American Mineralogist, Volume 73, pages 818-82I , 1988

NHo* in pegmatitic from the southern Black Hills, South Dakota

G. Cr,nvn SolovroNrr Gnoncn R. RossNr.q.N Division of Geological and Planetary Sciences,2California Institute of Technology, Pasadena,California 91125, U.S.A.

Ansrru,cr Trace amounts of ammonium ion (0.7 molo/oNHo* substitution) have been identified from the infrared spectra of alkali feldspars from pegmatites occurring in the southern Black Hills district, South Dakota. The spectroscopicdata indicate that the ammonium is structurally bound, most likely in the M site of the microcline. These results indicate that ammonium was likely a component of metamorphic fluids present during anatexis of metasedimentaryrocks in the southern Black Hills district.

INrnonucrroN cent materialintimately mixed with sinuouspatches of turbid materialin a typicalperthitic intergrowth. In the clearregions, Buddingtonite, the in which NHf, substitutes no inclusions-fluid or solid-were observedmicroscopically. for K+ in the M site, was first discovered in a low-tem- The slabs,mounted over brassapertures that rangedfrom 2.0 perature, ammonium-rich environment (Erd et al., 19 6 4). to 0.5 mm, werepositioned so that the beamavoided strongly Both buddingtonite and K-feldspars with variable am- turbid zones.The slabsranged in thicknessfrom 0.20 to 0.80 monium contents have subsequentlybeen found in a va- mm for measurementsin the 4000- to 2500-cm-' regionand riety of localities where rocks have been hydrothermally weregreater than 1.3mm for measurementsin the near-infrared or diagenetically altered (Krohn and Altaner, 19871, region(>4000 cm-'). Loughnanet al., 1983;Gulbrandsen,1974). Spectrawere measured at room temperatureusing a Perkin- Honma and Itihara (1981)studied distribution of trace Elmermodel 180double-beam spectrophotometer in the rn re- ', NHo+ in metamorphic and granitic minerals using wet- gion from 2500to 4000cm a Cary 17 double-beamspectro- photometerin Nrn regionfrom 4000 to 7000 cm-', or a chemical techniquesand discovered 0.06 molo/oNHf, in the Nicolet 6osxFrrR spectometer in both ranges.Polarized beams pegmatitic K-feldspar from the Ryoke belt, The Japan. wereproduced with eithera Au grid (on AgBr)or LiIO, polar- present specimensstudied in the investigation are the first izer.Spectra were measured polarized parallel to eachoptic ex- for which spectroscopicevidence shows structural am- tinctiondirection in the cleavageplane of the sample. monium contained in alkali feldspar from a pegmatitic environment. They are part ofa broader survey that de- IR AND NIR SPECTRA scribesthe speciation, geologic occurrence,and concen- polarizations mea- tration of trace hydrous components in feldspars (Solo- Figure I showsthe three different rn parallel (001) mon and Rossman,1982, and unpub. data; Hofmeister sured to optic extinction directions on and (010) from microcline from the Bob Ingersoll and Rossman, 1985). These works discovered that the slabs the in microcline most common hydrous speciesin feldsparsare (l) lattice- mine. The major absorption bands the and 3067 cm ' are aniso- bound hydroxyl and water and (2) water contained in spectraat 3645,3328,3220, pegmatite fluid inclusions. tropic. Other specimensfrom the Black Hills district show similar behavior (Table l). Absorption near ExpnmunNTAL METHoDS 2850 cm-' indicatesminor organiccomponents that either were in the original sample or were introduced during Infraredabsorbance spectra were obtained on cleavageslabs of alkalifeldspar from threepegmatite localities in the southern sample handling. The unpolarized NrR spectra of micro- BlackHills district,South Dakota. Table I liststhe localityand cline from the Bob Ingersoll mine (Fig. 2) have absorp- spectroscopicdata for eachsample. For comparativepurposes, tions at 5261, 4990, ar'd 4753 cm t. spectrawere also measured on a sampleof microcrystallinebud- dingtoniteprovided by R. C. Erd. InnNtmrcluoN oF rR-AcrrvE spEcIES KBr pelletsof buddingtonitewere prepared with sample:KBr The absorptionat 3645 cm-' is similar to the molec- ratiosof both 1:100(n) and 1:10(Nrn) and were dried under previously identified in feldspars vacuumat 75 "C for severalhours prior to measurement.The ular water absorptions pegmatiticsamples are (001) and (010) fragments spalled by Solomon and Rossman(1979) and Hofmeister and from largermicrocline crystals. Their texture is characterizedby Rossman(1985). This peak showsstrong anistropy that millimeterto submillimeterzones of opticallyclear to translu- indicates structurally bound molecular water, as opposed to the broad, isotropic absorption found for fluid inclu- (1964) zeolitic water I Present address:Converse Environmental Consultants Cal- sions. Erd et al. have suggestedthat ifornia, Pasadena,California 9l105, U.S.A. occurs in buddingtonite, on the basis of chemical and 'zContribution number 4470. thermal analyses.We have found that structurally bound 0003-004x/88/0708-08l 8$02.00 818 SOLOMON AND ROSSMAN: NH/ IN PEGMATITIC FELDSPARS 819

TneLe1. Spectroscopicdata for ammonium-bearingfeldspars from South Dakota

Peak Location and comments Orientation (cm') Abs/cm

Bob Ingersollmine (001) a' 3645 28 PenningtonCounty 3402 4.O (0.05wt% NHf) 3328 6.4 3221 12.0 3102 7.4 3069 7.8 2920 Weak 2851 Weak (010)p' 3645 Veryweak 3323 9.8 3227 8.6 3098 6.0 3067 6.2 (001)"y' 3645 1.6 3328 9.2 3199 10.8 3105 o.o 3067 I-O 2920 Weak 2851 Weak 0 Erd et al. (1964) 3626 3.0 4ooo 2soo 1.75 mg buddingtonite 3295 12.7 'i8l,?rrn#ff in 175mg KBr 3074 75 2850 26 Fig. l. Polarizedinfrared spectra of microcline(Bob Inger- soll mine. SouthDakota). Absorbance normalized to 0.5-mm Unspecifiedlocality, (001) a' 3200 Keystone County' (010) B', 3210 7.3 thickness.From top to bottom, polarizationsare a' (001);B' (001) r' 3180 5.8 (010);r' (001).Unpolarized infrared spectrum ofbuddingtonite Unspecifiedlocality, (010) a' 3210 +.o powderin KBr at bottom. CusterCounty. (010) B' 3210 3.2 Unspecifiedlocality, (001)a' 3200 9.8 unknown county' (001) 7' 3290 7.8 little doubt that NHo+ is responsible for the features in . Only major absorptionsare given. the microcline spectra. The anisotropic nature ofthe spectra (Fig. l) suggests that the NHo* is structurally bound in the microcline. The water occurs in feldsparswith evidenceindicating lattice interpretation that ammonium resides in the M site of strain (e.g.,abnormally high extinction angleof a' on [010]; the microcline is likely becauseof the analogy to bud- Solomon and Rossman, unpub. data). Perhaps ammo- dingtonite, where NHo+does occur in the M site and be- nium substitution in alkali feldspar causeslattice strain causethe fundamental NH absorption frequenciesindi- that results in defects accommodating water molecules. cate hydrogen-bondingdistances that are characteristicof There is no evidencefor HrO+ substitution in the M site, the M site. In K-feldspars, most of the M-O distances becausehydrogen bonding would causerR absorptions at are between2.99 and 2.70 A (Smith, l974,Table 4-4, p. energieslower than 3645 cm-t (Williams, 1976). 9l). The data of Nakamoto et al. (1955) (Fig. 3) predict The absorptions between 3000 and 3350 cm ' have stretchingmodes betweenabout 3000 and 3300 cm-l for not been previously reported in microcline spectra and NHf in such a site. Although the ionic radius for NHo+ have been found only rarely in our survey of feldspar is approximately 0. I A hrger than that for K*, the size spectra (unpub. data). Tabulated infrared spectroscopic of the M site will be correspondingly larger when am- data discussedbelow indicate that the most likely chem- monium is substituted for K (Gulbrandsen, 1974). Thus ical entity to absorb in this region is the N-H bond. N-O distancesare probably similar in both buddington- NHo* in silicateshas at least two fundamental stretches ite and K-feldspar. (2, and zr) and a combination mode (v2 * u) between We observe a number of other features in the near- 3300 and 2800 cm-' (Vedder, 1965; Erd et al., 1964; infrared portion ofthe spectrum (Fig. 2) that substantiate Bokij and Arhhipenko,19771,Chourabi and Fripiat, l98l). our assignments.The band at 5261 cm-'results from the The spectrum of buddingtonite, in particular, compares linear combination of the molecular-water bending (typ- closelywith that of microcline. With the exception of the ically near 1620 cm-', but not known precisely for mi- single, broad band at approximately 3250 cm ' in the crocline) and the observed asymmetric stretching (3645 buddingtonite spectrum, the microcline specimen ex- cm ') modes. hibits absorption at nearly the same frequenciesas the In a similar manner, NHf residing in an alkali feldspar buddingtonite (Fig. l). Where the buddingtonite has rhe M site resultsin at leastthree absorptionsnear 4750 cm I broad absorption, the microcline shows two narrower when the bend at -1420 cm I is combined with the peaksat 3221 and 3327 cm-t. Thesecomparisons leave stretchesbetween 3300 and 3150 cm-'. Similar features 820 SOLOMON AND ROSSMAN: NHo* IN PEGMATITIC FELDSPARS

IIAVELENGTH,pm HYDROGENBOND DISIANCES 2.O

N- H..O sj 004 O o-H o z 'n u'" 3 s.u

g4 2400 2800 3200

o.2 uvrHUmBEn,cm-l Fig. 3. X-H-O bond distance(A) plottedagainst frequency 5700 5400 5100 4800 4500 4200 3900 of fundamentalX-H stretch(cm-'). Diamondsymbols for X : WAVENUMBER,cm-l N; trianglesfor X : O. Datafrom Nakamotoet al. (1955).

Fig. 2. (Top) Unpolarized near-infrared spectrum of micro- cline (Bob Ingersoll mine, South Dakota). High-frequency oscil- district, California; Perth, Ontario; Spruce Pine, North lations are due to interference fringes. Slab thickness (010) : Carolina: and others) did not show evidence for lattice- 1.367 mm. (Bottom) Buddingtonite powder in KBr. bound ammonium (Solomonand Rossman,unpub. data). The majority of pegrnatitic feldspars contain either water- are found in the near-infrared spectrum of buddingtonite rich fluid inclusions or lattice-bound HrO. This result (Fie. 2). indicates a fundamental difference between the fluids in equilibrium with pegmatitic melts of the southern Black NHo+ lNo HrO coNcrNTRATroNS Hills and those of the other localities. The concentrationof ammonium was determined with The Li-rich pegmatitesfrom the southern Black Hills the Beer-Lambert law: absorbance: € x path x concen- are generally not closely associatedwith large massesof tration, where absorbanceis the height ofthe absorption granite, and they intrude regionally metamorphosedter- peak in a specified optic direction within the mineral, e ranes (Sheareret al., 1986). The district consists of is the molar absorptivity for the species,path is the thick- PreCambrian terrane with many zoned Li-rich pegma- nessof the mineral slab in centimeters,and concentration tites that outcrop I to 2 mi (1.6 to 3.2 km) distant from of the speciesof interest is in moles per liter. The density the central outcrop area ofthe Harney Peak granite. The of alkali feldspar that contains trace quantities of am- pegmatitesoccur as isolated intrusive massesin region- monium is taken as 2560 g.L-t. ally metamorphosedquartz-mica schist and quartz-stau- The e value for NHo+in alkali feldspar has not previ- rolite schist with subordinate graphitic schists and am- ously been determined. We estimatedthis value from the phibolites. In many cases,recrystallized tourmaline-rich absorption intensity of a known concentration of bud- granulite has developed within several feet of pegmatite dingtonite in a KBr pellet and the 7.95 wto/o(NH.)rO in contacts,and sillimanite is present. In contrast, many of buddingtonite reported by Erd et al. (1964). Our estimate the other pegmatitesthat contain predominantly water- ';Ella'(001)l of epertaining to NHf, absorption13220 cm bearing feldsparhave developedin closeassociation with is 162 L/mol.cm. Our estimateis subjectto some error, large batholithic massesand appear to be late-stagedif- becausethe buddingtonite received from Erd may not ferentiatesof batholithic magma ([,arsen, 1948). contain precisely 7.95 wto/o(NH.)rO (S. P. Altaner, pers. Stewart (1978) has suggestedthat the isolated occur- comm.). renceof small pegmatitesseparated by severalmiles from By this method, we calculate an ammonium content the nearestgranitic body (Harney Peak granite) is com- for microcline at the Bob Ingersoll mine to be 0.050 wto/o patible with the genesisof the pegmatitesby anatexis of NHf. This value is higher than the 6-196 ppm range Li-rich metasedimentaryrocks. He also presentedexper- found in K-feldspars from the Ryoke belt in Japan (Hon- imental work showing that the anatexis could occur at ma and Itihara, 198l). Our value convertsto 0.0066mol least 75"C or more below the minimum melting temper- buddingtonite component in this particular feldspar.The ature for the "simple" granite system (KAlSirO'-Na- concentration of the molecular water, indicated by the AlSi3Os-SiOr-HrO)at equivalent fluid pressures.At a P'"o 3645-cm 'band (calculatedfrom the calibrationdata of around 2 kbar, not an unreasonablevalue for the two- Hofmeisterand Rossman,1985), is 0.02 wto/oHrO. feldsparpegmatites, Li-rich metasedimentaryrocks would melt near 640"C. This would explain the isolated nature DrscussroN of the pegmatites,and their occurrencewithin high-grade The occurrenceof ammonium in microclines from the metamorphic rocks. Black Hills pegmatitesis of interest becausern spectraof This scenario for petrogenesisof the southern Black feldsparsfrom other pegmatitic environments (e.g.,Pala Hills pegmatites is consistent with the inclusion of am- SOLOMON AND ROSSMAN: NHo* IN PEGMATITIC FELDSPARS 821 monium in the pegmatitic fluids, becausemica-rich me- uate fellowshipand NSF Grants EAR 77-23147,EAR 83-13098,and tasedimentaryrocks, especiallythose with graphite, have EAR 86-18200. relatively large amounts of ammonium (up to 500 ppm RrrnnBxcns crrpo N with 900/ofixed as NHo+)compared with other crustal rocks (Stevenson,1962). Clearly, in pegmatitesassociated Bokij, G.B and Arkhipenko, D K (1977) Infrared spectra of oxonium ions in layer aluminosilicates.Physics and Chemistry very and ammonium with magmatic differentiation, fluids are water-rich. of Minerals,1, 233-242. From the studies of Hallam and Eugster(1976), it is ap- Chourabi, B., and Fripiat, J.J. (1981) Determination oftetrahedral sub- parent that under geologicconditions similar to those dis- stitutions and interlayer surfaceheterogeneity from vibrational spectra cussedby Stewart(1978), NH, fugacitymust exceedl0-a of ammonium in smectites.Clays and Clay Minerals, 29,260-268. Lee,D.E. (1964)Buddingtonite, for buddingtonite to be a stable phase. Erd, R.C., White, D.E., Fahey,J.J., and an ammonium feldsparwith zeolitic water. American Mineralogist, 49, The occurrenceof 0.7 molo/obuddingtonite component 83l-850. in the potassium feldsparsfrom the southern Black Hills Gulbrandsen, R.A. (1974) Buddingtonite, ammonium feldspar, in the probably indicates NH, fugacities that are much lower Phosphoria Formation, southeasternldaho. U.S. Geological Survey than l0-a, although possibly only by two orders of mag- Joumal of Research,2, 693-697. Hallam, M., and Eugster,H P (1976) Ammonium silicate stability rela- nitude. These discussionsshow that it may be important tions. Contributions to Mineralogy and Petrology, 57, 227-244. for petrologists to consider ammonium as a component Hofmeister, A.M., and Rossman,G.R. (1985) A spectroscopicstudy of of the fluid phasepresent during regional metamorphism, irradiation coloring of amazonite: Structurally hydrous, Pb-bearing at least in terranes similar to the southern Black Hills, feldspar. American Mineralogist, 70, 79+804. (1981) of ammonium in minerals South Dakota. In addition, laboratory calibration of am- Honma, H., and Itihara, Y. Distribution of metamorphic and ganitic rocks. Geochimica et Cosmochimica Acta, monium solubility in alkali feldspar may provide a means 45,983-988. for estimating NH, fugacity in regionally metamorphosed Krohn, M.D, and Altaner, S.P. (1987) Near-infrared detection of am- rocks. monium in minerals. Geophysics,52,924-930 Larsen, E.S. (1948) Batholith and associatedrocks of Corona, Elsinore, CoNcr,usroNs and San Luis Rey quadrangles,southern California. GeologicalSociety of America Memoir 29, 182 p. Our rn and NrR investigation of alkali feldspars from Loughnan, F.C., Roberts, F I., and Lindner, A.W. (1983) Buddingtonite pegmatitesin the southern Black Hills region, South Da- (NHo-feldspar)in the Condor oilshale deposit, Queensland,. kota, reveals trace amounts of a buddingtonite compo- Mineral Magazine,47, 327-344. (1955)Stretching fre- nent in alkali feldspar from a pegmatitic environment. Nakamoto,K., Margoshes,M, and Rundle, R.E. quenciesas a function of distancesin hydrogen bonds. Joumal of the From our absorbancedata, we estimate that these feld- American Chemical Society,77, 648U6486. spars contain approximately 0.050 and 0.024 wto/oof Shearer,C.K., Papike,J J., Simon,S.8., and Laul, J.C (1986)Pegmatite- NHo* and HrO, respectively, or 0.7 molo/oof the bud- wall rock interactions,Black Hills, South Dakota: Interaction between dingtonite component. We also estimate the value of the pegmatite-derivedfluids and quartz-mica schist wall rock, American Mineralogist,71, 518-539. molar absorptivity for NHo+ in microcline to be 162 Smith, J.V. (1974) Feldspar minerals: l. and physical L/mol.cm for the 3220-cm-t absorptionpeak with po- properties.Springer-Verlag, New York, 627 p. larizationparallel to a' in (001). Solomon.G C . and Rossman,G.R (1979)The role of water in structural A buddingtonite component in pegmatitic feldsparsap- states of K-feldspars as studied by infrared spectroscopy.Geological pears to occur when the pegmatites have formed from Societyof America Abstractswith Programs, I 1, 521' - (1982)Water in feldspars.Geological Society of America Abstracts melts of regionally metamorphosed mica-rich metasedi- with Programs,14, 622. mentary rocks. Its occurrence probably indicates the Stevenson,F.J . (1962) Chemical state of nitrogen in rocks. Geochimica presenceof NH, in the metamorphic fluids, although fur- et Cosmochimica Acta, 26, 797 -809. ther laboratory calibrations are necessaryto relate the Stewart, D.B. (1978) Petrogenesisof lithium-rich pegmatites.Amencan Mineralogist,63, 970-980 buddingtonite content of alkali feldspars to ammonium Vedder, W. (1965) Ammonium in muscovite. Geochimica et Cosmo- fugacity. chimica Acta. 29, 221-228. Williams, J.M. (1976) Spectroscopicstudies of hydrated proton species, AcxNowr,nncMENTS H+(H'O),, in crystalline compounds. In P. Schuster,G. Zundel, and C Sandorfy,Eds., The hydrogenbond: II. Structureand spectroscopy, Feldspar samples were provided by T. Giordano (New Mexico State p. 657-682. North-Holland Publishing Company, Amsterdam. Unrversity), and R. C. Erd (U S. GeologicalSurvey). H. Taylor, Jr., pro- vided the original impetus to study water and other hydrous speciescon- tained in feldspar.Helpful discussionof the manuscript was provided by MeNuscnrpr REcEIvEDApnll 29, 1987 S. P. Altaner. This researchwas supportedin part by a CONOCO grad- MArquscrrpr AccEPTEDFpsnuenv 29, 1988