American Mineralogist, Volume 71, pages 343-355, 1986

Lamellar and patchy intergrowths in :Experimental crystallization of eutectic silicates

JoN S. PrtnnsnN,I Glnv E. LorcnnN NASA JohnsonSpace Center SN4, Houston, Texas 77058

Ansrucr Coupled and noncoupled eutecticfeldspar intergrowths have been producedby dynamic crystallization experimentsin the ternary -water system.Fractionation during pla- gioclasecrystallization in nearly all compositions examined resultsin eutecticgrowth when a solute-enrichedboundary layer becomessupersaturated with K-feldspar. The growth rate of the eutectic product generally exceedsthat of monophaseefowth by a factor of l0 or higher and is an effective means of reducing thermal supercooling.Coupled intergrowths have lamellar structures that are normal to a planar, nonfaceted, solidJiquid interface. They are developedpredominantly in the {010} crystallographicplane of the host feldspar. The composite crystalline product is usually of-eutectic in composition. Variations in composition occur systematically along the growth direction and across single facets in responseto local changesin supercooling.Different crystallographicsectors show systematic compositional differencesdue to variable nucleation and growth kinetics, which often lead to hourglasspatterns superimposedon the intrgrowth structure.Uncoupled eutecticgrowth occursin the more K-rich melts. It is a result of the preferential groWh of sanidine in the crystallographica and c directions and simultaneous,coupled-lamellar oligoclase-sanidine intergrowth in the crystallographicb direction. This processleads to a patchy intergrowth with coarse,irregular phasedistributions and facetedinterfaces. The melt-grown composites have textures strikingly similar to feldspar intergrowths found in cumulus feldsparsof the Larvikite monzonite suite in the Oslo igneous province, and this similarity suggeststhat eutectic growth from near-cotectic melts may account for certain coarse feldspar inter- growths.

INtnoouctroN growth of silicatesand the high activation eneryiesasso- Intergrowth textures in igneous rocks, such as - ciated with nucleation (Swanson, 1977 lnfge4 1980). feldspar intergrowths in granophyre and graphic , Eutecticintergrowth ofquartz and feldsparare encouraged are often compared with eutectic crystallization products by low values of melt entropies for quartz, which reduce in metallic systems.These comparisons have led to the tendenciesfor facet formation (Jackson, 1958), but are conclusion that certain intergrowth textures form by si- complicatedby largedifferences in crystallographicstruc- multaneous crystallization from the melt (Barker, 19701, ture. In spite of the difficulties, limited successesin grow- Hughes,1972; Hibbard, I 979; Carstens,I 983).The growth ing these textures from the melt have been reported by mechanismsand the variable compositions of the inter- Swanson(1976) and Fenn (1979).High-temperature feld- growths, however, are not well understood. sparshave high melt entropiesbut sharea common crys- Eutectic crystallization has been studied in great detail tallographic structure,which allows an epitaxial relation- in metallic systems.Experimental studies in the last two ship between the constituent parts of an intergrowth decadeshave shown not only that eutectic-crystallization structureand thus a lower nucleationbarrier. Intergrowths products display great variability in growth properties and of two feldsparstherefore should occur more readily than resultingtextures (Chalmers, 1 964, chapter6;Elhott, 1977) those ofquartz and feldspar. but, more importantly, that off-eutecticcompositions are Convincing experimental evidence for simultaneous common as crystalline products and result from kineti- crystallization of two feldspars from the melt was first cally controlled growth. It is possible to vary the oFeu- reported by Lofgren and Gooley (1977). While the sug- tectic composition by controlling the growth conditions gestionthat compositefeldspars could grow from the melt (Mollard and Flemings, 1967a, 1967b; Flemings, 1974, was not entirely new (seereview by Smith, 1974, chapter chapters4, 6). l9), this experimental evidencebrought about some de- Experimental studies of eutectic crystallization in sili- bate as to its primary nature (Morse and Lofgen, 1978). cate melts are difrcult becauseof the strongly anisotropic Subsequentstudies have shown that simultaneous,two- feldspar crystallization occurs spontaneouslyin a wide I Presentaddress: Aarhus University, Aarhus, Denmark. rangeof ternary-feldsparcompositions at moderate cool- 0003-004X/86/03044343$02.00 343 344 PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS

Table l. Experimental run conditions, starting compositions, and descriptions of charges

SAMPLE F1 F7 Fll F10 F9 F8

An LZ 20 37 30 19 1t Ab 68 60 45 45 45 45 0r 20 20 18 ?5 36 44 wt% H20 4.0 6.0 10.0 8.0 6.0 4.0

Run No. 601 602 606 605 604 603 Type: Plagi oc'lase Plagioclase Plagioclase Plagioclase gtass Shape: Branching Branching Skeletal Skeleta l Skeletal (8750C) needles needles tabular pri snatic

Run No. 577 578 58? 581 580 579 TYPe: Plagioclase Plagioclase Plagioclase P'lagioc l ase Plagioclase Composite Shape: Eranching Branching Skeletal Prismatic + composi te Fan- (8000c) pl ates needles tabular spheruli tic

Run No. 6?5 626 630 629 628 627 TYPe: Plagioclase Plagioclase P'lagioc'lase Plagioclase Composite'l Composite Shape: Branching Praty Tabular Skeletal L ame'l a r Lame'lI ar (7400c) platy prt sms & patchy & patc hy

Run No. 589 590 594 593 59? 591 Type: Plagioclase Plagioclase Plagioclase Composite Composi'l te Composite Shape: pt aty Skeletal + composi te Lamell ar L ame'l a r Coarse (7000c) f an aggr. Plates enos & i rreg, & patchy patchy

Run No. 635 639 638 637 636 TYPe: Composite Composi te Composite Composite Conposite Shape: Lamel I ar LamelI ar LamelIar Lamellar Patchy (6500C) en0s al1 sides & i rreg. E patchy fan aggr.

Run No. 565 566 570 568 567 TYPe: Plagioc'lase Composite Composite Composite Composite Shape: Coarse Lamellar LamelI ar Lanellar Coarse (6000c) fan aggr. platy all sides & i rreg. patc hy

Run No 613 614 618 617 616 615 Type: Plagioclase Composite Conposite Composi te Composite Composite Shape: Pl aty LamelI ar LanelIar Lanellar LamelI ar Coarse (5000c) Dri snatic pri snatic tabular patc hy

ing rates, and the process must be considered a potential atures of starting materials and the approximate cotectic tem- origin for intergrowths of certain types of feldspars as well peratrue. The experiments in this study were not all water sat- as other silicate eutectic compositions. urated (at leastinitially), and the crystal-liquid systemsfractionated The purpose ofthis paper is to describe textural features continuously during the run so that the little we know of the phaserelations have only modest application. and mineral structures not observed in the earlier study A slow (for experimental studies),constant cooling rate of 2'C/h of Lofgen and Gooley ( I 977) and to present growth mech- was chosenin order to avoid the growth of highly branching or anisms for the composite intergrowths. skeletal crystals. In order to better examine the formation and evolution of the composite intergowth, the continuous-cooling ExpenrrvrrNTAl TEcHNTeuES experimentswere interupted at successivestages ofprogressive Ternary-feldspar starting materials were prepared by the gel solidification. The successivequench temperaturesand textural method (Luth and Ingamells, 1965),and their compositions are characterofeach run are given in Table l. listed in Table 1. Approximately 70 mg ofgel wascombined with Mineral compositionswere determined with an automatedCa- 4-10 utto/otriply distilled HrO and sealedin a platinum capsule. meca three-spectrometerelectron microprobe. The microprobe The capsuleswere weighed after the experimentto checkfor leaks. was operatedat 15 kV. A 2-3 pm-diameter beam with a sample The runs were completed in internally heated pressurevessels current of20 pA was applied to the feldspars,while the glassdata (Holloway, I 97 1). The chargeswere first melted (6-24 h), which were obtained by continuous rasteringat 1.6 magnification and producesa uniform liquid. The temperaturewas decreasedeither 15 pA samplecurrent, in order to avoid excessiveNa loss during at the linear cooling rate of 2'Clh or in 50"C steps, with the the measurements.Counting times were 3 x 30 s, and the anal- temperature being maintained at each step for 2-3 d. Pressure yses were calibrated against mineral standards.The technique was approximately 6 kbar during melting, but decreasedduring for analyzing the fine-scaleintergrowths is describedby Lofgen cooling according to the PW relations at the constant volume and Gooley(1977). ofthe pressurevessel to as low as 3 kbar for runs quenchedat 400"C. Thermocoupleswere calibrated againstthe melting tem- ExpnnnmNTAL RESULTS peratureofgold. Polished thin sectionsofthe run products were Microstructures preparedfor textural and chemical studies. The equilibrium phaserelations for the ternary-feldsparsystem The shapesof the feldspar crystals in the products of are not well known. The experimentsofYoder et al. (1957) give crystallization correspondbroadly to the generalpatterns the general form ofthe water-saturated liquidus surfaceat 5 kbar. previously described (Lofgren, 1974, 1980). Albite-rich These data provide a generalguide to predict liquidus temper- plagioclasesare predominantly acicular with frequent low- PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS 345

3OO um

Fig. l Lamellar-intergrouth texture along the rim of a normally zoned plagioclasecore. The inner, compositezone is dominated by sanidine (dark gray), and the margin, by albitic plagioclase0ight eray). angle branching, whereas the more Ca-rich creasingly wider outward and grade into an albite-rich are usually platy or tabular and the more K-rich feldspar plagioclaserim with planar and facetedcrystal interfaces compositions tend to have fan-spherulitic shapes.The (Fig. 2). More rarely, the compositional variation termi- individual crystal shape,however, changescontinuously nates with a K-rich composition. By contrast, the solid- during progressivesolidification as the cores ofinitially, liquid interface of the composite product is often ragged highly skeletalcrystals generally become filled. The irreg- in detail, although it appearsmacroscopically planar. ular externalfaces of most samplesend up as broad facets In addition to the systematiccompositional changein (seealso Lofgren, 1980,Fig. 8) in a processthat may be the growth direction, a complete textural variation from referred to as crystallization ripening. Two or three ini- monophaseplagioclase through intermediatesections vrith tially skeletal crystals occasionally fill the entire charge near-equal proportions of oligoclase and sanidine to and produce a complex interwoven texture with patchy monophasesanidine is occasionallyobserved across single extinction. crystal faces.This variation shows that the growth con- The microstructures describedbelow are mainly from ditions also differ along single growth planes (Fig. 3). the continuous-coolingexperiments, but are equivalent to The influence of crystallography on the growth mor- intergrowth textures in the mainly step-cooled experi- phology is revealedby the diferent textural characteristics ments described earlier by Lofgren and Gooley (1977). of the lamellar intergrowths in different growth sectors. Two particularly distinct types of intergrowth structures In the {010} plane the microstructure is usually regular are recognized.The most common is a fine-grained la- and consistsof oligoclaseand sanidine in lamellar or rod- mellar tlpe comprising l-1O-pm-wide lamellae of albite shapedintergrowths. The solidJiquid interface is planar, and sanidine, oriented in a parallel fashion roughly per- pendicular to the crystal-liquid interface. The other type consistsofcoarse, irregular patchesofoligoclase in a pre- dominantly K-feldspathic host. These patchesare on the order of 20-100 pm wide, substantially coarserthan the lamellar type. There is no gradation between these two types,and they often occur in different sectorsof a single crystal. Lamellar-intergrowth shuctures. Plagioclasecrystals in most solidification runs typically have normally zoned coreswith a composite rim of lamellar intergrowth (Fig. l). The lamellae consist of albitic plagioclasein a host sanidine,although often the lamellaeand host are of com- parable size and proportions. The transition from core plagioclaseto sanidinerim is typically discontinuous.The oligoclaselamellae do not have roots in the plagioclase core, but appearwithin the sanidine some few microme- (Fig. ters from the transition interface l). Fig.2. Continuous cooling experimentsshow a marked, dis- Another characteristic feature of the lamellar inter- continuous appearanceof sanidine (dark gray) at the plagioclase growth is the compositional variation toward one end interface and gradually diminishing amounts outward at the ex- member along the erowth direction. Narrow albite la- penseof plagioclase.Transverse banding is the result of minor mellae in the inner, sanidine-rich product become in- growth disturbances. 346 PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS

Fig.4. Patchyintergrowths become increasingly coarse in the growthdirection. Dark gray is sanidine;light gray, oligoclase with Fig. 3. Gradualcompositional changes along single crystal minor amountsof K-feldspar.Note colony structure and feather facet.Sanidine is dark gray;oligoclase, light gray. pattern. though the lamellae are often not exactly perpendicular usually at low anglesto major crystallographicplanes or to the plagioclasesubstrate but deflectedat a finite angle. facetswhen such are developed(Fig. a). In the {001} plane, on the other hand, the structuresare much more irregular, and the ratio of the two constituent Mineral compositions phaseshighly variable. The solid-liquid interface of this Results of microprobe analysesof the synthetic feld- type is usually nonplanar, and the secondphase may be sparsare presentedin Table 2. The compositional range rod shaped,vermicular, irregular, or platy. ofrepresentative, single feldsparsin each group is given Patchy-intergrorvth structures. Patchy intergrowths- togetherwith the co-existingglass, represented by the av- which seem to be confined to the most K-rich starting erageof5-10 points. The totals ofthe glassanalyses are materials, F-8 and F-9-are characterizedby a substan- low, about 96 volo/o,owing to dissolved water (analyses tially coarser grain size and irregular appearance.Indi- in Table 2 have beenrecalculated to 1000/ofor comparison vidual patchesmeasure from l0 to over 100 pm across. with starting material). Presenceof free vapor bubbles The product is dominated by K-feldspar over plagioclase indicate that most nrns were water saturated when the in a ratio of about 2:l or higher. Feldsparswith patchy experimentswere terminated. intergrowthsusually form a characteristiccolony structure Plagioclasesin the most An-rich composition (F-ll) Gig. a). Individual crystalsare aligned in parallel bundles show the widest rangeof compositional zoning before the that tend to expand in width along the growth direction appearanceof compositegrowth (Fig. 6a).The initial feld- and produce a radiating rosette structure consisting of spar (crystal core) is , about Anrr. In the ex- numerous individual crystal subgrains.Because of their periments cooled down to 740"C,the crystal-melt system simultaneousformation, these subgrain aggregatesoften has fractionated so that the rim is andesine(Anor) and at form a compact cellular structure with a curved growth 700t, oligoclase(Anro). In this range,the orthoclasecom- structure projecting toward the most nourishing residual ponent increasesfrom less than I to 5 mol0/0,which is melt (Fig. 5). Only the ends of the parallel feldsparsare within the solubility rangeof plagioclasefeldspars (Smith, exposedto unhindered growth and form well-developed 1974). At 650qC,the plagioclaserim compositions are facets,unlike the lamellar composites.The nonorthogonal about Anrr, but here some sectorshave composite rims appearanceofthe facetsshows that the facetingplanes are with an outer margin of about AnrAbrrOr,r. Becauseof {001}-{l0l} or {ll0}-{110}, thus following crystallo- the systematiccompositional variation along the growth graphic a and c directions (Fie. a). direction, the overall composition of the composite rim Within the crystal subgrainsare two feldspar compo- is lessOr-rich than the innermostcomposite section, which nents intergrown in a patchy structure. The central stem may be as high as AnrAbroOru, (Fig. 6b). Average melt is usually dominated by sanidine,whereas plagioclase ap- compositions at the time of appearanceof composite pearsnear the margins. Becauseofthe expanding nature growth correspondsto An,rAborOror. of the growth, the plagioclasepatches project toward the Initial plagioclase compositions in F-10 are roughly central stem and create a featherlike structure (Fig. 3). identical to those ofF-l I at 875"C, as is the overall range Mutual, internal boundaries between the component of zoning. At740C, the feldsparsin the two chargesare phasesin this type ofintergrowth are irregular and non- virtually identical, whereasin F-10 at 700"C a composite crystallographic.The elongation of individual patchesis margin forms on the oligoclaserim. This margin is ini- PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS 347

r;-!- .:

Fig. 5. Preferentialgrowth along the horizontal axis ofpatchy products;continuous widening ofindividual coloniesforces growth away from the principal growth plane in a fan-spherulitic manner. Back-scatteredelectron image. tially K-rich with an averageabout AnrAb,Orru, which cotectic boundary at 5 kbar according to Yoder et al. grades outward to a more intermediate composition of (1957). This occurs becausesolute accumulatesin the AnrAbooOrr,. In the subsequent650'C sample, the rim boundary layer at the liquid-solid interface which pro- has fractionated to An1?AbrrOrr,. The melt composition duces local conditions favorable for sanidine nucleation at the time the compositerim forms has a An:Ab:Or ratio of l6:36:48, roughly equivalentto that of the previous chargeat the equivalent stageoffractionation. The F-8 samplesshow no initial plagioclaseprecipita- tion and no crystalline phasesat 875"C. The early com- ponents of the composite crystals measureAnrrAbrnOr,, and AnoAbrrOr.n, respectively, at the solid-liquid inter- face.Integrated analyses at various positions in the elon- gate crystals show minor variations except for a slight decreasein the Ca content alongthe growth direction (Fig. 6c). Becauseof the fine lamellar intergrowth, minor amounts of sanidineare included in most plagioclaseanalyses, and the data tend to plot on the Or side of a mixing line betweenideal plagioclaseand sanidine (Fig. 6b). The dis- continuousappearance ofsanidine on the plagioclasecore Ab allows better definition of the Or-rich component of the F-1 1 intergrowth. If the crystallization product is exclusively a Gomposites composite, the plagioclasecomponent varies from An' in the root zone to An,, in the marginal portion of single crystals(Fig. 6c). The changingcomposition of coexisting feldsparsis in accordancewith decreasingtemperature.

Melt fractionation Ab The melt compositions vary in accordancewith the crystallizing phase assemblage;during initial plagioclase precipitation, the residual melt becomesenriched in Na and K following the expectedliquid line of descenttoward the cotectic field boundary @ig. 7). At the appearanceof sanidineand subsequentcomposite Btrowth, this trend be- comes deflectedtoward the eutectic minimum. The de- Ab zo 40 60 Bo wr% Or flection point, however,is systematicallydisplaced toward Fig. 6. Microprobe analysesof minerals grown from starting the An-Ab join and significantly above the equilibrium materials F- I I and F-8. 348 PETERSENAND LOFGREN:FELDSPAR INTERGROWTHS

Table 2. Electron-microprobeanalyses offeldspars and glassesin experimental charges soo'c F-11 'ou"--t^,oo"- OXIDE F-8 627GL 627PL 627KF 591G1 591P1 591K2 M"l, si02 64.17 66.46 60,65 64.37 69.75 62.39 65.28 A1203 20.84 ?1.82 24,3t 19,31 22,44 23,66 19.39 Cao 2.20 2,07 5.68 0. 60 1.65 4.73 0.58 Na Z0 5.35 3.59 7.22 2.84 2.56 8.01 3.25 KzO 7.45 6.05 1.90 t2.27 3.59 2.05 11.81

T0TAI 100.01 99.99 99.76 99.39 99.99 100,84 100.31 An 11 13.1 27.0 3.0 15.7 2i.8 2.8 Ab 47 41.2 62,2 25.3 43.9 66.9 28.7 0r 43 4s,7 10.8 71.8 40.4 11.2 68.5 oxlDE 636G1 536p1 536K3 F-10 605c1 605p3 605p7 si02 69.50 65,09 65.06 60.09 61.79 50.48 51,45 Ar203 23.52 22.3r 19.33 24,33 23.92 31.19 30.72 Cao 0.81 3.30 0.23 6.04 4,77 14.02 13.55 Na20 3.05 7.05 3,89 5.34 4. 55 3,42 3,76 Kzl 3.r2 2.14 1r.22 4,20 4.87 0.24 0.26 Ab F-8 ToTAL 100. 00 99.89 99.73 100.00 100.00 99.35 99.74 Melt An 8.1 I7.7 1.1 29.0 25.1 68.0 66.0 eso Ab 54.9 68.6 34.1 47,0 44.3 30.0 33.0 7oo' 0r 37.1 13.7 64.8 24,O 30.6 1.4 1.5 :::---;=-i}s@ 0xt0E 629cl 629p4 593G1 593C1 593p2 638G1 638PI si02 63.53 52, 04 68. t2 63.49 52. 06 66.48 51. 89 6e5 Al 20 60 80 wt% 203 23'36 30. 44 2L.45 20.62 30. 44 22.64 31.07 Ab 40 Or ca0 2,95 12, 95 2.25 1.99 L2.94 0.94 13.17 Fig.7. Microprobeanalyses of averageglass compositions at Na20 3.94 4.06 2.72 4.i1 3.88 4.84 3,97 variousstages of solidificationin startingmaterials F-8, F-10, K20 6.20 0.33 5.45 9.26 0.29 5.08 0, 33 and F-l1. Isothermsfrom Yoderet al. (1957)emphasize the TOTAL 99. 98 99 .82 99.99 99.47 99, 6r 99.98 100.43 nonequilibriumnature of the experimentallyproduced liquids. An 16.9 oz.o lo.5 9.7 63.7 6.0 63.5 Ab 40.8 35.5 35,9 35.4 34,6 55.9 34.6 0r 42.3 1.9 47.6 53.9 1,7 38.4 1.9 toward the cotecticminimum. In near-cotecticmelts such oxrDE 638K1 Fll 606c1 606p5 605p8 630G1 630p1 as F-8 (Fig. 7c), the composite product may therefore initially drive the coexisting melt away from the cotectic si02 65.47 58.57 61.47 51.80 49,r0 65.11 58.36 Alz03 19.25 25.59 24.52 31.13 32,83 23.31 26.57 thermal minimum, as a result of preferential growth of Cao O.22 7.45 5. 54 13.29 15,31 2,85 7.96 sanidine over plagioclase.After a transient period this NaZo 3.69 5.35 4,6L 3.85 2,80 3.62 6.88 trend becomesstabilized and is redirectedtoward the eu- Kzo i1.57 3.05 3,85 0.21 0.13 5.il 0.61 tectic minimum for the system (Fig. 7c). T0TAL 100.20 100.01 99,99 100.28 100.17 100.00 100.38 The timing and formation of composite growth in the An 1.1 36 30.0 64.8 74.6 18.4 37.7 ternary-feldspar system are thus closely related to melt Ab 32,3 47 45.? 34.0 24.7 42.3 58.9 0r 66.7 t7 24.8 1.2 0,7 39.3 3,4 composition. Analyzed feldsparsin a seriesof successive samplesof composition F-10 illustrate the evolution of OXIDE 630P5 594G1 594P3 594P6 639c1 639Cr 539P3 the contemporaneoussolid and liquid in terms of Ca and si02 51,30 66,67 50.77 6r . 58 65. 68 65.65 60, 38 K redistribution (Fig. 8). Initial plagioclasecrystallization Ar 203 31.73 22.04 32, 00 24.O4 22.76 18.96 25.45 depletesthe melt in Ca, whereas the corresponding en- Cao 13.94 2,29 14.24 s.90 1.07 0.16 6.33 richment in K eventually leads to supersaturationwith Na20 3.57 7.22 4.77 3.33 6.48 Kzl 0. 19 5.39 0.16 0.90 4.7| 12.04 1.20 respect to sanidine resulting in composite growth. The growth T0TAL 100.73 99.99 100.52 99.64 99.99 100.14 99.84 nucleationand ofsanidine affectthe residualliquid with respectto K in the same manner as Ca was affected An 68,0 15.0 69.5 ?9,4 7.0 0.8 32.5 Ab 31.0 42.8 ?9.6 65.2 56.4 29.4 60.2 by the earlier plagioclasefractionation. An interesting ef- 0r 1.1 42.2 0. 9 5.4 36,7 69.9 7.3 fect, however, is created by this process:Ca is enriched in the solid as the fraction of plagioclasein the composite and subsequentcomposite growth before the bulk melt increasesoutward. This createsan inverse zonation with reachescotectic composition. Once formed, however, the increasingCa outward and producesa plagioclaserim on modal proportions of the constituent phasesin the com- the K-feldspathic composite.This featureis similar to that posite product are determined only by the degreeof su- in the so-calledrapakivi texture, but opposite to that ex- percooling and the rate ofadvance ofthe interface. The pected from equilibrium-crystallization conditions. residual-liquid fractionation trend therefore reflects the In the most K-rich, near-cotecticmelt composition (F- kinetic conditions rather than continued diferentiation 8), plagioclaseis not the first phaseto precipitate. Instead, PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS t49

CaO wr% (a) \. 12.O :ETm € T,,o R o t CI "' 8.0 E itutional .o Te Const F Suporcooling 4.O

Dl!tanco

Hours K20 (b) C wt .9Ce .= 12.O o o o E Soll

Dlstence Fig.9. (a)Relations between growth rates, R, < Rr, and the 50 1O0 15O Hours correspondingliquidus-depression curves; (b) the diffusive Fig.8. Evolutionof contemporaneousmelt and solid com- boundarylayers are d, anddr, respectively.Z" andC. areeutectic positionsduring progressivefractionation in startingmaterial temperatureand composition,and Z- and C., initial melt tem- F-10;(a) showsthe CaOand O) the KrO distribution.The dis- peratureand composition. continuoussolid trends occur when a compositephase nucleates andstarts growing. tional supercoolingfor a given temperature gradient, yet high enough to produce a boundary layer with eutectic plagioclase albitic and sanidine apparently precipitate si- composition at the solid interface (Mollard and Flemings, multaneously, side by side, and becausethis assemblage 1967a;Flemings, 1974, p. 94). Theseconditions are met is closer to the melt composition than each of the indi- with a comparatively slow solidification rate (R' in Fig. phases, vidual there is little liquid fractionation. Domi- 9b), where the accumulated solute is distributed over a nant growth of K-feldspar initially enriches the liquid comparatively wide diftrsion zone (d'), and the temper- slightly in plagioclasecomponents, but after a transient ature gradient (f"",) is steeper than the tangent to the period, the eutectic growth is stabilized and brings the liquidus-depressioncurve (Z',o)at the interface (Fig. 9). residual liquid toward the eutectic minimum. The duplex Eutectic growth is termed "coupled" when the com- crystallization is therefore maintained throughout the so- posite solidification occurswith a planar frontl that is, the lidification range.By contrast, the most albite-rich com- gowth is coordinated so that neither phasegrows faster position, F-1, never precipitates phase a separateK-rich than the other, and the combined erowth is faster than during the course of crystallization. It appears that the eachof the constituent phasesalone (Rutter, I 977). Once more Na-rich (and Ca-rich) the melt, the greatermust be the composite product is formed, a planar, nonfaceted the fraction crystallized before composite growth is ob- interface is stabilized by transversediftrsion betweenthe served;i.e., the further the melt composition is removed constituent phases.This diffirsion rangeis on the order of from the cotectic, the more the liquid must fractionate to one-halfinterlamellar spacing,and very different from the reach the conditions for composite intergrowth. long-rangediffusion in the solute-enrichedboundary lay- er, which may have initially led to the duplex nucleation. Experimental and theoretical studies show that the Eurncrrc soLrDrFrcATroN composition of the composite product is related to solid- The composite feldspar intergowths have many fea- ificationrate (Mollard and Flemings,1967a,1967b).Ide- tures in common with eutectic solidification structures ally, when both the solid and the liquid have exactly a describedin the metallurgicalliterature. A briefdiscussion eutectic composition, solute is redistributed and imme- of eutecticsolidification mechanismsis thereforeincluded diately consumedat the solid interface by the two phases, here. A prime condition for simultaneous, coordinated and there is no boundary layer. In this specialcase, chang- growth of two phasesis that the solute enrichment in the ing growth rates clearly will not affect the overall com- boundary layer is sufficiently small to prevent constitu- position. However, significant changesin the interface 350 PETERSENAND LOFGREN:FELDSPAR INTERGROWTHS

producedby fluctuationsin composition alongthe solidifi- cation front (Fig. 2), shows that a roughly planar solidi- fication front is also present during growth of the com- posite product. The occasional, regular deflection of lamellae from a direction perpendicular to the solidifi- cation front indicates departurefrom an isotropic or con- tinuous glowth mechanism,possibly createdby the pres- ence offacets in one ofthe constituent phasesalong the liquid groove between two adjacent phases (Hunt and Hurle, 1968; Morris and Winegard, 1969). The lamellar intergrowth is formed when the solute- enriched boundary layer at the growing plagioclaseinter- face becomessupersaturated with K-feldspar, which nu- cleatesand startsgrowing. The subsequentheterogeneous nucleation ofalbitic plagioclaseon this sanidinehas a low energy barrier becauseof high structural compatibility (Lofgen, 1983)and thereforeoccurs readily asthe bound- ary layer becomesdepleted in K and enrichedin Na. From Fig. 10. Back-scatteredelectron image of lamellar inter- then on, the combined erowth of the two phasesis sta- growthshowing sector zoning and gradual compositional changes. bilized by transversediffirsion, which provides nutrients K-feldsparis white;albitic plagioclase,dark $ay; glass,black. for the complementaryphases and resultsin a more rapid growth rate. The discontinuousappearance ofsanidine on velocity will remove the averagesolid composition sub- the plagioclasecore as opposed to the continuous ap- stantially from the eutectic, provided the conditions for pearanceof albite lamellae in the K-feldspar suggeststhat a plane-front solidification are maintained (Mollard and the nucleationofsanidine on a plagioclasehost apparently Flemings, 1967a;Hunt and Jackson, 1967).When the hasa higher energybarrier than the precipitation ofalbitic solid is removed from eutectic composition, residual sol- plagioclaseon K-feldspar. ute accumulates in a diffusive boundary layer. This Systematic changes in the composition of the solid boundary layer, being on the order of Dr/R wide (Flem- product during progressivegrowth (zoning) are the result ings, 1974, p. 109)where D, is the diffusion coefrcient in of gradually changingsolidification conditions. When the the liquid and R the growth rate, can be affectedby con- isothermal $owth is unable to keep up with the contin- vective flow unlike the much narrower transversediffi.r- uous cooling rate, the interface becomesincreasingly su- sion zone which is largely within the static portion of the percooled and gradually changescomposition toward a boundary layer. Becausethe composite interface is iso- more albitic plagioclaseend member (Fig. l0). Further- thermal, changesin growth rate or thickness of the non- more, since the liquidus depressionis greatestat the cen- convective boundary layer will therefore result in com- tral portion oflarge facets (becauseofthe edgeeffect on positional changesin the composite product that follow solute distribution), the instantaneoussupercooling will a plane-front solute-redistribution pattern (Mollard and be highest there, and the bulk composition will accord- Flemings, 1967a). This can result in gradual composi- ingly be more Ab rich than the composite formed simul- tional changes, steady-state gtowth, or compositional taneously near the edges.Observed textures and com- bandingfollowing Tiller et al. (1953). positional relations clearly support this view (Figs. l, 3). The structure of eutectic composites is generally la- The composition of the composite product also varies mellar or rod shaped,depending on the proportion of the with crystallographicdirection. In the elongated,faster- secondaryphase (Mollard and Flemings, 1967b1,Elliott, growing direction of the feldspar crystals,the solid is sys- 1977). A volume fraction of l/r marks the theoretical tematically more polassicthan in the flattened {010} sec- stability of rod morphologiesalthough experimental data tor. This pronounced differencein composition must re- suggesteven lower values becauseofanisotropic surface sult from different growth properties in different energiesof the constituentphases (Flemings, 197 4, p. 104). crystallographic directions. Dowty (1976a, 1976b) em- Faceting behavior of one phase leads to more complex phasizedthe influenceof crystal structure on morphology patterns that reflect the anisotropy of the faceting phase and sector zoning, and, adding the influence ofdiferent (Hunt and Hurle, 1968;Croker et al., 1975a,1975b). conditions for heterogeneousnucleation of a secondary phase in different lattice planes (the wetting factor), the DrscussroN: ORrcrN oF THE TNTERGRowTHS prominent differencescan be ascribedto crystallographic Lamellar intergrowth factors. The optical continuity of host and lamellae in Evidenceof coupledeutectic solidification with lamellar different growth sectors of single crystals suggeststhat structuresis seenin many of the compositefeldspar zones. there is an epitaxial relationship between the host and Usually, the lamellar intergrowth is bounded by planar product phases. This allows principal crystallographic interfaces.A conspicuousbanding in somegrowth sectors, properties to be maintained in the different directions PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS 351

(a)

leteral monophace

_growth

Fig. 12. (a)Schematic presentation of thepatchy-intergrowth configurationin relationto main crystallographicplanes of a monoclinicfeldspar. Rules represent lamellar intergrowth; white Fig. I l. Sectoraldistribution ofpatchy intergrowths. (a) La- partsare the leadingsanidine. Facets may or may not develop mellarstructures dominate the {010} crystallographicplane, while on the leadingedge of a growthsubunit. If the sanidinezone K-feldsparsdominate the a-c crystallographicplane. (b) Faceted expandssimultaneously in two directions,a cuneiform-inter- growthin crystallographica and c directionsis predominantly growthpattern (as seen in Fig. 5 oflofgren and Gooley,1977) K-feldspar(dark), while lamellarintergrowths (light) form the is formed. Equivalentrelations in a syntheticfeldspar. San- sidesofeach subgrain. Note the lamellarcharacter ofthe struc- O) idine (darkgray) grows preferentially in lateral(horizontal) di- turesat thebase of the feldsparcolony when a compositesector rections,whereas a composite,oligoclase-rich product gray) is cut perpendicularto the growthdirection. Qight dominatesthe verticalgrowth, normal to the interface. during compositegrowth and commonly leadsto an hour- glasspattern superimposedon the intergrowth. Patchy intergrowths A convex interface with a radiating lamellar structure, The formation of patchy intergrowth results from an as seenin many end sectors,shows that growth of sanidine entirely different growth mechanism than the lamellar t1pe. is leading slightly over plagioclasein this direction (Jack- The irregular distribution of the constituent phasesshows son and Hunt, 1966). Ifthis differencein growth rate of that this growth is uncoordinated;one phasegrows struc- the two components increaseseven more, e.g., because turally independently of the other. As the solidification the differential growth anisotropy increasesfurther with proceedsthere is a significant coarseningof the patchy changing supercooling (Jackson, 1984), coupled growth intergrowth. Initial phasesegregations on the order of l0- can no longer be maintained, even with the convex in- pm width increaseto broad areasmore than 100 pm wide. terface.Only a singlephase will precipitate in this partic- The solid-liquid interface of the individual patchesoften ular crystallographicdirection, whereascomposite growth showsfacets, usually bounded by {001}-{ l0l } planesand is maintained in other directions. The relationship be- with maximum elongationalong c. Two phasesmay share tweencomposite structures in different sectorsand overall a common facetplane, but more often sanidinedominates melt differentiation in the studied samplesis significant facets in the elongated direction whereas albitic plagio- and is covariant with supercooling.In the most K-poor clase and/or composite lamellar products dominate the samples,composite growth occurs only in the elongated sidesor flanks of elongatedcrystal branches(Fig. 4). Patches a or c crystallographicdirections. In more evolved stages of facetedsanidine and lamellar plagioclase-sanidinecom- of solidification, when the K content of the melt is suffi- posites often co-exist in single crystals (Figs. I la, I lb). ciently high to provide conditions for composite growth The following gowth model for the patchy intergrowth in the {010} planes,the correspondiing{001} sectorsare is proposed.Heterogeneous nucleation ofsanidine occurs usually monophasesanidine. epitaxially on a facetof a normally zonedplagioclase crys- 3s2 PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS

Fig. 13. Step-cooledfeldspars with discontinuousgrowth Fig. 14. Lamellarintergrowth with a rhythmic,banded struc- zones.The composite zone is notablywider than the inner, mono- ture due to the discontinuousappearance of albitic plagioclase phasezones. lamellaein a predominantlyK-feldspathic host.

tal. Growth of this sanidine in the {010} direction even- Growth rates tually results in a coupled, lamellar plagioclase-sanidine Planar interfaces intuitively suggestgrowh by layer intergrowth as discussedabove, with the lamellae pro- spreadingcontrolled by interfaceattachment kinetics. The jecting away from the interface.The simultaneousspread- planar interfacesof lamellar eutectic structures,however, ing ofgrowth layersis influencedby the diffi,rsiveboundary are the result of the isothermal characterof the composite layer createdby the pre-existingplagioclase facet; i.e., the growth mechanism, which exceedsthe rate of faceted lateral, edgewisegrowth projects into a liquid that is al- growth by layer spreadingby many orders of magnitude ready enrichedin low-temperaturecomponents (Figs. l2a, (Flemings,1974, p.95; Rutter, 1977).Growth by layer l2b). The growth in this direction may thus occur as a spreadingoccurs by surface-edgenucleation or screwdis- noncoupled, eutectic solidification, where the re-entrant location and is usually controlled by the nucleation rate angle of the gowth step createsa stable phaseboundary. of new layers (Kirkpatrick, l98l). The latter is slightly This processis enhanced by anisotropic growth factors faster for a similar degreeof supercoolingbecause it has that encouragethe growth of sanidine in the crystallo- no nucleation barrier, and in the case of increasing su- graphic a and c directions. percooling, the screw dislocation may dominate the growth Since the lateral and the perpendicular growth occurs (Gilmer, 1977; Ilrkpatrick, l98l). Substantially faster simultaneously,the compositesector gradually returns to- growth rates, however, are achieved when supercooling ward albitic plagioclasewhile simultaneoussanidine pre- exceedsthe rougheningtemperature ofcertain facets,and cipitation occursin the lateral growth front. At somestage, the solid interfacebecomes atomically rough (Gilmer and renewed nucleation of sanidine will occur on top of the Jackson, 1977; Hartman, 1982). In this case,a planar composite plane and start spreadinga new layer. By this solid-liquid interface reflects the isotherms along the so- time the sanidine transgressesthe composite product of lidification front. The rate of growth in this caseis con- the previous layer, and the conditions for its further growth trolled only by the rate of attachment of constituent com- are diminished (Fig. l2a). Clearly, the above processcan ponents to the interface and removal of expelled solute occur repeatedlyand lead to overlapping zones of com- by diffusion. In coupled eutecticgrowth, this rateJimiting posite and noncomposite products, creating a highly ir- accumulationof soluteis minimized sincethe neighboring regular phase distribution. The growth processis com- phasesconsume complementary solute and thus mutually parableto layer speadingand may lead to facetedcrystals encouragetheir growth by transversediftrsion. This re- (Fig. l2b). ducesthe boundaryJayer vddth and allows very high so- The patchy intergrowth occursbecause the growth rate lidification rates because the rate-controlling diffusion ofthe sanidine exceedsthat ofthe composite product in rangeis reducedto the order ofthe interlamellar spacing. the crystallographica and c directions, and a plane front Coupled eutectic growth accordingly predicts a dra- cannot be maintained. In this case,the one phasegrows matic increasein the solidification rate when compared rapidly into the melt while the other precipitatesat some with the $owth of singlephases. This is most clearly seen distancebehind. When fast growth is directed in the elon- in step-cooled experiments, where the temperature is gated direction of the host crystal, slower growth in the dropped 50'C for constant time intervals of 2 d. Each transversedirection occurs simultaneously with a plane temperaturedrop createsa new growth zone whosewidth front and forms a coupled eutectic structure. reflects the relative growth rate for each time interval. PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS 353

2mm

Fig. 15. Natural, patchy intergrowth of two feldsparsin comb-textured lardalite feldspar. Photomicrograph shows the tip of a columnar feldspar with maximum elongation in crystallographicb direction. K-feldspar grows preferentially laterally (dark gray); the lighter-gray central stem is flame-structuredoligoclase.

Becauseof the closed-systemfractionation, the residual moderate to low supercooling rather than high super- liquid becomes fractionated and ultimately reachesthe cooling,and the formation of compositeintergroWhs thus conditions for cotectic crystallization. At this stagea dra- is expectedto occur in plutonic rather than in volcanic matic change in the growth conditions leads to a com- environments. The existenceof a complete solid solution posite growth zone more than ten times wider than the for K- and Na-rich feldspars at low pressures(Morse, previous monophasezones (Fig. l3). This fact emphasizes 1968) further restricts the occurrencein volcanic rocks the unique nature of the coupled eutectic gowth and se- and favors its occurrenceprimarily in plutonic rocks. verely restricts the possibility that the intergrowths may Plutonic feldspars with complex intergrowths are re- have precipitated as homogeneous phases and later ported by Barth (1945) from monzonitic larvikite occur- exsolvedinto a two-phase product. rencesin the Oslo region, . The two components Becausethe products of eutectic crystallization often of this intergrowh are plagioclase,An,r-rr, and alkali feld- solidify with off-eutecticcompositions, a limited bound- sparabout AnoAborOror,which are equivalent to the com- ary layer is usually present and affectsthe solid compo- ponents of eutectic intergrowths reported in the present sition. As the ofleutectic solid composition shifts toward experimental study. Oftedahl (1948) recognized patchy one end member under supercooledconditions, it may intergrowths in some of the more calcic members of the passthe extensionofthe liquidus curve ofthe other phase. same rock seriesand ascribed them to primary growth. Thus being outside the maximum solubility range of this Despitethese interesting observations, subsequent studies phase, it ceasesgrowth, and a monophase precipitation of larvikite feldspars have focused mainly on the spec- temporarily takes over. As this creates complementary tacular and intricate alkali feldspar unmixing patterns, solute enrichment in the boundary layer, the processcan believed largelyto derive from solid-statereactions (Muir repeat itself, creating a banded structure, as often seenin and smith, 1956;Smith and Muir, 1958; Smith, 1974). some metal eutectics(Hurle and Hunt, 1968). Composi- The influenceof late, solid-statereactions, however, would tional variations showingcyclic appearanceofmonophase not destroy any primary intergrowth, but rather enhance bands in the groWh direction are occasionally observed it and possibly smooth the initial phase boundaries. In in the composite feldspars. One such case of rhythmic view of the present results, therefore, a significant con- variations that could be the result of this mechanism is tribution to unusuallycoarse segregation in thesefeldspars shown in Figure 14. could result from primary growth. Strong support for this view comesfrom an occurrenceof comb-layeredfeldspars Appr,rc.q.rroN oF REsuLTs ro NATURALFELDSpARS in the Lardalite intrusion (Petersen,1985). The lardalite The wide range of crystallization conditions that pro- is a nepheline-rich variety of the larvikite, and the feld- duce eutectic structuresin the experimentswith ternary- sparsare clearly related though they are slightly more Or- feldspar melt compositions suggeststhat conditions for rich in lardalite than typical larvikite. The feldspars in similar growth behavior would be expected in natural thesecontact zonesgrow parallel to one another in a co- rocks. Owing to extensivesolid-state transformations and lumnar fashion perpendicularto the intrusion border (Pe- exsolutionin the natural feldspars,however, the structures tersen,I 985).Their lateralgrowth is constrainedby neigh- may not be readily recognized. boring crystals and the feldspars are elongated in the Eutectic solidification is primarily a result of growth at crystallographic b direction. These feldspars show con- 354 PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS

Fig. I 6. Larvikitefeldspar showing discontinuous nucleation Fig. I 7. Back-scatteredelectron image showing patchy inter- plagioclase ofK-feldsparalong diagonal facetsand flamestruc- growthin a larvikitefeldspar. Note incipient perthite exsolution gtowing (horizontal). tures alongthe compositesector next to the bordersof the coarseintergrowh patches. White is K-feldspar;gray, albitic plagioclase;dark-gray clots, nepheline. spicuously coarse segregationsof oligoclase and K-feld- spar(Petersen, 1985). The plagioclasecomponent is gen- erally heterogeneous>about An,rAbroOrr, with variable crystals with faceted outlines, in a fraction of the time Or content,whereas the K-feldspar is fairly homogeneous, that is usually indicated by growth ratesofthe singlephas- about AnrAbroOrnr.Patchy intergrowths of the two phases es. producea featherlike structure that demonstratesthat the CoNcr,usroNs K-feldspar occurs predominantly in the lateral plane forming the sidesof the feldspar dendrites and projecting Coupled and noncoupled eutectic solidification occurs toward the central stem (Fig. l5). Becausethese feldspars readily in ternary-feldsparmelts at moderatecooling rates grow constrainedpreferentially along the b axis, the pres- and near-cotecticconditions. The further the initial melt enceof composite plagioclase-alkalifeldspar in the crys- composition is removed from the cotectic,the more liquid tallographic b direction and predominant K-feldspar in fractionation is required to reachconditions for composite the crystallographica and c directions confirms that these $owth. Solute enrichment in a diffusive boundary layer data are in accord with the experimental observations at the primary plagioclaseinterface createsconditions for presentedhere. local supersaturationof sanidine and subsequentcom- Becausethe patchy-intergrowth mechanism presented posite growth, before the bulk melt reachescotectic com- here is related to layer-spreadingmechanisms, the devel- position. opment of directional, columnar growths is not an oblig- The composite crystallization occurs primarily as a la- atory condition for the formation of patchy intergrowths. mellar intergrowth of oligoclaseand sanidinewith a plane In fact, the main part of the Lardalite intrusion, as well front and a much higher rate of growth than singlephases. as most larvikite occurrences,contains cumulus feldspars This growth is controlled by transversediffusion across with intergrowth patterns that are similar to those of the the interfaceand removal of latent heat of crystallization. columnar feldsparspresented here. They display repeated, OFeutectic crystallization creates a diffusive boundary discontinuous occurrencesof K-feldspar along distinct layer of solute-enrichedliquid that can cause rhythmic plagioclasefacets and parallel growth in other crystallo- compositional changesand banding during growth. The graphic sectors(Fig. 16) and therefore may be the result lamellar products show systematiccompositional varia- of primary growth. Back-scatteredelectron imagesreveal tions in the growth direction as well as gradual changes that fine exsolution lamellae seemto increasetoward the acrossindividual facets following variations in the local irregular K-feldspar-rich patchesin accordancewith great- supercooling. er easeofnucleation at ihis location (Fig. l7). A patchy intergrowth is formed when the preferential The results of the present study suggestthat natural growth of sanidine in the crystallographica and c direc- feldsparintergrowths, previously consideredto be the re- tions is combined with simultaneous growth of a com- sult of solid-stateexsolution reactions,may be at least in posite product in the crystallographic b direction. De- part the result ofgrowth from the melt. This would clearly pending on the relative growth rates in thesetwo directions, overcome some obvious difficulties in providing suffi- the product developsa featherlike structure that is dom- ciently slow cooling rates to allow solid-state diffusion to inated by either K-feldspar or plagioclasewith subordi- account for extremely coarse grained occurrences.The nate amounts of sanidine intergrowth. Becauseof the si- results also suggestthat the considerably higher growth multaneousgrowth of two componentsin the noncoupled rates for this type solidification may lead to very coarse intergrowth, the boundariesbetween the constituentphas- PETERSEN AND LOFGREN: FELDSPAR INTERGROWTHS 355 es are irregular and, unlike the coupled intergrowth, show Jackson,K.A. (1958)Interface structure. In R.H. Doremus, B.W. perfection no reference to particular crystallographic planes. Roberts,and P. Turnbull, Eds. Growth and of crys- tals, 319-359.Wiley, New York. Eutectic, growth coupled and noncoupled in the exper- - (1984) Crystal growth kinetics. Materials Scienceand En- iments occurs over a wide range of temperature and com- gineering,65,7-13. positional conditions likely to exist in many natural, plu- Jackson,K.A., and Hunt, J.D. (1966) I-amellar and rod eutectic tonic environments at small supercooling. 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