). The pale green sample, which becomes a very intense teal upon irradiation, contains less than 3 ppm of all these trace

elements, perhaps explaining the paleness of its green color before irradiation. Both

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samples contain high concentrations of yttrium, 130 ppm in the dark green and 59 ppm in the pale green sample.

Another minor difference in the chondrite-normalized REE patterns for the Wise

Mine samples is in slope for HREE (Tm-Lu). The slope for these elements in the dark green sample is positive, while it is negative for the light green sample. A negative Eu anomaly exists in the REE patterns for both of the samples.

Long Lake

The different sectors and growth zones analyzed for the Long Lake fluorite crystals are distinctly different in color, but their REE concentrations and chondrite- normalized REE patterns (Figure 5b) are very similar. However, at Er, there is an increase in the separation of the data points, and the slopes remain positive for the green sample while becoming negative for the other samples. The REE concentrations for each sector are on the same order of magnitude, however all of the REEs are higher in concentration in the {111} sectors than in the {100} sectors. This is consistent with the

X-ray Fluorescence data of Bosze and Rakovan (2001). The blue zone does contain more yttrium than other color regions. The green zone contains more Ce and La than the blue, but so do the maroon and purple zones.

Bingham

DCP analyses were performed on whole crystals of fluorite from Bingham, NM by Bosze and Rakovan (in review). Chondrite normalized REE patterns (Figure 5c) reveal a correlation between REE chemistry and color groups (blue and purple fluorites

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in one group and colorless to green fluorites in the other), with the difference being in the slope of the light REE (LREE). The slope for the blue to purple fluorites is positive, while it is negative or zero for the green to colorless samples. The actual concentrations of the REEs in the Bingham samples are all very similar.

Optical absorption

The optical absorption spectra of each sample of fluorite before and after irradiation are in Figures 6a-g. The spectra of each sample changed with irradiation, most often in the form of new bands, but sometimes only in intensity.

Wise Mine

The optical absorption spectra of the dark green Wise Mine sample (WM2) before and after irradiation are nearly identical in peak location, but the irradiation intensified each band. The main peaks are at approximately 371 nm, 309 nm, 280 nm, 265 nm and

219 nm.

The OA spectra of the pale green Wise Mine sample (WM1) before and after irradiation changed significantly, as did the color. Very low intensity, unresolved structure is present in the spectrum of the fluorite before irradiation, while the spectrum of the fluorite after irradiation contains broad peaks at 225 nm, 335 nm, 400 nm, and 560 nm.

Long Lake

The OA spectrum for the colorless Long Lake sample (LL1) before irradiation contains a few low intensity peaks near 270 nm. The OA spectrum of the gray center

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contains significant noise, most likely due to the darkness of the sample, and thus reflects

the difficultly of light to pass through to the detector. Attempts were made to decrease the

thickness of the sample and so the concentration of absorbing centers, but were limited

by the fragility of the sample. The colored zones exhibited after irradiation (blue, green,

and maroon) show absorption in their spectra near 266 nm, 280 nm, 335 nm, 400 nm, and

570 nm. This same group of peaks is found in the bluish purple zone of a second Long

Lake sample.

Bingham

The optical absorption spectra of the Bingham samples changed upon irradiation

for every sample, whether in the form of new peaks, increased intensity of pre-existing

peaks, or both. All of the spectra contain parts or the entire Smakula spectrum (bands at

225 nm, 335 nm, 400 nm and 580 nm), with the exception of some of the colorless and pale green fluorite samples that, before irradiation, contain no resolvable structure. A band near 260 nm is prevalent in many samples, and is of greatest intensity in samples that became intense shades of blue and purple upon irradiation. As previously mentioned, this band has been observed in previous studies, but is not well characterized other than it does not appear to be part of the Smakula spectrum (Dantas, et al., 1996). In the darker samples, the 225 nm band is diminished or gone upon irradiation, while the 260 nm band appears, or grows in intensity.

Fluorescence

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Fluorescence spectra are shown in Figures 7a-g. All the spectra collected for each

sample are contained in one graph, with the intensity offset for comparison.

The background spectra noted in Table 2 for each excitation wavelength include

instrumental artifacts arising from reflection of stray light off of the solid sample holder.

Additional filters would have obscured the spectra and so were not used to correct the problem.

Wise Mine

Both the dark green and pale green Wise Mine fluorite samples contain a set of broadband emission peaks before irradiation at 430 nm, 640 nm, and 690 nm. WM1, the pale green sample, also contains sharp emission peaks around 460 nm, 485 nm, 495 nm, and 515 nm. All of the peaks are absent from the spectra of both of the irradiated samples.

Long Lake

The fluorescence spectra of the colorless sample of Long Lake fluorite contains broadband peaks at 430 nm, 640 nm, and 690 nm, as well as sharper peaks around 485

nm, 495 nm, and 515 nm. The sharper peaks are much less intense. All of the peaks are

absent from the spectra of the irradiated Long Lake sample.

Bingham

Most of the samples contain the peaks mentioned above, all of which are gone

upon irradiation. This includes the colorless sample. The dark blue and purple samples

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(B4, B5, B6, B1 and B12) contain less intense peaks than the lighter colored samples

(B3, B7, B9, B10 and B11).

EPR

The spectra for each sample are contained in Figures 8a-g. All the spectra for each

sample are contained on one page. The scales of the intensity (y-axis) are not equivalent

in order to enable study of the smaller, less intense structure.

Wise Mine

The EPR spectra of the pale green Wise Mine sample (WM1) before irradiation contains structure near g-factors of 6.98, 2.66, and 2. The spectra are noisy, and the

visible structure shifts slightly with change in position. The lines at a g-factor of 6.98 and

2.66 are more intense than the surrounding structure in the irradiated WM1 spectra, and

do not shift with change in position of the EPR tube in the instrument cavity. The single

line structure at a g-factor of 2 is very low in intensity, and also does not shift.

The spectra of the dark green Wise Mine sample (WM2) before irradiation are

much more complex than those of WM1, with more lines and structure around g-factors

of 6.13 and 2.66. The less intense lines appear to shift with change in position. The

spectra of the irradiated WM2 are still more complex, with more lines centered around g-

factors of 6.98, 6.6, 2.66 and 2. The surrounding smaller peaks shift slightly with changes

in position.

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Long Lake

The EPR spectra for LL1 before irradiation contains complex structure with many lines centered around g-factors of 7, 6.6, 2.6, and 1.95 that do not shift with change in position. The less intense lines do shift and disappear with change in position.

Three different color zones were cut from the same irradiated Long Lake fluorite crystal to compare EPR spectra among color zones, the maroon, green, and purple. The

EPR spectra of the green zone are all the same. Only minute shifts in the low intensity lines occur with change in position. Three large intensity lines are present at g-factors of

6.98, 2.66, and 1.97. The purple zone contains larger intensity lines at 6.65, 3.82, 2.66, and 1.94 g-factor, which change dramatically in intensity with change in position of the crystal. The same holds for the maroon zone, which contains structure centered near g- factors of 6.65, 2.6, and 1.96. A complex hyperfine structure appears at the first position near a g-factor of 2, but is missing from the other position spectra save for the most intense line.

Bingham

Most of the EPR spectra of the Bingham samples have a few common structures, both before and after irradiation. In these sample spectra there are complex structures centered near g-factors of 6.8, 2.6, and 2. The majority of these lines are surrounded by less intense structure in most of the samples. Samples B1, B2, B4, B5, B7 and B9 contain these structures with slight changes in line position and intensity before and after irradiation. This group encompasses all the color types. Those samples that do not

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contain these structures include B3, B6, B8, B10, and B11. This group also includes all the colors in which Bingham fluorite occurs.

Discussion

DCP

Wise Mine

Among those impurities analyzed for with DCP, yttrium was of the highest concentration in both Wise Mine samples. At close to 60 ppm in WM1, it is an order of magnitude higher in concentration than any other REE in this sample. Yttrium is associated with irradiation-induced blue color (Staebler and Schnatterly, 1971; Bill and

Calas, 1978; Wright and Rakovan, in review) in many fluorites, which is most likely the case for the irradiation induced blue color in this sample. WM2 contains relatively larger concentrations of such REEs as Ce (46 ppm) and La (21ppm), both of which were found to produce green color in synthetic fluorite (Wright and Rakovan, in review) and are known to produce color centers in other fluorites (Bill and Calas, 1978; Staebler and

Schnatterly, 1971). Perhaps the influence of these impurities on the color of the sample prevents the blue color normally associated with yttrium, which is 130ppm in WM2.

Long Lake

While the blue zone of the irradiated Long Lake fluorites does contain a slightly higher concentration of yttrium than the other color zones, it contains significant concentrations of other REEs responsible for color as well, such as La, Ce and Gd

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(Wright and Rakovan, in review; Staebler and Schnatterly, 1971). The concentration of each of these REEs is on the same order of magnitude in each of the color zones, but they differ by as much as 16 ppm, as in the case of Ce in the blue (25 ppm) and green (41 ppm) color zones. The actual color is the net result of all the absorbing centers in the sample, and in this case reflects slight differences in the ratios of impurity elements

(REEs) present.

Bingham

Bosze and Rakovan (2001) found the Bingham fluorites to be sectorally zoned with respect to the REE, but less so than the Long Lake samples. Sectoral zonation is defined as “compositional differences between time-equivalent portions of different growth sectors within a crystal” (Bosze and Rakovan, 2001). This is not the same as concentric zoning, which represents temporal changes in fluid (from which a mineral grows) chemistry. Irradiation experiments on various Bingham samples in this study yield well-defined concentric zoning of color, as well as less distinct sectoral zoning.

Like the Long Lake samples, this zoning is observed in cathodoluminescence images

(Bosze and Rakovan, 2001). With smaller concentrations of REEs and a greater number of sectors for the impurity elements to be divided between, any REE-associated color differences will be less distinct. Despite this, attempts were made to separate the color zones of an irradiated fluorite crystal from Bingham (pale green before irradiation) for

DCP analyses. After irradiation, dark teal zones appeared to be {100} sectors, while the green zones were most likely {111} and {110} sectors. The DCP analyses reveal slight, but possibly significant, differences in concentrations of a few key REE among the color

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zones. The green zone contained more La and Ce than the blue zone. A previous study on

synthetic fluorites (Wright and Rakovan, in review) found that La and Ce doped samples

produced green color upon irradiation. Both zones contained almost 100 ppm of yttrium, much higher than the concentration of any other REE, which could cause the color of the blue zone (Bill and Calas, 1978; Wright and Rakovan, in review).

The intensity of irradiation-induced purple color in Bingham samples produced by

SXRFMA and CL work by Bosze and Rakovan (2001) can be positively correlated with the concentration of REEs among the sectors of some crystals, but not all. In other words, they observed the darkest purple color in the sector (cubic) with the highest concentration of REEs. In this study, the darker purple color induced by irradiation is observed in the octahedral sectors of each Long Lake crystal, which Bosze and Rakovan (2001) also found to have the highest concentration of all the REEs. DCP data of the Long Lake color zones in this study comply with their observations. Yttrium was most highly concentrated in the blue color zone. Bosze and Rakovan (2001) did not analyze for yttrium in the

Bingham samples.

Wise Mine

The main peaks in WM2 are at approximately 560 nm, 371 nm, 309 nm, 280 nm,

265 nm and 219 nm, and do not correspond to any set or subset of peaks previously reported for fluorite. The main peaks in WM1 are at approximately 225 nm, 335 nm, 400 nm, 560 nm, which correspond to the four-band Smakula spectrum (Smakula, 1950;

Scouler and Smakula, 1960) found in many natural and synthetic fluorites that have

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undergone irradiation or additive coloration, and is the spectroscopic manifestation of a form of color center in fluorite (Staebler and Schnatterly, 1971). The blue color induced by irradiation would indicate that the REE responsible for the center is yttrium or gadolinium (Wright and Rakovan, in review), the former being well established as a cause of “light” colors in fluorite, including blue (Bill and Calas, 1978). The sample contains approximately 59 ppm yttrium and 0.49 ppm gadolinium, making yttrium the more likely cause of the irradiation-induced blue color. Following the Staebler and

Schnatterly model (1971) that explains this type of color center in fluorite, the trivalent yttrium is next to a fluorine vacancy, and charge compensation for both is non-local.

Upon irradiation, the two separate defects act as electron traps, trapping two electrons that are shared by both in a molecular orbital.

While the two Wise Mine samples have similar chondrite normalized REE patterns with one order of magnitude difference in concentration, their respective yttrium concentrations are of the same order of magnitude (WM1 has 59 ppm, WM2 has 130 ppm). The yttrium concentration is much greater than that of any of the REEs in the lighter green sample, corroborating the idea that an impurity-associated defect center involving yttrium is the cause of the irradiation induced blue color. In the darker green sample, yttrium is the highest in concentration, but is close in order of magnitude to La

(21.01 ppm), Ce (43.63 ppm), Nd (21.15) and Dy (11.4 ppm). La and Ce are two REEs found by Staebler and Schnatterly (1971) to be associated with the same type of OA bands and color center as yttrium, and subsequent irradiation experiments on synthetic fluorites doped with these REEs have yielded shades of green color (Wright and

Rakovan, in review). Divalent samarium is a well-established cause of green color (Bill

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and Calas, 1978), producing optical absorption peaks at 690 nm, 611 nm, 440 nm, and

422 nm, and is at a concentration of 4 ppm in the dark green Wise Mine sample. The

irradiation was energetic enough to produce F centers and REE3+-associated defects, and

reduce samarium to the divalent state. The optical absorption data does not support any one of these possibilities, however, the complex interplay between two or more of these

centers may produce shifts in the OA bands and produce the green color of WM2, and is

intensified by irradiation. Coexisting Ce- and Y-associated F centers are known to

produce deep yellowish green color in some fluorite, and produce OA bands near 306

nm, 590 nm, and 714 nm (Bill and Calas, 1978). The dark green Wise Mine sample

spectra do exhibit a peak at 309 nm, however, the longer-wavelength part of the spectra

do not contain any resolved structure.

Long Lake

The three peaks of these spectra near 335 nm, 400 nm and 580 nm correspond to

most of the four-band Smakula spectrum. The 225 nm peak may be present but is

swamped by the broad, high intensity absorption in the UV. As is expected for these

slightly different colors, (reddish purple to bluish purple to blue to green), the peaks are

also slightly shifted from one color zone to another. The OA peaks of the maroon zone

are all shifted about 20 nm from the others towards shorter wavelengths.

The peak around 270 nm has been observed in previous studies (Merz and

Pershan, 1967; Dantas et al., 1996 and others), however its origin is still unclear.

Annealing experiments by Dantas et al. (1996) indicate that the 270 nm band disappears

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at 206˚C, while the four bands of the Smakula spectrum are annealed away at temperatures above 256˚C, suggesting that they are unrelated.

Bingham

The prevalence of the Smakula spectrum, representative of the REE-associated color center modeled by Staebler and Schnatterly (1971) readily explains the color of many of the Bingham samples. Like the pale green sample (B11) analyzed by the DCP, many of these samples contain similar amounts of yttrium, which probably accounts for most of the blue color induced by irradiation. All of the colorless and pale green samples became blue with some green zonation after irradiation, and all exhibit the Smakula spectrum of an yttrium-associated color center with peaks at 225 nm, 335 nm, 400 nm, and 580 nm.

The purple and dark purplish-blue color exhibited by many of the Bingham samples is not likely the result of the REE-associated color center proposed by Staebler and Schatterly (1971), which Bill and Calas (1978) described as the cause of “light” or pale colors. The most accepted cause of purple color in fluorite is the F center, a fluorine vacancy occupied by a lone electron (Nassau, 2001). The spectroscopic evidence for these centers are broadband optical absorption peaks close to 560 nm. These peaks are present in almost all the samples after irradiation, and are close to the peak expected in the Smakula spectrum (560 nm to 580 nm). A previous study on synthetic fluorites doped with various lanthanides did not produce any purple color upon irradiation (Wright and

Rakovan, in review), and so the purple color of the Bingham samples is likely the result

119

of F centers. The purple and dark blue samples do contain parts of the Smakula spectrum in their OA spectra, and it is clear that the REE concentrations are similar among all of the samples. However, it is plausible that the intense absorption of the F centers would overprint any color resulting from absorption by REE-associated color centers, as their absorptions tend to be weaker (Naldrett, 1987). The reason why F centers are not produced in all the samples and whether it is related to REE chemistry is not clear. Merz and Pershan (1967) indicate that greater concentrations of trivalent impurities such as

REEs allow for the production of more color centers by providing an electron trap. One would expect more REE-associated colors such as green (Sm, Ce, and La) and blue (Y and Gd) rather than the F-center-associated purple, which is more prevalent in sectors with higher concentrations of REEs in both the Long Lake and some of the Bingham samples. Among whole crystals of Bingham fluorites, the chondrite-normalized REE patterns (Bosze and Rakovan, in review) indicate that the pale green and colorless samples contain slightly higher concentrations of La and Ce, however these REEs have been established as the cause of green irradiation-induced fluorite, while the samples in question become blue with small zones of green upon irradiation. Therefore, although the two color groups have strikingly distinct REE patterns, the relationship between the patterns and the color of the samples remains unclear. Spatially resolved yttrium concentration data for a wide range of Bingham samples and their sectors would be helpful in clarifying this relationship.

Fluorescence

Wise Mine

120

The peak at 430 nm falls into the range of possible emission by Eu2+ (430-450 nm) as

presented by Marfunin (1979) and Gaft et al. (2001). Divalent europium fluorescence

emission is known to produce blue-violet color in fluorite (Marfunin, 1979). This

combined with the longer wavelength emission at 640 and 690 nm may contribute to the

green color of the samples. Yb3+ fluorescence emission is known to cause yellow-green

color in some fluorites (Marfunin, 1979), but some authors only find this emission at low

temperatures (Huber-Schausberger et al., 1967). The additional peaks in these samples at

460 nm, 485 nm, 495 nm, and 515 nm are likely due to other lanthanides within the fluorite. All of the peaks in the spectra are absent in those of the samples after irradiation, and therefore have no bearing on the intensified green and new blue colors in the dark green and pale green samples, respectively, that result from irradiation.

Long Lake

As stated above, the only peak in the fluorescence spectra of the Long Lake samples that could have any bearing on the color (of which there is none visible to the human eye in this sample) at room temperature is the 430 nm peak representing divalent europium, and the other peaks are not known to cause color in fluorite. The lack of any of these peaks in the spectra of the irradiated samples indicates that whatever fluorescence is occurring does not contribute to the color of the sample.

Bingham

The lack of any peaks in the spectra of the irradiated samples again suggests that the fluorescence that is occurring in these samples does not contribute to the color seen

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under normal light conditions. There is a relationship between the intensity of these peaks and the color of the sample before irradiation. The dampening or elimination of the fluorescence emission of the irradiated samples is most likely due to the increased absorption of the luminescence emission by the darker colored samples (including those from the Wise Mine and Long Lake). If the unidentified peaks present were responsible for some color in the pale samples, this relationship between intensity and pre-irradiation color would explain the absence of the peaks in the spectra of the darker samples, and of all the irradiated samples. It seems unlikely that the peaks produce enough emission to induce color because the colorless emission is equally if not more intense than the emission from other samples before irradiation.

EPR

Wise Mine

The structure near a g-factor of 6.98 in the spectra of both of these samples is the result of Er3+ and perhaps Er3+ interacting with a neighboring fluorine ion (Morozov, et al, 1996). The greater intensity of this structure in WM2 would be due to the higher concentration of erbium in that sample. Many paramagnetic entities have a g-factor near

2, including F centers, La3+ in association with a fluorine ion, Lu3+ in association with a fluorine, and trivalent Gd (Anderson and Sabisky, 1971; Morozov, et al, 1996). These

REEs are all present in both samples, however the structure near a g-factor of 2 in their spectra is not well resolved for positive identification. No previous studies on fluorite have mentioned EPR structures with a g-factor of 2.6.

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Long Lake

The EPR spectra of the Long Lake sample before irradiation contains peaks in the

same general areas with g-factors close to 6.9, 2.66, and 2. Again, Er3+ and/or Er3+

interacting with a neighboring fluorine ion probably account for much of the structure

centered around a g-factor of 6.9. The complex structure at a g-factor of 2 is too noisy in

most of the Long Lake spectra, except for the 0o position of the maroon sector. This

spectrum contains a hyperfine structure that may be attributed to Gd3+ in the sample

(Morozov, et al, 1996), and the lack of a clear hyperfine structure in the rest of the spectra

for this sample may be due to the center’s anisotropy, or non-cubic symmetry.

Bingham

No clear relationship exists between the structures in the EPR spectra of the samples and any one color. They are present in samples of every color before and after irradiation. The samples in which these structures are fewer or absent also encompass the entire color scheme in which Bingham fluorites occur. The structures near a g-factor of

6.9 are due at least in part to Er3+ and/or Er3+ interacting with a neighboring fluorine ion,

and the structures around a g-factor of 2 could be any number of paramagnetic entities

previously reported in fluorite (see above). The intensity of all the structures increased

upon irradiation in samples B1, B3, B2 and B6. This group contains blue, pale green, and

purple fluorite. The increase in intensity of structure a g-factor of 2, could in part be due to the increase in the number of F centers present in those samples because F centers have a g-factor of 1.99 (Anderson and Sabisky, 1971). This center is isotropic, which is reflected in the lack of shifting of lines with change in crystal position in the EPR spectra

123

of these samples. The spectra of samples B4, B5 and B8 remained the same in intensity

after irradiation. All of these are shades of blue and purple. Upon irradiation, they

became extremely dark in color, presumably due to the great increase in F centers. It is possible that the F center population swamped the signal, and the instrumental limitations

lead to inaccurate spectra. The remaining Bingham samples for which EPR spectra were

collected are B7 and B9, pale green and colorless, respectively, before irradiation. In the spectra for both samples, some structure became more intense upon irradiation, while the

structure at a g-factor of 2 became less intense or remained the same.

Conclusions

The most prevalent spectroscopic evidence for specific color centers in fluorites from all three localities is the Smakula optical absorption spectrum, representing the

REE-associated color center model (Fig.8) proposed by Staebler and Schnatterly (1971)

(Fig.1). There have been many studies of synthetic and natural fluorites to investigate

these spectroscopic entities in fields of interest such as laser materials; however, few have

focused on the colors caused by the absorption. Wright and Rakovan (in review) looked

at synthetic samples similar to those studied by Staebler and Schnatterly (1971) for the

purpose of investigating the colors, and found that shades of blue and green result from

these centers containing different REE impurities (Y and Gd in blue, and Ce and La in

green irradiated fluorites).

The Smakula spectrum of the irradiated WM1 sample, a naturally pale green

sample that turned deep blue upon irradiation, has a high concentration of yttrium relative

to any other REE present in the sample. These facts suggest that a yttrium-associated

124

color center like that proposed by Staebler and Schnatterly (1971) is responsible for the

irradiation induced blue of WM1. The results for the darker green sample and WM1

before irradiation were not conclusive. The green color of the WM2 sample before and

after irradiation may be a result of the combination of several difference color centers, such as co-existing Y-, La- and Ce-associated centers analogous to the Y-associated center mentioned above, as well as divalent samarium. Fluorescence data indicates that fluorescence emission found in the samples before irradiation does not contribute to color. The intensity of REE-associated structure in the EPR spectra reflects the different concentrations of those REEs in each Wise Mine sample studied.

The Smakula four-band spectrum is very common in the OA spectra of the different color zones of the Long Lake irradiated sample, minus the 225 nm peak which is likely present, only swamped by higher energy UV absorption. The blue and green growth zones contain significant concentrations of REEs that have been previously established in color centers responsible for those colors, such as yttrium (Bill and Calas,

1978), La, Ce and Gd (Wright and Rakovan, in review). The colors of individual regions are likely a result of a combination of the trivalent REE-associated F centers like the blue and green zones, or F centers (purple regions). Fluorescence data, which could not be spatially resolved, is nonetheless evidence that fluorescence emission does not contribute to the irradiation-induced color of the samples. The EPR spectra indicate the presence of some REEs such as Er and Gd.

The OA spectra of the Bingham samples all contain parts or the entire Smakula spectrum after irradiation, and all but the very pale colored samples do before irradiation.

The yttrium DCP data of sample B11 indicates a very high concentration of yttrium in the

125

pale samples, which would account for their blue color upon irradiation. The green

irradiation-induced color of B11 may be a result of the higher concentration of Ce and La

(Wright and Rakovan, in review) or co-existing Ce and Y-associated defect centers (Bill and Calas, 1978). The purple color and dark blue color of many samples are most likely a result of F centers. The OA evidence for these centers (broadband peak at 560 nm) could be overprinted by the Smakula spectrum. The chondrite-normalized REE patterns for the color groups are distinct, but do not reveal a direct relationship with color. Yttrium was not analyzed for in the previous studies on the Bingham samples (Bosze and Rakovan (in review); Hill et al., 2000), however, and could play a crucial role in the explanation for the different colors of Bingham fluorites. The increase in intensity of EPR structures from the spectra of the samples before and after irradiation may indicate an increase in the number of F centers in those samples.

The interplay between absorbing centers evident from the data and the irradiation- induced color is complex and appears dependent on minute differences in concentrations of and ratios between different REEs. Many previous authors have commented on this considerable complexity and the difficulty of clarification (Bill and Calas, 1978; Naldrett et al., 1987). This study is the first to report spatially-resolved data on different sectors of individual crystals, in addition to concentric zones, in an attempt to further clarify the cause of color and its relationship to REE sectoral zoning. Improved experiments, including spatially-resolved EPR of more samples, may be necessary to distinguish the essential differences in structure and defects among sectors and concentric zones in fluorite.

126

The irradiation of the fluorite samples for the purpose of characterizing the causes

of color reveals and/or emphasizes the zonation of color or color intensity within the crystals. This includes both concentric zoning which represents temporal variations in mineral fluid chemistry, and sector zoning of symmetrically non-equivalent growth zones that grew concurrently. Both types of irradiation-induced color zonation found in the

Bingham and Long Lake samples correspond to the cathodoluminescence images reported by Bosze and Rakovan (2001) and Richards and Robinson (2000), respectively.

This provides further evidence that single crystals can be very heterogeneous with respect to trace element composition, and irradiatin-induced color/color intensity is highly dependent on the REE composition and concentration in fluorites. Like the CL images of

Richards and Robinson (2000), the color zonation of the Long Lake samples just as easily allows for the reconstruction of the growth history (change from cubo-octahedral growth to octahedral) and compositional changes in the mineralizing fluids (interface between the colors of concentric zones). Thus, the irradiation of fluorites, and perhaps other minerals that are also susceptible to irradiation-induced color such as alkali halides

(Nassau, 2001), may be useful for sector and concentric zoned-resolved analyses.

127

References

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Alig, R., Kiss, Z., Brown, J., McClure, D. (1969) Energy levels of Ce2+ in CaF2. Physical Review, 186, (276-284).

Anderson, C. and Sabisky, E. (1971) EPR studies of photochromic CaF2. Physical Review B, 3, (527-536).

Berman, R. (1957) Some physical properties of naturally irradiated fluorite. American Mineralogist, 42, (191-203).

Bill, H. and Calas, G. (1978) Color centers, associated rare-earth ions and the origin of coloration in natural fluorites. Physics and Chemistry of Minerals, 3, (117-131).

Bill, H., Sierro, J., and Lacroix, R. (1967) Origin of coloration in some fluorites. The American Mineralogist, 52, (1003-1009).

Botinck, W. (1958) The hydrolysis of solid CaF2. Physica, 24, (650-658).

Chapin, C.E. (1979) Evolution of the Rio Grande Rift-a summary. In Rio Grande Rift: Tectonics and Magmatism (ed. Reicker, R.E.). American Geophysical Union, Washington D.C. (1-5).

Constantopoulos, J. (1988) Fluid inclusions and rare earth element geochemistry of fluorite from South-Central Idaho. Economic Geology, 83, (626-636).

Dantas, N.O., Watanabe, S., Chubaci, J.F.D. (1996) Optical absorption (OA) bands in fluorites by heavy gamma irradiation. Nuclear Instruments and Methods in Physics Research B, 116, (269-273).

Dickson, J. (1980) Artificial colouration of fluorite by electron bombardment. Mineralogical Magazine, 43, (820-822).

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Gorlich, P., Karras, H., Symanowski, C., and Ullmann, P. (1968) The colour center absorption of x-ray coloured alkaline earth fluoride crystals. Phys. Stat. Sol., 25, (93- 101).

Loh, E. (1968) 4f n-1 5d spectra of rare-earths in crystals. Physical Review, 175, (533- 536).

Merz, J. and Pershan, P. (1967) Charge conversion of irradiated rare-earth ions in calcium fluoride. Physical Review, 162, (217-235).

Murr, L.E. (1973) Ordered lattice defects in colored fluorite: direct observations. Science, 183, (206-208).

Putnam, B.R., Norman, D.I., and Smith, R.W. (1983) Mississippi valley-type lead- fluorite-barite deposits of the Hansonburg mining district. In New Mexico Geological Society Guidebook 34th field conference, (253-259).

Spectroscopy, Luminescence and Radiation Centers in Minerals, A. S. Marfunin, Springer-Verlag, New York, 1979.

Naldrett, D.L., Lachaine, A., and Naldrett, S.N. (1987) Rare-earth elements, thermal history, and the colour of natural fluorites. Canadian Journal of Earth Science, 24, (2082-2088).

Nassau, K. (1978) The origin of color in minerals. American Mineralogist, 63, (219-229).

The Physics and Chemistry of Color, K. Nassau, John Wiley & Sons, Inc, 2001.

O’Conner, J. and Chen, J. (1963) Color centers in alkaline earth fluorides. Physical Review, 130, (1791-1795).

Recker, K., Neuhaus, A., Leckebusch, R. (1968). Fluorite. Proc. I.M.A. Cambridge. (145- 152).

Richards, R. and Robinson, G. (2000) Mineralogy of the calcite-fluorite veins near Long Lake, New York. The Mineralogical Record, 31, (413-422).

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Scouler, W. and Smakula, A. (1960) Coloration of pure and doped calcium fluoride crystals at 20oC and –190oC. Physical Review, 120, (1154-1161).

Sierro, J. (1965) Paramagnetic resonance of the Vf center in CaF2. Physical Review, 138, (648-650).

Smakula, A. (1950) Color centers in calcium fluoride and barium fluoride crystals. Physical Review, 77, (408-409).

Staebler, D. (1969) Optical studies of a rare-earth F center complex in rare-earth doped calcium fluoride. Ph. D. thesis, Princeton University (unpublished).

Staebler, D. and Schnatterly, S. (1971) Optical studies of a photochromic color center in rare-earth-doped CaF2. Physical Review B, 3, (516-526). Alig, R. (1971). Physical Review B, 3, p. 536-545.

Wright and Rakovan (in review) Spectroscopic investigation of lanthanide doped CaF2 crystals: implications for the cause of color

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Table 1. Sample names, localities, and color before and after irradiation. Sample B1 B2 B3 B4 B5 B6 B7 B8 name Locality Bingham, Bingham, Bingham, Bingham, Bingham, Bingham, NM Bingham, NM Bingham, NM NM NM NM NM NM Mine Royal Sunshine Sunshine Sunshine Sunshine Sunshine 2 Mex Tex Sunshine Flush 2 2 2 2 2 Natural Dark teal Bluish Pale Dark Dark Dark purple Pale green Pale color green green blue purple green Color Dark Dark teal Pale blue Very Very Very dark Pale blue and Pale blue after blue and and green dark blue dark purple green zones and green irradiation green zones purple zones zones Sample B9 B10 B11 B12 B13 WM1 WM2 LL1 name Locality Bingham, Bingham, Bingham, Bingham, Bingham, Westmoreland, Westmoreland, Long NM NM NM NM NM NM NH Lake, NY Mine Royal Royal Mex Tex Sunshine Royal Wise Wise N/A Flush Flush 2 Flush Natural Colorless Mint Pale Pale Colorless Pale green Dark green Colorless color green green purple wgc Color Pale blue Dark teal Pale blue Very Pale blue Dark teal Darker green Blue, after with with dark with green, irradiation green green purple green maroon, purple Wgc means “with gray center”.

131

Table 2. DCP data for natural samples. REE and Y Long Lake Long Lake Long Lake Long Lake Wise Mine (ppm) {100} {100} {100} {111} (WM2) Color Green irr Blue irr Maroon irr Purple irr Dark green La 17.68 14.43 16.23 18.88 21.01 Ce 41.63 25.04 40.39 46.57 46.63 Nd 16.21 9.88 23.36 26.46 21.15 Sm 3.37 3.06 6.24 7.04 4.4 Eu 0.32 0.34 0.6 0.66 1.2 Gd 6.01 8.6 12.23 12.73 7.9 Dy 14.97 19.73 19.18 21.51 11.4 Er 16.01 28.33 25.79 28.24 9.3 Yb 16.68 22.74 16.93 17.61 9.9 Lu 2.63 2.69 2.60 2.85 2 Y 136.7 185.63 137.97 143.65 130 +/- 8 REE and Y Bingham (B3) Bingham (B3) Bingham Bingham Wise Mine (ppm) {111} {100} (RF3) (RF25) (WM1) Blue irr Green irr Colorless Dark blue Pale green Color La 0.91 5.37 3.19 0.63 1.2 Ce 2.38 11.29 6.31 2.18 2.8 Nd 1.41 5.48 3.62 2.55 1.8 Sm 0.68 0.71 0.81 1.20 0.25 Eu 0.33 0.21 0.26 0.52 0.08 Gd 2.07 1.33 2.38 3.63 0.49 Dy 4.16 1.90 4.01 6.27 0.73 Er 2.37 1.27 2.21 2.86 0.54 Yb 1.41 0.78 0.94 1.32 0.42 Lu 0.14 0.07 0.09 0.13 0.05 Y 99.96 99.28 N/A N/A 59 +/- 4 DCP data for the sectors of up to 10 Long Lake crystals, two Wise mine whole crystals, the blue and green sectors of irradiated Bingham crystal (B3), and average crystals from the two Bingham color groups (modified from Bosze and Rakovan, 2001). The error for each number is less than +/- 1 unless otherwise indicated. “Irr”=following irradiation. “N/A”=not available.

132

Figure 1. Schematic of photochromic center in fluorite involving a molecular orbital with two electrons shared by a trivalent REE and a fluorine vacancy. Modified from Staebler and Schnatterly, 1971.

133

Figure 2. Slices of a single crystal of fluorite from Long Lake, New York. The top row is before irradiation, the bottom row is following irradiation.

134

Figure 3. Slices of fluorite from Long Lake, New York. The top two rows are before irradiation and the bottom two are following irradiation.

135

Figure 4. Fluorite from the Mex-Tex Mine, Bingham, NM. The crystal exhibits {100} and {321} faces, and natural sector zoning of color.

136

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 100

WM2 10 WM1

1 ppm (chondrite normalized) ppm (chondrite

0.1 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Atomic number

Figure 5a. Chondrite normalized REE patterns of fluorite from the Wise Mine, dark green (WM2) and pale green (WM1).

137

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 1000

LL1 green cubic concentric zone LL1 blue cubic concentric zone LL1 maroon cubic concentric zone LL1 purple octahedral sector 100

10 ppm (chondrite normalized)

1 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Atomic number

Figure 5b. Chondrite normalized REE patterns of fluorite from Long Lake, New York.

138

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 100

Average for colorless-pale green Bingham fluorites Average for blue-purple Bingham fluorites

10 ppm (chondrite normalized)

1 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Atomic number Figure 5c. Chondrite normalized REE patterns of fluorite from Bingham, NM. Modified from Bosze and Rakovan (2001).

139

4.0

3.5

3.0

2.5 Absorbance Units WM1, irradiated 2.0 WM1, natural

1.5 200 300 400 500 600 700 800 nm Figure 6a. Optical absorption of fluorite WM1 before and after irradiation.

140

3 Absorbance Units

WM2, natural 2 WM2, irradiated

1 200 300 400 500 600 700 800

nm Figure 6b. Optical absorption of fluorite WM2 before and after irradiation.

141

LL1, natural LL1, irradiated, blue octahedral concentric zone 8 LL1, irradiated, maroon cubic concentric zone LL1, irradiated, green cubic concentric zone LL1, natural, gray cube center

4 Absorbance Units

0 200 300 400 500 600 700 800

nm Figure 6c. Optical absorption of fluorite LL1 before and after irradiation.

142

B1, natural 8 B1, irradiated, dark blue zone B1, irradiated, green and blue zone B1, irradiated, blue zone B1, irradiated, green zone 6

4 Absorbance Units

0 200 300 400 500 600 700 800

nm Figure 6d. Optical absorption of fluorite B1 before and after irradiation.

143

B2, natural B2, irradiated 4

3 Absorbance Units

1 200 300 400 500 600 700 800

nm Figure 6e. Optical absorption of fluorite B2 before and after irradiation.

144

3 B3, natural B3, irradiated

2 Absorbance Units

0 200 300 400 500 600 700 800

nm Figure 6f. Optical absorption of fluorite B3 before and after irradiation.

145

B4, natural B4, irradiated 8 B5, natural B5, irradiated

4 Absorbance Units

0 200 300 400 500 600 700 800

nm Figure 6g. Optical absorption of fluorite B4 and B5 before and after irradiation.

146

3 Absorbance Units 2 B6, natural B6, irradiated, dark zone 1 B6, irradiated, light zone

0 200 300 400 500 600 700 800

nm Figure 6h. Optical absorption of fluorite B6 before and after irradiation.

147

B7, natural B7, irradiated 3

2 Absorbance Units

0 200 300 400 500 600 700 800

nm Figure 6i. Optical absorption of fluorite B7 before and after irradiation.

148

1.8

1.6

1.4

1.2 B9, natural B9, irradiated 1.0 Absorbance Units 0.8

0.6

0.4 200 300 400 500 600 700 800

nm Figure 6j. Optical absorption of fluorite B9 before and after irradiation.

149

4.4

4.2

4.0

3.8

Absorbance Units 3.6

3.4 B10, irradiated

3.2 200 300 400 500 600 700 800 nm Figure 6k. Optical absorption of fluorite B10 after irradiation.

150

4.5

4.0

3.5 Absorbance Units

3.0

B11, natural

2.5 200 300 400 500 600 700 800 nm Figure 6l. Optical absorption of fluorite B11 before irradiation.

151

3 Absorbance Units 2 B12, natural B12, irradiated 1

0 200 300 400 500 600 700 800

nm Figure 6m. Optical absorption of fluorite B12 before and after irradiation.

152

Relative Intensity Relative

400 500 600 700 nm WM1, 225 nm Irradiated, 225 nm WM1, 300 nm Irradiated, 300 nm WM1, 335 nm Irradiated, 335 nm

Figure 7a. Fluorescence emission of fluorite WM1 before and after irradiation at various excitation wavelengths.

153

Relative Intensity Relative

400 500 600 700 nm WM2, 260 nm Irradiated, 260 nm WM2, 280 nm Irradiated, 280 nm WM2, 300 nm Irradiated, 300 nm WM2, 370 nm Irradiated, 370 nm

Figure 7b. Fluorescence emission of fluorite WM2 before and after irradiation at various excitation wavelengths.

154

Relative Intensity Relative

400 500 600 700 nm LL1, 225 nm Irradiated, 225 nm LL1, 300 nm Irradiated, 300 nm LL1, 335 nm Irradiated, 335 nm

Figure 7c. Fluorescence emission of fluorite LL1 before and after irradiation at various excitation wavelengths.

155

Relative Intensity Relative

400 500 600 700

B1, 225 nm nm Irradiated, 225 nm B1, 300 nm Irradiated, 300 nm B1, 335 nm Irradiated, 335 nm

Figure 7d. Fluorescence emission of fluorite B2 before and after irradiation at various excitation wavelengths.

156

Relative Intensity Relative

400 500 600 700

B2, 225 nm nm Irradiated, 225 nm B2, 300 nm Irradiated, 300 nm B2, 335 nm Irradiated, 335 nm

Figure 7e. Fluorescence emission of fluorite B2 before and after irradiation at various excitation wavelengths.

157

2000

1500

1000

Relative Intensity Relative B3, Irradiated, 225 nm 500 B3, Irradiated, 300 nm B3, Irradiated, 335 nm

0 400 500 600 700

nm Figure 7f. Fluorescence emission of fluorite B3 after irradiation at various excitation wavelengths.

158

Relative Intensity Relative

400 500 600 700

B4 and B5, 225 nm nm Irradiated, 225 nm B4 and B5, 300 nm Irradiated, 300 nm B4 and B5, 335 nm Irradiated, 335 nm

Figure 7g. Fluorescence emission of fluorite B4 and B5 before and after irradiation at various excitation wavelengths.

159

Relative Intensity Relative

400 500 600 700

B6, 225 nm nm Irradiated, 225 nm B6, 300 nm Irradiated, 300 nm B6, 335 nm Irradiated, 335 nm Figure 7h. Fluorescence emission of fluorite B6 before and after irradiation at various excitation wavelengths.

160

3000

2500

2000

1500

Relative Intensity 1000

500

0 400 500 600 700 nm B7, 225 nm Irradiated, 225 nm B7, 300 nm Irradiated, 300 nm B7, 335 nm Irradiated, 335 nm

Figure 7i. Fluorescence emission of fluorite B7 before and after irradiation at various excitation wavelengths.

161

B9, 265 nm Irradiated, 265 nm B9, 300 nm Irradiated, 300 nm Irradiated, 225 nm Relative Intensity Relative

400 500 600 700

nm Figure 7j. Fluorescence emission of fluorite B9 before and after irradiation at various excitation wavelengths.

162

B10, 210 nm Irradiated, 210 nm B10, 240 nm Irradiated, 240 nm B10, 265 nm Irradiated, 265 nm Relative Intensity Relative

B10, 290 nm Irradiated, 290 nm B10, 300 nm Irradiated, 300 nm Relative Intensity Relative

400 500 600 700

nm Figure 7k. Fluorescence emission of fluorite B10 before and after irradiation at various excitation wavelengths.

163

4000

3000

Intensity 2000 Relative

1000

0 400 500 600 700

B11, 230 nm nm Irradiated, 230 nm B11, 270 nm Irradiated, 270 nm B11, 300 nm Irradiated, 300 nm B11, 310 nm Irradiated, 310 nm

Figure 7l. Fluorescence emission of fluorite B11 before and after irradiation at various excitation wavelengths.

164

Relative Intensity Relative

400 500 600 700

B12, 225 nm nm Irradiated, 225 nm B12, 300 nm Irradiated, 300 nm B12, 335 nm Irradiated, 335 nm

Figure 7m. Fluorescence emission of fluorite B12 before and after irradiation at various excitation wavelengths.

165

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

WM1, 225o

WM1, 135o

WM1, 90o

WM1, 0o

WM1, irradiated, 225o

WM1, irradiated, 135o

WM1, irradiated, 90o

WM1, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8a. EPR of fluorite WM1 before and after irradiation at various angles.

166

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

WM2, 225o

WM2, 135o

WM2, 90o

WM2, 0o

WM2, irradiated, 225o

WM2, irradiated, 135o

WM2, irradiated, 90o

WM2, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8b. EPR of fluorite WM2 before and after irradiation at various angles.

167

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

LL1, 225o

LL1, 135o

LL1, 90o

LL1, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8c. EPR of fluorite LL1 before irradiation at various angles.

168

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

LL, irradiated (maroon), 225o

LL, irradiated (maroon), 135o

LL, irradiated (maroon), 90o

LL, irradiated (maroon), 0o

LL, irradiated (green), 225o

LL, irradiated (green), 135o

LL, irradiated (green), 90o

LL, irradiated (green), 0o

LL, irradiated (purple), 225o

LL, irradiated (purple), 135o

LL, irradiated (purple), 90o

LL, irradiated (purple), 0o

1000 2000 3000 4000 5000 6000 Gauss Figure 8d. EPR of fluorite LL1 color zones after irradiation at various angles.

169

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B1, 225o

B1, 135o

B1, 90o

B1, 0o

B1, irradiated, 225o

B1, irradiated, 135o

B1, irradiated, 90o

B1, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8e. EPR of fluorite B1 before and after irradiation at various angles.

170

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B2, 225o

B2, 135o

B2, 90o

B2, 0o

B2, irradiated, 225o

B2, irradiated, 135o

B2, irradiated, 90o

B2, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8f. EPR of fluorite B2 before and after irradiation at various angles.

171

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B3, 225o

B3, 135o

B3, 90o

B3, 0o

B3, irradiated, 225o

B3, irradiated, 135o

B3, irradiated, 90o

B3, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8g. EPR of fluorite B3 before and after irradiation at various angles.

172

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B4, 225o

B4, 135o

B4, 90o

B4, 0o

B4, irradiated, 225o

B4, irradiated, 135o

B4, irradiated, 90o

B4, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8h. EPR of fluorite B4 before and after irradiation at various angles.

173

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B5, 225o

B5, 135o

B5, 90o

B5, 0o

B5, irradiated, 225o

B5, irradiated, 135o

B5, irradiated, 90o

B5, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8i. EPR of fluorite B5 before and after irradiation at various angles.

174

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B6, 225o

B6, 135o

B6, 90o

B6, 0o

B6, irradiated, 225o

B6, irradiated, 135o

B6, irradiated, 90o

B6, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8j. EPR of fluorite B6 before and after irradiation at various angles.

175

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B7, 225o

B7, 135o

B7, 90o

B7, 0o

B7, irradiated, 225o

B7, irradiated, 135o

B7, irradiated, 90o

B7, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8k. EPR of fluorite B7 before and after irradiation at various angles.

176

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B8, 225o

B8, 135o

B8, 90o

B8, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8l. EPR of fluorite B8 before irradiation at various angles.

177

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B9, 225o

B9, 135o

B9, 90o

B9, 0o

B9, irradiated, 225o

B9, irradiated, 135o

B9, irradiated, 90o

B9, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8m. EPR of fluorite B9 before and after irradiation at various angles.

178

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B10, 225o

B10, 135o

B10, 90o

B10, 0o

B10, irradiated, 225o

B10, irradiated, 135o

B10, irradiated, 90o

B10, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8n. EPR of fluorite B10 before and after irradiation at various angles.

179

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B11, 225o

B11, 135o

B11, 90o

B11, 0o

B11, irradiated, 225o

B11, irradiated, 135o

B11, irradiated, 90o

B11, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8o. EPR of fluorite B11 before and after irradiation at various angles.

180

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B12, 225o

B12, 135o

B12, 90o

B12, 0o

B12, irradiated, 225o

B12, irradiated, 135o

B12, irradiated, 90o

B12, irradiated, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8p. EPR of fluorite B12 before and after irradiation at various angles.

181

g-factor

6.75 3.38 2.25 1.69 1.35 1.12

B13, 225o

B13, 135o

B13, 90o

B13, 0o

1000 2000 3000 4000 5000 6000

Gauss Figure 8q. EPR of fluorite B13 before and after irradiation at various angles.

182