Surface studies of feldspar dissolution using surface replication combined with electron microscopic and spectroscopic techniques

Ceochimico er Cosmochimica Acro Vol. 46. pp. 503 10 512 Q Pergamon Press Ltd. 1982. Printed in U.S.A.

Surface studies of feldspar dissolution using surface replication combined with electron microscopic and spectroscopic techniques PATRICK C. FUNG Geological Research and Services Department, Petro-Canada,

P.O. Box 2844, Calgary, Alberta T2P 2M7

and GILLIANO G. SANIPELLI Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment,

Pinawa, Manitoba ROE 1LO

(Received February 26, 1981; accepted in revised form November 17, 1981) Abstract-The replica of a microcline cleavage surface was examined before and at various stages of interaction with water and acid solutions at 70°C. For up to 14 weeks in demineralized water the surface as a whole underwent very little change, except some micrometre-sized particles were found on parts of the surface after only one week. Similar particles were found on the actual cleavage surface and on the surfaces of other microcline powders similarly leached at 22°C. These particles were made up of aluminum and silicon with little or no potassium. They were likely formed in some preexisting activated feldspar lattice, either by solid transformation or by local supersaturation and precipitation from solution. Further leaching for 48 hours in a 0.01 mol. L-’ solution of hydrochloric acid caused only minor pitting of the same cleavage surface, probably due to enhanced dissolution, while contact with a 0.01 mol. L-’ solution of hydrofluoric acid caused extensive formation of dissolution pits and channels along crystal defects leading to the removal of large portions of the surface. Neither acid appeared to dissolve the newly formed aluminum silicate particles appreciably. Hence during the incongruent dissolution of a feldspar, most of the reactions, dissolution and formation of authigenic Al-silicate phases, occur preferentially along crystal defects. Since the authigenic phases occur as discrete particles occupying only a small fraction of the parent surface their presence will not affect the bulk composition or the overall dissolution rate of the surface. INTRODUCIION SURFACE texture and mineralogy of leached feldspars have been studied by several workers to elu-

cidate the mechanism of feldspar dissolution, e.g., Lagache (1965), Parham (1969) and Tsuzuki and Suzuki (1980). Recently determination of surface composition has become possible by the development of X-ray photoelectron spectroscopy (XPS) e.g., Petrovic et al., 1976, Holdren and Berner, 1978 and Fung et al., 1980. However, these studies must be performed either on the final product of an experiment, mainly due to the destructive nature of the

techniques (electron microscopy), or on the bulk samples, e.g., by X-ray diffraction (XRD) and XPS. This work is an attempt to examine the same feldspar surface before and at various stages of interaction with aqueous solutions and to locate and determine the composition of individual secondary phases that were observed in previous studies. A combination of techniques were used including surface replication, Scanning and Transmission Electron Microscopy (SEM and TEM respectively) and Energy Dispersive X-ray spectrometry (EDX). Advantage of the replication technique is that it is non-destructive, permitting repeated examination of the same surface at any desired stage of leaching. In addition, actual surfaces of leached powders were also studied for comparison. This work is part of the on-going basic geochemical research for the Canadian Nuclear Fuel Waste Man503

agement Program to investigate the feasibility of disposal of these wastes in repositories mined in crystalline rocks (Boulton, 1978). The objective of this report is to investigate the mechanism for Al and Si removal during the incongruent dissolution of feldspars and its effect on dissolution rate. EXPERIMENTAL TECHNIQUE Material and leaching procedure All the samples used for this study were microcline crystals collected from the Perth Feldspar Mine, Lanark County, Ontario, and have been described previously (Fung et al., 1980). For our work on surface replication, a (010) cleavage fragment, approximately 2 cm X 4 cm and 0.5 cm thick was used, obtained by carefully cleaving a large single crystal. The fragment was put in a stoppered plastic bottle containing 200 ml of demineralized water and maintained at -70°C in a water bath. After each interval of 1, 3, 7 and 14 weeks, replicas were made of the (010) surface as described in the next section. At the conclusion of this first stage of the experiment the specimen was transferred to a second bottle containing 200 cm3 of 0.01 mol. L-’ HCl solution at 70°C for another 48 hours. As a second stage of the experiment, another series of replicas was made and the specimen was then transferred into a third bottle containing 0.01 mol. L-’ HF solution at 70°C for another 12 hours. In this third stage, the replication process was repeated after which the specimen itself was finally coated with gold or graphite for examination bv SEM. Crushed powders, 0.25-0.42 mm in size were similarly leached in demineralized water at 22’C for 3 months. No replica was made for these powders which were coated with gold or graphite for SEM examination.

504

t’ f.

FIJNG

AND G. G. SANIPELLI

Surface replication

The two-step replica technique, e.g. see Phillips er at,, (1976), was adopted for this study using Bioden R.F.A. acetylcellulose replicating film (Okenshoji Co. Ltd., Tokyo). Figure la is a schematic of the technique. In the first step, a plastic replica is produced in which the topography is inverted from that of the specimen. In the second step, the final rephca is produced by depositing a thin layer of carbon and a small amount of platinum (to enhance resolution) onto the plastic replica from a source held at approximately 45”. The plastic is then dissolved away, leaving the final carbon replica, Replication was performed at desired intervals by removing the sample from the solution and blotting excess solution from the surface with a tissue paper. Several ‘blank replicas were made each time until a replica was obtained that was relatively free from foreign particles such as dust or fine-grained microcline powders. The clean replica was cut and some of the cut up pieces were mounted on a numher of circular copper grids -3 mm in diameter and stored in a specimen grid box (by LKB, Stockholm-Bromma, Sweden) to aid in sample location. Electron microscopy

A replica can be examined directly by optical and electron microscopy without further coating. Since they are very thin, and also because TEM has higher resolution at higher magnifications, all final examinations of the replica were made with a TEM (Hitachi EM model 1IA), All of the selected powders, the actual (010) surface at the end of the leaching ex~~ment, as well as some of the replicas were examined by SEM.

All SEM studies were carried out with a Cambridge Mark 2A Stereoscan unit interphased with a Si (Lif X-ray detector and an EDAX 707 A X-ray spectrometer for microanalysis. The computer program package EDAX-EDIT (supplied by the manufacturer) was used to compute the integrated area of the major element peaks for calculation of relative concentrations. These peak areas were corrected for overlapping peaks (deconvolution). Peak intensities were also measured directly from spectra without correrrion for overlap or peak shape. Relief of ihe replicas

Since the replication technique is not well known in the geologic literature, a brief discussion on texture interpretations follows. For a more detailed description, the interested reader should refer to reports by Phillips et al.
PLASTIC REPLICA FIRST STEP

FINAL REPLICA SECOND STEP

SPECIMEN SURFACE

a

b FIG. I. Schematics of the two-step replica technique (a) and interpretation Modified from Phillips et al., (1976). See text for discussion.

of the topology (bj.

505

FELDSPAR DISSOLUTION

a

d C

FIG. 2. TEM micrographs of the (010) surface before (fresh) and after reacting with demineralized water at 70°C for various periods of time, (a) fresh surface (b) to (d) after 1, 3 and 7 weeks respectively of reaction with demineralized water. Note the protrusions P (on the actual surface) are present only in the reacted surfaces and in the same location for all replicas.

best indication of the direction of shadowing. A depression

in the specimen surface can easily be distinguished from a trapped particle since a depression usually occurs in all the replicas at the same location. Finally, since the topography of the replica is reversed from that of the original specimen, a depression in the replica is caused by a protrusion in the surface, and vice versa for a protrusion in the replica, as illustrated in Fig. la. RESULTS

Study of the replicas Figure 2 shows TEM micrographs of the replicas from part of the (010) microcline cleavage surface before (a) and after (b-d) reacting with demineralized water for up to 7 weeks. Following the arguments presented earlier, the direction of shadowing

is from the SE or E as indicated by the direction of the white shadows (tails) adjacent to a depression in the actual surface. Comparison between the replicas of the fresh and reacted surface shows that the sur-

face as a whole underwent verv* little change. Greatcat change was confined to areas of crystal defects, such as cleavage fractures (NW corner). The most interesting features are those such as the ones marked P, which were protrusions. These protrusions were absent from the replica of the fresh surface, but were present in all replicas of the leached surface and all invariably at the same locations. Also, the shapes of these protrusions, which were very poorly defined after one week of reaction, appeared to have become more distinct with time. Hence, there is very little

Wb

I’ I. FUNG AND G. G. SANIPELLI

FIG. 3. Similar to Figure 2 but after further reaction with (a) 0.01 mol.dm-3 HCI solution 48 hours and(b) 0.01 moldm-’ HF solution for 12 hours at 70% Note sandy pitted surface in (a), extensively damaged surface in (b) and presence of protrusions (P) in both.

doubt that these protrusions were formed during the reaction of demineralized water with the microcline surface, and not particles that were present on the original surface, or foreign grains embedded onto the leached surface. Figure 3a shows the same surface after further reacting with a 0.01 mole L-r HCI solution at 70°C for an additional 48 hours. Parts of the surface, especially along the cleavage fractures, became slightly pitted, which is evidence of enhanced attack by the HCl. However, little other change can be observed and the particles were still in place. Finally, Fig. 3b shows the same surface after an additional 12 hours in a 0.01 mol. L-’ HF solution FIG. 4. (a) and (b) 7’kXf micrographs of replicas of a part of a (010) surface: (a) fresh, (b) after the final HF reaction. (c) SEM micrograph of the actual surface in the area shown in (a) and (b). Note the new NE trending etch channels and the newly exposed subhorizontal surface (triangular area in the centre) in (b) and fc) created by the reaction.

FELDSPAR

507

DISSOLUTION

at 70°C. The very severe attack of the surface by the HF can be seen in several places. Many of the ex-

isting depressions were enlarged and new ones created in the top left corner along pre-existing surface defects. For example the wide channel in the left corner of the photograph was originally the intersection of two cleavage surfaces (Fig. 2a). A new surface was created as seen at the top of the photograph, probably due to the complete removal of the original surface. Surprisingly, some of the particulates survived this severe attack. Figure 4 shows another part of the surface of the same specimen before and after the final HF attack which further demonstrates the severe action of HF on the surface. On the original surface (Fig. 4a) there was only one major channel system trending NW and two rows of NE trending pits. After the HF treatment (Fig. 4b), another prominent channel system, running NE, developed from the pits. At the upper central part of Fig. 4b, the new etch channel trending NE bent slightly to join up with the existing NW trending fracture and in the centre of the picture a triangular area bounded by the fractures was completely removed, exposing a new surface with the cleavage direction running NS. Figure 4c (slightly rotated anti-clockwise from Fig. 4a and 4b) is the actual HF leached surface showing the fracture systems at least partly developed from coalescence of pits. This and Fig. 5 verify our earlier interpretation of the topography of the replicas in that we were able to interpret the relief from both the replica and the actual surface of the same area. Figure 5 shows another area of the (010) surface. Figure Sa is the replica of the original surface while Fig. 5b shows the actual surface after the final HF attack. Again three major channel systems were created by the action of HF, one running NS (middle of Fig. 5b), one running NNW-SSE (lower middle) and the other running E-W (top left corner). These channels were obviously developed from pre-existing cleavage fractures or dislocation arrays. Pits were also seen in many parts of the surface. Hence while little change to the surface was produced by water and the HCl solution, extensive damage was done by the HF solution, mostly in preexisting defects.

a

b

Particles extracted ortto the replicas The early blank replicas normally contain many fine particles picked up from the specimen surface. Most of these particles were found to be microcline. Occasionally, other particles were observed and were subjected to microanalysis using the EDX attachment to the SEM. Figure 6a shows a fibrous particle extracted from the specimen surface after reacting with water at 70% for one week. It is composed entirely of Si and Al, without any noticeable amount of alkalis (Fig. 6b). Figure 6c shows several particles extracted from

FIG. 5. (a) TEM micrograph of a replica of part of the fresh (010) surface (b) SEM micrograph of the actual SUTface as shown in (a) after the final HF reaction. Note the two (newly created) etch channels running NS (centre) and EW (top left corner).

the specimen after reacting with water at 70°C for three weeks. These particles were again Al-silicate phases without any alkalis (Fig. 6d and Table 1, a). Some of these particles were unstable in the electron beam (in vacuum) and the area examined by EDX was destroyed.

508

I’ i:. FUNG

AND

G. G. SANlPELLl

a

c

d

FIG. 6. (a) SEM micrographs of replicas of part of the (010) surface after one week in water at 70°C showing a flaky particle picked up from the actual surface, (b) EDX spectrum of the particle in (a). (c) similar to (a) after three weeks, (d) EDX spectrum for the particle in (c).

Particles on the actual surface after the HF attack

Upon examination with the SEM, many areas in the actual specimen surface were found to have particles attached. Thus Fig. 7a shows several particles found on a highly uneven surface after the HF attack. EDX micro-analysis of these particles showed that they were composed almost entirely of Al and Si (Fig. 7d and Table lb). Some of the particles contained traces of K (e.g. particle 4, Fig. 7a has an EDX Si/K/Al peak area ratio of 7.0/O. 18/ 1.O). Similarly Fig. 7b through 7c show other areas containing particles with little or no K (Table 1, b). Leached powders

Upon intensive searching of microcline grain surface reacted with water at 22°C for three months, many areas were found which contained minute par-

titles firmly attached to the surface (Fig. 8). Microanalysis with the EDX showed that they were depleted in K and Si relative to the surrounding microcline surface, e.g., for the particle shown in Fig. 8a, %/K/Al peak ratios were --6/2/l, whereas for the microcline surface the ratios were 9.313.411. DISCUSSION The mechanism of feldspar dissolution has been a subject of considerable debate, e.g., see Helgeson,

197 1, Lagache, 1975 and summary of Petrovic, 1976. Currently there are two main groups of models. Briefly stated, they are as follows: 1. Diffusion model. Dissolution rate is controlled by diffusion of feldspar components through a thickening layer of secondary products which have been variously hypothesized to be composed of: (a) amorphous precipitate (Correns and von Engelhardt, 1938

FELDSPAR DISSOLUTION TABLE EOAX “ICROANALYSIS ON no3

niCrOCliM

(b)

grab,

(010)

in Figure 6(a) partfcle Pigure

Largest Largest

particle particle

after

the

Figure Figure

on the final

for

leached

Particle Largest

Particle

“ICrmcLlNE

6(c) 6(c) 6(e)

trolled by the rate of reaction at the feldspar-solution interface and accumulation and interaction of dissolved components (Lagache, 1965, 1975).

1

OF l-68

PArcrICLBS

ClaN‘vx

lOoN0

SoRFAC8

1 week

3.1/2/l 1.8lOll

“.d.

right

3.4/O/l

left

1.5/o/1

middle

1.9/o/1

15.0/011 2.6/O/l 3.0/O/L

n.d.

specimn surface HP leaching

particle particle particle particle particle

n.d. 2.9/O/l n.d. 1.4/0.48/l “.d.

Figure Figure Pigure vigure Figure

7(a), 7(a), 7(a), 7(c). 7(c),

4 5 6 7 8

Pigure Figure

7(d).

particle

n.d.

7(d),

microeline

n.d.

7.OlO.18ll 7.4/O/l 6.8/0.07/l 1.6/0.27/1.0 2.9/0.44/1.0 6.41011 8.8/1.7/1

and Wollast, 1967); (b) residual Al-silicate (Pa&s, 1973) or (c) crystalline precipitates (Helgeson, 1971). 2. Surface reaction model. Dissolution is cona

509

With the recent advent of surface analytical techniques such as XPS, semi-quantitative information on the bulk composition of the outermost few monolayers of a surface can readily be determined. XPS has been applied to feldspar dissolution studies by PetroviC et al. (1976), Holdren and Berner (1979) and Fung et al. (1980). It has become increasingly clear that after ten or more days of dissolution (depending on experimental conditions) in aqueous solutions, surface composition of a variety of feldspars is not significantly different from that of the fresh surface. Moreover, Berner and Holdren (1979) showed that surface composition of elastic feldspar grains removed from weathered soils is very similar to that of fresh feldspars of similar composition. Hence no continuous layer of secondary products, devoid or depleted in K, Na and Ca could have been formed on the surface of weathered feldspars. Yet b

d FIG. 7. (a) to(c) SEM micrographs of several parts of the actual (010) surface after final HF reaction containing several particles, and (d) EDX spectra for particle 4 in (a).

1’. C FUNG

AND

a

G. G. SANIPELLI

b

d

C

FIG. 8. SEM micronraohs showing particles present on surfaces of microcline grains leached in water at 22°C for three months. 1

.

__

all experimental evidence strongly suggests that at least for freshly prepared powders at room temperature, the composition of the leachant is always greatly different from that of the feldspar, with the leachant strongly depleted in Al and Si, e.g., see Busenberg and Clemency, (1976). Hence the question arises as to the fate of these network-forming elements, if they were not retained at the surface (as suggested by XPS data). Busenberg and Clemency (1976) and Busenberg (1978) observed minute amounts of an Al-phase (probably amorphous gibbsite) of unknown origin in some dissolution experiments. They also postulated the formation of gibbsite and Al-silicate phases such as metahalloysite, kaolinite and montmorillonite forming a continuous layer covering each grain surface. PetroviC (1976)

after reviewing a large volume of data on textural changes accompanying leaching of feldspars, concluded that for temperatures of 22°C to 200°C various Al and Al-silicate phases could have formed according to the thermodynamic scheme of Helgeson (1971), although disequilibrium effects may be important. In this study, particles were observed on the replicas of a microcline surface made at different stages of leaching in water at 70°C on several parts of the surface originally devoid of any particulates. From their mode of occurrence and composition as determined by EDX they are likely to be Al-silicate phases formed by the interaction of water and the microcline surface. Similar particles were also found on the actual surface of the same specimen, or others similarly

FELDSPAR

DISSOLUTION

leached at 22°C. These particles, which were firmly attached to the surface were also made up of Al and Si with little or no K. The small amount of K detected by EDX for some particles on a feldspar surface probably came from the microcline under-lying or surrounding the particles, since they were all very small and not all of them contained K. Hence they were probably also Al-silicate phases formed by the interaction of the surface and the aqueous solution. These observations suggest strongly that both Al and Si were released by the leaching process and at least part of them were fixed on the surface as Alsilicate phases. The particles could have been formed either by precipitation of dissolved Al and Si or by solid transformation of the feldspar lattice. Precipitation from solution in the normal sense requires supersaturation of the solution with respect to one or more Al-silicate phases such as kaolinite, which is unlikely because of the extremely low solid/water ratio for the experiment with the (010) cleavage fragment. Precipitation is further hampered by the notoriously slow kinetics of almost all Al-silicates under the conditions of the experiment, e.g., see La Iglesia et al. (1976). On the other hand, transformation of a feldspar lattice into, e.g., a kaolinite lattice requires rearrangement of the Al-$0 bond from a three-dimensional network into a sheet structure and elimination of Si. This process is also energetically and probably kinetically unfavourable. Hence formation of these secondary minerals must have occurred in some preexisting activated sites created during sample preparation or otherwise, on the freshly cleaved surface. Upon dissolution, local supersaturation leads to precipitation in these activated sites acting as nucleation centers. Alternately, upon contact with water, these high energy sites change to phyllosili~ates by direct transformation. There is presently no available data that warrants a choice between these two models, although the latter simpler model is favoured by the authors. A small amount of pitting of the (010) surface was observed after contacting HCl solution for 48 hours. Much more extensive pitting and channelling leading to the removal of relatively large pieces of the surface were observed after contacting the HF solution for only 12 hours. These textural changes occur mainly along pre-existing fractures and pit and hence imply preferential leaching of the surface along preexisting surface defects. This conclusion is consistent with the observations made by e.g. Berner and Holdren ( 1979) on feldspar surfaces leached in acid solutions and in nature (soils). Moreover, alteration products such as clays, simple oxides and hydroxides are common occurrences in altered feldspar surfaces without obvious signs of dissolution and precipitation processes. If pitting and channelling in feldspar surfaces, commonly observed in experimental and natural environments were due to preferential leaching along crystal defects (as is the normal practice), it is not

511

unreasonable to assume that some high energy sites also exist in a surface that are favourable for solid transformation into or nucleation centers for the precipitation of a clay mineral. Regardless of how these secondary phases were formed, they always occur as discrete particles (clusters). In areas not covered by any particles, very little change appears to have taken place. Hence the bulk composition of the surface, as determined by XPS, would not be affected appreciably by the presence of these particles which occupy such a small fraction of the total parent surface. In this regard, formation of authigenic phases also will not affect the overall dissolution rate of a surface by forming a diffusion barrier, as is required by the diffusion model. These observations support a modified surface reaction model, a subject for another paper in preparation {see Fung and Sanipelli, 1980). Finally, because of the extensive damage done to the surface by HF solution in such a short time and low strength, caution should be exercised in interpreting kinetic data obtained from dissolution experiments using mineral powders treated with HF solution. CONCLUSIONS

Using a surface replication technique, textural changes of a microcline feldspar cleavage surface were examined non-destructively when fresh and at various stages of dissolution in water. Small micrometer-sized particles were observed on several parts of the surface originally devoid of any particulates. They were found in all replicas of the same surface leached for various periods of time, all invariably at the same locations and with similar overall morphology. Spot analysis of these particulates picked up by the replica showed that they were composed entirely of Al and Si with no alkalis. Similar alkali deficient particles were also found in the actual surfaces of this and other similarly leached powder surfaces. Hence they are most likely authigenic Al-silicate minerals formed by water-solid interaction. Under the experimental conditions (extremely low solid/water ratio) these authigenic phases were likely formed in pre-existing activated sites, either by solid transformation or by local supersaturation and nucleation. Hence phase transformation, like dissolution occurs preferentially along crystal defect on the surface of a feldspar. These secondary minerals, regardless of their formation mechanism, always occur as discrete particles occupying only a very small fraction of the total parent surface. Hence their presence would not affect the bulk composition or, in this regard, the overall dissolution rate of the feldspar (by fo~ation of diffusion barriers). Acknawledgemems-The authors wish to thank G. W. Bird for his interest in this work and staff of the Analytical Science and Material Science Branches at Whiteshell Nu-

clear Research Establishment for assrstanct: with the clectron microscopy measurements. Our colleagues at WNRE, G. W. Bird, R. J. Lemire and B. Goodwin criticalIS reviewed the early version of the manuscript. The constructive comments from these and the referees, T. P&s, R. Berner. R. Petrovi? and anonymous are greatI? apprecialeti.

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Beachem C. D. (1969) Microscopic Fracture Processes. In Fracture, Vol. 3, Chap. 4. Academic Press, New York. Berner R. A. and Holdren G. R. Jr. (1979) Mechanism of feldspar weathering-II. Observations of feldspars from soils. Geochim. Cosmochim. Acta 43, I 173- 1186. Boulton J. (Ed.) (1978) Management of Radioactive Fuel Wastes: The Canadian DisDosal Proaram. Atomic Energy of Canada Limited Rhport, AEeL14. Brammar I. S. and Dewey M. A. P. (1966) Specimen Preparation for Electron Metallography. Blackwell, Oxford. Busenberg E. (1978) The products of the interaction of feldspars with aqueous solution at 25°C. Geochim. Cosmochim. Acta 4i, 1679- 1686.

Busenbern E. and Clemencv C. V. ( 1976) The dissolution kineticsof Feldspars at 25°C and 1 atm. CO2 partial pressure. Geochim. Cosmochim. Acta 40, 41-49. Correns C. W. and Von Engelhardt W. (1938) Neue Untersuchungen iiber die Verwitterung des Kalifeldspats. Chem. Erde 12. l-22.

Fung P. C., Bird.G. W., McIntyre N. S., Sanipelli G. G. and Lopata V. J. (1980) Aspects of feldspar dissolution. Nuclear ~e~~~o~og~ 51, 188- 196. Fung P. C. and Sanipelli G. (1980) Repeated leaching of microciine at 25°C. Proc. 3rd Internat. Symposium on Water-Rock Interaction, pp. 67-68. Helgeson H. C. (1971) Kinetics of mass transfer among silicates and aqueous solutions. Geochim. Cosmochim. Acia 35, 42 1.

Holdren G. R. Jr. and Berner R. A. (1979) Mechanism of

feldspar weathertng -. I Lxper~mental Cosmochiar ..4cla 43, i ltrl -1 171.

studte,

f .t~~~r./l~tti

Ladd M. W. ( 19731 The Electron Microscope Handbooh Specimen Preparation and Related Laboratory Procedures. Ladd Industries, Burlington, Vermont Lagache M. (1965) Contribution i l’etude de I’altiratwn des feldspaths, dans l’eau. entre 100 et 200°C. :rous diverses pressions de CO,; et application A la svnthtse des mini&x angileux. Bull Sb;, Fran. Mini&i. i’risruilow. 88. 223. 253

Lag&he &. t t975) New Data on the kinetics of ~he dissolution of alkali feldspars at 200°C in carbon dioxide charged water. Geochim. Cosmochim. Acta 40, I Y- 161. La lglesia A.. Martin-Vivaldi J 1.. Jr. and Aguayo 1. I_. (1976) Kaolinite crystallization at room temperature b) homogeneous precipitation. III. Hydrolysis of fefdspars. CiayL~~ClayMiner.-24, 36.42 _ Pates T. ( 1973) Steadv state kinetics and eauiiibrIum between ground water and granitic rocks. Geochim Cosmochim. Acta 37, 264 1 2664 Parham W. E. (1969) Formation of halloysite from feldspar: low temperature. artificial weathering versus natural weathering. Clays Clay Miner. 17, 13-22. Petrwi? R. (1976) Rate contioi in feldspar dissolution. Il. The Drotectice effect of nrectnitares. Geochim (h,mrcr&in*: +ra 40, I509 t 55i . Petrovii: K.. Berner R. :\ ,Ind Goldhaber M. B I 1976) Rate control in dissolution of alkali feldsparb. 1 Study ol’ residual l’eldspar grain> in X-ray photoelectron spectroscopy. Geochim. Cosmochim. Acta 40, 537--548. Phillips A., Kerlins U.. Rawe R. A. and Whiteson B. V. (1976) Electron Fractography Handbook. Metals and Ceramics information Center, Battelle Columbus Lahoratories, Columbus, Ohio. Tsuzuki Y. and Suzuki K. (1980) Experimental study oi the alteration process of labradorite in acid hydrothermal solutions. Geochim. Cosmochim. Acta 44, 673-683. Wollast R. ( 1967) Kinetics of the alteration of K-feldspar in buffered solutions at low temperature. Geochim. Cos~?fff~i~l Ac-fci 31, 635 64X.