Lithiophilite Formation in Granitic Pegmatites:A Reconnaissanceexperimental Study of Phosphatecrystallization from Hydrous Aluminosilicatemelts

American Mineralogist, Volume 71, pages 356-366, 1986

Lithiophilite formation in granitic pegmatites:A reconnaissanceexperimental study of phosphatecrystallization from hydrous aluminosilicatemelts

Jrvrns E. Snrcr,nv,* Gonnou E. BnowN, Jn. Department of Geology, Stanford University, Stanford, California 94305

AssrRAcr Crystallization of the pegmatite phosphate mineral lithiophilite, Li(Mn2+,Fe2+)POo, from hydrous aluminosilicate melts has been investigated experimentally to place some constraints on the conditions under which it can form from pegmatite melts. The starting materials for these reconnaissanceexperiments consisted of a mixture of minerals from the Li-rich Stewart pegmatite at Pala, California, plus varying amounts of water, chosen to model an inferred granite pegmatite melt composition. P-Z conditions during the experiments were chosen to approximate those inferred for granite pegmatite formation at shallow depths (3-5 km). A Mn-Fe phosphate mineral, identified as lithiophilite on the basis of optical properties, microprobe data, and electron-diffraction patterns, was crystallized over a temperature range of 500-400'C (at 1.5 kbar) only from water-saturated melts (containing 3-l I wto/oHrO depending on temperature). Lithiophilite was accom- panied by quartz but not by feldspar in these runs. Small amounts of sicklerite, Li,-,(Mnl1,,Fel*)POo, were also identified in some run products on the basis of optical properties. These reconnaissanceexperiments suggestthat coarse-grainedlithiophilite crystallizes in the interior portions of complex granitic pegmatitesas a primary phasefrom volatile- saturatedmagmas. No evidence was found for phosphate-silicateliquid immiscibility in the experimental run products, suggestingthat such immiscibility is not necessaryfor lithiophilite formation in water-saturatedgranitic melts. The absenceof feldsparin the run products is not consistentwith the observedassociation of lithiophilite, q:uarlz,and abundant K- and Na-feldspar in the upper intermediate microcline-quartz zone of the Stewart pegmatite. This discrepancybetween natural and synthetic crystallization products may be due to (l) the complexing of Na, K, andlor Al by P in our simplified melt compositions, which should lower the activities of theseelements and could suppressthe crystallization of alkali feldspar; (2) the use of Pt capsulematerial in the experiments;andlor (3) the large amount of undercooling (300-400'C) in the crystallization runs. The presenceof P in the melts appearsto have had little effect on quartz crystallization, morphology, or growth rates relative to its crystallization in P-free melts of simpler composition. These experimental results support the hypothesis that lithiophilite, q\artz, and alkali feldspar crystallized in the upper intermediate zone of the Stewart pegmatite from a water-saturated magma at temperatures of about 500'C or less and at pressuresof about 1.5 kbar. The structural role of phosphorusin hydrous pegmatite melts is discussedand related to some of the experimental observations.

fNrnooucrroN commonly associatedwith alkali feldspar and quartz in Important among the accessoryminerals in some gran- the intermediate and core portions of internally zoned itic pegmatites are the phosphate species lithiophilite- granitic pegmatites (Moore, 1973, 1982; Shigley and triphylite Li(Mn2+,psz+)POo,which can occur as giant Brown, 1985).On the basis of their textural relationships crystalsup to severalmeters across and as smaller crystals and modes ofoccurrence, it is probable that these phos- or massesof intergrown crystals. Field observations in- phatescrystallized during the latter stagesof granitic peg- dicatethat thesephosphates are primary phaseswhich are matite formation. According to the geneticmodel ofJahns and Burnham (1969), crystallization of mine&ls during period presence * Presentaddress: Research Department, Gemological Insti- this is facilitated by the of a coexisting, tute of America,1660 Stewart Street, Santa Monica, California exsolved,volatile-rich fluid within the pegmatite magma 90404. system. Only a few experimental studies (e.g., Stewart, 0003404x/86/03044356$02.00 356 SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES 357

1978;London, 1984, 1986)have attemptedto relateac- Optically homogeneousmineral fragmentswere selected,sep- cessorymineral paragenesisto the Jahns-Burnhammodel arately pulverized, and ground under acetone,resulting in grain for crystallization of granitic pegmatitesand to assessthe sizes of 10-20 pm. Each sample was fired at 500-600"C in an roles of water and elements such as Li, F, Be, and B in open Pt crucible to remove any organic contaminants, and the powdered proportions the formation of accessoryphases. No past experimental minerals were mixed in the desired in an agatemortar and a McCrone Mill. Optical examination revealed studies have provided information concerning the con- the mixture to consist of irregular, broken fragments with an ditions under which primary phosphates crystallize in a averagediameter of less than l0 pm and a maximum diameter pegmatite system. of lessthan 30 pm. To minimize the presenceof Fe3+and Mn3+, The presentexperimental study was undertakento gain the mixture washeated for 24 h at 600"Cin a reducingatmosphere a better understandingof the paragenesisof lithiophilite of forming gas(900/0 N, + 100/oHr). Following this treatment, no in pegmatites.It follows a recentfield and analytical study Fe3+could be detected by wet chemical analysis. To prevent of the well-known Stewart pegmatite in the Pala district, absorption of water, the mixture was kept in an evacuateddes- San Diego County, California (Shigleyand Brown, 1985), sicator until capsuleswere loaded. which documented phosphate mineralogy and the alter- Ten milligrams of this run mixture were loaded into 3-mm Ft or AgPd capsules,each of which contained a known ation history of lithiophilite. This Li-rich pegmatiteis one diameter amount of freshly distilled and deionized water (l-l 10/oof the of the largerand more important of the numerousgranitic total weight). These water contents were chosen to ensure that pegmatitesin the Pala genetically area that are related to the aluminosilicate melts in different experimentalruns spanned the Mesozoic PeninsularRanges (or Southern California) the range from water-undersaturated to water-saturated condi- batholith (Larsen, 1948) and are thought to have crystal- tions. The capsuleswere sealedimmediately after loading by arc lized at relatively shallow depths of several kilometers welding and were placed in a vacuum oven for 12 h to test for (Jahnsand Wright, 1951 ; Taylor et al., 1979).Crystals of leakageand to ensure an initially homogeneouswater distribu- altered lithiophilite up to 40 cm in diameter occur in the tion. Capsulewater contentsare believedto be accurateto within microcline-quartz upper intermediate zone of the Stewart 0.5 wto/oof the quoted values. pegmatite. Crystallization experirnents were carried out in the Tuttle-Jahns The experimental conditions of this study were chosen Laboratory for Experimental Petrology at Stanford University using an inrernally heated pressurevessel modified after the de- to approximatethe inferred P-T-X conditions ofthe Stew- sign of Yoder (1950); the vessel was oriented in a horizontal art pegmatite (P: during its formation 1.5 kbar; Z: position during the runs. An Inconel-sheathedPt,oo-PtroRh,o 800-500'C or lower; Taylor et al., 1979). Our objectives thermocouple,periodically calibrated againstthe melting points were to place some constraints on the conditions under of Au (1064.4"tC)and NaCl (801.2qq, was used for temperature which lithiophilite crystallization could take place from a measurement.Pressure was transmitted using Ar hydraulically pegmatitic melt, to compare the phase assemblagefrom pumped from a Harwood intensifier.Reported temperatures and the experimental runs with the observed lithiophilite- pressuresare believed to be accurateto + 10"C and + 100 bars, bearingassemblage at the Stewartpegmatite, and to relate respectively.The l.S-kbar pressure chosen for all runs corre- the experimental results to the paragenesisof primary sponds to that for an estimated depth of formation of 3-5 km pegmatite(Jahns, phosphateminerals in this pegmatite.Because of the rel- for the Stewart 1953; Taylor et al., I 979). The of the chargewas not purposelyconstrained during eachrun atively small number of experimental runs and the lack ./O, and is unknown. The steelpressure vessel and Mo heater wind- of experimental reversals, this investigation should be ings should result in an.p, below the FMQ buffer in the charge viewed as a reconnaissancestudy. if it were in equilibrium with thesecomponents. This was prob- pressure ExprnrlrnNr,ql MrrHons ably not the caseowing to the lower temperatureofthe vesseland to the presenceofwater in the capsules,which could Startingmaterials consisted of a suiteof naturalminerals, most result in an increase in /O, if H, diffused out of the capsule. of which werecollected at the Stewartpegmatite. Owing to a lack Therefore,the oxidation state ofFe in the chargeis not known, of suitable,unaltered Stewart lithiophilite, a sampleof freshlith- atthough it is thought to be predominantly Fe'?+on the basis of iophilitefrom theBranchville, Connecticut, pegmatite (Stanford past experiencevrith Fe-bearingruns in thesepressure vessels (P. Universitycollection #6072) was used for the phosphatecom- M. Fenn, pers.comm., 1985).Small amounts (<200loof the total ponent.The compositionsof eachof thesephases are given in phosphate)of sicklerite, Li, -"(Mni1,,Fei*)POo, were identifred Table l. The followingmixture, used in eachrun, wasselected in some of the run products on the basis of optical properties, to approximatethe modalcomposition of the lithiophilite-bear- indicating that some Fe3+was produced during the experiments. ingintermediate zone that forms a majorportion ofthe pegmatite Twelve successfulexperimental runs consistingofat leastfour body (volumepercents): qtartz, 35o/o;microcline, 300/o; albite, capsuleseach were carried out and were typically several days to 150/o;muscovite, 80/o; lepidolite, T0/o; and lithiophilite, 590. These a week or rnore in duration. This relatively small number of proportionsalso yield a bulk compositionfor thestarting mixture successfulruns and the lack of experimental reversalslimit the thatapproximates Jahns'(1953) estimate of thebulk composition accuracyof phase-assemblageboundaries. The liquidus temper- of the Stewartpegmatite (Table l). Comparedto most earlier ature was found to be approximately 800'C for the compositions studiesofpegmatite crystallization, the startingmixture used here used in these experimentsat 1.5 kbar. A1l ofthe runs involved representsa more complex composition that shouldbe closerto an initial melting of the capsulecontents at temperaturesof 800"tC naturalpegmatitic melts than one based on syntheticoxides; even or higher for periodsranging from 2 I to 48 h (Table 2). Following so,it is missingminor modalconstituents such as tourmaline initial melting and a heating interval during which a vapor phase and probablydoes not accuratelyreflect the minor elementsor exsolvedfrom the melt (as indicated by bubblesin the glassand volatilespresent during the formationof the Stewartpegmatite. dimples on the glass surfaces),the run was rapidly cooled iso- 358 SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES

Table l. Chemical compositions of starting materials and the run mixture

Microcllne Albite Muscovite Lepidolite I-ithiophllite Starting Ster.Tart pegmatite Mixture Jahns (1953)

SiO2 b4.J ot.I 46.2 64.8 o.52 74.7 74.9

A12O3 r9.3 20.4 36.4 18.3 0.1 12.l 14.9

Ie2O3 <0.04 0. 06 1 1a <0.04 13.33 0.68 **

Mn0 0. 02 0. 02 0. 38 0.45 36.25 7.76 **

Mgo <0.10 <0.l0 <0.10 <0.10 <0.10 <0.10 **

CaO <0.02 <0.07 <0.02 <0.02 <0.07 <0.02 0.1

Na20 2.20 10.6 0.61 0.32 0.15 2.30 3.6

Kzo IJ.U 0. 38 9.86 7.32 0. 03 4. 89

Li20 0. 05 0. 06 0.37 3.29 8.78 0.66 0.7 ** T102 <0.02 <0.02 <0.02

2.79 ** - z") 0.14 o. L2 0. 05 0. 05 44.85

other 0.12 0.47 5.09 2.71 0.23 0.86 0.8

total 99.31 r00.0 t00.22 96.82 104.13 100.26 100.2

* A11 values ln weight percent oxldes. Analyses performed using X-ray and atomic absorPtion spectroscopy by J.S. Wahlberg, J. Taggart' J. Baker, and K. King of the USGS AnalYtical Branch, Lakewood, Colorado (Job //KS41).

** Not reported.

barically to a predetermined subliquidus temperature at which in phaseidentification, the crystallinephases in onecapsule were it was held for periods of 40-140 h to permit crystallization.The examinedusing a Philips8M400 transmissionelectron micro- time required for this temperature drop varied with the amount scope,which was equipped with anEDAX 7l I energy-dispersive of undercooling of the system but was on the order of several X-ray analyzerand operatedat 100kV. minutes. At the end of eachrun, final quenchingat pressurewas accomplished by turning offthe furnace power; the pressure ves- Rrsur,rs oF cRysrALLIzATroN ExPERTMENTS sel reachedroom temperature within 20 min. Finally, the cap- exper- suleswere reweighedto check for possible volatile loss during Run conditions and resultsof the crystallization the courseof the run. iments are summarized in Table 2. Phase assemblages At the end ofa run, the contentsofeach capsulewere examined presentin each of the successfulruns are depicted in the with a polarizing microscope. Powder X-ray diftaction data on pseudobinary temperature-composition diagram shown run products were obtained using a Gandolfi camera and Ni- in Figure l. Phase-assemblagefield boundaries are be- filtered, Cu Ka radiation. Phases in several capsules were ana- lieved to be accurateonly to +50oC and +0.5 wto/oH.O lyzed using an ARL-EMX-SM microprobe with wavelength-dis- and thus are shown as dashedlines. persivespectrometers. Well-analyzed natural spessartine,apatite, Capsules from most of the crystallization runs con- were An orthoclase, albite, and olivine used as standards. accel- tained a with vapor bubbles and one or more crys- erating potential of 15 kV, an emission current of 0.02 pA, and Elass talline phasesidentified as qvartz>lithiophilite, and minor a beam diameter of lessthan l0 pm were normal operating con- properties, ditions. ZAF corections were made using the program MAGrcrv sicklerite on the basis of optical microprobe (Colby, 1968). The relatively small size of the crystalline run data, and electron-diffraction patterns. Figure 2 shows an products (l-30 pm) resulted in some uncertainty about micro- optical photomicrographof run l-9 which containedden- probe compositions of the smaller grains. Therefore, as an aid dritic groups of quartz crystals and vapor bubbles in glass. SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES 359

'ra--- 900 \,: . . O "---1.^. . . 800 a fi--r-'---. -----,l --i--,i-- T("c) L+Q i 700 Ai AA i L.r-v+e 600 ,'AAA AAA

500 --+----+-----a-----e-----t--. L+V+Q+Ph 400 I I

1 2 t e 10 11 12 il""i, 't"."t", ,iro Ol al+v aLre al+v+e IL+v+e+Ptr Fig. l. Pseudobinarytemperature-composition diagram showingthe phase assemblages in the run productsfrom 12crys- tallizationexperiments. Boundaries separating phase-assemblage fieldsare marked by dashedlines and are believed to beaccurate to +5ffC and +0.5 wt9oHrO. The lettersrefer to the following phases:L: liquid (melt),V: vapor,Q : quartz,p6: phosphate(primarily lithiophilite-triphylite).

An example of a run containing lithiophilite and quartz Fig. 2. Photomicrographof dendriticgroups of interlocking in glass(2-3) is shown in the back-scatteredelectron pho- quartzcrystals together with vapor bubblesin aluminosilicate phases tomicrograph in Figure 3. These are describedbe- glassfrom run capsulel-9. Scalebar : 100pm' low in order ofdecreasing abundance.

Glass cibility using similar rnu techniques (James and Mc- The glassin each capsuleis colorlessto pale brown. A Millan, 1970) readily showed evidence for such immis- representativemicroprobe analysisofthis glass(from cap- cibility in quenched liquids from the Li2O-SiOr-PrOs sule l-9, which contains quartz but no other recognizable system.The rnn observationsof the presentstudy do not crystallinephase) gave (in weight percent)7 4o/o SiOr, l2o/o identify the causeof the cloudy areasbut do tend to rule AlrOr, 50/oKrO,2o/o NarO, l0loPzOr, 0.50/o FeO, and 0.50/o out silicate-phosphateliquid immiscibility. MnO; the remainder is presumably HrO, LirO, and F. All of the chargeswith greaterthan I wto/owater contain Some variations in glasscomposition were noted during numerous bubbles which vary in size, number, and dis- microprobe analysis,with P2O5ranging from I to 3.5 wto/o tribution betweencharges (see Table 2 andFig.2)' There (average: 2 wto/o)for 46 spot analysesfrom glassesin I 5 is a significant increasein the number of vapor bubbles capsulesthat lacked phosphatecrystals. These composi- between chargeswith 3 and 5 wto/owater, suggestingthat tional inhomogeneitiesin some mns may indicate lack of the water-saturationlevel for melts ofthe run composition equilibrium or they may have been caused,in part, by is near 4 wt0/owater. This value is similar to the saturation element diftrsion during microprobe analysis. The glass level of 4.60lowater in granitic pegmatitemelts at 1.5 kbar is usually clear, but some areasexhibit cloudinessthat is found by Burnham and Jahns (1958). causedby numerous,small (< I pm) objectsthat could be crystals,vapor bubbles, or globulesof an immiscible liq- Quartz uid. Microprobe analysesof several of the cloudy areas Quartz is the most abundant crystalline phase present showed them to have essentiallythe same composition in the runs, occurringin all capsulesthat containedcrystals as nearby clear or colored areas.rEM examination ofone and forming over a wide range of water contents (l-l I such cloudy area at high magnification (200,00Qx) failed wto/o)and temperatures (800C and below). This phase to revealany indication ofliquid phaseseparation or small was positively identified by electron difraction and en- crystals.Past studies of phosphate-silicateliquid immis- ergy-dispersiveX-ray analysisusing the rEM; seeShigley 360 SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES

Table 2. Conditions and results of crvstallization exoeriments

Run Initial Initial Duration Water Final Final Durat ion Est. Products % of 'C remp u rressure nrs Content Z Tenp Pressure hrs A Crystals kbar kbar

1-1 800 1.5 52 600 L.3-L.2 120 400 L+Qz 7 5 800 52 600 1.3-1.2 720 300 L+V+QZ 15 800 52 600 1.3-1.2 120 200 L+V+QZ <5 L-1 800 52 600 r.3-r.2 120 200 L+V+QZ <5 r-9 800 52 600 1.3-1.2 t20 200 L+V+Qz 10 1- 11 800 1.5 52 11 600 r.3-r.2 120 200 L+V+QZ 5

2-l 800 47 I 2-3 800 1.5-1,4 47 3 500 1.2-1.1 ll7 400 L+V+Qz+Ph <5 800 1.5-1.4 47 5 500 L.2-r.l r17 300 I fI'TA-JDL ] A 2-7 800 r.5-1.4 47 7 500 1.2-1. 1 ll7 300 L+V+Qz <5 2-9 800 L.5-1 .4 41 9 500 r.2-t.r rr7 300 L+V+Qz 20 2-7r 800 r. 5-1.4 47 11 500 1. 2-1.I tL7 300 l+V+Qz+Ph(?) 10

4-3 800 r. 5-1.4 3 800 r. 5-I.4 5 500 L4 120 300 L+V+Qz+Ph 25 4-7 800 46 7 4-9 800 1.5-1.4 46 9 4-11 800 r. 5-1.4 46 11 500 r.4 120 300 T +\/+n?+Ph ) \

5-3 800 1. 6 70 3 800 1. 6 10 5 400 I. 5 100 400 L+V+Qz+Ph 20 800 1. 6 70 7 400 1.5 100 400 T +tl+nz+Ph 20 5-9 800 1. 6 lo 9 800 1. 6 70 11

6-3 800 1. 5 164 3 800 1.5 100 L+Qz 30 6-5 800 1.5 r64 5 800 1. 5 0 L+V+QZ 30 6-7 800 1. s rb4 7 800 1 ,5 0 l+V+Qz 20 6-9 800 1. 5 r64 9 800 1. 5 0 I,+V+QZ 20 6-11 800 1. s t64 11 800 1. 5 0 L+V+QZ 20

8-3 1000 48 1000 0 L+V O 8-5 1000 48 1000 0 L+V 0 8-7 1000 48 1000 0 L+V O 8-9 1000 48 1000 0 L+V O

9-3 1100 7.6 2I 1100 1 ,5 0 L+V 0 9-5 1100 1.6 2l 1100 1.5 0 L+V 0 9-7 1100 1.6 2L 1100 1. 5 0 L+V O 9-9 I 100 1.6 2T 1100 1.5 0 1-+V 0

10-1 1100 1. 6-1. 5 28 900 1 .5 95 100 L+Qz 15 10-4 1100 1 . 6-1.5 28 900 l. 5 95 0 L+v+Qz IO 10-7 1100 1. 6-1. 5 2a 7 900 I .5 95 O L+V 0 10-10 1100 1 . 6-1.5 28 10 900 1 .5 95 O L+V 0

1r-t 1100 r.4 28 1 850 r.4-r.2 95 150 L 0 17-4 1100 28 4 850 r.4-r.2 95 0 l+v 0 II-7 1100 7.4 28 7 850 1.4-1.2 95 0 L+v 0 11- 10 1100 r.4 28 10 850 L.4-t.2 95 0 L+v 0

L2-I 1100 30 I 800 )..4 42 20O L+Qz 20 12-4 1100 30 4 800 r.4 42 0 L+v 0 12-7 I100 1.5 30 7 800 r.4 42 O L+V 0 12-10 1100 1.5 30 10 800 r,4 42 O L+V 0

14-1 1100 1. 7-1.5 47 1 700 1.5-1.4 t37 300 L+Qz 80 L4-4 1100 t.7-r.5 47 4 700 1. 5-1.4 137 100 L+Qz 50 1100 1. 7-1. 5 7 700 1. 5-1.4 137 100 L+v+Qz 20 14-10 1100 1 .7-1. 5 47 10 700 1.5-1 .4 L37 100 L+V+QZ 5

16-1 1100 2.0-1.9 24 1 600 |.7 L4I 400 L+Qz 10 1100 2 .0-1. 9 24 4 600 r.7 2O0 L+V+QZ 10 r6-7 1100 2.O-1.9 24 7 600 1.1 T4L 2OO L+V+QZ 10 16-10 1100 2.O-t.9 10 600 t.7 rqa 200 L+V+QZ 5

Colunn headings; Initial temp = homogenization temperature; Initlal pressure = homogenization pressure; Final temp = temperature durlng crystallization; Final pressure = pressure during crystallization; Duration = time capsule held at tenperature; est. A = eatimated undercooling below liquidus; Products L=liquid(g1ass),V=vapor,Qz=qlartz,Ph=phosphatemineral;%ofcrystals=totalpercentage of crystals in capsule at end of run. Pt capsules - runs /l 1-7; AgPd capsules - runs /l 8-16.

Column entries: runs /l 6,8, and t held at hlgher temperature throughout experiment; * = capsule lost its volatlle content and lts constituents were umelted at the end of the run, SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES 361

Fig.4. Back-scatteredelectron photomicrograph (upper left) andcorresponding KaX-ray mapsfor phosphorus(upper right), manganese(lower left), and iron (Iowerright) for lithiophilite grainsin aluminosilicateglass from run capsule2-3 (seeFig. 3). 25 pm. Thewhite, Fig. 3. Back-scatteredelectron photomicrograph of a pol- Scalebar, applicable to all four images,equals The ishedthin sectionofaluminosilicate glass from run capsule2-3. irregularblebs are lithiophilite-triphylite crystals. dark-gray glass electron Grainsof theMn-Fe phosphate mineral identified as lithiophilite objectswithin the shownin the back-scattered q:uartz featuresare holes appearas white, spherical-,irregular-, or equant-shapedareas micrographare crystals,and the black in photomicro- distributedthroughout the glass.The dark-grayareas within the formedby vaporbubbles. The sharplines this glass polishing. glassrepresent tabular quartz crystals. Scale bar : 50 pm. graphare scratches on the surfacecaused by

(1982) for details. The following types of quartz mor- rangedfrom 0.1 x l0-8 to 5 x l0-a cmls dependingon phologieswere observed:(l) tabular crystals of 5-25 pm crystal habit, with relative growth rates parallel to the c in length and 5 prmin width; (2) acicular crystals of 25- crystallographicaxis being greater than those parallel to 7 5 pm in lengthand 5-10 pm in width; (3) dendritic groups a. These rates are comparable to the growth rates for of interlocking crystals up to several millimeters in size quartz(1.2 to 3.4 x 10-8cm/s) crystallizedfrom melts of and showing uniform extinction; and (4) multicrystalline, Harding pegmatite and AbuoQoocompositions (Swanson spherulitic aggregatesto several millimeters in size with and Fenn, 1986). radiating habit and wavy extinction. These different quartz habits were not found together Phosphates in the samecapsule in most runs. The acicularand tabular Small (up to 30 pm), light- or reddish-brown grains of crystalsare more common in runs with higher water con- Mn-Fe phosphatewere observedin runs 2, 4, and 5 (Table tents and smaller undercoolings,whereas the spherulitic 2), which also contain quartz. Theseruns were at or above crystalsare more common in runs with higher water con- water saturation and were undercooled by 300-400'C. tents and larger undercoolings.A generaldecrease in the The grains generally appear as white, irregularly shaped number of quartz crystals was observed with increasing blebs in back-scatteredelectron images(Fig. 3) and range water content. Theseobservations are consistent with those in size from lessthan 5 pm to 30 pm. Some of the grains from other crystallization studies of qtartz and feldspar are tabular to euhedral crystals. Optical examination (Swansonet al., 1972; Fenn, 1973, 1977 ; Lofgren, 1974, showedthat most of thesegrains display moderate relief, 1980; Swanson,1977;Fenn and Jahns, 1979; Swanson moderatebirefringence, uniform, straight extinction (not- and Fenn, 1986).With the exceptionof someof the spher- ed for euhedral crystals), and little pleochroism. X-ray ulites, most of the quartz crystals from the growth runs maps (Fig. 4) and microprobe analysis of these types of are suspendedin the glassand are not attachedto vapor grainsindicate the presenceof Mn, Fe, and P in amounts bubblesor the capsulewall, suggestingthat they nucleated consistent with the lithiophilite-triphylite solid-solution internally. Minimum growth rates observedin theseruns series (Table 3). Electron-diffraction patterns of one of 362 SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES

Table 3. Comparison of chemical data for lithiophilite

phosphate grains starting stoi-chlometric in glass * material- *t* composition ***

Pzos 44.85 4). ZL

FeOl 11.99 0.0

Mn0 36.25 .1) . ZZ

L|20 ,++ 8.78 9.56

otherr I I v.oz 1.03 0.0

total QO 17 102.90 r00.00

roral - Li2O QO l7 94.12 90.44

(I,lnO+FeO) 43.03 48.24 t,< )t

(I,lnO/MnO+I'eo) 0.87 0.75 1.00

average of five rnicroprobe analyses of various phosphate grains in capsule 2-5 ** analysis of Branchville lithiophllite starting material with iron shown as FeO - see Table I *** ldea1 compositi.on for lilvlnPO4 f total iron reported as FeO f+ not determined tf+ SiO2, A1203r K2O, Na2O

thesegrains gave d-spacingsand relative intensities that Fenn's ( 19 73, I 9 77) observation that experimentally grown are consistentwith X-ray-ditrraction data for lithiophilite alkali feldspar crystals generally elongate along a rather (Blanchard,l98l); seeShigley (1982) for details.These than c. In addition. rEM examination of several of these observationsindicate that most of the phosphategrains crystalsshowed that they are not feldsparbut did not lead (> 800/o)produced in the experimental runs are members to a unique identification. of the lithiophilite-triphylite solid-solution series.Micro- probe traversesacross the boundariesbetween lithiophi- DrscussroN AND coNcLUSroNs lite and surroundingglass generally showed a sharp break Lithiophilite paragenesis with no evidenceof an element depletion or enrichment halo around phosphategrains. Measured minimum growth The experimental results indicate that lithiophilite and rates for lithiophilite crystals,which are roughly equal in minor sickleritecocrystallizedwith quartz from a hydrous sizeto the tabular quartz crystals,are on the order of I x pegmatitic melt containing about 2 wto/oPrO, over the l0-t cm/s. temperatureinterval 500'C to at least 400"C at about 1.5- A secondtype ofphosphategrain, representingless than kbar pressure.Water was above or near the saturation 200/oof the total phosphatecrystals present, is distinguish- level (about 4 wto/oHrO) in all of the lithiophilite-pro- able from lithiophilite only on the basis ofoptical prop- ducing runs; in the water-undersaturatedruns (l wto/oHrO) erties.These grains show high birefringenceand reddish- no lithiophilite was produced. These results suggestthat brown pleochroism and are believed to be sicklerite. crystallization of lithiophilite from a pegmatite melt is facilitated by the presenceof an exsolved aqueous fluid phases Other phase;thus they are consistentwith the presenceoflith- A small percentage(<50/0) of the smaller, isolated, col- iophilite in the upper intermediate zone of the Stewart orlesscrystals present in the run products clearly display pegmatitewhich, accordingto the Jahns-Burnhammodel, inclined extinction and initially were thought to be alkali is believed to have formed in the presenceof an exsolved feldspars.Subsequent examination showed that none of volatile fluid during the latter stagesof pegmatite crys- thesecrystals exhibited a negative sign of elongation, which tallization. These results are also consistentwith the ab- would be expectedif they were alkali feldspars,based on sence of lithiophilite from the lower, albite-rich inter- SHIGLEYAND BROWN:LITHIOPHILITE IN GRANITIC PEGMATITES 363 mediate zone below the quartz core of the Stewart melts over a similar range of P-?" conditions and water pegmatite,which is thought to have formed at the same contents(Fen n, | 97 3, I 97 7 ; Swansonet al., | 97 2; I-nfgr en, time as the upper intermediate zone but not in the pres- I 9741Fenn and Jahns, I 979;Fenn, I 986).Possible factors enceof an exsolvedfluid phase(Jahns, 1982). The tem- responsiblefor the absenceof alkali feldsparsin any of perature interval reported above should be viewed as a the crystallization runs from the presentstudy include (l) minimum estimate for the formation of lithiophilite in the presenceof phosphorus and its ability to form com- the Stewart pegmatite; this is becauseof the failure of plexeswith alkalis and aluminum; (2) the type of capsule feldspar to nucleateand grow in the lithiophilite-produc- material (Pt) used in most of these experiments; and (3) ing runs may have causeda lowering of the temperature the largedegree of undercoolingemployed for most of the at which lithiophilite was saturated in the experimental crystallization runs. The discussion below suggeststhat melts. the last two factors are more important than the first one Field examination of the phosphate-containing,upper in preventing the nucleation and growth of alkali feld- quartz-microcline intermediate zone of the Stewart peg- spars. matite (Shigleyand Brown, 1985)showed that individual Raman spectra of quenched, anhydrous, aluminosili- phosphatecrystals or severalintergrown crystalsare sep- cate melts with I to 5 molo/oPrO' (Mysen et a1.,l98l) arated from each other by I m or more. The larger the have beeninterpreted as indicating the presenceof M-PO4 crystalsor intergrowths were, the geater their separation (M : Na,Ca) and AIPO. complexesin melts of different from other phosphatecrystals was. This scattereddistri- nonbridging oxygento tetrahedralcation ratios (NBO/T). bution suggestsa relatively low density of lithiophilite If such complexes,including K-PO4, do form in P-con- nucleation sites in the pegmatite melt. The spacingsamong taining pegmatite melts, the activities of Na, K, and Al crystalssuggest that even though the concentrationsof Li, should decrease,resulting in a reduction in the activities Mn, Fe, and P in the pegmatite melt at this stageof crys- of feldspar components. The abundance of alkalis and tallization were sufficient to permit lithiophilite forma- aluminum, compared with the small amount of PrO, in tion, theseelements were dispersedover a relatively large the melts of this experimental study, however, indicate volume around eachgrowing crystal.As componentswere that this proposed mechanism may not be the primary transported to the nucleation site, they were effectively reasonfor the absenceof alkali feldsparsin the run prod- depletedfrom this volume, which increasedwith the size ucts. of each crystal. Lithiophilite crystalsin the experimental A more important reason for the absenceof feldspars run products were scatteredin a similar fashion (seeFig. in the run products may involve the use of Pt capsule 3), although their separationsand sizesare much smaller. material for most of the crystallization runs. Fenn's un' The hypothesisconcerning element dispersion can be re- published work on feldspar growth from hydrous alu- lated to the inferred role of an exsolved fluid phase in minosilicatemelts (P. M. Fenn, 1985,pers. comm.) has lithiophilite growth from pegmatitemelts. Although little shown that feldspars almost invariably nucleate on and is known about the partitioning behavior of Li, P, Mn, grow from capsulewalls; suchgrowth is relatively easyon and Fe betweensilicate melt and a coexistingvapor phase, Au capsule walls, occurs to a limited extent on AgPd this water-rich phasecould effectrelatively rapid transport capsulewalls, and is quite difficult on Pt capsule walls, of these elements to the lithiophilite nucleation sites if especiallyfor melts of pegmatite composition. Thesedif- they do partition into the vapor phase.In addition to this ferencesmay be related to observeddifferences in H, dif- hypothesized mechanism, water dissolved in the melt fusion rates through Au (low H, permeability), AgPd (in- would almost certainly lower viscosity and increasethe termediate H, permeability), and Pt (hrgh H, permeability) diffirsivity of these elements.Production of lithiophilite and the resulting diferences in catalytic activity of H, at crystalsonly in experimental runs with water at or above capsulewalls, or to differencesin the catalytic activity of the saturation level, however, favors the former mecha- the different metals.If differencesin H, permeability cause nism. The element-dispersionhypothesis is also consis- a grcarerbuild-up of H, near the Au capsulewall than tent with the lack of evidencefor phosphate-silicateliquid near the Pt capsulewall, for example, there could be dif- immiscibility in the experimentalruns without phosphate ferencesin depolymerization reactionsof the aluminosil- crystals, suggestingthat such immiscibility is not neces- icate melt, involving H, or protons as catalysts,near the sary for lithiophilite formation in water-saturated peg- walls of different capsulematerials. Recent work by Luth matite melts. and Boettcher (1986) suggeststhat small amounts of Hr, from the dissociation reaction of HrO, can dissolve in Absenceof feldspar in run products hydrous silicate melts, resulting in increasesin solidus Considering the bulk composition of the run mixture, temperatures;however, little is known about the way in which is peraluminousand alkali-rich, the lack of feldspar which H, interacts with the melt structure. If this inter- crystalsin the run products is surprising and inconsistent action enhancesmelt depolymerization and the produc- with the close associationof lithiophilite, quartz, and al- tion of silicate monomers in the vicinity of Au capsule kali feldspar in the Stewart pegmatite. Both alkali and walls relative to Pt, the nucleation and growth of feldspars plagioclasefeldspars have been crystallized from phos- could be facilitated (cf. Taylor and Brown, 1979; deJong phate-free, natural and simplified, synthetic pegmatite et al., l98l; Turnbull, 1985).Although highly speculative, SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES

this mechanism could account for the relative differences plexes as well as "sheetlike" PrOr complexes. The sug- in ease of feldspar nucleation on the walls of different gestedpresence of AIPO. complexesimplies that Na and capsule materials and for the lack of nucleation in the Ca would be freed from their charge-balancing,network- experimentsreported here. forming role to become network modifiers, resulting in It is also likely that the large amount of undercooling some depolymerization of the melt. In neither depoly- in our phosphate-and quartz-producingruns (300-400"C) merized nor fully polymerized aluminosilicate melts was inhibited alkali feldspar nucleation. Fenn (1973) found Raman spectralevidence found for Si-O-P linkages(My- that very small (25'C) or very large(465'C) undercoolings senet al., 1981). produced a significant kinetic or energeticbarrier to the The above discussionis for anhydrous,P-bearing melts nucleation of feldsparsfrom hydrous melts in the system and doesnot apply directly to hydrous melts suchas those Ab-Or-H,O. from which pegmatitesform. Water in such melts is thought to occur, in part, as OH- and, in part, as dissolved mo- Shuctural role of P in aluminosilicate melts lecular HrO below water-saturationlevels (Stolper, 1982); Although typically a minor element in most alumino- above saturation levels, excesswater exsolves from the silicate melts, phosphoruscan have a dramatic influence melt to form a separatefluid phase.Raman spectralstud- on phaserelations and other melt properties.In synthetic ies of chemically simple, hydrous, aluminosilicate melts silicate melts, the presenceof PrO, has been shown to with and without nonbridgingoxygens (Mysen et al.,1982) reduce liquidus temperatures(Tien and Hummel, 1962; were interpreted as indicating that water causesdepoly- Wyllie and Tuttle, 1964\, to increasesilicate tetrahedral merization. Thus the presenceof phosphorus and water polymerization and enlargethe liquidus stability fields of together in pegmatite melts should result in less depoly- more polymerized silicate minerals (Kushiro, 1975),and merization than with water and no phosphorus;this pre- to promote silicate liquid immiscibility and associated diction assumesthat pegmatite melts are not fully poly- trace-elementpartitioning (Watson, 1976; Ryerson and merized and that phosphorus does not form complexes Hess,1978; Visser and Koster van Groos, 1979a,1979b; with OH- or HrO. This latter assumption may not be Mysen et al., 1982).These effectsshould be magnified in correct in light of the occurrenceof P-OH bonds in nu- the late-stagemagmas, such as pegmatite melts, because merousorthophosphate esters (see, e.g., Corbridge, l97l) of phosphorusenrichment in thesemelts as documented and in some orthophosphateminerals such as hureaulite in field investigations (e.9., Murata and Richter, 1966; (Moore and Araki, 1973). Henderson, 1968) and in experimental crystallization In the caseof water-saturatedpegmatite melts, such as studies(e.g., Rutherford et al., 1974;Hess et al., 1975). those from which lithiophilite was grown in the present In silicate and phosphatemelts and quenchedmelts, all study, there is permissive evidencethat phosphorusmay available evidence suggeststhat phosphorus is tetrahe- partition into the exsolvedaqueous-fluid phase. Ifthis is, drally coordinatedby oxygen(van Wazer, 1950;Westman in fact, the case, some of the phosphorus would be re- and Crowther, 1954; Westman and Gartaganis, 1957; moved from the melt and that fraction would no longer Meadowcraft and Richardson, 1965; Fraser, 1976; Ryer- causethe structural changessuggested above. son and Hess, 1980; Mysen et al., 1981). In relatively This oversimplified and speculative picture of the depolymerized natural magmasand synthetic silicate melts "structure" of HrO- and P-containingpegmatite melts can (high NBO/T), where a variety of cations in addition to be used to help rationalize some of the experimental ob- Si are present, the addition of Pr+ is believed to cause servations from the present study. The proposed forma- formation of M-PO. orthophosphate complexes with tion of AIPO. and M-PO. complexesin pegmatite melts nontetrahedral(M) cations suchas Li, Na, K, Ca, Fe, Mn, discussedearlier may account, in part, for the lack of which etrectively removes them from their normal net- feldspar nucleation and growh in the experimental runs. work-modifier roles (Ryerson and Hess, 1980; see also It was stated earlier that the addition of small amounts van Wazer and Callis, 1958). This assertionis based on of phosphorusto a granite pegmatite melt has little effect a comparison of the enthalpiesof formation of crystalline on the growth rate of quartz. Becausephosphorus is not phosphates and silicates, which indicates that P-O-M predicted to form P-O-Si linkages and may partition into bonds are energeticallymore stablethan Si-O-M bonds. the exsolved aqueous-fluid phase, it should have little This preferential bonding of M cations with POo tetra- efect on the mobility of SiOotetrahedra or on the efowth hedra in relatively depolymerized melts would produce rate of quartz in water-saturatedmelts. an increasein Si-O-Si linkagesand increasethe tendency The phosphatesproduced in the present experimental for silicate-phosphateliquid immiscibility (Hess, 1977). work and occurring in the Stewartpegmatite and in other Mysen et al. (1981) suggesteda different role for phos- natural phosphateparageneses are orthophosphates,i.e., phorus in highly polymerized framework structure melts their structures are made up of isolated POo tetrahedra such as those of NaAlSirO, and CaAlrSirO, compositions which arelinked by nontetrahedralcations. Byrappa (l 983) (NBO/T:0; seeTaylor and Brown, 1979);they inter- has shown in hydrothermal synthesis experiments that preted Raman spectraldata for quenched melts of these condensedphosphates are not stable in the presenceof compositions as indicating that addition of phosphorus water. In contrast, synthesis of phosphates from anhy- resultsin the formation of three-dimensionalAIPO4 com- drous melts resultsin pryo-, ring-, chain-, and sheet-phos- SHIGLEY AND BROWN: LITHIOPHILITE IN GRANITIC PEGMATITES 365 phate structures(Corbridge, l97l). Thus water must de- Corbridge,D.E.C. (1971)The structuralchemistry ofphosphates. polymerize phosphatestructural units such as the proposed Bulletin de la Soci€tdfrangaise de Min€ralogie et de Cristal- 94, 27l-299. "sheetlike"PrO, units (Mysenet al., l98l) and other pro- lographie, deJong,B.H.W.S., Keefer, K.D., Brown, G.E., Jr., and Taylor, posed polymerized units (see Fraser, 1976) in hydrous C.M. (1981) Polymerization of silicate and aluminate tetra- silicate melts, resulting in the formation of orthophos- hedra in glasses,melts, and aqueoussolutions-Ill. I-ocal sil- phates;this depolymerization in HrO-containing silicate icon environments and internal nucleation in silicate glasses. 45, 1291-1308' melts also should promote the dispersalof POotetrahedra Geochimicaet CosmochimicaActa, Fenn,P.M. (1973)Nucleation and growth ofalkali feldsparsfrom and reduce the probability of phosphate-silicateliquid melts in the system NaAlSi3Os-KAlSi3O8-H,O.Ph.D. thesis, immiscibility. No evidence was found for such immis- Stanford University, Stanford, California. cibility in quenchedmelts from the present study. - (1977)The nucleationand growth ofalkali feldsparsfrom Additional experimental and spectroscopicwork, es- hydrousmelts. Canadian Mineralogist, 15, 135-161. - (1986) On the origin of graphic granite. American Min- pecially on the partitioning behavior of phosphorus be- eralogist,7 l, 325-330. tween melt and exsolved aqueous-fluid phase,is needed Fenn, P.M., and Jahns,R.H. (1979) Experimental crystallization to verify some ofthe ideaspresented above and to further of Harding, New Mexico, pegmatite. Geological Society of elucidate the structural role of phosphorus in hydrous America,Abstracts with Programs,11,77-78. properties silicate metls. aluminosilicatemelts, including thosethat contain boron. Fraser,D.G. (1976)Thermodynamic of In D.G. Fraser,Ed. Thermodynamicsin geology,301-325. D. The recentexperimental study by London (1986) suggests Reidel Publishing Company, Dordrecht-Holland. that boron and lithium are fractionated into residual peg- Henderson,P. (l 968) The distribution ofphosphorus in the early matitic fluids. This fractionation causesan increasein the and middle stagesof fractionation of some basic layered in- silicatemelt-Hro miscibility to a point wherethere could trusions.Geochimica et CosmochimicaActa, 32,897-911. P.C. (1977) Structure of silicate melts. Canadian Miner- be extensive rehomogenization of silicate melt and an Hess, alogist,15, 162-178. exsolved aqueous-fluid phase during the latter stagesof Hess, P.C., Rutherford, M.J., Guillemette, R.N., Ryerson, F.J., pegmatite crystallization. Thus certain aspectsofthe Jahns- andTuchfeld, H.A. (1975)Residual products offractional crys- Burnham model that pertain to the role of an exsolved tallization of lunar magmas:An experimental study. Proceed- volatile fluid during pegmatite crystallization may need ings ofthe 6th Lunar ScienceConference, l, 895-909. Jahns, R.H. (1953) The genesisof pegmatites:II. Quantitative re-evaluation. Additional experimental data for P-bear- analysis of lithium-bearing pegmatite, Mora County, New ing, hydrous silicate melt systemsalso are neededbefore Mexico. AmericanMineralogist, 38, 1078-11 12. predictions of equilibrium properties of melts like those - (1982) Internal evolution of granitic pegmatites. In P. made by Burnham and Nekvasil (1986) will be possible eern!, Ed. Granitic pegnatites in scienceand industry, 293- Short Course Hand- for P-containing pegmatite melts. 327, Mineralogical Association of Canada book 8. AcrNowr,BocMENTS Jahns, R.H., and Burnham, C.W. (1969) Experimental studies of pegmatitegenesis: I. A model for the derivation and crys- Weare deeply indebted to our latefriend and colleague, R. H. tallization of granitic pegmatites.Economic Geology,64, 863- Jahns,for introducingus to the fascinatingrocks known as peg- 864. matitesand for stimulatingour interestin this project.He pro- Jahns,R.H', and Wright, L.A. (1951) Gem- and lithium-bearing vided someof the startingmaterial for experimentalwork and pegmatitesof the Pala district, San Diego County, California. alsoread and commented on an earlyversion of thismanuscript. California Division of Mines SpecialReport,TA, l-70. We alsowish to expressour thanksto P. M. Fennand M. F. James,P.F., and McMillan, P.W. (1970) Quantitative measure- phase in glassesusing transmission elec- Hochella,Jr. for their help with experimentalproblems. In ad- ments of separation microscopy.Pan2. A study of lithia-silica glassesand the Fennis for helpful providing tron dition, thanked discussionsand for influence ofphosphorus pentoxide. Physicsand Chemistry of unpublishedexperimental data. Discussions with P. B. Moore Glasses.11.64-70. andJ. Stebbinshave also been helpful. The presentmanuscript Kushiro, Ikuo. (1975) On the natue of silicate melt and its sig- hasbenefited substantially from reviewsby an anonymousref- nificance in magma genesis:Regularities in the shift of the eree,R. Ewing,M. Hochella,J. Krumhansl,C. Ponader,and J. liquidus boundaries involving olivine, pyroxenes,and silica Stebbins.This studywas supported, in part,by NSFGrant EAR minerals. American Journal of Science,27 5, 4Il-431. 80-16911 (to Brown). Larsen, E.S. (1948) Batholith and associatedrocks of Corona, Elsinore and San Luis Rey quadrangles,Southern California. America Memoir 29. RrrunnNcrs Geological Society of Lofgren,G.E. (1974)An experimentalstudy of plagioclasecrystal Blanchard,N. (1981)X-ray powderdiffraction data for lithio- morphology: Isothermal crystallization. American Journal of philite.Florida Scientist, 44, 53-56. Science,274,243-273. Burnham,C. Wayne,and Jahns, R.H. (1958)Experimental stud- - (1980) Experimental studiesof dynamic crystallization of ies of pegmatitegenesis: The solubilityof water in granitic silicate melts. In R.B. Harglaves, Ed. Physics of magmatic magmas.[abs.] Geological Society of AmericaBulletin, 69, processes,487-55 1. PrincetonUniversity Press,Princeton, New 1544-1545. Jersey. Burnham,C. Wayne,and Nekvasil, Hanna. (1986) Equilibrium London, David. (1984) Experimentalphase equilibria in the sys- propertiesof granitepegmatite magmas, 71,239-263. tem LiAlSiOc-SiO,-HrO: A petrogeneticgrid for lithium-rich Byrappa,K. (1983)The possiblereasons for the absenceofcon- pegmatites.American Mineralogist, 69, 995-1004. densedphosphates in nature.Physics and Chemistryof Min- - (1986)The magmatic-hydrothermaltransition inthe Tan- erals,10,94-95. co rare-elementpegmatite: Evidence from fluid inclusions and Colby,J.W. (1968) Quantitative analysis of thin insulatingfilms. phase-equilibria experiments. 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