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A Spectroscopic Study of Irradiation Colodlg of Amazonite: Structurally Hydrous, Pbbearing Feldspar

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A Spectroscopic Study of Irradiation Colodlg of Amazonite: Structurally Hydrous, Pbbearing Feldspar AmeficanMineralogist, Volume 70, pages 794-804, 1985 A spectroscopicstudy of irradiation colodlg _ofamazonite: structurally hydrous, Pbbearing feldsPar ANI.le M. Horwlstrnr AND GEoRcE R. Rossurc'N Diuision of Geological and Planetary Sciences2 Califurnia Institute of Technology, Pasailena, California 91125 Abetract Irradiation-inducedcolor in amazonitecan developonly in potassiumfeldspar having both structurally bound HrO and Pb impurities. Amazonite color is controlled by either (1) an absorption minimum in the p spectrumbetween three overlapping bands in the ultraviolet and a broad band at 625to 643nm, resulting in a blue color, (2) a combination in p of one UV band and a broad band at 720 nm, resulting in a greencolor, or (3) both of the above superimposed,resulting in a blue-greencolor. All optical variations correlat€dwith an EPR p"it".n indicative of Pb3+ or Pbr *. The different types of color are associatedwith a limited range in Pb content and structural stgte. For constant Pb content, the intensity of color is linearly relatedto the amount of struct-urallybound H2O, up to a limiting value.Dependence of color intensity on both Pb and HrO concentrationstrongly suggeststhat lead and water occur in a 1:1 ratio in the color centers.The first order reaction kinetics ofamazonite color formation by irradiation and the observation that water is not consumedin the process suggeststhai water plays a catalytic role in the irradiative transformation of Pb2+ to the amazonitechromophore. Introduction with optical absorption spectra of amazonitesand other potassiumfeldspars in an attempt to establishthe origin of The color of the blue-greenvarieties of microcline and color and its relationshipto water in the feldspars. orthoclase (amazonite) is radiation-induced (Przibram, 1956,p. 253).Chemical data indicate lead to be the color- ExPerimental ing agent (Foord and Martin, 1979),yet Pb2* cannot be Optical absorptionspectra wcre obtained from opticallyorient' becauseall electronictransitions the sole causeof the color ed polishedslabs of feldsparusing calcite polarizers in a Caty l7l (pers. of Pb2+ occur in the ultraviolet region. E. E. Foord spectrometer.Infrared spectra were obtained with gold wire grid comm.) has shown that amazoniteswith less than 10fi) polarizersin a Perkin Elmer 180 sp€ctrometer.Both visibleand ppm Pb are blue, whereasthose with a higher lead content IR spectrawere digitized and scaled,but only the visible data are green,and that the apparentintensity usually increases requireda baselinesubtraction. Data from computerpeak-fitting with lead enrichment. However, some feldspars with as did not give accurateareas for the peaksbecause the baseline much as 1fiX) ppm Pb are not colored (Foord and Martin, (whichis a combinationof a UV tail and scattering)could not be drawn in by 1979).Using electron paramagneticresonance (EPR) tech- adequatelymodelled. Therefore, the baselinewas + was estimated by multiplying niques,Marfunin and Bershov(1970) concluded that Pbr hand, and the integrated intensity the peak height by the full width at half height (Wr,r). Becauscthis centers are present in amazonites,but not in other feld- approach precludesresolution of two overlapping componentg Tarashchanet al. (1973)found that the intensity of spars. averageintensity and total areasare reported for doublets' an ultraviolet absorption band in amazonite,attributable 78 K on a Varian EJine spec- + EPR spectra were taken at to a Pb2 transition, increasedupon heating but decreased trometer at about 9.2 GIiIz' All samples werc coarsely-ground upon subsequentirradiation, and suggestedthat radiation powders weighing about 100 mg. Doubly integrated intensities converts Pb2* to Pbr*. However, Spelt and Lehmann were calculated from the empirical formula + (1982) did not find the Pbl EPR signal in Australian DII amazonite.Equally mysteriousis the correlation between loss of color and weight loss during dehydration of ama- (signalheight)' (signal width)' r/2 zonite (Plyusnin, 1969) becauseneither H2O nor OH- (g"tr). (r"-pte weigbt)'(modulation amplitude)' (power) alonecan producecolor. (1) The present paper presentsand correlatesEPR spectra (Eaton and Eaton, 1979).Also, a line integxalwas applied twice to r Present Address: GeophysicalLaboratory, CarnegieInstitu- doubly integrate digitized and baseline-subtractedfirst derivative tion of Washington,Washington, D.C. 20008. spectra. Becausethe numerically calculated value dependslinearly 2 Contribution Number 4050. on thc formula-calculated value, and the numerical method is less 0003-{xxx/8 5/0708-0794$02.00 794 HOFMEISTER AND ROSSMAN: COLORING OF AMAZONITE 795 precisedue to baselineapproximation, the data -l frorn equation (l) WAVENUMBER,cm are given. 3@OO 2SOOO t5000 Samples were irradiated at room ternperature in air with a 13?Cs source which produces0.66 MeV g:mrma rays in dosesof AMAZONTTE l.4l MRads/day.Dehydrations were done in air at 200to lm0.C. Loko Grcgc Water contents of selectedsamples were measured with a hy- UJ O f . o.3nn drogen extraction apparatus(e.g. Friedman, 1953).The measuie- 2 08 ments are precise(approximately 1%), but the amount f of water P.u evolved may involve a blank of I pmole HrO (R. E. Criss, pers. o comm., 1982) which increasesthe uncertainty a to about *5%. 6 04 Three crystals were measured several times to determine the re- producibility. Samples5 and 25 gave consistentrcsults, but thc water content varied by a factor of two for sample#12 which was inhomogeneouslyturbid from fluid inclusions.Water contents of all other feldspars were determined by comparison of their infra- AFIAZONITE red spectra to that of a feldspar with measured H2O content and Broko Hl I I td \ #3" r.o.5nn similar spectral features. (J Most of the feldsparswere provided by E. E. Foord and are L u" \ dcscribedby Foord and Martin (1979)and Foord et al. (in prep). dt \ The feldspars embrace the known range of color and lead content I eu o (Table 1),and include the exceprions(# 14, lS, tB, 22, 9, 2O,261 to Q o. trends of hand-spec'imencolor with chemistry(E. E. Foord, pers. commun.). Visible spoctrmcopy of amazonite AMAZONITE General properties Greenish-blueamazonites are the most common,but the apparentcolor encompassesvalues between this and green with a slight amount of yellow (Table 1). Occasionally superimposedon the green is a grey cast due to various oxygen-cation hole centers (Hofmeister and Rossman, 1985). AHAZONTTE Absorption spectraof all amazonitesexamined have the lo d Koivy,USSR following featuresin common: (l) The lrJ t2l rrO.5nn color is intrinsic. (2) (J 08 All bands are strongly polarizedin the beta direction (per- z pendicular o to (001)),and weakly polarizedin alpha,perpen- d 06 o dicular to (010).Very weak componentsin the gamma di- o (n g4 rection were ignored becausethese are due to scattering, being highest for the most turbid microclinesand zero for g2 the nearly gem quality orthoclases.(3) The spectra are dominated by a single band or two overlapping broad bands with total width W,,, of 3000 to 4500 cm-l which 209 3gg 400 sog 69,0 700 aoo 90,0 togg ttoo }JAVELENGTH, are slighly asymmetric and centered at 625 to 720 crrn.(4) nm The combination of near-IR bands with UV absorption Fig. 1. Polarized absorption spectra of natural amazonites at 23"C. (a) producesa transmissionminimum in the blue-greenregion, Blue, type B amazonite #5 from Lake George, Col- orado. The giving amazoniteits rangeof colors. finely spaced dots indicate where the alpha spectrum was estimated from a spectrum of the same chip after irradiation. There are four distinct color types (Table 2): (l) type B, The slow rise toward the UV is due to scattering from turbid common blue, low Pb microcline-perthites possessingthree regions, and from the perthitc lamellae. (b) Green, type G amazon- broad polarized absorptionsat625,385, and 330 nm along ite #3' from Broken Hill, Australia This sampleis close to gem with a UV tail (Fig. 1a); (2) type G, end-membergreen, quality so that very little scattering contributes to the spectra. The high Pb orthoclase with Arrr,r5, - 0 (Cech et al., l97l) weak band near 360 nm was induced by baselineoorrections. The consistingof one band at 72Onm plus a UV tail (Fig. lb); UV tail is markedly polarized in the same schemeas the amazon- (3) type D: superpositionof thesetwo end-memberspectral ite peak at 720 nm. (c) Medium grecn amazonite #32 trom peatl, types gives a doublet (Fig. lc) in intermediatepb-content Routt County, Colorado. The spectraare the superpositionof the B and microclines; the variation in relative intensity of the 220 G types such that the relative intensity of the 625 and 72O nm bands gives one apparent very broad band. Spectra and 630 nm peaksproduces a gradual shift in the transmis- taken at 77 K resolvethe two bands.(d) Blue-greentype T amazonite sion minimum, resultingin the continuousvariation #21 of hue from pegmatites near Keivy, USSR. The most intense absorption from blue-greento green; (4) type T, a turquoise color is for the type T color is shifted to 643 nm and is broader than thc produc€d peak by a at 643 nm (Fig. ld) which is slightly singlet of the blue amazonites. Broad bands are also present at broader than the 630 nm peak but is not resolvableinto a 385nm (shownhere) and revealedat 330nm by irradiation. 796 HOFMEISTER AND ROSSMAN: COLORING OF AMAZONITE Table l. Sampledescription and chemistryof amazoniteand other feldsparsin order of increasingPb content ity Color Description Eleclron llicroprobeAnalyses EmissionsPect.t Hz-lilanometry! Sample
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