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(1974) the Optical Absorption Spectra of Tourmaline

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(1974) the Optical Absorption Spectra of Tourmaline Canitdian Mineralogist Yol. 12,pp. 370-380(1974) THE OPTICAL ABSORPTION SPECTRA OF TOURMALINE: IMPORTAIUCE OF CHARGE-TRANSFER PROCESSES G. H. FAYE*, P. G. MANNTNG*'!, J. R. GossELIN* eNo R. J. TREMBLAY'I'*,!. AssrRAcr interpretation. It is generally accepted that the two prominent absorptions at -9,00O Optical absorption spectra resolved into Gaus- and sian bands, Mdssbauer -14,0@ cm{ in tourmaline spectra are compo- spectra, and electron-mi- 5Tz ,E(D) croprobe analyses are collated for several tourma- nents of the -> transition in Fe2+ ions lines in the elbaite-schorl and dravite-schorl series. located principally in the brucite-type D-sites This work clarifies the scattered and previously (Faye et al. 1968; Townsend 1970; Wilkins er pttzziiag information on spectral featureJ that af- al. 7969; Manning I969a). However, it has been fect the colours pleochroism and of tourmaline. argued that the two bands arise Emphasis is placed jm- from Fe2+ ions on the identification and located portance of the O2-+Fex+ ->Fea+, in the b- and Al c-sites, respectively 1Fe:+;, Fet+ (Burns and Fe2+ + Ti4+ charge-transfer processes. These L972; Burns & Simon 1973), but this processes are mainly responsible for the dichroism seemsimprobable becausethe relative intensities of tourmaline. The metal-metal interactions are of the two bands are remarkably constant for facilitated when the electric vector of incident lisht differently-coloured varieties. vibrates in the 0001 plane. Ultraviolet-centeied The origin of the numerous dichroic absorp- Ot + Fe2+ and 02'+ FeE+ processesare intensified tion bands in the visible region spectra (-700 by tho replacement of OH- by Or, for example nm to p400 nm) of tourmalines is poorly un- wheq fir+ substitutes into octahedral sites of tour- maline. The former process is important derstood. Strongly-polarized broad bands at in the col- -22,0OO ouring of Ti-rich dravites whereas the latter stronelv cm-r (45O nm) in spectra of green and i4fluences. tle spectrum of schorls. The resolvid, brown varieties have been assignedto Tie+ -> visible-region spectra of blue and black tourmalines Tia+ intervalence charge-transfer in the O001 are qualitatively remarkably similar and both have plane (Manning 1969b; Faye et al. 1968). prominent 1 absorption at -18,500 cm-1 which, Townsend (1970) feels the dichroism of tour- tlrough a heating experiment and Mijssbauer meas- malines arises partly frorn charge-transfer tran- urements, has been identified as the Fes+ -> Fes+ sitions between metal ions sharing octahedral process. The brown colour of Ti-rich dravites rs edges, and cited the suggestionthat a dichroic also _influenced by strong Fe2+ + Tia+ charge- ttanrfer absorption at 22,000 - 24,OOOcm-t, and a shoulder at - l7,5OA cm-1 in spectra of some weaker version of the band contributes to the green blue-green varieties may well represent Fe,+ -> colour of elbaites. Fe3+ interaction. On the other hand, Wilkins polarization _The properties of the Fe2+ d-d batds, et al. (7969) attribute the colours of tourma- which have maximum intensity rn Elc spectra oi lines to crystal-field transitions within Fe,+, colourld^ varieties,. arise from intenifti borrowing Fe3+ and Mn ions, but they ignored absorptions from O'2' -> Fes+ charge-transfer. Near-infrared due to Ti and intervalence spectra of specimens in the elbaite-schorl series charge-transfer.Some show, on curve-resolution, previously undiscovered of their assignmentsare suspect; for example, bands that mark the presen-e of Fe* in the c-sire. the prominent polarized band at 22,0O0 crtl These "new" features are strong in the spectra of in spectra of brown tourmalines is assignedto black _(schorl) specimens but weak for specimens the field-independent transition $At--> aAfE(G) near the elbaite end of the elbaite-schorl ioin. in Fe8+, and yet the band half-width of 5,500 cm-l is unacceptably large (Manning 1969b). INrnoDucrrolq Crystal-field transitions within Cr3+ and Mns+ are responsible for the colours of some deep- Although the chemistry and structure of tour- green and pink tourmalines (Manning 1969a,b); maline are well-known, the optical absorption these varieties are not considered here because spectra continue to defy a satisfactory unified their spectra are understood reasonably well. The complex structure of tourmaline inevi- tably introduces a considerable speculative ele- *Research Scientists, and ti*'s Technician. Mineral ment into spectral interpretation. Division, Mines Branch, Department Crystal-field $iences of bands Energy, Mines and Resources, 555 Booth Street. of Fe, Mn and Ti ions often present in Ottawa, Canada; and **Research Scientist. Inland two valence states and distributed over two dif- Waters Branch, Department of the Environment, ferent octahedral sites and possibly a tetrahe- 562 Bootb $treet, Ot[awa, Canada, KIA 0G1. dral site, are superimposed on strong interval- 370 THE OPTICAL ABSORPTION SPECTRA OF TOURMALINE 37r 'I. TABLE ELECTRONI'{ICROPMBE AMLYSES AND CELL PARAIiIFTERS OFMURXALINES' peclnen Locality Col our 'f6.010 chorl Villseuve, Quebec black 12.0 0,2 0.6 (7)* 7.172(4') lbalte IBI -l MinasGerais, Brazil bl ue 6.4 <0.01 0 .3 ]s.e08(6) 7.127(51 '15.886 tbalte TGr-2 unknoun green 5.0 0.01 0.8 {5) 7.1m (4) mn dravite iD. 1 Gouverneur,Ng York brtrn t .t 0,7 0,05 r5.e66(5) 7.204(3) rNn toumllne rc. unknown yel low-brcwn 4.9 I .0 lbalte TZ - zoneA unKmHn brcwnlsh-green 2.7 0.09 2,2 15.862(5) 1.121(3) lbalte TZ - zoneB unkmwn blue 2,8 0.02 2,0 rs.857(7) 7.rE (71 lbaite TZ - zoneD unknoIn brown 3.3 0.10 1.9 r5.890(7) 7.134(5) 'llght ibalte TZ - zone F UKrcWN green 2,0 0.04 1.7 15.832(7) 7.\a G) ves TABLE2. dssBAuERDATA FoR CERTAIN TouRlmlrilEs lsmer shlft, m/s luadrupole splitting, 2e line width at relatlve inten- oxidatlon ite dlstr. of FeZ (rel. to Fe o@der) ;r1€-hainh' lm/c slti6- % bra chor'l 1.1 2.2 40 2+ 8:7 0.7 0.6 0.3 0.8 'I -24t1 *15:I 'tBl-1lbalte '| .0 0.4 90 .0 , ,_n o:n 'I tbal te .1 2,4 53 2+ 2.1 9tz mt-], 1.0 1.7 l2 6ted 0.3 0.9 0.5 23 0.4 1.l 0.5 'l lbal te .1 0.4 80 2+ fcr-2 2,2 m <2 enco charge-transferbands arising from different The results of partial analysisby electron micro- combinations of adjacent donor-acceptor metal probe (D. R. Owens), and the cell dimensions ions (Fezn and Fe"+oFez+ and Tin+, Ti3+ and of powdered samples,as measuredby r-ray dif- Tinr) located in b-b sites (absorption polarized fraction using a 114.6 mm Debye-Scherrer ca- Elc), b-c sites (polarized E1c) and c-c sites mera (J. M. Stewart) are given in Table 1. (maximum intensity ,Ellc), as well as on oxygen The varietal names were selectedaccording to -> metal charge-transfer bands originating in the cell parameters and the nearest end-member the uv region. Insufficient attention has been in the elbaite-schorl or dravite-schorl series. paid to accurate curve-resolution of spectra in the optical region. Further, chemical analyses by Mdssbauer spectra electronmicroprobe do not yield metal valence statesdirectly. The M6ssbauer spectra are shown in Figures In the current work, the optical spectra of a L and 4 they were obtained at room temperature suite of brown, green, blue, and black tourma- with a spectrometer having a velocity drive of lines and of a crystal zoned in most of the above the constant acceleration type, synchronized by uTCo colours, have been collated, resolved into com- a 512-channel analyzer, The source was ponent absorptions, and related to Fe and Ti diffused in copper. The velocity scale was calib- content Mijssbauer obsorption and heating ex- rated, using uon powder as a standard, immg- periments are also described.A basisis presented diately ofter the measurement of the spectrum for a unified theory of interpretation of spectra of each sample. The linewidth from the calibra- of feunnalinss of the dravite-schorl series; tion spectrum varied from 0.3 to O.4 mm/sec, charge-transfer processes are shown to be pre- averaging0.37 mm/sec. dominant. The spectra were computer-fitted (D. W. Carson) to a selected number of l-orentzian lino profiles by a least-squaresmethod and cor- ExpnntvmNrer, Dsre[s rection was made for the parabolic background (intensity variations at the detector due to source Specimens motion). The typical linewidth resulting from these fits was appreciably larger than the calibra- The tourmaline specimens were obtained tion linewidth. This implies a spread in the through the courtesy of Mr. H. R. Steacy, cura- Mijssbauer parameters of the underlying spec- tor of the reference seriesof the National Min- tra and this bears directly on the interpretations eral Collection, Geological Survey of Canada. given here. Because the measurements were 374 TIIB CANADIAN MINEMLOGIST Simon 1973; Hermon et al. 1973) demonstrate that Fe'* is present in both l- and c-sites of tourmalines in the dravite.-schorl and elbaite- schorl series. The b:c occupancy ratio of Fe2' varies from - I : 1 for the black Villeneuve spe- cimen to -15:1 for some elbaites (Table 2, Hermon et al. L973). Despite M6ssbauer evidence for Fe'+ occu- pancy of the two sites, absorption spectra seem to show only the two commonly-observed near- U() infared bands arising from components of the z oTq-)'E o transition of Fez+ in one distorted oc- G o tahedral site. Becausethe average dimensions of 6 6 tho c-sites (Al-O = 1.93A) in dravites, for ex- ample, are substantially smaller than those of the D-sites (Mg-O - 2.054) (tsuerger er a/.
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