Clay Minerals (1976) 11, 235.
MINERAL REACTIONS AT GRAIN CONTACTS
IN EARLY STAGES OF GRANITE WEATHERING
A. M E U N I E R
AND B. V E L D E *
FacultO des Scienees--Laboratoire de Pedologie, 40, avenue du Recteur Pineau
86022 Poitiers, E.R.A. no. 220 du C.N.R.S. and *Universit~ de Paris VI,
Laboratoire de Petrographie, 4, Place Jussieu, Tour 26, 75230 Paris C E D E X 05
(Received 5 February 1976)
ABSTRACT: Two outcrops were studied which showed granite in the early stages
of weathering. Reactions between specificmineral species were observed, and especially
the formation of an illite-like phase which is similar to that of sedimentary illite. This
phase has a composition distinct from that of the initial micas in the rocks, notably
in its iron and magnesium content. This suggests solution-solid equilibria which
involve ionic transfer beyond the grain bondary region where the mineral is found.
INTRODUCTION
Granite alteration has been studied for quite some time so that we now know the
chemical variation of granite as a function of weathering intensity. Numerous authors
have detailed the evolution of global rock chemistry as the assemblage quartz-potassium feldspar--plagioclase--mica becomes kaolinite--expanding mineral--oxide
or hydroxide. It is generally assumed that the fine-grained micaceous phase present
in the different stages of weathering is simply the product of the mechanical destruction of muscovite or biotite. However, if we consider the hydrolysis reaction
established experimentally by Hemley (1959)
3 KA1Si30 8 + 2 H 2 0 -+ KAlaSiaO 1o(OH2) + 2 K + + 2 (OH)- + 6 SiO 2
Orthoclase
water
muscovite
it should be possible to destabilize orthoclase (potassic feldspar) and to produce
muscovite. One of the early equilibria which one will encounter during the hydrolysis
of a granite is that between muscovite and orthoclase. In fact, we have observed
evidence of this reaction during microscopic examination of granites in the early
stages of weathering. The following is a description of the occurrence and compositions of these and the primary micas of two granites.
The samples studied come from two profiles found in the granite massif of
Parthenay (Deux-S~vres, France). At the La Pagerie outcrop the initial rock has an
equant structure; its modal composition is 37"7~o quartz, 41.3 ~ potassic feldspar,
13.69/o plagioclase (An 10-15), 5 - 2 ~ muscovite, 2.3 ~ biotite. The second outcrop,
La Rayrie, which has been affected by post intrusive tectonism, contains 32"0~o
quartz, 37.7 ~o potassium feldspar, 15.8 ~o plagioclase (An 25-30), 5.2 ~ muscovite,
236
A. Meunier and B. Velde
4.6 ~ b i o t i t e a n d 4-7 ~ s e c o n d a r y sericite m i c a p r o d u c e d by r e c r y s t a l l i z a t i o n d u r i n g
t h e t e c t o n i c e p i s o d e . T h e b i o t i t e is a l t e r e d slightly to k a o l i n i t e ( f o u n d as b a n d s o r
l a m e l l a e in t h e grains) a n d in places to v e r m i c u l i t e . V e r m i c u l i t e is also f o u n d as r a r e
i s o l a t e d grains in the r o c k m a t r i x .
T h e initial b u l k r o c k c h e m i s t r y o f t h e t w o o u t c r o p s is q u i t e s i m i l a r as are t h e
c o m p o s i t i o n s o f the initial u n a l t e r e d m i n e r a l s p r e s e n t . T h e m u s c o v i t e s a r e slightly
silicic ( T a b l e 1), t h e b i o t i t e s are similar in b o t h o u t c r o p s f o r M g / F e + M g r a t i o value.
A p a t i t e is f o u n d in b o t h o u t c r o p s as a l a t e - f o r m i n g a c c e s s o r y m i n e r a l ( < 1 % m o d a l ) .
I n t h e L a R a y r i e o u t c r o p , a s e c o n d a r y m i c a (called sericite h e r e ) is p r e s e n t
p r i n c i p a l l y a r o u n d the p h e n o c r y s t s . T h i s w h i t e m i c a o c c u r s as s m a l l veinlets c o n t a i n i n g apatite. T h e a p p a r e n t v o l u m e o f this m i c a is 4.7 ~ , i n d i c a t i n g a significant late
TABLE 1. Calculated formulas for the white micas analysed, assuming 020 (OH)4, (a) = phenocryst
muscovites; ( b ) = reactional micas at muscovite-orthoclase interface; ( c ) = secondary sericites.
For sections a, b and c: R = samples from La Rayrie, P = samples from La Pagerie
a
R.1
R.2
R.3
R.4
R.5
R.6
R.7
R.8
P.I
P.2
P.3
P.4
P.5
Si
A1
Fe
Mg
Ti
Ca
Na
K
6.034
5.970
0.176
0"122
0.006
0-006
0.084
1.782
6.212
5-544
0.224
0'128
0.004
0"002
0-048
1-746
6"174
5.524
0,206
0,168
0,040
0,004
0.096
1.724
6'212
5-494
0.184
0"172
0"038
0-006
0-072
1.718
6"090
5.654
0.190
0'132
0.030
0-006
0.130
1-780
6.213
5.549
0-244
0.108
0
0
0.084
1.714
6-150
5.462
0.291
0.232
0
0
0.154
1.813
6.126
5"590
0.226
0"189
0
0
0"113
1-784
6"252
5.370
0"284
0-243
0
0-014
0-075
1'722
6"209
5"543
0"197
0'220
0
0
0"085
1-616
6-164
5'471
0"421
0'199
0
0
0"073
1'617
6"023
5"566
0'223
0"278
0"044
0"014
0'197
1"806
6"091
5"290
0"337
0"450
0"028
0"041
0"138
1"860
b
R.1
R.2
R.3
R.4
R.5
R.6
R.7
P.1
P.2
P.3
P.4
P.5
P.6
Si
A1
Fe
Mg
Ti
Ca
Na
K
6"464
5"203
0'270
0"066
0
0
0
1"862
6-889
4"510
0"349
0"165
0
0"001
0"013
1"874
6.679
4.960
0"359
0.213
0
0'045
0
1"169
6"395
5"254
0"646
0"048
0
0
0
1-271
6"365
5"218
0"326
0"263
0
0"014
0'020
1"657
6.404
5.364
0"301
0.166
0
0"007
0
1"343
6"127
5"049
0-334
0"189
0"386
0
0
1"757
6"148
5"686
0"839
0"154
0
0'053
0"071
0"187
6-947
4.593
0"224
0.487
0
0"115
0"010
0"773
6"422
5"038
0'421
0'281
0
0"037
0"001
1"720
6-264
5"004
0"574
0"460
0"005
0"029
0"105
1"683
7"095
4-221
0"310
0"321
0"010
0"051
0"166
1"387
6-264
5"226
0"409
0"417
0-012
0-043
0-I00
1"384
C
R.1
R.2
R.3
R.4
R.5
R.6
R.7
Si
A1
Fe
Mg
Ti
Ca
Na
K
6"388
5"458
0"082
0.080
0
0.014
0.012
1.702
6"560
5"154
0"204
0.152
0"004
0"014
0.024
1"522
6"380
5-510
0-224
0.168
0.002
0-010
0.032
1.102
6"910
4"692
0.180
0.148
0.004
0-008
0.026
1.586
7"510
3"928
0-126
0"134
0.002
0-006
0"392
1.210
6.266
5"336
0.206
0.196
0.006
0.008
0.242
1.838
6"612
4'976
0.240
0.204
0-020
0.008
0'054
1.590
M i n e r a l reactions in granite weathering
237
phase of recrystallization: it is undoubtedly associated with the partial destabilization
of the biotite and plagioclase. One observes the possible reaction:
plagioclase -I-biotite --> kaolinite + vermiculite + sericite + N a +.
The vermiculites are potassic, as are the sericites.
Mineral reactions at grain contacts
These are evident in early stages of alteration. The new phase produced is a
function of the species reacting. Fig. 1 indicates the nature of these reactions. We
find that contacts between quartz and all other minerals produce no new phase.
Such contacts present a Becke fringe even in zones where weathering is well advanced.
This demonstrates the inactive nature of quartz during the early stages of the
weathering process in granites.
Other than these inactive zones, one can divide mineral contacts into two sorts;
those where one mineral is destabilized and the other is unaffected. This type is
Quortz
Biotite
Orthoclase
Plagioclase
M uscovite
FIG. 1. Sites of secondary minerals occurrence: (1) Neutral interface; (2) Simultaneous
destabilization; (3) Growing micaceous phase; (4) Entrapped minerals; (5) Diffusing
zone; (6) Cutane; (7) Internal destabilization.
238
A. Meunier and B. Velde
seen when muscovite and orthoclase are in contact. The muscovite (or sericite in the
case of La Rayrie samples) is unaffected whereas the orthoclase is 'invaded' by mica
grains growing at right angles to the grain contact. These new phases can attain
50/zm in length. These growth zones are particularly important where muscovite is
in contact with orthoclase. Sericite-othoclase contacts seem less active.
The other type of grain contact reaction shows both phases to be unstable where
a new mineral grows at the expense of both minerals. Muscovite-plagioclase contacts
are typical of this situation: the phase produced seems to be kaolinite at La Pagerie
and an expanding mineral at La Rayrie.
Internal destabilization is seen in plagioclase and muscovite at more advanced
stages of weathering. Here, the grains are transformed at different sites at their
interiors (Bisdom, 1967).
Diffusion of iron and titanium oxides is seen as accumulations of these oxides at
biotite grain edges. These accumulations are seen at times to have entered adjacent
muscovite grains along cleavage zones.
Clay mineral transport is seen to have occurred in the formation of cutanic structures
along small fractures and zones of mineral dissolution (Brewer, 1964). These zones
are found at grain contacts or within the grains when alteration is advanced. The
mineral deposited is largely kaolinitic, as shown by microprobe analysis. These
structures and their origin have been described by Tardy et al. (1973).
White mica compositions
A Cameca microprobe with a Tracor energy-dispersive analyser was used to obtain
X-radiation measurements on the micas. The results were processed using the Empdar
programme (Rucklidge & Gasparini, 1969). Tables give these results as relative
numbers of atoms calculated on a basis of forty-four negative charges (O2o (OH)4)
per unit formula. The beam current was 1-5 nA which maintained most minerals
stable, i.e. no alkali ion loss was noted over the counting period. The estimated
oxide totals for different minerals were often less than would be expected, as low as
66 wt ~. However, the ionic ratios calculated were usually quite reasonable. For
example, the AI-Si value for iron and alkali-free cutans was quite near 1, indicating
the presence of kaolinite. Mica oxide totals ranged from 90-95 ~ . Usually, larger
grains gave higher totals. This indicates that the density of the minerals is an important
factor in microprobe analysis, as should be expected. The finer grained products of
alteration are probably less dense, containing voids, than are well crystallized micas
or orthoclase. Thus oxide totals are low but ionic ratios remain constant.
Table 1 gives the results of the microprobe determinations for the three types of
micas found in the two outcrops. Primary white micas, muscovites, from La Rayrie
(R) and La Pagerie (P) are listed under part a of the table. Secondary sericites due
to recrystallization during tectonism from La Rayrie are found in part c. The micas
due to weathering in the first stages of granite alteration are listed for both outcrops
in part b of the table. Inspection of these numbers indicates that the primary
muscovites are rather consistent in composition; the elements K, Si and A1 show
less than 1 0 ~ variation between extreme values. The secondary sericites from La
Mineral reactions in granite weathering
\\o
0,5
0~
1
239
o
\
\
\
\e
o
\
oO
O
0
0.3-
9
O
~
9 "o~\]
-t-
eO / . ~ ._..." D
9 ~,L-j oD
/
0"2
~ " - -........O
/
D
0"1
0
I
1,0
I
I
}
I
r
i,5
I
[
t
I
i
2.0
Si/AI
FIG. 2. Fe and Mg amounts of the three white micas analysed. Relative amounts of
Fe+ Mg versus Si/A1 atomic ratios for the three mica types. Larger circules weathering
micas, squares sericites from La Rayrie.
Rayrie and the weathering micas from both outcrops show > 1 0 ~ variation for
these elements. The last two types of minerals show a distinct tendency toward low
alkali contents, well below that of ideal muscovite (2-0 atoms) and high silica values.
They can enter into the category of illitic minerals. One can distinguish between the
last two groups (Table 1 b and c) on the basis of total M g + F e ions (Fig. 2).
The 'weathering' micas are richer in iron and magnesium and thus more closely
approach low temperature illite compositions (Hower & Mowatt, 1966).
It is interesting to note that no mineral type is completely homogeneous in a rock.
There is considerable scatter of compositions for various grains in a thin section.
However, the average composition for each mineral type is different in each sample;
the muscovites in each outcrop are on the average different as are the weathering
minerals.
CONCLUSION
We see here that the reaction between mica and orthoclase during weathering
produces a mineral which contains considerable amounts of Fe and Mg, elements
present in lower quantities in the muscovites and virtually absent in the orthoclase
( < 0 - 1 % Fe and < 0.05 % Mg was found in these feldspars). This means that the
240
A. Meunier and B. Velde
new mica-like phases (or illites) grew in equilibrium with solutions which contained
elements derived from m i n e r a l s o l u t i o n other than at the mica-feldspar contact.
Yet the m a j o r chemical control during crystallization is the mica-feldspar equilibrium
b o u n d a r y . Thus the equilibria between these two minerals are more complex in a
n a t u r a l e n v i r o n m e n t t h a n those in the K-A1-Si-HzO system described by Hemley
(1959) a n d subsequently used as a model by n u m e r o u s authors. One m u s t consider
the full possibilities of mica solid solution toward phengites a n d toward illites when
describing the phase equilibria of these minerals.
F u r t h e r we see that micas or illites can be produced during the early stages of
granite weathering. They do n o t have the same composition as m e t a m o r p h i c sericites
b u t this difference would be difficult to detect should the minerals be mixed in a
geological process such as soil f o r m a t i o n . The two types of mica-like phase could be
mixed with mechanically b r o k e n muscovite to produce soil illite, a mineral species
which will have in this case c o m p o n e n t s with three different origins.
REFERENCES
BISDOME.B.A. (1967) Leid. Geol. Med, 37, 33.
BREWERR. (1964) Fabric and Mineral Analysis o f Soils, vol. 1,470 pp. John Wiley and Sons, U.S.A.
HEMLEYJ.J. (1959) Am. J. Sci. 257, 261.
HOWERJ. & MOWATTT.C. (1966) Am. Miner. 51, 825.
RUCKLID6E J. & GASPARINIE.L. (1969) Electron microprobe analytical reduction EMPADR VII.
Department of Geology, University of Toronto (Internal publication).
TARDY Y., BOCQUIERG., PAQUETH. & MILLOTG. (1973) Geoderma, 10, 271.
S O M M A I R E : On a 6tudi6 deux branches de couche qui rdv616rent la pr6sence de
granite dans les premiers paliers de d6gradation. On observa alors les r6actions entre
les esp6ces min6rales sp6cifiques et en particulier la formation d'une phase similaire ~t
celle de l'illite s6dimentaire. Cette phase poss6de une composition distincte de celle des
micas d'origine se trouvant dans les roches et, en particulier, sa teneur en fer et en
magn6sium. Ceci suggbre alors un 6quilibria entre le solide et la solution ce qui entraine
un transfert ionique au-del~, de la rdgion limitrophe des particules off le min6ral a
6t~ trouv6.
K U R Z R E F E R A T : Es wurden zwei Ausbisse untersucht, die Granit in den frfihen
Stadien der Verwitterung zeigten. Man beobachtete die Reaktionen zwischen spezifischen
Mineralarten und insbesondere das Entstehen einer illitartigen Phase, die tier yon
sediment/irem Illit ~ihnlich ist. Die Zusammensetzung dieser Phase ist anders als die
der ursprtinglichen Glimmer in dem Gestein, vor allem was den Eisen- und Magnesiumgehalt anbelangt. Dies deutet auf LOsung/Feststoff-Xquilibrien hin, die fiber den
Korngrenzbereich, in we!chem das Mineral gefunden wird, hinausgehende Ioneniibertragung bedingen.
R E F E R A T A : Se estudiaron dos afloramientos que mostraban granito en las etapas
iniciales de intemperizaci6n. Se observaron las reacciones entre especies minerales
especificas, y especialmente la formaci6n de una fase parecida a la ilita que es similar a
la de la ilita sedimentaria. Esta fase posee una composici6n distinta de la de las micas
iniciales en las rocas, particularmente en su contenido de hierro y magnesio. El[o
sugiere equilibrios soluci6n-s61ido que implican transferencia i6nica m~s all~ de la
regi6n de trabaz6n del grano en donde se encuentra el mineral.