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Iligh Resolution Solid-State Sodium-23, Aluminum-27, and Silicon-29

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Iligh Resolution Solid-State Sodium-23, Aluminum-27, and Silicon-29 American Mineralogist, Volume 70, pages 106-123,I9E5 Iligh resolution solid-statesodium-23, aluminum-27, and silicon-29nuclear magnetic resonancespectroscopic reconnaissance of alkali and plagioclasefeldspars R. JeMes Ktnrpernrcr Department of Geology University of lllinois at Urbana-Champaign l30l West Green Street. Urbana. Illinois 61801 Ronnnr A. KrNsev. KeneN ANN SuIrn School of Chemical Sciences University of lllinois at Urbana-Champaign 505 South Mathews Avenue, Urbana, Illinois 61801 Donelo M. Hr,NoBnsoN Department of Geology University of lllinois at Urbana-Champaign 1301 West Green Street. Urbana. Illinois 61801 eNo Enrc OlorrBI-o School of Chemical Sciences University of Illinois at Urbana-Champaign 505 South Mathews Avenue. Urbana, Illinois 61801 Abstract We presentin this paperhigh-resolution solid-state magic-angle sample spinning sodium- 23,aluminum -27 , andsilicon-29 nuclear magnetic resonance (NMR) spectraof a varietyof plagioclaseand alkali feldsparsand feldsparglasses. The resultsshow that MASS NMR spectracontain a great deal of informationabout feldspar structures. In particular,we observethe following.(l) AUSiorder/disorder and exsolution effects in alkalifeldspars are readily observable.(2) To our presentlevel of understandingthe publishedstructure refinementsof anorthiteare not consistentwith the NMR of spectraof anorthite.(3) The spectraofthe intermediateplagioclases are interpretable in termsofe- and/-plagioclases, with much of the signalapparently coming from strainedsites between albiteJike and anorthite-likevolumes in the e-plagioclases.(4) The nearest-neighbor atomic arrangements in albite and anorthiteglasses are very similar to thosein the correspondingcrystals, althoughthe rangeof cation-oxygendistances is largerin the glasses.(5) Thereare no aluminum-27peaks in the rangeof 0 ppm that would indicateoctahedrally coordinated aluminumin eitherglass. Introduction spectroscopicaspects of this problem' Future work will examine the mineralogical and petrological applications We present in this paper high-resolution solid-state in more detail. Both magic-anglesample spinning (MASS) sodium-23,aluminum-27, and silicon-29nuclear magnetic and variable-angle sample spinning (VASS) techniques resonance(NMR) spectroscopic results for a variety of have been used. plagioclaseand alkali feldsparsand feldspar glasses.The The samplesdiscussed here include low albite, micro- objective of this work is to explore the applicability of line perthite, cryptopenhite, sanidine, a range of plagio- high-resolutionsolid-state NMR to the examinationof the clase compositions, and albite and anorthite glasses.The structuresof complex silicate materials, such as feldspars feldspar samplesare listed in Table 1. and feldspar glasses.The emphasisin this paper is on the In this paper we briefly summarizefeldspar nomencla- m03-004)vE5/0102-0 r06$02.00 106 KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS t07 Table l. Silicon-29 NMR chemical shifts of feldspars and spar mineralogy are given in the compendia of Smith feldspar glasses (1974) and Ribbe et al. (1983). For the benefit of those unfamiliar with the feldspars, CHEMI6L ruTERIAL LOCALIN SHTF1 COffiNTS we summarize their main features (following Smith, 1974 and Ribbe et al., 1983), state some of our assumptions, Alblte &elta, Vlrglnla l2n (Ab99) -96.: T20 and define some of our nomenclature and symbolism. -104.4 Tlo AIblte Class Synthetlc -98 l. Feldspar structures are based on a framework of corner- (Abr00) linked SiO+ and AlOa tetrahedra. The ratio of aluminum to Sanldin€ Cyonlng -97 poorly separated (or60Ab38An2) -IOI silicon rangesfrom l:3 to l:1. We use the notationQ4(nAl) to Mlcrocllne -93 Ab T2tr describe the silicon sites, where the superscript indicates the Perthlte -96.0 T2b four bridging oxygen atoms and the integer n (0 to 4) denotesthe T2o + Ab T2o -l0l,8 Tln number of aluminum (AlOo) tetrahedra to which a silicon (SiO+) -106,0 Ab Tlo tetrahedronis connected. CryptoperthlEe Ab T2E ( Spencer R) -96-o AbDo+0rT2 We assume,as is customary in dealingwith tetrahedralsilicon ( or43,4b,53,k4) Nomay) -98.0 Or Tl and aluminum occupancies,that the "aluminum avoidancerule" - 103,5 Ab TID (Loewenstein, -47 1954; Goldsmith and Laves, 1955)holds for all 01lgocla6e Bakersvllle, Shoulder (An Fak) ( An23 ) North Carollna -93.2 feldspars.That is, no aluminum tetrahedron is linked to another -97.2 - 105.3 Ab peak aluminum tetrahedron,only to four silicon tetrahedra.A// alumi- Labradotlte Sydney Ene6, -83 num sites, therefore, should be A4 (4Si). We will show, however, (Ans2 ) kbrado! -88 -91 that for our synthetic anorthite this is not completely true. - 101 -107 2. Feldsparframeworks have inherent monoclinic or pseudo- monoclinic symmetry. Because AlO4 tetrahedra are slightly Eytomlte Crystal kyl -84 Corlelatlon to (An70) Mlnnesota -89 larger than SiO4 tetrahedra, some patterns of AllSi distribution - 101 give monoclinic frameworks, whereasothers causeslight distor- tions and triclinic symmetry. The pattern of tetrahedral site Pacaye Volceno -83.0 Flrst tro peaks (h92) GuatgMla -84.6 poorly reeolved -89.2 occupancy leads to the important concept of AVSi tetrahedral site order/disorder.Other things being equal, high AVSi disorder -42.9 Synthetic Cortelatlon to promotes (Anr00) -84.8 publlshed Etructure monoclinic frameworks, whereas increasing order -89'6 reflnerentB poor promotes triclinic frameworks having increasing cell obliquity Anorthlte Glass Synrherlc -87 (indicated by the (hroo) magnitude ofthe departure ofthe B-axis from B*). The large, irregular cages in the tetrahedral framework are occupiedby potassium,sodium, or calcium ions. When occupied ture and structure and previous NMR work on feldspars, by the larger potassium ions, the cages are stretched and the framework tends review the theory of MASS NMR (especially as it per- to retain its monoclinic symmetry if it has a proper AUSi distribution. Ifthe AUSi pattern is not appropriate, tains to the study of quadrupolar nuclei), and present and the symmetry is triclinic. When occupied by the smaller, sodium discussour results. Theseresults are, we believe,only a or calcium ions, the cages crumple a little and distort the beginning of the application of MASS NMR to feldspar framework slightly so that the symmetry is triclinic, regardlessof mineralogy, and we present our conclusions with the the AVSi order/disorder (except for the highest temperature recognition that they are in someinstances tentative. It is sodium feldspar, monalbite). clear from the work we have done, however, that MASS 3. Feldsparcompositions are conveniently describedin terms NMR spectraof alkali and plagioclasefeldspars contain a of percentagesof the endmembercomponents KAlSi3Os (Ortho- great deal of information about Al/Si orderidisorder in the clase, Or), NaAlSi3Or (Albite, Ab), and CaAl2Si2Os(Anorthite, tetrahedral sites, about chemical bonding in the sodium, An). The rock-forming feldspars occur in two series, the alkali feldspars(Or-Ab) plagioclases(ALAn). aluminum and silicon sites, and also, we believe, about and 4. The principal potassic feldspars include, from low to high distortion of these sites due to exsolution. Our results temperatureforms, microcline (triclinic, Cl), orthoclase or low indicate that solid-state MASS and VASS NMR. when sanidine (macroscopicallymonoclinic; monoclinic by polarized combined with X-ray and electron difraction and trans- light microscopy but likely to be triclinic (low obliquity) by X-ray mission electron microscopy (TEM) can provide new and electron diffraction), and high sanidine (monoclinic, C2im). insights into the structures of a wide range of mineral The principal sodic feldspars are low (temperature) albite and phases. high albite, both triclinic, Cl; high albite has the lower cell obliquity. High sanidineand high albite form a continuous solid Previous work solution seriesdespite their different symmetries. 5. In the triclinic alkali feldspars(microcline, low albite, high Feldspar structure and nomenclature albite) there are four sets of similar but not identical tetrahedral sites conventionally labeled Tlo, Tlm, T2o, T2m. At high Feldsparsare the most abundantminerals in the earth's temperatures, the aluminum and silicon ions are randomly crust and have received corresponding attention from distributed over these sites. If a high temperature feldspar is mineralogistsand petrologists. Excellent reviews of feld- quenched, this disorder is retained. In the lowest temperature KIRKPATRICK ET AL,: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS forms, maximum low albite and maximum microcline, AVSi type that appears to be body centered by diffraction, even at order is, ideally, complete,with all the aluminum in the Tlo sites room temperatures.It is consideredto have a composite struc- and all the silicon in the other three sites. ture made from domains of P-anorthite. Adjacent domains are 6. In the monoclinic alkali feldspars there are two sets of "out-of-step" to one another (antiphasedomains) with respectto similar but not identical tetrahedral sites conventionally labeled AVSi ordering, which, coupled with the earlier mentioned 1- Tl and T2. High sanidineideally has one-quarterof an aluminum pseudosymmetryof each domain, causessystematic difiraction and three-quaxtersof a silicon in each site on the average absences.These diffraction absencesgive an apparent1l symme- (complete Al/Si disorder). Orthoclase (low sanidine) is more try for the averagestructure. ordered, with equal numbers of aluminum and
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