Mineralogical Magazine, February 1998, Vol. 62(1), pp. 113–119

Manganese valency and the colourof the

Mn2AsO4(OH)pol ymorphs eveiteandsarkinite

ULF HAÊ LENIUS Department of Mineralogy, Swedish Museum of Natural History, P.O.Box 50007, SE-10405 Stockholm, Sweden

AND

ERIKA WESTLUND Department of Geology and Geochemistry, Stockholm University, SE-10691 Stockholm, Sweden

ABSTRACT

Polarizedoptical absorption spectra of single crystals of the two dimorphic Mn 2AsO4(OH)-minerals eveiteand sarkinite show that minor concentrations of Mn 3+ attheoctahedral site determine the colour andcause the distinct pleochroism in green and yellow of eveite. In the sarkinite spectra only absorptionbands due to spin-forbidden d­ d transitionsin Mn 2+ atsix- and five-coordinated sites are observed,which produce a weakflesh-red mineral colour and a veryfaint pleochroism.

KEY WORDS: eveite,sarkinite, optical spectroscopy, La Ê ngban,Sweden.

Introduction range 2.19­ 2.23 AÊ .Thetrigonal bipyramidal sitesin the two minerals also show comparable EVEITE andsarkinite, two naturally occurring featuresincluding the presence of oneOH-ligand polymorphsofMn 2 AsO 4 (OH),display and mean M­ L distancesranging from 2.11 to contrastingproperties which reflect their different 2.15 AÊ .Theobserved difference in colour was structuralcharacter. Eveite, which is a memberof commentedon by Moore (1968), who ascribed it theorthorhombic adamite group and thus is toa possiblyhigher crystal field strength at the structurallyrelated to the rock-forming silicate Mn-sitesin eveite as comparedto theMn-sites in andalusite(Moore and Smythe, 1968) has, e.g. sarkinite.In the present study polarized crystal considerablylower density than the closer packed fieldspectra of chemically well-characteriz ed monoclinicsarkinite (Fig. 1), which is structurally singlecrystals of eveite and sarkinite have been relatedto wagnerite (Dal Negro et al., 1974). recordedwith the aim to shed some light on the However,the causes for the contrasting colours effectsresponsi blefor the observed colour observedfor the two minerals are less obvious. In difference. spiteof large similarities with respect to Mn 2+ coordination,sarkinite is faintly pleochroic in Materialsand methods flesh-redhues while eveite displays a distinct pleochroismwith X(a) = Z(b)= greenand Y(c) = Naturalsinglecrysta lsof eveite (NRM # yellow.In eveite there is one Mn -centred 390271)andsarkinite (NRM #741004)from octahedralsite and one independent five-coordi- LaÊ ngban,Sweden were ground and polished to natedMn-site, while in sarkinite there are four produceself-supp ortingdouble-s idedpolished independentoctahedral Mn-sites as well as four thinsections. The crystals used were selected on uniquefive-coordinated Mn-centered sites. The thebasis of theircrystal morpholog y,transpar - individualoctahedral sites in the two minerals are encyand lack of inclusion s.Two eveitecrystal comparable,each displaying two OH-ligands in sections(cut perpendic ularto (010) and (001)) cis-configuration,and mean M­ L distancesin the wereprepared,whilethreesections( cut

# 1998The Mineralogical Society U.HÐLENIUS AND E.WESTLUND b b Eveite Sarkinite

a a

FIG.1. The crystal structure of eveite and sarkinite projected on (001). Strongly shaded, intermediately shaded and open polyhedra represent five-coordinated Mn-centred sites, Mn-octahedra and As-tetrahedra, respectively.

perpendicularto (100), (010 )and(001 ))were programunder the assumption of Gaussian band preparedfromthree sarkini tecrystal s.The shapes. orientationofthecrystalsectionswere Subsequentto the optical absorption measure- confirmedbymeans of crystal morpholo gy mentsthe five absorbers were analysed at the andconoscop icmicroscop y.As determine dby spotsused for these recordings by means of a meansof digital micromete rmeasurementsthe CamecaSX50-microprobe running at an accel- thicknessof theeveite absorber swere265 and erationvoltage of 20kV anda samplecurrent of 266 mmandthe sarkinite absorbers were 530, 12.0nA. At eachspot 7 ­ 8analyseswere 560 and 753 mm.In addition to this, approxi- performed.Standard samples included synthetic mately2 mgof each mineral was separated CaSiO3 (Ca),Cu-metal (Cu), Fe 2O3 (Fe), GaAs undera binocularmicroscop eandground to a (As),MgO (Mg), MnTiO 3 (Mn,Ti), ZnS (Zn)and powdersuitable for FTIR-measur ements. naturalvanadinite (Cl). Neither Ti norCl were Theself -supportingcrystal section swere foundin detectable concentrations. Corrections of measuredin polarized light by optical micro- rawdata were performed by means of the ZAF- scope-spectrometryin the range 333 ­ 2,000 nm relatedPAP-program (Pouchou and Pichoir, (30 000­ 5000 cm ­ 1)usinga ZeissMPM 800- 1984). instrument.The general instrumental set up (light FourierTransform Infrared (FTIR) spectraof sources,lenses, monochromators, detectors and theseparated mineral powders were obtained with polarizers)in the different wavelength regions has aBiorad-instrumentusing Nujol emulsions placed beendescribed by HaÊ leniusand Lindqvist (1996). ona transparentCaF 2 disc.The spectra were Inthe present study the spectral slit widths recordedduring 64 cyclesat aspectralresolution adoptedwere 1 nmin the range 333 ­ 800 nm of 2 cm­ 1. and5 nmin the range 800 ­ 2,000nm. The field apertureand the object aperture were in all Results measurements100 and 60 mm,respectively. The measurementswerep erformeda tambient Thechemical analyses of the two eveite and three temperatureand air served as a reference sarkinitecrystals are summarized in Table 1. The medium.Raw spectrawere analysed by means obtainedanalyses show that the crystals used in ofpeak resolution using the Peak Fit 4 computer thepresent study are compositionally very close

114 MN2ASO4(OH)POL YMORPHS

TABLE 1.Electron microprobe analyses of sarkinite and eveite

SarkiniteA SarkiniteB SarkiniteC EveiteA Eveite B N = 8 N = 8 N = 8 N = 7 N = 8

As2O5 44.09 44.41 44.23 43.34 42.44 MgO 0.19 0.15 0.19 1.78 0.50 CaO 0.29 0.25 0.29 0.32 0.68 MnO 51.77 51.52 51.49 49.26 51.32 FeO 0.02 0.02 0.02 0.02 0.00 CuO 0.01 0.00 0.01 0.01 0.00 ZnO 0.15 0.08 0.10 0.00 0.02 H2Ocalc 3.40 3.40 3.40 3.38 3.34 Total 99.93 99.84 99.72 98.10 98.29

Cations on the basis of 9negative charges As 1.015 1.022 1.019 1.006 0.994 Mg 0.013 0.010 0.013 0.118 0.033 Ca 0.014 0.012 0.014 0.015 0.032 Mn 1.931 1.920 1.922 1.852 1.948 Fe 0.001 0.001 0.001 0.001 0.000 Cu 0.000 0.000 0.000 0.000 0.000 Zn 0.005 0.003 0.003 0.000 0.001

tothe nominal mineral chemistry. The analyses absorptionbands in the UV-region are less well- alsoconfirm the chemical homogeneity of the resolveddue to stronger L ­ Mabsorption,which individualcrystals. Substitutions at the Mn-sites pushesthe UV-absorption edge further into the arelimited and mainly restricted to Ca and Mg visiblespectral region. In addition to these proxyingfor Mn. The total extent of the absorptionbands, which are common to the two substitutionsis less than 7 % inthe present samples.The eveite analyses also show a low 80 oxidesum, apart from the presence of detectable concentrationsof Mg,which were not observed in 70 ) 1 previousanalyses of the mineral (Moore, 1968). -

m 60 Theonly microprobe analyses of eveite so far c ( published(Moore,1968) similarly display low t n e

i 50

totals.A seriesof WDS-scans on the present c i f eveitecrystals down to Z =10did not reveal f e 40 o

detectableconcentrations of any elements other c

n

thanthose reported in Table 1. o

i 30 t Z Therecorded polarized optical absorptio n p r o spectraof eveite (E/ /X(a),E/ /Y(c)and E/ /Z(b)) s 20 b andsarkinite (E/ /X,E//Y(b),E/ /Z,E//aandE/ /c) A X areillustrated in Figs 2 and3. Due to the non- 10 orthogonalityof the sarkinite cell five polarized spectrawere recorded in order to obtain a 0 Y completeabsorption character (see, e.g. Dowty, 30000 25000 20000 15000 10000 5000 1978).The sarkinite spectra which are weakly Wavenumber (cm -1) polarizedshow a numberof sharp to relatively FIG.2. Polarized absorption spectra of eveite. Arrows sharpabsorption bands in the UV ­ VISspectral indicate absorption bands assigned to trivalent manga- range.A verysimilar set of absorption bands are nese. The X-and Z-spectra are for the sake of clarity alsoobserved in the eveite spectra, although the offset with respect to the absorption baseline.

115 U.HÐLENIUS AND E.WESTLUND

70 0.8 0.12 Sarkinite 0.7 Eveite 0.10

60 e t e i

0.6 t ) n i i 1 e - k

0.08 v r e m a 0.5 s e c 50 ( c e

c n t a n 0.4 0.06 n a b e i r b c r 40 o

i 0.3 s o f s

0.04 b f b e a A o 0.2 A c 30 0.02 n 0.1 o i

t X p

r 20 0.0 0.00 o s b b=Y 3600 3500 3400 3300 3200 A 10 -1 Z Wavenumber (cm ) c 0 FIG.4. Unpolarized powder FTIRabsorption spectra of 30000 25000 20000 15000 10000 5000 eveite and sarkinite. Wavenumber (cm -1)

FIG.3. Polarized absorption spectra of sarkinite. All spectra except E//care offset with respect to the absorptionspectra of the two minerals to be very absorption baseline. similar,with possibly only minor shifts in peak positionsand band intensities. With the exception oftheabsorptionbandsat22160and 16485 cm­ 1,anda strongerUV-absorption in minerals,the eveite spectra reveal two absorption theeveite spectra, the strong similarities in the bandsat 22 160and 16 485 cm ­ 1. These two spectralcharacter of the two minerals are indeed broaderbands are distinctly anisotropic and they evident.The absorption bands in common to the determinethe colour and pleochroism of eveite. twominerals are all relatively weak ( e < 0.8 Inaddition to the bands in the UV andvisible l/mole cm)and have energies and band widths spectralranges there are two strongly polarized comparableto those recorded in alargenumber of narrowabsorption features in the NIR-range of Mn2+-bearingminerals (Rossman, 1988 a; Burns, theeveitespectraat6965cm ­ 1 and at 1993).These bands are tentatively assigned to 5200 cm­ 1.Inthe sarkinite spectra just one differentspin-forbidden d­ d transitionsin diva- polarizedNIR-absorpti onfeature at approxi- lentmanganese at six- and five-coordinated sites mately6920 cm ­ 1 isrecorded. This feature is, (Fig.5 andTable 2). In this assignment scheme, fromdetailed recordings at aspectralresolution of theRacah B- andC-parameters for six-coordi- 2nm,demonstrated to be composed of at least nated Mn2+ ineveite are calculated at 639 and fourabsorption bands at 6855, 6875, 6915 and 3583 cm ­ 1,respectively,and in sarkinite at 646 6940 cm­ 1.Allthe NIR-bands in the spectra of and 3578 cm ­ 1,respectively.For the five- thetwo minerals are distinctly anisotropic. coordinateddivalentmanganese,B-and TheFTIR-spe ctrumof eveite (Fig. 4 )is C-valuesared eterminedtobe623and characterizedby one sharp absorption band at 3550 cm ­ 1 ineveite and 619 and 3563 cm ­ 1 in 3560 cm­ 1 andtwo broad and less intense bands sarkinite.The crystal field splitting parameter Dq at3340and 3450 cm ­ 1.IntheFTIR-spectrum of fordivalent manganese at the five-coordinated sarkinite(Fig. 4) a setof overlapping sharp sitesin eveite and sarkinite, 556 and 569 cm ­ 1, absorptionbands centred at 3505, 3515, 3525 arefound to be considerably smaller than for the and 3535 cm­ 1 areevident. six-coordinatedmanganese, 907 and 902 cm ­ 1, in thetwo minerals. For this assignment the resulting 2+ Discussion Dq-ratiofor Mn atthe two different types of coordinationsites is ~0.62, which is veryclose to Inview of the nominally identical set-up of thetheoretical ratio of 0.52 (Burns, 1993).This transitionmetal cations (Mn 2+)andthe simila- theoreticalvalue is based on coordination sites ritiesin symmetry and geometry of the Mn- havingequal M ­ L distances.As the M ­ L centredsites in the two minerals one wouldexpect distancesof the five-coordinated sites in eveite

116 MN2ASO4(OH)POL YMORPHS

bondeddivalent manganese, indicate a slightly higherdegree of covalent bonding, which is consistentwith the shorter M­ L distancesfor thefive-coordinated sites. AlternativeMn 2+ -assignmentmodels, includingthe eveite spectral bands at 16 485and 22 160 cm ­ 1 wereexplored in additionto theone presentedin Table 2 andFig. 5. These additional ) 1 - schemesyielded unreasonably high Dq-values for

m 2+ c five-coordinatedMn ineveite, which equalled (

y oreven exceeded those obtained for divalent g r

e manganeseat the six-coordinated sites of the two n 2+ E arsenates.Considering the strongly similar Mn - coordinationin terms of site geometries, ligand typesand next-nearest neighbours in the two minerals,as well as the theoretically predicted DqV/DqVI-relationship,the parameters obtained fromthe alternative assigment models seem unrealisticand so these models are judged to be 550 905 Dq (cm -1) lesslikely than the one presented in Table 2 and Fig. 5. Thetwo additional absorption bands observed at 2+ 5 FIG.5. Energy level diagram of Mn (d ) with B = 664 22160 and 16 485 cm ­ 1 inthe eveite spectra, and -1 and C=3560 cm (Modified from Moore and White, whichcause the colour and pleochroism of the 1972). Vertical lines and open and filled circles indicate mineral,are thus obviously not related to divalent the approximate positions of bands caused by six- and 2+ manganeseat the five- or six-coordinated site. five-coordinated Mn ,respectively, in eveite and Furthermore,the EMP-analyses of the present sarkinite. eveitecrystals show that no other 3 d-elements otherthan Mn are present in concentrationslikely toproduce detectable absorption bands. Even an andsarkinite are slightly shorter than those of the assumedand highly overestimated e-valueof 200l/ 2+ six-coordinatedsites, the Dq V/DqVI-ratiosin the mole cmfor a potentialFe -bandin eveite, in presentcases are expected to be 10 ­ 30 % higher. combinationwith the observed maximum FeO- Thelower B-values found for Mn 2+ at the five- concentrationof 0.02 wt. % wouldresult in bands coordinatedsites as comparedto the octahedrally withextinct ioncoeffici entsof < 2.0cm ­ 1.

TABLE 2.Band positions and assignments

Bandenergy (cm ­ 1) Bandassignment Sarkinite Eveite Cation Transition

2+ 6 4 28 870 28 770 Mn [VI] A1g(S)? E(D) 2+ 6 4 28 330 28 340 Mn [V] A1g(S)? E(D) 2+ 6 4 27 610 27 630 Mn [V] A1g(S)? T2(D) 2+ 6 4 27 070 26 950 Mn [VI] A1g(S)? T2(D) 2+ 6 4 4 24 350 24 300 Mn [VI] A1g(S)? A1 E(G) 2+ 6 4 4 24 000 23 980 Mn [V] A1g(S)? A1 E(G) 2+ 6 4 23 150 23 570 Mn [V] A1g(S)? T2(G) 3+ 5 5 ­ 22 160 Mn [VI] B1g(D)? B2g(D) 2+ 6 4 21 800 ­ Mn [VI] A1g(S)? T2(G) 2+ 6 4 21 000 21 470 Mn [V] A1g(S)? T1(G) 2+ 6 4 18 190 18 130 Mn [VI] A1g(S)? T1(G) 3+ 5 5 ­ 16 485 Mn [VI] B1g(D)? A1g(D)

117 U.HÐLENIUS AND E.WESTLUND

Obviouslythec ausefort hetwost rong Thesharp IR-band recorded at 3560 cm ­ 1 in ­ 1 (approaching10 cm ),relatively broad ( o1/2 thepowder spectrum of eveite is in accordance approximately2000 cm ­ 1)andanisotropic bands withprevious studies on the isostructural Zn- inthe visible region of theeveite spectra must be arsenateadamite assigned to astretchingmode of soughtelsewhere. In the absence of significant thestructural OH-group (Braithwaite, 1983). The concentrationsof additional 3d-elements only one slightlyhigher energy of the OH-stretching band likelyalternative remains, i.e. a fractionof the ineveite as compared to adamite (3540 cm ­ 1, manganeseis present in the trivalent state. The fromBraithwaite, 1983) is ascribed to the lower broadnessof the two bands, their strong polariza- electronegativityof Mn relative to Zn. The tionas well as their energies are also consistent absorptionbands found between 3400 and withan assignmentto spin-allowed d­ d transitions 3600 cm ­ 1 inthe sarkinite IR-spectrum are also intrivalent manganese. The band energies are assignedto OH-stretching modes. The more comparableto those observed for spin-allowed complexIR-absorptioninsarkin ite,which d­ d bands in Mn3+ atdistortedoctahedral sites in displaysa numberof overlapping OH-bands, is anumberof minerals (Burns, 1993), and in explainedby the fact that in sarkinite there are particularto the Mn 3+-bands(22 100a nd fourindependent OH-ligands as comparedto one 16 000 cm­ 1)recordedin the spectra of the inadamite. The absorption bands recorded in the isostructuralmineral kanonaite (Smith et al., NIR-spectraof both minerals between 6800 and 1982).In analogy with the observed ordering of 7000 cm­ 1 representovertones of the funda- theJahn-Teller cation Cu 2+ (3d9)inisostructural mentalOH-stretchingmodes.Thevery Cu-substitutedadamite (Chisholm, 1985) it is pronouncedpleochroism of the OH-stretching proposedthat Mn 3+ (3d4),which is also susceptible overtonein eveite (E||a>>E||b>E||c) is in agree- tolarge Jahn-Teller distortions, has a strong mentwith the major alignment of the O ­ H vector preferencefor the octahedral site in eveite. along the a-directiondetermi nedby X-ray Taking an e-valueof 200 l/ mole cm, which structuralrefinement of the isostructural mineral correspondsto theupper limit of e-valuesobserved adamite(Hill, 1976). forspin-allowed Mn 3+ d­ d bandsat theoctahedral Inaddition to the sharp bands representing sitein theisostructural mineral kanonaite (Smith et fundamentalfrequencies and overtones of the al.,1982),the concentration of Mn 2O3 in the OH-stretchingmodes there are two absorption presenteveite samples is calcula tedto be featuresobserved in the NIR andIR-spectra of ~0.10 wt.%.Basedon the present assignment of eveitethat suggest that structurally bound H 2O- trivalentmanganese exclusively to the octahedral moleculesare present. The distinctly polarized sitein eveite, this implies that only 0.4 % of the absorptionband at 5200 cm ­ 1 observedin the octahedraare Mn 3+-centredand the remaining singlecrystal spectra and the two broad bands at 99.6% aremainly occupied by Mn 2+. A number of 3340and 3450 cm ­ 1 inthe eveite powder coupledcation substitutions for incorporating spectrumdisplay band features (energies and trivalentcations under charge balance conditions bandwidth s)typica lforH 2O-combination ineveite are possible. These alternatives involve modes(bend+stretch) and OH-stretching modes, thepresenc eofreduced As-catio nspecies, respectively,in water (Rossman, 1988 b). This dehydroxylationand cation vacancies. The IR- indicationof additional water molecules in the spectrumof eveitedoes not show any evidence for structuremay be one explanation for the low As3+-clusters,which possess IR-active modes that oxidesums obtained from the EMP-analyses of shouldgive rise to absorption bands at energies eveite. lowerthan bands caused by As 5+-clusters.On the otherhand it is questionable if the low concentra- Conclusions tions of As3+ requiredto enable the minor degree of Mn3+-incorporationwould be detectable in the Thecolour and pleochroism of eveite is mainly IR-spectrum.It is concluded that it isnotpossible determinedby the presence of two distinctly ont hebasis of the p resentda tasets to anisotropicabsorpti onbands at 22 160and unequivocallyassign a mechanismfor the substitu- 16 485 cm ­ 1 ,whichare caused by spin-allowed tionof trivalent Mn in eveite. An on-going laser d­ d transitionsin six-coordinated trivalent Mn. Ramanstudy with the aim to obtain information on Remainingabsorption bands in the UV ­ VIS theAs valence state distribution in eveitemay give spectralregion in eveite are caused by spin- somefurther useful results. forbidden d­ d transitionsin Mn 2+ atthefive- and

118 MN2ASO4(OH)POL YMORPHS

six-coordinatedsites. In the optical absorption Dal Negro, A., Giuseppetti, G.and Pozas, J.M.M. (1974) spectraof sarkiniteall bands in the same spectral The crystal structure of sarkinite, Mn 2AsO4(OH). rangeare assignable to divalent manganese at Tscherm. Mineral. Petrol. Mitt ., 21, 246 ­ 60. comparablefive-andsix -coordinatedcation Dowty, E.(1978) Absorption optics of low-symmetry positions.The main cause for the distinc t crystals -applications to titanian clinopyroxene differenceincolourbetweenthetwo spectroscopy. Phys. Chem. Minerals , 3, 173­ 81. HaÊ lenius, U.and Lindqvist, B.(1996) Chromophoric Mn2AsO4(OH)-polymorphsis the presence of smallamounts of Mn 3+ in eveite. divalent iron in optically anisotropic magnussonite. Thepresence of structurally bound H O in Eur. J.Mineral. , 8, 25­ 34. 2 Hill, R.J. (1976) The crystal structure and infrared eveitehas beenindicated by NIR-spectraof single properties of adamite. Amer. Mineral ., 61, 979­ 86. crystalsas well as by IR-spectra of powdered Moore, P.B.(1968) Eveite, anew mineral from samples.The low analytical sums recorded for the LaÊ ngban. Ark. Mineral. Geol. , 4, 473 ­ 6. mineralby EMP-methodsmay be partially due to Moore, P.B.and Smythe, J.R. (1968) Crystal chemistry thepresence of water molecules. of the basic arsenates: III. The crystal chemistry of eveite, Mn2(OH)(AsO4). Amer. Mineral ., 53, Acknowledgements 1841 ­ 6. Moore, R.K.and White, W.B. (1972) Electronic spectra Wethank the Swedish Natural Science Research of transition metal ions in silicate garnets. Canad. Council(NFR) fora grantto one of us(UH).Our Mineral., 11, 791 ­ 811. thanksare also due to H. Harrysson,Uppsala Pouchou, J.L. and Pichoir, F.(1984) Anew model for Universityfor microp robeanalys esand J. quantitaive X-ray microanalysis. I. Application to Lindgrenfor providing access to the FTIR facility the analysis of homogeneous samples. La Re´cherche atUppsala Universit y.The manuscrip twas Ae´rospatiale, 3, 13 ­ 36. improvedby a constructivereview by Prof. G. Rossman, G.R.(1988a) Optical Spectroscopy. In Rossman,which we greatly appreciate. Spectroscopic Methods in Mineralogy and Geology (F.C. Hawthorne, ed.). Mineral. Soc. Amer., Rev. Mineral., 18, 207 ­ 54. References Rossman, G.R.(1988b) Vibrational spectroscopy of hydrous components. In Spectroscopic Methods in Braithwaite, R.S.W.(1983) Infrared spectroscopic Mineralogy and Geology (F.C. Hawthorne, ed.). analysis of the olivenite-adamite series, and of Mineral. Soc. Amer., Rev. Mineral ., 18, 193­ 206. phosphate substitution in olivenite. Mineral. Mag ., Smith, G., HaÊ lenius, U.and Langer, K.(1982) Low 47, 51­ 57. temperature spectral studies of Mn 3+-bearing anda- Burns, R.G.(1993) Mineralogical Applications of lusite and epidote type minerals in the range Crystal Field Theory .Second edition. Cambridge 30,000 ­ 5,000 cm ­ 1. Phys. Chem. Minerals , 8, University Press, Cambridge. 136­ 42. Chisholm, J.E. (1985) Cation segregation and the O-H stretching vibration of the olivenite–adamite series. [Manuscript received 8January 1997: Phys. Chem. Minerals, 12, 185­ 90. revised 2April 1997 ]

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