American Mineralogist, Volume 78, pages 1056-1065, 1993
Barian titanian phlogopite from potassiclavas in northeast China:
Chemistry, substitutions,and paragenesis
MrNc ZnaNc*
Department of Geology, Imperial College,University of London, London SW7 2BP, U.K.
and Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A.
P,rur Suoo.Lnv
Department of Geology, Imperial College,University of London, London SW7 2BP, U.K.
RonBnr N. TrrorrpsoN
Department of Geological Sciences,University of Durham, Durham DHI 3LE, U.K.
MrcH.q.nL A. DuNc,lN
Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A.
Ansrnl,cr
Magmatic barian titanian phlogopite(BaO : 4.2-11.2 wto/oand TiO, : l0.l-13.1 wt0/0)
occursin the groundmassand in a magmatic inclusion of olivine leucitites from northeast
China. In addition, titanian phlogopite (BaO < 0.3 wto/oand TiO, < 6.4 wto/o)formed by
reaction betweenolivine phenocrystsand late fluids also occursas coronasrimming olivine
phenocrystsin associatedleucite basanites.Compositions of the barian titanian phlogopite
vary systematically;AlrO. and TiO, increaseand SiOr, MgO, FeO, and KrO decreasewith
increasingBaO. Substitutions deduced from such variations include Ba * (Al,Fe3*) : K
+ Si and 2(Mg,Fe'?*) : Ti + n. Ti-O and Ti-Tschermak's substitutions may also be
present in subordinate amounts. The Ba + Al : K + Si substitution is common to most
magmatic barian micas, except for some lamproitic micas. In contrast, no general Ti
substitution scheme is applicable, as Ti substitutions vary with melt compositions and
intensive variables.
Thesephlogopite samplescrystallized as a late phaseat low pressuresand temperatures.
Differencesin mineral assemblages
betweenolivine leucititesand leucite basanitescontribute significantly to the observed disparities in chemical composition between the barian
titanian phlogopite and the corona phlogopite and, in general, among phlogopite from
other alkalic magmatic rocks.
INrnoouc:ttoN
Titanian phlogopite occursin lamproites and other potassicrocks(e.g.,Mitchell, 1985;Wagnerand Velde, 1986;
Mitchell and Bergman, l99l), as well as in some alkalic
basalticrocks(e.g.,Ryabchikovet al., 198I ; Qi and Xiao,
1985), and varies widely in composition. Magmatic
phlogopite enriched in both Ba and Ti is rare. The occurrencesofbarian titanian phlogopite have been reported in somepotassicrocks (e.g.,Thompson, 1977;Wendlandt,1977;-Birch, 1978, 1980;Holm, 1982;O'Brien et
al., 1988)and in a few alkalic rocks suchas nephelinites,
olivine melilitites, and aillikites (e.g.,Mansker et al., 1979;
Velde, 1979;Mitchell and Platt, 1984;Barnettet al., 1984;
Boctor and Yoder, I 986; Cao and Zhu, 1987; Rock, I 99 l).
There is no generalagreementregarding the substitution
schemesin barian titanian phlogopite (e.g., Mansker et
t Presentaddress:School of Earth Sciences,Macquarie University,Sydney,NSW 2109, Australia.
0003-004x/93109l 0- l 056$02.00
Bol et al., 1989;Guo and Green,1990).Little
al.,19791,
attention has been paid to their chemical variations and
parageneticimplications. In this paper, we report chemical data for barian titanian phlogopite samplesin potassic lavas and a magmatic inclusion from northeast China,
compare them with other magmatic phlogopite samples,
and discusstheir substitution schemesand parageneses.
sETTTNGAND pETRoLocy
Three adjacentvolcanic fields ofpotassic volcanic rocks,
Wudalianchi, Erkeshan, and Keluo (WEK), in northeast
-126'45'E, 48'00'-49"30'N),are located
China (125"30'
between the Songliao Basin (a late Mesozoic basin) and
the Xing'an Mountains (a Paleozoicfold belt) (Hsii, 1989).
These lavas, which cover an area of > 1400 km2, were
erupted during three main episodes:late Miocene (9.6(0.56-0.13Ma), and Recent(AD
7.0 Ma), late Pleistocene
1719-1721)(Zhanset al., l99l).
The WEK potassicrocks are porphyritic, with groundmasstexturesvarying from vitric to holocrystalline.They
1056
Gnolocrc.rl
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
lo57
Fig. 1. Backscatteredelectron imagesof the WEK barian titanian phlogopite (a) in an olivine leucitite sample and (b) in DZ2n.
Abbreviations: phl : phlogopite; o1 : olivine; cpx : clinopyroxene; af: hyalophane; ne : nepheline; lc : leucite; sp : Fe-Ti
oxides.
are divided into three types; olivine leucitite, leucite basanite, and trachybasalt.Both olivine leucitite and leucite
basanitecontain phlogopite. Olivine (8-130/o),clinopyroxene (2-4o/o),and leucite (14-20o/o)phenocrystsin olivine leucitites are dispersed in a matrix of olivine, clinopyroxene,leucite, Fe-Ti oxides, nepheline,phlogopite,
and rare sodalite. Leucite basanites have a phenocryst
of olivine (4-9o/o),leucite(l-7o/o),and clinoassemblage
pyroxene (0-l lol0).Matrix phasesare potassium feldspar
(up to 500/o)and clinopyroxene (up to 350/o),plus minor
leucite, olivine, and Fe-Ti oxides.
Olivine leucitites have lower SiO, but higher MgO and
Mg/(Mg + Fe2*) ratios (43-45 wt0/0,14-10 wto/0,and
0.78-0.71)than leucitebasanites(47-55 wto/0,3.8-8.0
wt0/0.and 0.68-0.52). Most olivine leucititesand some
leucite basanitesare primary or nearly primary, formed
by partial melting of the mantle sourcesat different pressures(Zhanget al., 1991).K.O content is high and variable (3.6-7.l wto/o),and most sampleshave KrO/NarO
> l. TiO, and Ba concentrationsare 2.0-3.1 wt0/oand
1300-2200ppm, respectively.Olivine leucititesand leucite basanites are not systematically different in Ba,
whereasthe former generallycontain lower TiO, and KrO
than the latter. For additional details, see Zhang et al.
(1991,1993).
OccunnsNcEs oF PHLocoPrrE
Phlogopite occurs in the WEK potassic lavas as (l) a
matrix phase in olivine leucitites, (2) a constituent of a
magmatic inclusion (DZ2n), and (3) coronasrimming olivine phenocrystsin a leucite basanite (WZKI6). Phlogopite samplesfound in the first two occurrencesare Barich (BaO > 4.2 wo/o),whereas the phlogopite coronas
contain much lower BaO (<0.3 wto/o).
Interstitial phlogopiteplatelets(0.01-0.1 mm, Fig. la)
constitute up to 3 modalo/oof olivine leucitites. A few
larger anhedral grains, up to 0.2 mm, enclose early
groundmassphasessuch as clinopyroxene and Fe-Ti oxides. They show very strong, distinctive red-brown to
pinkish yellow pleochroism, which may be attributed to
high Tio, contents(Mitchell, 1985).
DZ2n \s a small inclusion (l cm) composedof barian
titanian phlogopite, hyalophane (9-20 molo/o celsian),
leucite, and sodalite. Subhedral phlogopite grains (0.20.3 mm, Fig. lb) exhibit the same distinctive pleochroism as those from olivine leucitites.
Phlogopite coronas rimming olivine phenocrysts are
found in a holocrystalline leucite basanite(WZKI6) sampled from the central portion of a thick lava flow (>20
m thick). Thesemica flakesare <0.05 mm in the direc-
1058
wt
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
SiOz
rt0
^l l^
^
o.^^i*&.,
|
l'4;"
(a)
320
r60
l.^^^
(d)
l4 0
t20
300
1 50
l0 0
160
^
wt% N,OJ
l4 0
.t
A^^
140
^"iffi"
13.0
i{^Tfi,.
t20
r ^r rl
l^
120
100
^
ll 0
80
(b)
l0 0
140
60
90
''*''o'
,r^i*fto
t20
r00
80
^l i^t
^ ;f.ir
80
10
60
. {fr.
6.0
50
(0
40
00
20
4_0 60
80
wt',6 BaO
roo rzo00 20
40
60 80
wt%BaO
100
i.60
Fig.2. (a-f) BaO vs. orher elementsfor the WEK phlogopite.
Solid triangles : olivine leucitite; solid circles : DZ2n; open
triangles : corona.
tion perpendicular to (001) and show pale red to light
yellow pleochroism.
AN,q.Lyrrc,{L TEcHNIeUE
Analysesof mica chemistry were mainly performed at
Imperial College (IC), University of London, using a
CambridgeInstruments Microscan 5 electron microscope
fitted with a Link energy-dispersivedetector. Analytical
conditions were acceleratingvoltage : 15 kV, fixed specimen current on a Co standard : 4.0 nA, and counting
time : 100 s. ZAF corrections were used, and synthetic
BaTiO, was analyzed to monitor interference between
BaZa, and TiKa, lines, which proved to be negligible.
A subsetof barian titanian phlogopite sampleshas also
been analyzedusing a Jeol 733 Superprobeequipped with
wavelength-dispersivedetectors at Southern Methodist
University (SMU), in order to analyze F and Cl contents
and to crosscheckthe Ba-Ti interference.Analytical conditions were acceleratingvoltage : 15 kV, current : 20
nA, and beam spot diameter : l0 pm. The X-ray counting rates were corrected using the a matrix correction
(Benceand Albee, 1968).Syntheticfluor-phlogopiteand
natural scapolite were used as standards for F and Cl.
Benitoite (BaTiSi.On) analysesgave BaO 37.15-38.38
wto/0,
TiO, 18.35-18.86wt0/0,
and SiOr43.4l-43.51 wt0/0,
in good agreementwith the theoretical values of 37.08,
19.32, and 43.58, respectively.The two setsof analyses
are generally comparable.
Traue 1, Representativephlogopiteanalyses
No.
Host
sio,
Alro3
Tio,
FeO
MnO
Mgo
CaO
BaO
Naro
KrO
62
DZ4
2a
DZ4
82
DZ6
94
DZ6
2d
DZ2n
252
OZ2n
WZK16
34.94
12.73
10.44
10.64
n.a.
13.76
0.55
32.87
13.43
11.09
10.54
n.a.
12.87
36.75
12.35
10.28
11.64
n.a.
o.b /
9.36
0.93
6.89
U.OJ
34.16
12.56
10.54
10.57
012
1 45 7
0.23
7.70
0.83
o.5c
1.34
98.61
0.711
33.24
13.57
11.47
11.36
0.01
12.46
0.34
969
0.48
6.16
n.a
98 78
0.662
31.52
13.86
12.48
1 00 9
0.11
12.47
0.12
9.49
0.87
5.20
0.93
96.36
0.688
31.60
14.47
13.06
10.24
0.09
11.82
0.13
10.91
1.01
5.18
n.a.
98.51
0.673
39.59
10.31
6.25
9.03
0.09
17.99
0.04
0.24
0.90
8.55
n.a.
92.99
0.78
5.066
2.437
1.314
1.448
0.001
2.831
13.097
0.056
4.905
2.542
1.461
1. 3 1 3
0.014
2.893
13.128
0.020
0.579
0.262
1.032
1.893
0.458
46.934
4.849
2.617
1.507
1.314
0.012
2.704
13.002
0.021
0.656
0.300
1.014
1.991
n.a.
47.377
5.896
1. 8 1 0
0.700
1.125
0.011
3.994
13.530
0.006
0.014
0.260
1.624
1.905
n.a.
45.507
ot
oz4
f
TotalMg'
JI
AI
Ti
FE
Mn
Mg
Total
Ca
Ba
Na
K
Total
F
Charge'.
97.54
0.698
5.259
2.258
1.182
1.340
n.a.
3.088
12.982
0.089
0.393
0.272
1.322
2.O77
n.a.
46.871
n 1R
608
n.a.
97 03
0.685
5.075
2.444
1.288
1.361
n.a.
2.962
1 3 . 13 0
0.025
0 566
0 190
1. 1 9 8
1.979
n.a.
46.916
5 181
2.24s
1.203
1.341
0.015
3.294
13.279
0.037
0.4s8
0.244
1.267
2.006
0.643
46.411
tJ.co
0.35
4.48
0.90
7.70
98.01
u.o/c
o:22
5.439
2.154
1.144
1.441
n.a.
2.992
13.169
0.05s
0.260
0.258
1.454
2.027
n.a.
46.777
u.5/v
0.142
1.198
1.974
n.a.
47.033
A/ote:numbers2aand2d analyzedat SMU; others at lC. Febtas FeO; n.a. : not analyzed.
-LessO=F.
" Calculatedwith t€trahedral + octahedral: 14.
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
Mrcl
crrnrvnsrnv
This study is based on about 80 phlogopite analyses.
Representativeresults are given in Table l. Figure 2 illustrateschemical variations of the phlogopite samplesin
a series of variation diagrams that show BaO vs. other
compositionalvariables.
Barian titanian phlogopite
1059
TiO, for the WEK phlogopite samplesand the other samples from the literature.
Mica samples in Hawaiian nephelinites are the most
TiO' (13.0-14.5wto/o),
enrichedin BaO (14.3-20.4wto/o),
and Al,O. (15.8-17.6 wto/o)but have the lowest Mg' (as
low as 0.40) among all the samplesconcerned.This is in
agreement with their reported occurrence as a Iate
groundmassphase.On the other hand, those in aillikites
and melilitites show wide variation in BaO (3-15 wto/o)
but relatively low TiO, (<4.0 wto/o).Their Al'O, contents
are variable, but generally high (up to 19.3 wto/o).High
Mg' values (>0.84) reflect the ultramafic nature of the
hosts (Rock, l99l). The composition of barian phlogopite in the silicate bands ofJacupiranga carbonatites(Brazil) is characterizedby extremely high Mg' (0.94-0.96)
and low TiO, (<0.2 wto/o).The high Mg' values are consistent with the dolomite-dominant host carbonatites
(Gasparand Wyllie, 1987).Al,O3 is high (16-20 wto/o),
but BaO is variable (<1.0-10.2 wto/o).Chemicalcharacteristics of barian titanian phlogopite in potassic rocks
are apparently related to tectonic settings (e.g., Barton,
1979).The phlogopitefrom potassicrocks associatedwith
subduction (e.g., Roman Province, Italy, and Montana)
shows a wide range in BaO contents (2-9 wto/o)and Mg'
(0.61-0.89)but, as expectedfrom the host magmas,contains lower TiO, (<6.0 wto/o)than the phlogopite from
potassic rocks of intraplate settings(e.g.,WEK and East
Australian leucitites, >8.0 wto/o).The Australian phlogopite samples are similar to the WEK ones in terms of
TiOr, BaO, and AlrO. contentsbut are lower in Mg' (0.560.61) than the latter (Fig. 3). Mica samplesin lamproites
rarely have BaO > 3.0 wto/0,despite strong enrichment of
Ba in the hosts.
Barian titanian mica samples found in WEK olivine
leucitites and in DZ2n conlain 4.2-11.2 wto/oBaO and
l0.l-13.1 wto/oTiOr. Mg' [: atomic ratio Mg/(Mg +
Fe,",)lvalues are 0.627-0.71l. Thus, most of them are
phlogopite (Deer et al., 1962). These phlogopite samples
have moderate but variable SiO, (31.7-36.8 wto/o)and
AlrO3 (l1.5-14.6 wto/o),
whereasMgO and FeO (Fe,.,)are
ll.3-14.2 and,9.2-14.2 wt0/0,respectively.KrO is low
and variable(5.2-7.7 wto/o).
NarO and CaO are below l.l
and 0.4 wt0/0,respectively,and do not correlatewith BaO.
They contain <0.06 wto/oCl, but moderate amounts of F
(0.50-1.39, av. 0.90 wtolo).Although many of the analyzed phlogopite grains are heterogeneous,no systematic
zoning patterns have been detected.
SiOr, MgO, FeO, and KrO decreasewhereasAlrO, and
TiO, increasewith increasingBaO in the barian titanian
phlogopite (Fig. 2). Phlogopite samplesinDZ2n are generally more enriched in BaO than those in olivine leucitites; thus they have higher AlrO. and TiO, but lower
SiOr, MgO, FeO, and KrO. However, the coherentelement trends and overlapping Mg' values demonstrate
similar substitution schemesand suggesta geneticaffinity
between the two groups of phlogopite, despite their different occurrences.
Si and Al cations (O : 22) are <8.00 (7.38-7.73; av.
7.5 + 0.06) pfu, implying that Al is confinedexclusively
phlogopite from other potassicrocks
to the tetrahedral occupancy.The sums ofoctahedral cat- Comparisonwith
TiO2, Al2O3,and FeO contents of the WEK phlogopite
ions are <6.0 (5.29-5.93,av. 5.6 + 0.09),lower than
samples
are plotted in Figure 4 for comparison with those
those for an ideal trioctahedral mica. In contrast, the occupancy in the l2-fold-coordinated interlayer site is al- from other potassicrocks, including lamproites, minettes,
Italian and Ugandan potassic rocks, and Mongolian leumost stoichiometric(av. 1.99 t 0.006).
cite basanites.The WEK phlogopite samplescan be readily
Phlogopite coronas
distinguished by their generally higher Al'O. and TiO,
Phlogopite coronas in WEKI6 are chemically distinct contents from those hosted by lamproites and the Uganfrom the barian titanian phlogopite. They have very low dan kamafugites.The latter generallyevolve toward very
BaO (<0.24 wto/o)and moderate TiO, (6.38 wto/o),but low AlrO, at various TiO, and FeO contents, forming a
high Mg' value (0.78), similar to the olivine phenocrysts late-stagetetraferri phlogopite. This chemical signatureis
(For,-,n, Zhang, unpublished data). The phlogopite coro- apparently imparted by the peralkaline nature of host
nas contain higher SiO, (39.5 wto/o)and K,O (8.6 wto/o) magmas.Although most of the WEK samplesplot in the
but lower AlrO3 (10.4 wto/o)than the barian titanian field distinguishedby micas from minettes and the Italian
phlogopite. They have fewer vacanciesin the tetrahedral potassic rocks in the AlrO. vs. FeO diagram (Fig. 4a),
and octahedralsites(t4rR:7.72, I6rR: 5.79) but do not they have much higher TiO, contents (Fig. 4b). The combination of a negativecorrelation betweenAlrO, and FeO
have sufficient interlayer ions (A site : 1.90).
and a positive correlation between AlrO, and TiO, for
Comparisonwith other barian phlogopite
the WEK samples(Fig. a) is distinctive and has not been
The compositions of barian phlogopite (arbitrarily set observed elsewhere.The WEK phlogopite samples are
as BaO > 2 wto/o)differ significantly among three cate- similar to the Mongolian mica megacrystsin terms of
gories ofhost alkalic rocks: (1) nephelinites,(2) aillikites, TiO, content(Fig.ab) and Mg' (0.63-0.71and 0.59-0.67,
melilitites, and carbonatites,and (3) maflc potassicrocks. respectivel/, but WEK phlogopite sampleslower in AlrO,
Figure 3 shows variations of BaO, AlrOr, and Mg' vs. and FeO than the mica megacrysts(Fig. 4a).
1060
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
16
Fig. 3. TiO, vs. (a) BaO, (b) Al,O3, and (c) Mg' for magmatic
barian micas. For the sake of clarity, one-half to two-thirds of
the WEK data points have beenomitted from Fig. 3, 4,6, and
7. Dala sources: nephelinite-Hawaii, Mansker et al. (1979);
southeastChina, Cao and Zhu (1987); aillikite and melilititeSouth Africa, Velde (1979), Boctor and Yoder (1986); Greenland, Rock ( I 99 I ); carbonatite - Brazil, Gaspar and Wyltie ( I 987);
potassic rocks-East Australia, Birch (1978; 1980); Roman
Province, Thompson (1977), Holm (1982); Montana, Wendlandt (1977), O'Brien et al. (1988); lamproite-Mitchell and
Bergman(1991).
t2
(\l
o
irg
iR
l
4
'sro
;t* o
U
V
ts
(a)
wt% BaO
r6
the possibility that Ti, Fe, and even Mg (Foley, 1990)
may occur in tetrahedral coordination. However, as the
WEK phlogopite sampleshave geneticaffinities, the substitution schemes can be evaluated by their coherent
chemical departuresfrom end-member phlogopite (Hewitt and Abrecht, 1986) in light of relevant experimental
data.
o
l2
(\l
o
l'. 8
!R
=
Ba substitutions
4
(b) "
V
05101520
r r 2 t B+
a rr2tE: 2rr2tK.
wt% NOs
l6
0.398
tr
rr2tBa+ r4iAl: rt2tK+ t4rsi.
t2
x
o
F-8
s
o
i
o
4
(c)
o
9o
oo
,V
T
o " "V* '
0r_
0.50
0.60
(l)
Another, proposed by Wendlandt (1977), Mansker et al.
(1979),and Bol et al. (1989),is
n""n
(\l
There are two alternative substitutions involving Ba
by Shmakin (1984)
and the interlayer sites.One, suggested
and Wagner and Velde (1986) and implied from coupled
substitutionsproposedby Mitchell (1981) and Guo and
Green(1990),is
0.70
0.80
0.90
1.00
Ms/(Mstft)
SussrrrurroN scHEMES
Assigning a unique set of substitutions to the WEK
barian titanian phlogopite is hindered by (l) the complexity of substitution schemesin micas, which can be
representedin terms of various linear combinations of
end-members(Hewitt and Abrecht, 1986),(2) the unconstrained Fe and Ti valencies and HrO contents, and (3)
(2)
The WEK phlogopite samples have almost stoichiometric interlayer occupancyand plot along the line of l: I
substitution of Ba for K + Na + Ca (Fig. 5a), which
favors Substitution 2 over l. As Ba cations per formula
unit (0.25-0.70)are >r4rAl- 2.0 (0.15-0.62),Al is insufficient to compensatefor the chargeimbalance. Thus,
even ifSubstitution 2 describesthe generalform ofcoupled Ba substitution, another cation, probably Fe3+,must
also be presentin tetrahedral coordination to compensate
for both the tetrahedral site deficiency and the charge
imbalance.
Chemical data for the other barian trioctahedral micas
from the literature also define a similar negative correlation between Ba and K + Na + Ca, as shown by the
WEK samples. Tho sums of the interlayer cations are
nearly 2.0 pfu for all but a few samplesfrom the Leucite
Hills and Smoky Butte lamproites, whose interlayer cations are as low as 1.8 pfu and whose BaO contents are
<3 wto/o(Ba < 0.2 pfu) (Mitchell and Bergman, l99l).
Therefore, Substitution 2 may be applicable to the majority of magmatic barian micas, except for those from
lamproites.
1061
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
(b)
(a)
o inclusionDZ2n
A
A
MO
)"^s
I
^o
R
Rto
;e
t
TBENDS
+
e
MO
U
M
I
LC
0
5
l0
15
l0
5
ZO0
ol-lcucititc
olivine corona
kamafugite
mincttc &
Italian high-K.
KEYS
Mongolian bas.
Ugandanhigh-K.
minettc
Italian high-K.
core, lamproite
15
wt%TiOz
wt%tuOt
Fig. 4. AlrO. vs. (a) FeO and (b) TiO, for phlogopite from potassicrocks. Data sources:lamproite, Mitchell (1981), Mitchell
and Bergman(1991);kamafugites,Edgar(1979);Italian potassicrocks, Thompsot(1977), Holm (1982);minettes,Bachinskiand
Simpson(1984);Mongolian leucitebasanites,Ryabchikovet al. (1981).
2.4
Ti substitutions
The role of Ti in barian titanian phlogopite depends
upon Ti valence and occupancy. The relationship between Ti cations and excesspositive charge (Fig. 6a) on
the basisof 14 tetrahedral* octahedralcations(e.g.,Bol
et al., 1989)lends strongsupportfor Tia+, as most of the
WEK samplesplot close to or above the line defined by
2 ionic charge- 44 :2Ti. Valuesof /o, calculatedfrom
Fe-Ti oxides in the WEK olivine leucitites are 0.5 log
units above the FMQ buffer (Zhang, unpublished data),
further suggestingthat all Ti in the potassicmagma could
be quadravalent.
Another controversy concernsTi occupancyin micas.
In light of the tetrahedral deficienciesin all WEK samples, Substitutions
ralSi: ratTi
(a)
*
2.0
;
1.8
{
+
1.6
$
(!
(4)
are theoretically possible.
Optical absorption and Miissbauer spectral results fail
to provide conclusive evidence in favor of t4rTiin micas
(seesummariesof Edgarand Arima, 1983;Dymek, 1983;
Abrecht and Hewitt, 1988; Brigatti et al., l99l). In particular, the presenceof torTiin barian titanian phlogopite
would increase lattice energy by electrostatic repulsion
between t4lTi and t"lBa, and hence decreasephlogopite
stability (Bol et al., 1989).With regardto the WEK micas,Substitution 3 cannot explain observedvariations in
the sum of tetrahedral * octahedral cations (about 0.5
pfu), nor does the positive correlation betweenAl and Ti
Gig. ab) support Substitution 4. For these reasons,oc-
a
\
a
t.2
1.0
13 . 8
(b)
14
rrMg + 2t41{l: t6rE+ 2t4rTi
"N-:-^+or^
'o;
Q r.4
(3)
and
1",t"s ? a
= 13.G
(T+o)-site
13.6
o
I
13.4
q)
.=
U)
13.2
+
b 13.0
12.8
0.0
0.1
0.4
0.3
Ba2*
0.6
0.7
Fig. 5. Ba vs. (a) K + Na + Ca and (b) tetrahedral + octahedral site cations for the WEK phlogopite.Calculatedwith O :
22. Symbolsas in Fig. 2.
\062
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
4.0
Ti-Tschermak's:
t0l11ylg,Fe2*)
+ 2t4rsi: I6rTi + 2t4r(Al,Fe3+) (5)
\t
T
rA^
3.0
)z'(a)
Ti vacancy:
o
2tol(Mg,Fe2+): I6lTi + I6rE
g
$
Ti-o:
6 z.o
o
t6t(Mg,Fe2+)
+ 2(OH) : t61Ti+ 2O2- + H2.
(f'
B 1.0
a.
6'{q'
r6tTi+ r61Fe2+
+ r6tE+ r41si+ r4rE
:3r61Mg + 2totMg.
(b)
3.0
$
2's
ss 2.o
.rl
A\
IA
1.5
6.0
(c)
5.0
e
g40
=
{9.
3.0
2.O
0.0
0.5
Q)
In addition, Foley (1990) suggestsanother substitution
for lamproitic micas formed under reducing conditions,
0.0
3.5
F
(6)
1.0
Ti4+
l.5
2.0
Fig. 6. Ti vs. (a) excesspositive charge, (b) tar(Al * Fe3+),
and (c) t6t(Ivtg+ Fer*) for the WEK phlogopite; a, calculated
with tetrahedral + octahedral : l4:b and c, with O : 22. Sufficient Fe3+is calculatedfrom Fe,",and added to Si + Al to fill
the T sites and is subtractedfrom Fe,".on the O sites. Symbols
as in Fig. 2.
tahedral occupancy ofTi, and thus tetrahedral occupancy
ofFe3*, is strongly preferred.
Assuming that Ti occurs as t6lTi4+, three basic substitutions involving Ti are frequently proposed (e.g., Dymek, 1983). They are
(g)
This substitution is, however, inapplicable to the WEK
micas becauseof the antipathetic correlation between Si
and Ti (ref. Fig. 2a,2c).
Although the data for the WEK barian titanian phlogopite define a slope broadly parallel to the line representing Ti-Tschermak's substitution [Ti : 2(Al + Fe3+)
- 21, the amount of Ti is much too high for this to be
the only or even dominant substitution (Fig. 6b). Distinction betweenTi vacancy(Tronnes et al., 1985) and
Ti-O (Bol et al., 1989)is equivocalbecausethe correlation shown in Figure 6a is consistentwith both interpretations, and becausethe importance of an oxy substitution cannot be evaluated without independent Fe3+and
HrO determinations (Dymek, 1983; Abrecht and Hewitt,
1988).A well-definednegativecorrelation betweenTi and
Mg * Fez+ for the WEK samples (Fig. 6c), which plot
parallel and closeto the line at zTi + (Mg + Fe'z+;: 6,
is strong evidence for the Ti-vacancy to be a dominant
substitution. The tetrahedral * octahedralsite deficiency
(Fig. 5b) also points to the same conclusion. An alternative explanation is that the deficiency is an artifact of
normalizing to O : 22.The Ti-O substitution could produce a positive correlation between O and Ti and, as O
> 22,higher calculatedtetrahedral * octahedralcations.
If this were the case, and a fixed-cation normalization
tetrahedral * octahedral : 14 were used instead, the calculated interlayer cations per formula unit would be on
average >2. 13 (up to 2.35), rather than being stoichiometric. The extra interlayer cations, however, must be
used with great caution, as evidenceagainstTi-O substitution becauseit may result from an effectof substitution
of F for OH upon the normalization procedure.This substitution results in O < 22 and thus increasesthe sum of
the interlayer occupancy calculated from fixed cations.
Since the maximum F content in the WEK phlogopite is
1.4 wto/o,correspondingto [(OH)3rrFor,]ooo,
the calculated interlayer cations can only be increasedby 0.04 pfu
relative to an F-free stoichiometric phlogopite, far less
than necessaryto compensatefor the apparent excessin
interlayer cations. In other words, ifthe calculatedexcess
of the A-site occupancywere to solely reflect the presence
of F, the WEK barian titanian phlogopite should contain
1063
ZHANG ET AL.: BARIAN TITANIAN PHLOGOPITE
at least4 w{/oF. Therefore,although no conclusivechoice
from the alternatives can be made, the data favor a Ti
vacancy substitution.
The above arguments are based on coherent chemical
variations of the WEK phlogopite using simple parameters such as the amounts of cation(s) or site occupancy.
Some plots with complex parametersrepresentinglinear
combinations of various substitutions have been employed(e.g.,Manskeret al., 1979;Wendlandt, 1977;Guo
and Green, 1990) in an attempt to identify specificcombinations of the basic substitutions for barian titanian
phlogopite. Nevertheless,the resultsfail to provide better
discrimination and sometimes could be misleading becausethe apparently linear relationships might be an artifact derived from correlationsamong simple parameters
(Foley, 1990). Three caseswill be considered below to
elucidate the problems associatedwith this approach.
1. In Figure 7 2(Al + Fe3+) * Ti is plotted vs. 2Si +
(Mg + Fe'?+
) for the WEK phlogopite.Wendlandt(1977)
interpreted a relationship similar to that shown in Figure
7 as evidencefor the Ti-Tschermak's substitution in barian phlogopite from the Montana potassicrocks. In fact,
compositions that are parallel to the join defined by
phlogopite and Ti-Tschermak's end-member can be derived from a linear combination of Substitutions 2 and
7. Moreover, the offset of data points below the join requires some type of vacancy substitution. Even Substitution 6 cannot significantly changethe slope ofthe trend
becauseSi and Al, which assumegreater weight than Ti
and Mg in the plot, apparently control the trend. Thus,
the slope of the trend in Figure 7 is not a sensitive indicator for particular substitutions.
2. From a near ideal linear trend with a slope of - 1
betweenBa + 2Ti + 3Al and K + 3(Mg,Fe) + 3Si, Mansker et al. (1979) argued for a combined substitution of
10.0
Ti-Tsch
8.0
$uo
,t
oo
R
A
Phl
2.0
0.0
10.0
r2.0
14.0 16.0
23i+{Mg+ft2.)
18.0
20.0
Fig.1. Aplotof 2Si+ (Mg + Fe'?t)vs.2(Al+ Fer*) + Ti
with O : 22. Symbolsasin
for the WEK phlogopite.Calculated
Fig.2.
as a best fit for natural and synthetic phlogopite from
potassic rocks (mainly lamproites). It is a linear combination ofSubstitutions l, 6, and 5 in the proportions of
l: l:2. Therefore,it cannot be verified for the samereason
the Hawaiian biotite cannot be. As Substitution l0 involves a significantproportion of the Ti-Tschermak'sendmembers, it is very unlikely to be applied to phlogopite
samplesin lamproites becauseof their Al-deficient character (e.g.,Mitchell, 198l) and a negative correlation between AlrO, and TiO, (Fig. ab). The Hawaiian data
(Mansker et al., 1979) fit both Substitutions 9 and l0
well, further illustrating that complex parameterscannot
discriminate sensitively among alternative substitutions.
rr2rBa+ t6r! + 2r6rTi+ 3t4lAl
Ti substitution schemesvary among micas crystallized
from
diverse magma compositions and under different
: rr2tK_u3ror(Mg,Fer+)
(g)
+ 3ratSi
physical conditions (Edgar and Arima, 1983). Therefore,
no general applicability of a particular scheme can be
for the Hawaiian barian titanian biotite. As Substitution
: t4lAlsubassumed.For instance,Ti vacancy and talFe3+
9 involves an O-site vacancy,the l:1 replacementshould
stitutions(Mitchell, l98l), but not Ti-Tschermak's,may
indicate a deviation from the proposed substitution rathbe predominant in lamproitic micas for the reasonsdiser than an accordancewith it. This trend doesnot parallel
cussed above. In contrast, Tschermak's substitution is
the join linking phlogopite and an end-member created
likely to prevail, accompaniedby minor Ti-Tschermak's,
which is a
by Substitution9, KBaMg3Ti2Si3Al5O,o(OH)",
in phlogopite in potassicrocks related to subduction (e.9.,
linear combination of Substitutions 2, 5, and 6 in the
Roman Province).
proportions l:1:1. Whereasa constantBa:Ti ratio of 0.5
is specifiedby Substitution 9, the Hawaiian biotite samples have variable Ba:Ti ratios (0.54-0.81, Mansker et al.,
Pln-tcnNnsrs AND MICA coMPosrrloN
1979).Therefore, the data indicate that all the three subTextural relations indicate that the phlogopite in the
stitutions operatein the Hawaiian biotite samplesat variWEK lavas is a late phasecrystallized from residual poable ratios.
tassic melts at near-surfacepressuresand low tempera3. Recently,Guo and Green (1990) proposedanother
tures,e.g.,<960'C in olivine leucitites(Fe-Ti oxide temsubstitution,
perature, Zhang, unpublished data). The presence of
leucite in inclusion DZ2n also suggeststhat it formed at
rr2lBa + rr2tD + 3t61Ti+ t61[]+ 4t4rAl
low pressures(probably <4 kbar, Taylor and Mackenzie,
:2'l2tK 1 4rel(Mg,Fe2+)+ 4r4rsi
(10)
1975).As Mg' valuesfor the phlogopiteinDZ2n (62.7-
to64
ZHANG ET AL,: BARIAN TITANIAN PHLOGOPITE
70.1) are slightly lower than for the phlogopite in the
groundmassof olivine leucitites(66.5-71.1),DZ2n may
have crystallized from a more evolved potassic magma.
Differencesin composition betweenthe barian titanian
phlogopite in olivine leucitite and the corona phlogopite
in leucite basanite cannot be attributed to the different
host magma compositions. The corona phlogopite shows
lower BaO, AlrOr, and TiO, but higherMg'than the barian titanian phlogopite, whereasthe host for the former
(leucite basanite wZKl6) in higher in Ba, Al,O., and
TiO, but lower in Mg' than for the latter (olivine leucitites). Differences in paragenesisbetween WEK olivine
leucitites and leucite basanitesmay provide an explanation for the contrast. Neither leucite nor nepheline, both
of which are present in the olivine leucitites, is the repository for Ba. Ba is hardly above 0.06 wto/oin leucite
and even lower in nepheline (Zhang, unpublished data).
Therefore after crystallizing >850/o Ba-free phases, the
residual olivine leucititic melts are so enriched in Ba that
barian titanian phlogopite can crystallize (Zhang et al.,
1993).In contrast,potassiumfeldspar,a major groundmass phasein the leucite basanites,incorporates a moderateamount of BaO (0.2-1.8 wto/0,av. 0.5 wto/o;Zhang,
unpublished data). Thus, the residual liquids in WZKl6,
with which olivine phenocrystsinteracted to form phlogopite, must have undergoneBa depletion. Higher Mg' in
the phlogopite coronas is attributed to the forsteritic olivine phenocryststhat dominate the MgO budget of the
reaction, whereas low AlrO. content reflects the combined effects of AlrO.-absent olivine phenocrysts plus
Al,O.-depleted residual liquids causedby potassiumfeldspar crystallization. Therefore, the chemistry of magmatic phlogopite depends on not only the composition of
host magmas and intensive variables but also mineral
(Foley, 1990)and crystallizationsequences.
assemblages
The latter factors can be employed to explain the lack of
barian phlogopite in extremely Ba-rich lamproites because Ba-rich accessoryphases, such as priderite, jeppeite, and shcherbakovite,compete for Ba.
AcxNowr,nncMENTS
Financial support for this study was generouslyprovided to M.Z. by
the late Stephen Hui through the Anna and Stephen Hui fellowship at
IC. Support at SMU was provided by the Department ofGeological Sciencesand by NSF grantEAR-90-04812to M.A.D. Discussions
with Mike
Holdaway are appreciated.We thank N. Royall and D. Deuring for their
assistancein microprobe analysis.The manuscript was improved by the
review of D.A. Hewitt and R.F Wendlandt
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