Phase Relations of Titan-Acmitein the System Na,O-FerOr- AlrO

13 Pages·2007·1.4 MB·English

American Mineralogist, Volume 59, pages 536-548, 1974
PhaseR elationso f Titan-Acmitein the System Na,O-FerOr-
AlrO'-TiOr.SiOat, l(X)O BarsTotalWater Pressure
MenrN F. J. Frowrn
Institutf i)r Mineralogied er Ruhr-Uniuersitd.t,
463 Bochwn, West Germnny
Abshact
Phase relations in the two pseudobinary joins NaFeSizOo-NaTiFeSiOo (acmite-.tf') and
NaFeSieoo-NaTiAlSioe (acmitejtx') in the systemN a2o-FezorAlzos-TiorSioz have beens tudied
at 1000 bars Ps"6, and oxygen fugacities controlled by the MnzO3-Mn3Or buffer. At 700.C
binary pyroxene solid solutions containing up to 28 mole percent tf and 15 mole percenrr x, respec-
tively, are stable, although miscibility is greatly reduced at the Ni-Nip buffer. The soild solution is
interpreted in terms of the coupled substitutionsT iFer+ : Fes+Sii n 1f-pyroxenesa nd TiAl : Fe3+Si
in tx-pyroxenes.S table subsolidusp hasesi n the acmite-tfjoin are pyroxene"" * hematite * freuden-
bergite (a non-stoichiometric member of the seriesN azO. FerOa. (6 * x)TiO). In the acmite-txj oin,
however, nephelinei s an additional subsolidusp hase; the extent to which hematite is attributable to
vapor-phase leaching is uncertain. The pyroxene solid solutions melt incongruently over intervals
of -100oC (for the most tf-rich pyroxene)a nd -130'C (for the most tx-rich pyroxene),t he respective
solidi being at775(+lO)oC and 745(+10)oC. In tf- and tx-rich parts of the joins, pseudobrookite
has a field of stability above 850(+10)oC and 79q+10)'C respectively,c oinciding with final dis-
appearanceo f pyroxene"". High temperature favors incorporation of tf, and probably tx, whereas
entry of titanium into acmite is greatest at high oxygen fugacities. Zoned aegirines from a Gran
Canaria ignimbrite (silica-oversaturated) and Tenerife lavas (silica-undersaturated) show strong
enrichment in tf (approaching 25 mole percent), those from Tenerife also showing significant contents
oftx (up to 8 mole percent). In nepheline-syenitesf rom the Katzenbuckel (Odenwald), freudenbergite
has been found in association with titaniferous aegirine, hematite, nepheline, and alkali feldspar
(Frenzel, 1961). These occurrences of titan-pyroxene indicate that conditions of extremely high
oxygen fugacity prevailed during the late-stagec onsoLidationa nd/or alteration of such peralkaline
rocks.
fntroduction (Yagi and Onuma, 1967),C aMgTi,Ou( Barth, l93l),
It has generally been accepted that pyroxenes and CaTiFe'*rOu (Huckenholz, 1969);h owever, the
evolving within fractionating serieso f alkalic magma sharp increase of titanium with increasing sodium
have followed a compositional trend of the type: and ferric iron in aegirinem ust be explainedi n terms
augite-soda-augite-aegirine-augite-aegirine,w ith a of sodium-bearinge nd-members pecies.
possibled ivergencef rom soda-augitet o soda-heden- Verhoogen's( 1962)v iew that partition of titanium
bergite. For many examples of such trends the betweens ilicatea nd oxide phasesi s very sensitivet o
titanium contents of early augites are high (up to oxygenf ugacity( lo,) with a high lo, generallyf avoring
6 percent TiOr) but decline as the later acmitic its incorporation by silicates, at least below the
pyroxenes are formed, e.g., Morotu (yagi, 1953), temperatureo f pseudobrookites tability, is substan-
Iki (Aoki, 1959;1964)A, tumi dolerite (Kushiro, 1964) tiated by the occurrenceo f titan-aegirines'i n highly
and Takakusayama( Tiba, 1966).I n other seriess uch oxidized natural mineral assemblagesF. luctuations
as the Miocenel avasa nd ignimbriteso f Gran Canaria of 1", during crystallization of evolved, and even
(Schmincke and Frisch, in preparation), the Las relatively primitive, magmas is a well-recognized
Canadas lavas of Tenerife (Scott, l9Z0), and the phenomenon,a nd there is evidenceo f considerable
nepheline-syeniteosf Katzenbuckel( Frenzel,p ersonal variation between ear$-formed and later-formed
communication), titanium contents in pyroxene crystallization products. An excellent example is the
increased ramaticallyi n the latest (peralkaline)s tages
of magma evolution. In calcium-bearingp yroxenes r The term'titan-aegirine' (Schmincke, 1969; Schmincke and
Frisch, in preparation) is proposed for aegirine with more than
titanium occurs to variable extents as CaTiAlrOu 0.1 atoms of titanium per formula unit.
535
PHASE RELATIONS OF TITAN-ACMITE AT IOOO BARS s37
Picture Gorge Basalt (Lindsley and Haggerty, 1970)
wheret he effectso f introducedw ater and atmospheric
oxygen into joint selvagesd uring late-stagec rystal-
lization arer eflectedb y the presenceo fpseudobrookite
(Fe'.TiO5) in the selvage,a nd a decreaseo f mean
magnetite oxidation index into the host rock until
such mineralsa s fayalite (Fe'*rSiOn)a nd aenigmatite
*
lNarFe' uTisi.Oro)a re encountered.
The present experimentals tudy of such problems
began with an investigation of two joins between
acmite and the theoretical end-membersN aTiFe"*
SiO. (t0 and NaTiAlSiO' (tx). The acmite-tf join
is contained by the four-oxide component system
NarO-FqOs-TiOr-SiOr,( NFTS), shown schematically
in Figure l. The acmite-txj oin is containedb y the five-
oxide component system NarO-FerO'-AlrOa-TiO2-
SiO, and may be representedg raphically in terms of
the four components NaFe"*SiOn-NaAlSion-TiOr-
SiO, (Fig. 2). This work is essentiallya study of
subsolidus relations in these joins, particularly of Frc. 1. The quaternary system Na2O-FerOa-TiOz-SiOr
pyroxenem iscibility. Attention has been confined to (NFIS) indicating relations of the join acmite-NaFeTisioo
phase relations at high fs" levels at total water (ac-tf) and the phases narsarsukite (nars), ramsayite (rams),
hematite (hm), pseudobrookite (ps), freudenbergite (fr) and
pressureso f 1000b ars.
rutile (ru). The range of sodium titanates (NT-NT") is also
Previous Work on Acmite shown.
Acmite has been the subject of extensive experi-
(NaAlSiOn),'tf (NaTiFeSiOu),'tx' (NaTiAlSiOu),
mental investigation.B ailey (1969) confirmed that it
ramsayite (NarTirSirOn),a nd for intermediatec om-
is stableb etween2 and 5 kbar within the rangeo f 16, positionso n the joins acmite-tfa nd acmite-tx.S everal
defined by the hematite-magnetite and quartz- compositions were prepared by mixing gels (see
fayalite-magnetiteb uffers, and Gilbert (1969) has
Tables I and 2).
shownt hat acmitem elts incongruentlyt o hematite *
liquid up to at least 45 kbar. Complete miscibility
probably existsb etweena cmitea nd jadeite at 40 kbar
(Gilbert, 1967), and has been confirmed between
acmite and diopside at 1 atmosphere( Yagi, 1966)
and at l0 kbar (Cassie, 1970). There is probably
substantial miscibility between acmite and heden-
bergite( Carmichael,1962;N olan, 1969),a nd possibly
also with ferrosilite (Yagr, 1966).R ecently,P opp and
Gilbert (1972) studied the join NaFe'*SirOr-
NaAlSi'O' at 4 kbar to determine the position of
the curve separating the stability fields of clino-
pyroxene * albite f quartz from clinopyroxenef
quartz. At 1", approximating to the Ni-NiO buffer
at 500oC they found 5-6 mole percent NaAlSi,Ou --_ l1
solid solution in acmitic pyroxenes.
Experimental Procedure
Starting gels were prepared by the method of
FIc. 2. The pseudoquaternarys ystem NaFeSiOa-NaAlSiO4-
Hamiltona nd Henderson(1 968)f or the'end-member' TiOr-SiO2 showing relations of the joins acmite-tf, and acmite-
compositions acmite (NaFeSLOo), nepheline tx. Note that the joins acmite-jadeitea nd jadeite-tx are coplanar.
53E MARTIN F. I. FLOWER
Tasln l. Selected Results from Experiments on the Join Tasls l. Continued
NaFeSizOe- NaTiFeSiOo for P11,s: 1000B ars (+50)f
Startlng - l^It.Z TemD. Time Conden€ed MSatatertrilnagl L^ oEPn' WHt.zZo CTc@tioD). (tTrfouuer s) pChoansdeo sAesdssblage
Materlal uompn' H^o ('cirb) Grours) Phase Asaenblage
Oqgen Buffer : Iln 20 3-l'Ing1 4 px+hEFgl* tf2l 5 700 L64 px
ger ..100 .10 700 I92 px pxfhcrgl* tfZO 5 775 150 px+(hn)
gel ,f ZO 10 700 L92 px px+hFFg1* ,fZO 5 800 98 pxlhrl'gl
geL ,f 2O 3-4 800 72 px+hn+gl px+lE+91* af3o 5 75O 128 px
gel .f40 3-4 7OO 96 px+fr+hn
px+hn+9l* af3o 5 775 L24 px+(hn)
ger tf 40 3-4 800 68 px+frlhFrgl p)d-hFFgl* af 5 700 168 px+fr
gel .flOO 3-4 77O 118 px+fr+hDl-(NS) prlhrFfrtNs* ,f4030 5 775 160 pxlfr+hn
ger xfzo 3-4 775 rLz Px px+frfiln+Ns* af3O 5 750 50 px
ge1 .f3O 3-4 775 LLz px
gel tf35** 3-4 840 72 px+hmfgl
gel '- 30 3-4 800 Lt2 px+hn+gl gel .f35** 3-4 7OO 168 px*fr
gel -'40 3-4 775 LL2 px+fr+ (tur)
.f45** 3-4 840 70 px+frlhnlgl
gel - -10 3-4 775 84 Px
gel .f45** 3-4 750 80 prff r|-hn
gel - -10 3-4 700 r84 Px
ac+ru+hE --100 700 130 px+fr+hc+(NS) Oxygen &tffer : Fe 30 4-Fe2O g
gel ."lOO 5 890 8 px+hrlot+(gf)
ac+ma+hn r-f- 30 S 700 r24 px+fr gel acl'O 5 880 8 px
ac+m9thn 700 I24 px*fr+hn
gel rf^^ 3-4 900 23 (px) +lu!+gl OeAgen Buffe" : Ltn2 0 g-ltn g0 4
ge1 .f4O 3-4 900 ta hrrgl gel acl'O 5 900 23 px
gel .floo 3-4 900 f r+Qs)+91
Oqgen Buffe?: Ni,-Nij
gel .ftoo 3-4 700 330 px+f r+llt}(NS?) Required proportions of the constituent elements
gel .f10 3-4 700 330 px+(iln) werei ntroducedi nto 65 percentn itric acid as follows:
gel ,f3O 3-4 700 330 px+fr+iln*(NS? ) (1) Na as NarCO, (Merck analytical grade),d ried at
ge1 .f5O 3-4 7OO 330 pFrfr+11r1(NS)
350'C; (2) Al as aluminum metal (Schuchardta naly-
Orggen Buffen : I'tn2o 3-14n304 tical grade), dried at I l0"C; (3) Fe as iron filings
.f1O 3-4 79O 136 px
ger tfIO 3-4 800 96 px+hrr(el) (Merck analytical$ ade), under ice-coolingt o prevent
hydrolysis and precipitation of iron hydroxide, this
gel .fro 3-4 850 72 px+h+gl
gel .f2O 3-4 790 72 px+hrr(gl) stageb eing precededb y evaporationt o near-dryness;
(4) Ti as a previously standardized solution of
ger -- 50 3-4 800 12 px+fr+hFlgl TiOr.HrO in 65 percent nitric acid, also under
ge1 --50 3-4 830 96 px+fr+hFl-gl
- -r0 5 900 18 (px)+hclgl ice-coolingt o avoid precipitation of TiO,; TiOr.H,O
ge1 5 900 18 hfrl.gI wasp recipitatedf rom a saturateda queouss olution of
titanyl sulfate (Riedel de Ha€n analytical grade) by
gel .f45o* 5 900 18 p s+hlrlgl
.f5O 5 900 18 psffr+91 addition of 25 percent ammonia solution. The
gel tf4o 3-4 850 70 tm-(fr)+(ps)+g1 precipitatew asw ashedw ith distilledw aterb y repeated
ger .f5O 3-4 850 70 hFl.f r+ps+gl centrifuginga nd the TiO, of the resultinga cid solution
ge1 .f35** 3-4 175 112 px+(hn?) was determined gravimetrically.
ge1 ,f3O 3-4 75O t6 px To check the accuracy of the gel preparation
ge1 .f35o* 3-4 750 76 px+fr technique,f our gels were analyzedf or NarO, TiO,
ge1 .f3O 3-4 720 160 px
(colorimetrically),a nd HrO, the resultsf or NarO and
gel ,f35o* 3-4 800 56 px+hE+gl TiO, (correctedt o a water-freeb asis)b eing as follows:
gel .f45** 3-4 800 56 px+fr+tu+91
gel .f3O 3-4 850 48 px+hslgI
gel wt percent wt percent
gel .f35** 3-4 850 48 (px)+hrl-gl
composition (Na:O) (Tiot
+A vqou" phase is inferred in a|,L we. Abbreuiations used:
ac affiLlte iln ilrenite ps bseudobrooktte obs calc obs calc
fy freudenbetgite nt nagnetite px pyno*n
gL glres ne rephelire ms nmsagite &CeotXor 13.0 13.5 3.4 3.5
hn henatite NS eodiw netreilicate ru rutilb OCootXlo 13.6 13.6 13.8 t4.O
tf NaIiFeSia5
*P"eDiow w products, aCsotfro 13.0 13.2 7. O 6.8
a*AeLm iws. 4Ce otfro r2.8 13.0 13.0 13.3
PHASE RELATIONS OF TITAN-ACMITE AT LOOOB ARS 539
TrsLB 2. Selected Results from Experiments on the Join
NaFeSizOs- NaTiAlSiOo for P11.6: 1000B ars (+50)t TseLn 2, Continued
Conpn. Wt. Z Tenp. TlEe Condensed CoEpn. llt. z Tenp. Tine Condenseil
Hzo ("ctlo) (lloure) Phase Mole H20 ('crlo) (Hours) Phase
fii Assenblage 7tx Asseublage
1aygen Buffer: 14n203-14n304;S ta"ting natertal: geL Oxygen Buffer: I4n2O3-I4n3O4;S tarting naterial: lm+gl*
t0 10 700 72 p)+? 40 5 700 68 px+ne*f*hn
20 5 700 72 px+ne 40 5 79O 76 (px)+ne+(fr)+hrrgl
30 10 700 168 px.+ne+fr+hFl?
40 10 700 I7O pr+ne+fr+HNs oxAg en Buffer : I'hr2 0 3-l4ng 04 ; Stnting naterLal : pe+Lm+gl*
5** 5 750 138 Px
40 10 750 I50 px+ne+frFhn+Ns+(91) 5** 5 780 48 px
50 3-4 700 L44 px+ne+fr+hn
30 3-4 750 I44 px+ne+fr+hsF(g1) oeVgen Buffer: Mnzoz-l'tnlc4i starti.ng naterLal: gel
10 3-4 800 82 px+hclgl 25** 4 175 52 px+ne+hD+gl
25** 4 810 36 px+hn+gl
30 3-4 800 82 (px)+ne+hErgl 30 4 810 36 hrl'(ne)+gl
40 3-4 800 10 ne+(fr)+helgl 40 4 8f0 36 hcFne+gl
50 3-4 800 12 ne+bEFps+fr+91
50 J-4 75O L2O pxFne+fr+br+(g1) 45** 4 810 36 hEFne+gl
50 3-4 775 LzO px+ne+fr+hrlgl 50 4 810 36 hdre+(ps)+91+(fr)
unbuffered; Stmting mteria1,: geL 10 4 750 48 px
100 20 700 L64 ru.+ne+(Ns)?
)rygen Buffer: Mnz)A-ltht|)4; Starti,ng nateriaL: gel
40 3-4 775 120 px+ne+fr+tn+91 Additional starting compositionsw ere prepareda s
20 3-4 800 1I0 px+hFlgl
20 3-4 750 LzO px*ne{hn}gL mixes in required proportions (describedla ter) of the
35 3-4 75O L24 px+ne+fr+hrr(91)
45't* 3-4 750 124 px+ne+f r+M(gl) following synthetic crystalline phases: acmite'
35** 3-4 800 76 ne+hn+gl ramsayite,n epheline,h ematite, and rutile. The first
45** 3-4 800 76 ne+hFlfrlgl three had eachb een synthesizedh ydrothermallyf rom
15** 3-4 850 56 prFhE+gl
20 850 56 hrlgl gelsa t 1000b ars and 750"C, during runs of about one
30 3-4 780 97 px'+ne+(fr)+hn+gl week's duration, while hematite and rutile were
48't't 780 97 px.+ne+hFFfr+gl obtained as laboratory reagents( Merck, analytical
48** 720 px+ne+fr+hE
48** 800 84 ne+fr+hn+gl grade, and Kronos, reagent grade' respectively).
l155**** J3-C-4' 800 a928 ppxx++hhErltgslgI All mixes were ground under acetonef or 3/4 hour.
Cold seal hydrothermal pressure vessels were
L5** 3-4 750 98 px+hrr(91? )
50 3-4 790 98 (px)+ne+frlhtrtgl utilized for experimentsu p to 850oC at 1000 bars
25** 3-4 800 68 (px)+(ne)+hElgl
20 3-4 775 px+hDlgL pressureP. ressurem easurementws ere accuratew ithin
15** 2 740 120 Px =t50 bars,a nd temperaturec ontrol, by chrome-alumel
1255**** 3-42 775000 LI22L2 ppxx+oe+fr+hEr(g1) thermocouple,i s believedt o be within lOoC of the
25** 3-4 740 L44 px+ne+(fr)+hb given temperature.E xperiments at temperatureso f
25*x 3-4 700 163 px+ne+(fr)+hn
5x* 3-4 800 56 px+hDF(gl) 900oCw erer un in an internally heatedp ressurev essel
oTgen Buffer: I4n20Z-I4nP4; Stuting naterial: ac+ne+ru modified by F. Seifert from the design of Yoder
20 r0 700 668 px+ne+? (1950).P ressurew as read from a Bourdon gaugea nd
50 10 700 668 px+ne+frlhn
1rggen Buffer: I'tu203-l,tn{4; Steting nateriaL: gel was accurate within t5 bars. Temperaturesw ere
20 4 900 19 hElgl read from Pt - PtnoRhroth ermocouplesa nd kept
50 4 900 19 px+hcFgl constant to within +5oC. In most experimentsf o"
5** 4 900 19 px+hr+gl
40 4 900 18 hctsgl was buffered by the assemblage MnzOg-MnaOn
45** 4 900 18 (ps )+hrlgl (bixbyite-hausmannite)u sing the double-tube tech-
13.00 44 990000 2149 hhErl-+gg1l nique of Eugstera nd Wones (1962).T he samplep lus
Oagg en Buffen : I'rn2 0 g-lht gp4 ; S tuting mterial : p c+ln+gl* HrO was encapsulatedin a sealedP t or Ag/Pd tube,
20 4 740 L22 Px'Ine+(hu) around which the buffering assemblag(ein most cases
20 5 750 LO2 pa+oe+hnr(91) a 9: I ratio of MnrO. to MngO. plus about 15 wt
OE!g en Buf fer : lln 20 g-lhng o4 ; s t@ting naterial : tmne+g|+ (pd ' percentH rO) was enclosedb y a sealedo uter Au tube.
30 5 780 95 px'+ne+fr+hel-91
30 5 750 IL2 Pxrne+fr+hE+(91) A few synthesise xperimentsw erea lsoc onductedu sing
the Ni-NiO and FerOn-FerObr uffer assemblagesT'h e
Se+eA T Dabqloeu r 1,-p h+a sfoeo Lien otien feftoret d abtbor ehuaiareti onbae en upsreeda'ent in aLL ntts, oxidation conditionsa t the MnzOr-MnsOnb uffer may
*^*Pcerel oimorlew e.rw praducte. be expressedb y the equation:
540 MARTIN F. ], FLOWER
q?6s /p - r\ are in reasonablea ccord( seeT able 3) with pyroxenes
toel o, : 7.34- + 0.0051\-
; r ), synthesizedd irectly from gels.
Phasesw ere determined optically and by X-ray
(Huebnera nd Sato,1 970). powder diffraction. Pyroxene cell parameters were
determined from X-ray diffraction patterns by a
Preliminary experimentsw ere carried out on the
least-squaresre finemento f 2d measurementsu sing a
MnrOr-MnrOo buffering mixture to investigatet he
computer program modified from that of Burnham
limits of its effectivenesws ith time at varying temper-
(1962). The region from 70" to 5" 20 was scanned
atures.A t 850"C and 1000b ars the buffering mixture
at l/4o per minute, using Nifiltered Cu radiation.
was largely convertedt o Mn'On after about 4 days;
Twelve peaks were measured by reference to an
at 800oC,6 days were required for completec onver-
internal silicon standard and indexed by comparison
sion, and at 700'C some9 days.A t 600'C the buffering
with the pattern given by Nolan and Edgar (1963)
assemblagea ppeared virtually unchanged after 14
for pure acmite.
days.' For experimentsw hoser un time exceededth e
buffering capacity at the particular temperature,t he CrXstalline Phases
run was interrupteda nd the charger econstitutedw ith
(l) Acmitic pyroxenes crystallized in the system
fresh buffering material. Some corrosion of Ag/Pd
NarO-FerOr-AlrO'-TiOr-SiO, at the Mn"Oa-MnrOn
capsulesb y manganeseo xidesw as observeda lthough
buffer are pale ruddy-brown: Well-formed prismatic
this wasn ot a seriousp roblem.P t capsulesw eref ound
or acicular crystals,u p to l00p in length, exhibit the
suitable for some higher temperaturer uns. Iron loss
pleochroism:X , yellow-brown; I, straw-yellow;a nd
to Pt was probably negligible at high f6, levels,a nd Z, foxy-brown. Pyroxenesc rystallizeda t the Ni-NiO
no discrepanciesw ere noticed betweenr esults from
and FerOn-FezO,b uffers are pale green, suggesting
using one or the other capsulem aterial.
the presenceo f Fe'* ions. Refractive indices are
The phase relations were obtained largely from
comparable to those given by Nolan (1969) and
synthesise xperimentsu sing the gel starting materials.
Yagi (1966).
HrO contents of gels were determinedt o be in the
(2) Hematite forms as small but highly conspicuous
region of 2-3 wt percenta nd so the amount of water
hexagonalc rystals up to 50p across that are dark
added to charges( no more than 5 percent)w as kept
red-brown in plane polarized light and blood-red
low to minimize vapor-phasel eaching effects.H ow-
under crossedn icols.
ever, even under melting conditions an aqueous
(3) Freudenbergitea, sodium-iron titanate belonging
vapor-phasew as invariably present, as indicated by
to the solid-solutions eriesN arO.FerOe'(6* x)TiOr,
small amounts of an amorphous quench product.
occursa s subhedralp rismsu p to about l00p in length.
Glassq uenchedf rom liquid is red-brownt o colorless,
It showss traight extinction and has refractivei ndices
and is distinguishable( with some difficulty) from the
(of about l.5l) that are substantially lower than
amorphous quench material. Needles tentatively
averagep yroxenev alues.D ependingo n the thickness
identified as sodium metasilicateb y X-ray diffraction
of crystals,i t variesf rom straw-yellowt o ruddy-brown
were sometimesn oticed among the latter. Due to
in color and showsf aint pleochroism.I ts presencew as
vapor-leaching,it must be assumedt hat the total of
initially confirmed from its unique X-ray pattern
condensed-phasceo mpositionsf rom any run is never
(McKie, 1966) and optical properties, which differ
exactly equivalent to the bulk composition of the
slightly from those of the isostructural sodium-
starting material, although it would appear that this
titanate series( Wadsley, 1964;B ayer and Hoffmann,
effect is of lessi mportancei n acmite-richp ortions of
1965,1966\.
the system.
(4) Pseudobrookitei s yellow to reddish-brown and
For accurated eterminationo f the pyroxenes olidi,
is faintly pleochroic. It was only observed in the
previousr un productsf rom the hyper-solidusr egions
presenceo f glassa nd of the phasesn epheline,h ema-
were re-run at consecutivelylo wer temperaturesT. he tite, and freudenbergiteb, ut never with pyroxene.I t
cell parameters of pyroxenes formed as exclusive
is easily identifiable from its high refractive indices,
single phases( occasionallyp lus negligible hematite)
birefringence,a nd very strong dispersion.
(5) Nepheline appearsa s small prismatic crystallites
2 There was no evidencet hat the outer capsule burst during of characteristicallylo w birefringenceF. rom the X-ray
theser uns. diffractionp attern, the 27.290"2 0<zfio>a nd 29.795"
PHASE RELATIONS OF TITAN-ACMITE AT |OOO BARS 541
20 tzorzpt eaks( Cul(a radiation)f or nephelinef ormed
in the'four-phase'regionp yroxene* hematite( ?) *
nepheline * freudenbergites how significant shifts o bs t \- xi-\:-0hlm44r.p'ffe*++rrl/ -../+*i
from the values (27.240" and 29.69O") for pure hm+l
'/--t---
ssyonlidth estoiclu ntieopnh etoliwnea.r Tdsh easne sirhoinft-sn epprohbealibnley m inodleiccautlee \ o-t'/ / -h=m- +P1s+ fr +- |
(NaFe3*SiOn).I ncorporation of excessS iO, in the A
nephelines tructurei s a further possibility (Hamilton
and MacKenzie, 196O;B ailey and Schairer, 1966).
Experimental Results
(l) The Join NaFes*S i"Ou-NaTiFeu*^S(iOAcum ite-tfl
This join proceeds from acmite composition
towards the silica-poor part of the four-component
0
NarO-FerOg-TiOr-SiOs,y stema nd, if extended,in ter-
'basal' Acroo
sectst he silica-free plane Na2O-FerOr-TiO,a t MOLEP ERCENNT oTiFeSiO6
compositionN arO'FerOg.4TiOr;( seeF igs. I ar.d2).
FIc. 3. Schematicp haser elations for part of the join acmite-tf
The experimentalr esults are presentedi n Table 1,
at 1000 bars total water pressure. Symbols as follows: px,
and the isobaric phase diagram for 1000b ars Ps,o
pyroxene solid solution; hm,'hematite; fr, freudenbergites olid
is given in Figure 3. solution; ps, pseudobrookite; and I, liquid (glass).
At temperaturesle sst han about 700"C, crystalliza-
tion of gel starting materials was sluggish, and, succeeded by the association, pyroxenea" * freuden-
exceptf or the acmitee nd-memberit self, there was no bergite * hematite.D espiteo ur imprecisek nowledge
direct confirmationt hat equilibriumh ad beenr eached, of both pyroxenea nd freudenbergites olid solutions,
even for runs of severalw eeks.T he solid-line phase it is unlikely that each pair of compositionsl ies with
boundaries in Figure 3 have been confirmed by hematitei n the samet riangular plane as the respective
experiments employing products and synthesized bulk compositions( see Fig. l). The implied fourth
phase assemblageso f previous runs as starting phase is not present, unless we assumei t to be a
materials( seeb elow). Theser esults( Table 1) are not sodium silicate that remains largely dissolvedi n the
at variancew ith thosef rom synthesise xperimentsa, nd vapor phaseo n quenchingo fthe charge.
a close approacht o equilibrium may be assumedf or The composition NaFeTiSiO' (tf) may be repre-
runs above 700oC. Charges from runs on acrotfro sented by equal mole proportions of NarTirSirOs
(900"C), acrotfro( 800'C) and acuotfno(8 00oC) were (ramsayite)a nd FerOa( hematite);h ence three com-
each held at consecutivelyl ower temperatures( see positions on the join ramsayite-hematite-namely,a t
Table l), and from the disappearanceo f hematite it ramsnohmror,a msrohmroa, nd ramsrohmuo(t f)-were
was possible to locate the solidus and part of the made using synthetic ramsayite and hematite. The
solvusc urves. resulting phasea ssemblagefr om runs on rams56hm5q
The melting temperature for pure acmite at the at 1000 bars P",6 and in the temperature range
MnrOr-MnaOnb uffer is greatert han 900oC,w hich is 650"-770"Ci s identical to that from gel composition
at least lO'C higher than that indicated by Bailey tfroou nder thesec onditions:u L pyroxenes*s freuden-
(1969) and confirmed by the present author at the bergite + hematite (plus amorphous quench),
Fe.On-FerO,b uffer at 1000b ars. This may be taken ramsayiteh aving entirely disappearedH. owever, run
as a probablem aximumf or crystallizationi n naturei n products from ramsnohmraon d ramsrohmroc ontained
the presenceo f excessH rO, becausein troduction of ramsayiteg pyroxene""* freudenbergitein differing
Fe'* would reduce the melting temperature.A t the proportions, and ramsayitei tself has beens ynthesized
700"C isotherm, binary solid solution of the tf without diffrculty from a gel of composition
component in asmite exists up to about 28 mole NarTirSirOn( Flower, in preparation).F urther experi'
percent.A t more tf-rich compositions,a n undefined ments carried out on ac76tf3a6n d acuotfuom, ade up
but small interval of ternary solid solution exists, from synthetic acrnite, ramsayite, and hematite
where pyroxene""c oexistsw ith freudenbergiteo nly, resultedi n the breakdowno f ramsayite,a nd confirmed
MARTIN F. I. FLOWER
the narrow two-phasef ield of pyroxenes'* freuden- (2) The Join NaFes*S irOu-NaTiAlSiO6(A cmite4x)
bergite (Fig. 3). The assemblagep yroxeness + Within the pseudoquaternarys ystem, NaFeSiO.-
freudenbergite* hematite( * Na : silicate? ) is also NaAlSiOn-TiO,-SiO,( Fig. 2), the acmite-txj oin lies
confirmed as the exclusives ubsolidusa ssemblagefo r within the triangular join acmite-TiOr-NaAlsio4,
titanium-rich parts of the join. becauset x is equivalent to equal mole proportions of
The solidus-liquidusr egion is more complicated.
nephelinea nd rutile. It appearst hat aluminum, with
The most titanium-rich pyroxenes olid solutionsb egin titanium, is able to enter the acmitic pyroxenes truc-
meltingi ncongruentlya t775"C (+10) to hematite* ture as the tx molecule. The experimental results for
liquid and, within an interval of 80-100'C, react the acmite-txj oin are given in Table 2, and the isobar
completelyt o hematite * pseudobrookite* freuden- phase diagram at 1000 bars is shown in Figure 4.
bergite * liquid. This region has not been studiedi n Binary solid solution of tx in acnrite at the MnrOs-
detail as it is largely beyondt he temperaturer ange of MnrOn buffer extendsu p to about 15 mole percenta t
the hydrothermal equipment,a lthough a brief study 700oC; more tx-rich bulk compositions result in
was made of the 900" isotherm using the internally pyroxene solid solutions (possibly toward tf)
heated gas-media apparatus. Results from these accompaniedb y nepheline.I n this two-phasef ield, a
experiments show that pseudobrookite, coexisting silica-rich phasb (e.g., albite) might be expected,
with liquid + hematite and/or freudenbergite, although none is actually encountered.I t must be
probably has a wide field of stability at high tempera-
assumedt hat silica and NazO are taken up by the
tures for the more titanium-rich compositions. vapor phase, although nepheline may take some
Pseudobrookitei s clearly not stable below a certain SiO, and FerOg in solid solution. At more tx-rich
(as yet undefined) temperature in this join, and compositions, pyroxene and nepheline coexist with
probably shows a reaction relation with tf-rich freudenbergitea nd hematitej The presenceo f hematite,
pyroxenes olid solutions. as for the acmite-tf join where hematite is almost
At the high ambient fs", hematite probably shows without doubt a subsolidus phase for tf-rich bulk
minimal solid solution towards titanium-bearing compositions,m ay be a consequenceo f the vapor-
species(c f Haggerty,l97O);t hus it would follow that, leaching, but this has not been unequivocally con-
below the temperature of pseudobrookite stability, firmed. In any case, with or without hematite,
titanium would be largely partitioned between
pyroxeness* freudenbergite* nephelinec omprise
pyroxene,f reudenbergitea, nd the liquid phase.I f the
the subsolidusb etweenc ompositionsr icher in tx than
pyroxene solid solution essentially involves the
acmite and tf molecules,t he freudenbergitem ust be
richer in both NarO and TiO, but poorer in FegOa,
as comparedt o the composition of natural freuden-
bergite( Fig. l) given by Frenzel( 1961).
Experiments on compositions 8cs6tf1s,a crotf.o,
acsstf5as nd tftoob uffered by the Ni-NiO assemblage
(Table l) indicate that pyroxene miscibility is con-
siderably restricted relative to that at the MnrOa-
'acmite'
MngO. bufrer. The melting point of was
measuredt o be 830'C (+10) at the Ni-NiO buffer,
some 20oCl esst han that measuredb y Nolan (1966).
The coexistenceo f ilmenite (Fe'*TiO3) with pryoxene
and freudenbergitef or composition acrotfroa t sub- a(taPx+ne + fr (+hm?) O
solidus temperatures,t ogether with the restricted
extent of pyroxenec ell parameterv ariation (Fig. 5),
suggestst hat miscibility is no greater tlan about o
18 mole percentt f in acmite. Solid solution at lower Ac.o
MotE PETENTN oTiAlSiO6
16, is almost certainly more complex than at the
MnrOr-MnrOn buffer and may involve the molecules
*Tin* Frc. 4. Schematic phase relations for part of the join acmite-tx
NarFe' SinO, or NaTit* sirOu( Prewitt, Shannon, at 1000 bars total water pressure. Symbols same as in Figure 3
and White, 1972). with the addition of ne, nepheline.
PHASE RELATIONS OF TITAN-ACMITE AT IOOO BARS 543
9.
I I
I I
9.
o
oA ,---'1"-lll--l
9.
o _l
bA
l,''tlrl
8'81
8.80 Y
-1---
l----l'
5.31
o --l'- -' j - -l- - -l-
j--'--li ---t----l---
CA
t-
5.30
F:-l!
1o7.45
[3" 1o7.40 I F--l--l-, l--- ---1---l---l---l-
r07.35
107.3C
-Fl----l: I
'|-l----l----l--
432.Q )--ll -
vA' I
)-" I I J-'--l-
431.0
-'1
430.0 ?'' V.
429.0
0102030405001020304a50
Actoo
.A"'oo Mr.nO. LEr hP?E6 RCEEr.N, T rNr-a FeTiSiOo MOLE PERCENT NaAlTiSiQ
Frc. 5. Variation of cell parameters a, b, c, B and Z of pyroxenes formed from bulk compositions along the joins acmite-tf and
acmite-tx. All data are presented in Table 3, wher: coexisting phases are indicated for non-binary parts of each join. Solid lines are
for pyroxenes formed at the MnzOa-MnaOr buffer, and dashed lines for those formed at the Ni-NiO buffer.
abott 24 mole percenta nd an as yet undefinedp oint spite of thorough grinding of starting materials,t he
where rutile is stable.T he compositiont x crystallizes rutile proved relatively unreactive. For acrotxroi t
as nepheline* rutile only, at all subsolidust empera- requireda month at 700oCa nd 1000b arsf or complete
tures and over a wide pressurer ange.N o information disappearance of rutile and recrystallization to
is availablef or the region between? cs6tx7a6n d txroo. pyroxene f nepheline * freudenbergite * hematite
To verify the stablec oexistenceo f subsolidusp hases ({ a trace of Na-silicate). Leakage reduced the
within this join, the compositionsa crot&s, acsqtX2q,ac7qtx36a fter one month and, although rutile had
and acuotxuwo ere made up from the pure synthetic disappeared, pyroxenes" cogxisted with nepheline,
end-memberp hasesa cmite, nepheline,a nd rutile. In ilmenite, freudenbergite,a nd nepheline.T he acrotxro
MARTIN F. T, FLOWER
charge run under the same conditions reacted to solution is metastable.T he restricted miscibility at
producep yroxeness* nepheline( seeT able 2). the Ni-NiO buffer is also apparentf rom cell data for
The interval for complete reaction of the most acmite-tf pyroxenes( Fig. 5).
tx-rich solid solutions is -l20oC, substantiallym ore For solid solutionsb etweena cmitea nd tf pyroxene,
than that for exclusively tf-rich pyroxenes. In other the increasei n cell volume is attributed to the need
parts of the join, phase relations are very complex, for either Ti'* (ionic radius : 0.680 A';, or Fe'*
particularly in view of vapor-phasel eaching effects, (0.645A ) to substitutef or Sin* (0.42 A) in the tetra-
and the interpretationsin Figure 4 area pproximations. hedral-site,w hile substitution of Tin* for Fe3* would
Investigations of the 900'C isotherm using the additionally increaset he a and b cell edges.I n tx-
internally heated vessel( Table 2) indicate the wide- bearing pyroxenes, entry of Al'. (0.530 A; into
spread occurrenceo f pseudobrookitec oexistingw ith tetrahedral sites would also expand the cell volume,
nepheline * hematite f liquid, with nepheline f in contrast to Al3* entry into octahedral sites as
hematite* freudenbergitef liquid, or with hematite observed in acmite-jadeite pyroxenes (Popp and
+ freudenbergite f liquid. Incompatibility of Gilbert, 1972\,w here cell volumes are reduced.T he
pseudobrookite with pyroxene was again evident. overall variation of cell data thereforei s assumedt o
Pseudobrookiteis clearlyu nstableb elow 790oC( + l0), reflect the coupled substitutions FeSi : TiFe and
some 200oCa bove its lower breakdown temperature FeSi : TiAl in tf- and tx-bearing pyroxenesr espec-
in the systemF eO-FezOg-TiO(,H aggertya nd Lindsley, tively. Hartman (1969) has argued convincingly
1970;H aggerty, 1970).I t is again of interest to note againstt etrahedralc oordination of Tin* as a general
the persistenceo f freudenbergitet o relatively high rule. Alt* and Fet* (in that order) would seemm ore
temperaturesin the five-components ystem. likely candidates.
(3) Cell Parameter Variation in Pyroxene Solid Petrogenetic Applicatio'ns
Solutions
The lattice parametersa , b, c, I and V have been PhaseR elations
determined for pyroxenes crystallized at l0 mole The experimentald ata presenteda bove define the
percenti ntervalsf rom acmite to 50 percentt f and tx, extent of titanium and aluminum substitution in
including pyroxenesf ormed in non-binary parts of the acmitic pyroxenes in terms of the molecules
joins. The data (Table 3) are plotted against bulk NaFeTiSiOua nd NaAlTiSiOo. Phaser elations in the
composition of chargesi n Figure 5. There is reason- investigatedr egion of the NarO-FerO.3-AlrOs-TiOr-
able correspondenceb etween data for pyroxenes SiO, system are of most interest with respect to
synthesizedf rom gel starting materials and those peralkaline igneous rocks-peralkalinity defined as
formed from crystallinea ssemblageosf the sameb ulk the ratio (Na * K)/Al exceeding unity-in which
composition (Table 3). Compared to the overall acmite is the most significant pyroxene component.
variation, the standard deviation is rather large and Peralkaliner ocks are frequently associatedw ith high
must be attributed to inaccuraciesin troduced by the oxidation levels during their genesis (Bailey and
poor crystallinity of the material. Scanning the Schairer, 1966), and the MnzO'-MnsOn buffer is
samplem ore than two times at l/4" per minute did believedt o be an appropriateu pper lo, boundary for
not noticeably contribute to a better cell refinement. their experimentals tudy.
However, entry of the tf moleculei nto acmite seems From the present investigationi t is important to
to increaset he unit cell volume (V) and parameters note (l) the fair$ extensivep yroxene miscibility in
a, b, and c, while decreasingt he angle F. A similar this system,( 2) the stable coexistenceo f the phases
effect is observedf or inclusion of tx. There is no pyroxeness* freudenbergite* hematite with liquid
conclusivei ndication of the nature of ternary solid (plus an aqueousv apor phase)i n the more titanium-
solution that exists in the two phase (pyroxene""f rich parts of the acmite-tf join, and (3) thesep hases
nepheline)f ield. plus nephelinei n the acmite-tx join. In both joins
Due to the poor crystallinity of the pyroxenes, pseudobrookitei s stable only at temperaturesa bove
optical data are not availablef or the most titanium- that at which pyroxened isappears.
rich members.V ariation of the cell data, however,
does give a fairly exact indication of the miscibility s All ionic radii quoted in this paper are the values from
limits in the two joins, provided none of the solid Shannon and hewitt (1969) for octahedrally-coordinatedi ons'
PHASE RELATIONS OF TITAN.ACMITE AT IOOO BARS 545
Titanium and Aluminum Contentso f Natural Acmite been reported from the Katzenbuckel( Odenwald)b y
Acmitic pyroxene (aegirine) is rarely observeda s Freudenberg( 1919)w ith a whole-crystaTl iO, content
primary phenocrysts in lavas, ignimbrites, etc. of 5.88w t percent.
The Canariess odic pyroxenesa re from rock types
Petrographic evidence suggestst hat it forms as a
that are typical products of protracted magmatic
late-stagec omponent,'oftenm antling earlier-formed
aegirine-augitein the groundmass,r ather than as a fractionation, and their strong zonation and enrich-
reaction product formed between magma and pre- ment in tf (and in the Tenerife aegirines,t x), may
viously-crystallizedir on-titanium oxides (see Nolan, suggestth is to be a more widespreadp henomenonth an
1966).M any, but not all, of suchl ate-stagep yroxenes whole-crystal aegirine analyses and CIPW norm
recalculationsw ould otherwise suggest.I n alkaline
are moderately titaniferous, although there is scant
'whole complexes such as the Lovozero Intrusion (Kola
indication from crystal' analysesth at titanium
content is relatedt o the proportion of acmitep resent. Peninsula), metasomatic introduction of soda and
silica may be an important factor in the formation of
Detailed electronm icroprobes tudieso n lavasf rom
Tenerife (Scott, 1970), and ignimbrites from Gran Na-titanates and Na-Ti-silicates by reaction with
Canaria (Schminckea nd Frisch, in preparation)h ave oxides under conditions of high 16,. Late-stage
shown an enrichmento f up to -25 mole percentt f in aegirinesin alkaliner ocks aref requentlyy ellow-brown
outer zoneso f groundmassa egirine.T his approaches in color and may themselvesb e significantly titani-
ferous (e.g., Katzenbuckel),a lthough few analytical
the experimentaml iscibility limit at the MnrOr-Mn3On
buffer (Fig. 3). The relative extent of Ti-enrichmenti s data, particularly for compositional zoning, are
available. For less oxidized intrusions such as the
illustrated in Figure 6, where atomic proportions of
Tin* are plotted against Na* for severalc ontrasting Qoroq nepheline-syenite in South Greenland
(Stephenson,1972p)y, roxened atad o not indicatea ny
pyroxene series. In outer calcium-free zones of
late-staget itanium enrichment( Fig. 6), and tend to
titan-aegirinesf rom Gran Canaria, titanium content
suggestt hat titanium is more exclusivelyc ontained
is almost double that of typical zoned titan-augites
from Tenerife (Scott, 1970),w hile soda-augitesa nd in oxidem inerals.A luminum contentso f volcanica nd
plutonic aegirinesa re hardly significant when com-
aegirine-augiteso f intermediate soda content are
pared to the high contents of the soda-poor( calcic)
conspicuouslylo w in titanium. Titan-aegirineh as also
pyroxeneso f basic and intermediater ocks (Fig. 7).
There is a slight indication of relative aluminum
Taer.n 3. Cell Parameters of Pyroxenes* enrichment with increasingN a* in pyroxenesf rom
Itapirapua (Gomes, Moro, and Datra, 1970) and
8:ff:: S;::::'*' a(A) b(r) c(r) B' v(i3) Tenerife (Scott, 1970),b oth from thoroughly under-
saturatedr ock seriesP. ositivec orrelationo f aluminum
,.100 none 9.66s(2) 8.800(1) 5.299(t) L07.42(2) 430.0(z) and titanium in the Tenerife aegirines suggests
tflo incorporation of the tx molecule (up to -8 mole
none 9.667(5, 8.807(4) 5.299<3) ro: .4O(4) 430.4(4)
,fzo none 9.666(4) 8.82r(3) 5.305(3) 107.4r(4) 43r.6(L) percent),w hile the high aluminum and low titanium
.fjo none 9.570<4) 8.828(3) s.308(3) 107.39(4) 432.5(4) in the Itapirapua pyroxenes (Gomes et al, 1970)
,f4o f!+hr 9.670(4\ 8.831(3) 5.309(3) 107.38(4) 432.7(4) probably reflects solid solution towards a jadeitic
.fso f+hn 9.672(s) 8.827<4) 5.310(3) 107.39(4) 432.6<4)
end-member.
.*rO nof,e 9.673(3) 8.8r2(3) 5.300(2) r07.33(3) 43r.3(3)
.r2o ne 9.673(5, 8.817(4) 5.303(3) toj.34(4) 43L.7(4\
.*30 ne+fr(hn) 9.676(5) 8.813(4) 5.303(2) 107.33(5) 431.7(s) Natural Occurrenceo f Freudenbergite
t*40 ne+f*hn+? 9.674(5) 8.8r5(3) 5.304(3) L07.36(4J 4].r,7G, Natural freudenbergiteh asb eenf ound in nepheline-
.*so ne+fr+h#? 9,673(5) 8.8I4(4) 5.304(3) 107.36(4) 43r.7(4>
syenites from the Katzenbuckel (Frenzel, 1961)
orygen Buffe!: Ni-M1; Z00oC ! U; St@ting naterial,: geL
.fro 1ln 9.663(4) 8.814(3) 5.300(2) 107.39(4) 430.8(4) commonly accompanied by zoned aegirine and
.f3o f*llm 9.674(6) 8.81r(5) s.304(4) r07.39(s) 431.s(6) hematite.T he appearancee lsewherein Katzenbuckel
.fs' fr+ltn 9.676(3, 8.810(3) 5.306(2) 107.4r(2) 431.5(3) rocks of both hematite and pseudobrookitet estifies
,rAgen Buffe!: Iln203-tht304; ?S1oC ! 10; St@ting natep.Lal: p#lLa+gl
t*5 none 9.669(5) 8.810(4) s.3Oo(4) 107.38(5) 43o.s(s) to the high ambientl e, during their late-stagec rystal-
lization and/or alteration, while groundmassa egirine
o,lgen Buffep: 1,4n2o7t4n304; ?S1oC ! 10; Sta?ting Mteria'l: aene+lu
.*2o 9.667(3, 8.808(3) s.299(2) 107.36(3) 430.0(3) and aegirine-augitec rystals are rimmed with titan-
rcell pay@tep' slban tith ore stmdzrd ewr (in patentlesea) in tem€ of acmite (Frenzel,p ersonalc ommunication,1 972).T he
Zast deeinal aited, thue 9.665(2) indicates a stadard. e"pop of !0.002, occurrenceo f freudenbergitein tergrownw ith hematite