American Mineralogist, Volume 75, pages 529-538, 1990

Ion exchange reactions between dehydroxylated micas and salt meltsand the crystal chemistry of the interlayer cation in micas

HlNs Knppr.Bn*Mineralogisches Institut der Universitiit, KaiserstraBe 12, 7 5OO Karlsruhe, Federal Republic of Germany

AssrRAcr

The interlayer cations in dehydroxylated micas can be exchanged by reaction with saltmelts (chlorides and bromides) at temperatures of 700 to 950 'C under atmospheric pres-

sure within a few hours. Generally, during the reactions air and moisture must be excludedto prevent decomposition of the micas. By this method, dehydroxylated mica phases ofthe type M*Alr[AlSi3OloO] with M : Li, Na, K, Rb, Cs, Tl were prepared as well as micasof the type M%tr,,AIr[AlSi3OroO] with M : Ca, Sr, Ba. Additionally, a sodium mica wasprepared with part of the Na* ions replaced by La3*, probably according to the reaction 3Na+ : La3+ * 2tr. Most of these micas are compounds not described previously. For allmicas except the Sr and Ba phases, complete sets of lattice constants are given. The latticeconstants are almost exclusively determined by the ionic radius and not by the charge ofthe interlayer cation. The tetrahedral rotation angle can be varied by about l0'solely bythe interlayer substitution. In micas with very small interlayer cations like Na* or Li*,repulsive forces between the O atoms across the interlayer region cause an interlayerovershift, resulting in anomalously high basal spacings and a smaller B angle. The interlayerspacing of the lithium mica can expand by reversible uptake of water. The other dehy-droxylated micas do not react with water at room temperature. The differences in thestructure of micas with small and large interlayer cations explain many features of thegeochemistry of these phases, such as the miscibility gap between paragonite and musco-vite, the preferential uptake of potassium in micas and structurally related clay minerals,and the smaller thermodynamic stability range of paragonite compared with muscovite.

INrnonucrroN

Interlayer cations of micas are not easily exchanged, incontrast to the structurally related clay minerals. Even athigh pressures and temperatures, reaction times of sev-eral weeks are necessary to reach chemical equilibriumbetween fine-grained micas and aqueous salt solutions(Iiyama, 1964; Scott and Smith, 1966; Krausz, 1974; Fluxand Chatterjee, 1986).

White (1954) and Franz and Althaus (1976) tried toexchange the interlayer cations of micas by reaction withsalt melts. White obtained only decomposition products.Franz and Althaus were able to partially replace the K inphengites by Na, but some decomposition of the micasto other phases was always observed. Fujii (1966) ob-tained solid solutions between dehydroxylated paragoniteand dehydroxylated muscovite by ion exchange betweensalt melts and muscovite. During the experiments, partof the mica reacted to nepheline, hydroxysodalite, andother phases.

In this paper, ion-exchange reactions between water-free, dehydroxylated micas and salt melts are described

* Present address: Bayerisches Geoinstitut, Universitiit Bay-reuth, Postfach l0L25l, Universitiitsstra8e 30, 8580 Bayreuth,Federal Republic of Germany.

that allow the introduction of a wide variety of cationsinto the interlayer of micas (cf. Keppler, 1988). Duringthese reactions, the mica structure is completely pre-served and no decomposition occurs. The reaction prod-ucts are used to study the influence of the interlayer sub-stitution on the crystal structure of these phases.

ExprnrlrBNTAL METHoDS

Starting materials

Paragonite-2Mr and muscovite-2M, were prepared byhydrothermal treatment of stoichiometric mixtures ofamorphous SiO, (spec. pure), chemically pure AI(OH)3,and 4N solutions of NaOH or KOH at 7 kbar for 14 days.The reaction temperature was 550 to 600 'C for parago-nite and 650 to 700 "C for muscovite. X-ray diffractionpatterns indicate the presence of small amounts of thelM polytype in the paragonite samples, but no other im'purities were detected. The diffraction patterns are simi-liar to those given by Chatterjee (1970) and Chattedeeand Johannes (1974). According to sEM investigations,the samples consist of aggregates of irregularly shapedplatelets ranging from I to 5 pm in size. Dehydroxylatedparagonite and muscovite were prepared by heating thecorresponding mica at 850 'C for 5 h. IR spectra showedthat the products obtained were completely water-free.

0003-o04x/90/0506{529$02.00 529

530 KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

EXPERIMENTALMETHOD

rubber bolloon(orgon reservoir)

three-woy top

furnoceT= 700- 950 'C

somDte.dehydroxytoted Porogonite ormuscovite + melt of onhydrouschtor ides or bromides ( lorge

excess of mett)

Fig. L Experimental scheme used for performing ion-ex-change reactions in an inert-gas atmosphere.

Ion exchange between micas and salt melts

The interlayer cations in dehydroxylated muscovite-2M, or paragonite-2Mr were exchanged by reaction withmelts of anhydrous chlorides or bromides at 700 to 950{ under atmospheric pressure within a few hours. Bothdehydroxylated paragonite and dehydroxylated musco-vite can be used as starting materials for all reactions. Tofacilitate X-ray identification of reaction products, themica that was expected to show the greater change oflattice constants after the ion exchange was usually cho-sen.

The exchange reactions between micas and KCl, RbCl,and CsCl were performed in a Pt crucible in air using anelectric furnace. In these cases, both hydroxyl-containingand dehydroxylated micas could be used as starting ma-terials. In all other cases, however, it was necessary toexclude moisture and oxygen by working in an inert gasatmosphere and to use absolutely water-free starting ma-terials (dehydroxylated micas and anhydrous salts). Oth-erwise, reactions between the salt melt and HrO or O,would occur that would lead to the formation of stronglyalkaline substances (e.g., CaO and LiOH), according to

equations of the type 2 CaCl' + O, - 2 CaO + 2 Cl, orLiCl + H,O - LiOH + HCI (Gmelin 1927, 1957). Thesealkaline compounds would attack the silicate frameworkof the mica and lead to decomposition. Reactions underinert gas were performed in a quartz-glass tube filled withargon (Fig. l).

A high excess of salt melt compared to the amount ofmica was always used in order to obtain an almost com-plete cation exchange. After the run, the excess of saltwas dissolved away with water (for NaCl, KCl, RbCl,CsCl), very dilute aqueous HCI (CaCl,, SrClr, BaClr,BaBrr, L,aClr), aqueous solution of sodium thiosulfate(TlBr), or absolute ethanol (LiCl).

Investigation of reaction products

Chernical analyses. Quantitative analyses were per-formed by means of atomic absorption spectrometry af-ter dissolving the samples in HF + HCIO4. SiO, was de-termined by difference, with the sum of all oxides being100 wt0/0. Analyses of the reaction products by electronmicroprobe gave only semiquantitative results becausethe small grain size and perfect cleavage of the micasmade the preparation of polished surfaces impossible.

IR spectra. Pellets were prepared by pressing a mixtureof l-2 mg of sample and 200-300 mg of KBr. After heat-ing in a vacuum at. 200 "C for several hours to removeadsorbed water, the IR spectra of the samples were mea-sured using a Beckman spectrometer n 4250.

X-ray diffraction. X-ray diffraction patterns of powdersamples were obtained by means of a Philips diffractom-eter PW 1050/25 (proportional counter, Ni-filtered CuKaradiation, scan rate r/ao 20/min, time constant 2 s,5-70o20, internal standard of high-purity silicon). Lattice con-stants based on the powder data were refined by a least-squares procedure.

ExpnnrvrBNTAL RESULTS AND DlscussroN

Synthesis of mica phases by ion exchange

The conditions for the synthesis of the mica phasesdescribed in this paper are summarized in Table l. Thereaction products are fine-grained white powders that aresoft to the touch. Figure 2 shows the X-ray diffractionpattern ofthe product obtained by ion exchange betweendehydroxylated paragonite and KCI melt. The pattern isvirtually identical with that of dehydroxylated musco-vite. The substitution of alkalies (and thallium) in theinterlayer causes a significant shift ofthe (002) basal re-flection (Fig. 3). The products of the ion exchange withhalogenides of calcium, strontium, and barium do notshow a strong (002) basal reflection. However, (006) al-ways has high intensity in the diffraction patterns, and itsposition strongly depends on the size of the interlayercation (Fig. 4). Table 2 gives the chemical analyses ofthree alkali micas prepared by ion exchange between al-kali halogenide melts and dehydroxylated paragonite. Themeasured compositions are almost identical with the cal-culated composition of dioctahedral potassium, rubid-

quorlzf i t t ed(1o tm)

gtoss tulrewith orgon

KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS 531

/.4 35 30 25 20 15 10 5

" 2 0 C u K .

Fig. 2. X-ray diffraction pattern of potassium mica preparedby ion exchange between dehydroxylated paragonite and KCImelt. The pattern is virtually identical with that of dehydroxyl-ated muscovite.

ium, and cesium micas. This confirms the stoichiometricnature of the ion exchange process. Samples of the rubid-ium and cesium mica were treated with a large excess ofNaCl at 850'C for 2 h. Dehydroxylated paragonite wasobtained as a reaction product, demonstrating the re-versibility of the reaction. The rR spectra of the dehy-droxylated paragonite used as starting material and thecesium mica obtained by ion exchange are compared inFigure 5. Aside from small frequency shifts, the spectraare almost identical, demonstrating that the silicateframework of the mica was not altered by the ion ex-change. For the lithium mica, quantitative analyses can-not be given because the samples obtained always con-tained some decomposition products (strong X-rayreflections al 4.55 and 3.51 A; a phase very similar to,but probably not identical with B-eucryptite, LiAlSiOo).The decomposition is probably caused by the presence ofsome water in these experiments. The LiCl used as saltmelt is extremely hygroscopic and takes up a considerableamount of water, even during the short time necessaryfor handling the chemical in air during the preparationof the experiments. Under absolutely water-free condi-tions, the pure lithium mica could probably be obtained.In the absence of quantitative chemical analyses, thechemical formula of the lithium mica given in Table I isbased on the lattice parameters (Table 3) that correspondto a dioctahedral mica. The magnitude of D for the lith-ium mica is smaller than that of D of the starting material(dehydroxylated paragonite), which precludes the incor-poration ofl-i in the vacant sites ofthe octahedral layer.Therefore, all Li must be in the interlayer, which is inagreement with the very low value of c (Table 3) andleads to the formula given in Table l. The calcium, stron-tium, and barium mica samples usually contained a smallamount of unreacted starting material. The microprobedata confirm that the Al-Si ratio of these samples was notchanged by the ion exchange process, indicating an un-

L i9.50

NoK9.11 8.78

\

Cs8.20

9.5 9.0 8.5 8.0"20 CuK.

Fig. 3. Position ofthe (002) basal reflection ofdehydroxyl-ated dioctahedral micas with univalent interlayer cations. Thevalue for Rb is calculated from the lattice parameters becausethe reflection has low intensity and is difrcult to measure.

changed composition of the tetrahedral and octahedrallayer. To maintain charge balance, the interlayer of thesemicas must contain vacancies, as indicated in the chem-ical formulas given in Table l. If the charge balance wereachieved by substitution of Al for Si in the tetrahedrallayer, the total Al-Si ratio of the mica would have tochange from l: I to 2:1. Although the microprobe dataare only semiquantitative, they clearly rule out this pos-sibility. An analogous substitution mechanism with va-cancies in the interlayer probably accounts for the incor-poration of a small amount of lanthanum (La3*) into themica phase when it is treated with a NaCl-LaCl, melt(Table l). Treatment of dehydroxylated paragonite or de-hydroxylated muscovite with pure molten LaCl, yieldsonly decomposition products.

Co Sr27.t 8 27.28

Bo26.6s

27.5 27.0 26.5"29 Cu K .

Fig. 4. Position ofthe (006) reflection ofdehydroxylated di-octahedral micas with divalent interlayer cations.

TI Rb8.60 8.56

532

Taau 1. Data for synthesis of micas by ion exchange

KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

Startingmaterial Salt melt

ReactionWeight ratio salt temperature Reaction

melt to mica fC) time (h)

Lithium micaLiAt,IAlsi30loo]

Sodium micaNaAlr[AlSi3OioO]

Potassium micaKAlr[AlSirOloO]

Rubidium micaRbAl,[Alsi30loo]

Cesium micaCsAlr[AlSi3O,oO]

Thallium micaTtAl,[AtsLoloo]

Calcium micaCaos tr osAl,[AlSi3OloO]

Strontium micaSro 5 ! o sAl2[AlSi3OloO]

Barium micaBao5 n osAlr[AlSisoloO]

Sodiumlanthanum micaSodium mica containing1-2 wlo/o La"O"

P

M

P

P

P

P

M

P

P

LiCl (under Ar)

Nacl (under Ar)

KCI (in air)

RbCl (in aio

CsCl (in air)

TlBr (in air)

CaCl, (under Ar)

SrBr. (under Ar)

BaBr, + BaGl, (2:1) (under Ar)

NaCl + Laols (9:1) (under Ar)

5:1 to 10 :1

100:1

100:1

1 00:1

1 00:1

700-750

900

850

900

900

750

850

850

950

850

20:1

5 :1

1 0 : 1

1 0 : 1

2O:1

'I

2-5

c

c

8

'|

2,5

I

Note: P : dehydroxylated paragonite, M : dehydroxylated muscovite. The composition ol salt mixtures is given in weight ratios.' Contains decomposition products.

'- Product contains some unreacted starting material.

Crystal chemistry of the interlayer cation in micas

In Figure 6, the basal spacing d(001) is plotted againstthe radius ofthe interlayer cation for the dehydroxylatedalkali micas (compare Table 3; complete X-ray powder-diffraction data of the micas are given in Appendix l).Surprisingly, the data points do not form a straight line;there is a jump between Na and K. The monoclinic angleB of the alkali micas exhibits a similar behavior; althoughit is about 96' for the potassium, rubidium, and cesiummicas, it decreases to about 9lo for the lithium and so-dium micas. Both observations point to a fundamentaldifference in the coordination of the small and the largealkali ions in the interlayer. The unexpectedly high basal

TABLE 2. Chemical analyses of micas prepared by ion exchangebetween dehydroxylated paragonite and alkali halo-genide melts

Measured wt% Theoretical wt%

spacings and low values of the angle 0 for the small in-terlayer cations can be understood on the basis of thecrystal structure of paragonite-2M'. Because of the smallsize of the Na* ion, in the paragonite structure the Oatoms forming the boundary of the interlayer region comeclose enough that repulsive forces result (Lin and Bailey,1984). The difference in the coordination of the small andlarge alkali ions in the interlayer of a mica is schemati-cally depicted in Figure 7. The large cations are coordi-nated by two sixfold rings of O atoms of the tetrahedralsheets (which appear as brackets in the cross section shownin Fig. 7). If the size of the interlayer cation is reduced,repulsive forces between the tetrahedral sheets allow anovershift ofthe tetrahedral layer over the interlayer cat-ion (Lin and Bailey 1984). This causes an increased basalspacing and reduced value ofB.

The pseudohexagonal oxygen rings in the tetrahedrallayers of micas always show some ditrigonal distortion.The amount of distortion is directly correlated to themagnitude of b according to the relation cos a : b/bt*(Bailey 1984), where a is the tetrahedral rotation angle.It gives the amount by which the tetrahedra in the sixfoldrings are rotated from their positions in ideal hexagonalgeometry. The parameter b,*r is the b lattice constant fora mica containing undistorted hexagonal rings (6,*" :

9.335 A for Al:Si: l:3 in the tetrahedral layer, Bailey1984). Both b and a of the alkali micas are plotted inFigure 8 as a function ofthe radius ofthe interlayer cat-ion. Apparently it is possible to change the tetrahedralrotation angle by about l0'(from 8.96'for Cs to 18.88'for Li) solely by the interlayer substitution. This dis-proves earlier assumptions that the tetrahedral rotationis essentially controlled by the composition of the octa-

K.oNaroAl2o3sio,

Rb?oNaroAlrossio,

csroNaroAl2o3sio,

Potassium mica KAlr[AlSi"O,oO]12.890.49

39 6247 00

Rubidium mica RbAl,[AlSisoloO]21.82

u,c\t35.6941.96

Cesium mica CsAl,[AlSisO,oO]29.070.55

32.4137.97

12.38

40.2247.40

21.91

35.8542.24

29.72

32.2638.02

Note; Total wt% : 100 because SiO, wtTo was determined by difference.

KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS 533

!

zoaa

(nz.

EF

WAVE NUMBER cm-1

Fig. 5. rR spectra ofdehydroxylated paragonite (Na) and themica obtained by ion exchange between dehydroxylated parago-nite and molten CsCl (Cs).

hedral layer (McCauley and Newnham, 197l).In contrastto this idea, Radoslovich and Norrish (1962) consideredthe interlayer cation to be the primary control of the tet-rahedral rotation angle: "For the micas in particular thesurface oxygen triads rotate . . . until halfofthe oxygens'lock' onto the interlayer cation." This interpretation is

1 2 1 3 1 r , 1 5 1 6 ' t 7 1 8 1 9

r (X i l ) A

Fig. 6. Plot of the (001) basal spacing against the radius ofthe interlayer cation for the alkali micas. Data from Table 3.

consistent with the experimental data presented in thispaper. As in the case ofd(001) and B, the greatest changein b and a occurs between Na and K. This can be ex-plained by the fact that the small interlayer cations arepartially pulled out ofthe sixfold oxygen rings ofthe tet-rahedral sheets (see Fig.7), which causes a higher tetra-hedral rotation angle and a correspondingly smaller valueof b.

The plot of the cell volume of the alkali micas againstthe radius of the interlayer cation for l2-fold coordina-tion (Fig. 9) is not a straight line. The nonlinearity iscaused by the increasing tetrahedral rotation with de-creasing size ofthe interlayer cation, leading to a distor-tion ofthe coordination polyhedron around the interlayercation and a reduction in the effective coordination num-ber. For that reason, the radii for 12 coordination usedin Figure 9 are slightly too high for the small cations witha lower effective coordination number, explaining the de-viation of the curve from a straight line. No discontinuityis observed between Na and K because the anomalouslv

Tmle 3. Lattice parameters, basal spacings d(001), and tetrahedral rotation angles a of dehydroxylated, dioctahedral micas(monoclinic, assumed space group C2lc)

Interlayercation Err (A) a (A) b (A) c (A) Bf ) d(oo1) (A) a (') Y(A')

700 600 500 400 300

18.71519.45920.17220.65221.49820.61019.47219.61420.069

/Vofe: The sodium and potassium micas were prepared by dehydroxylation of paragonite-2M, or muscovite-2M,, respectively; the other micas wereobtained by ion exchange (Table 1). Complete X-ray powder data are given in the appendix. The effective ionic radius for 12 coordination is t'21r, accordingto Shannon (1976). The value for Li was calculated by linear extrapolation of the radii for lower coordination numbers.

Li 1.25Na 1 .39K 1 .64Rb '1.72

Cs 1 .88Tl 1.70Ca 1.34Sr 1.44Ba 1 .61

5.244(20)5.244(08)5.225(04)5.243(04)5.263(04)5.234(04)5.251(06)

8.833(22)8.893(1 1 )9.1 63(05)9.1 98(1 0)9.221(12)9.1 77(09)8.891(07)

1 8.71 6(29)1 9.463(0s)20.275(Os)20.764(06)21.604(08)20.720(0511 9.480(1 7)

90.50(33)91.20(08)9s.78(03)95.98(04)95.68(04)95.s1 (04)91 .64(1 2)

18.8817.7011.029.838.96

10.5617.74

866.9907.4965.8996.0

1043.3990.0909.1

534 KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

te t rohedro l sheet

lorge inter loyer cot ion (K,Rb.Cs)p= e6 'd(00'1) regulor

smol l Inter loyer cot ion (L i , No)p : 9 1 'd(00' l ) h igher thon expected

Fig. 7. Simplified model for the coordination of the interlay-er cation in micas.

large basal spacings and small values of b for the smallcations mutually compensate in the calculation of the cellvolumes.

The lithium mica exhibits a behavior which is uniqueamong the alkali micas; its interlayer spacing can expandby a reversible uptake of water. Figure l0 shows the X-raydiffraction pattern of a sample of the lithium mica thatwas exposed to humid air for several days. In addition tothe usual X-ray reflection at 9.5" 20 CvKu, a line appearsaI 7.3 which corresponds to an expanded basal spacingof l2.l A. fns reflection disappears after heating for I hat 700'C in a vacuum. The ability of the lithium mica totake up water can be explained by the existence ofrepul-sive forces in the interlayer region and by the high hy-dration energy for Li* (5 l9 kJ/mol, compared to 264 kI/mol for Cs*; Cotton and Wilkinson, 1980).

The size of the Tl* ion is almost identical to that ofRb* (Table 2), but Tl* is much more easily polarized thanRb* (Cotton and Wilkinson, 1980). The lattice parame-ters of the rubidium and thallium micas are very similar(Table 2), as expected on the basis of the ionic radii.However, a closer examination of the data shows that bof the thallium mica is significantly reduced compared tothat of the Rb phase. The value of b for thallium is closerto that for K than to that of the Rb mica. It is possiblethat the easily polarized electron shell ofthe Tl* ion canbe deformed by the oxygen atoms ofthe tetrahedral layer,causing an increased tetrahedral rotation angle and a re-duced value ofD.

The lattice parameters of the sodium and calcium mi-cas are virtually identical, and d(001) of the barium mica

te t rohedro l sheet 20

15

10

9 3

9 2

J I

_o

1 2 1 3 1 t 1 5 1 6 1 . 7 1 . 8 1 9r ( X l l ) A

Fig. 8. Plot ofb and the tetrahedral rotation angle (a) againstthe radius ofthe interlayer cation for the alkali micas'

is very close to the value for K. Both observations dem-onstrate that the charge of the interlayer cation and theexistence of vacancies in the interlayer have very littleinfluence on the lattice parameters.

The influence of ilehydroxylation on the mica structure

The crystal structure of dehydroxylated muscovite hasbeen determined by Udagawa et al. (1974). The maindifference from the structure of muscovite is in the co-ordination of Al in the octahedral sheet. Whereas Al isoctahedrally coordinated by O and OH in muscovite, itis surrounded by five O atoms in trigonal-bipyramidalconfiguration in the dehydroxylated phase. The interlayerand the neighboring tetrahedral sheets are little affectedby the dehydroxylation. Therefore, conclusions concern-ing the crystal chemistry of the interlayer cations in de-hydroxylated micas very probably also hold for hydroxyl-containing micas. This idea is supported by results ofFujii (1966), which show that the miscibility gap betweendehydroxylated paragonite and dehydroxylated musco-vite is virtually identical with the two-phase region ofnalural, hydrous sodium-potassium micas.

In Table 4, the lattice parameters ofthe dehydroxylatedand the corresponding hydrous mica phases are com-

TABLE 4. Lattice parameters of hydrous dioctahedral micas and their dehydroxylated equivalents

a (A) b (A) c (A) B f )Sodium micas

Paragonite-2Mr (Chatterjee, 1974) 5.1304(10)Dehydroxylated paragonite-2M1 (this papeo 5.244(8)

8.8927(1 5)8.893(1 1 )

1 9.2698(28)19.463(5)

94.220(16)91.20(8)

95.780(1 0)9s.776(26)

96.540(3)95.98(4)

Muscovite-2Ml (Chatteriee and Johannes, 1 974)Dehydroxylated muscovite-2M1 (this paper)

5.1 871 (8)5.225(4)

Potassium micas8.9927(12) 20.1490(22)9.163(5) 2o.27s(s)

Rubidium micas9.059(s) 20.s9(1)9.198(10) 20.764(6)

Rb analogue of muscovite-2M1 (Voncken et al., 1987) 5.215(3)Anhydrous rubidium mica (this paper) 5.243(4)

Note: The lattice parameters for the water-free micas are from Table 2 of this paper. For the dehydroxylated muscovite, similar data have beenpublished by Udagawa et al. (1974), Vedder and Wilkins (1969), and Eberhart (1963).

535

I

1t,l l l l l

12 1086r."29 Cu K*

Fig. 10. Portion ofthe X-ray diffraction pattern ofpartiallyexpanded lithium mica.

in the mica structure. This causes the preferential uptakeof K* compared to Na* in micas and clay minerals ofrelated structure, which ultimately leads to the enrich-ment of Na* over K* in sea water (e.9., Krauskopf, 1967).The thermodynamic stability field of paragonite is muchsmaller than that of muscovite. Paragonite decomposesto feldspar and corundum at much lower temperaturesthan muscovite (Chatterjee, 1970 Chatterjee and Johan-nes, 1974). In addition, the assemblage paragonite +quartz has a lower thermal stability limit (Chatterjee,1973), whereas muscovite + quartz is stable under sedi-mentary P, Z conditions. It is possible that these differ-ences are partially caused by the relative destabilizationof the paragonite structure by repulsion across the inter-Iayer.

CoNcr-usroNs

Ion-exchange reactions between micas and salt meltscan be used to introduce many different cations into theinterlayer of a mica. Very unusual phases can be ob-tained, such as micas with the extremely small Li* ion inthe distorted l2-fold-coordinated site of the interlayer.The variation in the lattice parameters with the size ofthe interlayer cation indicates the existence of repulsiveforces across the interlayer of micas containing small in-terlayer cations such as Na* or Li*. The appearance ofrepulsive forces in the structure when the size of the in-

KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

1000

950

900

1 2 1 3 1 t , 1 5 1 6 1 7 1 . 8 1 9r ( X l l ) A

Fig. 9. Plot of the cell volume against the radius of theterlayer cation for the alkali micas.

pared. The magnitudes of a, b, and c apparently increaseduring dehydroxylation of the sodium, potassium and ru-bidium micas. Only the change in the value of b of parag-onite seems to be insignificant. At first sight, this is sur-prising, because the decrease in the coordination numberof Al from 6 in the octahedral sheet of a mica to 5 in thedehydroxylated phase and the short Al-O distances in thelatter structure (Udagawa et al., 1974) favor a decrease inthe lattice parameters during dehydroxylation. In addi-tion, the orientation of the O-H bond in dioctahedralmicas points to repulsive forces between the proton andthe interlayer cation (Bailey, 1984). Removal of the pro-ton should therefore cause a contraction of the structure.The reason for the observed expansion of the mica struc-ture during dehydroxylation is probably the disappear-ance of H bridges. On the basis of the fine structure ofthe OH valence vibration, Langer et al. (1981) postulatedthe existence of trifurcate H bridges in the structures ofmuscovite and paragonite. Because the H bridges havecomponents in all three crystallographic directions, theirdisappearance during dehydroxylation could explain theincrease in a, b, and c.

Geological applications

The geologically most important result of this study isthe difference in the coordination of small and large in-terlayer cations in micas. In paragonite, strong repulsiveforces across the interlayer seem to be present, whereasno such effect exists in muscovite. The correspondingstructural differences explain the existence of a miscibilitygap between paragonite and muscovite. According to Fu-jii (1966), the basal spacing ofmuscovite-paragonite solidsolutions as a function of the Na-K ratio is not a straightline, but is strongly sigmoidal, a relation that is in agree-ment with the structural model outlined in this paper.The existence ofrepulsive forces across the interlayer ofparagonite weakens the chemical bonding of sodium ions

730

536 KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

terlayer cation is less than that of a critical value may beimportant for the understanding of many features of thecrystal chemistry of micas, e.9., the miscibility gap be-tween paragonite and muscovite and the preferential up-take of K in micas and structurallv related clav minerals.

Acxuowr-rrGMENTS

The author thanks E. Althaus for the permission to conduct this studyin the laboratories of the Mineralogical Institute of the University ofKarlsruhe, Federal Republic of Germany. My colleague Laurinda Cham-berlin (Caltech) kindly corrected the English ofthe original manuscript.The quality of the manuscript was improved by reviews and critical com-ments by Niranjan D. Chatterjee, Petr Cernj', and Michael F. Hochella.

RrrrnnNcns crrEDBailey, S.W. (1984) Crystal chemistry of the true micas. Mineralogical

Society of America Reviews in Mineralogy, 13, 13-60.Chattedee, N.D. (1970) Synthesis and upper stability ofparagonite. Con-

tributions to Mineralogy and Petrology, 27,244-257.Chatte{ee, N.D. (1973) Low-temperature compatibility relations of the

assemblage quartz-paragonite and the thermodynamic status ofthe phaserectorite. Contributions to Mineralogy and Petrology, 42,259-271.

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MeNuscnrpr RECETvED Jurv 12, 1989MeNuscnrvr AccEprED FssnuARv 6, 1990

AppnNorx 1. X-nav piowDER-DrFFRAcrroN DATAFOR ANHYDROUS MICAS

The lattice parameters of all micas mentioned in this appendixare given in Table 3. The following abbreviations are used in theappendix tables (Tables Al-A7): a : asymmetrical reflection,dff: difiLse reflection, * : reflection used for refinement oflattice constants.

TABLE A1. X-ray powder-diffraction data for lithium mica Li-Al,[Alsi30'oo]

hkt ah 4* (A) d* (A)

0020041 1 01 1 1021112022i 1 3114006

1 l1 4283848

o

o1 2

100

9.3394.6834.4914.4114.3144.0773.9813.6673.2563.120

9.3574.6794.5094.3914.2984.0743.9943.6683.2593.1 19

Note.' The sample contains considerable amounts of decompositionproducts. Therefore, only the reflections are listed that can be unambig-uously assigned to the mica.

KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS 537

TABLE A2. X-ray powder-diffraction data for sodium mica (de- Tlet-e A4. X-ray powder-diffraction data for rubidium micahydroxylated paragonite) NaAlr[AlSi.O,oO] RbAlrIAlsi3o,oo]

hkt ilh 4* (A) 4* (A) hkt ilh 4". (A) d-" (A)

0020041 1 01 1 1112022i 1 30231140060251 1 51 1 6i301312020081172200,0,100,0 ,12

1 61 91 9233772

24100

o

21 86Io24q

2

9.7074.8644.5094.4114.1314.0353.7393.6483.3313.2412.9542.9232.6612.600z.cco2.5232.4312.3802.2561.9451.622

9.7294 8654 .5164.4174.1254.O443.738J . O O /

3.3403.2432.9282.9222.6572.580:z.5552 .5182.4322.3872.2581.9461.622

0020041 1 11 1 1o22112i 1 30231 1 3114006o24114o251 1 5i 1 60260080271 1 82212220,0,10i37206139314156

,l

1 2

4

1 8204

1 1271 21 21 01 4

8100

1 52129

1 71 7

10.2585.1434.5294.3434.1464.0203.9s83.8263.6423.5693.4403.4003.2653.0722.9242.8702.7582.5862.4842.3362.2772.1742.0652.0231.9861.6891.574I . J Z T

10.3265.1634.5174.3494.2014.0213.9563.8243.6403.5723.4423.4343.2643.0732.9252.8702.7562.5812.4832.3362.2772 1752.0652.0221 9811.6891.5741.528

dff

dff

dffdff

Note. Several weak and broad reflections at higher angles.

TABLE 43. X-ray powder-diffraction data for potassium mica(dehydroxylated muscovite) KAlr[AlSi30loO]

Ih 4"" (A) d*" (A)

dffdffdff

hkt

0020041 1 11 1 11'12i 1 30231140061140251 1 5i 1 6131008133221

0,0 ,10137

2,0,10139156315

1 3234322

d

6

1001 0

9J

37247

1 3za

o

I

10.0715.0394.5114.3263.9883.9143.7893.5243.3613.2283.0322.8872.8192.5952.5232.4122.2652.0171.9941.6771.666

1 .516

10.0685.0434.4974.3323.9973.9143 7863.5233.3623.2293.0282.8872.8192.5942.5212.4112.2692.0171.9951.6771.6651.5171 . 5 1 6

0020041 1 11 1 1i12o221121 1 3023i140060241141 1 5008131027221223

0,0 ,10137206o,0 ,122400,2 ,122,O,12331

zaI8

1 2Ic

1 62125224

1 81 2o

10041

1 3

10.7825.3694.5274.3814.3344.1874.0644.0063.8783.6333.5793.4853.3353.0032.6872.612z.cc4

2.2882.2282.1482.0592.O231.7911.7301.6701.5541.524

10.7495.3744.5354.3794.3294.2374.0694.0033.8773.6323.5833.4993.3363.0002.6872.6152 . 5 5 b

2.2852.2262.1 502.0582.0211.7911.7301.6701.5521s23

TABLE A5. X-ray powder-diffraction data for cesium micaCsAl,[AlSi.O,oO]

ilh 4," (A) d* (A)hkt

dff

dffdffdffdffdff

2338846

24

41 8

538 KEPPLER: ION EXCHANGE BETWEEN MICA AND SALT MELTS

Trele A6, X-ray powder-diffraction data for thallium mica Tl- TABLE A7. X-ray powder-diffraction data for calcium micaAlr[AlSi3OloO] Caoutro"Alr[AlSi.O'oO]

nh 4," (A) d*" (A) ilh d.* (A) d*. (A)hkl0020041 1 11121121 1 3o231 1 31140060241 1 50251 1 5i 1 6008o271 1 80410422040440,0 ,10i37i ,1 ,1 o

10.2825.1 434.3454.3034.0203 9433 8143.6353.563

3.422

3 2073.0682.9192.8622.5742.4812.3282.278

2.236

2.0972.0612.0201.948

10.3055.1534.3424.2904.0153.9473.8163.6343.5633.4353.4273.1 943.0662.9212.8632.5762.4782.3312.2802.2392.2332.0962.0612.O171.943

1 1 1022i 1 30231 1 31140060251312020270411 , 1 , 1 0

4.4354.0353 739

3.642

3.3563.2142.9482.5662.5162.3592.2071.773

4.4274.O443.7533.668J.Ob/

3.3543.2452.9292.5632.5162.3582.2081.772

10027JC

1 23952JC

2140

1 8

25353427

100

923't2

1 3381 06

dff

1 3I Q

72

1 3

26r001 0201 3

dff

dff

ondtf

Note.'Some very weak and ditfuse reflections are not listed.

A/ote: The sample contained some unreacted starting material (dehy-droxylated paragonite) Therefore, it is possible that some weak reflectionsof the thallium mica could not be observed because of the overlap withstrong reflections of dehydroxylated paragonite.