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Intergrowths between anthophyllite, gedrite, calcic amphibole, cummingtonite, talc and chlorite in a metamorphosed ultramafic rock of the KTB pilot hole, Bavaria

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A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36 m, contains the assemblage Caamphibole - anthophyllite - chlorite - talc, formed at about 630degreesC/10 kbar under conditions of the high-P amphibolite facies. EMP analyses of the Ca-amphibole yielded magnesiohastingsite (to pargasite) compositions with (Al+Fe3++Cr+Ti)([6]) of 1.0-1.2 p.f.u. and (Fe2++Mn)/(Mg+Fe2++Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe3++Cr+Ti)([6]) less than or equal to 0.16 and (Fe2++Mn)/(Mg+Fe2++Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al-[4]/(Al-[4]+Si) of 0.27-0.30 and Fe2+/(Fe2++Mg) of 0.12-0.14. TEM investigations revealed, for the first time, a complex lamellar intergrowth in the sequence talc --> anthophyllite anthophyllite/cummingtonite intercalations --> anthophyllite --> Ca-amphibole. This lamellar intergrowth is about 15 mum in width, but individual cummingtonite lamellae intercalated with dominant anthophyllite are only 1 mum wide. All the amphiboles share the directions b* and a*; anthophyllite and the monoclinic amphiboles are intergrown along (100); anthophyllite and talc are intergrown with (100)(Ath)//(001)(Tlc). In other areas anthophyllite and chlorite are intergrown with a*(Ath)//c*(Chl), and b*(Ath)//b*(Chl) and Ca-amphibole exsolves lamellae of cummingtonite//(100). With one possible exception, cummingtonite has the space group C2/m. Anthophyllite displays chain multiplicity faults //(010), anthophyllite and cummingtonite chain arrangement faults //(100). Microstructures suggest that anthophyllite was formed at the expense of cummingtonite that is interpreted as a high-T precursor phase testifying to a possible earlier, granulite-facies metamorphic stage. During the retrograde P-T path, on cooling presumably below 500-550degreesC, anthophyllite exsolves platelets or lamellae of gedrite with {hk0} composition planes. Orientations close to {230} and {110} have been recorded, so far not described in the literature. Alteration products of biotite are composed of clinochlore and a sheet silicate intermediate in composition between chlorite and biotite, i.e., with partly high K contents, but with chlorite metric.
Content may be subject to copyright.
This paper is dedicated to the memory of
Luciano Ungaretti who has made important
contributions in the field of amphiboles
Intergrowths between anthophyllite, gedrite, calcic amphibole,
cummingtonite, talc and chlorite in a metamorphosed ultramafic rock of the
KTB pilot hole, Bavaria
WOLFGANG FRIEDRICH MÜLLER
1
, ESTHER SCHMÄDICKE
1,2
, MARTIN OKRUSCH
3
and ULRICH SCHÜSSLER
3
1
Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstr. 9,
D-64287 Darmstadt, Germany
e-mail: wmueller@geo.tu-darmstadt.de
1,2
Mineralogisches Institut, Universit Erlangen, Schlossgarten 5, D-91054 Erlangen, Germany
3
Mineralogisches Institut der Universität Würzburg, Am Hubland, D-97074 Würzburg, Germ any
Abstract: A metamorphosed ultramafic rock, penetrated by the KTB pilot hole at a depth of 1382.36 m, contains the assemblage Ca-
amphibole anthophyllite chlorite talc, formed at about 630°C/10 kbar under conditions of the high-P amphibolite facies. EMP
analyses of the Ca-amphibole yielded magnesiohastingsite (to pargasite) compositions with (Al+Fe
3+
+Cr+Ti)
[6]
of 1.0-1.2 p.f.u. and
(Fe
2+
+Mn)/(Mg+Fe
2+
+Mn) ratios of 0.10-0.16, while coexisting anthophyllite has (Al+Fe
3+
+Cr+Ti)
[6]
0.16 and
(Fe
2+
+Mn)/(Mg+Fe
2+
+Mn) ratios of 0.21-0.29. Chlorite is clinochlore with Al
[4]
/(Al
[4]
+Si) of 0.27-0.30 and Fe
2+
/(Fe
2+
+Mg) of
0.12-0.14.
TEM investigations revealed, for the first time, a complex lamellar intergrowth in the sequence talc
®
anthophyllite
®
anthophyllite/cummingtonite intercalations
®
anthophyllite
®
Ca-amphibole. This lamellar intergrowth is about 15 µm in width,
but individual cummingtonite lamellae intercalated with dominant anthophyllite are only 1 µm wide. All the amphiboles share the
directions
b
* and
a
*; anthophyllite and the monoclinic amphiboles are intergrown along (100); anthophyllite and talc are intergrown
with (100)
Ath
//(001)
Tlc
. In other areas anthophyllite and chlorite are intergrown with
a
*
Ath
//
c
*
Chl
and
b
*
Ath
//
b
*
Chl
and Ca-amphibole
exsolves lamellae of cummingtonite //(100). With one possible exception, cummingtonite has the space group
C
2
/m
. Anthophyllite
displays chain multiplicityfaults //(010), anthophyllite and cummingtonite chain arrangement faults//(100). Microstructures suggest
that anthophyllite was formed at the expense of cum mingtonite that is interpreted as a high-T precursor phase testifying to a possible
earlier, granulite-facies metamorphic stage. During the retrograde P-T path, on cooling presumably below 500-550°C, anthophyllite
exsolves platelets or lamellae of gedrite with {hk0} composition planes. Orientations close to {230} and {110} have been recorded,
so far not described in the literature. Alteration products of biotite are composed of clinochlore and a sheet silicate intermediate in
composition between chlorite and biotite,
i.e.
, with partly high K contents, but with chlorite metric.
Key-words: amphibole-chlorite-talc intergrowths, anthophyllite, gedrite exsolutions, cummingtonite, Ca-amphibole, orientation
relationships, chain multiplicity faults, chain arrangement faults, TEM.
Introduction
Since the classical work of Sundius (1933), the coexistence
of Ca-amphiboles with monoclinic and/or orthorhombic Fe-
Mg amphiboles, testifying to miscibility gaps in the amphi-
bole composition space, has been described repeatedly (e.g.
C. Klein, 1968; Robinson et al., 1969, 1982; Deer et al.,
1997). Robinson (1961) was the first to observe an assem-
blage with hornblende, cummingtonite and anthophyllite in
metabasites in the upper sillimanite zone of southern New
England, U.S.A.; however, he was not sure about the stable
coexistence of these three amphiboles. From a more rigor-
ous evaluation of the chemographic relations, C. Klein
(1968) and Robinson & Jaffe (1969) suggested the three-
amphibole assemblage to occur in rocks with a bulk
Fe
2+
/(Fe
2+
+Mg) ratio centered on 0.39, forming a slim
three-phase triangle, e.g. in the AFM projection. In the same
metamorphic belt, Robinson et al. (1969) collected a meta-
basite sample with a bulk Fe
2+
/(Fe
2+
+Mg) ratio of 0.35, con-
taining four different amphiboles: magnesio-hornblende
with fine exsolution lamellae of cummingtonite, primitive
cummingtonite with fine hornblende lamellae and antho-
phyllite with submicroscopic gedrite lamellae. Hollocher
(1991) interpreted the three-amphibole assemblage in me-
tabasites of southern New England as being due to the pro-
grade reaction
anthophyllite + hornblende + ilm enite + quartz
= cummingtonite + plagioclase + rutile + H
2
O.
Eur. J. Mineral.
2003, 15, 295307
DOI: 10.1127/0935-1221/2003/0015-0295
0935-1221/03/0015-0295 $ 5.85
2003 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
James et al. (1978) recorded the assemblage hornblende-
cummingtonite-anthophyllite in mafic gneisses of the high-
er sillimanite zone in Ontario, Canada.
The widespread occurrence of magnesiocummingtonite
in homotaxial intergrowths with tremolite was described
from ultramafic schists of the kyanite-staurolite zone in the
Cima di Gagnone area, Ticino, Switzerland (Evans et al.,
1974; Rice et al., 1974). Anthophyllite of almost identical
composition, often intergrown with cummingtonite along
lamellae //(010) and //(100), was explained by these authors
as an inversion product during cooling. Single-crystal
precession photographs show magnesiocummingtonite
(P2
1
/m) as the main phase intergrown with a small amount
of anthophyllite (Pnma) mutually sharing their a* direc-
tions, and about 5 % of tremolite sharing its c* direction
with that of cummingtonite. Streaks //a* in the precession
photographs indicate submicroscopic anthophyllite lamel-
lae (Evans et al., 1974; Ghose, 1981). Similar anthophyllite-
cummingtonite-actinolite intergrowths were recognized by
Kamineni et al. (1979) in metamorphosed ultramafic rocks
from Baffin Island, Arctic Canada. At metamorphic P-T
conditions corresponding to the lower sillimanite zone, the
anthophyllite-gedrite solvus starts to open, giving rise to the
assemblages cummingtonite-anthophyllite-gedrite (Schnei-
derman & Tracy, 1991) and Ca-amphibole-cummingtonite-
anthophyllite-gedrite (Stout, 1971, 1972; Hawthorne et al.,
1980; Schumacher, in Robinson et al., 1982). The same
four-amphibole assemblage, however, with a still wider an-
thophyllite-gedrite solvus, was r ecorded by Spear (1980) in
metabasites of the kyanite-staurolite zone in Vermont.
In many cases, intergrowths of different amphiboles
range down to submicroscopic dimensions and, therefore,
cannot be evaluated r easonably without transmission elec-
tron microscopy (TEM). In an excellent paper, Smelik &
Veblen (1993) present the results of TEM studies on 17 ex-
solved orthoamphiboles from 6 different localities. Some of
the exsolution lamellae are visible with the conventional
petrographic microscope. With TEM, optically homo-
geneous amphiboles show exsolution lamellae ranging in
width between 200 nm down to Guinier-Preston zones, i.e..
to unit-cell dimensions. The exsolution lamellae are mostly
oriented parallel (010), but orientations parallel {140},
{130} and {120} were also recorded, as well as bent lamel-
lae with orientations scattering around (010). Smelik &
Veblen (1993) show that these orientations conform to opti-
mal phase boundaries (Robinson et al ., 1977) with composi-
tion surfaces, minimizing the surface energy and leading to
best fit of both crystal structures. Obviously, the orientations
are controlled by the differences of the lattice constants b
and a of anthophyllite and gedrite which are sensitive to dif-
ferences in Ca and Na contents, to the Fe/ Mg ratios and to
the P-T conditions of formation. The authors observed ho-
mogeneous and heterogeneous nucleation, but also lamellar
fabrics indicating coarsening of spinodal exsolutions.
Other TEM studies were performed on anthophyllite-ge-
drite (Gittos et al., 1976; Treloar & Putnis, 1982), horn-
blende-grunerite (Gittos et al. 1974, 1976) and hornblende-
cummingtonite pairs (U. Klein et al. 1996). Carpenter
(1982) observed antiphase domain boundaries with a dis-
placement vector of ½[110] in a cummingtonite with space
group P2
1
/m forming exsolution lamellae //(001) in tremo-
lite, first described by Bown (1966). The space group of
cummingtonite mostly found is C2/m (Ghose 1981; Deer et
al., 1997). Exsolution phenomena in amphiboles are de-
scribed in great detail by Smelik & Veblen (1991, 1992,
1993, 1994) and Smelik et al. ( 1991).
Like other chain silicates, amphiboles may theoretically
display three different types of planar faults (e.g. Maresch &
Czank, 1988; Putnis 1992): (i) chain periodicity faults,
CPFs, (ii) chain multiplicity faults, CMFs, and (iii) chain ar-
rangement faults, CAFs. (CMFs are also w ell described by
the term “chain widths defects as used by some authors.)
CPFs are conceivable in amphiboles but have not been ob-
served so far (see Czank, 1994). CAFs and CMFs are both
common in amphiboles and have been observed in this
study. Like in the pyroxenes, CAFs are typically oriented
//(100) and CMFs //(010), cf. Veblen (1981, 1992).
In this paper, we present a transmission electron micro-
scope (TEM) and electron microprobe (EMP) study of the
assemblage Ca-amphibole-cummingtonite-anthophyllite-
gedrite-chlorite-talc encountered in a metamorphosed ultra-
mafic rock of the pilot hole of the Kontinentales Tiefbohr-
Programm (KTB; German Continental Deep Drilling Pro-
gram), northeastern Bavaria, Germany. E arlier studies of
this rock were aimed at its protolith characteristics and
phase relationships (Matthes et al., 1995) and the stability
field of the mineral assemblage observed (Schmädicke &
Okrusch, 1997). In the present study, the dominant points
are the chemical composition of coexisting amphiboles,
their exsolution phenomena and mutual orientation relation-
ships, and those with associated chlorite and talc within the
same ultramafic rock. So far, most TEM studies are devoted
to one or two minerals only, and few were performed on pet-
rologically well-characterized samples. We therefore hope
to contribute to a better correlation between microscopic
and nanometer-scale petrology.
Experimental techniques
Coexisting amphiboles and sheet silicates were analyzed us-
ing a CAMECA SX50 microprobe at Mineralogisches Insti-
tut, Universität rzburg, with three wavelenth dispersive
spectrometers. Operating conditions were 15 kV accelera-
tion voltage, 15 nA beam current, and counting times of 20
s except for Fe (30 s). Natur al and synthetic silicates, oxides
and metals were used as reference standards; matrix correc-
tion was carried out with the PAP program of CAMECA.
Mineral formulae were calculated on the basis of 23 oxy-
gens. Fe
3+
was estimated by the midpoint method (Papike et
al., 1974) for Ca-amphiboles and by normalization to 15
cations excluding Na and K (Leake, 1997) for Fe-Mg am-
phiboles. Detection limits are between 0.05 and 0.1 wt.%,
and the analytical errors are less than 1 % relative for major
elements except Na (2 %).
Most of the TEM studies were carried out on a Philips
CM 12 transmission electron microscope operated at 120
kV; a few were performed on a Philips CM 20-UT transmis-
sion electron microscope operated at 200 kV (both instru-
ments are in the Department für Material- und Geowissen-
296 W.F. Müller, E. Schmädicke, M. Okrusch, U. Schüssler
schaften, TU Darmstadt). Specimens suitably thin for TEM
were prepared by Ar
+
ion milling as described in more detail
elsewhere (e.g. Müller, 1991). Mineral compositions were
determined by energy dispersive X-ray spectrometry (EDX)
using an EDAX 9900 attached to the CM 12 electron micro-
scope.
Sample provenance and petrography
The ultramafic rocks under consideration are part of two
amphibolite-metagabbro successions, penetrated at depths
of 1160-1610 m and 3575-4000 m by the pilot hole of the
German Continental Deep Drilling Program (KTB) near the
town of Windischeschenbach, Oberpfalz, NE Bavaria. In
these sequences, massive, medium to coarse-grained meta-
gabbros with ophitic to subophitic relic textures (e.g. Pat-
zak, 1991, 1996; Patzak, et al., 1991; Schalkwijk, 1991;
O’Brien et al., 1992) are repeatedly intercalated with layers
and lenses of ultramafic talc-chlorite-amphibole rock. Con-
tact relationships, relic textures, relics of igneous minerals
and bulk rock compositions indicate that these ultramafic
layers were derived from mafic cumulates, differentiated
from a basaltic magma, either in the lower parts of dolerite
sills or small gabbro intrusions (Matthes et al., 1995).
The investigated sample, derived at a depth of 1382.36 m,
is a massive ultramafic rock (Fig. 1). It consists predomi-
nantly of various amphiboles (about 65 vol.%), chlorite
(about 25 vol.% + traces of altere d biotite), talc (10 vol.%),
serpentine (<1 vol.%), opaques, mainly magnetite and il-
menite (1-2 vol.%), and rutile, apatite and calcite as acces-
sory phases. The rock is inhomogeneous on a thin section
scale. Two different domains can be distinguished: domain
(i) is dominated by medium- to coarse-grained porphyroc-
lasts of calcic amphibole, domain (ii) consists of fine-
grained, randomly oriented matrix minerals (Fig. 1; also see
Matthes et al., 1995, Fig. 3 to 7, and Table 1).
The porphyroclasts of pale-brown titano-magnesioha-
stingsite to pale-green magnesiohastingsite/pargasite are
dusted in a patchy manner with tiny platelets of ilmenite.
These features are interpreted as relics of late-igneous am-
phibole, originally derived from magmatic Ti-augite. The
porphyroclasts are spatially rela ted with fine, colourless ag-
gregates of talc, opaque phases and serpentine interpreted as
pseudomorphs after olivine (Matthes et al., 1995). The
pseudomorphs mostly occur in mutual contact with the por-
phyroclasts, in many cases as inclusions. Rare brownish,
submicroscopic aggregates (of ?chlorite + iron oxides), in-
cluded in amphibole, are interpreted as pseudomorphs after
orthopyroxene. Porphyroclastic amphibole shows a pro-
nounced undulatory extinction grading into subgrains and,
preferably at the margins, into neoblasts.
The matrix is composed of pale-green calcic amphibole
(magnesiohastingsite to pargasite, 50 vol.%) with fine exso-
lution platelets of cummingtonite, pale-green chlorite (40
vol.%), colourless anthophyllite (5 vol.%), very fine-
grained talc flakes, mostly forming aggregates ( 5 vol.%),
and subordinate opaque phases (Fig. 1). Chlorite and some-
what larger talc grains show undulatory extinction and kink-
bands. The granular (to columnar) Ca-amphibole has an in-
Fig. 1. Photomicrograph of the investigated sample. Calcic amphi-
bole (Cam) with fine exsolutionplateletsin coexistencewith antho-
phyllite (Ath) and chlorite (Chl). Plane-polarized light. Horizontal
field of view = 0.8 mm.
homogeneous extinction, in contrast to the columnar to ar-
row-shaped anthophyllite. Calcic amphibole, anthophyllite
and talc may form oriented intergrowths with chlorite
(c
Ath
//(001)
Chl
). At least in some domains, textural equilibri-
um between the matrix phases is indicated. In contrast to the
description of Matthes et al. (1995), cummingtonite was not
recorded as a major matrixphase in the sample investigated.
Mineral chemistry
Selected microprobe analyses of coexisting amphiboles
from sample 1382.36 m are listed in Table 1 and graphically
presented in Fig. 2 and 3. In the discrimination diagram of
Leake et al. (1997), all Ca-amphiboles intergrown with an-
thophyllite plot in the field of pargasite and magnesioha-
stingsite (Fig. 2b); in most cases, the Fe
3+
/Al ratios are > 1,
thus conforming to the magnesiohastingsite composition.
Chemical characteristics (in cations p.f.u.) are Si 6.09-6.35,
octahedral (Al+Fe
3+
+Cr+Ti) 1.01-1.18, (Ca+Na) in M4
1.88-1.92, (Na+K)
A
0.70-0.78, Ti 0.050-0.075; the
(Fe
2+
+Mn)/(Mg+Fe
2+
+Mn) ratios range between 0.10 and
0.16 (Fig. 3). In one of the two areas analyzed, the Ca-am-
phibole displays Cr
2
O
3
contents of 0.16-0.29 wt.%, well
TEM of amphibolesin a metamorphosed ultramafic rock 297
Table 1. Selected microprobe annalyses (wt.%) and structural formulae of coexisting Ca-amphibole, anthophyllite and chlorite.
Ca-amphibole Anthophyllite Chlorite
SiO
2
44.45 43.68 43.78 55.88 55.28 54.38 SiO
2
29.21 30.16
TiO
2
0.59 0.47 0.60 0.02 0.04 0.11 TiO
2
0.10 0.04
Al
2
O
3
12.52 12.94 13.07 0.46 0.94 2.22 Al
2
O
3
20.47 19.05
Fe
2
O
3
4.29 4.51 4.36 1.93 2.28 3.53 Fe
2
O
3
0.35 0.49
Cr
2
O
3
0.29 0.07 0.09 0.08 0.04 0.11 Cr
2
O
3
n.d. n.d.
MgO 16.75 16.10 15.98 24.46 23.76 23.14 MgO 28.97 29.88
CaO 11.35 11.72 11.75 0.68 1.11 2.73 CaO 0.00 0.01
MnO 0.08 0.13 0.17 0.35 0.43 0.32 MnO 0.06 0.01
FeO 4.36 5.16 5.31 13.34 13.15 10.94 FeO 8.25 7.88
Na
2
O 2.94 3.07 2.95 0.14 0.21 0.53 Na
2
O 0.04 0.02
K
2
O 0.14 0.14 0.17 0.03 0.00 0.01 K
2
O 0.00 0.02
Total 97.76 97.97 98.23 97.36 97.24 98.00 Total 87.46 87.55
cations, O = 23 cations, O = 28
Si 6.354 6.273 6.271 7.853 7.796 7.613 Si 5.633 5.800
Al 1.646 1.727 1.729 0.075 0.157 0.366 Al 2.367 2.200
Cr 0.000 0.000 0.000 0.009 0.005 0.012 8.000 8.000
Fe
3+
0.000 0.000 0.000 0.062 0.042 0.010 Al 2.285 2.118
8.000 8.000 8.000 8.000 8.000 8.000 Fe
3+
0.051 0.071
Al 0.464 0.463 0.478 0.000 0.000 0.000 Ti 0.015 0.006
Cr 0.033 0.008 0.011 0.000 0.000 0.000 Fe
2+
1.340 1.270
Fe
3+
0.462 0.487 0.470 0.141 0.200 0.362 Mn 0.010 0.000
Ti 0.063 0.051 0.064 0.002 0.004 0.012 Mg 8.330 8.560
Mg 3.568 3.446 3.411 4.856 4.796 4.626 Na 0.015 0.006
Fe
2+
0.410 0.545 0.565 0.000 0.000 0.000 K 0.000 0.005
Mn 0.000 0.000 0.000 0.000 0.000 0.000 12.039 12.045
5.000 5.000 5.000 5.000 5.000 5.000 Total 20.039 20.045
Mg 0.000 0.000 0.000 0.268 0.199 0.202
Fe
2+
0.111 0.076 0.071 1.568 1.551 1.280
Mn 0.009 0.016 0.020 0.041 0.051 0.038
Ca 1.739 1.803 1.803 0.103 0.167 0.409
Na 0.140 0.106 0.105 0.020 0.031 0.071
2.000 2.000 2.000 2.000 2.000 2.000
Na 0.675 0.748 0.715 0.017 0.026 0.072
K 0.025 0.025 0.031 0.004 0.000 0.002
0.700 0.773 0.746 0.021 0.026 0.073
Total 15.700 15.773 15.746 15.021 15.026 15.073
Fig. 2. Amphibole compositions in sample 1382.36 m from micro-
probe analyses; classification after Leake
et al
. (1997). (a) Fe-Mg
amphiboles. (b) Ca-amphiboles with (Na+K)
A
> 0.5 p.f.u. and Ti
< 0.5 p.f.u..
above the detection limit, whereas the other area yielded
lower values of around 0.1 wt.%.
As compared to coexisting Ca-amphibole, anthophyllite
shows distinctly lower octahedral (Al+Fe
3+
+Cr+Ti) values
of 0.02-0.16 (Fig. 3a), whereas the (Fe
2+
+Mn)/
(Fe
2+
+Mn+Mg) ratios (0.21-0.29 p.f.u) are higher (Fig. 3b),
which is commonly the case in anthophyllite-Ca-amphibole
assemblages (e.g. Robinson et al., 1982) . In contrast to the
Ca-amphiboles analyzed, anthophyllite did not reveal any
titanium and chromium contents above the detection limit.
Chlorites in the investigated ultramafics are clinochlores
to Mg-pycnochlorites with Al
[4]
/(Al
[4]
+Si) ratios of 0.25-
0.31 and Fe
2+
/(Fe
2+
+Mg) ratios covering a wide range be-
tween 0.12 and 0.29 (Matthes et al., 1995). The clinochlores
of sample 1382.36 are at the Mg-rich side of this range with
Fe
2+
/(Fe
2+
+Mg) ratios at 0.12-0.14, i.e. similar to those of
the coexisting Ca-amphiboles, the Al
[4]
/(Al
[4]
+Si) ratios be-
ing at 0.27-0.30 (Matthes et al., 1995, Fig. 9). The clinoch-
lore investigated did not reveal K
2
O and Na
2
O contents
above the detection limit.
Rare alteration products of biotite appear as interlayers of
chlorite (clinochlore to Mg-pycnochlorite) and a sheet sili-
cate, optically resembling biotite. However, EMP analyses
of the latter yielded compositions intermediate between bio-
tite and chlorite, rather than proper biotite. For sample
1410.80, high but extremely variable alkali contents of
298 W.F. Müller, E. Schmädicke, M. Okrusch, U. Schüssler
Fig. 3. Compositions o f coexisting amphiboles
in sample 1382.36 m f rom microprobe analy-
ses. (a) (Ca+Na) in M4
vs
. [6] coordinated
(Al+Fe
3+
+Cr+Ti). (b) (Ca+Na) in M4
vs
.
(Fe
2+
+Mn) /(Mg+Fe
2+
+Mn).
Fig. 4. Lamellar intergrowth of anthophyllite(A) and cummingtonite
(C) parallel to (100).Orientationof the amph ibolesis [010]. The grain
boundary of anthophyllite bulges into cummingtonite (arrow). TEM
bright field image. This and the following imag es and diffraction pat-
terns (Fig. 4-10) are taken from an area about 15 µm in diametershow-
ing an intergrowth in the sequence talc, anthophyllite, anthophyllite/
cummingtonite,anthophyllite,Ca-amphibole.The electrondiffraction
pattern taken from the area of Fig. 4 is shown in Fig. 9.
Fig. 5. Lamellar intergrowthof anthophyllite(A) and cummingtoni-
te (C) parallel to (100). Same grain as in Fig. 4. H RTEM image dis-
playing (100) lattice fringes of anthophyllite (d
100
18.6 Å) and
(200) lattice fringes of cummingtonite (d
200
4.6 Å) by which they
can be easily distinguished.
0.77-4.73 wt.% K
2
O and 0.07-0.44 wt.% Na
2
O, with Fe
2+
/
(Fe
2+
+ Mg) ratios of 0.21-0.26 and Al
[4]
/(Al
[4]
+Si) ratios of
0.16-0.33 were recorded by Matthes et al. (1995, Fig. 9, Table
5),who assumed that these sheet silicatesrepresent in fact sub-
microscopic intergrowths of biotite and chlorite. By contrast,
Eggleton & Banfield (1985) described alteration products of
biotite, which mainly consist of chlorites with 0.08-0.17 wt.%
K
2
Oand 0.22-0.25 wt.%Na
2
O, intergrown with relicsof prop-
er biotite with 9.49-9.66 wt.% K
2
O and 0.09-0.16 wt.% Na
2
O.
TEM observation s and interpretation
In agreement with the petrographic observations, am phi-
boles, chlorite and talc are by far the most frequent minerals
encountered by TEM in the sample investigated. Rarely ob-
served were biotite, pentlandite, magnetite and calcite. The
dominant amphibole recognized in the two TEM specimens
studied is anthophyllite.
TEM of amphibolesin a metamorphosed ultramafic rock 299
Fig. 6. Lamellar intergrowthof cummingtonite(C) and anthophyllite
(A). TEM bright field image. In contrast to the other cummingtonite
lamellae, the electrondiffractionpattern from the cummingtonitela-
mella C
P
contains reflections of the type h + k = odd (see Fig. 10).
The lamella C
P
is also distinguished from the other by its disloca-
tions (arrows).
Fig. 7. Lamellar intergrowth of anthophyllite(A) and Ca-amphibole
(Cam) parallelto (100). Same area as in Fig. 4. TEM bright field im-
age.
Fig. 8. Lamellar intergrowth of anthophyllite (A), cummingtonite
(C) and talc (Tc) parallel to (100). Same grain and orientation as in
Fig. 4. TEM bright field image.
Lamellar intergrowths of anthophyllite,
cummingtonite, Ca-amphibole and talc
Lamellar intergrowths of anthophyllite and Ca-amphibole
parallel to (100) as well as anthophyllite and talc, with
(100)
Ath
//(001)
Tlc
, have been observed in the TEM. Very
spectacular was an area of about 15 µm in diameter display-
ing a lamellar intergrowth in the sequence talc
®
antho-
phyllite
®
anthophyllite/cummingtonite intercalations
®
anthophyllite
®
Ca-amphibole. TEM-images and electron
diffraction patterns of this area are shown in Fig. 4-10. The
orientation of the amphiboles is [010]. The amphiboles dis-
play a strict crystallographic lattice relationship by mutually
sharing a* and b*; the composition planes are (100). The
planes of intergrowth between anthophyllite and talc are
(100)
Ath
and (001)
Tlc
, i.e. (100)
Ath
//(001)
Tlc
.
As can be seen from the TEM micrographs, the antho-
phyllite and cummingtonite lamellae are mostly smaller
than 0.5 µm. The grain boundary of anthophyllite bulging
into cum mingtonite indicates that replac ement of cumming-
tonite is under progress (Fig. 4). According to the electron
diffraction patterns, the cum mingtonite lamellae have al-
most always the space group C2/m and not P2
1
/m (see be-
low). Anthophyllite and cummingtonite lamellae can be
easily distinguished by imaging of the (h00) lattice fringes:
their distances d
100
of anthophyllite are about 18.6 Å, the
d
200
of cummingtonite about 4.65 Å (Fig. 5). Anthophyllite
lamellae attain a width of only a fe w tenths of micrometers;
cummingtonite lamellae are rarely wider. TEM images at
300 W.F. Müller, E. Schmädicke, M. Okrusch, U. Schüssler
Fig. 9.
a
*-
c
* electron diffraction pattern of the lamellar intergrowth
anthophyllite-cummingtonite shown in Fig. 4.
Fig. 10.
a
*-
c
* electron diffraction pattern of the cummingtonite la-
mella C
P
shown in Fig. 6. Note the sharp but very faint reflectionsof
type h + k = odd (arrows). See text for discussion.
higher magnification of the anthophyllite/cummingtonite
intergrowth area show that perfect stacking sequences along
a* are rare (Fig. 5). Chain arrangement faults //(100) with
1.5 x (d
100
18.6 Å) have been observed.
Fig. 11. Anthophyllite with platelets of exsolved gedrite. The orien-
tation of anthophylliteis near [001]. The grain containsa chain mul-
tiplicity fault ( CMF) // (010) which serves as site for heterogeneous
nucleation of gedrite platelets. Left and right of the CMF, there is a
precipitation-free zone (PFZ) which is followed by an ar ea with nu-
merous g edrite p latelets probably due to homogeneous nucleation.
The main composition planes of the platelets are near {110}. TEM
bright field image.
In one case, a cummingtonite lamella of about 1 µm in
width showed very sharp, but very weak reflections of the
type h + k odd. These reflections suggest a primitive Bravais
lattice and the space group P2
1
/m. However, high resolution
lattice images of this lamella did not reveal any periodicity
corresponding to d
100
of 9.3 Å. It is possible that the reflec-
tions suggesting a primitive Bravais lattice are an artefact
due to chain multiplicity faults parallel to (010) which
would lead to reciprocal lattice spikes parallel to b*, i.e., re-
flections from the first order Laue zone could be present in
the diffraction pattern. These would be reflections of the
type h1l (with h + k odd) appearing exactly at the positions
where the weak reflections show up. The EDX analyses did
not reveal chemical differences between the possible P- and
the C-cummingtonite.
Exsolution of gedrite from anthophyllite
Although the anthophyllites from sample 1382.36 m are op-
tically homogeneous, most of them show exsolution phe-
nomena in the submicroscopic range as revealed by TEM.
Examples are given in Fig. 11-15. The electron diffraction
TEM of amphibolesin a metamorphosed ultramafic rock 301
Fig. 12. Anthophyllitew ith plateletsof exsolvedgedrite. Same g rain
as in Fig. 4. In the present micrograph,the true shape of the platelets
is blurred by the strain contrast around them. However, the CMF (ar-
row) and the p recipitation-free zone a re very apparent. TEM bright
field micrograph.
patterns of the anthophyllites with exsolution never dis-
played reflections indicating a different space group or a
significantly different unit cell. Theref ore, we assume that
the exsolved phase is gedrite. Since anthophyllite and gedri-
te have the same space group and very similar unit cell pa-
rameters, a separation of their individual diffraction spots
becomes apparent only at higher Bragg angles (Fig. 14b).
The gedrite lamellae were too sm all for an EDX analysis
with the instrument available. In general, anthophyllite
grains with gedrite exsolution revealed significantly higher
Al contents than those without precipitates.
Dimensions and orientations of the gedrite exsolution ex-
hibit a considerable variability. The precipitates have platy
or lamellar morphology. If the orthoamphibole crystals are
orientated //[001] or close to this direction, two symmetri-
cally equivalent orientations of the lamellar or platy precipi-
tates can be seen (Fig. 11-14), a feature already described by
Smelik & Veblen (1993). Since the anthophyllites usually
contain CMFs //(010), the orientation of the composition in-
terfaces of the exsolution features is readily determined.
The platelets are frequently between 30 and 160 nm in
length and 10-15 nm in width (Fig. 11-14), but larger gedrite
lamellae attaining a l ength of about 1 µm and a width of 40-
60 nm occur, too. In Fig. 11, the angle between (010) and the
Fig. 13. High resolution TEM image of gedrite platelets in antho-
phyllite. As shown by the continuous(010) and (210) lattice fringes
across anthophyllite (A) and gedrite (G), the precipitates are f ully
coherent with their host. Or ientation [001].
long composition interfaces of the platelets is at about 40-
43°. This means that they are close to {110} with an angle of
43.9° with (010). In another grain, the lamellae formed an
angle of about 35° with (010), i.e. they are close to {230}
(32.6°). I n Fig. 14a, the orientation of the lamellae with an
angle of about 25° with (010) is almost exactly at {120}
(25.6°). Since orientations of gedrite lam ellae conform to
optimal phase boundaries which are controlled by differ-
ences in lattice dimensions (cf. Robinson et al., 1977; Sme-
lik & Veblen, 1993), an ideal crystallographic orientation
cannot be expected.
Fig. 11 and 12 display an excellent example for heteroge-
neous nucleation on a CMF (cf. Smelik & Veblen, 1993),
along which the platelets appear to be lined up. To both sides
of the CMF, an exsolution-free zone about 0.2 µm in width
is recognized, outside of which the exsolution frequency is
distinctly increased. In these areas, we assume that homo-
geneous nucleation occurred since there is no indication for
heterogeneous nucleation at a crystal defect, and modulated
structures characteristic for spinodal decomposition are
missing. (Modulated structures are no proof for spinodal de-
composition, but their missing makes the action of this
mechanism very unlikely.) The textural evidence clearly
shows that homogeneous nucleation was later than the het-
302 W.F. Müller, E. Schmädicke, M. Okrusch, U. Schüssler
Fig. 14. (a) Gedrite lamellae with compositionplanes close to {120}. In addition,CMFs (arrows) parallelto (010) are seen. TEM bright field
image. (b) Electrondiffractionpattern
b
*-[401]* from the area shown in Fig. 14a.Note the pair of we akerreflectionsdue to the gedrite lamel-
lae around the reflections of the anthophyllitehost.
erogeneous one; otherwise, an exsolution-free zone would
not have formed. On the other hand, the platelet orientations
of both types of exsolution differ but slightly: the platelets of
the homogeneous nucleation form somewhat wider angles
with (010) than those at the CMF. Presumably, there was no
large temperature difference between the onset of both
mechanisms and the growth of the gedrite platelets.
As shown in Fig. 13 by lattice imaging, the gedrite plate-
lets are fully coherent with the anthophyllite host. No dis-
locations were obse rved along the interfaces. H owever,
Fig. 12 shows considerable strain-field contrast around the
gedrite platelets. Especially in the thicker areas of the TEM
foil, the outlines of the gedrite platelets are blurred by the
strain contrast; therefore, their true or ientation at the CMF
is hardly visible. In general, the strain contrast at the CMF
and in other areas is most pronounc ed _ (010). Since antho-
phyllite and gedrite reveal the la rgest differenc es in their
lattice constants b, intergrowth surfaces different from
(010) are selected in order to minimize the free surface en-
ergy.
Obviously, the CMFs did not always serve as a nucleation
site for the gedrite lamellae, as shown in Fig. 14a. On the
other hand, there are exsolution-free areas around the
CMFs. As a possible explanation, alumina enrichment at the
CMFs might be assumed. Another interesting observation
are contrast effects between the coarser lamellae of Fig. 14a
(indicated by arrows) which point to incipient exsolution.
Exsolution lamellae of cummingtonite in Ca-amphibole
In two different TEM-specimens two Ca-amphibole grains
were found which contain exsolution lamellae of cumming-
tonite //(100) (Fig. 15a), a feature frequently described in
the literature (e.g. Zingg, 1996; Deer et al., 1997). The
widths of the lamellae shown in Fig.15a vary between 18
and 75 nm, their distances between about 110 and 280 nm,
on average about 180 nm. The corresponding electron dif-
fraction pattern indicates a strict lattice orientation relation-
ship with both amphibole phases sharing a* and b* (Fig.
15b). The splitting of the reflections of the type h0l and h00
seen in the pattern is due to their different angles (
Cum
>
Cam
) and different interplanar spacings d
h00
(d
100 Cum
<
d
100 Cam
). The amphibole grain under consideration is situat-
ed close to, and has the same orientation as, the anthophyl-
lite-cummingtonite-Ca-amphibole intergrowths described
above.
Oriented intergrowth of anthophyllite and chlorite
A textbook-like example of an oriented intergrowth be-
tween anthophyllite and chlorite is shown in Fig. 16a. Ac-
cording to the corresponding diffraction pattern, the orienta-
tion re lationship is a*
Ath
// c*
Chl
and b*
Ath
// b*
Chl
. This is the
orientation relationship to be expected for an intergrowth of
TEM of amphibolesin a metamorphosed ultramafic rock 303
Fig. 15. Exsolutionlamellae on (100) of cummingtonite(C) in Ca-amphibole (Cam). (a) TEM bright field image. The orientationof the grain
is [010]. (b)
a
*-
c
* electron diffraction pattern showing that
a*
Cam
is parallel to
a*
C
.
Fig. 16. Intergrowth of anthophyllite(A) and chlorite (Chl) with the orientationrelationship
a
*
Ath
//
c
*
Chl
and
b
*
Ath
//
b
*
Chl
. (a) TEM bright
field image. ( b) Electron diffraction pattern.
304 W.F. Müller, E. Schmädicke, M. Okrusch, U. Schüssler
amphiboles and sheet silicates, as illustrated by the well-
known schematic drawing for the structural relation be-
tween pyroxene, mica and amphibole (Thompson, 1970,
1978, 19 81) and shown by Veblen & Buseck (1980) and
Veblen (1980), e.g. for anthophyllite and talc. However,
as far as we know, it is the first time that this oriented in-
tergrowth is documented for anthophyllite and chlorite.
The electron diffraction pattern of chlorite (Fig. 16b)
shows nearly continuous streaks parallel to c* at the posi-
tions of reflections of type 0kl with k = ± 2, ±4, ± 8, ± 10,
etc., i.e. for reflections k = even which cannot be divided
by 3. This indicates largely rand om stacking of the pseu-
do-hexagonal chlorite unit cells parallel to (001), rotated
by about 60° around c*. It should be mentioned that b
Chl
is
a pseudo-orthohexagonal axis, see e.g. Nespolo et al.
(1997) f or crystallographic setting of sheet silicates, and
Eggleton & Banfield (1985) for stacking disorder in chlo-
rite.
The chlorite investigated contains about 2.2 atom-% of K
indicating that it was form ed by alteration of biotite (Mat-
thes et al., 1995). However, in contrast to, e.g., the observa-
tions of Eggleton & Banfield (1985) the electron diffraction
pattern of this chlorite does not contain any reflections
which could be assigned to biotite. Therefore, the presump-
tion by Ma tthes et al. (1995) that these K-rich chlorites are
in fact submicroscopic intergrowths between chlorite and
biotite does not hold in this case. It is interesting to note that
a breakdown product of biotite, still containing consider-
ably high amounts of potassium, is structurally proper chlo-
rite.
Crystal defects
In addition to exsolution lamellae and their corresponding
internal interfaces in anthophyllite and Ca-amphibole, the
anthophyllite grains typically display chain multiplicity
faults (CMFs) on (010) and chain arrangement faults
(CAFs) on (100). Some Ca-amphibole grains are deformed
and contain numerous dislocations which are often concen-
trated in bands or cell walls. By contrast, dislocations in an-
thophyllite are very rare. In chlorite, kink bands were ob-
served. Therefore, the TEM observations correspond to the
light-optical characteristics of these minerals.
Conclusions and discussion
The most important results of our TEM investigations
are:
1. For the first time, a complex lamellar intergrowth of
anthophyllite, cummingtonite, Ca-amphibole and talc was
detected by TEM methods. The spatial sequence of phases
in a range of 10 to 15 µm, perpendicular to the lamellae, is
talc
®
anthophyllite
®
anthophyllite/ cummingtonite inter-
calations
®
anthophyllite
®
Ca-amphibole. All the amphi-
boles have the directions b* and a* in common; anthophyl-
lite and the monoclinic amphiboles are intergrown along
(100), anthophyllite and talc with (100)
Ath
//(001)
Tlc
. Cum-
mingtonite was only recorded in lamellae of up to 1 µm
width. In general, cumm ingtonite displays the space group
C2/m. Only one lamella was detected which may have the
space group P2
1
/m.
2. Two types of planar faults were recorded in the Mg-Fe
amphiboles investigated with TEM: anthophyllites display
chain multiplicity faults (CMFs) // (010), anthophyllite and
cummingtonite chain arrangement faults (CAFs) //(100).
3. Anthophyllites relatively rich in Al exsolve platelets or
lamellae of gedrite with composition planes of the type {hk0}.
Orientations close to {230} and {110} have been recorded, so
far not described in the literature. In the same anthophyllite
grain, homogeneous and heterogeneous nucleation has been
observed; the latter one occurred on CMFs //(010).
4. Ca-amphibole displays lamellar exsolution of cum-
mingtonite //(100). Some grains show numerous disloca-
tions. In one case, a cellular structure was observed.
5. Intergrowths of anthophyllite and chlorite with the ori-
entation relationship a*
Ath
// c*
Chl
and b*
Ath
// b*
Chl
have
been recorded.
6. Alteration produc ts of biotite, although still containing
considerable K-contents, are metrically chlorite.
Since the microstructures observed in the ultramafic rock
are the result of the petrological conditions, we will try to
connect our TEM observations with the metamorphic and
deformational history. As indicated by rare eclogitic relics,
the amphibolite-metagabbro succession, penetrated by the
KTB pilot hole, experienced a high-pressure stage (Röhr et
al., 1990; Schalkwijk, 1991; Schalkwijk & Stöckhert, 1992;
O’Brien et al., 1992) with maximum P-T conditions of
about 15 kbar/750°C (O’Brien et al., 1992). For a metagab-
bro sample recovered at a depth of 1268 m, not far from the
ultramafic rock investigated, these authors calculated about
13 kbar and 580-650°C. On uplift and cooling, the succes-
sion underwent a partial overprint leading to assemblages of
the high-P granulite facies at about 670-72C and 10-12
kbar. Subsequently, a pervasive amphibolite-facies meta-
morphism took place at still high pressures of 8-11.5 kbar
and a poorly defined temperature range of 575-675°C
(O’Brien et al., 1992).
In the ultramafic rocks associated with the metagabbros,
the assemblage Ca-amphibole-anthophyllite(-cumming-
tonite)-chlorite-talc was presumably formed during the am-
phibolite-facies event (Matthes et al., 1995). No mineral rel-
ics unambiguously testifying to the eclogite and granulite
stages have been observed in the ultramafics. Textural evi-
dence recorded by the optical microscope and the TEM sug-
gests a stable coexistence of Ca-amphibole, anthophyllite,
chlorite and talc. This is corroborated by a petrogenetic grid,
calculated for the system CaO-MgO-FeO-Al
2
O
3
-SiO
2
-H
2
O
(CMFASH), in which this par agenesis covers a wide stabili-
ty field (Schmädicke & Okrusch, 1997, Fig. 5). A pseudo-
section in the CMFASH system, calculated for the average
bulk-rock composition closely conforming to that of the
sample investigated displays a distinctly reduced stability
field of this assemblage. This, in combination with conven-
tional geothermometry, leads to peak metamorphic condi-
tions of 630 50°C and 10 1 kbar (Schmädicke &
Okrusch, 1997, Fig. 6), corresponding to the high-P am-
phibolite facies and well in accord with the estimates of
O’Brien et al. (1992).
TEM of amphibolesin a metamorphosed ultramafic rock 305
The stable coexistence of Ca-amphibole, anthophyllite,
chlorite and talc, observed in petrographic thin section and
supported by the calculated phase diagram, was confimed
by TEM on a submicroscopic scale. The interfaces encoun-
tered testify to textural equilibrium. Cummingtonite was de-
tected only in lamellar intergrowths with anthophyllite, but
never in mutual contact with Ca-amphibole, talc and chlo-
rite. This fact, together with a bulging grain boundary of an-
thophyllite into cummingtonite indicates that cummingtoni-
te is not part of this stable assemblage. However, it may have
been a stable or metastable precursor phase to anthophyllite,
formed at higher temperatures of the (?) granulite-facies
stage. Obviously, anthophyllite with X
Fe
0.25 is stable with
respect to cummingtonite at about 630°C/10 kbar. So far,
experimental results on the stability fields of cummingtoni-
te
ss
and anthophyllite
ss
in the system (Mg,Fe)
7
[(OH)
2
/
Si
8
O
22
] were only achieved by non- reversed experiments of
Schürmann (1967) and Hinrichsen (1967), respectively,
broadly conforming to our results. The crystallographic
continuity between the intergrown anthophyllite and cum-
mingtonite points to a topotactic nucleation. In analogy with
pyroxene, this inversion is generally favoured by higher
cooling rates and/or lower temperatures compared to mas-
sive tr ansformations, where no crystallographic continuity
exists (e.g. Putnis, 1992). O ur results seem to suggest that C-
centered cummingtonite was transformed directly into the
stable anthophyllite. Alternatively, a two-step inversion C2/
c cummingtonite
®
P2
1
/n cummingtonite
®
anthophyllite
may have been preferred for kinetic reasons, again ana-
logous to pyroxene, i.e. high-pigeonite
®
low-pigeonite
(
®
orthopyroxene).
Judging from the empirical results of Spear (1980) and
Smelik & Veblen (1993), exsolution of gedrite from antho-
phyllite occurred on cooling below 500-55C. The large
angle of the gedrite platelets with (010) indicate a large dif-
ference in the lattice parameters of anthophyllite and un-
mixed gedrite. This may indicate that the exsolution temper-
atures were lower than the 460-52C estimated by Smelik
& Veblen (1993). Moreover, the difference in orientation
between the heterogeneously and, later on, homogeneously
nucleated platelets suggests an extended exsolution process
over a larger temperature interval.
Acknowledgments: WFM is most grateful for his few per-
sonal meetings with Luciano Ungaretti and will remember
him as a very kind and noble-minded colleague and friend.
Thanks are due to Hartmut Fuess for granting access to the
CM20-UT transmission electron microscope at the Fachbe-
reich Material- und Geowissenschaften, Fachgebiet Struk-
turforschung, Technische Universität Darmstadt, and to
Gerhard Miehe for his help and advice with this instrument.
We thank the technicians Thomas Dirsch, Josef Kolb and
Gerlinde Seifert at TU Damstadt and Klaus-Peter Kelber at
Würzburg University for their competent help. Support by
the Deutsche Forschungsgemeinschaft (grants Mu358/15-2
and Ok2/40-1, -2) is gratefully acknowledged. We thank Gi-
ancarlo Della Ventura, Annibale Mottana and an anony-
mous colleague for careful reviews.
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Received 26 March 2002
Modified version received 27 August 2002
Accepted 13 November 2002
TEM of amphibolesin a metamorphosed ultramafic rock
307
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Reports comprehensive data on mineral assemblages and mineral chemistry for 34 orthoamphibolite gneisses from Orijarvi, Traskbole, and Pernio in southwestern Finland, classic areas first reported on by Eskola 75 years ago. In addition, an analysis of phase relationships in these samples is presented. Most of the protoliths are apparent altered mafic volcanics of Archean age. By far the most common assemblage is quartz+plagioclase+cordierite+anthophyllite+biotite+ilmenite, although five samples were observed containing coexisting anthophyllite and gedrite and several containing almandine-rich garnet. -from Authors