Rare-earth elements, thermal history, and the colour of natural fluorites

D. L. NALDRETT' Ottawa-Carleton Centre for Geoscience Studies, Geology Department, University of Ottawa, Ottawa, Ont.,

Canada KIN 6N5

ANDRE LACHAINE Physics Depanment, Royal Military College of Canada, Kingston, Ont., Canada K7K 5.50

AND

S. N. NALDRETT Chemistry Department, Royal Military College of Canada, Kingston, Onr., Canada K7K 5L0

Received December 1, 1986

Revision accepted March 23, 1987

The contents of La and of all 13 naturally occurring rare-earth elements (REEs) in samples of natural fluorites were deter- mined by neutron activation analysis. The samples were chosen to test the relation of REE content to colour. Total REE contents of the samples ranged from 3.46 to 97.2 ppm; the range of the least abundant REE, Lu, was 0.031 -0.57 ppm and the range of the most abundant REE, Ce, was 0.94-33.3 ppm. No relation was found between the absolute amounts nor the enrichment or depletion of a particular REE and colour. The absorption spectra from 400 to 700 nm were determined by photo- acoustic spectroscopy using a few milligrams of powdered fluorite sample. Optical absorption spectra were obtained for all samples including those that were too small or too opaque for transmission or reflectance methods. The absorption maxima obtained are similar to those reported by others for samples of similar colours. It is concluded that ionizing radiations from incorporated radioactive elements produce the divalent REE ions that account initially for purple colour. From the description given in the literature on the thennoluminescence of fluorites it is then shown that the thermal history can account for the ulti- mate colour. However, other factors, such as the presence of water vapour, oxygen, or hydrogen; variation in the growth rate of crystals; colloidal particles; and exposure to light, can contribute to the colour.

Les concentrations de La et de tous les 13 ClCments de terres rares dans des Cchantillons de fluorites naturelles ont CtC dCter- mindes par I'analyse d'activation neutronique. Les Cchantillons ont CtC choisis dans le but de tester la relation entre la concen- tration des terres rares et la couleur. Les concentrations totales des terres rares des Cchantillons variaient de 3,46 B 97,2 ppm; la variation de la terre rare la moins abondante, Lu, Ctait de 0,031 -0,57 ppm, et celle de la terre rare la plus abondante, Ce, de 0,94-33,3 ppm. Aucune relation n'a CtC observke entre les quantitks absolues, ni de l'enrichissement ou de l'appauvrisse- ment, de terres rares particulitres et la couleur. Les spectres d'absorption dans la garnme 400 B 700 nrn ont CtB dCterminCs par spectroscopie photoacoustique en utilisant quelques milligrammes d'tchantillon de poudre de fluorite. Les spectres d'absorp- tion ont CtC obtenus pour tous les Bchantillons incluant ceux qui Ctaient trop opaques ou dont la quantitC Ctait trop petite pour les mCthodes en transmission ou en kflectance. Les absorptions maximales obtenues sont analogues B celles dCjB rapportCes pour d'autres Cchantillons de couleurs sirnilaires. L'Ctude m&ne ?I la conclusion que les radiations ionisantes des ClCments radioactifs incorpoks produisent les ions de terres rares bivalentes qui sont responsables B I'origine de la couleur pourpre. I1 est dCmontr6, B partir de la description donnCe dans la IittCrature sur la thennoluminescence des fluorites, que l'histoire thennale peut expliquer la couleur finale. Cependant, d'autres facteurs, telle que la pksence de vapeur d'eau, l'oxygbne ou l'hydrogbne; la variation du taux de croissance des cristaux; les particules colloidales; et l'exposition ?I la lumikre, peuvent contribuer ?I la couleur.

[Traduit par la revue]

Can. I. Ealth Sci. 24, 2082-2088 (1987)

Introduction Fluorite, CaF,, occurs naturally in a wide variety of colours

ranging from colourless through yellow, green, blue, violet, purple, blue-black, brown, and occasionally pink to red and sometimes iridescent. In addition, its marked fluorescence gave the name to the phenomenon. Two decades before the term "colour-centre" was introduced to explain the origin of colour in alkali halides, a mineralogist recognized the impor- tance of ionizing radiation in the production of colours in minerals (Doelter 1909). He also reported that the colour could be bleached out by heating (Doelter 1911, 1925) and first studied the thermoluminescence of fluorite (Doelter 1924), which now appears to offer the most promising explanation of the ultimate colour of natural fluorite. The colour-centre phenomena are more complex in fluorites than in alkali

'Present address: Geography Department, Queen's University, Kingston, Ont., Canada K7L 3N6. Rinted in Canada 1 Imprim6 au Canada

halides. Defects and aggregates of defects intrinsic to the fluorite crystal lattice occur, and in addition other colour- centres are produced by the occurrence of impurities in the crystal lattice. The trivalent ions of yttrium, lanthanum, and rare-earth elements (REEs) have similar dimensions to the divalent calcium ion and displace a significant number of cal- cium ions with various types of charge compensation, e.g., YF, and CaF, form a series of solid solutions up to 55 mol% YF,, as in the mineral yttrofluorite (Short and Roy 1963; Adarns and Sharp 1972). Fluorites that are cerium-rich also occur (Sverdrup 1968).

The experimental effort of this investigation has been con- centrated on two problems. The first is the content of Y, La, and REEs of different fluorites. Sufficiently extensive use of neutron activation analysis methods was made so that the amounts of La and all 13 REEs were determined for samples of various colours. An approximate determination of Y was made by atomic absorption spectroscopy. The second is the first use

NALDRETT ET AL.

of photoacoustic spectroscopy to obtain the absorption spectra of the samples. Techniques that depend upon the transmission or reflectance of light are hampered by the cracks, bubbles, "feathers," etc. that occur in natural materials. The method of photoacoustic spectroscopy measures only the light of each particular wavelength that is actually absorbed, and is not affected by scattering or reflectance.

Method Determination of REE content

We have chosen samples to test the relationship of colour to REE content, e.g., samples F1, F2, and F3 are from colour- less, brown, and purple areas, respectively, of a specimen of fluorite from Barbaraschact, Wolsendorf, Bavaria, and are aggregations of fairly well formed cubic crystals of these different colours. Sample F4 was cut from the edge of a 5-cm cube of yellow fluorite appearing as a "phantom" inside a 9-cm cube of blue fluorite from Cave-in-Rock, Illinois. Sample F5 was blue fluorite removed from close to the edge of the yellow phantom cube. F6 was a 1.5-cm cube of clear pink fluorite from Uri, Switzerland (Dana 9.2.1). F7 was a sample of purple - black iridescent fluorite from the Auglaise quarry, Junction, Ohio; the 1-mm crystals were separated from a car- bonate matrix using hydrochloric acid. F8 was green fluorite

E a V

cubes, about 6-8 mrn, from the Rugerly quarry, Stanhope, - c Weardale, England. F9 was very dark purple fluorite from B c Cave-in-Rock, Illinois. F10 was green octahedral crystals 8 from the Kola peninsula, U. S. S. R. Sample F3 1 was octahedral crystals of intense purple fluorite from the Burro Mountains, 'd

Sierra County, New Mexico. F32 was yellow fluorite from the 5. May Stone quarry, Fort Wayne, Indiana. F33 was green fluor- 3

* ite from the Austrian Alps. All of the samples with the excep- 0

tion of F7, iridescent fluorite, were clear crystals free of matrix. 4

Q

The amounts of lanthanum and of each of the 13 naturally 2 occurring rare-earth elements were determined by neutron ? activation analysis. A neutron flux of 2 x 1013 neutrons 9 s-' ~ m - ~ was used and the resulting gamma activity up to d

3

2049 keV was scanned with a Ge(Li) detector. To determine w the presence of each isotope with a minimum of interference -1

from others, five independent scans were made for each 3 sample: a short irradiation of a few minutes and the immediate measurement of short-lived activities; a medium irradiation of 1-2 h and measurement of activities after about 3 h and 12 h; a long irradiation of several days followed by measurement after 12 days and after about 30 days. For each scan, a plot of activity versus gamma ray energy, with the isotope responsible for each significant peak identified, and a number printout of the activities at 1 keV intervals was provided. Corrections were made for geometry and for the Ge(Li) detector efficiency at each voltage. From these data we calculated the amounts of each element present, whenever possible using supporting data for the same isotope at different garnrne ray energies, and for different isotopes of the same element. The tables of Bujdoso et al. (1973) were used for the calculations. Table 1 shows the contents in parts per million and the average deviation of lan- thanum and the REEs in 13 fluorite samples. Yttrium could not be determined by this method because the product isotope does not emit gamma rays. It was determined instead by atomic absorption spectrometry. This method is less precise but is probably satisfactory as the earlier attribution of Y to the colour of fluorites has been shown to be improbable (Ark- angelskaya et al. 1964; Birsoy and Murr 1979). The content of

2084 CAN. J. EARTH SCI. VOL. 24, 1987

2 0 Ydlow.Bmwn Yallow-Brown

1 0 . ------3 -Y

0.6

--. 2 -- \ ------3!\ . 2 0 Blue-Purple --.

(I) w

k l o n z p 0.6 U ui > I-

t 2 0 Green

z g 10 z w 06

2-01 Various I

FIG. 2. Optical absorption spectra of fluorites of various colours average chondrites (Henmann 1970) is also shown. Figure 1 determined by photoacoustic spectroscopy. shows the proportion of REEs in these fluorites compared with the proportion in chondrites.

I

- -

Determination of absorption spectra The optical absorption curves of the fluorites in this study

were determined by photoacoustic (also called optoacoustic) spectroscopy (Pao 1977; Rosencwaig 1980). The sample (a few milligrams of powder) was placed inside a closed photo- acoustic cell (EG&G/PARC Model 6003, Princeton Applied Research, Princeton, New Jersey) containing air and a sensi- tive microphone. The sample was then illuminated with mono- chromatic light, which was intensity-modulated by a Bentham Model 218 mechanical chopper. Nonradiative deexcitation processes convert part or all of the light absorbed by the sample into heat. The periodic flow of this heat into the gas (air) of the cell produces pressure variations in it; this is how the sound originates. These pressure variations are then detected by the microphone, fed to a lock-in amplifier, and stored in a microcomputer. The signal is recorded as a function of wavelength of the incident light, which is varied by passing the light from a 450 W xenon arc lamp through a Kratos GM 252 monochromator. To remove all spectral structures of the light source, monochromator, etc., the spectra are divided, point by point, by a reference spectrum that is obtained by using a perfect absorber (carbon black powder) as a sample. Each spectrum is thus internally normalized, with the highest peak in the spectrum arbitrarily set at 100% absorption. The samples of purple black iridescent fluorite, F7, and of intense purple octahedral crystals. F3 1, were too dark to give a reflec- tance reading (Macbeth ' ' Color-Eye" , Kollmorgen Corpora- tion, Newburgh, New York). The crystals of most of the samples in this study were also too small to be sectioned for transmission spectroscopy. The optical absorption curves of the fluorites in this study are shown in Fig. 2 in the same

. .

grouping as used in Fig. 1 in which the enrichment or depletion of REEs is shown. The peaks observed are broad, as expected for crystalline material. They may also be broadened by absorption of several divalent REEs contributing to the peak. The locations of the peaks are similar to those reported using methods that measure transmission or reflectance of light by natural fluorite samples (Smakula 1950, 1954; Barile 1952; Przibram 1956; Bill et al. 1967; Hunt et al. 1972; Birsoy and Murr 1979; Bill 1982).

Discussion

I 1 1 I La d a 6r Nd plrn S& Eu Gd TL Dy Ho Er Tm Y b Lu

fie role of REEs Table 1 shows that there is a wide variation in the amounts of

a particular REE in the various fluorites, and also in the total REE content. The lighter REE, La to Gd, are generally more abundant than the heavier elements, Tb to Lu. There does not appear to be any association between the presence of a particu- lar REE, even in comparatively large absolute amounts, and the colour of the fluorite. A useful test is the effect on colour of the enrichment of a particular REE, i.e., an increase in the pro- portion of that REE to the total REE content of the sample compared with the proportion of that REE to the total REE content of average chondrites (which are assumed to approxi- mate the original cosmic composition). Figure 1 shows these enrichment (or depletion) ratios for the 13 fluorites in this study. Fluorites of similar colour are grouped together and are separated from other colours. The curves are similar for all colours with an enrichment of light REEs and a depletion of heavy REEs, with no particular colour associated with enrich- ment or depletion of any or all REEs.

Direct experiments (Marchand et al. 1976) showed that

'- - 7 2 k o n t -_ \ I

FIG. 1. Enrichment or depletion of REEs in fluorites compared with I I I I I

the proportion of each REE to the total REE content in chondrites. 400 460 520 580 640 700

WAVELENGTH. nm

NALDRETT ET AL. 2085

when fluorite is deposited from a mother solution of CaF2 plus REEs, the composition of the fluorite deposited parallels the composition of the solution but with an enhancement of REEs relative to CaF2 and with the partition coefficient of crystal to liquid increasing from La to Lu. A pattern of REE abundances compared with that in chondrites consistent with these findings has been reported for many fluorites (Schneider et al. 1975, 1977; Marchand et al. 1976; Grappin et al. 1979; Birsoy and Murr 1979; Lenk et al. 1982; Chatagnon and Galland 1982; Strong et al. 1984; Dill et al. 1986). Explanations for the ano- malous behaviour that is occasionally observed for Eu, Ce, Sm, and Tb have been offered by Fryer (1977), Kemch and Fryer (1979), Bilal and Becker (1979), Grappin et al. (1979), and Schneider et al. (1975, 1977). None of these investiga- tions shows a particular colour associated with the enrichment of a particular REE, and the colour of fluorites must be accounted for in some other manner.

Evidence from studies of thermoluminescence The probable explanation of the colour of natural fluorites

appears from a study of thermoluminescence (TL). The charge compensation required when a trivalent REE ion has substi- tuted for a divalent calcium ion is provided, for example, by an extra F- occupying an interstitial position in the fluorite lattice, or 02- substituting for a lattice F-, or Na+ replacing a lattice Ca2+, etc. The colour of fluorite is initially produced by ioniz- ing radiation that has removed an electron from a fluorine ion, F-, or from an oxygen ion, 02-, the released electrons then having reduced trivalent REE ions to divalent REE ions, most of which absorb visible light. The remaining neutral fluorine atom, FO, or monovalent oxygen ion, 0-, represent "holes" that can attract electrons, but that are held in various "traps" in the fluorite lattice. As the temperature is raised, as in the TL process, traps are successively emptied as their activation energies a& reached. On bekg reieased, the "hole" can extract an electron from a divalent REE ion, leaving a trivalent REE ion in an excited state, which then reverts to a stable state by releasing energy as light (see, for example, Kiss and Staebler (1965) and Marfunin (1979 p. 234) ).

The colour due to the divalent REE disappears and the absorp- tion of trivalent REEs is very much less and does not con- tribute to colour (Men and Pershan 1967~) . The absorption spectra of divalent REE ions in a fluorite lattice have been determined by McClure and Kiss (1963) and by Men and Pershan (1967~).

The TL glow peaks of fluorite have been reported by many observers, e.g., Iwase (1933), Hill and Aron (1953), Moore

1 (1965), Blanchard (1966, 1967), Ratnam and Bose (1966), Men and Pershan (1967a, b), Kaufhold and Herr (1968), and Sunta (1970). The TL glow peaks fall into five main groups:

I 60-80, 95 - 110, 130- 160, 190-270, and 290-375°C. It is noteworthy that the wavelengths of light emitted at these

various TL peaks show a regular change with increasing tem- 1 perature, e.g., Iwase (1933) showed that natural fluorites emitted bands at 574, 542, and 478 nm below 200°C and that bands appeared at 436,420, and 380 nm only at temperatures above 200°C. It has been confirmed by the other observers mentioned above that the colour-centres involving energy towards the red end of the colour spectrum are involved first, and that the higher energy centres towards the blue and violet are discharged only at higher temperatures.

R e role of ionizing radiation The initial stock of divalent REE ions is produced by ioniz-

ing radiation. There are many references in the literature to the use of gamma or X-rays and a few to the use of radium, elec- trons, neutrons, protons, or deuterons. Roentgenequivalent amounts of all radiations appear to be equally effective in pro- ducing colour in fluorites. These irradiations generally produce a blue to purple colour. However, prolonged intense radiation can also produce crystal damage and bleaching.

The effect of irradiation on the colour of fluorite rises very rapidly and almost asymptotically to about 10 kR, after which there is only a minimal colour change up to about 250 kR, after which structural damage may occur (Sabisky 1965; Kaufhold and Herr 1968). These amounts of irradiation can certainly occur in natural fluorites as a result of radioactive elements that are present, e. g . , naturally occumng potassium, gadolinium, lutetium, neodymium, and samarium all have some long-lived radioactive isotopes that contribute to ionizing radiation. How- ever, most of the irradiation will be contributed by uranium that is present in all fluorites in the proportion of a few hundredths of a part per million up to several parts per million (Doelter and Sirk 1910; Henrich 1920; Hoffmann 1924; Haberlandt 1934; Pnibram 1956; Blanchard 1966; Kaufhold and Herr 1968; Birsoy and Murr 1979). Because the presence of 1 ppm of uranium will contribute about 353 rnR per year (Houtermans 1960), it is evident that over tens of thousands of years an adequate reservoir of divalent REE ions will be pro- vided to produce colour in natural fluorites. Thermal bleaching studies (Calas 1972a, b) showed that half the colour would dis- appear in 200-450 years at O°C and that the process is reversible; thus, the colour exists because of continuing irradiation due to the presence of radioactive elements, not to a single irradiation event.

Efiect of thermal history on colour All of our samples of fluorite that were irradiated with

gamma rays from a 'OCo source became blue to purple. Other observers also found similar effects on irradiation, e.g., yellow fluorite (Eysank 1936; Bill et al. 1967), green fluorite (Keller- mann 1937; Pnibram 1956, p. 193; Bill et al. 1967), and red fluorite (Bill et al. 1967). Blue to purple is the usual colour of irradiated fluorite, whereas other colours can result from expo- sure to light or heat. Purple is a nonspectral colour in which there is a dominant absorption in the green to yellow, about 550-580 nm, and reflectance or tlansmission of both red and violet in various proportions, giving a possible range in colour from blue purple to purple to red purple. As absorption in the red is reduced, the yellow absorption dominates and the fluor- ite is green. As the yellow absorption is deactivated, leaving absorption mostly in the violet, the colour is yellow to brown. Finally, as the violet absorption is deactivated by heat, all colours are reflected or transmitted and the fluorite becomes colourless.

E#ect of other variables on colour Several complications can add to the above simplified

account, e.g., oxygen can enter the fluorite lattice either directly or by way of hydrolysis (Bontinck 1958; Muto and Awazu 1968). Bill (1982) offers evidence that the origin of the colour of yellow fluorite is an 0; centre. Pink fluorite is com- paratively rare and seems to require particular conditions, one of which may be the presence of oxygen (Bill and Calas 1978).

The intensity of colouration increases with the REE content, but no particular colour is favoured. The fraction of trivalent

2086 CAN. J. EARTH

REE ions reduced by ionizing radiation is only a small per- centage (Kiss and Yocum 1964; Men. and Pershan 1967~) . The reason for this is that a trivalent REE ion compensated by an extra fluorine ion in an adjacent interstitial site (axial sym- metry) is a stable entity and is not reduced by ionizing radia- tion. Only trivalent REE ions compensated at distant sites (cubic symmetry) are reducible. Kirton and McLaughlan (1967) reported that 10 different symmetry sites were possible and that each produced a somewhat different optical absorption spectrum. Which REE is reduced will depend on the energy supplied (Men, and Pershan 1967~) . Light of a single wavelength can affect specific centres and the effect of light can be complex and can produce colour changes or bleaching (Przibrarn 1956, pp. 191 - 193).

The interstitial fluorine ions can migrate and the thermal his- tory will determine which trivalent REEs are left reducible (Friedman and Low 1960). A further effect of thermal history reported by Ratnam and Bose (1966) was that for irradiation near normal temperatures a small rise (30°C) in the tem- perature of irradiation markedly affected the TL glow curve pattern.

Factors such as changes in temperature or composition of the mother solution can affect the rate or pattern of growth of fluorite crystals, which in turn can affect colour. Natural fluorite is often banded or layered, and in some cases micro- scopic examination shows colour to be distributed in patches along [ I l l ] (cleavage) planes in growth zones. The banding along growth zones suggests that the colour is affected by the growth conditions, a temporary faster growth rate producing more defects (Braithwaite et al. 1973) and adsorption of impurities causing a slower growth rate (Calas and Zarkas 1973).

Application of pressure, either before or after irradiation, has been found to affect the colour of some fluorites (Pnibram 1956, p. 193). On the other hand, a shock, as from explosives, bleaches some fluorites (Braithwaite et al. 1973).

Still another variable that can contribute to the colour of fluorites is the presence of particles of colloidal size proposed by Doelter (1925), and later specifically for Blue John type fluorites by Allen (1952), MacKenzie and Green (1971), and Braithwaite et al. (1973). It is now recognized that the presence of calcium colloids is responsible for the blue colour of Blue John type fluorites, which have a somewhat different absorption spectrum than the majority of blue fluorites (Bill and Calas 1978). Calcium colloids have been shown to be pro- duced in fluorites by bombardment with 75 kV electrons (McLaughlan and Evans 1968) but not by 10 kV electrons (Rao and Bose 1970). Proximity to collophane deposits con- taining U is suggested as the origin of the colloid formation in Blue John type fluorites (Braithwaite et al. 1973). Scattering of visible light by particles of colloidal size may also account for the interferencecolour pattern of iridescent~fluorites, and for some very dark purple fluorites that are more resistant to thermal bleaching.

All of these fluorites showed fluorescence under ultraviolet light. Samples F2, F7, and F32 showed yellow to brown fluorescence; all other samples showed purple fluorescence, although sample F9 was almost black under ultraviolet light. It is proposed to make a spectroscopic determination of the fluorescences.

Summary The contents of Y, La, and all 13 naturally occurring REEs

in samples of natural fluorites of various colours were deter-

SCI. VOL. 24, 1987

mined. No relation was found between the enrichment or depletion of any particular REE and colour. The optical absorption spectra were determined by photoacoustic spec- troscopy for each of the fluorites, including samples that were too small or too opaque for transmission or reflectance methods. The absorption maxima are similar to those reported by others for samples of similar colours. From the observations reported in the literature on TL glow curves, it is shown that after ionizing radiation has produced the familiar violet to purple fluorite, the thermal history of the fluorite can account for other colours. Contributing factors, such as the presence of oxygen, hydrogen, and water vapour; variation in the growth rate of crystals; production of colloidal particles; and exposure to light, can also affect the colour of fluorites.

Acknowledgments We are grateful for the help of the Atomic Energy Research

Establishment, Harwell, England, and in particular Dr. D. Gibbons and Dr. J. W. Haynes in providing the neutron activa- tion of the samples, and identifying the isotopes responsible for the activity peaks. We thank Dr. John Stone of Queen's University, Kingston, Ontario, for the gamma irradiation of fluorite samples. Dr. Paul Rochon of the Physics Department of the Royal Military College, Kingston, Ontario, initially pro- vided very helpful assistance with the photoacoustic spectros- copy. We are also grateful to the Department of National De- fence for Academic Research Program grant F 4140. Referees' comments are appreciated by adoption.

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