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a Institut des Sciences de l'Environnement, Univ. du Québec à Montréal, C.P. 8888, succursale Centre-ville, Montréal, QC, Canada H3C 3P8, present address: Univ. de Moncton, Moncton, Nouveau-Brunswick, Canada E1A 3E9
b CAMPARIS, Laboratoire de Minéralogie-Cristallographie, Univ. de Paris 6, 4 Place Jussieu, 75252 Paris, France
c Univ. du Québec à Montréal, C.P. 8888, succursale Centre-ville, Montréal, QC, Canada H3C 3P8
jolicos{at}umoncton.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Abbreviations: AMS, Archer Mountain Suite • RVF, Rockfish Valley Fault • SEM, scanning electron microscopy • XRD, x-ray diffraction
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Given the abundant rainfall and warm temperature of the southeastern USA, well drained acidic materials experience pedoclimatic conditions responsible for a high weathering intensity (Rebertus et al., 1986). The pseudomorphic weathering of biotite may lead to the formation of sand-sized vermicules of kaolinite within saprolites. These books provide clay-sized particles to pedoturbated horizons upon the breakdown of the less elastic kaolinite grains (Cady, 1950, Rebertus and Buol, 1985). The exact process of kaolinization is unclear. Some authors consider vermiculite to be a necessary intermediate (Rebertus et al., 1986), others do not (Harris et al., 1985b). Grant (1964) proposed two-unit cells of biotite to one-unit cell of kaolinite conversion, while Harris et al. (1985b) proposed a layer-to-layer replacement of biotite by kaolinite, with an Al deficit compensated by Fe, yielding an Fe-rich kaolinite. Rebertus et al. (1986) proposed a one-unit cell of biotite to 1.5-unit cells of kaolinite transformation model, with a necessary addition of Al released by weathering and precipitated within interlayers of the altering biotite. Ahn and Peacor (1987) proposed a one biotite layer to two kaolinite layers transformation involving dissolution of biotite and recrystallization of kaolinite at linear boundaries. According to Graham et al. (1989b), the formation of gibbsite could result from the weathering of pseudomorphs of kaolinite after biotite, following the complete loss of Si.
Studying the influence of biotite weathering on soil development and mineralogy, Rebertus et al. (1986) consider that it results in an anti-gibbsite effect (Jackson, 1963a, 1963b) which would be responsible for the low content of gibbsite in soils developed from biotite-rich parent rocks. The presence of mica and its weathering to vermiculite would provide preferential interlayer absorption sites for aluminium released by mineral weathering, thereby keeping the Al-activity at low levels in the soil solution and inhibiting gibbsite formation. Losche et al. (1970) explain the absence or occurrence of gibbsite in soils developed on granitic biotite gneiss of North Carolina by an anti-gibbsite effect associated with slope aspect. According to them, warmer south-facing soils show higher gibbsite contents while cooler conditions on north-facing soils are conducive to kaolinite and Al-interlayered mica–vermiculite formation, which thus prevents the formation of gibbsite. However, Daniels et al. (1987) did not recognize this relationship in a study of soils from an adjacent area.
Jolicoeur et al. (1995) investigated the mineralogical and chemical characteristics of the saprolites and soils developed from granulitic, monzonitic, and charnockitic gneisses and mylonites of the Blue Ridge Mountains and western Piedmont foothills of central Virginia. Biotite was found to pseudomorphically weather to kaolinite, or to a kaolinite and gibbsite assemblage. Although this latter has been mentioned in the literature, the development of mainly gibbsitic pseudomorphs has not been observed before in these soils. Our objectives are (i) to document the weathering of biotite to kaolinite and gibbsite within saprolites and soil profiles developed from felsic rocks on south-facing slopes of the Piedmont and Blue Ridge Mountains, (ii) to investigate the factors governing the weathering of biotite to gibbsite at specific sites, rather than the more widespread weathering of biotite to kaolinite, and (iii) to discuss the implications of these findings on the proposed anti-gibbsite effect, and the kaolinization of biotite in southeastern soils.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Profile C4 saprolite and Ultisol have developed in a granoblastic to ultramylonitic charnockitic gneiss (Henika, 1981). The parent-rock mineralogical composition includes orthoclase, plagioclase, quartz, myrmekite, hypersthene, hornblende, chlorite, biotite, apatite, zircon and ilmenite. The ultramylonitic fabric is composed of subhorizontal chloritic foliation planes. Profile G1 saprolite and Inceptisol have developed in the Hills Mountain Granulite Gneiss (Sinha and Bartholomew, 1984). This latter contains perthitic feldspar, plagioclase, quartz, hypersthene, hornblende, chlorite, biotite, muscovite, clinozoisite, zircon, and ilmenite. Profiles M2 and M3 have developed in the Archer Mountain Suite (AMS) proto-mylonitic monzonitic gneisses (Sinha and Bartholomew, 1984); Profile M2 is a Typic Hapludult, formed in saprolite while Profile M3 is a Typic Dystrochrept formed in saprolite. The AMS gneiss contains perthitic feldspar, microcline, plagioclase, quartz, myrmekite, biotite, muscovite, apatite, clinozoisite, zircon, and ilmenite. Profile M4 saprolite and Ultisol have developed in AMS mylonites, which is closed to the AMS gneiss in composition but within which feldspar is absent at the sampled site. The parent-rock and saprolites of the three profiles show near vertical biotite-rich foliation planes.
Bulk samples were taken in each soil horizon and nonsaprolite (no preservation of the parent-rock fabric) and saprolite residuum units, for physical, mineralogical, and chemical analyses. Undisturbed samples were taken for preparation of thin sections and petrographic observations. Profiles were sampled with hand tools, excavating as deeply as practicable into the saprolite. Profiles were described by procedures outlined in the Soil Survey Manual (Soil Survey Staff, 1951).
Laboratory
Thin sections for soil profile horizons and nonsaprolite and saprolite residuum units were described, using terminology from Brewer (1976).
Part of the XRD analyses were accomplished at Agriculture Canada (Ottawa, Ontario), using CoK
radiation and a Scintag PADV automated powder diffractometer equipped with a graphite monochromator. Clay-size samples (<2 µm) separated by sedimentation were saturated with Mg and K and freeze-dried. Potassium-saturated samples were used for glycolation and heating treatments. Clay- and silt-size samples, and sorted mineral grains and soil material were analyzed at the Laboratoire de Minéralogie-Cristallographie (Univ. de Paris 6 et 7, Paris, France), using CuK radiation and a Philips PW1710 diffractometer. Random powder mounts were analyzed at 25°C. Oriented samples were analyzed at 25°C and after glycolation and heating treatments (300 and 550°C).
Residuum fragments and aggregates of mineral grains were examined by SEM using a JEOL-JSMT 20 stereoscan electron microscope (Laboratoire de Minéralogie, Univ. de Paris 7). The SEM examination was accompanied by elemental analysis by means of an energy-dispersive x-ray analyzer (SEM-EDXRA) (Institut français de recherche scientifique pour le développement en coopération, Bondy, France).
Quantitative microprobe analyses were performed using a CAMEBAX microprobe (CAMPARIS, Univ. de Paris 6) equipped with four wavelength dispersive spectrometers, operating at 15 kV, 10 nA and with counting times of 10 s per element. The microprobe was equipped with an energy dispersive spectrometer which was used for qualitative elemental analyses of samples.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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X-ray Diffraction Analysis
Weathered biotite flakes from the saprolite of profile C4 have been mechanically sorted using sieves, and analyzed by XRD (Fig. 3a) . The XRD pattern show a main peak at 10.2 
and a broad, lower intensity peak at 7.1-7.4 . The 7 peak indicates kaolinite and/or 7 halloysite. A slight widening of the 10 peak may be noticed at its base. This widening could be due either to 10 halloysite or to incipient vermiculite layers. Nevertheless, no halloysite particles (glomerules or tubes) were observed with SEM. Sand-sized pseudomorphs (vermicules) have also been sorted and analyzed by XRD (Fig. 3b), which indicated mainly well ordered kaolinite and minor amounts of mixed layer mica–vermiculite. Similar pseudomorphs from the saprolite of profile G1 were nearly pure kaolinite (Fig. 3c).
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Microprobe Analysis
Microchemical analyses of biotite and its weathering products from saprolite of profile C4 are given in Table 1
. As the optical properties of biotite are lost and the layers resemble vermiculite and kaolinite, there is a correlative evolution from a mica composition to a kaolinite composition. This is done through a gradual loss of K, Fe, Mg, and Mn, and a relative enrichment in Si and Al. Kaolinization is also indicated by a decrease of the Si/Al atomic ratio to 1. In the Si-Al-Ca + Na + K system, the trend towards the kaolinite pole is obvious (Fig. 4a)
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Compared to the biotite crystals present in the parent-rocks, biotite crystals in the saprolites of Profiles M2, M3, and M4 show less regular cleavage planes, which gradually widen and are filled at places with opaque oxides. At this stage, a white to grey secondary product is associated with the biotite as thin coatings around and between adjacent biotite crystals. The secondary product has weak optical relief and birefringence, and on SEM analysis was found to be composed of juxtaposed prismatic crystals <1 to 2 microns in diameter (Fig. 5 , Photos 1 and 2). Qualitative elemental of the secondary product, although including part of the host mica (Si, K, Fe, Ti), indicates an Al-rich mineral phase (Fig. 6a) . Microprobe analysis indicates that these coatings are essentially aluminous. Thus, the morphological and chemical evidence indicate that the secondary product is gibbsite. No crystallized iron oxides were observed by SEM, hence the only possible secondary iron products are amorphous or fibrous products.
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X-ray Diffraction Analysis
On XRD traces, slightly weathered biotite (Fig. 7a)
showed a significant kaolinite content, and the presence of goethite. The latter is probably located within cleavage planes. The XRD traces from transparent mica flakes (Fig. 7b) show the presence of biotite, a mixed layer mica–vermiculite, kaolinite, and gibbsite. This is in agreement with the petrographic observation of several mineral type–bearing crystals disposed in a sandwich pattern (Fig. 2, Photos 3 and 4). In the M4 Ultisol Bt horizons, however, the mixed layer mica–vermiculite is absent from the biotite mica pseudomorphs (Fig. 7c).
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The microprobe analyses confirm the mineralogical evolution of biotite to kaolinite to gibbsite (Table 1). The Si–Al–Ca + Na + K ternary diagram indicates that kaolinite is only an intermediate stage, the ultimate secondary end-product resulting from the weathering of biotite being gibbsite (Fig. 4b). Except for a weak 4.84- peak on an XRD diagram of sand-size biotite grains reported by Graham et al. (1989b), there is no prior evidence of gibbsite being a weathering product of biotite in the southeastern USA. Kaolinization is rather considered to be the ultimate weathering process (Schroeder et al., 1997). Our data indicate that in the weathering mantle developed from monzonitic proto-mylonitic gneisses and mylonites of the Piedmont foothills of Central Virginia, biotite crystals are transformed to gibbsite, through a kaolinite intermediate.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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Biotite Alteration-derived Kaolinite–Gibbsite Assemblage
In the southeastern USA, the kaolinization of biotite to sand-size kaolinite pseudomorphs has been documented for a long time (Cady, 1950). Kaolinite is not usually considered to be a major biotite weathering product in middle latitudes, where vermiculitization or the formation of other 2:1 phyllosilicates are well known in saprolites and soils (Seddoh and Pedro, 1974; Wilson, 1970, 1975; Meunier and Velde, 1978; Williams et al., 1986; Moresi, 1987; Velbel, 1988). Kaolinization of biotite is a common mineralogical evolution under "more extreme weathering conditions" (Wilson, 1975, p.352), particularly in tropical environments, either humid or arid (Novikoff et al., 1972; Ojanuga, 1973; Eswaran and Heng, 1976; Eswaran and Bin, 1978a, 1978b; Penven et al., 1981). However, the formation of kaolinite and gibbsite from biotite has been reported in humid cold temperate regions (Wilson, 1966), and the formation of kaolinite from biotite has even been reported in Wisconsinian tills in Québec (Wilhelmy, 1983). Therefore, kaolinization of biotite might very well represent a process characteristic of some humid temperate environments, particularly warm temperate climates. The intense leaching in freely drained residuum and soils is probably an important factor in the alteration of biotite in these environments (Wilson, 1966; Macias Vazquez, 1981; Velbel, 1988). Unfortunately, recent studies of biotite weathering in southeastern saprolites and soils do not address the question of the age of the residuum and soils, and recent work specifically dealing with the kaolinization of biotite do not mention the contemporaneous or ancient character of the pedoclimatic conditions associated with this process (Harris et al., 1985a, 1985b; Rebertus et al., 1986; Graham et al., 1989a, 1989b; Kretzschmar et al., 1997).
In the limits of this study, the detailed process by which gibbsite crystals develop perpendicular to the biotite 001 cleavage planes has not been studied. The aluminium might be of allochthonous origin (Wilson, 1966, 1967; Tsusuki and Nagasawa, 1968; Stoch and Sikora, 1976; Bisdom et al., 1982; Graham et al., 1989a). The gibbsite prismatic crystals may precipitate from circulating Al-rich weathering solutions within the biotite cleavage planes. If such is the case, contrary to SEM observations of the topotaxial development of halloysite after biotite (Fig. 5, Photo 3), the geochemical link between biotite and gibbsite would not strictly be one of a primary parent mineral to a derived secondary product. Acid weathering environments (Graham et al., 1989a; Graham and Buol, 1990) and high concentration of organic matter in the surface horizons, eventually leading to translocation of aluminium as complexes with organic acids and its movement towards deeper horizons (Macias Vasquez, 1981), have been proposed to explain the formation of Al-rich percolating solutions.
The gibbsite coatings which characterize the early stage of the alteration of the biotite crystals have not been observed elsewhere in our saprolite and soil profiles, either by optical microscopy or by SEM. Although feldspar weathering in some of these rocks could be a source of aluminium and the allochthonous origin of Al cannot be discarded, this specific occurrence strongly suggests some dissolution–precipitation process at the scale of the parent biotite crystal. If this is the case, we are indeed in the presence of a gibbsitization process, either directly from biotite (Martin Patino et al., 1985; Novikoff et al., 1972), at early stages, or from kaolin pseudomorphs after biotite (Graham et al., 1989a), at later stages. This last mode of formation is in accordance with the polymineral assemblage observed within the pseudomorphs, particularly in the final stages of weathering, where 2:1 type clay minerals are absent and gibbsite is taking more importance relative to kaolinite and halloysite (Fig. 2, Photo 4).
Gibbsitization of biotite is a common process in tropical environments according to the literature (Novikoff et al., 1972; Gilkes and Suddhiprakarn, 1979a, 1979b; Bisdom et al., 1982). The formation of gibbsite has also been reported in temperate climatic environments such as Scotland (Wilson, 1966), Japan (Kato, 1965; Tsuzuki and Nagasawa, 1968), and New Zealand (Lowe and Percival, 1993; Bakker et al., 1996). In the southeastern USA, except for two mentions of gibbsite as a minor secondary product associated to biotite weathering (Grant, 1964; Graham et al., 1989b), the formation of gibbsite has been related to the alteration of feldspars. While kaolinization implies a monosiallization regime, that is, to a weathering regime leading to the formation of 1:1 phyllosilicates, gibbsitization corresponds to allitization (Kato, 1965), that is, a weathering regime leading to the complete mobilization of Si and to the formation of aluminium oxyhydroxides. This extreme geochemical evolution has been observed at the scale of biotite mica crystals within the Central Virginia residuum and is reminiscent of agressive humid tropical-like pedoclimatic conditions.
The kaolinite and gibbsite pseudomorphs have been observed within both Dystrochrepts and Hapludults. They have been observed within both gruss-type saprolites and saprolites showing nearly complete alteration of feldspars and biotite. This has been interpreted elsewhere as indicating that the weathering of biotite to gibbsite is contemporary with present pedoclimatic conditions in materials developed from acid rocks on south-facing slopes of Central Virginia (Jolicoeur et al., 1995). Although we lack data to constrain the soil exposure ages or rate of weathering in these profiles, we believe that the development of gibbsite after biotite in relatively weakly weathered (gruss-type) material suggests that biotite weathering to kaolinite and gibbsite is still operative. Wilke and Schwertman (1977) suggest a Tertiary age and conditions conducive to extensive leaching of silica to explain the development of gibbsite in similar material of the Bayerischer Wald (Germany). However, the geomorphological setting of our profiles, which are present on slopes up to 28° in gradient, does not seem compatible with such an ancient age. We consider that the porous nature of the residuum and warm and humid temperate conditions similar to the present can be held responsible for the developement of these weathering products. On the other hand, kaolinite and gibbsite pseudomorphs have been observed only in saprolites and soils developed from proto-mylonitic and mylonitic rocks of the AMS. Since this suite has a chemical and mineralogical composition similar to that of the other parent-rocks considered, the decisive factor for the weathering of biotite to gibbsite seems to be the mylonitic fabric of the parent-rocks of profiles M2, M3, and M4. These profiles are developed in rocks showing near-vertical micaceous foliation planes. The susceptibility of biotite to weathering could be enhanced by its concentration in vertical planes along which intense leaching by soil solutions can proceed. In other words, kaolinite is the most widespread weathering product formed from biotite in saprolites and soils developed in acid rocks on south-facing slopes of the central Virginia Blue Ridge Mountains and Piedmont foothills, but on mylonitic to protomylonitic rocks showing biotite-rich vertical foliation planes, weathering can proceed towards gibbsite.
Biotite Kaolinization and Gibbsitization and the Anti-gibbsite Effect
According to Rebertus et al. (1986), the kaolinization of biotite is responsible for a strong anti-gibbsite effect. The soils in which biotite is being pseudomorphically altered to hydroxy-interlayered vermiculite and to kaolinite would be characterized by a low content of gibbsite. Losche et al. (1970) also mentioned that hydroxy-interlayer formation act as sinks for Al, the result being the absence of gibbsite in soils on north-facing slopes of the southern Appalachians Mountains. Other workers have also explained the decrease in gibbsite contents towards surface horizons of soils (Graham et al., 1989a), or its absence in these horizons (Glenn and Nash, 1964) by the anti-gibbsite effect.
Besides the results presented in this paper, Kato (1965), Wilson (1966), Eswaran and Heng (1976), and Graham (1986) have observed the transformation of biotite to multicomponents mineral assemblages including hydroxy-interlayered 2:1 type clay minerals, and 1:1 type clay minerals and gibbsite. The formation of hydroxy-interlayered vermiculite, kaolinite and/or halloysite, and gibbsite within the same soil horizon and within the same mineral grain seems to contradict the reality of the anti-gibbsite effect. In this study, HIV–kaolin–gibbsite pseudomorphs after biotite have been observed in Bw, Bt, and C horizons of Typic Dystrochrepts and Hapludults (Jolicoeur et al., 1995).
Multimineral pseudomorphs after biotite have been explained by the existence of microenvironments of weathering within the soil profile and within mineral grains (Kato, 1965; Eswaran and Heng, 1976; Meunier, 1980; Penven et al., 1981; Graham, 1986). Such microenvironments are characterized by open to restricted drainage and by specific chemical composition of weathering solutions or primary and secondary minerals. Such a model of the weathering system seems more appropriate to explain the juxtaposition of several mineral types resulting from the weathering of biotite and the mineralogy and chemistry of soil microsystems (Ildefonse et al., 1979). It does not contradict the anti-gibbsite effect nor the role of organic matter in the mobilization of Al within soils but it suggests that while these mechanisms may be operative at the scale of the soil macrosystem, specific mineral weathering systems may be far more complex. The weathering paths of biotite in the soils of central Virginia are a good example of this.National Oceanic and Atmospheric Administration. 1982
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ACKNOWLEDGMENTS |
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Received for publication March 29, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionREFERENCES |
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