sign in sign up
<<
Home , Elbaite, Pleochroism, Tourmaline

American Mineralogist, Volume 73, pages 822-a25, 1988

Role of natural radiation in coloration

L,nNe M. RorNrrzr* Groncn R. Rossm.lNl Division of Geological and Planetary Sciences, Institute of Technology,Pasadena, California 9 I 125, U.S.A.

Ansrn-lcr The optical spectraof subjectedto large, controlled dosesof gamma radiation have been compared to those of natural specimens.Both naturally pink and laboratory-irradiated elbaites show the same spectroscopicfeatures. Optical absorption features of Mn2* in nearly colorless elbaites are lost during laboratory irradiation, indi- catifig a Mn2+ r Mn3+ transformation during the radiation process.Measurements of the radiation levels in tourmaline pockets in southern California pegmatiteshave been used to compute the dosesthat natural samples should have experiencedover geologic time. Thesedoses generally correspond to the dosesrequired to restorethe color to elbaitesthat have been decolorizedby laboratory heat treatment, indicating that color in naturally pink tourmaline is a product ofnatural radiation. This radiation could have been effective only after the cooled below the decolorizing temperature of tourmaline, suggesting that most pink elbaites originally grew nearly colorless in the pegmatitesand only later attained their pink color through oxidation of Mn via ionizing radiation.

INrnorucrroNr observedthat both gamma and X-radiation develop and High-energy ionizing radiation is capable of changing intensify pink color in elbaitesand that heat treatment at the color of several and inducing a variety of 600'C removes both the natural and irradiation-induced radiation-damage centers that include trapped electrons color. A variety ofstudies have presentedthe visible and and oxidized or reducedcations and anions. The detailed near-infrared spectra of pink tourmalines and proposed atomic processesaccompanying these changesare often explanationsfor the colors (Bershov et al., 1969; Wilkins poorly understood.It has long been recognizedthat many et al., 1969; Manning, 1969, 1973; Bakhtin et al., 1975), minerals change color when they are irradiated in the but have not dealt with the role ofradiation. In particu- Iaboratory. The new colors that develop frequently re- lar, none of these studies address the issue of whether semble the colors of naturally occurring varieties of the laboratory irradiation duplicates or even approximates . Furthermore, the natural color of many minerals natural processesin detail. can be cyclically removed by heating and restored by ir- ExprnruBNTAL DETATLS radiation. Therefore, it is often stated that the colors in- duced by laboratory irradiation are identical to natural Absorptionspectra were taken using procedures described in colors with the tacit assumption that the underlying Rossman(1975). Samples (Table 1) werechosen to represerta wereperformed in a 190-Curie atomic-level color origins are also identical. varietyoflocalities. Irradiations '3?Cssource at a rateof 1.4Mrad/d of 0.66MeV gammarays. There are few quantitative studies that have examined Heatingexperiments were performed in air. these concepts in detail. Two questions need to be ad- dressed:what are the detailed transformations that occur Rnsur,rs AND DrscussroN when a mineral is irradiated, and do the changesinduced Dose considerations in the laboratory correspond to those that occur in na- ture? These questions can be answered in part from a To consider the role of natural radiation in tourmaline comparison of the spectroscopicdata from natural and coloration, two approacheswere used. The first was to laboratory-irradiated minerals. It is possible that the an- calculate the theoretical dose a should receive from swer will be different for different minerals. It is also nec- the host minerals of a typical pegmatite. The secondin- essary to estimate the radiation doses experienced by volved actually measuring the radiation level in tour- minerals in nature in order to determine whether the ra- maline pockets. (1954) diation doses applied to laboratory samples are similar The theoreticaldose was calculatedusing Jahns's granitic peg- to those that a mineral experiencesin its natural setting. description ofa typical southern California In this paper we considerthe coloration ofpink elbaite. matite. The crystal was modeled as a sphere 1.8 cm in Bershov et al. (1969), Nassau(1975), and others have radius in the center of a 1.0-m radius sphere of compo- sition 60 molo/opotassium , 30o/oquartz, and l0o/o * Present address: Department of and Geophysics, albite. Becausethe pink tourmaline are often in Yale University, New Haven, Connecticut06511, U.S.A. part surroundedby , the calculation was concerned 0003404x/88/0708{822$02.00 822 REINITZ AND ROSSMAN: RADIATION AND TOURMALINE COLORATION 823

0.30 TJAVENUMBER,cm- I zsggg lsooo

i 0.6 r Tourmaline ;, td rrrrn2*bands eO20 o I z o o f; o.4 C d. j o o o o < o.? 2src

350 4SO I'IAVELENGTH,nn 0gg oo 2.O 3.0 Fig. l. Opticalabsorption spectnrm ofnearly colorless, high- Mn elbaite(807) from the SanDiego mine, which showsMn,+ (moles/l features.Polarizations: solid line: Ellcl dashed line: EIc. Sam- [l.|nz*] i ter) ple7.06 mm thick. Fig.2. Beer'slaw plot for theMn'z+ bands at 414nm, EJ-c ();412 nm, Ellc (cross); and 414 nm, Ellc(rectangle). A linearfit to the first two is indicated. only with aoK gamma radiation that is capable of pene- trating to the interior of the pocket. From the mass at- tenuation coefrcients for gamma rays (Davisson and Ev- tures, as did the yellow-green, high-M1z+ Z.ambian el- ans, 1952)the half-path length of the 1.5-MeV 4oKga.mma baites discussedby Rossmanand Mattson (1986) and ray through the idealized pegmatite is 5.25 cm. Under Shigleyet al. (1986). the simplifying assumption that the mass of 40K A Beer's law plot (absorbanceat 414 nm vs. Mn con- is constant throughout the , the calculation indi- centration) of a number of samplesrendered colorless by cates that the mineral receives a dose of 30 Mrad over heat treatment yields the molar absorptivity (e value) of 100 m.y., the approximate ageof pegmatitesin the Mesa Mn2+ in elbaiteof 0.10 t 0.01 L/(mol'cm) for EIc and Grandedistrict (Foord, 1976). e: 0.044 + 0.004Ellc (Fig.2). To compare these estimates with the radiation level actually encounteredin a pegmatite, a calibrated Geiger Responseto irradiation counter probe was placedin four tourmaline gem-pockets The intensities of the Mn2+ peaks decreasewhen the in the Himalaya mine, Mesa Grande, California, as they sample is irradiated. The heights of the peaks fall off were being mined, and in an emptied pocket in the Maple roughly exponentially with increasing dose. Simulta- Lode mine, Aguanga Mountain, California. In all cases neously,new bands appearand grow with increasingdose these measurementsindicated that dosesof the order of (Fig. 3). They are at 480 and 700 nm for Ellc and at 390, 35 to 45 Mrad would be accumulatedover 100 m.v. 515, and 700 nm for EIc (Fig. a). In some samplesthe 390-nm band is obscured by an intense ab- Oxidation states of Mn in elbaite sorption. The 700-nm band grows on top of any pre- The oxidation statesof Mn in tourmaline must be de- existingFe2+ bands in the 700-nm region (Smith, 1978) termined before the details of the radiation-induced that may be present in the heat-bleachedcrystals. These changescan be discussed.Several authors have associated samebands are observedin naturally pink elbaites.These the color of natural pink elbaites with Mn3+ (Manning, results demonstrate that Mn2+ initially present in the 1973), but no one has identified Mn2+ features in their crystal is lost during the course of irradiation. The post- optical absorption spectra.The spectroscopicfeatures of Mn'?* should be easily recognizedby their narrow, weak TABLE1. Localitiesand minor-element concentrations (in wt%) absorption bands between 410 and 420 nm (Keesterand White, 1966).Because previous spectroscopicstudies have Sample Locality MnO FeO CaO TiO, been concernedwith pink to red tourmaliues, we exam- 565-4 Himalayamine, M-esa Grande, 1.19 0.02 0.60 0.03 ined a nearly colorless,Mn-rich core of an elbaite (sample California 565-8 Himalayamine, Mesa Grande, 0.24 0.08 0.34 0.01 807) from the San Diego mine (Foord, 1976).Its spec- California trum shows the weak, spin-forbidden Mn2+ absorption 595-A NortheasternAfghanistan 0.69 0.73 0.54 006 bands at 414 nm in the EIc and at 419, 614-54 Stewartmine. Pala. California 0.39 0.11 0.15 0.01 743 Mozambioue O28 0.07 0.28 0.01 414,412, and 408 nm in the Ellc polarizationplus other 745 0 17 0.07 2.65 0.01 broad features that are due to the combined features of 807' San Diegomine, Mesa Grande, 3.57 0.03, 1.08 0.o2 (Fig. California Mn and the other minor constituents l). The spectra 808-A MinasGerais, 0.24 0.09 0.29 0.01 of other naturally colorlesssamples. or samplesrendered . (1986). colorless by heat treatments show the same Mn2+ fea- Analysisfrom -Rossman and Mattson 824 REINITZ AND ROSSMAN: RADIATION AND TOURMALINE COLORATION

IJAVENUMBER,cm-' ?ASOO TSOOO

Tourmaline l.o lrj Mn3*bands O z o.8 u@ o.6 o a o o.4

4.2

':: aao eoo ,i"r'-ri,'&n. Fig. 3. Opticalabsorption spectrum of elbaite(614-54) from 'C the Himalayamine, 3.64 mm thick, first heatedto 600 to Dose(Mrods) decolorizeit, thenirradiated with 32 Mrad to turn it red. Solid Iine:Ellc; dashedline: EIc. Mn2+bands at 414 nrn are not Fig. 4. Saturationcurves for sample565-4; opticalabsor- observedaft er irradiation. bancevs. dose:squares, 51 5 nm (o);cross, 480 nm (e);diamond, 390nm (ar). irradiation oxidation state of the Mn that is responsible for the color ofpink elbaiteshas previously beenassigned the tourmaline pockets formed over the range of 565 to by Manning (1973)and othersto Mn3+. Our resultssup- 525 'C. Our bleaching experiments on sample 565 show port this assignment.The fate of the electron that is driv- that the color is rapidly lost at 600'C and that partial en from the Mn2+ is not established. bleachingoccurs in 5 d at 500 "C. Sample 614 showed partial bleaching after 6 h at 250 "C. Our conclusion is Concentrationof Mn3+ that the pink color in theseelbaites could not have existed To determine the amount of Mn3+ that forms in pink at the temperatureof crystallization but rather developed tourmaline, we must calibrate the intensity of the Mn3+ postcrystallization under the influence of external radia- absorption bands to derive the molar absorptivity of tion at a lower temperature. Mn3+. The molar absorptivity was estimated from ex- periments where elbaites were subjected to a series of Comparisonof natural and laboratory-irradiated moderate dosesof radiation. The decreasein Mn2+-band elbaites intensity and the growth of Mn3+ bands were measured Elbaites from a variety of localities were examined both under the assumption that all the Mn2+ lost was con- in the natural state and after heat treatment and/or lab- verted to Mn3+. This experiment was done most readily oratory irradiation to determine whether all elbaites dis- with the sample 807 from the San Diego mine because played identical spectroscopic behavior. These results of its high Mn2* concentration.This samplewas stepwise show that bands in the visible region ofthe optical spec- irradiated to 79.8 Mrad. Although the sample had not trum occur in the same positions for all samples.Some reachedsaturation at this point, the 4 I 4-nm band (E I c) variation was observed in the ultraviolet region, but it decreasedin intensity by 24.3o/o.Assuming that 24.3o/oof appearsto be related to the Fe content of the samples. the Mn2+ had been oxidized in the irradiation, the fol- Thus, we conclude that irradiation causesthe oxidation lowing e valueswere computedfor Mn3+: e : 7.5 at 515 of Mn2+ to Mn3+ in all elbaites and that laboratory irra- nm for EIc; e : 1.9at 480 nm for Ellc. diation duplicates the natural process. Added support for the natural irradiation origin ofthe THn nl,or,l,uoN cYCLE color of pink tourmaline comes from irradiation experi- The cycle of oxidizing Mn2+ in elbaite with gamma ments. A portion of the Himalaya mine sample was first rays to yield a deep pink or reddish color and reducing decolorized at 600 .C and irradiated with 30 Mrad, the the Mn3+ with heat treatments to remove the color is calculatedlevel ofnatural exposures.The resulting depth reversible. All of the samples used in this study were of color in the irradiated crystal was essentiallyvisually brought through the full cycle at least once. The easewith identical to a control sample that had been left in its which the color is lost at moderate temperatures in the natural state. laboratory indicates that the color could not have sur- vived under the temperatureof crystallization. The fluid- Problems remaining and phase-stability studies of London (1986) Some elbaitesfrom pegmatite districts are pale pink to indicate that the tourmalines in the southern California nearly colorlessbut contain Mn and readily develop deep pegmatitescrystallized at about 450 "C. The isotopic and pink colors upon exposure to gamma radiation in the fluid-inclusion studiesofTaylor et al. (1979) indicate that laboratory. Sample 807 is a casein point. In our experi- REINITZ AND ROSSMAN: RADIATION AND TOURMALINE COLORATION 825 ence,these examples are relatively few in number. Their RnronpNcts crrno existencedoes not contradict the conclusions developed Bakhtin,A.I , Minko, O.Y., and Vinokurov, V.M. (1975)Isomorphism above.Nassau (1975) has previously noted that after lab- and colour of tourmaline. Izvestiia Akademii Nauk SSSR,Geologrc oratory irradiation, some tourmalines that develop red Series,6, 73-83 (in Russian). colors are susceptibleto fading after severalweeks ofex- Bershov, L.V, Martirosyan, V O , Marfunin, A.S., Platonov, A.N ' and A.N. (1969) Color centersin tourmaline (elbaite). posure there is arange ofstability to Tarashehan, to sunlight and that Soviet Physics-Crystallography,13, 629-630. elevated temperatures.These observations indicate that Davisson,C.M., and Evans,R.D. (1952)Gamma-ray absorption coeffi- not all electron-receptorsites in tourmaline have the same cients.Reviews ofModern Physics,24, 79-107. stability. Consequentially,even though all tourmalines in Foord, E E. (1976) and petrogenesisof layered pegmatite- San Diego County, California, a given pegmatite are exposedto natural levels ofradia- dikes in the Mesa Grande district, p. 123-177.Ph D. thesis,Stanford University, Stanford, California. tion and are experiencing the radiation-induced transi- Jahns,R H. (l 954) Pegmatitesofsouthern California. California Division tion of Mn2+ to Mn3+, some will be unableto maintain of Mines and Geology Bulletin 170, chapter 7. an accumulation of Mn3+ becauseof the intrinsic insta- Keester,K L., and White, W.B. (1966) Crystal-field spectraand chemical bility of the electron receptors. The identity and nature bonding in manganeseminerals. Proceedings,5th International Min- AssociationMeeting, Cambridge,England, 22-35- ofthese receptorsare not yet known. eralogical London, David. (1986) Formation of tourmaline-rich gem pockets in miarolitic .American Mineralogist, 7 l, 396405. Manning, P.G (l 969) An optical absorption study of the origin of colour CoNcr,usroNs and in pink and brown tourmalines. Canadian Mineral- Pink color in elbaite from a variety of pegmatite local- ogist,9, 678-690 - Mn3* ab- ities is the result of natural radiation. When these tour- (1973i)Effect of second-nearest-neighborinteraction on pink black tourmaline.Canadian Mineralogist, I l, 97l- grew, sorption in and malines originally they contained Mn2* and were 977 very pale colored. Over geologictime, exposureto back- Nassau,K (1975) Gamma-ray irradiation induced color changesin the ground levels of gamma radiation from aOKcaused the color of tourmaline American Mineralogist, 60, 7 l0-713. gradual formation of Mn3*, which is responsiblefor the Rossman,G.R (1975) Spectroscopicand magnetic studies offerric ofcolor in ferric iron clustersbridged pink to red colors that the crystalsnow possess.In some hydroxy sulfates:Intensification by a singlehydroxide ion. American Mineralogist, 60, 698-704. pale-colored elbaites this processhas not gone to com- Rossman.G.R , and Mattson,S M. (1986)Yellow, Mn-rich elbaitewith pletion so that it is often possibleto oxidize the remaining Mn-Ti intervalencecharge transfer. American Mineralogist, 71, 599- Mn2+ to Mn3* by laboratory irradiation, duplicating the 602. (1986) Mn-rich natural irradiation processand producing results that are Shigley,J.E , Kane, R E , and Manson,D.V. A notable, gem elbaite tourmaline and its relationship to "tsilaisite." American 'l spectroscopicallyidentical to the natural crystals. Mineralogist, l, 121+1216. Smith. G. (197S)A reassessmentofthe role ofiron in the 5,000-30,000 spectraof tourmaline. Physics AcrNowLnocMENTS cm-r region of the electronicabsorption and Chemistry of Minerals, 3, 343-373. Tourmaline samples for this study were provided by R. H. Currier Taylor,B.E., Foord, E.E., and Friedrichsen,H. (1979)Stable isotope and (Arcadia, Cal.), E E Foord, (Denver, Col.), M E Gray (Culver City, fluid inclusion studies of gem-bearinggranitic pegmatite-aplitedikes, Cal.), and W. F. Larson (Fallbrook, Cal.). S. M Mattson provided tour- San Diego County, Califomia. Contributions to Mineralogy and Pe- maline analysesin advance of publication and valuable suggestionsre- trology,68, 187-205. sulting from her work on some of the samesamples. The cooperation of Wilkins. R.W.T., Farrell, E.F., and Naiman, C.S. (1969) The crystal field W. F. Larson in allowing us to conduct radiation measurementsin a nch spectraand oftourmaline. Journal ofPhysics and Chemistry gem-pocketwhile he was actually mining it is especiallynoteworthy and of Solids. 30, 43-56. appreciated This researchwas supported in part by NSF grants EAR- 8313098and EAR-8212540.Caltech Division ofGeologicaland Plane- MlNr;scntrr REcEIvEDNoveN{sen 6, 1987 tary SciencesContribution 4377. MANUsCRIPTACCEPTED Mmcn 15, 1988