1
Abstract
Interleaved phyllosilicate grains (IPG) of various compositions are
widespread in low-grade Verrucano metasediments of the northern
Apennines (Italy). They are ellipsoidal or barrel-shaped, up to 300400µm long and they are often kinked and folded; phyllosilicate
packets occur as continuous lamellae or as wedge-shaped layers
terminating inside the grain. Using electron microscopy techniques
(SEM, TEM) six types of IPG have been distinguished on the basis of
their
mineralogical
composition:
(1)
Chl+Ms±Kln;
(2)
Chl+Ms+Pg±Kln; (3) Ms+Prl±Pg; (4) Ms+Prl+Su; (5) Ms+Prl+Chl+Su;
(6) Su+Ms. Types (1) and (2) are mainly composed of chlorite, with
Ms and Pg as minor phases; Kln grows on Ms in highly weathered
samples. Types (3), (4), (5), and (6) are composed of muscovite, with
intergrown Prl, Chl, Su and new-formed muscovite. IPG show all
kinds of contacts: from coherent grain boundaries with parallel basal
planes and along-layer transitions to low- and high-angle grain
boundaries.
IPG formed on pristine minerals such as chlorite and muscovite.
The transformations took place during the prograde and retrograde
metamorphic path of the rocks: they were facilitated by deformation
and they occurred in equilibrium with a fluid phase, which allowed
cation diffusion. Prograde reactions (Chl=Ms (or Pg); Ms=Prl;
Ms=Chl) involve dehydration and sometimes a decrease in volume,
whereas retrograde reactions (Ms=Kln; Ms=Su) involve hydration
and an increase in volume. These transformations do not simply
occur through an interchange of cations, but often involve deep
structural changes: transitions from one phyllosilicate to another
2
generally proceed through dissolution-recrystallization reactions. In
conclusion, Verrucano IPG represent microstructural sites which
have not completely equilibrated with the whole rock and whose
mineral assemblage depends on the original composition of the
microstructural sites.
3
Introduction
Composite grains of chlorite and muscovite have been frequently
recognized in sedimentary and low-grade metamorphic rocks; they
are commonly called stacks, intergrowths, or aggregates. Since
Sorby (1853) first described them, these grains have been observed
in pelitic and psammitic rocks of different age (Craig et al., 1982; van
der Pluijm and Kaars-Sijpesteijn, 1984 and bibliography therein), and
various hypotheses have been proposed for their origin: 1) stacks
are detrital grains, i.e. clasts which may or may not be modified
during weathering and transport (primary origin) (Beutner; 1978); 2)
stacks originate from the mimetic growth of chlorite (with or without
new-grown muscovite) on a detrital nucleus during diagenetic to
metamorphic stages (primary-secondary origin) (Craig et al., 1982;
Woodland, 1982; 1985; Dimberline, 1986; White et al.,1985;
Milodowski and Zalasiewicz,1991; Li et al., 1994); 3) the aggregates
are
strain-controlled
porphyroblasts
formed
entirely
during
metamorphism (secondary origin) (Weber, 1981). This diversity of
opinions probably reflects the actual variability in the origin of the
aggregates.
This study re-examines the fine-scale interleaved phyllosilicate
grains (IPG from Franceschelli et al., 1991) of Verrucano rocks
(northern Apennines) which constitute a quartz-arenite facies
referred to the beginning of the Triassic rift process (Franceschelli et
al., 1986 and bibliography therein). This group of formations
experienced low-grade metamorphism (Franceschelli et al., 1986;
Giorgetti, 1995) during the Apennine orogeny (Carmignani and
Kligfield, 1990). Franceschelli et al. (1989; 1991) suggest that IPG
4
originated during deformation and metamorphism and formed on a
detrital precursor. The envisaged mechanism supports the idea that
IPG result from an equilibrium process among detrital muscovite, the
matrix mineral assemblage and an internally buffered fluid phase.
The proposed model for IPG formation on detrital muscovite can also
be extended to detrital chlorite.
Our purpose is to clarify the origin of the Verrucano IPG and their
evolution during metamorphism using SEM-EDS and TEM. These
methods allow detailed textural and chemical analyses.
Specimen description and analytical techniques
Samples examined for this study belong to the pyrophyllite+quartz
zone (Franceschelli et al., 1986) and were collected from the
Verrucano formations of the Monticiano-Roccastrada Unit which
crops out along the mid-Tuscan ridge (Fig.1). The metapelites and
quartzites consist of different modal proportions of detrital minerals
and new-formed, metamorphic minerals (tab.1).
Detrital minerals include quartz, muscovite, chlorite, minor
feldspars and rare carbonates. Syn- to post-tectonic metamorphic
minerals (mainly phyllosilicates and chloritoid) crystallize in different
microstructural sites: phyllosilicates such as Ms, Chl, Prl, Su
(sudoite), Kln (mineral symbols from Kretz, 1983) constitute the rock
matrix and grow as fine-grained (<10 µm) crystals underlying the
main schistosity, or replace detrital muscovite or chlorite, forming
finely intergrown aggregates (IPG; Franceschelli et al., 1991). IPG
are ellipsoidal or barrel-shaped, up to 300-400 µm in length. They
5
often maintain characteristics (such as microfolds, kinks, and
pressure-solution effects at their borders) which suggest a detrital
origin; furthermore, IPG are more abundant in coarse-grained
samples (quartzites) than in fine-grained ones (metapelites).
Polished thin sections were prepared for optical observations,
SEM study using back-scattered electron (BSE) imaging and X-ray
energy dispersive (EDS) analyses. Analyses were performed with an
EDAX 9100/70 attached to a Philips 515 scanning electron
microscope. The operating conditions were: 15 kV, 20 µA emission
current, and a 0.2 µm beam spot size. Natural minerals were used as
standards. In particular, several analyses were performed on an
homogeneous muscovite crystal during the entire working period of
the instrument.
IPG for TEM observations were selected from polished sections
using a petrographic microscope; the sections were glued onto Cu
grids with a single central hole of 200-400 µm in diameter and
thinned by argon ion mill (Gatan Dual Ion Milling 600 at Granada
University). Two ion-milling conditions were used: 1) 6 kV, 1A, and
15° incident angle while perforating; 2) 6 kV, low-angle (12°) and low
current (0.4A) final milling for ~4 hrs to clean the sample surface.
Samples were analysed with two transmission electron microscopes:
a Philips CM20, operating at 200 kV, with a LaB 6 filament, and a
point to point resolution of 2.7 Å (Granada University), and a Philips
EM 400T, operating at 120 kV, with a W filament, and a point to point
resolution of ~4 Å (Siena University).
6
Diffraction patterns were obtained from selected areas (SAED);
high-resolution images (HRTEM) were obtained following the
procedures suggested by Buseck et al. (1988) and Buseck (1992).
Both microscopes were equipped with a X-ray energy dispersive
(EDS) EDAX DX4 which allowed semi-quantitative analyses of areas
with a minimum size of 200 Å.
Optical, SEM, and TEM results
IPG (up to 300-400 µm in length) are always coarser than the
matrix phyllosilicates, detrital quartz and feldspars. Most IPG are
randomly oriented with respect to the bedding and the main
schistosity and are sometimes strongly deformed. IPG are composed
of different phyllosilicate packets from one to tens of microns thick;
although the different phyllosilicates can sometimes be identified
optically, they are more easily identified in BSE images. Most
packets are intergrown parallel or subparallel to (001); some have a
lenticular shape or grow as wedges, which terminate inside the
stacks, forming semi-coherent boundaries with the neighboring
phyllosilicates. New minerals in the stacks grow preferentially at
grain boundaries, in extensional directions and along microfold
hinges. The IPG are often weathered; they are coated by reddish
hydroxides and Ti-Fe oxides grow along stack borders or between
the phyllosilicate lamellae (Fig. 2).
SEM-EDS analyses allowed the identification of 6 types of IPG
with different phyllosilicate assemblages (tab.1).
7
Types (1) and (2) are composed of chlorite packets, which are
optically continuous and separated by mica lamellae; types (3), (4),
(5), and (6) show complex textures, in which continuous muscovite
packets are intersected and split by other phyllosilicates with variable
orientations.
Type (1) and type (2) IPG
Type 1 stacks (M62) belong to highly weathered quartzites
containing limonitic haloes along veins and fractures and euhedral
crystals
of
siderite,
which
are
almost
totally
replaced
by
calcite+dolomite+hematite. The IPG are composed of weathered
chlorite with parallel or subparallel intergrown packets of muscovite
and lamellae or wedges of kaolinite; hematite can grow at the IPG
borders. Type 2 IPG (M669) are made up of chlorite (60 to 70%),
muscovite, and minor paragonite. Micas preferentially grow at the
stack borders or in packets with (001) parallel or subparallel to the
chlorite basal plane. Type 1 and 2 IPG are always embedded in a
matrix comprising the same phyllosilicate assemblage (tab.1).
Both IPG types show similar characteristics at the TEM scale, and
they are summarized in table 1.
Chl is the most abundant phase and occurs in packets several
hundred Å thick (Fig.3a). Electron diffraction patterns show a onelayer polytype with sharp 00l reflections; rows with k≠3n sometimes
have weak satellites indicating a 56Å spacing. A 4-layer polytype
may therefore be present together with the dominant one-layer
polytype
(Fig.3b). Lattice fringe
images show packets
undeformed and defect-free 14 Å layers (Fig.3a).
with
8
Muscovite occurs as a two-layer polytype either as isolated layers,
interstratified with chlorite (Fig.3a), or as packets of variable
thickness (Fig.4a). Muscovite in M62 IPG occurs as defect-free
packets, whereas in M669 IPG lattice fringe images show packets
with a typical "mottled structure" (Fig.5a), and defects such as layer
splitting or termination can be observed. The textural relationship
between chlorite and mica is analogous in the two types of IPG. Chl
and mica are generally coherently intergrown with parallel (001)
planes. A zone of disordered, interstratified chlorite and mica packets
is shown in figure 4a; rarely, low- or high-angle grain boundaries can
occur. The lateral transformation from chlorite layers to muscovite (or
paragonite) layers is evident in figure 3a. AEM analyses of these
mixed-layer packets fall on line between the two end-member mica
and chlorite (Fig.4b). The presence of smectite layers, which would
result in a higher Si content (as described by Nieto et al., 1994), can
therefore be ruled out.
Lenses ~50 Å thick without resolved basal planes are present
within the muscovite packets: they are probably paragonite, as
electron diffraction patterns display split pairs of 00l reflections with
10.0 Å and 9.6 Å lattice spacing corresponding to Ms and Pg (Fig.
5b). In addition, AEM analyses indicate the presence of muscovite
and paragonite. Paragonite can also occur in muscovite-free packets
or with subordinate potassium mica. Lattice fringe images are difficult
to obtain because paragonite is easily damaged by the electron
beam.
Muscovite and paragonite are parallel (Fig.5b) intergrown or form
low-angle grain boundaries. Neither compositionally intermediate
9
sodium potassium mica nor basal reflections with an intermediate
periodicity are recorded. These features suggest the presence of two
single phases rather than an intermediate phase, as described by
Jiang and Peacor (1993).
In M62 IPG, kaolinite is present as a one-layer polytype and the
presence of dickite or nacrite (two-layer polytypes) can be ruled out.
SAED patterns show different degrees of stacking order: reflections
may be sharp or weakly streaked along c  (Fig. 6a). Kaolinite occur
in packets of 7 Å-layers intercalated with muscovite packets (Fig.6b).
Muscovite-kaolinite have either parallel basal planes or slightly
different orientation (Fig. 6a). The lateral transition from 10 Å-Ms
layers to 7 Å-Kln layers occurs without the formation of an
intermediate phase (Fig.6c). Furthermore, relics of 10 Å-layers can
be observed inside kaolinite packets, causing deformation contrast of
7 Å-layers (Fig.6b).
Type (3), type (4), type (5) , and type (6) IPG
Type 3 IPG (M7) belong to quartzites with a muscovite,
pyrophyllite, ±sudoite, and paragonite (recognized only at the TEM
scale) matrix. The IPG generally contain more than 60% muscovite;
pyrophyllite grows in packets or wedges inside the Ms crystal
whereas paragonite is present in small (<10 µm) lenticular inclusions.
Type 4 IPG (M29) are rare and consist of intimately intergrown
Ms+Prl+Su with parallel or subparallel basal planes. The matrix
contains only Ms+Prl and rare Pg.
Type 5 (M8) is the most complex type of IPG. These stacks, which
may be embedded in a Ms+Prl or Ms+Prl+Su matrix, are
10
characterized by an extremely variable relative abundance of the four
phyllosilicates. Chlorite is generally present in IPG as weathered,
discontinuous packets surrounded by pyrophyllite and sudoite.
Textural relationships between chlorite and muscovite are variable;
hematite is present at the border of or inside the IPG. Type (3) and
(4) IPG consist of stacks formed on muscovite. Type (5) IPG might
actually have formed on an older muscovite or chlorite.
Type 6 IPG are mainly composed of sudoite and only occur in the
M39 quartzite where they coexist with a Ms+Su+Prl matrix.
The similar features and relationships among the intergrown
phases observed in the different IPG belonging to types (3), (4), and
(5) allow for a summary description (tab.1).
Muscovite is characterized by two distinct microstructures: i)
lattice fringe images show that Ms is present either as several
hundred Ångstrom thick, defect-free packets with straight lattice
fringes or as ii) small discrete packets with slightly different
orientations along c  (Fig.7a-b). Packets with bent, split or
interrupted basal planes are characterized by a higher dislocation
density. Chemical analyses in different muscovite packets of a single
IPG reveal variable compositions; given that the analized areas are
structurally homogeneous, we can exclude that this variability is due
to contamination by other phases.
Pyrophyllite occurs both as the 2M polytype (Fig.8a) and as the 1T
polytype. Prl packets are several hundred Ångstrom thick, often
undeformed
and
defect-free
(Fig.8b).
Pyrophyllite
is
easily
distinguished from muscovite for its different contrast, lack of
"mottled structure", and its lattice fringe spacing (9.2 Å Prl; 10 Å Ms).
11
Sudoite electron diffraction patterns show both a one-layer and a
two-layer polytype (IPG type (4), (5), and (6)). Both the relative
intensity of 00l reflections in SAED and the chemical analyses allow
sudoite to be distinguished from trioctahedral chlorite. Sudoite forms
packets can be parallel intergrown with Ms (Fig.9a-b) or wedgeshaped and terminating inside a muscovite packet (Fig.10).
A one layer polytype of chlorite occurs as a minor phase in type
(5) IPG. In some SAED patterns, satellite reflections are visible along
c  , both in 00l row and in rows with k≠3n. These satellite reflections
indicate an ordered stacking sequence: long-period polytypes with 4or 6-layer periodicity can be recognized.
The presence of paragonite in type (3) IPG (M7) was determined
through microanalysis: electron diffraction patterns of Ms with high
Na content revealed the presence of a phase with 9.6 Å-spaced
basal planes parallel intergrown with 10 Å-spaced basal planes.
Muscovite
forms
coherent
boundaries,
low-angle
grain
boundaries, or incoherent boundaries with the other phyllosilicates
both .
The lattice fringe image in figure 11 shows a deformed packet of
Ms: at the microfold hinge, new-formed muscovite grows with a
different orientation.
Mineral chemistry
Representative chemical analyses are reported in tables 2 and 3.
The chlorite composition in the IPG (tab.2) varies with the host
rock composition and the mineral assemblage of the IPG (Giorgetti,
12
1995). The X Mg (=Mg/Mg+Fe) is linearly correlated with the rock
X Mg and ranges from 0.49 (M62) to 0.58 (M33); total Al varies from
5.50 (M62) to 6.24 (M8): Al-rich chlorite coexists with Prl and Cld,
whereas Al-poor chlorite is associated with Ms (and Pg) only. Chl
has a constant composition within each individual sample, both in
IPG and in the matrix, no matter what the microstructural sites in
which it occurs.
The X Mg of sudoite (tab.2) is independent of the rock X Mg .and
varies from 0.75 to 0.82, with a mean of 0.79. As observed for
chlorite, the Al content varies according to the mineral assemblage:
Al-rich sudoite coexists with Prl and Cld; Al-poor sudoite is also
associated with Chl.
Unlike chlorite, muscovite has an extremely variable composition:
in a single sample, different muscovite compositions can be found in
both the matrix and individual IPG (tab.3a and b). There are
differences between matrix Ms and IPG Ms: the compositional range
in matrix muscovite is smaller than that in IPG muscovite and, in
almost all samples the interlayer occupancy is higher in IPG
muscovite.
Paragonite and pyrophyllite have an almost ideal composition.
Transformation mechanisms
The textural relationships described in the previous paragraph are
consistent with dissolution and crystallization mechanisms, and no
indication of exsolution or spinodal decomposition have been
13
observed. Pristine phyllosilicates give rise to new ones as direct
replacement products, without the formation of intermediate phases.
Although more than one mechanism can operate during
replacement, reactions among phyllosilicates have been modelled
after Veblen and Ferry (1983) considering the observed "along-layer"
transitions.
Chlorite  mica. The observed lateral transitions from one 14 Ålayer to one 10 Å-layer or from one 14 Å-layer to two 10 Å-layer are
solid-state reactions accompanied by significant chemical changes.
In the former, for example, we observe the loss of a brucite-like layer


which is replaced by interlayer cations (K , Na ) and the reordening
of the octahedral cations from a trioctahedral arrangement to a
dioctahedral one. The latter mechanism implies a volume increase
with a gain of tetrahedral-like layers and interlayer cations. The
possible structural relationship has been schematically illustrated by
Veblen and Ferry (1983). The reaction has been modelled for the
former mechanism considering chemical compositions observed in
IPG M62:


Mg 4.5 Fe 4.8 Al 2.7 (Si 5.3 Al 2.7 )O 20 (OH) 16 +1.7K +0.1Na +
0.9Si
4

+12H 
2
K 1.7 Na 0.1 (Mg 0.2 Fe 0.2 Al 3.7 )(Si 6.2 Al 1.8 )O 20 (OH) 4 +4.3Mg
4.6Fe
2
+
+12H 2 O
(1)
This transformation entails a loss of divalent cations (Mg
Mn
2
) and of H 2 O, and a gain of K


(or Na ) and Si
2
4
, Fe
2
,
. As an
14
influx of H  ions is needed, the reaction is favoured by an acidic
environment. Furthermore, the presence of hematite inside and at
the boundaries of IPG -a feature observed in all IPG types- indicates
that iron oxides precipitated at high f O 2 conditions. Reaction (1)
occurs at constant Al contents; in this particular case, it seems that
Al is relatively immobile in the metamorphic environment (Veblen and
Ferry, 1983).
It is likely that the Chl  Pg transformation depends on Na

activity in the metamorphic fluid. This reaction, as all the ionic
reactions discussed in due course, must occur in equilibrium with a
fluid phase which allows the transport of ions.
Muscovite  Paragonite. Muscovite and paragonite always form
discrete phases and a metastable precursor, such as sodiumpotassium mica (Jiang and Peacor, 1993), has never been observed.
HR images have never highlighted the direct transition from
muscovite to paragonite because the volume change is so small that
it can remain undetected. Nevertheless, a nucleation and growth
mechanism can be invoked for paragonite formation. A spinodal
decomposition mechanism can be ruled out as there are no zones
with a modulate texture or composition. The Ms  Pg reaction not

only involves the Na  K

substitution, but also a chemical change
in the dioctahedral layer. As muscovite is far more phengitic than
paragonite, there must also be loss of Mg
2
and Fe
2
and changes
in the Si/Al ratio.
Muscovite  Pyrophyllite. The lateral transition from one 10 Ålayer to one 9.2 Å-layer results in a decrease in volume, as in the
case of the Chl transformation. The substitution of Ms by Prl gives
15
partial interlayer (see Fig. 8b) (Page, 1980), and slightly differing
lattice constants result in strain contrast along the interfaces (Fig.8b).
Reaction (2) has been modelled according to the chemical
composition of type 4 IPG (M29):
K 1.7 (Mg 0.2 Fe 0.1 Al 3.8 )(Si 6.3 Al 1.7 )O 20 (OH) 4 + 1.7Si 4  
 Al 4 Si 8 O 20 (OH) 4
+1.7K  +0.2Mg 2  +0.1Fe 2  +1.5Al 3
(2)
The Al and K produced in reaction (2) are incorporated in a new,
low celadonitic muscovite.
Muscovite  Sudoite. The tansformation between Ms and Su
may occur either through a topotactic or a dissolution-crystallization
mechanism. Textural evidence indicates that sudoite forms after
muscovite: if a 1:1 transition occurs, the dehydration reaction
involves the substitution of interlayer K

for a trioctahedral layer with
a consequent increase in volume. The reaction modelled for sample
M8 is:
K 1.8 (Mg 0.2 Fe 0.1 Al 3.7 )(Si 6.2 Al 1.8 )O 20 (OH) 4 +12H 2 O+3Mg
+0.7Fe

2
+2.2Al
3
2

(Mg 3.2 Fe 0.8 Al 5.9 )(Si 6.2 Al 1.8 )O 20 (OH) 16

+1.8K +12H

(3)
In these types of IPG there is probably also a Chl  Su
transformation in which a talc-like layer of the former is substituted by
a pyrophyllite-like layer of the latter.
Muscovite  Kaolinite. At the Ms-Kln boundary, two types of
layer transition can occur:
16
1 Ms (10 Å)  2 Kln (14 Å)
2 Ms (20 Å)  3 Kln (21 Å)
Although both mechanisms involve an increase in volume, the
latter is the most likely. Volume change in the second reaction is
minor, in agreement with the lack of strain contrast at the Ms-Kln
boundary.
Considering the structure of the two phyllosilicates, this reaction
can occur via a dissolution-recrystallization mechanism. Structurally,
it implies the addition of gibbsite-like sheets and a reversal of the
orientation of some tetrahedral sheets of the T-O-T units (Ahn and
Peacor, 1987).
2 (K 1.8 (Mg 0.2 Fe 0.2 Al 3.7 )(Si 6.2 Al 1.8 )O 20 (OH) 4 )+
+6H 2 O+4H  +Al 3 
3
(Al 4 Si 4 O 20 (OH) 8

)+3.6K +0.4Mg
2
+0.4Fe
2
+0.4Si
4
(4)
Reaction (4), modelled for IPG M62, produces K
cations while Si
4

, H , H 2 O, and Al
3

and bivalent
are consumed. It is favoured
by an acidic environment; the high chemical activity of H

ions could
explain the direct transition from Ms to Kln without the formation of
mixed layer illite/smectite observed by other authors (Meunier and
Velde, 1979; Beaufort and Meunier, 1983; Banfield and Eggleton,
1990).
Origin of interleaved phyllosilicate grains
17
Optical and SEM-BSE observations suggest a detrital origin for
IPG. Size distribution, morphology, and textural relationships
between stacks and cleavage suggest that the IPG originated from
pristine grains that were present prior to the development of
cleavage. Furthermore, deformation textures such as kinking,
bending and cleavage which crosscuts stacks can be interpreted as
the result of "weathering" through a combination of metamorphic and
tectonic events (Li et al., 1994). The difference in mineralogy
between some IPG and the matrix of corresponding samples is
further evidence of the detrital origin of the IPG. In addition, we have
observed that, unlike chlorite, muscovite has not equilibrated at the
sample scale and preserves different detritic compositions. The
stacks behave like a microsystem: although the IPG exchange
matter with the surrounding matrix, neither textural nor chemical
equilibrium is achieved, reflecting a sluggish diffusion rate at low
temperature, which determines the small size of the equilibrated
domains (Li et al., 1994).
TEM data
confirms the proposed model and allow the
characterization of reaction mechanisms involving phyllosilicates.
Topotactic growth is facilitated along the basal planes, but some
transformations occur through dissolution-crystallization reactions.
Prograde reactions (as Chl  Micas, Ms  Prl, Ms  Pg) lead to a
decrease in the volume of solids and, generally, to the production of
H 2 O; on the contrary, retrograde reactions (Ms  Kln, Ms  Su)
involve an increase in volume and the consumption of water.
Transformations are facilitated by deformation: microfolding and
shortening of coarse-grained crystals cause dissolution of detrital
18
phases; new phyllosilicates precipitate in extension sites (splitting of
basal planes and hinge regions) originating an assemblage in
equilibrium with the new P-T conditions. This process can be clearly
observed in figure 11, where a muscovite packet is deformed by a
microfold. In this highly deformed region, starting from the fold hinge,
new formed muscovite and pyrophyllite crystallize during prograde
metamorphism; sudoite topotactically grows on muscovite during the
retrograde phase.
SEM and TEM images have never revealed the presence of relict
phases other than muscovite or chlorite. This feature suggests that: i)
either chlorite crystals are detritic or they derive from the complete
weathering of an undetected precursor during diagenesis or before
the metamorphic peak of the rocks; ii) muscovite crystals represent
detrital grains or grains which equilibrated at P-T conditions different
from those characterizing peak metamorphism; this interpretation is
also confirmed by the different compositional ranges of matrix
muscovite and muscovite in the IPG of a single sample. There is no
evidence of older precursors, which are often observed by other
authors. Many have described chlorite-mica stacks which form on
detrital biotite of igneous origin (Dimberline, 1986; White et al.,
1985): the shape of stacks sometimes resembles that of amphiboles
or pyroxenes, probably derived from volcanic detritus (Milodowski
and Zalasiewicz, 1991; Roberts and Merriman, 1990). The chemical
and textural characteristics of Verrucano IPG and the lack of
expandable layers (corrensite, smectite) strongly indicate that stacks
developed
during
prograde
and
retrograde
paths by direct
replacement of pre-existing minerals whose origin remains uncertain.
19
Deformation and fluid circulation triggered the growth of new-formed
(i.e. metamorphic) minerals and the new mineral assemblage
depends on the chemical composition of the microstructural site.
Acknowledgements
The authors are grateful to M. M. Abad Ortega from the Scientific
Instrument Center of the University of Granada for his help with
HRTEM work. Financial support was supplied by the Italian Ministry
of University and Scientific and Technological Research grants
(MURST to I.M.) and Research Projects nº PB92-0961 and PB920960 of the Spanish Ministry of Education as well as Research
Group 4065 of the Junta de Andalucia.
The authors also thank B. Grobéty and M. Mellini for their
constructive reviews of the paper.
20
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1
sample
IPG
other matrix minerals
Ms-Kln-Chl
matrix
(<2µm)
Ms-Kln-Chl
Type 1
(M62)
Type 2
(M669)
Chl-Ms-Pg
Chl-Ms-Pg
Qtz+Rt
Type 3
(M7)
Ms-Prl
(+Pg)
Ms-Prl-Su
(+Pg)
Qtz+Cld+
Hem+Rt+Ilm
Chl, defect-free and
well crystallized (1, 4layer); hundreds Å
thick
Ms, defect-free (2M);
hundreds Å thick
Type 4
(M29)
Ms-Prl-Su
Ms-Prl (+Pg)
Qtz+Cld+
Hem+Rt
Ms, defect-free (2M);
hundreds Å thick
Type 5
(M33)
Ms-Prl-Chl-Su
Ms-Prl-Su
Qtz+Cld+
Hem+Mag+Rt
Ms, defect-free (2M);
hundreds Å thick
Type 5
(M8)
Type 6
(M39)
Ms-Prl-Chl-Su
Ms-Prl
Ms-Su
Ms-Su-Prl
Qtz+Cld+
Hem+Rt
Qtz+Hem+Rt+
Ilm
Ms, defect-free (2M);
hundreds Å thick
Qtz+Cal+Dol
+Hem+Rt
TEM data: main
mineral in IPG
Chl, defect-free and
well crystallized (1, 4l.); hundreds Å thick
TEM data:
integrown mineral
Ms (2M); isolated l. or
hundreds Å thick
-lateral transtitions with Chl,
l.a.g.b. with Chl
Ms (2M); isolated l. or
hundreds Å thick
-lateral transtitions with Chl,
l.a.g.b. with Chl
Ms, highly defective (2M),
tens Å thick; -l.a.g.b. with the
defect-free Ms
Su (1, 2-l.); tens, hundreds Å
thick;
-c.g.b. or h.a.g.b. with Ms
Su: as in type 4 IPG
Kln (1T); tens, hundreds Å thick;
-lateral transtitions with Ms, and l.a.g.b. with Ms
Pg (2M); isolated l. or hundreds Å thick
-c.g.b. with Ms, l.a.g.b. with Chl
Prl (1T, 2M), hundreds Å thick;
-lateral transtitions with Ms, l.a.g.b. with Ms
Pg (2M), tens Å thick
-c.g.b. with Ms
Prl: as in type 3 IPG
Prl: as in type 3 and 4 IPG
Chl (1, ,6-l.); hundreds Å thick;
-lateral transition and l.a.g.b.
with Ms
Su (1-l.); tens, hundreds Å
thick;
-c.g.b., l.a.g.b., and h.a.g.b.
with Ms
tab.1. Mineral assemblages of IPG and associated matrices of the studied samples. Minerals in brackets are extremely rare.
Mineral symbols are from Kretz (1983); Su = sudoite. l. = layer; in brackets: polytype (es.: 1 l.= 1 layer polytype; 2M = 2M
plytype); thickness refers to mineral packet thickness. l.a.g.b. = low angle grain boundary; h.a.g.b. = high angle grain
boundary; c.g.b. = coherent grain boundary; i.g.b. = incoherent grain boundary.
1
Tab.2
Chl
M62
24.97
21.94
26.17
14.21
Chl
M669
25.63
23.55
24.68
13.33
87.29
87.19
Si
AlIV
AlVI
Fe
Mg
Mn
5.31
2.69
2.81
4.65
4.50
XMg
0.49
SiO2
Al2O3
FeO
MgO
MnO
total
Chl
M33
25.58
25.54
19.10
15.42
0.27
86.01
Chl M8 Su
M29
26.75 37.14
26.33 35.29
19.58 4.60
14.91 11.08
0.46
88.03 88.11
Su
M33
33.66
35.27
6.31
11.89
Su
M39
33.25
36.01
5.24
12.18
87.13
86.68
5.38
2.62
3.22
4.33
4.17
5.28
2.72
3.48
3.29
4.74
0.05
5.38
2.62
3.62
3.29
4.47
0.08
6.63
1.37
6.05
0.69
2.95
6.17
1.83
5.79
0.97
3.25
6.10
1.90
5.88
0.80
3.33
0.49
0.58
0.57
0.81
0.77
0.81
tab.2. Representative chemical analyses of chlorite and sudoite in the
studied IPG. Structural formulae are calculated on the basis of 28 oxygens;
all Fe as FeO. XMg= Mg/(Mg+Fe).
Tab.3
SiO2
TiO2
Al2O3
FeO
MgO
CaO
Na2O
K2O
total
Si
AlIV
AlVI
Ti
Fe
Mg
Ca
Na
K
M62
47.42
0.96
36.13
1.56
0.85
M62
48.80
0.40
33.11
2.88
1.36
M669
46.64
0.67
35.82
1.77
0.65
M669
48.75
M7
48.18
0.16
35.11
1.37
0.78
M29
47.08
0.51
37.35
0.44
0.45
M29
47.75
32.67
2.67
1.58
M7
47.14
0.16
36.47
1.42
0.49
0.57
10.24 10.68
97.73 97.23
0.43
10.07
96.05
0.60
9.57
95.84
1.11
8.78
95.57
0.49
9.39
95.48
1.54
8.64
96.01
0.76
9.22
95.96
6.15
1.85
3.67
0.09
0.17
0.16
6.40
1.60
3.51
0.04
0.32
0.26
6.15
1.85
3.72
0.07
0.19
0.13
6.45
1.56
3.54
6.33
1.67
3.77
0.02
0.15
0.15
6.13
1.87
3.87
0.05
0.05
0.09
6.31
1.70
3.70
0.29
0.31
6.19
1.81
3.84
0.02
0.16
0.10
0.14
1.69
1.79
0.11
1.69
0.15
1.61
0.28
1.47
0.12
1.57
0.39
1.43
0.19
1.61
36.28
1.41
0.54
0.29
0.11
M33
46.45
0.64
36.00
1.03
0.52
0.11
1.22
8.77
94.74
M33
50.30
6.16
1.84
3.78
0.06
0.11
0.10
0.02
0.31
1.48
6.59
1.41
3.60
32.39
1.89
1.54
0.14
0.29
9.18
95.73
0.21
0.30
0.02
0.07
1.53
M8
46.09
0.15
37.27
1.21
0.47
0.11
1.27
8.42
94.99
M8
47.84
0.53
33.41
2.27
1.66
M39
47.07
0.49
37.04
0.69
0.79
M39
48.66
0.59
32.97
2.21
1.69
0.39 1.41
10.32 8.79
96.42 96.28
0.27
10.57
96.96
6.08
1.92
3.88
0.01
0.13
0.09
0.02
0.32
1.42
6.31
1.69
3.51
0.05
0.25
0.33
6.13
1.87
3.81
0.05
0.07
0.15
6.38
1.62
3.48
0.06
0.24
0.33
0.10
1.74
0.36
1.46
0.07
1.77
tab.3. Representative chemical analyses of muscovite in the studied IPG: for
each sample, muscovites with the maximum and with the minimum
2
celadonitic substitution are reported. Structural formulae are calculated on
the basis of 22 oxygens; all Fe as FeO.
FIGURE CAPTIONS
Fig.1. Sketch map of Verrucano outcrops in the northern Apennines. Studied
samples are from Monticiano-Roccastrada and M. Leoni outcrops.
3
Fig.2. Back-scattered electron image showing an IPG (sample M8)
composed of Ms-Prl-Chl-Su with some hematite crystals along the rim; the
grain is strongly altered.
Fig.3. Representative chlorite from type (1) IPG (sample M62). a: lattice
image of Chl showing 14Å-layers with coherently intergrown single 10Å-
layer (indicated by the arrow); b: corresponding SAED pattern.
Superstructure reflections are visible in the rows with k≠3n; probably L4 type
sequence according to Jullien et al. (1996) may be recognized; c: transition
from one 10Å-layer to two 14Å-layer (enlargement of the area indiacted by
the arrow in a).
4
Fig.4. a: image of intergrown chlorite and mica packets in type (2) IPG
(sample M669); b: AEM analyses of chlorite and muscovite (type 2 IPG)
plotted in the Si-Al-Mg+Fe diagram (based on cation proportions); the mixedlayer compositions lie on the line between mica and chlorite.
5
Fig.5. a: lattice image of muscovite (10Å and 20Å periodicities are visible)
with the tipical "mottled structure" (sample M669); b: SAED pattern of parallel
intergrown Pg (9.6Å) and Ms (10Å) (2-layer polytypes); splitting of 00l
reflections is evident.
6
Fig.6. Textural relationships between Kln and Ms (sample M62). a: SAED
pattern showing 7Å-reflections of two differently oriented Kln crystals; one of
them has [001] parallel with that of a Ms crystal; b: corresponding lattice
image of a several hundred Å-thick packet of Kln, parallel intergrown with a
muscovite packet; relicts of 10Å-layers are indicated (arrow). c:
enlargement of the circled area in b showing the lateral transition from 7Ålayers to 10Å-layers (thin arrow); thick arrow indicate a relict of a 10Å-layer
inside the Kln packet.
7
Fig.7. a: lattice image of muscovite packets showing low-angle grain
boundaries (arrow; type 4 IPG, sample M29); b: corresponding SAED pattern
showing at least three Ms crystals with different orientations; splitting of 00l
reflections indicates the presence of discrete Pg packets.
Fig.8. a: image of pyrophyllite in type (3) IPG (sample M7) showing 9.2Ålayers with relicts of 10Å-layers (arrows); strain contrast is visible in relation
to layer terminations. b: SAED pattern of Prl (two-layer polytype) parallel
intergrown with muscovite (10Å reflections along the 00l row).
8
Fig.9. a: lattice image of type 6 IPG showing alternating Su (14Å periodicities
are visible) and Ms packets; these packets are generally parallel intergrown
with coherent contacts: on the right side of the image a sudoite packet abuts
inside a muscovite packet. b: SAED pattern of parallel intergrown sudoite
and muscovite.
9
Fig.10. Image of a wedge-shaped packet of sudoite (14Å-layer) terminating
inside a muscovite packet (type 4 IPG): basal plane of muscovite are bent at
the wedge termination (sample M29).
Fig.11. TEM image of a microfold in a Ms packet (sample M29): from the fold
hinge new muscovite grows, a low-angle grain boundary between Ms and Prl
is also visible (arrow) in the corresponding SAED pattern which shows two
Ms and one Prl crystals with slightly different orientation.