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Lamellar and Patchy Intergrowths in Feldspars: Experimental

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Lamellar and Patchy Intergrowths in Feldspars: Experimental American Mineralogist, Volume 71, pages 343-355, 1986 Lamellar and patchy intergrowths in feldspars: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 feldspar-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 quartz- ciated with nucleation (Swanson, 1977 lnfge4 1980). feldspar intergrowths in granophyre and graphic granite, 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 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
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