weathering of glauconites - Mineralogical Society

Clay Minerals (1981) 16, 231-243. 

WEATHERING OF GLAUCONITES: REVERSAL OF 

THE GLAUCONITIZATION PROCESS IN A 

SOIL PROFILE IN WESTERN FRANCE 

C. COURBE, B. VELDE* AND A. MEUNIER 

Laboratoire de Pddologie de l'Universitd de Poitiers, E.R.A. n ~ 220 du C.N.R.S., 40 avenue du Recteur Pineau, 

86022 Poitiers Cddex, and *Laboratoire de Pdtrographie de l'Universitd de Paris VI, 4 Place Jussieu 75230 

Paris Cddex 05, France 

(Received 24 November 1980) 

A BST RACT: Polarizing microscope, electron microprobe and X-ray diffraction examination of 

minerals in a soil profile developed on a gtauconite sand indicate that destabilization of glauconite 

can be a progressive process which appears to be the reverse of glauconitization. Glauconite in 

these soils appears to be destabilized into a mixed-layer glauconite-nontronite phase, which 

crystallizes as a plasma mineral. This material in turn is transformed into smectite+kaolinite 

+ oxides. Loss of K and Fe is evident in whole rock as well as microprobe analyses of the samples. 

Thus glauconite can lose both Fe and K to aqueous solution during weathering, leaving 

aluminous clay minerals in the soil. 

It has been noted that plants growing on glauconite-bearing subsoils in a sector of the 

Loire Valley vineyards in France (Chinon, Bourgueil, Saumur) do not show symptoms of 

chlorosis. This suggests that soluble iron is available to the plants and that this iron comes 

from glauconites destabilized during weathering. The present investigation was under- 

taken to elucidate the chemistry of the destabilization process. The soil profile studied (at 

Chace, near Saumur, Maine-et-Loire) was developed on a Turonian (Middle Cretaceous) 

sand which contained considerable quantities of glauconite. 

EXPERIMENTAL METHODS 

Samples from different parts of the soil profile were treated in the following manner: 

(1) Whole rock analyses were made by flame and atomic absorption photometry 

(Jeanroy, 1972). 

(2) Glauconite grains were separated by electromagnetic means into magnetic and less 

magnetic fractions arbitrarily selected (Odin, 1969) within the grain size range 100-200 

#m. 

(3) X-ray diffraction traces (Co-K~) were obtained from samples treated with sodium 

dithionite/citrate (De Coninck & Herbillon, 1969) and saturated with K, Mg and Li 

(Robert & Tessier, 1974). Ethylene glycol was used to test the expandability of 

Mg-saturated material. 

(4) Differential thermal analyses were carried out in order to determine the nature of the 

iron oxides and hydroxides. 

(5) Thin sections were made of samples from various parts of the profile and after 

0009-8558/81/0900-0231502.00 9 1981 The Minerological Society

232 C. Courbe et al. 

examination using a polarizing microscope, chemical analysis of 2-3 #m diameter spots 

were made using a Cameca electron microprobe equipped with a Tracor-Northern 

solid-state detector and data treatment system. The beam current used at 15 kV excitation 

potential of the electron beam was 1-1.5 nA. This prevented alkali loss and breakdown of 

the clay minerals concerned. Data were treated using a ZAF type program. 

Description of the profile 

RESULTS 

Fig. 1 is a sketch of the profile. The lowest zone is unaltered glauconite rock, fissured by 

oxidized and argillized material. 

Zone 1. The unaltered rock is a fine grained sand with 77.3% of the grains between 100 

and 200 #m in diameter. Glauconite comprises about 30% of the sand. Most of the 

glauconite is present as grains but a small portion occurs as glauconite matrix material 

~ ~ 411 ~ ~ -- 

- - -- :3b _ - - _ _ - - 

a; 

=,.'...::...::%: 

10 c m 

%:..~c:: "'2")' ' 

FtG. 1. Weathered profile developed on glauconitic sand at Chace, near Saumur. Five zones are 

recognizable under an allochthonous layer of Tertiary sand (circles); a, b, c, d: colour or textural 

modifications inside the level as described in the text. Drawings at the side of zones 1 a, 3b, 4a and 

5d show the microstructure of each level. Q=quartz; G =glauconite; A= argillan; P=pore; 

GP = glauconite plasma; SAP = secondary argillaceous plasma.

Weathering of glauconite 233 

filling interstices. This green material is termed glauconite plasma of secondary origin. 

Zone 1 is subdivided into 1 a, the least altered horizon, which contains fissures lb and lc, 

and an ochre zone 1 d. 

Zone 2 (50 cm) is characterized by a yellow-brown colour which intensifies towards red 

hues in fractures. The grain size distribution is similar to that of zone 1. 

Zone 3 (40 cm) represents a transition between the lower and upper, argillized, portion 

of the profile. This uppper part (3b) shows an increase in secondary argillaceous plasma 

materials, present as veinlets. It is brown in colour. 

Zone 4 (30 cm) is characterized by ferruginous concretions especially at the top (4b) 

where they represent about 159/0 of the sample; the clay fraction represents 25~ of the 

sample. Secondary veins of green glauconite plasma pervade the zone. 

Zone 5 (55 cm) shows prismatic structure. The clay content increases to 35~ of the 

sample and the general colour is less intense, contrasting with the green and red-brown 

hues of the lower zones. 

Photomicrographs of samples from different zones are shown in Fig. 2. 

In general, on moving up the profile there is a decrease in quartz content and also in 

grain glauconite content, these changes being balanced by formation of the argillaceous 

plasma and, to a lesser extent, by an increase in porosity. These transformations are 

reflected in changes in the bulk composition at each level (Table 1). 

Three samples were chosen for detailed study following preliminary XRD examination 

of the glauconite and argillaceous portions of the profile samples. These were the least 

altered sample (la), a sample from a horizon rich in glauconite plasma (4a), and a sample 

from the top of the profile (5d), where alteration was well advanced. 

FIG. 2. Photomicrographs of glauconites and argillaceous plasma: (1) zone la, round-grain 

glauconites; (2) zone 3b, formation of glauconite plasma; (3) zone 4a, formation of secondary 

argillaceous plasma; (4) zone 5d, secondary argillaceous plasma and argillans. G = glauconite; 

Q = Quartz; Fk =potassic feldspar; M = Muscovite; GP =glauconite plasma; SAP = secondary 

argillaceous plasma; Ox = Oxide/hydroxide concretions; A = argillans.


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Least altered horizon ( la) 

This sample contained normal granular glauconite as well as secondary plasmic mica, 

both of which had a green colour under the microscope. A small portion had a fibrous 

appearance and higher birefringence. Microprobe analyses (Table 2) indicated that the 

minerals were rich in K atoms (0.5-0.7) while Fe atoms ranged from 1.10 to 1.30. XRD 

indicated 1Md+minor 1M polymorphs (Burst, 1958) and hence a disordered structure. 

There was a scatter in the composition of the glauconites, which appears to be normal for 

these minerals (Velde, 1976). A 15-20~ variation was noted between grain centres and 

edges. The centres tended to be rich in K and Fe. Some glauconite grains showed a brown 

colour but were not distinguished by either K20 or Fe203 contents. Even though they 

appeared altered and oxidized, they had not undergone significant chemical trans- 

formation. 

Zone of glauconite plasma (4a) 

Microscopic observation of this zone indicated that the glauconite grains destabilize to 

give a plasmic green glauconite and iron oxide/hydroxide concretions. Goethite appeared 

in X-ray patterns. In places the plasma was transformed into argillaceous matrix which 

contained highly birefringent platelets of clay minerals, sometimes oriented around 

quartz grains. Pore spaces were frequently coated with argillan material (Brewer, 1964). 

The glauconite grains had much the same composition as those in the less altered 

horizon 1 a (Table 3). Sometimes they contained more Fe, probably in hydroxide form. 

DTA curves indicated goethite (endotherm at 280~ and amorphous iron oxide 

(exotherm at 340~ XRD of separated grains showed slight structural differences when 

compared with material of the same electromagnetic properties from sample la. The 

diagnostic hkl reflections for the 1M structure were less evident and the 001 band was 

displaced to larger spacings. Thermogravimetric analyses indicated a 10.13~ weight loss 

compared to 8"46~o in sample la. Glauconite plasma was much more abundant than in 

sample 1 a and its composition was clearly less potassic and iron-rich than the glauconite 

grains in zone 1 a, or those still present in the sample. Several showing especially low K and 

Fe contents were probably mixtures of glauconite mixed-layer mineral (Burst, 1958) and 

another clay mineral. 

The less magnetic and non-granular material extracted represented, in large part, the 

argillaceous matrix. XRD showed smectite, kaolinite and some mica, this probably 

representing the destabilization of the mixed-layer glauconitic mineral. Microprobe 

analysis indicated further loss of K20 and Fe203 compared to glauconite grains and 

glauconite plasma (Table 3). 

Highly altered zone (5d) 

In this zone the glauconite grains and glauconite plasma were largely destabilized, 

being replaced by argillaceous plasma. 

The separated glauconite grains were more expandable than in the less altered zones. 

Weight losses of these grains were still greater than those from lower levels (12.18~o), 

indicating the presence of more expandable material; microprobe analyses showed lower 

K20 and Fe203 contents (Fig. 3, Table 4). The glauconite plasma also showed a potassium

e atom s 

.. 

Weathering of glauconite 

o 

x 

x 

o 

o o x 

x o 

o OoO 

o 

0 

0 

05 Katoms 

FIG. 3 Fe/K composition of glauconite from zone 5d. Circles=green glauconite grains; 

crosses = brown glauconite grains; triangles = green glauconite plasma; dots = secondary argilla- 

ceous plasma. 

and iron loss. The argillaceous plasma contributed to the prismatic structure of the 

horizon. Analyses of this material suggested the presence of significant amounts of 

kaolinite and iron oxides as well as smectite. XRD confirmed this but no pure kaolinite 

zones (AI = Si, lack of alkalis and iron) were found using the microprobe. Appreciable 

amounts of Ti (1~ as oxide) were contained in the iron concretions of zone 5b. 

Small amounts ( < 2~) of C 1 and S were found occasionally in the plasma zones. The 

secondary plasma zones nearest to the argillan material coating pore zones showed, in 

some instances, evidence of a micaceous or mixed-layer mineral which contained little Fe 

(Fig. 2). These zones contained minerals of high birefringence which were almost 

colourless or slightly yellow. Using the Hoffman-Klemen test (1950) on fine clays from 

this zone, it was shown that the smectite present was montmorillonitic, in contrast to the 

beidellite-nontronite found in other parts of the profile. Thus a new aluminous potassic 

mineral was formed during weathering in areas of high porosity. 

DISCUSSION 

The data indicate that there is a loss of K and Fe in the glauconitic materials towards the 

top of the weathering profile. It is also evident that weathering changes the mineralogy 

progressively, i.e. glauconite (micaceous) is replaced by a mixed-layer phase then smectite 

plus kaolinite and oxides. Iron is concentrated in concretionary form in mid-profile (zone 

4). The uppermost parts of the profile contain little Fe, yet newly formed potassic minerals 

are present. A general increase in kaolinite content is noted. 

With the aid of the phase diagram proposed by Velde (1976) it is possible to construct a 

reasonably coherent sequence of events (Fig. 4). By placing the microprobe analyses for 

239


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MLAI - MLFe 13.0x 

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AI Fe 

FIG. 4. Composition of green and brown glauconite grains, glauconite plasma, secondary 

argillaceous plasma and argillans in zone la (dots), zone 4a (crosses) and zone 5d (circles) on 

K A1-Fe coordinate phase diagram (Velde, 1976). Mo=montmorillonite; N=nontronite; 

Ox=iron oxide; Kaol=kaolinite; MLal=aluminous mixed layered phase; MLFe=iron-rich 

mixed-layer phase; I = illite; G = glauconite. 

K, A1 and Fe as coordinates, a reasonably tight grouping of the original glauconite grains 

is seen, this being followed by a larger compositional spread for the green glauconite 

plasma. This new material contains a greater portion of expandable layers compared with 

initial sedimentary glauconites of the sand. The probe analyses of plasma material 

(smectite plus kaolinite and oxides) plot in a sector of the diagram which should contain 

these minerals. It is interesting to note that potassic minerals of high birefringence found 

in the secondary argillaceous plasma of the upper zone fall into the mixed-layer 

illite-montmorillonite compositional sector of the proposed phase diagram. Thus in some 

zones the activity of Fe in solution seems to be greatly diminished while potassium activity 

remains at a relatively high level in order to produce an aluminous illite or mixed-layer 

mineral. There seem to be zones in the upper levels of the profile which have higher Fe 

activity in solution than others. Generally, the following sequence of mineral parageneses 

occurs as alteration becomes more intense: 

(i) glauconite; 

(ii) glauconite + mixed-layer glauconite + iron oxide; 

(iii) smetite + kaolinite + Fe, Ti-oxide or mixed-layer glauconite + aluminous mixed-layer 

illite mineral. 

CONCLUSION 

The most significant point to emerge from the present investigation is the progressive 

change in glauconite mineralogy which produces mixed-layer minerals similar to those 

encountered in sedimentary glauconites. Other workers (e.g. Wolff, 1967) have shown 

that weathering of glauconite produces a kaolinite+ oxide reaction product. Yet the 

profile studied here gives a sequence which is the reverse of glauconitization (Hower, 

241


1961; Velde, 1976). This suggests that glauconitization is a reversible process under 

low-temperature conditions and thus should be considered as a stable reaction or at least 

a reversible metastable reaction. 

The result of glauconite weathering in the profile is to release K and Fe ions into the 

altering solution. These solutions react to produce new mixed-layer minerals in a 

step-wise process which gives an iron-rich or aluminous trend depending upon the 

position in the rock fabric. Materials in close contact with glauconitic minerals change 

into nontronitic minerals while those near pores, in closer proximity to solutions from the 

surface, produce iron-poor phases. Two mineral trends are then evident as shown in 

Fig. 3. Much Fe is lost to solution as shown by bulk 'rock' analyses. 

Returning to the initial observation concerning the vineyards: since glauconitic 

weathering in these profiles releases iron to the solutions, this can be incorporated by the 

plants thus preventing chlorosis. It is necessary that the roots of the plants reach levels just 

above the glauconitic plasma zone where iron release is greatest. Also, the high potassium 

activity should be beneficial to the plants and thus one might consider glauconites as a 

natural mineral fertilizer, at least for vineyards. 

REFERENCES 

BPJ~WER R. (1964) Fabric and Mineral Analysis of Soils, p. 470. Wiley, London. 

BURST, J.F. (1958) Mineral heterogeneity in glauconite pellets. Am. Miner. 43, 481-497. 

DE CONINCK F. & HFa~BILLON A.J. (1969) Evolution min~ralogique et chimique des fractions argileuses dans les 

alfisols et spodosols de la Campine (Belgique). Pedologie XIX, 159-272. 

HOFF~AN U. & KLEIN R. (1950) Verlust der Austanochf~ihigkeit yon Lithiumionen an bentonit durch 

Erhitzung. Zeitsch. anorg, chem. 262, 95-99. 

HOWEg J. (1961) Some factors concerning the nature and origin of glauconite. Am. Miner. 47, 886-896. 

JEANROV E. (1972) Analyse totale des silicates naturels par spectrophotom6trie d'absorption atomique. 

Application au sol et ~ ses constituants. Chimie Analytique 54, 159-166. 

OolN G.S. (1969) M~thode de s~paration des grains de glauconie. Int6r~t de leur ~tude min6ralogique et 

structural. Rev. Gdol. Phys. Gdol. Dynam. XI, 171-176. 

ROBERT M. & TESSIER D. (1974) M6thode de pr6paration des argiles des sols pour les 6tudes min6ralogiques. 

Ann. Agron. 25, 859-882. 

VERDE B. (1976) The chemical evolution of glauconite pellets as seen by microprobe determinations. Miner. 

Mag. 40, 753-760. 

WOLFF R.G. (1967) X-ray and chemical study of weathering glauconite. Am. Miner. 52, 1129-1138. 

Rl~SUM]~: Les observations microscopiques, les analyses/t la microsonde et les d~terminations 

par diffractions de rayons-X sur un sol d~velopp~ sur un sable glauconieux, indiquent que la 

d~stabilisation de la glauconie intervient progressivement suivant le processus inverse de la 

glauconitisation. Elle s'alt~re en dormant un interstratifi~ glauconie--nontronite qui cristallise 

dans un plasma glauconieux. I1 se transforme/L son tour dans un plasma secondaire argileux 

compos6 de smectite + kaolinite + oxyde. La perte de potassium et de fer au niveau de la glauconie 

entralne la formation de phases argileuses alumineuses darts le sol. 

KURZREFERAT: Die Untersuchung yon Mineralen in einem aus Glaukonitsand entwick- 

elten Bodenprofil mittels Polarisations-mikroskopie, Elektronenmikrosonde und R6ntgenbeu- 

gungsanalyse zeigt, dab die Glaukonitzersetzung als ein fortschreitender Prozel3 gesehen werden 

kann, der scheinbar in Umkehrung zur Glaukonitbildung abl/iuft. Offensichtlich wandelt sich der 

Glaukonit dieser B6den in eine wechselgelagerte Glaukonit-Nontronit Phase urn, die als 

glaukonitisches Plasma auskristallisiert. Im weiteren Verlauf wird dieses Material in Smectit+ 

Kaolinit+Oxide umgebildet. Ein Verlust an K und Fe ist im Ausgangsgestein ebenso 

offensichtlich wie in den Mikrosondenanalysen der Proben. Daher kann Glaukonit Fe und K im 

Zuge der Verwitterung in w/issrige L6sung abgeben und dem Boden aluminiumhaltige 

Tonminerale hinterlassen.


RESUMEN: E1 examen de un perfil de suelo desarrollado sobre una arena glauconitica a trav6s 

del microscopio de polarizaci6n, microsonda electr6nica y difracci6n de R-X indica que la 

desestabilizaci6n de la glauconita es progresiva siguiendo un proceso inverso al de la 

glauconitizaci6n. La glauconita se altera dando lugar a un interestratificado glauconita-non- 

tronita que cristaliza en un plasma glauconitico que se transforma, a su vez, en una mezcla 

compuesta pot smectita, kaolinita y 6xidos. La p6rdida de potasio y hierro por parte de la 

glauconita conlleva a la formaci6n de fases aluminicas en el suelo. 

243

weathering of glauconites - Mineralogical Society

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