American Mineralogist, Volume 65, pages 970-980, 1980

The low-temperature inversion in sub-potassic nephelines

C. M. B. HnNoBnsoN

Department of Geology, The UniversityManchester MI3 gPL, England

AND ALAN BnucB THolrtpsoN

I nstitut fiir Krist all o gr ap hie und P etro gr ap hie, E T HZiirich C H - 809 2. Swit z erland

Abstract

Differential 5sxnning calorimetric (DSC) and high-temperature X-ray di-ffraction studieson seven nephelines belonging to the system NaAlSiOo(Ne)-KAlSiO4(Ks) show that sampleswith less than 2.5 mol percent Ks (sub-potassic nephelines) exhibit complex relations. AType-C nepheline containing 1.6 percent Ks has a 5ingle sharp DSC peak near 360 K thistemperature is close to that at which extra X-ray reflections characteristic of an orthorhombic3c superstructure disappear (365 K) and marks an inversion to hexagonal symmotry. Type-Bnephelines containing 0 to 0.36 percent Ks have two DSC peaks below 480 K which are sepa-rated by 2Oo to 40". The two peaks conflrm the existence of two phases in Type-B nephelinesand the presence of two inversions. For each sample, the lower-temperature DSC peak coin-cides with the disappearance of the orthorhombic superstructure (c/ Type-C) and the higher-temperature peak with the disappearance of extra X-ray reflections typical of a low-symmetrysecond phase. Type-B and -C nephelines have DSC scans sirnilar to that for tridymite whichsuggests that the low-temperature inversions in sub-potassic nephelines are caused by the col-lapse of the 'tridymite-like' framework about the relatively small Na* in the larger cationsites. In contrast, nephelines with more than 2.5 percent Ks are hexagonal at room temper-ature and show no inversions. Thus a Type-H nepheline eeat4ining 27 perc:ent Ks does nothave extra X-ray reflections and does not show any peaks on DSC scans between 340 and520 K.

The heat capacity (C) data for A.4O7 (pure-Na, Type-B) above 500 K show excellentagreement with the Ci equation fitted to the drop calorimetric data of Kelley et al. (1953) onsynthetic pure Na nepheline between 467 and ll80 K and can be used for thermochemicalcalculations on Ks-bearing nephelines above about 5M K.

Introduction

Although the natural occurrence of nepheline min-erals is restricted, they have been the subject of manydetailed experimental studies. [n particular, thenepheline (NaAlSiOo, Ne)-kalsilite (KAlSiOo, Ks)system has served as a useful analogue f61 6qdsllingactivity-composition relations of other imml5siUsNa-K phases (e.9. feldspars, micas, and halides). Inaddition, nepheline solid solution towards SiO, hasprovided necessary information on defect solid solu-tions, and the Na"KrAl8Si8O32 phase is an excellentexample of what may be an ordered compoundwithin a solid-solution series. However, nephelines of

the same composition may crystallize in differentspace-groups under different cooditions and theseforms may be related by phase transitions sf higherthan first-order. Very little is known generally of howsuch transitions affect phase equilibria in mineralsystems, but the large amount of information avail-able for nephelines makes them suitable candidatesfor such studies.

Henderson and Roux (1977) showed that neph-elines synthesized in the system Ne-Ks have differ-ent structures, depending upon composition andmethod of synthesis. At room temperature those withbetween 25 and 2.5 percent Ks (all compositions aregiven as mol percent) are hexagonal (Type-H). This

0003-004x/80/09 l M970$02.00 970

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES 971

composition range has x : 2.0 to 0.2, where -r is thenumber of K atoms in the unit cell Nar_,K,Al8Si8O3r.All X-ray peaks in this type of nepheline can be un-equivocally indexed using the Hahn and Buerger(1955) cell (HBC). In contrast, Henderson and Roux(1977) showed that nephelines with

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

Cell parameters at elevated temperatures were deter-mined using the method of Henderson and Taylor(1975) in which the Pt sample holder peaks wereused for internal standardization. The 2d values forthe peaks were measured with a vernier rule and arereproducible to within +0.005" 2d.

The room temperature cell parameters (Table l)for the sub-potassic nephelines refer to a pseudo-hexagonal cell, whereas those for Type-B nephelinesare averaged values for the two phases present (Hen-derson and Roux, 1977, p.293). This averaging is re-flected in the relatively high standard errors for thesesamples.

The low-temperature inversion was studied vdth aheating stage mounted on an X-ray diffractometer,using a heating rate of -3o min-' and a cooling rateof -5o min-t.

X-ray results

The room-temperature X-ray patterns for the threenew specfunens show that NE.l7 (1.6 percent Ks) isType-C whereas NE.l2 (0.36 percent Ks) andNE.l289 (0.30 percent Ks) are clearly Type-B. Allthree specimens have anouralous cell parameters atroom temperature (Table l) and plot below the 'nor-

mal nepheline lins' dsfiasd in Figure 3 of Hendersonand Roux (1977). Note that the average c parameterfor the two phases in NE.l289 is substantially smallerthan the c parameters for the specimens studied pre-viously.

The inversion in NE.l7 was monitored using theextra reflection at -28.2" 20 CuKa which is charac-teristic of the orthorhombic 3c superstructure ofType-C nephelines. The -reflection decreased in in-tensity and eventually disappeared over the temper-ature range 358-368+5 K, but it reappeared on cool-ing with a hysteresis of -15o.

Type-B nephelines have split (002) reflections, in-dicating the presence of two phases. The presence ofan extra reflection at -28.2"20 CuKa confirms thatone of the phases has the orthorhombic 3c super-structure whereas extra reflections at 22.6", 29.2",and 30.2" 20 CuKa presumably belong to the lower-symmetry second phase (Henderson and Roux, 1977,p. 293). Thus the inversions in NE.l2 and NE.l289(both Type-B) were monitored using the extra reflec-tions at 28.2" and 30.2" 20, and the split (002) reflec-tion. On heating NE.l2, the 28.2" 2d reflection de-creased and disappeared over the temperature range413428t5 K. The (002) peak was broad and asym-metric at 448 K, showing that two phases were stillpresent. The 30.2o 20 reflection disappeared over the

Table I. Synthesis condit ions, composit ion and room

temperature (293 K) cell parameters for different types ofsYnthetic nePhelines

KsSpecimen mol % T, K

Time

days a, A

Type -A

N , 4 *

Tvpe:E_

Schairer* +

A . 4 0 7 *

N 8 . 1 2

N E . 1 2 8 9

UP::.c-N E 1 7

TVoe-H_L . 4 r 1

1390 1 atm

rszo) -t l a m

r32ot870 2. 5 kbar

1170 1 l$ar1240 2 l$ar

990 I kbar

8?3 2l'dtrar

1

( sl " n

5043

0

00 . 3 60 . 3 0

9 . 9 6 3 G ) 8 . 3 4 1 ( 2 ) 7 r 1 . 7 ( 2 )

e . e 6 9 ( 3 ) 8 . 3 6 1 ( 4 ) 7 1 9 . 6 ( 5 )

9 .977 (2 , \ 8 . 33e(4) 718.9(5)e . 9 3 1 ( 6 ) 8 . 3 3 9 ( e ) 7 1 9 . 5 ( s )9 . 9 9 5 ( 3 ) 8 . 2 7 5 ( 6 ) ? 1 5 . 9 ( 5 )

s . 9 9 0 ( 1 ) 8 . 3 2 9 p \ 7 r 9 . 9 p l

10 .004(2) 8 . 390(3) 727. 2 (3)

* Data from Herderson and Roux (1977).

+ Glass start ing mater ial ; others synthesized from ge].

** Numbers in pereotheses.are est\mated standard deviations oI

D a € m e t e r s : a . c x 1 0 3 , v x 1 o ' .

range 428468+5 K. On cooling, the extra reflectionsreappeared at about l5o below these temperatures.On heating NE.1289, t}lre 28.2" 2d reflection dis-appeared over the nnge 413428+5 K, (002) wascomparatively sharp zt 425 K, and the 30.2" 20 extrareflection disappeared over the runge 423-444+5 K.Q1 ssqling, all of the extra reflections reappearedat -410 K but broadening of(002) could only be de-tected at -365 K.

In NE.12 and NE.1289 thLe 28.2" 2d reflection dis-appeared before there was any major decrease in theintensity of the 30.2" 2d reflection. Further study ofX-ray charts from heating experiments on Type-Bnephelines N.1042 (0.2 percent Ks) and N.1040 (0.5percent Ks) (Henderson and Roux, 1977) alsoshowed that the extra reflections at -28.2" 20 dis-appeared -10 K before the 30.2" 2d reflection beganto decrease. The same relationship was reported forthe pure-Na, Type-B nephelines (e.g. A.72, Hender-son and Roux, 1977, p. 289). The implication is thatthe two phases present in Type-B nephelines invertto hexagonal symmetry over different temperatureranges (see later discussion).

The X-ray patterns for NE.l2, NE.l l , andNE.l289 above their respective inversion temper-atures are entirely consistent with hexagonal symme-try and can be fully indexed using the HBC. Cell pa-rameters determined above the inversions are: NE.17(408 K) a : 9.981 esd O.OOI, c : 8.360 esd 0.00.21,NE.l2 (500 K) a : 9.992 esd 0.001, c : 8.366 esd0.002; NE.l289 (48 K) a : 9.983 esd 0.001, c :

8.360 esd 0.001A. These values plot close to the 'nor-

mal nepheline line' and in this respect the new speci-

Table 2. Change in proportions of the two phases present inType-B nephelines after annealing near the low-temp€rature

lnverston

Specim@ Mol. % Ks % of phase with lower q panmeterBeforc annal ing After anneal ing

973

were perfonned at a rate of l0o min-' with an in-strument RANGE setting of 5 mV sec-rcm-r. Temper-ature and energy calibrations were made relative tothe melting of indium metal (429.8 K) and the a-Bquafiz inversion (8214.0 K) (Table 3). Because of thelimited amounts of material available, only one ali-quot was studied for some nepheline samples. How-ever, two different aliquots studied for each ofNE.l2, NE.1289, and NE.l7 showed that the DSCscans were reproducible.

Because the voltage displacement on a DSC scan isa function of mass, the weights of empty gold pans,sapphire reference disc, and the powdered nephelinesamples in their gold pans were determined with adigital rricrobalance before and after each scan. Al-

Table 3. Experimental heat capacities of pure-Na, Type-Bnepheline A.407 (e.f.w- : 142 tjlP_aff.-ined by differential

Temp.

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

NE. Schairer4 .407N E . 1 2N E . 1 2 8 94 . 7 2

00

0 . 3 60 . 3 0

0

3 0505 08 0

303 03 08 040

mens are similar to other sub-potassic nephelines im-mediately above their inversion tefirperatures(Henderson and Roux, 1977, Fig. 3). Note thatNE.1289 shows an increase of 0.085A in c relative tothe room temperature value.

Henderson and Roux (1977, p.290 and 293) re-ported that the proportions of the two phases inType-B nephelines at room temperature changedwith thermal treatment. We have attempted to quan-tify this effect by comparing the peak heights of thesplit components of the (002) and (004) reflectionsbefore and after annealing the samples near the low-temperature inversion for 2 to 3 hours (Table 2). Be-cause the split components substantially overlap, theestimated proportions are not better than +20 per-cent (relative). After thermal treatment, A.72 andNE. Schairer (both pure-Na) and N8.12 (0.36 per-cent Ks) show increased proportions of the phasewith the lower c parameter whereas.this phase de-ueased in proportion in NE.l289 (0.30 percent Ks).All heat-treated samples show a marked increase inthe intensities of the 30.2" 20 extra reflection: thus itis the low-symmetry second phase that increases inamount. These relations allow the relative c parame-ters of the two phases in Type-B nephelines to be de-duced for all samples except A.407. Thus in A.72,NE. Schairer, and NE.l2 the low-symmetry secondphase has the lower c parameter while the phase withthe orthorhombic 3c superstructure has the higher cparameter. In NE.l289 the converse is true. Notethat the c parameters for the low-symmetry phases inA.72 and NE.1289, estimated from the split (002) and(004) reflections, are very similar, with values of8.325 and 8.315A, respectively.

Differential scanning calorimetric study

DSC procedures with the Perkin Elmer DSC-2

DSC analysis is a comparative technique involvingelectrical-thermal balance between a sample sealedin a gold pan against an empty reference gold pan inthe twin platinum holders of an adiabatic enclosure(O'Neill, 1966). The temperature scans with the DSC

Heat caprcity was determined with refermce to D( -Alroe(Ditmars and Douglas, 1971) and is believed to be accira"te towithin 1 percent m the basis of alternate scans on a sapphire discand o( -Alooo pwder,TemperatfirJs were calibrated against p3ak mset for indiummel t ing a t 429,8 K and the a-A quar tz inyers ion a t 844.0 Kandhave a prec is im o f -O.2o ,

TKeIvin

Heatcapac rty

sJ/(mo]. K)

Temp.

TKelvin

Heatcapa.city

upJ/(mol. K)

3 6 7 . 8372.8

382.73 3 7 , 6

3 9 2 . 63 9 7 . 5402.5407.44 r 2 . 3

41,7 34 2 2 . 24 2 7 . 2432.14 3 7 . 1

4 4 2 . 0447.04 5 1 . 9456.9461.8

1 3 5 . 51 3 6 . 3r 3 7 . 71 3 8 . 41 3 9 . 6

140. 51 4 1 . 6t42 .2143.5I 4 4 . 9

1 4 6 . 3r49 .41 5 C . 9L55.21 5 9 . 6

1 6 1 . 51 5 9 . 01 6 1 , 3t69 .71 7 8 . 0

4 6 6 . 8471.74 7 6 . 74 8 1 . 64 3 6 . 6

496.75 0 1 . 15 0 6 . 15 1 1 . 1521. t

5 3 1 . 15 4 1 . 05 5 1 . 05 6 1 . 05 7 1 . 0

5 8 1 . 05 9 1 . 06 0 1 . 06 1 0 . 062r.0

6 3 0 . 9640.96 5 0 . 96 6 0 . 9670.9

6 8 0 . I6 9 0 . 97 0 0 . I

1 6 5 . 5r 5 7 , 6

148, 1r 4 7 . 8

I 4 5 . 2t45.61 4 8 . 9146.7147 .0

t47 .61 4 9 . 1r49.21 5 0 . 01 5 1 . 0

15t.2152.41 5 3 , 1I O J . b

1 5 4 . 0

1 5 4 . 91 5 5 . I1 5 6 . 41 5 7 . 8

1 5 3 , 61 5 9 . 31 6 0 . 7

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

N.4 ( -O%Ks)

NE 1289 (fourth heoting) (0 30%Ks)

NE.17 (1.6%Ksl

NE. Schoirer (-O%Ks)

T'KFig. l. Tracings of DSC scans for various nepheline samples in the temperature interval 34() to 600 K. Scans were made at l0 K

min-r, with a RANGE setting of 5 mcal sec-r cm-r, chart range l0 mV (: 25 cm) and chart speed 20 mm min- r. The scan for NE.1289 is

the fourth heating scan (after the three scans in Fig. 2). Sample weights are between 20 and 30 mg. The solid vertical lines refer to the

temperatures (tlQ K) at which the extra X-ray reflections at -28.200 CuKa disappear, and the dotted vertical lines to those (+10 K) at

which the 30.2"20 CuKa extra X-ray rcflections disappear. The displacements are a function of sample mass and heat capacity of sample

and enclosing gold pans, and are measured relative to the low- and high+emperature isothermals bracketing the scans.

lI

.9Eo-coItcul

lowance was made for the small amounts of pow-dered samples that were lost during removal of thegold pans with a vacuum tool. Chart measurementswere used as input for a computer program (origi-nally written by K. Krupka, U.S. Geological Survey,Reston, and modified at ETH, Ztrich) to make thenecessary corrections for temperature calibration,differences in sample pan weights, energy calibrationconstant for the DSC instrument, and differences inlow- and high-temperature isothermal baselines. Theprogram was also used to compute the sample heatcapacity relative to corundum (the sapphire referencedisc), using data of Ditmars and Douglas (1971).

DSC results

Tracings of the chart recorder output for variousnepheline specimens are showr in Figure 1. A scaninterval of 340 to 600 K was used for N.4 (pure-Na,Type-A), NE. Schairer (pure-Na, Type-B), NE.1289and NE.l2 (both Type-B, 0.3 and 0.36 percent Ks,respectively), and NE.l7 (1.6 percent Ks, Type-C).The scan interval for A.407 (rure-Na, Type-B) was34V7W K and that for A.417 (27 percent Ks, Type-H) was 340-520 K.

The DSC sc,ans for the Type-A, pure-Na nepheline(N.4) show only a smooth, continuous displacementwithout peaks over the temperature range 340 to 600K (Fig. l); Type-H nepheline (,4..417) has a similarbehavior between 340 and 520 K. The DSC scan for

the Type-C nepheline (NE.17) has a single peak near360 K (Fig. l). The scans for the Type-B nephelinesare more complex. NE. Schairer (pure-Na) showstwo broad peaks, of approxinately equal intensity,near 430 and 470 K; the other pure-Na, Type-Bnepheline (A.,107) shows a similar overall behavior,but the lower temperature peak is less pronounced.The scan for NE.12 (0.36 percent Ks) also shows twobroad double peaks some 50 below those of NE.Schairer. The scan shown in Figure I for NE.l289(0.3 percent Ks) is the fourth suceessive fusating scanfor this specimen and shows two distinct peaks near370 and 390 K, each having a shoulder possibly con-cealing a second peak.

The nature of the DSC scans is noticeably depen-dent on temperature cycling, as is well illustrated forType-B specimen NE.l289 (Fig. 2). The first scan,made without initially cycling between the low- andhigh-temperature isothermals so that the sample wasnot annealed, produced a strong split peak near 410K with a minute irregularity near 425 K. The secondheating scan on NE.l289 included cycling the sampleacross the full temperature interval, 16ot anasaling it,and showed two distinct peaks. The lower-temper-ature peak contains shoulders corresponding to thesplit peak of the first scan. The higher-temperaturepeak, near 425 K, corresponds to the minute irregu-larity near this temperature observed in the first scan.Note that the combined area under the two peaks of

(cool . ing di rect tyafter B)

HENDERSON AND THOMPSON: SI]B-POTASSIC NEPHELINES 975

II

.9Eo

oE

IJJ

T,KFig. 2. Traces of DSC scans for Type-B nepheline sample

NE.1289, made under the same machine conditions as for Fig. l.Trace A represents the first heating without cycling the samplethrough the invenion. Curve B r€presents the second heating,including cycling the sample through the temperature range toadjust lower and upper isothermals. Curve C represents a coolingscan made directly after B. Curve D represents the third heating,including isothermal adjustments, made after C. See Fig. I for thefourth heating trace.

the second scan is similar to that under the singlepeak ofthe first scan. A cooling scan from 510 to 330K was made directly after the second heating scan.This cooling scan revealed a single peak about l0obelow and of comparable area to that of the first(unannealed) scan, and a small feature near 355 K.After the cooling scan, a third heating scan (againannealing between the low- and high-temperatureisothermals) was perfonned and also revealed thetwo peaks. There was an apparent increase in thearea under the higher temperature (-425 K) peak atthe expense of the area under the lower peak (-410K). The fourth heating scan on NE.l289 (Fig. l) issimilar to the third scan (Fig. 2) in terms of peak areaand shape.

The presence of peaks in the DSC scans confirmsthe occurrence of crystallographic inversions in thesenephelines, some of which may be lambda transitions(Rao and Rao, 1978, p. 22).However, it is not clearwhether the broad DSC peaks in some specimensrepresent lambda transitions, proceeding more slowlythan the a-B quartz inversion, or whether they reflectthe superposition of transitions due to different com-position domains in chemically zoned crystals (seelater discussion). If the temperatgls 5sanning ratekeeps pace with the rate of the nepheline inversion,the temperatures and areas under the DSC peakscould be used to obtain quantitative values for theheats of transition relative to the o'-B quartz in-version and indium melting peaks (see later).

Discussion

Relationship between DSC and X-ray results

The absence of DSC peaks in Type-H nephelineA.417 (27 percent Ks) and the absence of discontin-uities in its thermal expansion behavior (Hendersonand Roux, 1976) confirm that such nephelines havethe normal HBC structure at room temperature.

The absence of DSC peaks in Type-A nepheline(N.4, pure-Na) between 340 and 600 K is signifisallbecause an equivalent specimen (N.10) showed aninflection in a at 453 K. Henderson and Roux (1977)correlated this temperature with the low-temperatureinversion shown by pure-Na, Type-B nephelines, butthe extra X-ray reflections in N.l0 did not disappearuntil above 873 K. The DSC results confirm thatType-A nephelines do not show the same transitionnear 473 K as the pure-Na, Type-B nephelines.

The single DSC peak near 360 K for Type-Cnepheline NE.17 (1.6 percent Ks) can be correlatedwith the temperature (365+5 K) at which the extraX-ray reflection near 28.2"20 disappears (Fig. l). Inaddition, Type-C nepheline N.233 (0.3 percent Ks)showed inflections, but not discontinuities, in thethermal expansion of the cell edges close to the tem-perature at which the 28.2"20 peak disappears (Hen-derson and Roux, 1977, Fig. 6). It seems clear thatthe single DSC peak is caused by the inversion of theorthorhombic 3c superstructure, characteristic ofType-C nephelines, to hexagonal symmetry.

The occurrence of two peaks in the DSC scans forType-B nephelines is consistent with the presence oftwo phases, as suggested by the split (002) and (004)X-ray reflections. The temperatures of the lower-tem-perature DSC peaks in Type-B nephelines are within- l0 K of those at which the extra X-ray reflection at28.2"20 disappears (Fig. l). By analogy with Type-Cnephelines, this DSC peak is presumably caused byinversion of one of the phases from orthorhombic (3csuperstructure) to hexagonal symmetry. The higher-temperature DSC peaks in Type-B nephelines can becorrelated with the temperatures at which the30.2"20 extra X-ray reflection disappears (Fig. l);this reflection is characteristic of the lower-symmetryphase. Type-B nephelines also show discontinuitiesin cell volumes at this temperature (e.9. NE. Schai-rer, Fig. 6, Henderson and Roux, 1977). The higher-temperature DSC peak can, therefore, be correlatedwith inversion of the low-symmetry (second) phase tohexagonal symmetry.

The assignments of the DSC peaks in Type-Bnephelines are consistent with the proportions of the

HENDERSON AND THOMPSON: SUB.POTASSIC NEPHELINES

phases deduced from X-ray and DSC scans forNE.1289. The increase in intensity of the higher-temlperature DSC peak for NE.l289 after temperaturecycling suggests that the phase producing this peakincreased in proportion. This effect is also seen in thehigh+emperature X-ray results, which show that thephase with the higher c parameter increased inamount during annealing (Table 2). We have alreadydeduced that in NE.l289 the low-symmetry secondphase has the higher c parameter and, therefore, thehigher-temperature DSC peak can be correlated withthe inversion shown by this phase.

The DSC trace obtainsd s1 sesling NE.l289 im-mediately after the second heating (Fig. I, scan C)shows only a single peak near 400 K. This behavior isconsistent with the reappearanc€ of both the 28.2"and 30.2"20 extra X-ray reflections at 400+5 K uponcooling. Thus both inversions in NE.l289 appear tobe indistinguishable sn ss6ling. In addition, broad-sning, and ultfunately splitting, of the (002) reflectioncan only be detected at -363 K; this 6lendsling ma]be correlated with the small feature on the DSC scannear 355 K (Fig. 2, scan C).

Dependence of inverion temperature on composition

The work of Henderson and Roux (1977) and theresults of our X-ray and DSC studies demonstrate aclear relationship between increasing Ks content ofnepheline and decreasing temperature of the inferredorthorhombic to hexagonal inversion. In Type-Bnephelines this inversion has been correlated with thelower DSC peak (Fig. l). The upper DSC peak alsoappears to be displaced to lower temperatures withincreasing Ks.

The identification of two inversion peaks in theDSC study of Type-B nephelines could imply thatthe two phases in these samples coexist stably andhave different Ks contents. Henderson and Roux(1977) considered this possibility for Ks-bearingType-B nephelines but pointed out that it could notbe the case for the supposed pure-Na, Type-B speci-mens. They concluded that the two phases in the lat-ter specimens had the same composition, that the or-thorhombic 3c superstructure was metastable, andthat the lower-symmetry phase was the stable low-temperature form. Our results are not inconsistentwith this interpretation, but the presence of smallamounts of impurity KrO in the nominally pure-Nanephelines could have an important effect. Partialanalyses of A.407 and NE. Schairer gave

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

perature c parameters for the orthorhombic 3c super-structures in A.72 and NE.l289 differ bv as much as0. l2A, the lower-symmetry phases, *hi"h irr"r"ur"in amount on annealing in both specimens, havealmost identical room-temperature c parameters(8.320+0.005A, based on HBC-indexing). Thusphases with the orthorhombic 3c superstructure oc-cur in very different states of framework collapse.These structures are almost certainly metastable, andthe low-symmetry phase therefore appears to be thestable low-temperature form.

Relations between low-temperature inversions innep he line and t ri dymit e

Henderson and Roux (1977) related the occur-rence of the low-temperature inversion in sub-potas-sic nephelines to the collapse of the 'tridymite-type'

framework about the small Na atoms occupying thelarger cation sites (Dollase, 1970; Foreman and Pea-cor, 1970). They also drew attention to the similarbehavior of tridymite, which exhibits displacivetransformations near 470 K and which forms variouslower-symmetry (pseudohexagonal) cells below 470K (Sato, 1963a,b, 1964; Dollase, 1967 ; Kihara, 1977 ;Nukui et al.,1978).

Thompson and Wennemer (1979) measured theheat capacities of synthetic tridymite, cristobalite,and tridymite-cristobalite 'mixed phases' by DSC.Tridymite showed a sharp peak at 390 K and twobroad, overlapping peaks in the range 436470 K.These heat effects were correlated with the mono-clinic to low-orthorhombic and low-orthorhombic tointermediate-orthorhombic tridymite inversions ofNukui et al. (1978).

The DSC traces for sub-potassic nephelines andtridymite are broadly similar in that DSC peaks oc-cur within similar ranges of temperature. The singleDSC peak occurring in Type-C nephelines (NE.l7)has a similar form to the 390 K peak in tridymite.The two peaks occurring in Type-B nephelines (e.g.AA07) are similar in form and temperature range tothe broad heat effects between 436 and 470 K in tri-dymite. These similarities support the suggestion(Henderson and Roux, 1977) that the low-temper-ature inversions in sub-potassic nephelines are pri-marily a property of framework collapse and that theinterframework cations play a relatively subordinaterole. Further correlation between nepheline and tri-dymite is not possible because we do not fully under-stand either the nature of the structures of the stablelow-temperature forms or the role of 'structural cav-ities' (defects) in determining the structure. In addi-

tion, the nature of tridymite-cristobalite 'mixed

phases' is not yet known (Thompson and Wennemer,1979), but their occurrence suggests the possible exis-tence of analogous nepheline-carnegieite'mixedphases.' If such structural modulations exist as do-mains in sub-potassic nephelines (e.5. n the two-phase Type-B nephelines) the problem ofcharacter-izing the stable low-temperature form of nephelineswill become even more complex.

Correlation of heat capacity measurements withprevious data

Our data for the heat capacity (C) of nepheline4.407 (pure-Na, Type-B) are shown as a function oftemperature in Figure 3. Also shown are the low-temperature (50-298 K) adiabatic calorimetric mea-surements for synthetic nepheline (NaAlSiOo) andkaliophilite (KAlSiO4) from Kelley et al. (1953).(Note that the nepheline sample used by these work-ers was synthesized by J. F. Schairer and presumablyis similar to our specimen NE. Schairer.)Kelley et al.also measured the heat contents for the same syn-thetic nepheline sample from 387 to 1509 K by dropcalorimetry and presented three linear equations ofCi vs. Z for the ranges 298467 K (the low-temper-ature inversion), 467-1180 K (the high-temperatureinversion), and l180-1525 K. These equations wereadopted by Robie et al. (1978) in their recent com-pilation of thermochemical data. The fits for the 298-467 K and 467-1180 K ranges are shown by short-dashed lines in Figure 3. Our Ci data for A.4O7 above500 K agree well with the 467-1180 K equation ofKelley et al. (1953, p. l5). However, their equationfor the range 298467 K represents only an averageof measurements for sample A.407 because of thecomplex behavior of Ci at the inversions. The DSCmeasurements (summarized in Table 3) suggest thatthe peaks at 439 K and 462 K may represent lambda-type transitions (see also Kelley et al., 1953, p. l5).

The difficulties in evaluation of E-Idn" and SrSrn, from heat capacity measurements across broadtransitions are well known (see Rao and Rao, 1978).However, the Ci data obtained for nepheline ,{.407(Table 3) have been fitted to polynomial equationscovering various temperature ranges. The smoothedvalues of Ci and the integrated functions, If-Ifrn"and Sr-Sr,* are presented in Table 4. The heat in-volved in the transition (Fig. 3) for the inversions in4.407 was estimated by subtracting 14229 J mol-',the area between 400 K and 500 K below the extrap-olated 467-1180 K Co equation of Kelley et al., fromthe integrated areas in Table 4 (15397 J mol-'). The

978 HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

200.0

180.0

-

oE

oo-(J

160.0

140.0

100.0

120.0

200.0

Fig. 3. Heat capacity (Co) in joules mol-rK-r as a function of temperature for the specimen of pure-Na, Type-B nepheline A.407measured by DSC (O symbols between 3?0 and 700 K); note the presence of two peaks neat 440 and 460 K. Also shown are data pointsfor synthetic nepheline (tr, NaAlSiOo) and katiophilite (4, KAlSiO4) to 298 K determined by adiabatic calorimetry (Kelley el al., 1953).The short-dash lines, passing approximately through the transitions to 467 K and from 467 to 7N K, refer to the fits to the dropcalorimetric data for NaAlSiOa (Kelley et al., 1953). The long-dash curve refers to equivalent fits for KAlSiOa (see Robie et al.,1978' p.

405-416).

300.0 4oo.o 500.0 600.0 700'0

T,K

resulting value for the heat of transition (1168 Jmole ') is not noticeable on the scale of Figure 4 ofKelley et al., and has not been used to introduce dis-continuities into the integrated functions E-I{r",and S;-S;,8 presented in Table 4.

Although ttre 29846'7 K Ci equation of Kelley era/. represents an average of our measurements forsample A.407, it will only be valid for nominallypure-Na, Type-B nephelines because the heat capac-ity relationships for Ks-bearing Type-B and -C neph-elines will clearly be different from those for thespecimen of Kelley et al. (cf. NE. Schairer with Ks-bearing nephelines in Fig. l). In addition, we haveshown that Type-H nephelines do not exhibit any

DSC peaks between 340 and 520 K the only neph-elines likely to show such peaks in this temperaturerange are those with

Table 4. Smoothed molar thermodynamic propert ies fornepheline A.407 (NaAlSiOa)

979

inversion (cf. Type-C) and the upper-temperaturetransition with the low-symmetry (second phase)-hexagonal inversion. [n contrast, Type-H nepheiines(>2.5 percent Ks) show no DSC peaks indicative oftransitions or inversions between 340 and 520 K; theyare hexagonal at room temperature. Type-A neph-elines (pure-Na) also have no transitions between340 and 600 K.

Similarities between the DSC scans of sub-potassicnephelines and tridymite support the suggestion thatlow-temperature inversions in nepheline are primar-ily caused by collapse of the framework. The inter-framework cations appear to have only a modifyingeffect.

In experirnental studies of nephelines, the propor-tions of NaAlSiO.-KAlSiO4-trSiSiOo (tridymite)and the amount of framework or interframework or-der-disorder will influence the inversion character-istics and possibly the phase relations, especially ifmetastable structures occur. These complicating fac-tors should be considered when natural nephelinesfrom igneous and metamorphic rocks are studied.For studies attempting to obtain thermochemicalproperties for nepheline solid-solutions, the equationof Kelley et al. (1953) fitting the heat capacity datafor pure-Na nepheline above 467 K can be applied toKs-bearing nephelines. However, the Kelley et al.equation between 298 and 467 K is an averaged fitwhich, because of complexities introduced by the oc-currence of the low-temperature inversions, is spe-cific to pure-Na, Type-B nephelines.

AcknowledgmentsWe thank Dr. J. Roux for supplying the sample of N8.1289.

The differential scanning calorimeter is supported by the ETHSpecial Project Funds. We also thank K. Krupka for an extremelyhelpful review of the original manuscript. The work at Manchesterwas supported by the Natural Environment Research Council.

References

Buerger, M. G. (1951) Crystallographic aspects of phase transfor-mations. In R. Smoluchowski, Ed., Phase Transformations inSolids, p. 183-21L Wiley, New York.

Cohen, L. H. and W. Klement, Jr. (1976) Effect of pressure on re-versible solid-solid transitions in nepheline and carnegieite.Mineral. Mag., 40, 487 492.

Ditmars, D. A. and T. B. Douglas (1971) Measurements of the rel-ative enthalpy of pure a-Al2O3 (NBS Heat capacity and en-thalpy standard reference material No. 720) fron 273 to I 173 K.J. Res. Natl. Bur Stands., 75A, 401420.

Dollase, W. A. (1967) The crystal structure at 220"C of ortho-rhombic high tridymite from the Steinbach meteoire. ActaCrystallogr., 2 3, 617 -623.

- (1970) Least-squares refinement of the structure of a plu-tonic nepheline. Z. Kristallogr., 132,27-44.

HENDERSON AND THOMPSON: SUB-POTASSIC NEPHELINES

T

Kelv in

^oup

J/Gnol. K)

to -Ho

J/(m01. )

^o ^o

J/(rnol. K)

2 9 8 . 1 53 0 t . 03 5 0 . 03 6 0 . 03 ? 0 . 0

3 8 0 . 03 9 0 . 04 0 0 . 04 0 5 . 04 1 0 . 0

4 1 5 . 0420.0425.04 3 0 . 04 3 5 . 0

440.04 4 5 . 04 5 0 , 04 5 5 , 04 6 0 . 0

470.04 8 0 . 04 9 0 . 05 0 0 . 05 1 0 . 0

5 2 0 . 05 4 0 . 05 6 0 . 05 3 0 . 06 0 0 . 0

6 2 0 . 06 4 0 . 06 6 0 . 06 3 0 . 0? 0 0 . 0

r19 .21 1 9 . 7t3t.41 3 3 . 51 3 5 . ?

1 3 7 . 91 4 0 . 0742,01 L ' 1

t 4 2 , 9

r 4 4 , 31 4 6 . 31 4 9 . 0752.31 5 6 . 3

1 6 0 , 91 5 8 . 81 6 0 . 6166.2I 7 4 . 8

1 6 0 . 4150.2] 4 5 , 5r45 .7t46.4

747.71 4 8 . 51 4 9 . 91 5 1 . 3152.7

t 5 4 . 21 5 5 . 6t 5 7 , 01 5 8 . 41 5 9 . 8

02 2 1

650178269r72

r0540119301334014056I4775

15499r623116976t7 ' t351 8 5 1 3

1931220t1,9209132 r 7 2 722579

2+26325311272852873730197

316643+620376044061643657

4672649823529495610359285

00 . 7

2 0 . 1

3 1 . 23 4 . 83 8 . 34 0 . 14 r . 9

4 3 . 64 5 . 94 7 . 1€ . 950.7

54.45 6 . 1

5 9 . 8

6 3 . 46 6 . 76 9 , 77 2 . 77 5 , 5

7 8 , 48 4 . 08 9 . 49 4 . 79 9 . 8

1 0 4 . 91 0 9 . 81 1 4 . 61 1 9 . 3L 2 3 . 9

The loil tempenture adiabatic data for nepheline (Keliey q! q!. ,1953) was included in the smocthing of the data and thus 293.15 K isthe chosen referace tempenture. It is, therefore, assumed thatno further trositions occur between 298 md 370 K. Note that theintegnted functi@s have not been adiusted for heat or entropy oftrmsitiof,.

al. ovet the range 467-1180 K to temperatures abovell80 K, because the high-temperature inversion innepheline only occurs in specimens with

980

- and W. M. Thomas (1978) The crystal chemistry of silica-rich, alkali-deficient nepheline. Contrib. Mineral. Parol., 66,3 l 1 -318.

Donnay, G., J. F. Schairer and J. D. H. Donnay (1959) Ncphelinesolid solutions. Mineral. Mag., 32,93-t09.

Foreman, N. and D. R. Peacor (1970) Refinement of the nephelinestructur€ at several temperatures. Z. Kristallogr., 132,45-70.

Grossman, L. and I. M. Steele (1976) Amoeboid olivine aggre-gates in the Allende meteorite. Geochim. Cosmochim. Acta, 40,t49-155.

Hahn, T. and M. J. Buerger (1955) The detailed stnrcture of neph-eline KNa3AloSi4O 16. Z. Kristallogr., 1 06, 308-338.

Henderson, C. M. B. and J. Roux (1976) The thermal expansionsand crystallographic transformations of some synthetic neph-elines. Prog. Exp. Petrol., N.E.RC. Rep. 3,60-69.

- and - (1977) Inversions in sub-potassic nephelines.Contrib. Mineral. Petrol., 6 I, 279-298.

- and D. Taylor (1975) Thermal expansion by X-ray diffrac-tion: the use ofthe specimen holder as the internal standard andthe expansion of MgO (rcriclase) and MgAl2Oa (spinel). Trcns.Brit. Ceram. Soc., 74, 55-5'1.

Ishibashi, Y. and Y. Takagi (1975) Comments on specific heatmeasurements. Jap. J. Appl. Phys., 14,637-642.

Kelley, K. K., S. C. Todd, R. L. Orr, E. G. King and K. R. Bon-nickson (1953) Thermodynamic properties of sodium-alumi-num and potassium-aluminum silicates. U.S. Bur. Mines, Rep.Invest. 4955.

Kihara, K. (197'l) An orthorhombic superstructure of tridymite

HENDERSON AND THOMPSON: ST]B.POTASSIC NEPHELINES

coexisting between about 105 and l80oC. Z. Kristallogr., 146,185-203.

Nukui, A., H. Nakazawa and M. Akao (1978) Thermal changes in

monoclinic tridymite. Am. M ineral., 6 3, 1252-1259.O'Neilt, M. J. (1966) Measurement of specific heat functions by

differential scanning calorimetry. Anal. Chem., J& 133l-1336.Rao, C. N. R. and K. J. Rao (1978) Phase Transitions in Solids.

McGraw-Hill, New York.Robie, R. A., B. S. Hemingway and J. R. Fisher (1978) Thermody-

namic properties of minerals and related substances at 298. 15 Kand one bar (ld pascals) pressure and at higher temperatures.U.S. Geol. Sun. Bull. 1452.

Sato, M. (1963a) X-ray study of tridymite (l). On tridymite M andtridymite S. Mineral. J. (Japan),4' 115-130.

- (1963b) X-ray study of tridymite (2). Structure of low tri-dymite, type M. Mineral. f. Qapan),4, 13l-146'

- (1964) X-ray study of tridymite (3). Unit cell dimensionsand phase transition of tridymite, tyt/e S. Mineral. J. (Japan), 4'215-225.

Thompson, A. B. and M. Wennemer (1979) Heat capacities andinversions in tridymite, cristobalite, and tridymite-cristobalitemixed phases. Am. Mineral.,64, 1018-1026.

Tuttle, O. F. and J. V. Smith (1958) The nepheline-kalsilite sys-tem II. Phase relations. Am. J. $ci.,256,571-589.

Manuscript received, April 2, 1979;

accepted for publication, April 4, 1980.