Clay Minerals (2004) 39, 233–266

Weathering of the primary rock-forming minerals: processes, products and rates

M. J. WILSON

The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK

(Received 3 April 2004; revised 26 May 2004)

ABSTRACT: This paper describes the ways in which the major rock-forming primary minerals (olivine, pyroxenes, amphiboles, feldspars, micas and chlorites) break down during weathering, the products that develop during this breakdown and the rates at which this breakdown occurs. The perspective chosen to illustrate this vast topic is that of the residual soil weathering profile. Different physical and chemical conditions characterize the various parts of such a profile. Thus, in the slightly weathered rock at the base of the profile, mineral weathering will take place in microfissures and narrow solution channels and the capillary water in such spatially restricted volumes may be expected to be close to equilibrium with the primary mineral. In these circumstances, the weathering product formed may be closely related to the primary mineral both compositionally and structurally. The saprolite higher up in the weathering profile may or may not retain the fabric and structure of the original parent rock, but in either case the close relationship observed between primary mineral and weathering product in the slightly weathered rock may be lost. This part of the profile will usually be affected by freely flowing drainage waters, the composition of which will be far from equilibrium with specific primary minerals. Weathering products which do form are likely to reflect the interaction between bulk water and bulk parent material. In the soil profile, the situation will be further complicated by organic ligands derived from decomposing organic matter or from the direct activities of soil microbes or plant roots. Thus, biological weathering will assume a much greater significance in this part of the profile compared with the mainly inorganic processes dominating in the saprolite and the slightly weathered rock. The general nature of any particular weathering profile will reflect the interactions between climate, topography, parent material, soil biota and time and superimposed upon this complexity, when considering how individual primary minerals break down in detail, will be factors related to the nature of the mineral itself. Particularly important in this respect is the inherent susceptibility of the mineral to weathering, which is related to overall chemical composition and structure, as well as the distribution and density of defects, dislocations and exsolution features, which often control the progress of the weathering reaction.

KEYWORDS: weathering, olivine, pyroxene, amphibole, feldspar, biotite, muscovite, chlorite, soil, saprolite.

Weathering is a fundamental process in the the most important process in the geological cycle geological cycle that should be regarded as of as it most directly affects the living world in equal importance as the processes of meta- general and the life of man in particular. Thus, morphism, volcanism, diagenesis, erosion, etc., weathering is responsible for the formation of soils, that are much more extensively studied in most upon which nearly all terrestrial life ultimately departments of earth science. Indeed, a case could depends, playing a central role in controlling the be made for mineral weathering to be considered as inherent fertility status of soils through the supply of many of the nutrients that enable plants to grow. * E-mail: [email protected] Again, in more recent times it has been realized that DOI: 10.1180/0009855043930133 mineral weathering acts as a buffer against a variety

# 2004 The Mineralogical Society 234 M. J. Wilson of environmental threats that are of direct concern be preferentially adsorbed or fixed in the soil by to man. For example, the impact of acid deposition certain clay or Fe oxide minerals, which are on the terrestrial environment is mediated to a large themselves the products of primary mineral weath- extent by the primary mineral content of the soil ering. These pollutants are thus made less available (Nilsson and Grennfelt, 1988). If the soil contains for uptake by plants or are inhibited from being few weatherable minerals then incoming acidic dispersed into the wider environment. Again, deposition from the atmosphere will not be organic contaminants may be adsorbed on to the neutralized by basic cations from soil minerals surfaces of clay minerals, after which they may be and acidification of the soil will ensue, leading to broken down to more harmless compounds by undesirable changes in the terrestrial ecosystem microbiological activity. dependent on the soil (Sverdrup et al., 1994). Apart from the importance of weathering to life, Furthermore, this acidity in the form of dissolved the process occupies a unique place in the protonated Al ions may then be transferred from the geological cycle in that it is located at the interface soil to surface waters, in turn causing their between the atmosphere, the hydrosphere and the acidification and resulting in undesirable impacts biosphere. Indeed, life itself is an important agent on the aquatic ecosystem (Sverdrup and Warfvinge, of mineral weathering as will be shown later. In 1990). On the other hand, a soil may not be addition, weathering can occur at any stage in the sensitive to the impact of acid deposition because it geological cycle, effectively short-circuiting the contains an adequate supply of weatherable base- cycle as a whole. Thus, depending upon the rich primary minerals and is able to effectively timing of uplift to the Earth’s surface, weathering neutralize the incoming acidity. Mineralogy and may ensue directly after sediment deposition, weathering rates are therefore crucial in deter- diagenesis, metamorphism, intrusion of plutonic mining whether or not a soil is sensitive to the rocks or extrusion of volcanic rocks (Fig. 1). effects of acid deposition from the atmosphere. This Evidently, the composition of sedimentary rocks has been taken into account in the application of the will be strongly influenced by the nature of critical loads concept to the effects of acid weathering in the source area of the sediment. deposition (Sverdrup et al., 1990), and is now the Clearly, therefore, mineral weathering is a basis for determining the acceptable limits of acid process of prime importance to the earth sciences, emissions to the atmosphere in many countries as well as to agricultural and environmental (Hettelingh et al., 1991). sciences, from many different points of view. This Mineral weathering also helps to protect the has become much more widely realized in recent environment, directly or indirectly, from a variety decades as shown by the tremendous upsurge in of other threats. It is of direct importance, for research activity and in the volume of scientific example, in the problem of climate change, certainly literature dealing with weathering. In particular, on a geological time scale. Thus a direct negative there has been a great deal of work upon the feedback relationship has been proposed (Berner, detailed mechanisms of primary mineral weath- 1991) between increasing atmospheric CO2 levels ering, the products formed from such weathering and weathering rates of Ca-bearing silicate minerals and the rates at which the process occurs. In this like plagioclase feldspars, pyroxenes and amphiboles. paper, some (but by no means all) of the more The increasing Earth surface-temperatures resulting significant recent work on the weathering of from the greenhouse effect of higher atmospheric primary minerals, including olivines, pyroxenes, CO2 levels will also cause accelerated weathering amphiboles, feldspars, micas and chlorites, will be rates through higher rainfall and run-off, the calcium reviewed, from the particular context of location of so removed being converted to CaCO3 by reaction the mineral in the weathering profile. As will be with CO2 in the atmosphere, thus acting as an shown in the following discussion, position in the effective sink for this gas. According to Walker et al. weathering profile has a crucial influence in (1981) and Berner (1991), in these circumstances the determining the nature of the prevailing weathering long-term effect of geochemical weathering is to environment, and consequently on the mechanisms stabilize global surface temperatures. involved, the products formed and the rates at Mineral weathering may also exert an indirect which the process occurs. For the most part this protective effect on the environment. For example, review will deal with weathering as it occurs in some heavy metals and radioactive elements may acidic to circum-neutral conditions. Weathering of rock-forming minerals 235

FIG. 1. Weathering in the geological cycle.

THE WEATHERING PROFILE rock is perfectly coherent but is marked by weathering related to fractures and fissures along There are of course many different kinds of which the weathering agent (generally water) has weathering profile. Often such profiles are truncated progressed. This is then succeeded by a discontin- because of mass movement, erosion, etc., but the uous pattern of blocks of weathered and unweath- most complete form is represented by a residual soil ered rock, which gradually gives way to a situation developed upon a disaggregated, saprolitic parent where virtually the whole body of the rock is rock which itself overlies coherent, partly weath- weathered and disaggregated. This part of the ered bedrock. Delvigne (1998) has described the profile is termed ‘‘alterite’’ by Delvigne (1998) various components of the residual weathering and is broadly equivalent to what is known as profile in detail (Fig. 2) and shown how they may saprolite. It is usually found that in the lower part differ in terms of their fabric, texture and overall of the saprolite the original fabric and texture of the mineralogy. At the base of the profile, the parent bedrock are perfectly preserved, despite the almost complete disaggregation of the constituent mineral grains one from another. The coherence of the rock may be completely lost but it has maintained its A Horizon original volume, leading Delvigne to suggest the Soil B Horizon term ‘‘isoalterite’’ for this part of the profile. The C Horizon isoalterite may be several metres thick but eventually gives way to a situation where the Completely disaggregated original fabric and structure of the parent rock can rock, but fabric and texture lost no longer be recognized. This results from the alteration, movement and redistribution of certain Saprolite/alterite mineral components and is often accompanied by a Completely disaggregated colour change due to formation of Fe oxide rock, but fabric and texture minerals. Delvigne distinguishes this part of the maintained weathering profile as ‘‘alloterite’’. At the top of the weathering profile and often grading down into the saprolite there is what soil scientists generally think Weathered rock Hard coherent partly weathered of as the soil itself. This is usually characterized by rock with fractures and fissures a sequence of horizons designated as A, B and C. The A horizon occurs close to the surface and FIG. 2. Units of a fully developed weathering profile. consists of a mixture of minerals and humified 236 M. J. Wilson organic matter, although the eluviation of both factor in the soil horizons. Redox conditions too materials down the profile may have occurred. The may vary in the different parts of the weathering B is a subsurface horizon characterized by the profile, tending to be most oxidizing in the soil and illuviation and accumulation of minerals and least oxidizing in the weathered rock. Finally, it organic matter from higher up in the profile, may be expected that a direct microbiological effect whereas the basal C horizon consists of unconso- on mineral weathering through, for example, the lidated mineral material which does not show the excretion of organic acids, will be more important characteristic properties of the A and B horizons, in the soil compared with the weathered rock or Consideration of primary mineral weathering in saprolite. the context of the complete weathering profile is In addition to the variation in physical and important because quite different chemical and chemical conditions within the weathering profile, it physical conditions obtain in the various parts of should be borne in mind that the overall nature of the profile. This aspect has been discussed in some the soil, and to a large extent that of the saprolite detail in a perceptive review of the microscopic and weathered rock, will depend upon the five aspects of mineral weathering by Hochella & classical factors identified by Jenny (1941), namely Banfield (1996). For example, the surface area of climate, topographic position (influencing drai- mineral grains, whether measured geometrically or nage), parent material, time and biota. by BET methods (which take into account both An outline of the general mechanisms by which surface roughness and internal surface area) are at a mineral weathering operates in the different parts of minimum in the weathered rock, at a maximum in the weathering profile may now be given. In the the soil and at an intermediate level in the saprolite. weathering rock at the base of the profile, the Therefore, reactive mineral surface will similarly capillary/stagnant water may be close to equili- increase towards the surface, although it may be brium with individual primary minerals and will noted that grain size will not always correlate with tend to form weathering products that bear a close reactive surface. Again, porosity and permeability chemical and structural relationship to these will vary in different parts of the weathering minerals. Reactions may be slow and diffusion- profile, generally increasing upwards and being at controlled, often affecting the internal surface area a maximum in the soil. This means that hydraulic of the primary mineral. In the isoalterite part of the conductivity will increase in the same direction, saprolite, the minerals are more affected by freely although it should be borne in mind that structural flowing drainage waters whose composition will features in the weathering profile may lead to tend to reflect the interaction between bulk water channel flow. Nevertheless, the actual nature of the and bulk parent material. Therefore, equilibrium water may change within different parts of the between these waters and individual primary weathering profile. In the soil, gravitational, minerals will not be attained and a close structural drainage or freely flowing water will predominate, and chemical relationship between a specific whereas in the weathered rock at the base of the primary mineral and a weathering product may profile there will be a greater proportion of stagnant not expected to occur. In the alloterite, the or capillary water. This will mean that weathering destruction of a coherent rock fabric means that minerals will be closer to equilibrium with external such a relationship is even less likely. Within the fluids at the base of the profile but furthest away saprolite and in the lowest parts of the soil profile, from equilibrium in the soil layers. Moreover, mechanisms for the aqueous dissolution of the vegetation growing in the soil will act as a sink primary minerals assume greater significance and for cations released by the weathering minerals and the results of laboratory dissolution experiments are thus move the system even further away from of greatest relevance to natural weathering equilibrium. The pH value of the waters in the processes. Within the soil profile itself, the role of weathering profile is also likely to vary markedly. organic ligands derived from decomposing organic This is likely to be lowest in the soil horizons and matter, in addition to the activities of soil microbes highest in the weathered rock where the basic and plant roots, will be of relatively greater cations from the weathering minerals will be more importance in primary mineral weathering, in easily retained in the stagnant water. In addition, comparison with the mainly inorganic processes organic ligands resulting from the decomposition of dominating in the lower parts of the weathering organic matter will be a more important weathering profile. Weathering of rock-forming minerals 237

ROLE OF THE PRIMARY MINERAL phyllosilicates and Fe oxide minerals. In the latter IN INITIATING WEATHERING case, the most common product is iddingsite, which REACTIONS may be optically homogeneous and crystallographi- cally orientated with respect to the parent olivine In addition to the environmental factors discussed (Brown and Stephen, 1959). The transformation above, the nature and rate of the weathering process from olivine to iddingsite was studied in detail by clearly depends upon the nature of the primary Eggleton (1984) using transmission electron micro- minerals themselves. This was recognized by scopy of ion-beam thinned specimens. A two-stage Goldich (1938) who showed that the overall process was proposed. In the first stage, the olivine susceptibility to weathering of the common breaks down into thin lamellae consisting of a primary minerals was related to the discontinuous metastable hexagonal phase and elongated parallel and continuous reaction series identified in to the olivine c axis. This transformation opens up magmatic crystallization. Thus, olivine and solution channels which become infilled with lath- anorthite, the first formed minerals in the like crystals of saponite, whose (001) parallels the discontinuous and continuous reaction series, olivine (100), and crystallites of goethite. This early respectively, are the most susceptible to weathering. transformation is thus considered to be of an In both series, minerals that crystallize at succes- epitaxial or topotactic nature. In a second stage, sively lower temperatures are more stable to water migrates more freely through the solution weathering, with quartz, muscovite and K-feldspar channels permitting further growth of the smectite being particularly resistant. This variation relates to and goethite nuclei formed during early alteration the differential between conditions at magmatic and largely, but not entirely, maintaining the early crystallization and at the Earth’s surface during orientation relationship with the parent olivine. In weathering, as well as to overall mineral structure this instance, alteration was undoubtedly due to and chemistry. However, in recent decades it has deuteric/hydrothermal activity, although weathering become apparent that susceptibility to weathering was considered to result in similar mineral also depends very much upon the more detailed assemblages. features of mineral structure and chemistry. As will This point was amplified in a later study on the be shown later, dislocation and defect structures are weathering of olivine in basalt to iddingsite by particularly important in determining the sites Smith et al. (1987). Three different basalts were where weathering is initiated, as are the inhomo- examined. In each, the process of iddingsitization geneities in chemical composition induced by appears to have commenced before weathering, but micro-textural features. Furthermore, such details progresses to completion as a result of later have a direct bearing upon questions related to the weathering. Iddingsite formation again involves fundamental mechanisms by which weathering etching along lamellar zones parallel to (001) and proceeds. formation of smectite and oriented goethite within these zones. An important finding, however, is that some elements such as Al and Na, have been WEATHERING OF OLIVINE introduced from outside the weathering olivine Transformations in fresh and weathered rock crystal, showing that the process is not truly isochemical. In fact, in the later stages of Delvigne et al. (1979) reviewed the way in which decomposition, dioctahedral smectite and halloysite olivine weathers in a variety of situations mainly are associated with the weathered olivine. from a microscopic or micromorphological point of The initial stages of the alteration of olivine were view. They pointed out that usually olivine is explored in further detail by Banfield et al. (1990). destroyed or replaced during the early stages of It was found that in altered basaltic andesites the weathering and consequently is normally not found olivine consisted of fine intergrowths of forsterite- to any great extent in soils, except under unusual rich olivine and laihunite, an oxidized fayalitic circumstances. They also emphasized that in fresh phase where structural ferric iron is balanced by rocks olivine is often already altered by magmatic structural vacancies resulting in a distorted olivine and metamorphic transformations to orthopyrox- structure. Where the intergrowths are widely enes, serpentines or talc and by deuteric or spaced, the laihunite may act as a barrier to hydrothermal processes to mixtures of hydrous weathering, resulting in the preferential dissolution 238 M. J. Wilson and etching out of the forsterite to yield textures laboratory experiments. While such experiments do very similar to those reported by Eggleton (1984) not necessarily simulate natural weathering (Casey and Smith et al. (1987). The etched out channels et al., 1993), they do show how the rate of are first filled with semi-oriented aggregates of dissolution of olivine depends on the detailed saponitic clay and subsequently with aggregates of nature of the mineral itself, particularly its oriented hematite crystals. Banfield et al. (1990) composition, crystallography, dislocation density concede that there is no firm evidence to decide if and micro-textural features, as well as on the these products result from deuteric alteration or chemistry of the external solution, including pH, weathering, although in a further transmission ion activities and redox conditions. electron microscope (TEM) study (Banfield et al., The effect of overall structure on dissolution 1991), they definitely attributed these changes to kinetics under acid conditions of gem quality weathering. The occasional presence of appreciable forsteritic olivine has recently been elegantly Al in the smectites associated with weathering demonstrated by Awad et al. (2000). They showed olivine indicates that micro-environmental control that there was a much higher dissolution rate along is by no means complete and that ions diffusing the b axis (Table 1), attributing this to preferential into the decomposing olivine grains derive from protonation of the O atoms that surround the cations extraneous sources. This was clear from an earlier in the M1 site in the form of a distorted octahedron. study of lateritic weathering of olivine-bearing Dissolution occurred through the formation of etch rocks by Nahon et al. (1982). It was shown that pits parallel to (010), (hk0) and (h¯k0). early-formed smectites were trioctahedral but that The mechanism involved in the aqueous dissolu- as weathering progressed they were replaced by tion of forsteritic olivine at Earth surface-tempera- dioctahedral (beidellitic-nontronitic) varieties. tures in an open-system, mixed-flow reactor over a Further weathering results in the replacement of wide range of pH values and solution compositions these smectites by amorphous and well crystallized has recently been reviewed and investigated by oxyhydroxides. Pokrovsky and Schott (2000a,b). They concluded In the context of the weathering profile as a that at acid and slightly alkaline pH values, a silica- whole, there is no doubt that the smectitic minerals rich protonated precursor complex several unit cells that form from olivine in weathered rock are quite thick forms at the surface of the mineral through ephemeral. Once these products are removed from exchange of H+ with Mg2+ according to the the relatively closed near-equilibrium situation that reaction: exists in the weathered rock and placed within the Mg SiO +4H+ =>HÀSiO + 2Mg2+ more open, far-from-equilibrium situation that 2 4 (surf) (aq) 4(surf) (aq) prevails throughout the saprolite and the soil, then This is followed by polymerization of partially + they quickly disappear. Quantitatively, therefore, in protonated SiO4 tetrahedra, penetration of H into the context of the weathering profile as a whole the leached layer and adsorption of H+ on silica these early weathering products may be relatively dimers. Dissolution rate is non-stoichiometric in the unimportant. first ~10 h of the experiment but then becomes stoichiometric thereafter. The decomposition of the surface complex is the rate-limiting step controlling Weathering in basal saprolite forsterite dissolution under acid conditions. The The formation of early weathering products like forsterite grains are characteristically etched, those found in weathered rock may be completely bypassed in the situation where the fluid to mineral À1 ratio increases dramatically, as might be antici- TABLE 1. Dissolution rates (mm h ) of gem-quality pated, for example, in the disaggregated and olivine at 23ºC and 50ºC at pH 1 along the crystal- incoherent saprolite higher up in the weathering lographic axes (data from Awad et al., 2000). profile. The saprolite at the base of the profile (termed ‘gruss’ in German or ‘are`ne’ in French) is Temperature a axis b axis c axis often characterized by physical incoherence but À5 À4 À6 little or no chemical weathering. Here, disaggre- 23ºC 2.38610 1.46610 7.16610 À5 À3 À4 gated grains of olivine may be subjected to 50ºC 3.01610 1.53610 1.72610 dissolution effects similar to those described in Weathering of rock-forming minerals 239

FIG. 3. (a) SEM (100 mm wide) of surface of etched olivine from a montane soil developed on allivalite on the island of Rhum; (b) SEM (150 mm wide) of etched olivine from gabbro boulder beneath a lichen at Pitcaple, Aberdeenshire (Wilson & Jones, 1983). showing that dissolution occurs preferentially at These are (1) in sandy and rocky material in the specific sites that may be crystallographically vicinity of actively eroding ultrabasic or basic controlled (Awad et al., 2000) or that may mark outcrops, (e.g. Wilson, 1969); (2) in soils derived the emergence of dislocations or defects, as from recent volcanic ash deposits; and (3) in soils suggested by Grandstaff (1978). The relationship where olivine is protected by a cortex of iddingsite between etch markings and structural dislocations in or a reaction rim of pyroxene or garnet. Usually, olivine has been well illustrated by Kirby & olivine weathers quickly in the soil to mixtures of Wegner (1978). Weathered olivines are found to various hydrous clay minerals and Fe oxides/ be heavily etched (Fig. 3), suggesting that dissolu- oxyhydroxides. Thus Prudeˆncao et al. (2002) tion experiments like those described above are found, in a profile on weathered basalt, that relevant to some natural conditions, despite the olivine decomposed to a mixture of dioctahedral cautionary comments of Casey et al. (1993). aluminous smectite and halloysite, illustrating the Comparative dissolution experiments such as those of Franke and Teschner-Steinhardt (1994) clearly 2 À1 show that the weathering of olivine proceeds at a TABLE 2. Rates of mineral dissolution (mg m day ) substantially faster rate than the other ferromagne- obtained in the experiments of Franke & Teschner- sian minerals in the discontinuous reaction series Steinhardt (1994). (Table 2) Mineral pH 5.5 pH 4.0

Weathering in soils and upper saprolites Olivine 200À400 6000À8000 Olivine is rarely found in soils and the upper Pyroxene 14À25 20À200 parts of saprolites because of its extreme suscept- Hornblende 26À30 100À200 Biotite 1.5À1.8 42À53 ibility to weathering, but Delvigne et al. (1979) identified three situations where it may be found. 240 M. J. Wilson requirement for inputs of Al from external sources. Boland (1982) conducted further studies on the If olivine does survive into the soil then it is likely weathering of enstatite to talc in a mafic rock. This to be subjected to attack by microbial activity and transformation was considered to be a definite by organic acids in addition to inorganic chemical topotactic reaction with strict crystallographic weathering. Welch and Banfield (2002) showed that continuity where the (001), (100) and (010) of the the interaction between the Fe-oxidizing chemo- pyroxene become the (100), (001) and (010), autotroph Acidothiobacillus ferrooxidans and faya- respectively of talc. Further weathering results in lite resulted in distinctive dissolution textures, the orientation relationship being lost with progres- typically consisting of finely etched out channels sive replacement of the talc by smectite and on the (001) face and similar to features previously sometimes smectite-chlorite. Basham (1974) had observed in abiotically weathered samples. The previously described the weathering of augite and inter-channel strips contained a greater abundance hypersthene to pseudomorphs of vermiculite in of laihunite, which appeared to be less reactive to weathered gabbro, where the c axis of the weathering, thus leading to preferential attack on vermiculite was oriented at right angles to the the fayalitic olivine. This microbial alteration c axis of the pyroxene. It was concluded that the brought about an order of magnitude increase in host pyroxene exerted close structural control over surface area of the fayalite grains but at the same the development of the hydrous mineral. In a study time would probably make them so susceptible to of what was taken to be natural weathering of disintegration that they would be unlikely to survive pyroxenoid from a hydrothermal vein, Banfield et in a soil environment. The experiments of al. (1995) concluded that alteration in the form of Varadachari et al. (1994) show that olivine is etch pits commenced at grain boundaries and planar completely decomposed after shaking with 0.5 M defects between intergrown rhodonite and pyrox- oxalic acid after 30 days and Drever (1994), while mangite. These etch pits are filled with a Zn-rich minimizing the overall effect of chelation by manganous smectite (the Zn being derived from organic ligands on mineral weathering rates, associated weathered sphalerite) which is first well- concedes that in the case of olivine there could be oriented but which later becomes randomly oriented a significant effect. as the etch pits enlarge. Similarly, in a study of amphibole weathering Banfield & Barker (1994) found that reactions were focused in finely exsolved WEATHERING OF PYROXENES gedrite and anthophyllite lamellae which converted AND AMPHIBOLES isovolumetrically and topotactically to smectite. Transformations in weathered rock The reaction was inferred to be a diffusion- controlled, solid-state process, involving only In weathered rock, both pyroxene and amphibole partial depolymerization of the amphibole structure have been found to weather to hydrous layer and with no intermediate amorphous-type material. silicates in a similar way to that described for In contrast to these findings, Singh & Gilkes olivine, namely via a mechanism involving a high (1993) showed that the weathering of the pyroxene degree of structural inheritance by the secondary spodumene in coherent weathered rock at the base phase from the primary mineral. Thus, Eggleton of a lateritized pegmatite proceeded by replacement (1975) showed that, in a clay pocket in weathered by randomly oriented smectite in etch pits and skarn, the pyroxene hedenbergite converted to cleavage cracks via formation of some intermediate nontronite maintaining complete optical continuity non-crystalline material. It was concluded, there- and constant volume throughout. X-ray and electron fore, that weathering had proceeded by complete diffraction showed that the two phases were breakdown of the spodumene structure involving a crystallographically coherent and a topotactic solution process and precipitation of an amorphous mechanism was proposed involving disconnection phase. These findings are in accord with those of of pyroxene chains following changes resulting Nahon & Colin (1982) who identified amorphous from the loss of Ca2+,Mg2+ and Si4+, oxidation of intermediary products during the weathering of Fe2+ and hydration. Reconnection of tetrahedral orthopyroxene (enstatite and bronzite) at the base of silicic and octahedral ferric chains then occurred a lateritic weathering profile in the Ivory Coast. A through movement in the {010} pyroxene chain to possible reason for these different findings is that form the nontronitic ‘talc’-like units. Eggleton and the materials studied by Eggleton (1975), Eggleton Weathering of rock-forming minerals 241

& Boland (1982), Banfield & Barker (1994) and Schott et al. (1981) in aqueous dissolution Banfield et al. (1995) had previously been subjected experiments on enstatite, diopside and tremolite to hydrothermal or deuteric alteration involving suggested that the parabolic dissolution kinetics formation of hydrous phyllosilicates from the were an experimental artefact induced by the primary mineral, which was thus predisposed to creation of ultra-fine particles by grinding. continued transformation along the same pathway in Further, they proposed that dissolution is controlled the confined space of weathered rock at the base of by reactions at the mineral surface and does not the profile. proceed by diffusion through a residual layer. Calcium and Mg are initially released preferentially for structural reasons, after which dissolution Weathering in saprolite becomes congruent and linear. X-ray photoelectron In the saprolite, disaggregated pyroxene and spectroscopy (XPS) suggested a cation-depleted amphibole grains may be subjected to aqueous layer at the mineral surface only 5 to 20 A˚ thick, dissolution processes similar to those studied in too thin to be rate limiting with respect to diffusion. through-flow reactors. The processes and mechan- Similar conclusions were arrived at with respect to isms involved have been reviewed extensively by the aqueous dissolution of the Fe-rich bronzite, both Brantley & Chen (1995). Early experiments, such as in the presence and absence of dissolved oxygen those conducted by Luce et al. (1972) on the (Schott & Berner, 1983). Other evidence for a dissolution of enstatite, showed parabolic dissolu- surface-controlled mechanism for dissolution of tion kinetics, which were interpreted as being pyroxenes and amphiboles is the universal occur- consistent with diffusion of ions through a cation- rence of etch pits in weathered grains (Fig. 4), depleted, silica-enriched shell surrounding the which are thought to develop at points where mineral grains. Parabolic dissolution kinetics were structural dislocations occur at the mineral surface also found for bronzite (Grandstaff, 1977) and for (Berner et al., 1980, Berner & Schott, 1982). hypersthene (Siever & Woodford, 1979). However, However, this interpretation of the dissolution

FIG. 4. (a) SEM (200 mm wide) of deeply etched cleavage planes of clinopyroxene in soil derived from volcanic material, Italy (Violante & Wilson, 1983). (b) SEM (75 mm wide) of etched-out lamellar intergrowths of clinopyroxene separated from gabbro after incubation in a culture with Aspergillus niger (Wilson & Jones, 1983). 242 M. J. Wilson process is still open to question following further stable to aqueous dissolution when compared with research on altered grains using advanced techni- olivine. ques. Thus, Petit et al. (1987) showed by hydrogen- depth profiling using a resonant nuclear reaction Weathering in the upper saprolite and soil (RNR) technique, that diopside dissolution proceeds via surficial hydration through a thickness of about Turning to the nature of pyroxene/hornblende 1000 A˚ , coupled with a depletion of all elements. It decomposition in the upper parts of the weathering was proposed that the leached layer formed by the profile, Nahon & Colin (1982) showed that migration of molecular water through the structure orthopyroxene transformed directly to clay mineral along defects and that reaction with the silicate products under more intense weathering conditions. network created a more open porous structure Specifically, they identified smectites intermediate further enhancing water migration. These observa- in composition between dioctahedral (beidellite- tions were not thought to be incompatible with the nontronite) and trioctahedral (saponite) types, as earlier conclusions of Schott et al. (1981) that the well as a talc-like mineral with interstratified dissolution of diopside did not occur through a swelling layers. These minerals pseudomorphed leached surface layer. However, Mogk & Locke the original orthopyroxene but were not structurally (1988) used Auger Electron Spectroscopy (AES) to related to it. Further weathering resulted in the study naturally weathered hornblende and decomposition of the clay minerals and the concluded that there was systematic cation deple- formation of well crystallized goethite via a tion to depths of ~1200 A˚ , indicating that weath- ferrihydrite stage. Delvigne (1998) illustrates how ering cannot be accounted for by surface reactions pyroxene that is completely weathered to goethite alone, but must also involve depletion of cations can be infilled by gibbsite of allocthonous origin. through volume diffusion. They concluded that their Proust (1982) drew a clear distinction between the results support a non-steady-state diffusion model, weathering products formed at the base of the as proposed by Luce et al. (1972), but that the profile, where rock structure is preserved, and those kinetics of mineral dissolution in general, whether forming higher up in the profile where structure is by laboratory experiments or natural processes, is controlled by a balance of surface reactions and volume diffusion. The results of Zhang et al. (1993) TABLE 3. Dissolution rates for some common on the experimental weathering of hornblende of pyroxenes and amphiboles with respect to 2 À1 different size fractions also indicated surface silica (moles cm s ) at 25ºC between pH enrichment of Si compared with Ca, Al, Mg and 4.0 to ~6.0 (selected from data compiled by Fe. Following the conclusions of Blum et al. (1990) Brantley & Chen (1995); dissolution rates for olivine from Pokrovsky & Schott (2000b). for the dissolution of quartz, they discounted a direct relationship between dissolution rate and dislocation density, although surface etching of the Mineral pH Àlog RSi minerals did occur, clearly showing selective dissolution at high energy sites. Enstatite 4.8 À13.8 6.0 À16.4 The rates of dissolution of pyroxenes and Bronzite 4.0 À15.8 amphiboles under ambient conditions, together 6.0 À16.0 with all the many factors that may affect these Diopside 4.0 À13.3 rates, have been reviewed in detail by Brantley & 6.0 À14.8 Chen (1995). Table 3 shows selected dissolution Augite 4.1 À16.1 rates with respect to silica of some of the most 6.0 À14.9 Anthophyllite 4.0 À15.7 common pyroxenes and amphiboles in the pH range 6.0 À14.8 4.0 to 6.0 at ambient temperatures. It can be seen Tremolite 6.0 À14.8 that dissolution rates do not always necessarily Hornblende 4.0 À15.9 increase with decreasing pH, (in the case of augite, 6.0 À15.6 anthophyllite and hornblende), that enstatite (though Glaucophane 6.2 À14.6 not bronzite) and diopside are least stable to Olivine 4.2 À12.7 6.1 13.9 dissolution weathering under acid conditions, and À that both augite and hornblende are much more Weathering of rock-forming minerals 243 lost. It was found that saponite forms at the base of altered granodiorite by Banfield & Eggleton (1990). the profile but that this is replaced towards the Poorly crystallized (proto-crystalline) material was surface by vermiculite which eventually gives way again identified as an initial weathering product to a dioctahedral vermiculite-beidellite assemblage. forming within etch pits in both feldspars. In Earlier, Wilson & Farmer (1970) had found that plagioclase this material was later replaced by discrete Fe-rich lamellae of another amphibole randomly oriented smectite containing Fe, Ca and within hornblende weathered selectively to an K, and then later by halloysite. Similar weathering interstratified swelling chlorite-saponite, which products were associated with the K-feldspar. was not oriented relative to the parent mineral Eswaran & Bin (1978) had earlier observed in other than showing a tendency to alignment along slightly weathered rock at the base of a deep cleavage planes. This clay mineral decomposed weathering profile in Malaysia that amorphous completely in the upper parts of the profile. material accumulated in plagioclase trans-grain The consequences of more intense weathering of voids, later transforming to halloysite. Para-crystal- hornblende were revealed by Anand & Gilkes line clay precursors were also observed in the (1984). It was found that the hornblende in a weathering of K-feldspar in an alkaline igneous weathered meta-dolerite simply disappeared as there complex in Brazil (Tazaki & Fyfe, 1987). These was insufficient Al to form kaolin minerals, as was precursors showed primitive circular morphology, the case with plagioclase feldspar. Similarly, in a yielded diffuse electron diffraction patterns and weathered plagioclase-hornblende gneiss, Velbel probably transformed mainly into spheroidal halloy- (1989) found the amphibole to have dissolved site. On the other hand, Anand et al. (1985) could stoichiometrically, followed by the precipitation of find no evidence for the occurrence of non- goethite, gibbsite and kaolinite within and around crystalline material as an intermediate stage in the primary mineral. Complete removal of the latter their study of feldspar weathering in Western left a porous, ferruginous boxwork of weathering Australia. In this instance, the feldspar was products, with optically oriented pendants observed to weather directly to halloysite and projecting into the void spaces. It was concluded gibbsite, although this could represent a later that these textures had been formed both by stage of weathering as the material studied is neoformation and transformation mechanisms. In described as saprolite rather than weathered rock. soils both pyroxenes and amphiboles are frequently found to be heavily etched, as described in detail by Weathering in saprolite and soils Berner et al. (1980) and Berner & Schott (1982). Even in Arctic soils, such etching is prominent, Dissolution mechanism. As outlined above, the increasing with the age of the soil and enabling reactions in the saprolite may be considered as weathering rates to be calculated (Locke, 1986). comparable in some ways to the alteration process that occurs in an experimental through-flow reactor, about which there is still some controversy. The WEATHERING OF FELDSPARS main features of this debate concern the mechanism Transformations in weathered rock of feldspar dissolution, with particular points of interest as far as this paper is concerned, being the The earliest stages of feldspar decomposition in role of dislocations, defects and micro-textural the weathered rock stage were investigated by features in the dissolution process of the exposed Eggleton & Buseck (1980) in their high-resolution mineral surface. These and other areas of feldspar TEM (HRTEM) study of partly altered K-feldspar dissolution kinetics were comprehensively reviewed megacrysts in granodiorite. Weathering of the by Blum & Stillings (1995), but there have been feldspar was considered to take place at structural interesting developments since then. Early studies defects, particularly at the boundary of twinned and of the experimental dissolution of feldspar seemed untwinned domains, firstly yielding amorphous to show that the process was incongruent and was materials. These materials later crystallized to consistent with the hypothesis of a leached layer what was interpreted as interstratified illite-mont- (Wollast, 1967). This hypothesis requires that a morillonite, although no XRD evidence was cation-depleted coating, consisting of Si and Al, presented. A further TEM study was made of the forms around the weathering feldspar grain and weathering of K-feldspar and plagioclase feldspar in thence controls the rate at which cations are 244 M. J. Wilson released from the feldspar structure. These cations ering processes. These studies have made use of must first exchange with H+ ions and then diffuse depth profiling techniques to establish definitively by a solid state mechanism from the interior of the the nature of any residual surface layer. Using back- feldspar, through the leached layer into the external scattering spectrometry, Casey et al. (1989) showed solution. This leached layer maintains a constant that labradorite in acid solutions (pH 1 to 3) was thickness, because it too is gradually dissolving at a infiltrated by H+ ions to a depth of several hundred constant rate and tightly adheres to the feldspar Angstroms, resulting in depletion of Na+,Ca2+ and grain. This hypothesis was widely accepted until the Al3+ and in the production of an amorphous Si-rich work of Berner and his colleagues threw doubt surface. At higher pH values though, there is only upon the existence of a leached layer in both limited penetration of H+ ions and dissolution experimentally altered and naturally weathered occurs at the immediate mineral surface. Similar feldspars. Firstly, Petrovich et al. (1976) used conclusions were reached by Muir et al. (1989) X-ray photoelectron spectroscopy (XPS) to show after reacting labradorite in solutions of pH 3.5 and that on experimentally altered sanidine grains, if a 5.7, using secondary ion mass spectrometry (SIMS) leached layer did exist it was no more than ~17 A˚ and XPS. Furthermore, extensive etching was thick, which is far too thin to control the rate of observed on some of the mineral surfaces, diffusion of cations into the external solution. showing that dissolution was occurring at selected Holdren & Berner (1979) presented further XPS sites of weakness. A later SIMS and XPS and SEM evidence to show that there was no investigation showed that the thickness of the leached layer in an experimentally altered albite, siliceous leached layer depended directly upon the suggesting instead that feldspar dissolution was a composition of the plagioclase feldspar, as well as surface-controlled reaction. They accounted for the the pH of the altering solution (Muir et al., 1990). parabolic dissolution kinetics of feldspar dissolution The thickness of the layer increased in the order by proposing that the process occurred in two albite < oligoclase < labradorite < bytownite and stages, the first being due to the non-linear underlined the importance of the hydrolysis of dissolution of ultra-fine particles produced by SiÀOÀAl bonds in the mineral structure. Casey et grinding, and the second to linear dissolution at al. (1991b) quantified the influence of plagioclase sites of excess surface energy due to the emergence composition on dissolution rates at pH 3 and of dislocations or defects. These sites are revealed showed that these rates varied non-linearly for the by distinctive patterns of etch pitting both in Ca-rich feldspars and were more variable than the naturally and experimentally weathered feldspars Na-rich feldspars. Dissolution rates (Àlog R)of and were considered to be fundamentally incon- various plagioclases as a function of mole % sistent with the residual leached layer hypothesis anorthite in pH 3.0 solutions range from ~10À15 (Wilson, 1975, Berner & Holdren, 1979). The latter for albite, to 10À14 for bytownite, to 10À13 for authors also showed that XPS data on naturally anorthite. weathered albite, oligoclase and microcline did not Nesbitt et al. (1991) addressed the question of the support the existence of a residual coating, although influence of dissolved cations on the mechanism of the conclusions of Chou & Wollast (1984) dissolution of labradorite in acidic solutions. Using following their study of the laboratory weathering SIMS, they found that no residual layer formed of albite appeared to challenge this interpretation. after leaching with distilled water, that HCl However, in published correspondence between the leaching yielded a 1500 A˚ thick silica-rich layer two groups of authors (Berner et al., 1985; Chou & and that the addition of Na, K, Ca and Al to the Wollast, 1985) it was agreed that the XPS data acid could reduce this thickness to <75 A˚ .They could be reconciled with the solution chemical concluded that incongruent dissolution occurs in results if dissolution occurred preferentially at HCl solution but that congruent dissolution occurs dislocations which intersected only a small part of in most electrolyte solutions where dissolved the mineral surface. cations are more abundant than protons. For most Later studies, particularly focusing on plagioclase natural soil solutions this would be the expected feldspars, have added further detail to the situation and consequently extensive leached layers mechanism involved in their alteration and have would be unlikely to occur on naturally weathered also addressed the question of the extent to which feldspar grains, consistent with the observations of laboratory experiments can simulate natural weath- Berner & Holdren (1979). However, Stillings and Weathering of rock-forming minerals 245

Brantley (1995) and Brantley & Stillings (1996) quartz, all arrive at the same conclusion, namely found that most of the feldspars that they examined, that for the minerals studied there is a very poor, or except oligoclase and labradorite, dissolved stoi- even no, correlation between dislocation density chiometrically in the laboratory, even without salts. and dissolution rate in experimental systems. Thus, Recent evidence from Teng et al. (2001) using Blum et al. (1990) found virtually indistinguishable atomic force microscopy (AFM) and synchrotron dissolution rates of synthetic quartz with a X-ray reflectivity indicates that the (001) cleavage dislocation density of <105 cmÀ2 and the same surfaces of orthoclase feldspar dissolves stoichio- plastically deformed quartz with a dislocation metrically under acid conditions (pH 1) except for a density of ~561010 cmÀ2 and dramatically non-stoichiometric layer of a single unit-cell increased etch pitting in distilled water at 80ºC thickness. and in 0.2 M HF at 22ºC. It was concluded that The nature of naturally weathered feldspar etch-pit dissolution makes a minor contribution to surfaces is still a subject of debate. Nesbitt & overall dissolution rate of minerals, particularly in Muir (1988) examined by SIMS the surface of under-saturated systems such as soils. Despite such oligoclase from weathered stones taken from a experimental evidence, this is a rather difficult 10,000 year old till, and concluded that there was conclusion for a soil mineralogist to accept, faced an enrichment of Al and a depletion of Si. As this with feldspar grains under the SEM showing contrasts with what is usually found in laboratory intensively etched but otherwise clean and sharp experiments, it was suggested that the results from surfaces. Recent work by Teng (2004) on the such experiments may not be applicable to natural dissolution of calcite in relation to saturation state environments. But a more recent SIMS and AFM and etch pit formation may help to reconcile these study of the surfaces of albite after burial in soil for viewpoints. Using in situ fluid-cell AFM, Teng various periods of time, showed that they were (2004) found that three different dissolution modes depleted in Al (and Na), suggesting dissolution could be observed and related to saturation state. mechanisms similar to those observed in the Two of these modes occurred near-equilibrium and laboratory (Nugent et al., 1998). The elevated far-from-equilibrium, where there was only a weak Al/Si ratios observed by Nesbitt & Muir (1988) dependence of dissolution rate on dislocation were attributed to a thin patchy coating of an density. In the near-equilibrium situation, no etch alumino-silicate, undetectable by SEM but visible pit formation occurs and dissolution occurs at steps under the AFM. on the mineral surface. A sharp increase in etch pit formation occurs far-from-equilibrium, but this is uncontrolled in that it occurs in defect-free regions Influence of dislocations and microtexture on as well as along dislocations. Therefore, there is no feldspar dissolution significant relationship between dissolution rate and Many of the studies referred to above stress the dislocation density far-from-equilibrium, the region influence of dislocations and defects, as well as where most experimental mineral weathering compositional heterogeneities on the mechanism of studies have been conducted. At intermediate feldspar dissolution, as well as the role of these saturation levels, however, dissolution takes place features in increasing specific surface area. Wilson primarily at dislocations and along steps, so that (1975) drew attention to the deeply etched nature of dissolution rate should be more significantly related feldspar grains in some Scottish soils and, by to dislocation density in this saturation range. So far analogy with correspondence between etch pits and as feldspars are concerned, it is not known whether dislocations in metals, surmized that there was a the waters draining through saprolites and soils are similar relationship in weathering feldspars. Berner typically in this intermediate saturation range where & Holdren (1977) independently arrived at a similar dissolution rate relates to dislocation density on conclusion and provided compelling evidence to exposed surfaces, but this must certainly be indicate that such etching essentially controlled the considered as a possibility. natural feldspar dissolution process (Berner & The importance of micro-texture, in this case thin Holdren, 1979). However, investigations such as lamellae of albite in a host of K-feldspar (Fig. 5), as those by Casey et al. (1991a) for rutile, Holdren et sites of dislocation and foci of dissolution was al. (1988) for calcic plagioclase feldspar, Schott et illustrated for an experimentally altered-microcline al. (1989) for calcite and Blum et al. (1990) for perthite by Wilson & McHardy (1980). Similarly, 246 M. J. Wilson

FIG. 5. (a) SEM (40 mm wide) of the 010 surface of microcline-perthite after mild hydrothermal treatment showing perthitic lamellae transected by dislocations. (b) SEM (40 mm wide) of 010 microcline-perthite surface after further hydrothermal treatment showing selective etching of the perthitic lamellae (Wilson & McHardy, 1980).

Inskeep et al. (1991) emphasized the susceptibility feature of the work of Lee & Parsons (1995) is to weathering of Ca-rich exsolution lamellae, their use of a resin-impregnation technique to reveal ~700 A˚ thick, relative to the more sodic phase in the details of this micro-porosity and the way in their TEM/XPS study of experimentally altered which etch pits inter-connect beneath the mineral labradorite. This selective dissolution produced a surface. They concluded that experimental dissolu- corrugated surface and significantly affected the tion of the perthitic alkali feldspar is primarily XPS analyses. Oxburgh et al. (1994) also surmized focused on edge dislocations of the perthitic that the degree of exsolution in feldspars could lamellae and, in the case of natural weathering, on greatly affect their dissolution rate. This relation- the perthitic lamellae themselves. Lee et al. (1998) ship was explored in great detail by Lee & Parsons later sought to reconcile the lack of correlation (1995) for a perthitic alkali feldspar. This feldspar between dislocation density and experimental contained various types of micro-texture, including dissolution rate with the compelling SEM imagery cryptoperthites made up of <75 nm wide albite showing that micro-texture and associated disloca- exsolution lamellae, lamellar micro-perthites with tions essentially control the progress of dissolution >75 nm albite films, irregular patch perthites during natural weathering. It was concluded that consisting of semi-coherent and coherent inter- this inconsistency could probably be accounted for growths of albite and microcline with significant by the differences in the saturation state of the micro-porosity. This micro-porous texture is present weathering solutions in the laboratory and under in unweathered grains and is a feature of deuteric or natural conditions, as previously outlined. They hydrothermal alteration. A striking innovative attributed particular importance to dislocation Weathering of rock-forming minerals 247 density during the early stages of weathering and controlled by the number of defects cropping out on suggested that etching has a positive feedback effect the feldspar grain surface, and by the distances in that it promotes the disintegration of the grains, between these defects. In large grains this distance exposing further reactive surface to weathering. is much less than the size of the grain, so that if the density of defects per unit surface area remains constant over a range of grain sizes, then reaction Weathering rates of feldspar and surface area rate will correlate with specific surface area. With A compilation of experimentally derived dissolu- decreasing grain size, eventually the distance tion rates for the main feldspar minerals was between defects is the same or larger than the published by Blum & Stillings (1996) and a size of the mineral grain. In this case, no new selection of these data for dissolution at pH 3.0 defects are exposed and the relationship between and 5.0 are shown in Table 4. These rates are dissolution rate and specific surface area will be consistent with the relative order of stability of the lost. For similar reasons Anbeek (1992, 1993) also feldspar minerals to natural weathering as proposed found that the larger particles of freshly ground and by Goldich (1937). naturally weathered feldspars were more reactive However, the assumption that weathering rates of than their finer-grained counterparts. These obser- feldspar are proportional to exposed surface area is vations imply that some coarser particle sizes of not necessarily true, as indicated by Holdren & feldspar could be more sensitive to weathering in Speyer (1985). They considered that weathering soils than the finer grain sizes. Such a finding was occurred at selective sites of weakness and not over made by Bain et al. (1994). They found that the the grain surface as a whole, leading them to plagioclase feldspar content of the coarse sand distinguish between a surface reaction-controlled fraction (200À2000 mm) decreased considerably mechanism and a surface area-controlled model. from the C to the A/E horizons in two podzolic Holdren & Speyer (1985) showed that dissolution soils whilst that of the fine sand fraction rates of five different size fractions of an alkali (20À200 mm) remained constant (Table 5). The feldspar, the specific surface area of which spanned SEM observations showed that the coarse sand a range of a factor of 20, varied by less than a feldspars were more heavily etched. factor of 2. They accounted for this lack of correlation by suggesting that reaction rate was Weathering products of feldspar in saprolites and soils

TABLE 4. Dissolution rates (R = Àlog mol Experimental alteration of feldspars rarely yields 2 1 felds cm sÀ ) of feldspar minerals at pH 3.0 crystalline clay mineral products such as are and 5.0 from data compiled by Blum & frequently found in the varied environments that Stillings (1996). occur in saprolites and soils. The most common products yielded by feldspar weathering are Mineral pH R halloysite and kaolinite. Thus, Parham (1969a) showed that halloysite formed as a weathering K-feldspar 3.0 15.6 5.0 16.6 TABLE 5. Plagioclase feldspar (oligoclase) content (%) Albite 3.0 15.5 in coarse sand (200À2000 mm) and fine sand 5.0 16.0 (20À200 mm) fractions from basal and upper horizons Oligoclase 3.0 15.4 in the Allt Mharcaidh catchment, Cairngorm Moun- 5.0 16.2 tains, Scotland (Bain et al., 1994). Andesine 3.0 14.6 5.0 15.5 Soil type Horizon Coarse sand Fine sand Bytownite 3.0 13.4 Peaty podzol Eg 27 45 5.0 14.8 C4046 Anorthite 3.0 10.0 Alpine podzol AehR 23 51 5.0 12.1 C4452 248 M. J. Wilson product from orthoclase in weathered Hong Kong sufficiently slow to allow concentration gradients to granite, possibly via an amorphous precursor become established and for the system to reach (Parham, 1969b). Using SEM, Eswaran & Bin equilibrium in accordance with theoretical activity (1978) also demonstrated that feldspar weathered diagrams. primarily to halloysite in a weathered granite in It has often been asserted that illite is a common Malaysia, but additionally showed that both weathering product of feldspar in soils, but as yet halloysite and kaolinite could occur in the same there seems to be little unequivocal evidence to feldspar pseudomorph. Robertson & Eggleton support this contention. Bearing in mind the higher (1991) showed that plagioclase feldspar in deeply temperatures and pressures necessary to form illite weathered granite in Queensland was converted to during the diagenesis of smectitic sediments, this is kaolinite, which then altered to spiral halloysite perhaps not surprising. rods through a process involving hydration. Jeong (1998) also observed co-existing kaolinite and Role of organic acids in feldspar weathering halloysite in labradorite in a weathered anorthosite in Korea, but from SEM evidence interpreted the The role of organic acids in mineral weathering, halloysite as forming ellipsoids and tubes on the particularly the implications for feldspar dissolution plagioclase surface in the early stages of weath- in soils, is still the subject of some debate. Huang ering. Globular aggregates of halloysite later & Keller (1970) treated sand-sized grains of formed and then coalesced to form stacked kaolinite microcline and labradorite with weakly complexing plates. acids (0.01 M acetic and aspartic acids) and strongly Gibbsite too may be a product of feldspar complexing acids (0.01 M salicylic and tartaric weathering, even in its early stages in soils and acids) and found that the solubilities of all cations saprolites. Thus, in a young montane soil developed were generally higher than in distilled water or on allivalite (an ultra-basic anorthite-olivine rock) CO2-charged water. Again, Huang & Kiang (1972) on the island of Rhum, the clay fraction contained investigated the dissolution of fresh plagioclase an abundance of gibbsite which could only have feldspars – albite, oligoclase, labradorite, bytownite come from the feldspar (Wilson, 1969). Again, and anorthite – in water and in various organic Tazaki (1976) found that gibbsite formed from acids at 0.01 M concentration. They found that Ca- plagioclase feldspar in relatively young weathered rich feldspars dissolved more readily in organic volcanic ash and Kawano & Tomita (1996) acids than water, but that the opposite was the case observed that a mixture of halloysite and gibbsite for the Na-rich feldspars. The effectiveness of formed on the early weathered surfaces of organic acid dissolution was attributed to its K-feldspar in granitic rock in Yakushima Island, ability to complex Al from the mineral. However, Japan. In both instances an amorphous precursor Manley & Evans (1986) considered that the organic was involved. acid concentrations used in the above experiments Different weathering products may be formed were too extreme when compared with what are À4 from feldspar under more closed and alkaline actually found in soils. They used 10 M low conditions. For example, Wilson et al. (1971) molecular weight organic acids to dissolve labra- showed that in granite and granulite cobbles in a doritic plagioclase feldspar, albite and microcline deeply weathered boulder conglomerate in northeast and found that, while citric and oxalic acid were the Scotland, all the feldspars – orthoclase, orthoclase most effective of the organic acids used, the microperthite, albite and oligoclase – had trans- strength of the acid was more important than its formed to a Cheto-type montmorillonite poor in Fe. ability to complex metals. Similarly, Mast & Drever In this instance there appeared to be little or no (1987) found that oxalic acid at concentrations up structural control of the parent feldspar or evidence to 1 mM did not affect the rate of oligoclase of a precursor amorphous phase. Again, Rodgers & dissolution and that dissolution rate was also Holland (1979) investigated the weathering independent of pH over the range 4 to 7. In fact, products of micro-cracked feldspar in tonalite Drever (1994) minimized the effect of organic acids cobbles from a morainic deposit in Montana. on feldspar weathering, whether the acids are They found only kaolinite in orthoclase, whereas produced directly by land plants or indirectly in oligoclase there was both kaolinite and smectite. following the decomposition of organic matter in They inferred from this that transport of solutes was soils. Laboratory experiments showed that for alkali Weathering of rock-forming minerals 249 feldspars oxalic acid had a negligible effect on where mineral surfaces may be in direct contact dissolution rate, but that for anorthitic plagioclase with roots or fungal hyphae which may excrete feldspars dissolution rates increased by a factor of organic acids at much higher concentrations than ~2 to 3. For labradorite and bytownite, Welch & are found in drainage waters. Lichens represent Ullman (1993) found that dissolution rates in convenient model systems where such close organic acid solutions, particularly oxalic, citric, contacts between minerals and organic acid succinic, pyruvic and 2-ketogluconic acids, could be excreting organisms, in this case the mycobiont or up to 10 times greater than the dissolution rates in fungal partner of the lichen symbiosis, may be mineral acids at the same pH. The enhancement of studied easily. Jones et al. (1980) studied the ligand-promoted dissolution of Al and Si was weathering of basalt by the crustose lichen particularly strong at near neutral pH values, Pertusaria corallina and concluded that the probably due to the formation of strong bidentate labradorite-bytownite plagioclase feldspar in the chelates at the mineral surface. Similarly, the results rock was being etched directly by oxalic acid of Stillings et al. (1996) for the dissolution of excreted from the mycobiont. They reasoned that microcline, albite, oligoclase, andesine and bytow- accumulations of crystalline Ca oxalate at the nite in flow-through reactors in 1 mM oxalic acid lichen-rock interface resulted from this interaction, solutions showed 2À15-fold increases compared with the Ca being provided by feldspar. Similar with inorganic acids. This enhancement was observations were made following treatment of especially marked in the pH range 5 to 6. A plagioclase feldspar with 0.5 M oxalic acid and with review by Drever & Stillings (1997) concludes that the oxalic acid-producing-fungus Aspergillus niger. 1mM oxalic acid shows feldspar dissolution effects The link between the oxalic acid-producing ranging from none at all to enhancement by a factor mycobiont and substrate mineralogy was further of 15, and that humic acids do not significantly reinforced by the finding of unusual and new affect feldspar dissolution. The effect of organic oxalate minerals where lichens had weathered acids on feldspar weathering rates is thus consid- substrates of appropriate composition (Wilson et ered to be rather small, with the possible exception al., 1981; Wilson & Jones, 1984; Purvis, 1984). In of micro-environments immediately adjacent to addition, lichen weathering also resulted in the roots and fungal hyphae. formation of siliceous relics, aluminous silicate gel- Recently, van Hees et al. (2002) investigated the like material and poorly ordered Fe oxide minerals effects of naturally occurring soil organic acids on (Jones et al., 1980; Jones et al., 1981). However, microcline and labradorite dissolution and found later studies queried the role of oxalic acid in lichen that at pH 5 the rate increased from 2.4 to 5.7 times weathering of feldspars and other rock-forming from aqueous dissolution at the same pH, which minerals. Thus, Barker & Banfield (1996) investi- was considered to be a significant enhancement. gated the weathering of amphibole syenite by The evidence would appear to indicate, therefore, Rhizocarpon grande and Porpidea albocaerulescens that organic acids such as oxalic and citric at the and could find no evidence of ‘‘pervasive leaching’’ concentrations likely to occur in soils may increase of feldspars or other minerals, all of which the dissolution rates of feldspars, usually by less remained perfectly intact in the lichen thallus, or than an order of magnitude, through their ability to of the existence of siliceous relics such as were complex Al particularly around pH 5. At lower pH identified by Wilson et al. (1981). They attributed values increased dissolution effects are more what biologically mediated weathering there was, to attributable to the higher proton concentration extracellular organic polymers, most likely acidic rather than the complexing nature of the acid. mucopolysaccharides, and argued against the role of Calcium-rich plagioclase feldspars are more suscep- lichen and oxalic acid as important weathering tible to attack by organic acids than K-feldspars by agents. Further, they implied that the siliceous relics virtue of their higher Al content. observed by Wilson et al. (1981) could have been artefacts of the preparation procedure. These various findings and interpretations may quite Biological weathering of feldspars possibly be accounted for by the fact that different As realized by Drever (1994), the conclusions lichens were being studied. In fact, not all lichens drawn by laboratory experiments may not necessa- are able to excrete metal-complexing organic acids rily apply to the specialized environment of the soil and sometimes lichens may actually reduce weath- 250 M. J. Wilson ering rates. Lee & Parsons (1999) in their study of plagioclase in an unvegetated catchment. Such weathering by Rhizocarpon geographicum on Shap enhanced weathering suggested a major role for granite concluded that the alkali feldspars beneath plants in lowering atmospheric CO2 in geological the lichen, although extensively etched, had most history. likely been subjected to non-biochemical weath- The weathering of feldspars by mycorrhizal fungi ering before lichen colonization. Further, they has became a subject of some debate since the observed silica-rich material similar to that found paper of Jongmans et al. (1997) on rock-eating by Wilson et al. (1981) but concluded that this fungi. They described tunnel-like features within could actually help to retard weathering by the feldspar and hornblende grains in forest soils, some lichen. Wilson (2004) also drew attention to work of which were colonized by fungal hyphae. describing the protective effect of some lichens to Jongmans et al. (1997) hypothesized that the destructive weathering of ancient stonework. tunnels were actually formed by ectomycorrhizal Further evidence for the efficacy of microbial fungi which were capable of dissolving the mineral extracellular polysaccharides in weathering plagio- by virtue of excreting organic acids. The feldspars clase feldspar (bytownite) was provided by Barker thus acted as the direct source of mineral nutrients et al. (1998) who showed that release of Si and Al to forest trees (van Breemen et al., 2000; was increased by up to 2 orders of magnitude in Landeweert et al., 2001). However, another batch reactors to which bacteria were added, interpretation could be that the fungal hyphae compared with inorganic controls. In many cases were fortuitously occupying these spaces within the grains were covered by biofilms, the mineral the feldspar grains and that these tunnels, therefore, surfaces appearing to be more extensively etched pre-date weathering. The existence of such features beneath these biofilms. Again, Welch et al. (1999) in fresh feldspars is well documented by Lee et al. made similar observations with respect to an (1998). Furthermore, Wallander & Wickman (1999) enhancement of plagioclase feldspar dissolution could find no evidence that ectomycorrhizal fungi when the mineral was reacted with solutions of in Pinus sylvestris could effectively mobilize K acid polysaccharide, although at neutral pH these from microcline in a pot study, although the substances inhibited release of Si and Al. It is duration of experiment was rather short (33 uncertain at present whether these findings can be weeks). On the other hand, Hoffland et al. (2002) applied to natural environments because, as pointed showed that the intensity of tunnelling in feldspar out by Welch et al. (1999), water is unlimited in grains increased with the age of the soil in a podzol batch reactors but restricted and episodic in lichens. chronosequence, the implication being that the Although the lichen-rock relationship does tunnels were being formed during weathering and represent a convenient model for understanding that the fungal hyphae within the tunnels demon- microbially mediated mineral weathering in the soil strate a causal relationship. rhizosphere (Barker & Banfield, 1998; Banfield et al., 1999), increasingly, efforts are being made to WEATHERING OF MICAS characterize the nature of feldspar weathering within the rhizospheric soil itself. In part, this is A huge amount of work has been done on due to the realization that higher plants are capable weathering of mica minerals because of their of markedly enhancing weathering rates. Thus, importance as a potential source of K in the soil Cochran & Berner (1996) found that vascular for plants. It has long been known that there was a plants on Hawaiian basaltic lava flows increased sharp contrast in the susceptibility to weathering of weathering rates by at least a factor of 10 when biotite compared with muscovite. Biotite weathers compared with unvegetated or lichenized flows. very easily and may progress to advanced stages The dissolution of plagioclase feldspar accounted even at the base of the weathering profile. Thus, the for most of these increased rates. Lichenized mineral may be converted directly to kaolinite and Hawaiian basalts weather more quickly than earlier and less extreme transformations, such as where these rocks were not colonized (Brady et vermiculitization, may be bypassed altogether. In al., 1999), but only by a factor of ~2. Moulton et al. contrast, muscovite is often extremely resistant to (2000) found that plagioclase feldspar in a weathering and may persist unaltered to the top of vegetated catchment developed on tholeiitic lava the weathering profile, even in the old soils. In this flows in Iceland weathered twice as quickly as review, therefore, it is more convenient to consider Weathering of rock-forming minerals 251 mica weathering in terms of degree of alteration of exchange mechanism brought about by splitting of the structure rather than the position of the mineral the particles. This effectively released the stress in the weathering profile. brought about by expansion and bending of the flakes following K depletion of the layers from the flake edge, resulting in inhibition of further K Vermiculitization of mica exchange. The vermiculitization reaction is the simplest In the natural vermiculitization of biotite, the form of biotite weathering and merely involves process frequently involves the formation of exchange of the interlayer K of the mica for ions of hydrobiotite, a regularly interstratified biotite- the external solution. Otherwise, complete structural vermiculite yielding a basal spacing of ~24 A˚ integrity is maintained (Barshad, 1948). This (Coleman et al., 1963; Wilson, 1970). This reaction is a diffusion-controlled process and can interstratification is brought about when K is be accomplished easily in the laboratory (Rausell- removed from every other layer in the biotite Colom et al., 1964), although whether it proceeds structure and implies that there is some mechanism will depend, amongst other things, on the concen- by which K is preferentially retained in the layers tration of K in the external solution (Martin & adjacent to the vermiculitized layers. A plausible Sparks 1985). The critical solution K concentrations reason for this was proposed by Norrish (1973), at 25º C for the vermiculitization of 10À20 mm size who suggested that the regular interstratification flakes of biotite and muscovite in NaCl were found was due to the inclined hydroxyl orientation in to be 11 and 0.1 mg/l, respectively (Scott & Smith these adjacent layers, thus placing the K ions in 1966; Newman 1969). At higher K levels the these layers in a more negative and more stable vermiculitization reaction would stop, providing environment. Bassett (1960) had earlier proposed one possible reason as to why muscovite is so that the reason why phlogopite and biotite were much less susceptible to weathering than biotite. more susceptible to the vermiculitization than The natural weathering of biotite has been muscovite was the perpendicular orientation of the studied intensively since the classic work of dipole moments of the hydroxyl ions in the Walker (1949), focusing particularly upon the trioctahedral micas, compared with the oblique vermiculitization process itself, as well as asso- orientation of these dipole moments in the ciated aspects such as oxidation of octahedral Fe, dioctahedral mica. the mechanism involved in the reduction of layer The natural vermiculitization of biotite involves charge, the development of interstratified struc- not only the replacement of interlayer K by other tures, and conversion of biotite to ‘chlorite’ or cations but other structural changes too, in smectite. particular the oxidation of octahedral ferric ion. In the laboratory, vermiculitization of micas at This was noted by Walker (1947) in the early stages levels of K below equilibrium concentrations can be of biotite weathering in some Aberdeenshire soils maintained in the external solution by precipitating and was later thought to be one of the mechanisms the K as an insoluble compound such as potassium by which layer charge was reduced during the tetraphenylboron (Reed & Scott 1962). Using this vermiculitization process (Newman & Brown, method of vermiculitization, Raman & Jackson 1966). However, the two processes – Fe2+ oxidation (1965) and Newman & Brown (1966) observed that and vermiculitization – do not keep in step, the the alteration process proceeded not only from the former occurring early in the weathering process edges of the flakes, but also by penetration along and the latter increasing progressively as the cracks and other defects normal to the basal plane. weathering proceeds. Some other mechanism is Another complicating factor with respect to the required, therefore, to reduce layer charge (Scott & diffusion control model is the effect of particle size. Amonette, 1988). The ejection of Fe from the Mortland & Lawton (1961) found that K released octahedral sheet of biotite following oxidation is from biotite in NaCl solutions was slower from fine one such mechanism for which there is good particles than from large ones and Reichenbach & evidence in both laboratory and naturally vermicu- Rich (1969) made a similar observation with litized biotite. Farmer et al. (1971) presented respect to muscovite following exchange with infrared and other evidence showing that octahedral BaCl2 at elevated temperatures. They proposed Fe was irreversibly lost during experimental that this effect was related to differences in the vermiculitization of biotite, forming amorphous 252 M. J. Wilson interlayer oxides, and that the number of octahedral orientation such as probably occurs with hydro- sites in the structure increased considerably. Scott biotite. This long-range regularity was suggested as & Amonette (1988) considered that cation ejection being a reflection of the distance of dissipation of accounts for most of the reduced layer charge the stress field set up by an initially vermiculitized observed during vermiculitization of biotite and that layer. there is no need to invoke deprotonatation from The vermiculitization of biotite can occur at any hydroxyls as an additional mechanism. The fact that stage of the weathering profile and may involve oxidation of biotite does not necessarily proceed in other structural changes in addition to those tandem with vermiculitization is shown by the described above. Thus, Moon et al. (1994) provided occurrence of oxybiotites after natural and experi- evidence to show that the loss of layer charge mental alteration (Gilkes et al., 1972). during the vermiculitization of phlogopite involved In fact, the oxidation of octahedral ferrous Fe in loss of Al from the tetrahedral sheet and biotite may be an important factor in the ability of progressive increase in Al (replacing Fe2+ and the mineral to retain its interlayer K, as was earlier Mg) in the octahedral sheet during the early stages pointed out by Barshad & Kishk (1968). of weathering. A similar mechanism was earlier Convincing experimental evidence for the influence proposed by Fordham (1990b) to explain the of oxidation state on release of interlayer K from conversion of biotite to dioctahedral clay minerals biotite to salt solutions, and on overall dissolution in a soil developed on granite gneiss. Both rates of biotite in acidic solutions, was provided by dioctahedral vermiculite and smectite-like minerals Gilkes et al. (1973a,b). The capacity of biotite to were identified primarily on the basis of electron retain most of its interlayer K in weathered soils probe microanalysis, the qualification ‘like’ being (Denison et al., 1929) and in sediments deposited in necessary because of the uncertainty involved in oxidizing conditions, such as the Old Red identifying clay minerals in this way. However, Sandstones (Wilson & Duthie, 1981), may also be smectites may certainly form as a consequence of accounted for by the oxidation state of the mineral. biotite weathering as concluded by Ismail (1969, Recent work by Jeong & Kim (2003) confirms that 1970) under arid and alkaline conditions and biotite can be almost completely oxidized in a Kapoor (1972) under humid acidic conditions. weathering profile and yet retain most of its K in a Earlier, MacEwan (1954) described trioctahedral 10 A˚ structure. The occurrence of trioctahedral illite smectite derived by weathering of biotite in poorly in soil clays is consistent with such observations drained soils. (Fordham, 1990a). Nevertheless, active oxidation may also play an Kaolinization of micas essential role in the vermiculitization process as is shown by the formation of hydrobiotite. Thus, a It should perhaps be emphasized that vermiculi- biotite which weathered naturally to hydrobiotite tization may be bypassed under more intensive (Wilson, 1970) could be altered to hydrobiotite in weathering conditions and conversion of biotite the laboratory only under oxidizing conditions directly to kaolinite may occur (De Kimpe & (Farmer & Wilson, 1970). Gilkes (1973) also Tardy, 1968). In fact, biotite may be completely found that oxidation of structural iron in biotite pseudomorphed by kaolinite (Mitsuda, 1960) and and removal of K from alternate layers led to this transformation may take place within a few formation of hydrobiotite during experimental centimetres of fresh rock (Eswaran & Heng, 1976). vermiculitization. Optical observations of kaolinized biotite often The vermiculitization of biotite may not necessa- suggest an orientation relationship between the rily proceed through a hydrobiotite stage but in this two phases and this was confirmed by single case it still involves depletion of K from some crystal X-ray photographs (Wilson, 1966; Tzuzuki preferred layers which are always bounded by other et al., 1968). An epitaxial origin for the oriented layers which are not K-depleted (Banfield & kaolinite, where the biotite basal surface acts as a Eggleton, 1988). Using TEM these authors some- template for kaolinite growth, was suggested by times observed a long-range regularity, where Wilson (1966) and by Banfield & Eggleton (1988), vermiculite layers were separated by 4 to 5 biotite but other works indicate that a topotactic origin layers, and argued that such an arrangement could involving complete kaolinite replacement of the not be accounted for by a change in hydroxyl biotite is also feasible (Gilkes & Suddhiprakarn, Weathering of rock-forming minerals 253

1979a,b; Harris et al., 1985a,b; Rebertus et al., Weathering of micas in the soil 1986; Ahn & Peacor, 1987). Recent work seems to confirm the viability of these different modes of All the processes and products of weathering origin. Thus, Dong et al. (1998) provided TEM discussed so far can occur in any stage of the fully evidence to show that a single layer of biotite developed weathering profile. However, in the soil, altered to two layers of kaolinite. On the other micas may weather in a way that seems to be hand, Jeong (1998, 2000) showed that the weath- specific to this environment. Dioctahedral micas, ering of biotite to kaolinite involved up to a 9-fold for example, may be depleted of interlayer K to volume increase, suggesting an extraneous source yield what is essentially a dioctahedral analogue of of the Al (and presumably Si) and growth of vermiculite, a mineral first described from British kaolinite on templates provided by the biotite basal soils by Brown (1953). Dioctahedral vermiculite is surfaces. It would be logical to think that the almost always confined to soil clay fractions and kaolinite associated with exfoliated flakes of occurs particularly in intensively weathered soils muscovite also forms by an epitactic mechanism, such as Ultisols, Alfisols and even Oxisols. It has but TEM and analytical evidence favour a rarely, if ever, been identified as a weathering topotactic origin in two instances (Robertson & product of muscovitic mica in the saprolite or Eggleton, 1991; Singh & Gilkes, 1991). weathered rock parts of weathering profile. The reason for this may be that vermiculitization of dioctahedral mica can only take place from Multi-component weathering products of exceedingly thin crystals like those described from biotite soils (Robert et al., 1991). Particle microdivision Many weathered biotite flakes, which may or has been shown to be an important part of the mica- may not be vermiculitized, consist of a multi- weathering process in soils (Romero et al., 1992; component mineral complex involving halloysite Aouidjit et al., 1996) and there is evidence that (Eswaran & Heng, 1976; Kretzschmar et al., 1997), with decreasing grain size the 2M1 muscovite may gibbsite (Wilson, 1966; Tsuzuki et al., 1968; convert to the 1Md polytype (Martin-Garcia et al., Jolicoeur et al., 2000) and goethite (Eswaran & 1997). Aoiudjit et al. (1996) presented HRTEM Heng, 1976; Gilkes & Suddiprakarn, 1979a,b; evidence to show that vermiculitization of Banfield & Eggleton, 1988). Halloysite appears to muscovitic mica occurred in very fine particles. have grown preferentially on the biotite cleavage They showed micrographs of particles with a surfaces but there is no indication of a strong muscovite core and a 15 layer-thick vermiculitic orientation relationship. Gibbsite too, although rim, as well as muscovite-derived vermiculite oriented parallel to the basal cleavage of biotite, consisting of 2 to 5 layers in thickness. Fine- is randomly oriented in the ab plane (Tsuzuki et grained dioctahedral vermiculite may therefore be al., 1968). These observations suggest that both susceptible to vermiculitization in the soil because minerals are formed as a result of crystallization of its higher specific surface and enhanced from solutions not necessarily directly connected reactivity. However, this concept is inconsistent with the weathering of biotite itself, although with the experimental data of Reichenbach & Rich Jolicoeur et al. (2000) interpret SEM evidence as (1969) which suggests that muscovitic mica retains indicating topotactic formation of halloysite from its interlayer K more tenaciously with decreasing biotite. In contrast, both Gilkes & Suddiprakarn grain size. (1979b) and Banfield & Eggleton (1988) report that It may be noted that a dioctahedral analogue of goethite may occur as oriented laths on the biotite hydrobiotite has also been described from some soil surfaces indicating that the surface influences the clay fractions, where it clearly derives from the orientation of the Fe oxide mineral, although both weathering of dioctahedral mica (Churchman, 1978, sets of authors conclude that this does not 1980). A regularly interstratified rectoritic mineral necessarily indicate an epitaxial relationship. was also obtained from the alteration of sericite by However, in all probability, the source of the Fe Tomita (1977). In these instances the mechanism must at least in part be attributed to the ejection involved in the origin of the regular interstratifica- from the biotite structure of octahedral Fe tion remains to be elucidated, but it is clearly not following oxidation, as suggested by Eswaran & likely to involve the oxidation of octahedral ferrous Heng (1976). Fe. 254 M. J. Wilson

More usually, dioctahedral soil vermiculites are experimental system used they found that biotite converted to interlayered or intergradient minerals could be completely vermiculitized, where the fungi due to the introduction of non-exchangeable acted merely as K+ sinks and where K was hydroxy-Al polymers into the interlamellar space. exchanged for Na in external solution. There was The aluminous material may derive directly from even a small but significant vermiculitization of the decomposition of the tetrahedral or octahedral muscovite. Where the fungi were in direct contact sheets within the mineral structure, or indirectly with the micas, K-depletion of the minerals was not from the weathering of other aluminous components so marked but was still easily observable. The such as feldspars. Al-hydroxy interlayered vermi- effect of soya bean mycorrhizal fungi on biotite culites are exceedingly common in acidic soils, e.g. weathering was shown by Mojallali & Weed (1978) April et al., (1986) and Karathanasis, (1988), the to promote vermiculitization which occurred parti- stability of the interlayer material being favoured by cularly at the edge of sand-sized flakes. However, the pH range 4.0À5.8 approximately (Barnhisel & in addition to depletion of K there was also a loss Bertsch, 1989). The pH dependency of the of Al and Si, suggesting a more drastic structural interlayers of dioctahedral vermiculite in some rearrangement. Complete breakdown of biotite to an Scottish podzolic soils was demonstrated by Bain amorphous product following inoculation by et al. (1990), complete removal being detected in Aspergillus niger was reported by Boyle et al. E-horizons where pH fell below 4.3. (1967) and was attributed to the production of Although most Al-interlayered vermiculites are oxalic and citric acid. Mycorrhizal weathering of dioctahedral, trioctahedral vermiculites resulting biotite on Pinus sylvestris seedlings grown in pot from biotite weathering can also be interlayered in experiments using forest soils was also attributed to the same way. This was first shown by Kato (1965) oxalic and citric acids excreted by the fungus in Japan and by Wilson (1966) in Scotland and has (Wallander & Wickman, 1999; Wallander, 2000). since been described in Piedmont soils in the USA The ability of the mycobiont associated with (Rebertus et al., 1986; Jolicoeur et al., 2000). some crustose lichens to completely convert a trioctahedral mica to a siliceous relic through the complexing action of oxalic acid was suggested by Biological weathering of micas the work of Wilson & Jones (1983). However, Biotite can also weather in the soil as a direct Leyval & Berthelin (1991) reported greater K losses result of uptake by plants of interlayer K or by from phlogopite when Pinus sylvestris was inocu- decomposition by organic acids excreted by plant lated with acid-producing bacteria (Agrobacterium rootlets, fungi or bacteria. Mortland et al. (1956) sp.) compared with inoculation of a mycorrhizal showed that fresh biotite, applied as the sole source fungus (Laccaria laccata). Nevertheless, greater K of K in a pot experiment with wheat, was almost losses occurred where the phlogopite was closely completely vermiculitized to a 14 A˚ structure, a associated with the mycorrhizae, although that was transformation accompanied by depletion of total K not attributed to acid production. (5.8 to 2.8%) and an increase in CEC (11 to 54 mEq/100 g). A similar result was obtained by Rates of weathering of micas Spyridakis et al. (1967) when coniferous and deciduous tree seedlings were grown over a With regard to the rates of weathering of micas, 13 month period in sand cultures with biotite as a there is a difficulty in making comparisons with the sole source of K and Mg. Additionally, they other main rock-forming minerals because typically claimed that most of the species tested produced the trioctahedral micas do not dissolve stoichiome- kaolinite. However, while the XRD evidence for trically in aqueous acidic solutions. For example, identifying the clay mineral is convincing, it is Acker & Bricker (1992) found that in the pH range notable that such rapid formation of kaolinite has 5.0 to 5.5, Mg is the only cation detected in never been found by other researchers. (A possible substantial amounts in the altering solutions, explanation for the results of Spyridakis et al. indicating attack on the octahedral sheet and no (1967) is that the sand used for their experiments significant dissolution of tetrahedral sheet. At lower may itself have been contaminated with kaolinite). pH values there are indications that the whole of The direct effect of fungal species on weathering the biotite structure begins to decompose, but still of micas was tested by Weed et al. (1969). In the incongruently. It is only at pH 3 that increasing Si Weathering of rock-forming minerals 255 and Al concentrations indicate significant decom- involving, therefore, non-stoichiometric and prefer- position of the tetrahedral sheet, with increasing Mg ential dissolution of the brucite sheet. It was and Fe concentrations showing further attack on the proposed that there was a tendency for every octahedral sheet. Even at pH 1 the dissolution of second brucite sheet to be removed leading to a biotite is incongruent and anisotropic, with the edge semi-regularly interstratified vermiculite-chlorite. surfaces being attacked in preference to the basal This process developed through a change to a surfaces (Turpault & Trotignon, 1994). These lower energy IIa stacking arrangement of the authors found that short-term leaching occurred in vermiculitized interlayer, thus increasing the stabi- the order K (interlayer) > Fe, Al > Mg (octahedral lity of adjacent interlayers because of reduced layer) > Si (tetrahedral layer). Acker & Bricker repulsion between the interlayer cations and the (1992) suggested that at pH 4 the dissolved ion adjoining tetrahedral sheets. The oxidation and ratios were consistent with the formation of a ejection of Fe from octahedral sites in order to vermiculitic product where tetrahedral Al was reduce layer charge, as in the biotite to vermiculite conserved and where substantial amounts of transformation, was not thought to be involved in interlayer K and octahedral Mg, Fe and Al were the process. Aspandiar & Eggleton (2002a,b) also released. Such a product has previously been described the topotactic conversion of chlorite to a described in a natural weathering profile by regularly interstratified chlorite-vermiculite (corren- Velbel (1985). Further, Acker and Bricker deter- site) in weathered basaltic rock and invoked a mined octahedral dissolution for biotite at pH 6.7 at similar mechanism to account for the stacking 3.9610À12 moles/m2 sÀ1. This rate was some 30 arrangement. It should be noted, however, that all times higher than that determined by Velbel (1985) the above studies involved weathering of hydro- which was calculated by geochemical mass-balance thermally altered rock. In these circumstances and where the pH of the natural waters was ~6. At pH 5 bearing in mind the occurrence of interstratified the octahedral dissolution rate determined by Acker chloritic minerals in meta-basalts (Shau et al., and Bricker was 9.1610À12 moles/m2 sÀ1, which is 1990) and even vermiculite-like minerals in low- quite similar to the rates found later at this pH for grade metamorphic sequences (Ruiz Cruz, 1999), it both biotite and phlogopite by Kalinowski & is obviously prudent to distinguish unequivocally Schweda (1996). These authors also found that between the effects of hydrothermal alteration and dissolution of muscovite in the pH range 1 to 4 was those of weathering. close to stoichiometric, but two orders of magnitude slower than the trioctahedral micas. Weathering in saprolite In the saprolite, chlorite weathering continues to WEATHERING OF CHLORITES proceed by vermiculitization but also involves the Transformations in weathered rock formation of other minerals with increasing weath- ering intensity. Gilkes & Little (1972) demonstrated The alteration of chlorite in deeply weathered that vermiculitization of chlorite in a profile on rock was investigated by Murakami et al. (1996) weathered phyllites was accompanied by oxidation and Banfield & Murakami (1998). The alteration of ferrous iron and loss of Fe and Mg from the described was in a deeply weathered quartz mica brucite sheet. Vermiculitization of chlorite in an schist associated with a secondary uranium ore amphibolite saprolite was also shown by Proust deposit in Northern Territory, Australia. Murakami (1982). In this instance the vermiculite phase tended et al. (1996) found that chlorite progressively towards dioctahedral composition and was accom- converted to vermiculite, through chlorite-vermicu- panied by kaolinite at grain contacts. Further lite intergrades and interstratifications, and ulti- evidence for the formation of a dioctahedral mately to kaolinite and fine-grained Fe oxide. The interstratified vermiculite phase derived from vermiculitization process is characterized by a chlorite weathering in both weathered rocks and depletion of Fe and Mg and by a slight loss of saprolite was provided by Proust et al. (1986). They Al. This process was studied in further detail by concluded that vermiculitization did not proceed by Banfield & Murakami (1998) using atomic-resolu- oxidation of Fe2+ but by simultaneous leaching of tion TEM. Their evidence suggested vermiculitiza- Fe2+ and Mg from the brucite sheet with tion by a continuous solid-state mechanism concomitant enrichment of Si and Al. With 256 M. J. Wilson further weathering, however, vermiculitized chlorite chloritic meta-basalt similar to the parent material converts to a mixture of kaolinite and iron oxide in the soils studied by Johnson (1964). Structural minerals as found by Murakami et al. (1996) and control was again invoked to account for the Aspandiar & Eggleton (2002a,b). The latter authors regularity of the interstratified structure, although proposed that the nature of microsites in the Ross & Kodama (1973, 1976) showed that the same weathered rock and saprolite, within which the chlorite polytype can yield different products. chlorite occurred, profoundly influenced the Complete vermiculitization of chlorite has been mechanism of weathering and the sequence of suggested as occurring in podzolic soils in Wales secondary products. Where the fabric and texture of developed on chlorite-mica mudstones and phyllites the rock had been largely preserved and where few (Adams, 1976; Adams & Kassim, 1983). However, solution passageways had developed, then topo- these chlorites may be unusual as Evans & Adams tactic and epitaxial reactions occur, even where (1975) provided chemical evidence to show that the vermiculite converts to kaolinite and where direct chlorites in the parent material may contain both structural inheritance is not possible. On the other dioctahedral and trioctahedral layers. The weathered hand, where micro-fissures create solution passage- chlorites in Canadian podzols studied by Ross et al. ways within the regolith unit, then the inheritance (1982) showed a change towards a dioctahedral of structure and chemistry from the weathering structure concomitant with vermiculitization, loss of primary mineral to the secondary product is at a Al and Fe and crystallization of goethite. In minimum and a dissolution-precipitation Canadian Alfisols, Ghabru et al. (1990) proposed mechanism must predominate involving a variety that an Fe hydroxy interlayer vermiculite weathered of extraneous sources. from chlorite. In acidic lignitic shales, Senkayi et al. (1981) found that chlorite ultimately transformed into a smectite following dissolution of an Fe-rich Weathering in soils interlayer hydroxide sheet. Carnicelli et al. (1997) In soils, chlorite may yield a variety of weath- described the transformation of a ferruginous ering products some of which are similar to those chlorite to dioctahedral smectite in a podzol found in weathered rock and saprolite. Thus, profile where there was so little illite that this Johnson (1964) showed that a regularly interstrati- mineral could not have been the source for the fied vermiculite-chlorite weathered from chlorite in smectite. The various weathering products identi- a soil profile developed upon metamorphosed basalt fied in these studies emphasize the importance of in Pennsylvania. The regular interstratification was thoroughly characterizing the nature of the original suggested as being due to the original structure of chlorite itself. the parent chlorite where alternate brucite inter- With more intensive weathering in the soil, the layers were linked to adjacent tetrahedral sheets by vermiculitization stage of chlorite decomposition bonds of different strength. However, the fact that may be completely bypassed. Thus, chlorite weath- even the apparently unweathered chlorite yielded a ering in some Korean forest soils resulted in the high spacing (29 A˚ ) on the XRD diagram indicates formation of a mixture of tubular and crumpled that the chlorite was already interstratified to some lamellar halloysite along with fine-grained goethite extent in the meta-basalt, possibly as a result of and hematite (Cho & Mermut, 1992). In the acidic hydrothermal alteration as proposed by Shau conditions common in the organic horizons of (1990). The experimental formation of regularly podzols chlorite may be completely dissolved with interstratified chlorite-vermiculite was achieved by no intermediate product other than goethite (Bain, Makumbi & Herbillon (1972) and by Ross & 1977; Ross et al., 1982; Bain & Duthie, 1984). This Kodama (1976), both sets of workers emphasizing is consistent with the stoichiometric dissolution of the role of irreversible oxidation. However, it chlorite in mineral acids found by Ross (1968, should be noticed that both studies involved 1969) and Malmstro¨m et al. (1996), as well as the original chloritic material that may already have lack of vermiculitic product found after leaching contained some expansible layers. Thus, Herbillon with fulvic acid (Kodama et al., 1983). However, a & Makumbi (1975) record traces of vermiculite or recent investigation by Hamer et al. (2003) on the interstratified vermiculite-chlorite in the fresh dissolution of ripidolite (Mg-Fe-chlorite) in organic chlorite schist that they used and the material and inorganic acids showed that the process was used by Ross & Kodama (1976) was from a non-stoichiometric at low proton and ligand Weathering of rock-forming minerals 257 concentrations, with preferential release of Si environment and the solid-state reaction described relative to Al and Fe. For mineral acids at higher above may be bypassed altogether. With regard to concentrations, dissolution becomes almost stoi- the decomposition of feldspar in weathered rock, chiometric but non-stoichiometry is still maintained there is little evidence that the clay minerals formed in the case of oxalic and citric acids. For the former bear a close structural relationship with the parent there is preferential release of Al and Fe relative to mineral, which is understandable bearing in mind Si and for the latter of Fe. The organic acids the drastic rearrangement that must be involved in enhance dissolution relative to inorganic acids by a the conversion of a framework- to a layer-structure. factor of ~3. The log of dissolution rates of the The bulk of the evidence indicates that in weathered chlorite studied at pH 4.5 is ~10À12 moles m2 sÀ1 rock the formation of clay minerals, usually which is similar to the dissolution rate of biotite. smectitic or halloysitic, from feldspars involves the precipitation of a precursor amorphous phase, although this is not necessarily the case in saprolite. CONCLUSIONS The saprolite at the base of the weathering profile The processes, products and rates of weathering of is often completely physically disaggregated but the primary rock-forming minerals (with the altered chemically to only a minor extent. At this exception of quartz) have been reviewed in the stage, the environment is relatively open, allowing context of the different conditions characteristic of the free passage of drainage waters the chemistry of the fully developed weathering profile. In the which is influenced by reactions higher up in the special closed and near-equilibrium environment profile and with weathering of individual minerals in weathered rock at the base of the profile, olivine, taking place in a far-from-equilibrium situation. pyroxene, amphibole and chlorite have been found Here the weathering products formed in weathered to transform largely by a solid-state topotactic rock may undergo further more drastic transforma- mechanism to pseudomorphs of oriented expansible tions, usually involving the formation of a complex trioctahedral clay minerals (smectites, vermiculites of kaolinitic and Fe oxide minerals, and fresh and interstratified minerals), in addition to Fe oxide mineral surfaces are exposed to aqueous dissolution minerals (goethite and hematite). Orientation processes for the first time. Such fresh mineral relationships show that there is a close structural surfaces may continue to be exposed to weathering relationship between the host mineral and the higher up in the saprolite and even in the soil. It is weathering product. However, in many instances in these circumstances that the experimental weath- the weathered rock has also been hydrothermally ering of minerals in through-flow reactors have the altered, so that it is often not clear that the changes most relevance to natural weathering. There has described can be wholly attributed to weathering. been much debate concerning the nature of aqueous Because of the possibility that even slight dissolution at the surface of the non-layer silicate hydrothermal alteration may predispose a mineral primary rock-forming minerals. The pendulum has to weather along an already established pathway, it swung back and forth between incongruent and is obviously prudent to characterize the nature of congruent dissolution; between the formation of a the primary mineral as thoroughly as possible and residual surface layer, (the thickness of which to ensure that it is completely fresh. In any event, controls the rate of mineral dissolution) and the trioctahedral clay minerals forming at the base dissolution virtually at the immediate mineral of the weathering profile are usually ephemeral and surface in the absence of such a residual layer; seldom persist in the more open and far-from- and between preferential dissolution at the sites of equilibrium conditions characteristic of most structural dislocations and defects and the lack of saprolites and soils. In addition, the reactions any relationship between the density of dislocations occurring in weathered rock can only take place and the rates of dissolution. The current situation at micro-sites to which water has access via micro- seems to be that, depending on the particular cracks and other passageways, so that only a mineral being studied and the conditions of proportion (possibly a small proportion) of a weathering, there are elements of truth in all these particular mineral may be affected in this way. different interpretations. For olivines, pyroxenes Further, once the weathered rock disaggregates to and amphiboles, modern spectroscopic techniques saprolite at the base of the weathering profile then seem to indicate that a cation-depleted layer does the primary minerals are exposed to a more open form during aqueous dissolution. Such a layer can 258 M. J. Wilson also be observed during the dissolution of Ca-rich rhizosphere where there is direct contact between plagioclase feldspars but its thickness is much minerals and organic acid excretions from roots, reduced for the Na-rich and K-feldspars and it may fungi or bacteria and where organic acid concentra- not form at all where cations exceed protons in the tions are higher. The recent suggestion that acidic weathering solutions. Thus, dissolution of these mucopolysaccharides produced by bacteria are more primary minerals can be congruent or incongruent, effective agents of mineral weathering than organic depending upon the conditions of weathering. Even acids remains to be comprehensively evaluated in where a residual layer is detected, it is still natural weathering studies. observed that the mineral surface is etched, The decomposition of micas, particularly biotite, implying preferential dissolution at selected points may progress to advanced stages even at the base of of weakness such as dislocations and defects. With the weathering profile. Vermiculitization is the regard to the relationship between dislocation simplest form of weathering but, in addition to densities (as revealed by etching) on mineral loss of interlayer K, the process often involves surfaces, and the dissolution rates of these surfaces, oxidation of octahedral ferrous Fe, ejection of Fe current evidence suggests that this may depend from the octahedral sheet leading to a more upon the saturation state of the weathering dioctahedral structure and development of a solutions. The widespread occurrence of micro- regularly interstratified structure. Oxidation may textural features, such as exsolution lamellae of also increase the capacity of biotite to retain its another phase, is another feature of the primary interlayer K, thus inhibiting the vermiculitization minerals that may control the progress of weath- process. In these circumstances, biotite may weather ering. Where such lamellae are coherent with, and directly to kaolinite, either through epitaxial growth inherently more weatherable than, the host phase, as or topotactic replacement, without going through a in the case of Ca-rich lamellae in plagioclase preliminary vermiculite phase. In soils, weathered feldspars or Na-rich lamellae in K-feldspars, then biotite may consist of a multicomponent complex they become the foci of preferential weathering. involving halloysite, goethite and gibbsite. Within the upper parts of the saprolite and in Muscovite is much more resistant to weathering soils, the pyroxenes and amphiboles usually than biotite and is very difficult to vermiculitize, weathertoavarietyofclayminerals,the even in the laboratory. However, dioctahedral composition of which indicates inputs from vermiculites are a common component of some sources other than the weathering primary soils and are likely to have formed from a mineral, admixed with Fe oxide minerals. Under dioctahedral muscovite-like precursor. Often, such more intensive weathering conditions, the primary vermiculites contain non-exchangeable hydroxy-Al mineral may decompose altogether, without forma- interlayers in the interlamellar space and similar tion of a weathering product. Halloysite and interlayers are also found in vermiculitized biotites. kaolinite, sometimes co-existing, are the most Like biotite, muscovite too can be converted to common weathering products of feldspars in soils kaolinite in weathering profiles, apparently invol- and saprolites, but gibbsite is also found, sometimes ving a solid-state replacement mechanism. Biotite during the early stages of weathering. Weathering can be vermiculitized by plants where it is the sole in soils and, to some extent, in the upper parts of source of K and can be completely decomposed to the saprolite may be mediated by the effects of an amorphous product by oxalic-acid producing organic acids, produced indirectly by the decom- fungi. The dissolution of biotite in mineral acids is position of organic matter or directly by the incongruent over a wide pH range, with cations microbiota. The effect of organic acids at concen- from the interlayer and the octahedral sheet being trations such as are found in soil waters on primary released before those of the tetrahedral sheet. On mineral dissolution, particularly the feldspars, does the other hand, muscovite has been found to not appear to be as great as was once thought. dissolve congruently, but two orders of magnitude There is certainly an enhancement over the effect of slower than biotite. protons at pH values >5, but usually by less than an The weathering of chlorite also involves a order of magnitude. At lower pH values, such an process of vermiculitization, involving regular enhancement is very much lower, or even non- interstratification between chlorite and vermiculite existent. However, these conclusions do not layers, in weathered rock, saprolite and soil. necessarily apply to situations such as those in the Preferential dissolution of the brucite sheet Weathering of rock-forming minerals 259 following oxidation has been suggested as one chlorite. I. Reactions and products in microsystems mechanism by which this process occurs. With controlled by the primary mineral. Clays and Clay more intensive weathering, chlorite may weather to Minerals, 50, 685À698. a mixture of halloysite and fine grained Fe oxide Aspandiar M.F. & Eggleton R.A. (2002b) Weathering of chlorite. II. Reactions and products in microsystems minerals, or may even decompose altogether. In controlled by the solution avenues. 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