Lorenzoni, P.

Quantin, Paul

Horizon / Pleins textes

Miner. Petrogr. Acta 
Vol. XXXV-A, pp. 289-296 (,1992) 
Weathering of leucite to clay minerals in tephrites 
of the Vico volcano , 
$AUL QUANTIN" and PAOLO LORENZONI"" 
** Ist. Sper. Studio e Difesa Suolo, P.za D'Azeglio 30, 50121 Firenze, Italy 
Orstom, 72 Route d'Aulnay, 93143 Bondy, France 
ABSTRACT - The weathering of leucite has been studied in tephrites of the 
Vico volcano. The petrography was made on undisturbed samples by optic 
and scanning electron microscopy with microprobe analysis. We follow a 
weathering sequence of leucite crystals from a core of lava to a first altered 
cortex and then in sandy and clayey arene of weathering. Then we compare 
the alteration in the B and A horizons of an overlying andosol, deriving from 
tephrite pyroclasts under a perhumid climate. We can observe also the leu- 
cite alterationin a ((petriscos (leucotephrite) under a xeric climate. The clay 
minerals composition was obtained from a powder of-whole material and 
the less 2fim fraction, by X Ray diffraction and TEM. The alteration of leu- 
cite to analcime in the first cortex is inherited from a former hydrothermal 
process. While clay minerals are forming by further weathering in relation 
with drainage intensity, e.g.: allophane and imogolite in andosols under 
perhumid climate and with high drainage; halloysite under less humid cli- 
mate or  with slower drainage. 
Key words: Leucite, Weathering, Analcime, Allophane, Imogolite, Halloysite, 
Vico-volcano. 
Introduction in an incoherent and more altered tuff. 
On the impulse of L. Lulli and D. Bidi- 
ni we made a study of soils on the vol- A. Cundari and G. Graziani (1964 cano of Vico (Bidini and al. 1984; Lulli 
studied first the weathering products of .and al. 1986, and 1988). P. Quantin and 
leucite in the tephrites (leucotephrites P. Lorenzoni (1 980) made especially 
and petrischi = acid leucotephrites) of studies of mineralogy and micromor- 
the Vico volcano. They showed that leu- phology. We have namely observed the 
cite is generally altered into analcime, or alteration of phenocrysts and volcanic 
eventually to kaolinite (probably halloy- glasses in various parent materials (ig- 
site after new investigation) and al- nimbrites, tuffs and leucotephrites), ac- 
lophane. E. Azzaro and al. (1976) studied cording either to their situation in a 
later the alteration processes of leucite (( climotoposequencex or to the either 
in the volcanic tuffs of Bracciano. They loose or cemented texture of rock. 
observed the formation of r analcime Leucite is abundant in tephritic lava 
when the original features-of tuf€*are r
maining, whereas 10 Å halloysite appea 
laws or ash falls in and 
era.,The old leucotephr 
9. R.S.T.O.M. Fonds Docum6ntah. 
Mo 5 -31 4 4 6  Qp.4 
CDte : 8 
290 P. Quantin, P. Lovenzoni I ,  
a 
I I  *4 I D  11 1, IO . 7 0 . 1 2 
Degrees 28 
Inaltmred 
aphr i te  
IC 
Iw 
Fig. 1 - XRD on whole powder of ((arene, and andisol horizons. A analcime; I-Sm interstratified; L 
leucite; CP augite; Sa sanidine; P1 plagioclase. 
Weathering of leucite to clay minerals ... 29 1 
ly weathered at the soil bottom, under 
either the form of ccpetriscoa (altered 
acid leucotephrite) on the outer pied- 
mont of the volcano, or the form of 
<carene), on the upper edge of the caldera. 
The aim of this paper is to show some 
features of leucite alteration in the 
ccarenea or in the ccpetrisco), substratum 
and in an andisol as well. 
Materials and methods 
Mate vial 
The weathering of leucite has been ob- 
I 
E 
f 
I 
C1.y.y 
'arene' 
Bc 
Bw 
4 
I Fig. 2 - XRD on <2pm fraction of <carene), and andisol horizons. H 10 Å halloysite; mH 7 A halloysite; 
F feldspar; Q quartz. 
292 P. Qtinntin, P. Lorenzoni 
served in an andisol (melanudand) and 
the underlaying (carene), of leucotephrite 
(see soil profiles no 40 and 111, in Lulli 
and al 1990, p. 90-93). This andisol is situ- 
ated at the upper border of the caldera. 
We studied successively in the <tarene>,: 
the kernel of unweathered lava, the cor- 
tex of weathering, the sandy <carenen and 
the clayey ((arene),; then in the andisol: 
the B and the A horizons. We have also' 
studied the white clayey alteration of leu- 
cite in a <(petrisco,,, at the bottom of a 
A 
< 3.000 x : 2thetr I) : 2702. Linear 50.010 
18r&T Na8.61K0.39LlSi308 Sanidime wtassian disordered syn R12Si205(OH)4.2H20 Halloysite F) 
9-6453 Q12Si205(OH)¶ Halloyslte R 
H 10 I\ halloysite ; mH 7 À halloysite ; Sa sanidine B 
N normal - hydrated 
S dried at 100' C 
H 300 heated at 300' C 
H 400 heated at 400°C 
I L I 
< 3.800 OuPvlinr x : 2thcta u : 3070. Linear 50.010) 
Fig. 3 - XRD on altered leucite of ((petriscor; A whole powder; B <2pm fraction. H 10 A halloysite; 
mH 7 A halloysite; Sa sanidine. 
Weathesing of leucite to clay minerals ... 293 
brown soil, on the outer piedmont of the 
volcano. 
In the first site the soil has been 
formed under a temperate and regular- 
ly wet climate, while in the second site 
the soil has been formed under a climate 
with dry season. 
M e  tho& 
For mineralogy and petrography we 
have used the following methods. 
- X Ray diffraction: on a powder of 
whole material and on an oriented < 
2pm fraction (normal, dried at 100°C, 
heated at 5OO0C, glycolated). 
- TEM microscopy: on <2pm fraction. 
In the case of a petrisco, we have ana- 
lysed only the white powder of 
weathered leucite. 
- Optic microscopy: on thin sections of 
samples of the 4 layers of the ccarenea 
and the A and B horizons of the andisol. 
- SEM microscopy and ccin situ), chem- 
ical analysis by microprobe: on the same 
thin sections used for O.M. of the 
I <carene)) and the andisol; and in addition 
on a crystal of weathered leucite from 
the petrisco. 
Results 
Mineralogy  
1) Parent material: The leucotephrite, 
at the bottom of the andisol is constitut- 
ed of around 50% of alkaline basaltic 
glass, 30% of leucite and 20% of 
labradorite phenocrysts, as well as some 
microcrystals of apatite, Ti-magnetite 
and olivine. The petrisco (named acid leu- 
cotephrite by Cundari and Graziani, 
1964) is a leucitic phonolite where sani- 
dine is substituted to labradorite. 
2) Powder of whole material of the 
<carene>> and the andisol (Fig. 1): In addi- 
tion to the minerals of the parent materi- 
al, some products of weathering appear, 
like analcime and clay minerals, while 
the content in leucite is decreasing. In 
the <carene,> there are two main products: 
analcime and 10 Å halloysite; the last one 
is increasing in the clayey arene. In the 
andisol, the analcime and the leucite are 
diminishing; but there are only traces of 
crystallized clay minerals, because the 
clays are para-crystalline (allophane, im- 
ogolite). 
3) < 2pm fraction. (Fig. 2): In the san- 
dy and clayey cearene)), 10 Å halloysite 
partially dehydrated at 7 Å is the 
predominant mineral. In the C horizon 
at the bottom of soil we still observe the 
presence of analcime, mixed mainly with 
imogolite and with some traces of 10 A 
halloysite. In the B horizon of andisol the 
fibrous imogolite is predominant, while 
in the A horizon we observe mostly 
spherical allophane and a little of halloy- 
site. In this latter horizon we note some 
traces of quartz due to a probable con- 
tamination by rhyolitic ash fall, near the 
surface of soil. 
In the >>petriscon, the white product of 
leucite weathering is made of a pure 10 
Å halloysite, partially dehydrated at  7 Ål 
and mixed with some relicts of sanidine 
(Fig. 3). 
. 
Op fie microseop y 
We will describe only the various 
forms of alteration of leucite. 
294 P. Qtiaiztiiz, P. Lorenzoni 
- In the kernel of almost unaltered leu- 
cotephrite, leucite looks almost intact, 
polygonal, with polysynthetic twinning; 
it shows some inclusions of labradorite 
microlites. 
- In the cortex, the leucite is altered 
around its margin and along some radi- 
al fissures, leaving some islets of un- 
altered mineral. 
- In the sandy earenen as well as in the 
clayey one, the leucite is almost com- 
pletely altered, in a light yellowish 
product, which is isotropic in cross-light. 
It remains only few islets of unaltered 
mineral. Sometimes we observe some 
recrystallization of zeolites at the mar- 
gin of the crystal. We note also in the 
clayey (carene)), some palagonitized glass- 
es, which could indicate a product of 
phreato-magmatic outburst. 
- In the B horizon of the andisol, the 
leucite again is almost completely al- 
tered to a light brown and isotropic 
product, with some dull yellowish verge 
at the margin. 
- In the A horizon, the small crystals of 
leucite which are isolated in the soil 
matrix are completely altered to a dull 
or light brownish and isotropic product; 
while the phenocrysts of leucite which 
remain included in the pyroclasts of 
tephritic .lava are not completely altered, 
leaving some unaltered islets of the 
primary mineral. (That means a latter 
eruption of tephritic pyroclasts). 
Scanning Electron Microscopy (SEM) and 
microprobe analysis 
We have summarized some relevant 
results of microprobe analysis, after nor- 
malisation in percentage of the main ele- 
ments, on the anhydrous oxides basis 
(Tab. 1). The <(in situ), microprobe analy- 
sis allows us to specify the chemical 
composition of leucite and of its differ- 
ent forms of alteration we have observed 
by optic microscopy and further by SEM. 
The SiO,lA1,0, molar ratio (Ki) provides 
a good estimate of the corresponding 
mineral, either leucite for a value of 4.5 
and high contents in K,O, or analcime 
for 4.0 and high contents in Na,O, or 
halloysite for 2.0, and finally imogolite 
for 1.1 or allophane for 1.8, according to . 
the previous data of mineralogy. 
The pure leucite in lava kernel <ce, has 
a low content in Na. But in the cortex, 
<<da, the leucite center is altered to a /  
TAB. 1 , 
Microprobe analysis of leucite and its alteration products , 
SiO, Alzo, Fe,O, MgO Ca0 Na,O K,O Ki 
e kemel 58 22 0.4 0.0 o. 1 0,9 19 4.5 
d cortex 61 26 0.5 0.2 o. 1 1 1  0.9 4.0 
b sand-arene 51 43 1.5 0.3 0.3 1-2 1-2 2.0 
a clay-arene 52 44 1,7 02 0,3 0.0 1.1 2.0 
B horizon 39 59 0,7 0,1 0.8 0.0 o, 1 1.1 
A horizon 49 46 1,7 0.5 o. 1 0,4 O J  1.8 
P petrisco 53 44 0,7 0.2 0.7 0,9 0.0 2.0 
V . glass 52 19 10 1.7 5.8 4.1 4.3 4.6 
Weathering of leucite to clay minerals ... 295 
composition cprresponding to analcime 
(K replaced by Na). In sandy <carene b), 
a major part of leucite is weathered to 
a composition of halloysite, mixed with 
a little of zeolites. In clayey (carene a), 
and petrisco, leucite is almost complete- 
ly weathered to halloysite composition 
(K,=2.0). *In the B horizon of andisol, 
leucite is fully weathered to isotropic 
plasma of imogolite composition 
(Kl=l.l); but in the A horizon, leucite is 
less weathered to a more siliceous 
product of allophane (KI= 1.8) composi- 
tion. Around the leucite the volcanic 
glass w-V), is of Na and K rich 
trachybasaltic composition, which is the 
source of Na to form analcime. 
and with rather slow drainage (Quantin 
1991). However, some relicts of zeolites 
can rimam. That confirms the observa- 
tions of Cundari and Graziano (1964), 
about the presence of analcime, al- 
lophane and ((kaolinite)> in the altered 
leucite of a petrisco. 
3. In the B horizon of andisol, the leu- 
cite transformation to a fibrous imogo- 
lite, by a strong evacuation of Na, K and 
Si, is due to a weathering process under 
perhumid climate and high drainage 
(Quantin 1991). However in the A 
horizon, the more siliceous allophane 
formation means a less advanced stage 
of weathering, in the presence of relicts 
of volcanic glass and of a high humus 
content. 
Interpretation 
Conclusion 
1) In the ((cortex),, the diffusion of Na 
from the glass to the leucite center can 
explain the formation of analcime. That 
is the result of a hydrothermal and alka- 
line process of alteration (Azaro and all. 
1976), during a hydromagmatic eruption. 
2) In the (carene,) and the epetrisco)), 
the formation of halloysite, by evacua- 
tion of Na, K and a part of Si, is a 
weathering process under humid climate 
The alteration of leucite to analcime 
is inherited from a former hydrothermal 
process. Clay minerals are formed by 
further weathering, in relation with 
the rainfall drainage intensity: allophane 
and imogolite under perhumid climate 
and with high drainage; halloysite 
under humid climate, or with slower 
drainage. 
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Weathering of leucite to clay minerals in tephrites of the Vico volcano

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