American Mineralogist, Volume 78, pages 782-793, 1993

Formation and modification of metastable intermediate sodium potassium mica,paragonite, and muscovite in hydrothermally altered

metabasites from northern Wales

Wnr-Tnn JrlNc, Doxar-n R. PneconDepartment of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109-1063, U.S.A.

Ansrucr

Metastable intermediate sodium potassium mica (MuorParrMa,) is intergrown with pa-ragonite (MuuPan,Ma.) and muscovite (MurrPa,r), forming aggregates of randomly orient-ed crystals in the margin of the Ordovician Caradoc metabasites, northern Wales. Thecross sections of the mica aggregates resemble the morphology of euhedral plagioclase.Muscovite is the dominant phase with paragonite and intermediate sodium potassiummica being irregularly distributed within mica aggregates. The proportion of the inter-mediate sodium potassium mica increases as the distance from the sedimentary contactrncreases.

TEM data show that the intermediate sodium potassium mica occurs as chemicallyhomogeneous, thick stacks of layers that give only one set of 00/ reflections, with - 9.8 Aperiodicity in electron diffraction patterns. All micas are two-layer polytypes. Mixed lay-ering of micas is not observed. The X-ray mapping and lattice-fringe images imply thatsome interfaces between Na- and K-rich micas obliquely transect basal planes. Inter-growths ofsubparallel stacks are also present with straight interfaces parallel to the basalplanes of one of the adjacent crystals. Both types of interfaces are common. XRD patternsdisplay a second-order reflection having d: 10.02 A, with prominent tailing at high valuesof 20 and fourth-order reflections of 5.01, 4.93, and 4.82 A, consistent with muscovite,intermediate sodium potassium mica, and paragonite. The peak positions of the inter-mediate sodium potassium mica are nearly identical to those of so-called 6:4 regularmixed-layer paragonite-muscovite reported as occurring in slates from other localities,implying that micas that have XRD patterns lacking superlattice reflections have beenmisidentified.

The data imply that the intermediate sodium potassium mica formed as hydrothermallyderived pseudomorphs after plagioclase microphenocrysts. Subsequent interactions be-tween the intermediate sodium potassium mica and hydrothermal fluids resulted in theredistribution of Na and K in micas and formation of paragonite and muscovite. Thea**/e*^* and a*,/a* ratios of the fluids increased during formation, presumably becauseof interactions with pelites, giving partial replacement of the early formed micas by mus-covite and potassium feldspar. Redistribution of Na and K of the micas occurred in twoways: (l) dissolution of the intermediate sodium potassium mica and crystallization ofmuscovite and paragonite, resulting in lamellar intergrowths of parallel or subparallelpackets of micas, (2) alkali exchange between fluids and micas and diffusion within micaswithout significant dissolution and crystallization, giving rise to inclined irregular inter-faces between micas. The breakdown of the intermediate sodium potassium mica repre-sents a stepwise kinetic approach to equilibrium, analogous to the prograde transition ofmetastable illite to muscovite + pyrophyllite in low-grade rocks.

INtnonucrroN

Paragonite * phengite (or + muscovite) assemblagesare common in anchizonal to epizonal metapelites andin prehnite-pumpellyite to amphibolite or eclogite faciesmetabasic rocks (Guidotti, 1984; Frey, 1987; Shau et al.,1991, and references therein). The existence of a misci-bility gap in the muscovite-paragonite system has been

0003-004x/9 3/0708-078 2$02.00

well documented, based on many natural and experimen-tal paragonite + muscovite pairs. Relations among micaswhose compositions approximately correspond to the pa-ragonite-muscovite join have been utilized as a geother-mometer in spite of uncertainties arising from the pos-sible effects of pressure, interlayer compositions, andphengitic and celadonitic substitutions (Essene, 1989).Maximum solid solutions of paragonite and muscovite

782

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES 783

that have been observed in natural assemblages are ap-proximately MuroParo and MuuoPaoo, typically occurringin single-mica assemblages of high-grade metamorphicrocks (Guidotti, 1984). Apparent compositions interme-diate between paragonite and muscovite may have threepossible origins, however: (l) averaging ofvery fine-scaleintergrowths of paragonite with muscovite (e.9., Shau etal., l99l), (2) averaging of mixedJayer paragonite-mus-covite (Frey, 1969), or (3) a homogeneous metastable sol-id solution, as implied by phase segregations (Livi et al.,I 988) involving diffusion.

The presence of very fine-scale lamellar intergrowthsofparagonite and muscovite (or phengite) has been doc-umented by Ahn et al. (1985) and Shau er al. (1991) withtransmission electron microscopy (TEM). Shau et al.(1991) showed that compositions of sodium potassiummicas consistent with considerable solid solutions are ac-tually due to intergrowths unresolved by electron micro-probe analyses, whereas those that approach Na- or K-richend-members, as analyzed by analytical electron micros-copy (AEM), represent the true limits of solid solution.Based on the d values of mica 00/ reflections in X-raypowder diffraction (XRD) patterns, many authors haveinferred the presence of 6:4 regular mixed-layer parago-nite-muscovite (or paragonite-phengite) in low-grade an-chizonal to low-grade epizonal pelitic rocks (equivalentto prehnite-pumpellyite facies through low greenschist fa-cies for mafic rocks) (for reviews, see Kisch, 1983; Frey,1987). Such materials have never been directly observedor analyzed, however, because they are too fine-grainedfor conventional methods. Livi et al. (1990) observed pa-ragonite that has significant interlayer vacancies in sev-eral low-grade metamorphic slates from the Alps that weredetermined to contain mixed-layer paragonite-muscovitebased on XRD data. They were not able to make directcorrelation between XRD data and TEM observations,however. Livi et al. (1988) showed segregation bound-aries between paragonite and illite in Alpine black shalesthat imply either the prior existence of phases with inter-mediate solid solution between Na- and K-rich micas orreplacement across 00 I layers. Li ( I 99 I ) briefly describedthe occurrence of an intermediate sodium potassium mica(MuuoPaoo) in relation to chlorite-mica stacks in an an-chizonal slate from central Wales. To our knowledge, ho-mogeneous intermediate solid solutions (MuuoPaoo-MuroParo) have not been described in detail.

Dioctahedral sodium potassium micas have recentlybeen identified in the margin of metabasites of Caradocage that intruded a sequence of Ordovician pelitic rocksat Tremadoc, northern Wales, U.K. The micas have widevariations in Na and K contents, ranging from that ofmuscovite to paragonite. XRD patterns show features in-dicative of intermediate sodium potassium mica that aresimilar to those of so-called 6:4 regular mixedJayer pa-ragonite-muscovite, in addition to those of paragonite andmuscovite. Although the micas are relatively coarse-grained compared with those in low-grade rocks, theyexhibit complex intergrowths. TEM and AEM therefore

have been used to characterize them, demonstrating theexistence of metastable intermediate solid solutions andshowing that such material corresponds to much or all ofthe so-called 6:4 regular mixed-layer paragonite-musco-vite described by others.

ExprmnrnNTAL TECHNIeUES

XRD patterns of bulk rock samples were obtained withCuKa radiation at 35 kV and 15 mA on a Philips XRG3 100 X-ray generator. A step size of only 0.005' A2d anda long counting time of -6 s per step were used to resolvethe details of 00/ reflections of micas. Quartz was used asan internal standard. Thin sections of the samples wereexamined with backscattered electron imaging (BEI)techniques on a Hitachi S-570 scanning electron micro-scope operated at 15 kV to determine textural relationsbetween micas and other phases and to select areas forTEM observations. Ion-milled TEM specimens were pre-pared following the method described in Jiang et al.(1990). A Philips CMl2 scanning transmission electronmicroscope was operated at a voltage of 120 kV and abeam current of - l0 pA to obtain TEM images and se-lected-area electron diffraction (SAED) patterns. Crystalswere frequently rotated so as to eliminate the effects ofdynamic diffraction and to determine the absence or pres-ence of mixed layering. Lattice-fringe images were ob-tained with 00/ reflections (d > 4.7 A) included in theobjective aperture. X-ray energy-dispersive spectra wereobtained from thin edges of the specimens using a low-angle Kevex Quantum detector (X-ray take-off angle-34). A raster of -200 x 200 nm in scanning modewas used to minimize alkali diffusion and volatilization.Ion-milled standards of paragonite, muscovite, albite, cli-nochlore, fayalite, rhodonite, and titanite were used inthe thin film approximation (Lorimer and Clif;, 1976) forquantitative analyses of elements Na, Mg, Al, Si, K, Ca,Ti, Mn, and Fe from X-ray energy-dispersive spectra.

OccunnnNcE oF TNTERMEDIATE DIocTAHEDRALSODIUM POTASSIUM MICA

The samples are metabasites collected from the Or-dovician dolerite sills in a suite of metamorphosed pelitesnear Tremadoc, northern Wales. Studies of the regionalgeology suggest that the Ordovician igneous and sedi-mentary rocks in northern Wales have been tectonicallydeformed and affected by prehnite-pumpellyite to lowgreenschist facies metamorphism (or anchizone to epi-zone for pelitic rocks) during the early Devonian Acadianorogeny (Roberts, 1979, l98l; Bevins et al., 1984; Ko-kelaar et al., 1984; Merriman and Roberts, 1985; Bevinsand Merriman, 1988). The petrology of the sampled dol-erite sills has not been studied in detail, but the nearbyTal y Fan metabasite intrusion, emplaced in felsic andmafic tuffs, has similar mineral assemblages and has beeninvestigated by Bevins and Merriman (1988).

The metabasites consist of white micas, chlorite, tita-nite, bytownite, albite, calcite, potassium feldspar, rutile,quartz, and minor actinolite, pyrite, and Cu-bearing sul-

784 JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

Fig. 1. Backscattered electron images of a metabasite fromthe margin of the metabasite intrusion near Tremadoc, northernWales. (a) Aggregates (M; elongated, dark gray) of white micas(mainly muscovite) formed as pseudomorphs after plagioclasemicrophenocrysts, coexisting with potassium feldspar (K; me-dium gray) and chlorite (C) near the sedimentary contact. Scalebar :20 pm. (b) Close-up view of a part of a showing boundaries(inclined to the mica cleavage) that separate muscovite (Mu) andNa-rich mica (M) displaying different contrasts. Scale bar : 40

pm. (c) Partial replacement of white micas (M) by potassiumfeldspar (K) that are crosscut by chlorite (C) veinlets. The matrixis also mainly potassium feldspar. Scale bar : 20 pm. (d) Ag-gregates (ion-milled specimen) of white micas (M) approximate-ly 1 .5 cm from the sedimentary contact showing corroded edgesin contact with chlorite (C) and titanite (T). The matrix containscorroded lathlike bytownite (8, medium gray). Potassium feld-spar is not present. Scale bar : 50 pm.

fides, with textures implying virtually complete replace-ment of primary igneous minerals in the margin of thesills near Tremadoc. In the interior of the dolerite sills,clinopyroxene, actinolite, ilmenite, bytownite, albite, ti-tanite, rutile, chlorite, calcite, prehnite, and epidote-groupminerals are present with relict ophitic textures, locallywith the presence of very small amounts of white micas.

One of the metabasite samples was collected from thelower contact of a dolerite sill approximately I km north-west of Tremadoc (National grid reference SH 554409).The sample contains a relatively high proportion of whitemicas and is the focus of this study. Paragonite, musco-vite, and mica that has compositions intermediate be-tween those of muscovite and paragonite are intergrown,forming aggregates (80-250 pm in length; 15-100 pm inwidth) of randomly oriented crystals (Fig. 1). Such aggre-gates of micas have well-defined outlines that resembletypical euhedral crystals of plagioclase. Concentrations ofmica aggregates are locally present, with or without thepresence of potassium feldspar (e.g., Fig. la). It was notpossible to determine if the third mica is a true inter-mediate solid solution or a mixture of paragonite andmuscovite with BEI techniques, but the TEM results de-scribed below demonstrate the presence of a true inter-mediate sodium potassium mica, and it is therefore re-ferred to as such hereafter. Although some mineralboundaries in mica aggregates are obscured by possibleoverlaps of two or three micas, some of the interfacesappear oblique to the cleavage planes of adjacent micas(Fig. lb).

Muscovite is the dominant phase in mica aggregates.Subordinate paragonite and intermediate sodium potas-sium mica are irregularly distributed in mica aggregates(e.g., Fig. la and lb). The proportion of the intermediatesodium potassium mica increases as the distance fromthe sedimentary contact increases. In some cases, it ap-pears that mica aggregates consist solely of intermediatesodium potassium mica. In samples from the interior ofthe sill, very small amounts of irregular aggregates of mi-cas are present, with ragged contacts with clinozoisite andpartially albitized bytownite.

Potassium feldspar predominates in the fine-grainedmatrix and also occurs as large lathlike or irregular grainsthat partially replace mica aggregates (Fig. lc) in areas< - I cm from the sedimentary contact (designated as thecontact zone). No Na was detected in the potassium feld-spar. Chlorite, albite, q\arlz, and titanite are also majorphases composing the matrix. Fine veinlets of chloritecut across some of the potassium feldspar grains and micaaggregates (e.g., Fig. lc) and extend into the interior ofthe sill subnormal to the sedimentary contact. Mica ag-

$egates commonly display corroded edges in contact withchlorite and titanite outside the contact zone of the meta-basite (Fig. ld), where corroded lathlike bytownite isabundant and potassium feldspar is rarely present in thematrix. The bytownite is surrounded mainly by albite,calcite, chlorite, and quartz. Albitization ofplagioclase isubiquitous in both marginal and interior samples of the

785

metabasites. The albitization is likely a result of regionalmetamorphism in the prehnite-pumpellyite or prehnite-

actinolite facies, with the decomposition of bltownite andformation of calcite and albite. Similar processes were

also inferred to have occurred in the Tal y Fan metaba-sites (Bevins and Merriman, 1988).

X-n-c.v PowDER DIFFRACTIoN

XRD patterns of the bulk rock sample display promi-

nent splitting of the basal reflections of micas, indicatingthe presence of multiple micaceous phases (Fig. 2). Be-cause our electron diffraction data show that these mi-

caceous phases are invariably two-layer polytypes (see

below), XRD patterns were indexed on the basis of a two-layer polytlpe for all of the micas.

The strong 002 peak (d: 10.02 A) displavs a broadhigh-angle tail having a range of'2d, corresponding to dvalues from 10.02 to -9.60 A @ig. 2a). The spacing of10.02 A implies a dominant K-rich micaceous phase, as

consistent with the observation of muscovite in BEI im-

ages. The d(001) values of micas are strongly dependenton interlayer composition (Guidotti, 1984), for example,d = 10.0 A for muscovite and 9.6 A for paragonite. The

spacing ofthe high-angle tail suggests overlap ofthe basalreflections of sodium potassium micas of different inter-layer compositions, e.g., admixtures of intermediate so-

dium potassium mica, paragonite, and muscovite.Three principal fourth-order basal reflections with d:

5.01. 4.93. and 4.82 A were observed (Fig. 2b), implyingthe presence of three dominant micas and possibly minor

amounts of phases of intermediate compositions. Thefourth-order d values of the three micas correspond tomuscovite, intermediate sodium potassium mica, and pa-

ragonite, respectively. Similar features were also ob-served for the much weaker tenth-order basal reflections

@ = 2.00 A). fne sixth- and eighth-order basal reflec-tions overlap the peaks ofother phases (not shown). The

basal reflections of the intermediate sodium potassium

mica. however. are also consistent with the observationsfor so-called 6:4 regular mixed-layer paragonite-musco-vite that has been identified in clay separates of manylow-grade metamorphic pelites, for example, shales orslates from the Alps (Frey, 1970, 1978), northern Wales(Merriman and Roberts, 1985), and the southern Appa-lachians (Weaver and Broekstra, 1984). No first-orderbasal reflection or superlattice reflections (Fig. 2c) wereobserved for the micas, and none has been recorded fromsodium potassium micas elsewhere, except the poorly de-fined reflection with d = 50 A reported by Frey (1969).

ColrposrrroNs oF SoDIUM PoTASSIUM MICAS

Table I shows representative formulae of the inter-mediate sodium potassium mica, paragonite, and mus-covite normalized on the basis of 12 total tetrahedral andoctahedral cations. The analyses presented here were ob-tained only from areas that were characterized by elec-

tron diffraction and lattice-fringe images and shown to besingle phases. All of the micas have nearly the same Al

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

786 JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

OJo

I

ITABLE 1. Representative structural formulae of the intermediate

sodium potassium mica, paragonite, and muscovite inthe margin of the metabasite intrusion near Tremadoc,northern Wales

dvalues in A

1 0 . 0

17 .0 1 7 5 18 .0 18 .5 19 .0

2.5 3.5 4.5 5.5

Degree 20 (CuKcr radiation)

Fig.2. X-ray powder difraction pattern (CuKa radiation) ofthe bulk rock sample of a metabasite collected from the lowercontact of a dolerite sill near Tremadoc, northern Wales, show-ing (a) the second-order basal reflections, (b) the fourth-orderbasal reflections, and (c) the absence of low-29 superlattice re-flections of micas.

Nofe: Normalization is based on 12 tetrahedral and octahedral cationstotal. All Fe is assumed to be Fe,+. Formulae 1-2 : intermediate sodiumpotassium mica, 3-4 : paragonite, 5-6 : muscovite. Two sd on the basisof counting statistics are 0.08-0 1 4 pfu for Si, 0.10-0.1 5 pfu for Al, < 0.01pfu for Ti, Fe, Mg, and Ca in all analyses, and 0.05-0 08 and 0.01-0.03pfu for K and 0 02-0.04 and 0.08-0.12 pfu for Na in muscovite and pa-ragonite, respectively.

and Si compositions and minor Mg and Fe contents. Theintermediate sodium potassium mica has Na/K atomicratios spanning the range from 0.5 to 2 (not shown).However, most analyses of the intermediate sodium po-tassium mica have nearly equal amounts of interlayer Naand K, as typified by those shown in Table l. The micasthat do not have atomic ratios of Na/K = I commonlyoccur in areas with irregular interfaces inclined to micabasal planes. In stacks of parallel or subparallel packetsof micas, each packet is homogeneous in composition,consisting solely of intermediate sodium potassium mica(Na/K = 1), paragonite, or muscovite. The compositionsof paragonite or muscovite vary slightly in alkali and Cacontents from crystal to crystal but are homogeneouswithin each individual crystal. Most of the paragoniteanalyses give Na = 1.80, K < 0. 15, and Ca < 0.05 pfu,but some with Ca as high as -0.20 pfu were also ob-tained. Most of the muscovite compositions have Na/Kratios nearly equal to l/7 with or without minor Ca con-tents.

Tru.NsnrrssroN ELECTRON MlcRoscopEOBSERVATIONS

Aggregates of micas such as those shown in the BEIimages were examined by TEM to aftrm the identifica-tion of the intermediate sodium potassium mica and todetermine the intergrowth relations between differentmrcas.

Intermediate sodium potassium mica

The intermediate sodium potassium mica commonlyoccurs as thick stacks that are occasionally interleavedwith parallel thin packets of paragonite or muscovite. In-terstratification of thin packets of layers within thickpackets of muscovite is also present. Figure 3 shows alattice-fringe image and SAED pattern of the intermedi-ate sodium potassium mica. The mica is relatively easilydamaged by the electron beam, compared with musco-

6.085.800.020.070.0300.241.75

q i ( o 7

A r 5 9 3Ti 0Fe 0.06Mg 0.04C a 0 0 4Na 1 .03K 0.93

f.-

I6.035.860.020.060 0 301 0 30.97

6 1 05.8500.0500.041.750 1 2

5 9 06.0200.06o.020 0 61 . 8 10.13

6.04s.8500.070.040.020.231.73

1 . 5

(c)

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES't87

x

Fig. 3. (a) Lattice-fringe image of intermediate sodium po-tassium mica displaying lenticular pores typical of Na-rich whitemicas; (b) electron diffraction pattern of a showing sharp 00/reflections that are consistent with a single mica.

vite, giving rise to lenticular pores and a slight distortionof layers. The SAED pattern shows that the 00/reflectionshave slight spreading normal to c* but are sharp and haveno splitting along c*, suggesting the presence of severalcrystals of a single micaceous phase with slightly differentorientations. Where the intermediate sodium potassiummica is directly adjacent to muscovite or paragonite, elec-tron diffraction patterns display split pairs of 00/ reflec-tions, with the split distance approximately half of the

separation between corresponding 00/ reflections of pa-

ragonite and muscovite (e.g., Fig. 4), implying a period-

icity of -9.8 A along c*, which is consistent with theXRD results.

Intergrowth relations

Two principal types of interfaces between micas wereobserved in the sample: (1) straight interfaces that areparallel to basal planes (parallel interfaces), and (2) irreg-

Fig. 4. Electron diffraction patterns of (a) muscovite + pa-ragonite and (b) muscovite + intermediate sodium potasslummica showing the differences in the separations in therr corre-sponding 00/ reflections.

ular interfaces that are inclined to the basal planes ofparallel or subparallel packets ofadjacent micas (inclined

interfaces).Parallel interfaces. Figure 5 shows a TEM image of

coexisting muscovite and intermediate sodium potassr-

um mica with parallel or subparallel basal planes' The

interfaces are parallel to the basal planes of one or both

of the adjacent micas. The SAED pattern clearly displays

split pairs of00/ reflections corresponding to two sets of

bisafreflections with periodicities of - 10.0 and -9.8 A,

respectively. The 02/ reflections have periodicities of-20.0 and -19.6 A along c* for the respective phases,

suggesting that the intermediate sodium potassium mica

and muscovite are two-layer polytypes. No other micapolytypes or mixed layering were observed in the sample.

Similar relations also occur between paragonite and mus-

covite, analogous to those observed by Ahn et al. (1985)

and Shau et al. (1991).Inclined interfaces. Figure 6 illustrates interfaces that

obliquely transect the basal planes of adjacent domains

of paragonite and muscovite. Most lattice fringes are con-

tinuous across the interfaces. Terminations of layers and

strain contrast commonly occur at such interfaces, im-plying partial coherency. Figure 7 shows energy-disper-sive X-ray maps of elements Na and K of an intergrowth

of micas, demonstrating mineral boundaries that are sub-

normal to the cleavage and separate K-rich and Na-rich

areas. Quantitative analyses and electron difraction data(not shown) suggest that the micas are muscovite andparagonite, respectively, with compositions similar to

those listed in Table 1. Such relations are not observed

between intermediate sodium potassium mica (Na/K =

l) and paragonite or muscovite. However, some K- or

Na-rich micas that have slightly more Na or K than the

muscovite or paragonite are observed to have inclined

irregular interfaces in relation to paragonite or muscovite'

Figure 8 shows interfaces that are inclined to the basalplanes of muscovite and relatively Na-rich muscovite

20nm

60nm

788 JIANG AND PEACOR:LAYER SILICATES FROM ALTERED METABASITES

Fig. 5. Transmission electron microscope image of interstratified packets of muscovite (Mu) and intermediate sodium potassiummica (M). The interfaces are parallel to the basal planes of the micas. The splitting of the 00/ reflections along c* of the inset SAEDpattern reflects the differences in d values. Both of the micas are two-layer polytypes, as suggested by the - 20-A periodicity of the02/ reflections.

(MurrParr). Portions of the interfaces are defined by lineswith light contrast, but with no apparent strain contrast,probably simply reflecting a slight inclination of phaseboundaries from 90" to the imaged plane. A layer ter-mination (upper middle) and a parallel interface (lowermiddle) are also present. Although the difference in thespacings of different mica layers is very small, the SAEDpattern clearly shows a slight splitting of the 00/ reflec-tions. The split distance corresponds to a difference of-0. I A between the periodicities of the micas in the c*direction. The I 1/ (or lT/y reflections suggest that bothof the micas are two-layer polytypes.

Drscussrolq

Hydrothermal formation of intermediate sodiumpotassium mica

The similarity between the cross sections of mica ag-gregates and the morphology of euhedral plagioclase crys-tals suggests that the mica aggregates are pseudomorphsafter plagioclase microphenocrysts. The Na may have beenderived from the decomposition of plagioclase, but such

a reaction requires significant K input. The fact that themicas and potassium feldspar are concentrated in themargin of the dolerite sill suggests that Kt must havebeen leached out of the pelitic sedimentary rocks andtransported by the convecting fluids during emplacement.Alt and Honnorez (1984) and Bdhlke et al. (1984) re-ported the occurrence of potassium feldspar in responseto low-temperature reactions with sea water in alteredoceanic mafic rocks. Bevins and Merriman (1988) sug-gested that potassium feldspar is an early hydrothermalalteration product of plagioclase microphenocrysts in thecontact zone of the Tal y Fan metabasite. Sodium potas-sium micas were not reported in that study. Nevertheless,those authors demonstrated that K-rich phases may oc-cur in altered mafic rocks when affected by fluids of ap-propriate composition. The micas in the studied meta-basites are therefore inferred to be a result of reactionswith hydrothermal fluids that interacted with peliticcountry rocks and had relatively low 4.,o, and higha*,/e* compared with the Tal y Fan intrusion, whichwas emplaced within volcanic tufs. Such replacement offeldspars by micas is common in hydrothermal or deu-

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

Fig. 6. Lattice-fringe image and electron diffraction pattern of coexisting paragonite (Pa) and muscovite (Mu). Paragonite andmuscovite layers are continuous across the inclined interfaces (highlighted with black arrows). The interfaces are associated withstrain contrast and layer terminations (marked by white arrows).

789

teric alteration of granitic and pegmatitic igneous rocks(London and Burt, 1982; Monier et al., 1984; Speer eta I . , 1 9 8 0 ) .

The presence of wide variations in mica compositionsmay be attributed to the superimposed effects of differenthydrothermal events or to changing conditions ofa singlehydrothermal event. The intermediate sodium potassiummica is inferred to have formed during early interactionsbetween K-rich hydrothermal fluids and plagioclase,perhaps metastably at temperatures below the -550 "Crequired for equil ibrium conditions. The well-con-strained occurrence of white micas as pseudomorphs ofplagioclase and the high Na contents of the white micassuggest preferential alteration of plagioclase microphen-ocrysts in early stages of hydrothermal alteration. Super-saturation with respect to the intermediate sodium po-tassium mica may have occurred because of abruptchanges of fluid chemistry and temperatures during theemplacement and rapid cooling of the dolerite intrusion.Speer et al. (1980) studied igneous and secondary mus-covite in granitoid plutons of the southern Appalachianpiedmont. The secondary muscovite that occurred as

saussuritization products of plagioclase contains more Nathan the hydrothermal and magmatic muscovite in thematrix. The composition of that secondary muscovite isapparently affected by the composition of parent plagio-clase. Similar relations may also prevail in the meta-basites at Tremadoc; i.e., local composition as modifiedby hydrothermal fluids, rather than bulk rock composi-tion, controls the composition of white micas.

Metastability of intermediate sodium potassium mica inrelation to the forrnation of paragonite and muscovite

The solvus between paragonite and muscovite has beendocumented by many authors using experimental andnatural paragonite-muscovite pairs (for reviews, see Gui-dotti. 1984). The solvus closes with increasing tempera-ture and is asymmetric toward paragonite (greater solidsolution of Na in muscovite than of K in paragonite) butis truncated by the formation of alkali feldspars and alu-minosilicates at high temperatures. The paragonite fieldbecomes smaller, and the miscibility of paragonite inmuscovite decreases above the temperature at which al-kali feldspar first appears. In rocks, sodium potassium

790 JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

Frg. 7. X-ray maps of Na and K from a paragonite-musco-vite intergrowth displaying interfaces oblique to the cleavage;the {00 I } cleavage direction is indicated by black arrows. Framewidth: 3 um.

micas that have significant solid solutions occur only insingle-mica assemblages that have been buffered by alkalifeldspars, but the mica compositions are severely limitedby the solvus. These relations collectively show that theintermediate sodium potassium mica in the Tremadocintrusion is metastable or unstable with respect to parag-onite + muscovite.

As consistent with the Ostwald step rule, metastablephases commonly form prior to the formation of stablephases, especially in short-lived and low-temperature al-teration or formation processes. The mineralogical, tex-tural, and chemical heterogeneity in the studied samplesclearly reflects a lack of equilibrium wherein the inter-mediate sodium potassium mica is a metastable precur-sor of muscovite * paragonite. The intimate inter-growths of muscovite, paragonite, and intermediatesodium potassium mica suggest that simultaneous pri-mary formation of the micas due, say, to local variationsin composition and kinetic factors did not occur. Instead,the inclined interfaces between muscovite and paragoniteimply secondary interactions between fluids and micasand diffusion within some originally homogeneous micas,giving rise to the formation of paragonite + muscovite,as discussed below. In addition, the replacement of the

micas by potassium feldspar and the dominance of mus-covite near the sedimentary contact imply an increase ofK activity, consistent with muscovite being a later phaseformed during the same or a second alteration event. Thecompositions of the coexisting paragonite and muscovitedo not represent a paragenetic pair of Na- and K-richmicas that formed under equilibrium conditions. Thatimplies that most of the muscovite has been formed bythe alteration of intermediate sodium potassium mica orthat the alteration was not homogeneous but involvedpartial replacement and local chemistry.

Although we do not have data regarding the specificcompositions and temperature of hydrothermal fluids, thegeneral trend ofthe changes in fluids can be inferred. TheaK*/ aH* and e** / a*^, ratios of the hydrothermal fluids arelikely to have increased in the early stages of hydrother-mal activity because of increasing reaction with the pe-litic country rocks at high temperatures. Such conditionsresulted in partial decomposition of the intermediate so-dium potassium mica, forming muscovite and parago-nite. The low proportion of intermediate sodium potas-sium mica, the high proportion of muscovite in the contactzone, and the irregular distribution of micas in mica ag-gregates are consistent with such a process. In addition,the amount of muscovite is far in excess of that requiredto combine with the available paragonite to form the in-termediate sodium potassium mica; a source of K istherefore necessary in excess ofthat required for the in-termediate sodium potassium mica. The progressive in-crease of a*,/a* and a**/a*^, in hydrothermal fluids isalso evidenced by the presence of potassium feldspar inthe contact zone. Partial replacement of the micas by po-tassium feldspar in later stages ofhydrothermal alterationis consistent with the hypothesis. Such chemical gradientsmay have been in part due to different degrees ofphysicalinfiltration of fluids at different distances from the sedi-mentary contact.

Alkali redistribution of sodium potassium mica

There are two types of intergrowths among the micasin the metabasites: (l) lamellar intergrowths of parallelor subparallel packets of micas with interfaces that areparallel to one of the adjacent crystals, and (2) domainsof micas with interfaces obliquely transecting their largelycontinuous basal planes. These two textures are inferedto have formed through dissolution and crystallizationand alkali exchange and diffi.rsion, respectively, each ofwhich is discussed below.

Dissolution and crystallization. Lamellar intergrowthsofparallel or subparallel packets ofphyllosilicates are verycommon in low-grade metamorphic and hydrothermallyaltered rocks (e.g., Franceschelli et al., 1986; Jiang andPeacor, l99l; Shau et al., l99l). In such rocks, dissolu-tion and crystallization processes prevail in part becausesolid-state ditrusion is sluggish and because the compo-sitional changes of minerals require significant destructionand reconstruction of bonds. Dissolution and crystalli-zation processes are particularly important in low-tem-

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES 7 9 1

perature hydrothermal alteration of phyllosilicates be-cause of high HrO activit ies, which promote thereplacement of individual layers or packets of layers ofone phyllosilicate by another (e.g., Jiang and Peacor, I 99 l;Sharp et a1., 1990). Development of lamellar intergrowthsof micas by solid-state diffusion requires the transport oflarge alkali ions across the layer structures ofthe micas.Such a process is highly unlikely relative to diffusion alonglayers; it has never been observed, whereas replacementrelations along layers are common. In addition, solid-state diffusion implies the retention of original structuresand at least partial coherency between resultant phases,which generates significant strain for interfaces parallel tobasal planes (Veblen, 1983). Each mica packet in the la-mellar mica intergrowths is homogeneous, with interfacesthat are free ofstrain contrast, except for lenticular porescaused by beam damage. Furthermore, packets of mus-covite or paragonite are directly adjacent to or interstrati-fied with packets of intermediate sodium potassium mica.These data collectively imply that alteration of an origi-

nally homogeneous mica proceeded in part through dis-solution, with the subsequent crystallization of parago-nite and muscovite. As the period of alteration was short-lived, rapidly changing nonequilibrium conditions areinferred to have given rise to incomplete reaction, withsome original homogeneous Na-K packets being unaf-fected.

Alkali exchange and diffusion. As discussed above, dis-solution and crystallization processes would generate Ia-mellar intergrowths with straight interfaces parallel tobasal planes and with minimal structural coherency. Themica packets that have interfaces inclined to basal planeshave individual layers that are largely continuous acrossthe interfaces, with some layer terminations with or with-out strain contrast. They must therefore have formed bya different process.

The occurrence of inclined interfaces that separate twomicas having layers that are largely continuous across theinterface is consistent with the diffusion of alkali ionsalong layers of an originally homogeneous sodium potas-

Fig. 8. Lattice-fringe image of domains of muscovite (Mu) and relatively Na-rich muscovite (M). The curved interfaces (indicated

in part by black arrows) are subnormal to the basal planes. A layer termination is present in the upper middle (marked with a whitearrow). The electron diffraction pattern shows two sets of 00/ reflections corresponding to - 10.0 and -9.9 A periodicities. The I 1/(or 11/) reflections imply that the micas are two-layer polytypes.

792 JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES

sium mica. The presence of wide variations in interlayerNa and K contents with compositions for octahedral andtetrahedral cations that are equal within error is consis-tent with such a mechanism. Inclined interfaces were alsoobserved to occur between paragonite and illite in meta-morphosed shales from the Swiss Alps (Livi et al., 1988,1990), but no firm conclusions were reached regardingtheir origin. There are two reasons why diffusion in theTremadoc micas is inferred to have been driven by in-teraction with Na-K fluids rather than by simple exso-Iution: (l) No regular patterns of phase or compositiondistribution such as those observed by Veblen (1983) forexsolution ofwonesite to talc and sodium biotite are pres-ent, although they are normally observed for products ofexsolution in all such cases. (2) The compositions and thelarge differences in the amounts of paragonite and mus-covite within individual stacks are incompatible withexsolution from the intermediate sodium potassium mica.K input is required for the development of muscoviteplus subordinate nonparagenetic paragonite from initiallyhomogeneous intermediate sodium potassium mica.

Overgrowth mechanisms have been reported to occurfor phyllosilicates with inclined interfaces, e.g., antigo-rite-talc interfaces observed by Worden et al. (1991). Suchinterfaces commonly are straight and occur between sim-ple, homogeneous micas with rim-core relations. The ir-regular patterns of distribution of Tremadoc mica aggre-gates are not compatible with overgrowth, especially inthe case of the curved, inclined mica interfaces.

Interrnediate sodium potassium mica vs. mixed-layerparagonite-muscovite

This study demonstrates the existence of metastable,homogeneous intermediate sodium potassium mica. Thismica gave XRD patterns nearly identical to those of so-called 6:4 regular mixed-layer paragonite-muscovite (orparagonite-phengite) (Frey, 1969), except for the absenceof a reflection of d = 50 A. Although the identificationof 6:4 regular mixedJayer paragonite-phengite based ona very ill-defined low 20 reflection is questionable, manyresearchers have reported the presence ofmixed-layer pa-ragonite-muscovite in low-grade metapelites, based onone or two characteristic peaks that include intermediatebasal reflections but no superlattice reflections (for ref-erences, see Kisch, 1983; Frey, 1987). However, the in-termediate mica of this study satisfactorily is consistentwith indexing ofall peaks in all such published patterns.

Identification ofan ordered interlayered sodium potas-sium mica is based in part on the assumption that thealternative phase, homogeneous intermediate sodium po-tassium mica, is not thermodynamically stable in low-grade metamorphic rocks. However, Li (1991) recentlyhas also observed homogeneous intermediate sodium po-tassium mica (MuuoPaoo) in a low-grade slate from centralWales; it is inferred to have originated from smectite anddeveloped in early stages ofdiagenesis through anchizon-al metamorphism (Li et al., 1992), discrete paragoniteand muscovite occurring at higher grades, analogous to

muscovite and paragonite in pelites from northern Walesreported by Merriman and Roberts (1985). The occur-rence of intermediate sodium potassium mica demon-strates the existence of this phase even in low-grademetapelites occurring in ordinary prograde sequences. In-deed, it may be an early metastable phase in a stepwisekinetic sequence, much as illite and mixed-layer illite/smectite (Jiang et al., 1990) precede muscovite in pro-grade sequences.

We therefore conclude that many of the published XRDpatterns of so-called mixed-layer paragonite-muscoviteactually correspond to homogeneous sodium potassiummica of intermediate composition or random mixed-lay-er paragonite-muscovite. Unless well-defined superlatticereflections are observed in XRD or SAED patterns, asindicative of ordering of Na and K at the scale of indi-vidual layers, reports of such ordering should be viewedwith caution.

Acxuowr,BncMENTS

We are grateful to R.J. Merriman of the British Geological Survey andB. Roberts of the Birkbeck College for their aid in collecting samples andfor helpful discussions. We also acknowledge M. Frey and K.J.T. Livi fortheir critical reviews This work was supported by grants EAR-88-17080and EAR-91-04565 to D.R.P from the National Science Foundation. Thescanning transmission electron microscope and scanning electron micro-scope used in this work were acquired under NSF grants EAR-87-08276and BSR-83-14092, respectively. Contribution no. 495 from the Miner-alogical Laboratory, Department of Geological Sciences, The Universityof Michigan, Ann Arbor, Michigan 48109-1063.

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MnNuscnrn RECETvED Julte 5, 1992Meuuscnrpr ACCEPTED MmcH 25. 1993

JIANG AND PEACOR: LAYER SILICATES FROM ALTERED METABASITES