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O R I G I N A L P A P E R
The mechanism of cation and oxygen isotope exchange in alkalifeldspars under hydrothermal conditions
Dominik R. D. Niedermeier Andrew Putnis Thorsten Geisler Ute Golla-Schindler Christine V. Putnis
Received: 18 January 2008 / Accepted: 4 June 2008 / Published online: 21 June 2008 Springer-Verlag 2008
Abstract The mechanism of re-equilibration of albite in
a hydrothermal fluid has been investigated experimentallyusing natural albite crystals in an aqueous KCl solution
enriched in 18O at 600C and 2 kbars pressure. The reac-
tion is pseudomorphic and produces a rim of K-feldspar
with a sharp interface on a nanoscale which moves into the
parent albite with increasing reaction time. Transmission
electron microscopy (TEM) diffraction contrast and X-ray
powder diffraction (XRD) show that the K-feldspar has a
very high defect concentration and a disordered Al, Si
distribution, compared to the parent albite. Raman spec-
troscopy shows a frequency shift of the Si-O-Si bending
vibration from*476 cm-1 in K-feldspar formed in normal16O aqueous solution to *457 cm-1 in the K-feldspar
formed in 18O-enriched solution, reflecting a mass-related
frequency shift due to a high enrichment of18O in the K-
feldspar silicate framework. Raman mapping of the spatial
distribution of the frequency shift, and hence 18O content,
compared with major element distribution maps, show a
1:1 correspondence between the reaction rim formed by the
replacement of albite by K-feldspar, and the oxygen iso-
tope re-equilibration. The textural and chemical
characteristics as well as the kinetics of the replacement of
albite by K-feldspar are consistent with an interface-cou-
pled dissolution-reprecipitation mechanism.
Keywords Feldspar replacement Ion-exchange mechanism Interface-coupled dissolution-reprecipitation
Hydrothermal Fluid-mineral interaction Oxygen isotope exchange
Introduction
Understanding the redistribution of elements during
hydrothermal mineral replacement reactions is fundamen-
tal for many disciplines in geosciences, e.g. metamorphic
petrology, economic geology and geochronology. Reac-
tions induced by hydrothermal fluids are responsible for the
chemical and mineralogical differentiation of large parts of
the earths crust. In natural environments, fluid-rock
interaction results in changes of the bulk rock composition,
mineral assemblage and the isotopic composition. How-
ever, the exchange mechanisms of these large scale
geological processes are not yet fully understood. This
deficit is largely a result of the complexity of natural fluids
and the problem of reconstructing the physicochemical
conditions during re-equilibration reactions in natural
environments.
Natural feldspar minerals often show complex re-
equilibration or alteration textures such as lobate zones of
different composition and high porosity (e.g. turbid feld-
spars) that penetrate into primary feldspar (Putnis et al.
2007). Numerous oxygen isotope analyses have further
shown that many feldspars were partly re-equilibrated in
the presence of a fluid phase (e.g. Cole et al. 2004;
Elsenheimer and Valley 1993, Fiebig and Hoefs 2002). The
Communicated by J. Hoefs.
D. R. D. Niedermeier A. Putnis T. Geisler U. Golla-Schindler C. V. PutnisInstitut fur Mineralogie, Westfalische Wilhelms-Universitat,
Corrensstrasse 24, 48149 Munster, Germany
Present Address:
D. R. D. Niedermeier (&)
Institut fur Petrologie der Ozeankruste, Universitat Bremen,
Klagenfurterstrasse 2, 28359 Bremen, Germany
e-mail: [email protected]
123
Contrib Mineral Petrol (2009) 157:6576
DOI 10.1007/s00410-008-0320-2
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high content of alkali chloride solutions observed in many
fluid inclusions (Poty et al. 1974; Roedder 1972) has led to
the suggestion that metamorphic and igneous rocks have
crystallised in the presence of volatile-rich fluids, enriched
in Na and K (Orville 1962). Especially in areas of mag-
matic activity, where the transport of heat is partially
controlled by hot fluids. The impact of supercritical,
chloride-rich fluids is obvious. Hence, exchange reactionsbetween albite (NaAlSi3O8) or potassium feldspar (KAl-
Si3O8) and K- or Na-rich chloride solutions, respectively,
are important buffering reactions in feldspar-bearing rocks.
The hydrothermal re-equilibration of feldspar in aqueous
chloride solutions has thus been a subject of numerous
experimental studies (e.g. Hovis 1997; Labotka et al. 2004;
ONeill and Taylor 1967; Orville 1963; Schliestedt and
Matthews 1987; Weise and Schliestedt 1988). Based on
hydrothermal experiments made at 1 kbar and 350800C
ONeill and Taylor (1967) already suspected that the
exchange of cations and oxygen between alkali feldspars
and alkali chloride solutions proceeds via a reaction frontthat moves through the parent feldspar crystal. Later,
Labotka et al. (2004) found a strong spatial correlation
between the exchange of the cations and oxygen atoms
during their 18O tracer experiments at 2 kbar, 600C by
using modern micro-analytical tools. This observation is
evidence for a complete reorganization of the feldspar
framework and can be explained by a process that is based
on the spatial and temporal coupling of congruent albite
dissolution followed by immediate K-feldspar precipita-
tion, resulting in an enrichment of Na in the fluid. It has
been suggested that many re-equilibration processes,
whose mechanism had been described in terms of volume
diffusion in the solid state, might be attributable to such a
coupled dissolution-reprecipitation process (Putnis 2002;
Putnis and Putnis 2007).
On the other hand, dry exchange by homogenization of
molten salt and solid feldspar[800C can be explained by
solid state diffusion processes in which cations are
exchanged between the molten chloride and the solid
feldspar (e.g. Christophe-Michel-Levy 1967; Laves 1951).
During cation exchange in feldspars, the Al and Si atoms
do not necessarily need to be rearranged, because the cat-
ions are incorporated in large sites within the framework.
At high temperatures such diffusional ion exchange can
proceed at significant rates so that it can be observed on
laboratory time scales.
In a series of experiments, low albite has been treated
with an aqueous solution of KCl. The KCl solution was
prepared using H2O enriched with 95%18O as a tracer.
Confocal micro-Raman spectroscopy was used to investi-
gate the oxygen isotopic variation within experimentally
produced K-feldspar reaction rims. The heavier 18O iso-
tope, substituting into the feldspar structure for16O, lowers
the vibrational frequencies of the tetrahedral Si(Al)-O
framework. This principle was used to (1) determine
whether oxygen isotope exchange accompanies cation
exchange and (2) to study potential variations of the oxy-
gen isotope composition across the product K-feldspar
(see, e.g. Geisler et al. 2005; Putnis et al. 2007). Scanning
electron microscopy (SEM), electron probe micro-analysis
(EPMA) and transmission electron microscopy (TEM)were used to analyse the structural and chemical interface
between pristine Na-feldspar and converted K-feldspar on a
micro- and nanometer scale.
Experimental methods
The starting material for the experiments was a natural
sample of a low albite from Amelia County, Virginia
(USA). This pegmatitic albite is nearly 100% Na feldspar
and fully Al, Si ordered (Salisbury et al. 1987). Two
different grain sizes of feldspar material were prepared forexperimental runs. First, single crystals of feldspar, with
targeted grain size of 22.5 mm, were cleaved or broken
from a large crystal. After cleaving, the crystals were
cleaned with deionised water and dried in an oven at
120C. Second, a powder was produced by crushing the
material in an agate mortar under ethanol. The powder
was washed in deionised water and wet sieved to remove
grains finer than 100 lm and coarser than 200 lm. The
procedure of grinding, washing, and sieving was repeated
several times until enough material for several experi-
ments had been produced. The preparation methods of the
starting material correspond to those used by Labotka
et al. (2004).
Hydrothermal experiments were carried out at 600C
and 2 kbar for run durations between 3 and 14 days. The
albite was reacted with two molar aqueous KCl solution.
For the hydrothermal experiments, the sample was filled
into a gold capsule of 2.8 mm inner diameter and*25 mm
length. The gold capsules were filled with about 15 mg of
feldspar powder (grain size 100200 lm) and 30 mg of
fluid, yielding a fluid to solid ratio of about two. For the
preparation of TEM samples some experiments were con-
ducted with single grains of*2 mm in diameter. The
aqueous solutions for the experiments were prepared with
dry KCl (grade 99.5%) and deionised water. In some
experiments 18O enriched H2O (95%18O) was used for the
preparation of the KCl solution. The capsule was sealed by
carbon-arc welding and then loaded into a horizontally
mounted cold-seal vessel. The pressure-vessel was heated
externally from room temperature to 600 2C within
*1.5 h while water was used as the external pressure
medium to keep the pressure at 2 kbars during the
experiment.
66 Contrib Mineral Petrol (2009) 157:6576
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After the experiments, the gold capsules were opened
and the sample material was washed out with deionised
water. The reacted powder and the reacted single crystals
were washed several times with deionised water and dried
at 120C in an oven. The sample material was then sec-
tioned for analytical measurements. For electron
microscopy and Raman measurements, the powder sample
was embedded into araldite mounts and polished at thesurface. Petrographic thin sections of larger grains were
prepared for optical microscopy and electron microprobe
analyses. The specimens were carbon coated for analyses
by SEM and EPMA. For TEM, thin sections were prepared
with acetone-soluble adhesive and first were analysed by
light microscopy and SEM to identify areas of interest for
TEM specimen preparation. Copper discs with a slit of
500 lm width and 2 mm in length were glued with epoxy
resin on these areas, cut out with a Medenbach drill, and
removed from the thin section using acetone. The specimen
was ion-beam thinned by a Gatan DuoMill Ion Thinner
until it was electron transparent.
Analytical techniques
Powder X-ray diffraction analyses of the starting material
and the reacted products were performed with a Philips
XPert PW 3040 automated diffractometer. The samples
were finely ground and packed into standard sample
holders of 2 mm depth to minimize the effect of preferred
orientation. The operating conditions were 45 kV accele-
ration voltage at 40 nA tube current. The samples were
X-rayed using monochromatic Cu-Ka1 radiation
(c = 1.540598 A). For a good comparison, all samples
were analyzed under identical conditions with a step size of
0.02 (2h) within a range between 5 and 70 (2h), where
the most informative reflections for the feldspar structure
occur. The 2-theta position was calibrated using silicon as
an external standard. The counting time was set to 25 s per
step.
Samples of starting material and reacted phases were
analyzed with a JEOL 840 SEM. Element distribution
images were made with the integrated Oxford INCA EDX-
system. A JEOL JSM 6300F field emission scanning
electron microscope (FE-SEM) was used for observations
of the sample surfaces. The images were captured using the
analySIS (Soft Imaging System) software. For comparison
of the sample, grains of the untreated and of the reacted
feldspar were mounted together onto aluminium stubs with
Leit-C tabs and subsequently carbon coated.
Electron probe micro-analyses (EPMA) were performed
using a JEOL JXA 8600 MX Superprobe at 15 kV
acceleration voltage and 15 nA beam current. Altogether
nine elements were measured, using natural mineral
standards of diopside for Ca and Si, rutile for Ti, micro-
cline for K, jadeite for a Na, rhodonite for Mn, fayalite for
Fe, kyanite for Al and forsterite for Mg. Matrix effects
were corrected using the ZAF correction procedure. A
JEOL 8900 JXE Superprobe was used for mapping the
distribution of Na, Al, Si, and K in some reaction products.
Transmission electron microscope (TEM) investigations
of starting feldspar and reacted sample were carried outwith a JEOL 3010 TEM that operated at 297 kV. The
thinned samples were mounted in a double tilt specimen
holder. The TEM is equipped with a Gatan post column
energy filter, and an Oxford EDX-system, which enables
compositional probing of small areas of the sample. The
crystallographic orientation of the samples was determined
from diffraction patterns. Bright field images and diffrac-
tion images were taken on photo negative films.
A confocal LabRam Raman system was used for a series
of single Raman measurements and Raman mapping. A
HeNe (632.187 nm) laser was used for excitation. The
scattered light was collected in 180 backscatteringgeometry and dispersed by a grating of 1,800 grooves/mm
after having passed a 100 lm entrance slit. A 1009
objective with a numerical aperture of 0.9 and a confocal
hole of 400 lm was used for all measurements, yielding an
axial (depth) resolution of\3 lm while the lateral reso-
lution was better than 1.5 lm. The acquisition time for a
single measurement and for the mapping procedure was 60
and 24 s, respectively. All measurements were made at
room temperature.
Results
After experimental treatment, the cleaved feldspar grains
did not change in size, but the surfaces changed to a milky-
white colour and were fairly friable. The grains that were
optically transparent before the reaction turned translucent
and some grains coalesced during the reaction to form
clusters of numerous single crystals.
Secondary electron (SE) images of experimentally
treated albite display high porosity at the grain surface
(Fig. 1) whereas there was no porosity in the starting
material. Some areas protrude from the mineral surfaces,
while at the same time; some pits penetrate deep into the
host crystal. The surface topography has increased on the
reacted surface in comparison to the pristine albite surface,
which only shows smooth cleavage planes (Fig. 1df).
Substantial change can be observed at the cleavage sur-
faces, where the change in surface topography is the most
intensive. Obviously different crystallographic layers have
been exposed during the reaction. At lower magnification
these features appear as typical dissolution features of a
mineral surface. At high magnification the surface of the
Contrib Mineral Petrol (2009) 157:6576 67
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product K-feldspar (Fig. 1c) is characterised by fine texture
apparently made up from small clusters, quite unlike the
original albite surface at the same magnification. These
observations suggest new crystal growth is associated with
the reaction. Feldspar grains that have been experimentally
treated for longer periods show flat surfaces. In contrast to
the rough surface in Fig. 1, there is only a minor surface
roughness visible in Fig. 2. The crystal planes tend to form
an idiomorphic feldspar crystal habit.
In backscattered electron (BSE) images of sectioned
grains a dark grey rim and a brighter core, representing the
reacted rim and the pristine albite, respectively, can be
distinguished (Fig. 3). This difference in image contrast
indicates an abrupt change in the chemistry at the boundary
between the two phases. This boundary or interface is sharp
on the micrometer scale and is accompanied by numerous
pores (Fig. 3). The thickness of the K-rich feldspar rim
varies conspicuously, e.g. from 10 to 100 lm in a 7-day
Fig. 1 ac SEM images of the
surface of a reacted albite grain
from a 5-day experiment in
comparison with (df) the
surface of an untreated grain.
The images show the cleavage
surface (001). Note that the
starting material shows no
evidence for porosity, whereas
the reacted material shows a
rough surface topology
Fig. 2 SEM photomicrograph of treated feldspar after long-term
experiment conducted for 13 days. Obvious is a difference between
areas of high and low growth rates. In the upper right part of the
picture, plane surface areas have evolved, whereas on the lower left
side, coarser surfaces are prevailing. Oriented growth of surface areas
reflects roughly the habit of an idiomorphic single crystal
68 Contrib Mineral Petrol (2009) 157:6576
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experiment. EDX elemental mapping demonstrates that theconverted rim primarily contains K but also small amounts
of Na (Fig. 4). Relics of pristine albite are irregularly
distributed within the grain (Fig. 4).
Quantitative analyses of the distribution of Si, Ti, Fe,
Ca, Mn, Mg, Na, K and Al were made by electron probe
micro-analysis (EPMA). Due to the spread of the electron
beam, larger sample areas have been probed and do not
record the sharp chemical gradient at the interface. Due to
the loss of Na under the electron beam, higher concentra-
tions of Si and Al appear in the albite-rich areas when
normalised to 100%. A line profile across the interface
between the albite and the product K-feldspar was mea-sured by EPMA (Fig. 5a, b). For each analysis the molar
percentage of albite, orthoclase and anorthite was calcu-
lated (Table 1). At the interface the feldspar composition
abruptly changes from pure albite to pure K-feldspar.
Interestingly, the pure K-feldspar only occurs directly near
the interface in the reacted material (Fig. 5b). Further
behind the interface the product K-feldspar becomes more
Na-rich and approximates to an average composition
Or87Ab12An1.
The minor element Mg concentrations indicate varia-
tions similar to Ca with higher concentrations in albite-
bearing areas. The distribution of Ti, Fe and Mn did not
show any variation between the starting material and theproduct K-feldspar. Electron microprobe element distri-
bution maps give a broad overview of the distribution of
Na and K near the interface (Fig. 5c, d). These chemical
maps confirm the chemical anomaly in the product K-
feldspar near the interface and show that this anomaly
extends parallel to the reaction front. The black area in the
upper parts of Fig. 5c shows the purity of the albite starting
material, where no K could be detected. The sharp chem-
ical gradient at the boundary between the pristine albite
and the K-feldspar is again obvious.
Raman spectroscopy allows estimation of the 18O con-
tent and its spatial distribution inside the K-feldsparproduct that formed in a 18O-enriched solution due to a
mass-related frequency shift of those vibrations that
involve the motion of oxygen atoms. The most prominent
bands in the Raman spectrum of K-feldspar occur near 476
and 513 cm-1 (Fig. 6) and have been assigned to O-T-O
bending and stretching vibrations of the four-membered
tetrahedral rings (Matson et al. 1986; McKeown 2005).
These modes are characteristic for all alkali-feldspars and
their frequency is almost independent of the NaK ratio
(Mernagh 1991). However, the degree of crystallinity has
an influence on the mode frequencies (Matson et al. 1986).
Nevertheless, since we compare the mode frequencies of
K-feldspars that formed under identical conditions, it is
safe to assume that the degree of crystallinity in the K-
feldspar products is the same. Therefore, the observed red-
shift of the frequency of these modes in the K-feldspar
formed in 18O-enriched solution relative to the frequency
Fig. 3 SEM image of a cross section of the interface between pristine
albite in the core and reacted K-feldspar in the rim. Note the
occurrence of numerous pores at the interface
Fig. 4 a BSE image of a
reacted albite grain from a 5-day
experiment. Note the sharp
interface between the dark Na-
rich areas in the core and the
light K-rich areas in the rim.
The area shown in this imagewas chemically mapped by
EDX. b Na (white area) and (c)
K (white area) distribution of a
reacted albite grain from a 5-day
experiment
Contrib Mineral Petrol (2009) 157:6576 69
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observed for 16O-rich K-feldspar (Fig. 6) can directly be
linked to the 18O content in the feldspar lattice. It is evident
from Fig. 6 that the Raman bands of the K-feldspar formed
in the 18O-enriched fluid shift to significantly lower wavenumbers relative to bands of K-feldspar formed in nor-
mal 16O-rich solution. The observed shift Dx, of the
bending mode near 476 cm-1, for instance, is about
-18 cm-1, reflecting a strong enrichment of18O. The band
near 478 cm-1 was chosen for Raman mapping. Interest-
ingly, the Raman map of the frequency shift of this mode in
the K-feldspar formed in the 18O-enriched solution shows
even a higher frequency shift close to the interface (Fig. 7),
which would indicate a higher 18O concentration. An
important observation is that there is no gradient in the
frequency shift that could be related to inward diffusion of18O.
Transmission electron microscopy (TEM) in the bright
field imaging mode revealed a very different diffraction
contrast between the pristine albite and the converted K-
feldspar (Fig. 8a). The parent albite has smooth diffraction
contours, indicating a relatively defect-free crystal struc-
ture, whereas the product K-feldspar shows complex
diffraction contrast and numerous linear features, sug-
gesting a high defect density. EDX spectra from both sides
of the interface were taken to clearly identify the two
feldspar phases. The areas of product K-feldspar display
numerous structural features that cannot be observed in the
albite. Observations of the product K-feldspar show no
macroscopic porosity, but dense dark textures near theboundary to the pristine albite. Some of these structures
have an elongated appearance, tending into direction of the
fluid/solid interface, and might represent voids or channels.
The interface between the two phases is characterised by a
dark zone that may be caused by high density of disloca-
tions (Fig. 8a). This dark area generally appears at the
boundary of the two feldspar phases. Electron diffraction
patterns taken from both sides of the interface reveal that
both feldspar phases are structurally similar (Fig. 8b, d)
and the crystallographic orientation of the K-feldspar rim
coincides within a degree with that of the albite core. Only
a slight displacement can be observed in the diffraction
patterns of the interface area, with splitting between two
equivalent diffraction spots (Fig. 8c), from each feldspar
phase. Some parts of the reaction interface show dense
dislocation arrays suggesting a semicoherent interface,
while in others there appears to be an open porosity.
X-ray powder diffraction patterns of the product K-
feldspar agree very well with powder diffraction patterns of
a natural sanidine (Weitz 1972). The structural state of the
untreated and the product K-feldspar was estimated using
aK
lowhigh count rates
Nad
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35
Distance from edge (m)
Mol%
b
Albite
Orthoclase
cFig. 5 a BSE image of the area
within a reacted feldspar grain
that was chosen for electron
microprobe traverse along the
white line (detailed results in
Table 1) and for electron
microprobe mapping marked by
the white rectangle. b Profiles of
the albite, orthoclase and the
anorthite component across the
interface of the product K-
feldspar and unreacted albite.
Note the decrease of the albite
and anorthite component before
the reaction interface between
10 and 14 lm. Electron
microprobe distribution maps of
c K and d Na near the reaction
front. Note that the interface
between the pristine albite and
the product K-feldspar is sharp
on the micrometer scale. The
exceptionally low Na-values
within the red area are caused
by the line analyses (b), where
Na and K were lost under the
influence of the electron beam
70 Contrib Mineral Petrol (2009) 157:6576
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Table1
Electronmicroprobedata
ofaline-scanacrosstheinterfacebetween
pristinealbiteandtheK-feldsparreaction
product(seeFig.
5)
Electronmicroprobedata
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
31
SiO2
64.6
8
64.8
4
64
.31
64.5
3
64.2
7
64.0
2
64
.13
64.5
0
64.2
6
64.6
7
64.7
7
64.7
5
64.4
6
64.2
8
68.4
7
69.1
6
69.1
9
TiO2
0.0
0
0.0
0
0
.04
0.0
1
0.0
0
0.0
0
0
.00
0.0
4
0.0
0
0.0
0
0.0
4
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
1
Al2O3
18.3
6
18.5
6
18
.27
18.1
3
18.1
9
18.3
6
18
.05
18.3
4
18.3
8
18.4
0
18.0
2
18.0
9
18.2
7
18.4
4
19.1
0
19.1
1
19.4
3
FeO
0.0
1
0.0
3
0
.00
0.0
1
0.0
0
0.0
0
0
.01
0.0
1
0.0
0
0.0
0
0.0
3
0.0
3
0.0
0
0.0
0
0.0
0
0.1
1
0.0
1
MnO
0.0
0
0.0
2
0
.02
0.0
0
0.0
0
0.0
4
0
.00
0.0
1
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
1
MgO
0.0
0
0.0
0
0
.00
0.0
0
0.0
0
0.0
0
0
.00
0.0
0
0.0
0
0.0
0
0.0
0
0.0
2
0.0
0
0.0
0
0.0
1
0.0
1
0.0
1
CaO
0.1
3
0.1
9
0
.17
0.1
4
0.1
3
0.1
4
0
.12
0.0
7
0.0
8
0.1
2
0.0
2
0.0
0
0.0
4
0.1
6
0.2
4
0.2
0
0.2
5
Na2
O
1.4
0
1.3
9
1
.36
1.3
3
1.2
6
1.3
2
1
.12
1.1
8
1.1
2
0.7
9
0.3
7
0.2
2
0.1
4
0.1
9
7.8
2
9.9
4
10.2
2
K2
O
14.5
5
14.7
6
14
.46
14.5
7
14.6
9
14.7
9
14
.79
14.9
8
15
.00
15
.67
16.2
9
16.3
2
16.2
5
16.0
3
2.3
8
0.1
6
0.2
0
Sum
99.1
3
99.8
0
98
.64
98.7
2
98.5
2
98.6
6
98
.22
99.1
3
98.8
5
99.6
4
99.5
4
99.4
2
99.1
5
99.1
1
98.0
3
98.6
8
99.3
1
Normalizedtoeightoxygenatoms
performulaunitfeldspar
Mg
0.0
00
0.0
00
0
.000
0.0
00
0.0
00
0.0
00
0
.000
0.0
00
0.0
00
0.0
00
0.0
00
0.0
01
0.0
00
0.0
00
0.0
01
0.0
01
0.0
00
Ca
0.0
06
0.0
09
0
.008
0.0
07
0.0
06
0.0
07
0
.006
0.0
04
0.0
04
0.0
06
0.0
01
0.0
00
0.0
02
0.0
08
0.0
11
0.0
09
0.0
11
Mn
0.0
00
0.0
01
0
.001
0.0
00
0.0
00
0.0
02
0
.000
0.0
01
0.0
00
0.0
00
0.0
00
0.0
00
0.0
00
0.0
00
0.0
00
0.0
00
0.0
00
Fe
0.0
01
0.0
01
0
.000
0.0
00
0.0
00
0.0
00
0
.000
0.0
00
0.0
00
0.0
00
0.0
01
0.0
01
0.0
00
0.0
00
0.0
00
0.0
04
0.0
00
Na
0.1
26
0.1
24
0
.123
0.1
20
0.1
14
0.1
19
0
.102
0.1
06
0.1
01
0.0
71
0.0
34
0.0
20
0.0
13
0.0
17
0.6
67
0.8
41
0.8
61
K
0.8
60
0.8
69
0
.860
0.8
66
0.8
74
0.8
81
0
.884
0.8
88
0.8
91
0.9
26
0.9
66
0.9
68
0.9
64
0.9
51
0.1
34
0.0
09
0.0
11
Sum
A
0.9
92
1.0
04
0
.992
0.9
93
0.9
95
1.0
09
0
.992
0.9
98
0.9
96
1.0
03
1.0
02
0.9
90
0.9
79
0.9
77
0.8
13
0.8
63
0.8
84
Si
2.9
97
2.9
91
2
.996
3.0
05
3.0
00
2.9
90
3
.004
2.9
95
2.9
92
2.9
96
3.0
12
3.0
09
2.9
99
2.9
89
3.0
11
3.0
17
3.0
05
Al
1.0
03
1.0
09
1
.003
0.9
95
1.0
00
1.0
10
0
.996
1.0
03
1.0
08
1.0
04
0.9
87
0.9
91
1.0
01
1.0
11
0.9
89
0.9
83
0.9
94
Sum
B
4.0
00
4.0
00
4
.000
4.0
00
4.0
00
4.0
00
4
.000
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
4.0
00
Mole%
feldsparcomponent
An
0.6
0.9
0
.8
0.7
0.6
0.7
0
.6
0.4
0.4
0.6
0.1
0.0
0.2
0.8
1.5
1.1
1.4
Ab
12.0
11.7
11
.8
11.5
10.9
11.2
9
.7
10.1
9.6
6.7
3.2
1.9
1.2
1.7
81.3
97.8
97.3
Or
87.3
87.3
87
.4
87.8
88.5
88.1
89
.7
89.5
90.0
92.7
96.7
98.1
98.6
97.5
17.3
1.1
1.3
Theaverageanalyticaldatafrompristinealbiteanalyses17
31aregiveninth
elastcolumn.
Theelectronmicroprobeana
lyseswereconvertedtomolepercentofthefeldsparcomponentsby
normalizingtoeightoxygenatoms
performulaunit
Contrib Mineral Petrol (2009) 157:6576 71
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the refined lattice parameters and selected diffraction peak
positions (Kroll and Ribbe 1987). The lattice parameters
have been obtained by Rietveld refinement using the
fullprof computer program (Rodriguez-Carvajal 1990).
The state of Al, Si ordering in alkali feldspars is defined by
the Al content at the t1o site in the crystal structure. Fol-
lowing equation was used to determine the occupancy
value Rt1 for the albite starting material as well as for the
sanidine product feldspar (Kroll and Ribbe 1987):
Xt1 t1o t1m
b 21:5398 53:8405 c
2:1567 15:8583 c1
with the lattice parameters for the albite starting material
given in Table 2, Eq. 1 yields Rt1 = 0.996, i.e. the albite is
fully ordered. These calculated values are similar to those
obtained by Smith et al. (1986) for the Amelia low albite
0.997 Al in T1O, 1.001 Si in T1 m, 1.002 Si in T2O and1.006 Si in T2 m. For the product K-feldspar, we obtain
Rt1 = 0.569 from the lattice parameters given in Table 2.
The reaction product is thus highly disordered sanidine
feldspar.
Discussion
The results of this study suggest that a continuous
exchange by a coupled dissolutionreprecipitation process
leads to the replacement of albite by K-feldspar. The SEM
observations of the reacted surface and the electron dif-fraction patterns across the reaction interface demonstrate
that, although the K-feldspar has recrystallised during the
reaction, it essentially remains a single crystal with the
same crystallographic orientation as the parent phase. Such
an observation is consistent with an interface-coupled
dissolution-reprecipitation mechanism in which the nucle-
ation and growth of the product is epitaxial on the parent
surface (Putnis and Putnis 2007). The interface between
pristine albite and product K-feldspar is sharp on a nano-
meter scale, as seen by TEM (Fig. 8a). The TEM bright-
field image across the reaction interface shown in Fig. 8a
further reveals a difference in diffraction contrast betweenthe albite parent, showing virtually no extended defects,
and the K-feldspar product that is characterised by dense
linear features normal to the reaction interface. These
features may represent nano-channels that formed along
dislocations and allowed the fluid reservoir to keep contact
with the fluid at the reaction front. The linear features must
originate from the dissolution-reprecipitation reaction,
because no comparable crystallographic defects could be
observed in the albite starting material. When albite is
replaced by K-feldspar, a molar volume increase of 8%,
caused by the incorporation of the larger K-cation, has to
be compensated by the crystal lattice at the interface. It is
the coherent stress that leads to the formation of connected
dislocations, which provide pathways for chemical
exchange between the surrounding fluid and the fluid at the
reaction interface. Lee and Parsons (1997) observed fea-
tures similar to these nano-channels in natural orthoclase
that was partially converted to albite. They observed albite
crystals replacing orthoclase that were delineated by strain
shadows and proposed a relationship to closely spaced
dislocations. Also Worden et al. (1990) reported jagged,
Fig. 6 Representative Raman spectra of two experimentally
exchanged K-feldspars, one containing common 16O (grey) and one18
O (black) in the tetrahedral framework. Large-amplitude modes at
476 and 519 cm-1 shift to lower values near 457 and 497 cm-1,
respectively
-13-18 -14-15-16-17
100 m
Fig. 7 Raman contour map ofDx from the area marked in the BSE
image. Dx is the difference between the frequency of the bending
mode near 476 cm-1 in the K-feldspar formed in an 18O-enriched
fluid and the frequency of this mode in 16O-rich K-feldspar
72 Contrib Mineral Petrol (2009) 157:6576
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incoherent interfaces between albite-twinned albite and
albite-twinned microcline in naturally altered feldspars.
After experimental treatment larger open pores are com-
mon at the interface between the pristine albite and the
K-feldspar (Fig. 3). These pores must have formed during
the hydrothermal experiments as they only occur near the
reaction interface (Figs. 3, 8a) and it is therefore highly
likely that they are an integral feature of the replacement
mechanism. It is evident that the reaction rate should
depend on available fluid pathways through the product
1m
Na
Si
Al
O
O
Si
AlK
Na
a
b c d
Fig. 8 a Bright-field TEM
image of the interface between
K-feldspar on the left and albite
on the right (zone axis [112]).
The interface has a high contrast
and appears dark. The product
K-feldspar phase on the left side
is characterized by a much
higher structural complexity.
Diffraction patterns of (b) the
albite, (c) the interface area and
(d) the product K-feldspar (zone
axis [112])
Contrib Mineral Petrol (2009) 157:6576 73
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crystal, i.e. on interconnected porosity. In contrast to the
porous material seen in Fig. 1, where numerous pores
appear at the feldspar surface, the surface roughness of the
grain shown in Fig. 2 is obviously reduced.
The reaction between albite and a hydrothermal fluid is
also influenced by the crystallographic properties of the
host albite. Powder X-ray analyses reveal a change fromtriclinic (C-1) albite structure to monoclinic (C2/m) sym-
metry in K-feldspar which depends on the Al, Si order. The
X-ray powder diffraction of the albite starting material
indicate a very high degree of Al, Si order while the K-
feldspar product phase is highly disordered. At tempera-
tures of 600C K-feldspar is Al, Si disordered (Smith and
Brown 1988) and the difference in Al, Si order between the
parent and product provides extra thermodynamic driving
force, in addition to the chemical exchange. Kinetically,
the reaction between albite and a KCl solution is strongly
dependent on the crystallographic direction (Holdren and
Speyer 1985; Weise and Schliestedt 1988), as evidenced by
the irregular rim thickness (Fig. 3). The reaction rate is
greater along the (001) crystal surface that shows more
conspicuous structural changes than other crystal surfaces.
This observation can be rationalised by the fact that the
bonding parallel to the (001) lattice plane is weaker than
along other crystal planes, which is reflected by the pre-
ferential cleavage of albite along the (001) plane. In
ordered feldspars the proportion of Al-tetrahedra at the
(001) surface is higher than at other crystal surfaces
(Bondham 1967), and the Al-tetrahedra are preferred sites
of dissolution, because the AlO bonds are energetically
easier to break than the SiO bonds (Oelkers and Schott
1995; Xiao and Lasaga 1994).
Raman spectroscopy gives evidence for the incorpora-
tion of 18O into the feldspar lattice and thus provides
further evidence that the K-feldpar precipitated from
solution. Particularly, the most K-rich area near the reac-
tion interface shows the highest Raman shift of-18 cm-1
compared to K-feldspar formed in 16O-rich isotopically
normalsolution. The area approximate to the grain edge,
a few micrometer away from the interface, contains more
Na and has a smaller Raman-shift of-14 cm-1 due to the
lower 18O content (Fig. 7). The position of the interface
can be observed as a sharp boundary between albite start-
ing material and K-feldspar product in the Raman map as
well as in the electron microprobe map. Observations by
electron microprobe and Raman spectroscopy show a one-
to-one correlation between cation and oxygen isotope
exchange, which is consistent with data of Schliestedt andMatthews (1987) and Labotka et al. (2004).
The oxygen isotope exchange is not influenced by
fractionation between sodium and potassium feldspar.
Apparently both feldspars, Albite and K-feldspar, have
identical isotopic properties relative to one another
(Schwarcz 1966; Taylor and Epstein 1962). ONeill and
Taylor (1967) demonstrated that the isotopic composition
just tends to approximate equilibrium conditions between
the feldspar and the fluid. The high concentration of K and18O found close to the interface seems to be similar to 18O-
enriched alteration zones with sharp chemical gradients to
relict unreacted areas in polycrystalline, Ta-based pyro-chlore after hydrothermal treatment (Geisler et al. 2005).
Our results are also consistent with data on natural
hydrothermally altered plagioclase in volcanic rocks
which show a direct correspondence between cation and
oxygen isotope exchange. Cole et al. (2004) have studied
plagioclase feldspars from Rico Dome, Colorado with
naturally altered rims of mainly albitic composition. They
have observed narrow rims enriched in 18O in distal
feldspars and wide reaction rims or completely exchanged
feldspars that are depleted in 18O, proximal to the Rico
hydrothermal system. Cole et al. (2004) have noted that
simple volume diffusion could not have governed the
transport during hydrothermal exchange. They compared
the observed penetration values with hydrothermal based
volume diffusion coefficients for oxygen in Na- or K-
feldspars (Farver and Yund 1990). The durations to pro-
duce d18O profiles, approximating those measured for
Rico feldspars, are one to two orders of magnitude greater
than could be achieved by volume diffusion within the
age of the system.
Volume diffusion is definitely too slow as an effective
transport mechanism of K and Na ions and O (Brady and
Yund 1983; Christoffersen et al. 1983; Cole and Chakra-
borty 2001; Foland 1974) to explain the high exchange
rates that could be observed by Raman spectroscopy and
electron microprobe. The maximum diffusion length for K
in albite at 600C experiments is less than 0.300 lm within
67 days under anhydrous conditions (Brady and Yund
1983; Christoffersen et al. 1983; Giletti and Shanahan
1997), although under hydrothermal conditions the diffu-
sivity might be significantly higher (Cole and Chakraborty
2001, Giletti and Shanahan 1997). However, on small scale
volume diffusion may operate to equilibrate relict domains
Table 2 Refined lattice parameters for the albite starting material
and for the sanidine product phase
Cell parameter Starting albite Product K-feldspar
a 8.1430 8.5616
b 12.7920 13.0204
c 7.1610 7.1743
a 94.2425 90.0000b 116.5904 115.9713
c 87.7035 90.0000
c* 0.1565 0.1550
74 Contrib Mineral Petrol (2009) 157:6576
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of albite feldspar (Fig. 3), or to slowly homogenize areas
within the reprecipitated K-feldspar.
The observations of a chemically and structurally sharp
reaction interface, the correlation between cation and
oxygen exchange, the generation of defects or nanopores
associated with the reaction, the recrystallised texture of
the product K-feldspar and the rate of the reaction are all
not consistent with volume diffusion as a dominantmechanism, but are entirely consistent with an interface-
coupled dissolution-reprecipitation mechanism (Putnis
2002; Putnis and Mezger 2004; Geisler et al. 2005; Putnis
et al. 2007; Putnis and Putnis 2007).
The details of the reaction progress by such a mechanism
is likely to be complex as the product solid continuously
re-equilibrates with a changing fluid composition.
The approach towards equilibrium conditions between
the product feldspar and the composition of the fluid also
undoubtedly affects the reaction rate. As the replacement
of albite by K-feldspar proceeds, the composition of the
fluid changes to higher NaCl contents while the productfeldspar composition continuously re-equilibrates by
interacting with the fluid. Feldspar-fluid equilibria in the
system NaAlSi3O8KAlSi3O8H2ONaClKCl (Orville
1967; Lagache and Weisbrod 1977; Pascal and Roux 1982;
Rubie and Gunter 1983) define the coexisting compositions
(Fig. 9). Although the aim of our experiments was to
determine the mechanism of the replacement, rather than to
establish equilibrium, Fig. 9 may be used to show the
possible evolution of the rim compositions in our experi-
ment. Starting from the initial situation where the fluid
composition mole% K/(K+ Na) = 100, which would
coexist in equilibrium with pure K-feldspar, the fluid and
rim composition would successively re-equilibrate, until
the experiment was terminated, in our case with a rimcomposition of Or87Ab12An1. At each step of such an
evolution, re-equilibration is achieved by successive dis-
solution and reprecipitation reactions. A further caveat
however, is that such a description assumes that the rim
composition is in equilibrium with the bulk fluid compo-
sition, whereas in situ experiments on replacement in salt
systems have shown that the solid composition which
reprecipitates is determined by the fluid composition at the
reaction interface, and that large compositional gradients
may exist in the fluid while the reaction proceeds (Putnis
et al. 2005). This suggests that the rate limiting step in such
replacements may be the mass transfer through the fluidpathways in the solid reaction product.
Although we have no satisfactory explanation for the
enrichment of K and 18O in the K-feldspar at the reaction
interface (Figs. 5, 7), it is a consistent feature of these
experiments and, in terms of18O, also of other experiments
(Geisler et al. 2005) and suggests that the fluid at the
interface must also be enriched in K and 18O during the
reaction. The lower 18O enrichment away from the reaction
front may indicate that here the chemical composition of
the product feldspar re-equilibrated with an increasingly16O richer fluid by secondary processes.
Acknowledgments We are indebted to Peter Schmid-Beurmann
who helped with the Rietveld calculations, Arne Janen and Angelika
Breit for the powder X-ray diffraction measurements and Jasper
Berndt for the electron microprobe mapping. We are also grateful to
Herbert Kroll for his help in calculating the state of Al, Si order. We
thank an anonymous reviewer for a possible explanation for the K and18O enrichment at the reaction interface.
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