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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


123


5/12

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


123


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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


123


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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


123


8/12

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


123


9/12

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])


123


10/12

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


123


11/12

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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0

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http://dx.doi.org/10.2138/gsrmg.43.1.83http://dx.doi.org/10.2138/gsrmg.43.1.83http://dx.doi.org/10.2138/gsrmg.43.1.83http://dx.doi.org/10.2138/gsrmg.43.1.83


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