Nho* in Pegmatitic Feldspars from the Southern Black Hills, South Dakota
Total Page：16
File Type：pdf, Size：1020Kb
American Mineralogist, Volume 73, pages 818-82I , 1988 NHo* in pegmatitic feldspars 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 feldspar 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) cleavage 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,