Mineralogy and Petrology (1991) 45:1-9

Mineralogy ana

Petrology © by Springer-Verlag 1991 Printed in Austria

Variations in the OH Concentration of Rutiles from Different Geological Environments V. M. F. H a m m e r and A. Beran Institut fiir Mineralogie und Kristallographie, Universit/it Wien, Austria With 2Figures Received March 18, 1991; accepted July 9, 1991

Summary A suite of 19 rutiles form a wide range of geological environments shows a distinct variability in the OH concentrations. The analytical H2 O+ content determined by IR spectroscopy varies from 0.04 to 0.21 wt.% with the greatest amounts occurring in amphibolite facies rutiles. The main feature of the IR spectra in the region of the OH stretching fundamental is a single narrow band centered at 3280 cm-i; some of the spectra show an additional band, centered either at 3360 or at 3320 cm -~. Varying OH concentrations in rutiles are probably a function of the activity of hydrous components during crystallization and are weakly correlated to a minor content of tri- and pentavalent elements.

Zusammenfassung Die Variation in der OH Konzentration yon Rutilen unterschiedlicher geologischer Vorkommen

Eine Seric yon 19 Rutilen aus untcrschiedlichcn gcologischen Vorkommen zcigt eine deutliche Variabilitfit in den OH Konzentrationen. Der IR spcktroskopisch bestimmte analytische H20÷-Gehalt variiert von 0.04 bis 0.21 Gcw.%, wobei die hfichsten Gehalte in Rutilen der Amphiboliffacies auftreten. Das wesentliche Merkmal der IR Spektren im Bereich der OH Streckschwingung ist eine einzelne schmale Bande bei 3280 cm-1; einige der Spektren zeigen entweder bei 3360 oder bei 3320 cm -1 einc zusfitzlichc Bande. Wahrschcinlich stcllen die variierenden OH Konzentrationcn, die mit einem gcringcn Gehalt an drei- und fiinfwertigen Elementen schwach korrcliert sind, eine Funktion der "Wasser"-Aktivitfiten bei dcr Kristallisation dar.

Introduction The recognition of a structural O H content in rutile was initially based on the observation of a doublet of sharp bands centered at 3277 and 3322 cm -1 in the IR

2

V.M.F. Hammer and A. Beran

absorption spectrum of synthetic crystals (Softer, 1961; von Hippel et al., 1962). Beran and Zemann (1971) have established the presence of OH in rutile from natural occurrences and proposed a model for the structural OH incorporation. In the pleochroic scheme of the absorption band, maximum absorption occurs when the electric vector of the polarized IR radiation vibrates perpendicular to the c-axis, thus indicating OH dipoles on the O-site oriented perpendicular to the plane of the three coordinating Ti-atoms. The aim of this paper is to establish the relationship between OH concentration and geological provenance and to determine whether the OH content reflects the environment of formation.

Sample Description and Experimental IR spectroscopy provides an extremely sensitive method for detecting trace amounts of OH and only a few ppm analytical H 2 0 ÷ in an "anhydrous" mineral can produce significant absorption. Rutiles from a wide range of geological environment have been studied, including samples from pegmatitic and from low- to high-grade metamorphic rocks (Table 1). The metamorphic rutiles formed under conditions of the blueschist facies (sample no. 19), eclogite facies (no. 18) and amphibolite facies (no. 16, 17). Rutiles from alpine fissures in amphibolite and greenschist facies rocks formed under hydrothermal conditions which range from 1 to 2 kbar and from 200 to 500 °C (H6ck et al., 1981). The alpine fissure samples are from localities in the Penninic zone of the Eastern Alps; samples no. 10, 11, 12, 13 belong to the "Obere Schieferhiille", samples no. 9, 14 and 15 to the "Untere Schieferhtille". Metamorphic conditions for the blueschist facies sample from Dora Maira in the Western Alps (no. 19) are estimated to be > 28 kbar and approximately 700 °C (Chopin, 1984). The eclogite facies sample (no. 18) from Trescolmen in the Central Alps underwent p, T conditions of > 15 kbar and > 550 °C (Heinrich, 1982). The rutile samples in amphibolite facies are accessory minerals in amphibolites (no. 16) of the Middle Austroalpine "Otztal" crystalline basement (Purtscheller, 1978) and in graphitic gneisses (no. 17) of the "Bunte Serie", a metasedimentary unit in the Moldanubicum of the Bohemian Massif (Scharbert and Fuchs, 1981). The pegmatitic futile samples no. 1 and 2 are interpreted to have originated in pegmatitic veins found within basic intrusives and overlying gneisses (Bugge, 1978). Mica schists and gneisses of the Middle Austroalpine "Koralpe" crystalline basement are cut by both pegmatitic and quartz veins containing the rutile sample 3 (Meixner, 1965). The rutile bearing veins of all three localities contain abundant apatite. Sample preparation consisted of orienting isolated crystals by morphology and optical methods, followed by preparing plane-parallel self-supporting slabs polished parallel to the c-axis ranging from 0.015 to 0.030 cm in thickness. The selected rutile samples proved in small parts to be absolutely free from inclusions and impurities. Adjusting a micro-sampling accessory the diameter of the circular measuring area varies from 0.05 to 0.10 cm. IR spectroscopic measurements were performed with a computer-controlled IR spectrophotometer Perkin-Elmer 580 B, Interdata 6/16. Non-polarized spectra were recorded with 2 cm -1 resolution. The samples were analysed with an automated five-crystal spectrometer ARL-SEMQ electron microprobe at 15 kV and 15 .,

Table 1. Localities, Geological Environment, Colour and Analytical H2O+ Contents for Rutiles. HzO + values determined by IR spectorscopy are calculated on basis of Beer's law using the integral molar extinction coefficient = 6540 1-mo1-1" cm -z. Colour in round brackets refers to colour of polished slabs Sample no. 1 2

3

4

5 6 7

8 9 10

11 12 13

14

15 16 17

18 19

Colour

H2O+ content in wt.~o

Locality

Geological environment

black (deep red) black (deep orange-red) deep red (orange-red)

0.09

Snarum Norway Modum Norway

black (deep orange-red) red (orange-red) yellowish brown (yellow-brown) red (orange-red)

0.09

pegmatitic veins in basic intrusives pegmatitic veins (apatite-rich) in basic intrusives pegmatitic and quartz veins in gneiss isolated crystal

deep red (orange-red) deep red (orange-red) deep reddish brown (yellowish red) deep red (orange-red) deep red (orange-red) red (orange-red)

0.16

black (deep orange-brown) deep red (red) black (orange-brown) black (deep reddish brown deep red (red) red (orange-red)

0.11

0.13

0.10

0.09 0.16 0.09

0.10 0.18

0.18 0.14 0.15

0.19 0.21 0.16

0.08 0.04

Modriach Steiermark Austria Mlad/t Vo~ice Bohemia CSR Capao do Lana Brazil Campo Brazil Millhollands Mill Alexander Co. North Carolina ? South Africa Hollersbachtal Salzburg Austria Pr/igraten Osttirol Austria Pr/igraten Osttirol Austria Pr/igraten Osttirol Austria Pfitscher Tal Sfidtirol Italy Maurer Tal Osttirol Austria Habachtal Salzburg Austria Lisens Tirol Austria Amstall Nieder6sterreich Austria Alpe Trescolmen Tessin Switzerland Dora Maira Piemont Italy

isolated crystal isolated crystal isolated crystal

isolated crystal alpine fissure in phyllite alpine fissure in amphibolite alpine fissure in micaschist alpine fissure in amphibolite alpine fissure in chloritemicaschist alpine fissure in chloritemicaschist alpine fissure in amphibolite amphibolite graphitic gneiss (amphibolite facies) eclogite inclusion in pyrope (blueschist facies)

4

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o

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~ d c~ v

d

o

dc~o

n::l

c5 V •

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IN

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V

V

oo

V

o,~V ~

0

oov

. ~

e~

m

V

V

,5~o

V

o

,5o

O

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H a m m e r a n d A. B e r a n

Variations in the OH Concentration of Rutiles

5

nA using data reduction methods described by Bence and Albee (1968). The A1 and V contents of samples no. 5, 7, 8 and 11 were corroborated by neutron activation analyses (Table 2). Analytical H2 O+ contents were calculated by calibrating the IR band intensity against the water content of a "standard rutile" measured with a modified Du Pont moisture evolution analyzer (MEA) 903 H (cf. Wilkins and Sabine, 1973). Approximately 200 mg of gem-quality crystal fragments available from the rutile sample of Capao do Lana (no. 5) were used for MEA analysis. The rutile fragments were checked by IR spectroscopy. The H2 O+ content of ultra-pure Mg(OH)2 (Merck, art. no. 5870) was used for standardization of the MEA. The dried sample was heated at c. 500 °C per minute to 950 °C and held at this temperature for 30 minutes. The water evolved was taken as the OH concentration. The IR spectra obtained after dehydration indicateda decrease of the integrated band intensity to less than one tenth of its original value. According to the MEA analysis and supplementary IR spectroscopy the corresponding total analytical H 2 0 + content due to structurally incorporated OH amounts to 0.09 wt.% with a precision of + 0.02 wt.%. A powered sample of 200 mg in weight was also used for elemental analysis performed at 930 °C with an elemental analyzer Perkin-Elmer 240. The resulting value of approximately 0.01 wt.% H is in full agreement with the results of MEA determinations. On the basis of Beer's law (Absorbance = extinction coefficient, sample thickness. OH concentration) the calculated value for the integral molar extinction coefficient amounts to 6540 1. mol -~- cm -2 which was used for the determination of the structural OH concentrations in rutiles of the present study (Hammer, 1988). The validity of Beer's law has been confirmed by the linear relation obtained by plotting the absorbance values versus the thickness of crystal plates ranging from 0.005 to 0.10 cm (cf. Langer and Fl6rke, 1974; Beran and G6tzinger, 1987; Miller et al., 1987; Beran et al., 1989; Skogby et al., 1990).

Results All naturally occurring rutiles are characterized by a narrow absorption band at 3280 cm -1 (Fig. 1). Some of the samples show an additional band centered either at 3360 or at 3320 cm -1. The type of spectra consisting of a strong band at 3280 cm -1 and a weak band at 3360 cm -a is restricted to rutiles from pegmatitic veins in basic intrusives (no. 1, 2). Rutiles from high-grade metamorphic paragenesis show a split band with maxima at 3280 and 3320 cm -1 (no. 18, 19). A unique type of spectrum with a strong absorption band at 3280 cm -~ and two weak but broad shoulders centered at 3010 and 2920 cm -x is shown by the futile sample no. 3 from a pegmatitic vein in metamorphic rocks. Characteristic features of this sample are described in detail by Hammer (1990). Rutiles occurring in amphibolite facies rocks and in alpine fissures show a single band centered at 3280 cm -1. As evident from Table 1 the H 2 0 + content of the rutiles varies from 0.04 to 0.21 wt.%. Two types of factor may influence the H2 O+ content of natural rutiles: (1) crystal chemistry, specifically the presence of impurity cations and then charge balance; (2) environment of formation, which depends on the activity of hydrous components during the crystallization process.

6

V . M . F . Hammer and A. Beran •

15 14 13 12 17 16 15 18 17 16 15 14

,

,

,

,

l

,

no.2

no. 5

g 11

% 13 12 11 10

5

3 00

30'00' 2&0

Wavenumber

7(cm-1)

Fig. 1. Representative OH absorption spectra of rutiles demonstrating the variability observed for individual samples. Non-polarized IR radiation, doubly polished slabs cut parallel to the c-axis. Rutile no. 2, thickness t = 0.023 cm, no. 3, t = 0.030 cm, no. 5, t = 0.026 cm, no. 14, t = 0.030 cm, no. 16, t = 0.024, no. 18, t = 0.030 cm, no. 19, t = 0.030 cm. For sample no. see Table 1

1.0

o16 0.9

o5 ~o

0.8

0.7

17

~3°

°e o12 02

-~ 0.6 c~

11

06

0.5

o3

0.4

5. %7 .I

09

o18 0,3 0.2 0.1

o'i o12 o'.3 o; o'6 o'7 2(M203-M205)

(mot%)

Fig. 2. OH concentration in tool% H 2 0 plotted versus the difference of the summed tri-valent metal oxides (A1203 + VzO3 + Cr203 + Fe203) minus penta-valent oxides (Nb205) in mol% demonstrating a weak correlation. Sample no. refer to Table 1

Reported rutile analyses often c o n t a i n considerable a m o u n t s of A1, V, Cr, M n , Fe, Zr, N b , Sn, Ta (e.g. Rumble, 1976; Putnis and Wilson, 1978). The rutile samples in the present study c o n t a i n up to more than 1 wt.% of A1203, V203, C r 2 0 3 , FezO3

Variations in the OH Concentration of Rutiles

7

and Nb205 (Table 2). A M6ssbauer study of the rutile from Capao do Lana (no. 5) shows more than 90~o of Fe in trivalent state (cf. Lindsley, 1976) and trivalent Fe is reported in synthetic rutiles by Bente et al. (1985). A striking feature of the rutile included in a pyrope from a high pressure rock of the Dora Maira Massif (no. 19) is the high A1203 content. The high amount of V203 and Cr203 in contrast to the extremely low Fe/O 3 content of the rutile from a graphitic gneiss in the Moldanubicum (no. 17) represents also a characteristic feature of the accompanying dravites, kyanites and phengites (Beran et al., 1985). Most of the rutiles from alpine fissures (no. 9, 11, 12, 13, 14, 15) reveal very low concentrations of Nb. Figure 2 shows a weak correlation between H2 O+ content and the content of tri- and penta-valent metal oxides, suggesting that the OH incorporation is partly associated with the minor element content. It is evident that there is an environmental factor, distinct from crystal chemistry and charge balance effects. The highest OH concentrations are found in rutiles from alpine fissures, representing high activities of hydrous species during formation. Rutiles from alpine fissures in amphibolite facies rocks (no. 10, 11, 12, 15) show higher amounts of H2O+ than alpine fissure rutiles from greenschist facies rocks (no. 9, 13, 14), thus indicating different hydrothermal conditions in different metamorphic environments. Rutiles as accessory components in amphibolite facies rocks (no. 16, 17) also show high H2 O+ contents and have apparently crystallized in water rich environments. Eclogite facies rutile (no. 18) has a low OH concentration and the even lower H / O + content of the blueschist facies rutile (no. 19) supports the proposal of Chopin (1984) and Schreyer (1988) that minerals of the Dora Maira high-pressure zone formed under conditions of low water activity (cf. Rossman et al., 1989). Rutile no. 18 shows beside the absorption maximum at 3280 cm -1 a satellite band at 3320 cm -1. The absorption spectrum of rutile no. 19 is characterized by an absorption maximum at 3320 cm -1 and a weak absorption at 3280 cm -1. Rossman and Smyth (1990) reported an IR spectrum of rutile from an eclogite xenolith in kimberlite from Roberts Victor, South Africa, with a similar double sharp absorption at 3320 and c. 3300 cm -1. A doublet band centered at 3322 and 3277 cm -1 is prominent in the spectrum of Verneuil-grown rutile, which suggests that the band splitting is linked to high temperature growth conditions. Split bands with maxima at 3360 and 3280 cm -1 also occur in rutiles from pegmatitic veins (no. 1, 2, 3), where temperature plays a dominant role. There is no correlation between band splitting and minor-element content. It is possible to explain the presence of a doublet band in natural samples and in pure synthetic futile by vacancies in the cation lattice combined to slight structural changes. All the natural rutiles studied contain structurally incorporated OH groups and their H2 O+ content varies from 0.04 to 0.21 wt.~. The OH incorporation is partly related to the substitution of tri- and penta-valent cations for Ti, but also gives an indication of the activities of hydrous species during formation of the futile. The presence of split OH absorption bands is associated with formation under high p, T-conditions and is most likely related to crystallization at high temperature.

Acknowledgements Samples for this study were provided by F. Koller (University, Vienna), G. Niedermayr and R. Seemann (Museum of Natural History, Vienna). Thanks are due to F. Brandstf~tter

8

V . M . F . Hammer and A. Beran

(Museum of Natural History, Vienna) and H. Dietrich (University, Vienna) for assistance with the EMS and to F. Grass (Atominstitut, Vienna) with the NAA work. The elemental analysis was kindly performed by J. Zak (University, Vienna). G. Amthauer (University, Salzburg) is acknowledged for performing the M6ssbauer measurements. We are grateful to J. Zemann for helpful discussions. Comments from J. Chisholm substantially improved the manuscript. The investigation was supported by the "Fonds zur F6rderung der wissenschaftlichen Forschung" (P6324).

References

Bence AE, Albee AL (1968) Empirical correction factors for the electron microanalysis of

silicates and oxides. J Geol 76:382-403 Bente K, Kupcik V, Schmid P (1985) Spektroskopische und r6ntgenographische Unter-

suchungen Fe-haltiger Rutile. Fortschr Miner 63, Beih 1:20 Beran A, Fischer R, Pichlh6fer R (1985) Zur Mineralogie der Graphitlagerstfitten des

Moldanubikums in ()sterreich. Fortschr Miner 63, Beih 1:21 - - G&zinger M A (1987) The quantitative IR spectroscopic determination of structural

OH groups in kyanites. Mineral Petrol 36:41-49 - - Rossman GR, Grew ES (1989) The hydrous component in sillimanite. Amer Mineral

74:812-817 -

-

Zemann J (1971) Messung des Ultrarot-Pleochroismus yon Mineralen. XI. Der Pleo-

chroismus der OH-Streckfrequenz in Rutil, Anatas, Brookit und Cassiterit. Tschermaks Min Petr Mitt 15:71-80 Bugge J A W (1978) Norway. In: Bowie SHU, Kvalheim A, Haslam H W (eds) Mineral deposits of Europe, Vol 1: Northwest Europe. Inst Mining Metall, Mineral Soc London, pp 199-249 Chopin C (1984) Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences. Contrib Mineral Petrol 86:107-118 Hammer VMF (1988) Quantitative IR-spectroscopic determination of structural OH-groups in natural rutiles of various occurrences. Z Kristallogr 185:631 -(1990) IR-spektroskopische und chemische Untersuchungen an Rutil von Modriach/ Steiermark. Mitt Abt Miner Landesmuseum Joanneum 58:11-14 Heinrich CA (1982) Kyanite-eclogite to amphibolite facies evolution of hydrous marie and pelitic rocks, Adula Nappe, Central Alps. Contrib Mineral Petrol 81:30-38 Hippel A yon, Kalnajs J, Westphal WB (1962) Protons, dipoles, and charge carriers in rutile. J Phys Chem Solids 23:779-799 H6ck V, Koller F, Meixner H, Seemann R, Schramm JM, Steyrer HP (1981) Petrographie und Mineralogie der Grauwackenzone und der Nordabdachung der Hohen Tauern im Land Salzburg. Fortschr Miner 59, Beih 2:3-37 Langer K, Fl6rke OW (1974) Near infrared absorption spectra (4000-9000 cm -1) of opals and the role of"water" in these SIO2' nH20 minerals. Fortschr Miner 52:17-51 Lindsley DH (1976) Experimental studies of oxide minerals. In: Rumble D (ed) Oxide minerals. Short Course Notes, Vol 3. Miner Soc Amer, pp L61-L88 Meixner H (1965) Der Vivianitfund yon Modriach im Rahmen der Mineralvorkommen des Raumes Pack-Ligist, stidlich K6flach, Koralpe, Weststeiermark. Karinthin 52: 120-136 Miller GH, Rossman GR, Harlow GE (1987) The natural occurrence of hydroxide in olivine. Phys Chem Minerals 14:461-472 Purtscheller F (1978) Otztaler und Stubaier Alpen. Sammlung Geol Ftihrer 53. Borntraeger, Berlin Stuttgart, pp 128 Putnis A, Wilson M M (1978) A study of iron-bearing rutiles in the paragenesis TiO2-AI203P205-SiO2. Min Mag 42:255-263

Variations in the OH Concentration of Rutiles

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Rossman GR, Beran A, Langer K (1989) The hydrous component of pyrope from the Dora Maira Massif, Western Alps. Eur J Mineral 1:151-154 Smyth JR (1990) Hydroxyl contents of accessory minerals in mantle eclogites and related rocks. Amer Mineral 75:775-780 Rumble D (1976) Oxide minerals in metamorphic rocks. In: Rumble D (ed) Oxide minerals. Short Course Notes, Vol 3. Miner Soc Amer, pp R1-R24 Scharbert HG, Fuchs G (1981) Metamorphe Serien im Moldanubikum Nieder6sterreichs. Fortschr Miner 59, Beih 2:129-152 Schreyer W (1988) Subduction of continental crust to mantle depths: petrological evidence. Episodes 11:97-104 Skogby H, Bell DR, Rossman GR (1990) Hydroxide in pyroxene: Variations in the natural environment. Amer Mineral 75:764-774 Softer BH (1961) Studies of the optical and infrared absorption spectra of rutile single crystals. J Chem Phys 35:940-945 Wilkins R WT, Sabine W (1973) Water content of some nominally anhydrous silicates. Amer Mineral 58:508 516 -

Authors' addresses: Dr. Vera M. F. Hammer, Geotechnisches Institut, Bundesversuchs- und Forschungsanstalt Arsenal, Faradaygasse, A-1030 Wien, Austria. Prof. Dr. A. Beran, Institut f/Jr Mineralogie und Kristallographie, Universit~it Wien, Dr. Karl Lueger-Ring 1, A-1010 Wien, Austria.