Silt production by weathering of a sandstone under hot arid conditions: an experimental study

Journalof Arid Environments (1987) 12, 199-214

Silt production by weathering of a sandstone under hot arid conditions: an experimental study B.

J. Smith*, J. P.

McGreevyt & W. B. Whalley*

Accepted 25June 1985 Various mechanisms have been proposed for the formation of silt-sized material, of which loess deposits mainly consist. These range from glacial grinding to aeolian abrasion and, more recently, salt weathering of loose, sand-sized quartz grains which relies upon salt solutions penetrating and exploiting already existing cracks. If, however, the original grains are partly constrained within a rock mass it could be argued that the effectiveness of salt weathering would be enhanced, and the requirement for pre-existing cracks might possibly be negated. To test this contention, cubes of unbedded quartz sandstone were subjected, under laboratory conditions, to 60 diurnal cycles of heating and cooling between approximately 21 and 54°Cdirected through one exposed face and wetting with solutions of NaCl, Na2S04 and MgS0 4. Debris produced by the simulation was examined by scanning electron microscopy and thin sections were made from the sandstone blocks. Micrographs and particle size analysis show that most debris consists of more or less intact sand-sized grains liberated from the parent rock. However, coarse and medium silt-sized material, often characterised by fresh fracture surfaces, was also observed. Examination of the parent rock suggests that this silt-sized material originates from two sources: breaking away of silica cement and coatings of secondary silica from around sand-sized grains, and the microfracturing of quartz sand grains. Microfracture patterns observed in thin sections appear to result from point-loading by adjacent grains, which results in compressive and/or shear stresses sufficient to fracture individual grains. No attempt is made to quantify the amounts of silt produced, but it is suggested that the mechanisms described could, under suitable conditions of rock type and salt availability, provide a viable source of loessic silt.

Introduction The possibility that mechanisms specific to hot desert environments are capable of producing quartz silt in the size range typical ofloess deposits (20-60 I-Lm) has stimulated some controversy in recent years. One body of thought (Smalley & Vita-Finzi, 1968; Smalley & Krinsley, 1978) has asserted that although loessic silt may be produced in hot deserts by grain impacting and fracturing during aeolian transport, or by the direct release of silt-sized particles during weathering of suitable rock types, the amounts produced are minimal. Kuenen (1960, 1969) has gone further and suggested that silt generated by aeolian abrasion is characteristically < 20 urn and, therefore, finer than true loessic silt. These

* Department of Geography, Queen's University of Belfast, Belfast BT7 INN, Northern Ireland. t PublicRecordOffice of Northern Ireland, 66 Balmoral Avenue,Belfast BT9 6NY, Northern Ireland. 0140-1963/87/030199+ 16$03.00/0

© 1987 Academic PressInc. (London)Limited

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views have fostered attempts to account for occurrences ofloessic silt adjacent to deserts in terms of mechanisms which do not operate in deserts. The loess of western China has, for example, been explained as deriving from glacial grinding in mountain areas, fluvial transport of the resultant rock flour to the Gobi Desert and subsequent segregation and redistribution of silt particles by aeolian processes (Smalley, 1972; Smalley & Krinsley, 1978). Such explanations are now coming under increasing scrutiny as more information becomes available on the nature of desert and near-desert loess deposits. Substantial revisions have been made in reconstructing the limits of Quaternary glaciations in western China and Tibet (Derbyshire, 1983), and it now seems that in the areas proposed by Smalley & Krinsley (1978)as sources for Chinese loess, glaciations were less extensive than previously thought. If this is so, it would appear more likely that the quartz silt in these deposits actually originated within the Gobi Desert itself (Derbyshire, 1983). Other recent studies have indicated that in the drier savannas of West Africa, many soils contain significant percentages of loess-sized silt, and that the drift which blankets much of northern Nigeria, for example, may be considered to be of at least a reworked loessic origin, ifnot a true loess (Bennett, 1980; Smith & Whalley, 1981). In addition, it has been shown that the present-day Harmattan winds which blowout of the Sahara, carry an abundance of silt which is deposited south of the desert (Whalley & Smith, 1981; McTainsh & Walker, 1982). Similarly, Sarnthein & Koopman (1980) found that large quantities ofloessic silt « 40 11m) are deposited annually over the Atlantic in a broad zone of about 100 km to the west of the Sahara. It would therefore seem that, contrary to traditional ideas, considerable amounts of loessic silt have been, and are currently being, produced in, and perhaps adjacent to, the world's hot deserts. Because of this, geomorphologists have begun to consider the full range of geomorphic processes which could operate in these environments to produce silt. The role of aeolian abrasion, for example, has been reappraised and it has been demonstrated that grain collisions can produce medium and coarse quartz silt particles (20-60 11m) with many of the characteristics of those found in loess (Whalley, Marshall et al., 1982). Interest has also been focused on the possible roles of weathering in silt production. Although some evidence has been presented which points to the importance of grain fragmentation through silica solution (Nahon & Trompette, 1982; Pye, 1984), one process in particular has received much recent attention: the fragmentation of sand-sized quartz by salt weathering. Quartz silt production by salt weathering Krinsley & Doornkamp (1973) appear to have been the first to propose that salt crystal growth in hot desert environments could cause cracking and detachment of surface layers of quartz grains. Krinsley & McCoy (1978) subsequently elaborated on this idea, but the experiments conducted by Goudie, Cooke et at. (1979) and Pye & Sperling (1983), together with observaticnsrnade by Derbyshire (1983), have provided firm evidence of the effect. In their laboratory studies, Goudie, Cooke et at. (1979) and Pye & Sperling (1983) found that silt could be produced by subjecting loose (dune and grus) sand grains to simulated 'hot desert' salt weathering conditions (cf. Cooke, 1979). In seeking to account for the observed release of silt-sized fragments from quartz sand grains, these workers have invoked the presence of microfractures which could act as pathways along which salts could penetrate to cause particle disruption. Krinsley & Smalley (1972) questioned the existence of such weaknesses in quartz, but others have characterised structural defects within quartz sand grains, especially those of plutonic origin (Moss, Walker et al., 1973; Riezebos & Van der Waals, 1974). Moss and Green (1975, p. 485), for example, identified 'partially healed sub-planar micro-fractures dividing grains into sheets typically one to a few micrometres thick'. Following the release of sand grains from the parent rock, these microfractures appear to be exploited quite rapidly by processes of transport and weather-

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ing, resulting in relatively resistant 'core' grains (Moss, Green et al., 1981). Thus, Pye & Sperling (1983) found that salt weathering caused only slight damage to quartz dune sand grains but major damage to first cycle granitic regolith grains. Such a finding may not be strictly relevant in the context of desert silt production, in that such first cycle grains would be relatively rare, particularly in ancient desert environments such as the Sahara, where most quartz sand has experienced or is undergoing energetic transport, or is released not from plutonic rocks but from essentially surface sediments (such as the Nubian sandstones of the eastern Sahara (Said, 1962)). In these situations, loose sand grains might be expected to possess relatively few microfractures and thus exhibit a reduced susceptibility to saltinduced fragmentation. However, if the likelihood of breakdown by stresses generated by salts within grains were to be questioned, there remain other possible means whereby salt weathering could generate quartz silt. This paper now considers two such possibilities.

Grainfracture by external stress As long ago as 1938, Griggs & Bell demonstrated that sub-parallel fracturing of individual quartz grains could be induced by loading under laboratory conditions. Similarly, Smalley (1963) suggested that compaction of sands can cause grain fracturing and result in the production of substantial quantities of silt-sized material. Such loading of quartz grains to produce silt particles might possibly be achieved by an indirect effect of salts present in pores of rocks such as sandstone. It is suggested that fracturing could be caused during salt crystal growth, hydration or thermally induced expansion, not directly, but by the transmission of associated stresses through grains and their concentration at point contacts between them. The situation which is envisaged is depicted in Fig. 1. The fact that grains are constrained within a rock would allow this possibility-unlike the case of loose grains (as used by Goudie, Cooke et al. (1979) and Pye & Sperling (1983), for example), where stresses could be accommodated by grain movements. The basis of this idea is the experimental work of Gallagher, Friedman et al. (1974), which showed that when sands are sufficiently compressed, so-called 'microfracture chains' develop which run through the points of contact between individual grains. It is pertinent to note that fracturing of this kind does not require the pre-existence of planes of weakness within grains .

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Detachment of quartz overgrowths Goudie, Cooke et al. (1979) suggested that silt could be derived from the cracking, not of loose, primary grains, but of coatings of secondary silica precipitated on their surfaces.

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The present study expands upon this suggestion by considering the possibility that a major source of secondarysilica-quartz cement in rocks--could, under favourable circumstances, be exploited by salt weathering to produce silt. Authigenic quartz, occurring as overgrowths around detrital quartz grains, is one of the most common cementing materials in sandstones (Waugh, 1970; Pittman, 1972) and is the primary cement in orthoquartzites (Sibley & Blatt, 1976), silcretes (Summerfield & Whalley, 1980) and other silicified sands (Riezebos, 1974). The formation and characteristics of quartz overgrowths have been well documented by the workers mentioned, and need not be considered in detail here. It is pertinent only to draw attention to Pittman's (1972) observation that contacts between overgrowths and detrital grains are often discontinuous, the two being connected only at isolated points. He suggested that, under natural conditions, some of these voids could be filled by liquid. It would not be unreasonable to suppose that one such liquid could be a salt solution, which would present the possibility of detachment of overgrowths from host grains by salt weathering. A recent investigation by Anderhalt (1984) demonstrated that overgrowths can be removed from parent grains during fluvial transport and provides some support for the feasibility of this idea. In an attempt to study these possibilities, we carried out an experiment in which blocks of a sandstone were weathered by a variety of salts under simulated hot desert conditions.

Experiment

Rock type Freshly quarried samples of 'Darney Stone', a Carboniferous sandstone from near Otterburn, Northumberland, England, were used for the study. It is a relatively pure, apparently unbedded quartz sandstone which contains small amounts of kaolinite, muscovite and iron oxide. On the basis of previous evidence (Cooke, 1979), it was considered that its high porosity (18.1-18.7% (International Society for Rock Mechanics, 1979)) would render it susceptible to salt weathering. Thin sections of unweathered rock suggest that the constituent quartz grains are predominantly sub-angular, with a modal sizeof approximately 150-200 urn (although it is borne in mind that size measurements from thin sections often provide underestimates of true particle sizes). Grains are held together by limited interlocking of quartz overgrowths. The high porosity values provide an indirect indication that, although overgrowths are present; their development is not particularly extensive (see Whalley, McGreevy et al. (1982) for a more detailed discussion of relationships between porosity and stages of overgrowth development). Scanning electron microscope examination (see below) offresh fracture surfaces and loose grains gently brushed from these surfaces, shows that although some overgrowths have developed as broad crystal faces around host grains (Fig. 2a), it is much more common to find them in isolated patches. Figure 2b shows a single grain on which overgrowths have developed as isolated, well-defined projections within a mass of much smaller amorphous crystal forms (cf. Pittman, 1972). It is rare to see overgrowths and parent grains in contact with each other; more usually, the impression is of overgrowths 'resting' on grains with only a limited number of attachment points (Fig. 2b). Closer observation shows that contacts can be quite open (Fig. 2c) and could thus be exploited by salt weathering. Other discontinuities occur where adjacent projections begin to merge. Figure 2d shows that individual overgrowth units usually retain their character and are separated from neighbours by fine boundaries which could be subject to disruption by salts.

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Figure 2. SEM micrographs of quartz overgrowths on sand grains showing the extent to which some overgrowths interlock (a and ll), the wayin whichother overgrowths occuras isolatedpatches apparently restingon the host grain (b), and the spacebetweenovergrowths and host grain (c). Each scale bar represents 10 J.Lm.

Methodology Specimen preparation Six, 7-cm cubes of sandstone were cut using a diamond saw, washed thoroughly to remove loose particles and dried for 48 hours at 60°C. After cooling, five sides of each block were 'moisture-proofed' by coating with varnish. Each block was then embedded in expanded polystyrene leaving the unvarnished face exposed (Fig. 3). These measures were adopted to ensure that any moisture applied to the block surface would also be lost through it and that, during cycles of heating and cooling, rock temperature gradients would be established normal to the surface. In short, the experiment was aimed at reproducing weathering conditions experienced by large rock surfaces as opposed to small rock cubes (see McGreevy & Smith (1982) and Smith & McGreevy (1983) for further discussion of the experimental design).

Temperature conditions The experiment was conducted in a Gallenkamp humidity oven and the temperature regime used is shown in Fig. 4. Internal temperature gradients were established during both heating and cooling phases, with a reversal of the gradient occurring soon after cooling commenced. The gradient is highest during the heating phase, with a maximum temperature difference between surface and 6 em depth approaching 7°C. Similar temperature characteristics have been recorded for rocks exposed in the Sahara (Smith,

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1977; Kerr, Smith et al., 1984). The temperature range of 22-S4°C is also considered to be representative of temperatures which occur in hot deserts, and especially in shadowed areas (tafoni, for example) where salt weathering is particularly prevalent (Wilhelmy, 1964; McGreevy & Smith, 1983).

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Salt treatments and experimental procedures One hour before the beginning of each diurnal heating phase, the specimens were treated individually as follows. Two were used as controls. One was kept dry, whilst the exposed face of the other was lightly sprayed with distilled water until a thin film (c. I mm deep) had developed on the surface; this was then allowed to soak into the block. Three other blocks were similarly treated with 10% solutions (on a percentage weight basis) of NaCl, MgS04 and Na ZS04' Because sodium chloride has been shown to be relatively ineffective in causing rock breakdown under experimental conditions (Goudie, 1985, Table 9)although, curiously, .it is extremely effective in nature (Goudie & Watson, 1984)-the sixth block was treated with a saturated solution ofNaCl. However, to avoid excessive salt concentration in the specimen, the amount of salt introduced was kept constant throughout the experiment. This was achieved by first standing the block in a saturated NaCl solution for 24 hours and allowing absorption via the unvarnished face, drying it for a further 24 hours and, having placed it in its polystyrene jacket, spraying it daily with distilled water (as above). The experiment lasted for 60 days, after which loose debris was removed from the exposed surfaces by light brushing; this was not done at any other stage during the experiment. The debris was then thoroughly washed to remove salts. For samples from which sufficient amounts were generated, the debris was split. One half was used for particle size analysis by Coulter Counter. The remainder was sieved to obtain the <63 urn fraction, samples of which were mounted on aluminium stubs using double-sided adhesive tape, sputter-coated with gold and examined under a scanning electron microscope (JEOL 35CF) in the normal (secondary emissive) mode. Following removal of debris from the blocks, the exposed surfaces were vacuumimpregnated with resin (araldite) and thin sectioned for optical microscope examination. The sections were prepared normal to the exposed surfaces. Every effort was made to ensure thorough impregnation of resin in order to avoid grain disturbance during sectioning.

Results

Particle sizeanalysis ofweathering debris Of the six blocks used, only those treated with 10% MgS0 4, 10% Na ZS04 and saturated NaCI produced enough debris for particle size analysis. The block treated with distilled water and that which was left dry produced no measurable amounts of debris, whilst that treated with 10% NaCl produced a small quantity but insufficient for analysis. Because of possible inconsistencies in the process of removal by brushing, no attempt was made to determine accurately the relative amounts of debris produced. However, it was evident that 10% Na ZS04 caused the greatest degree of surface disaggregation, followed by 10% . MgS0 4 and saturated NaCl (Fig. 5). The particle size distribution curves of debris are shown in Fig. 6. There are marked differences in terms of both the range of particle sizes produced and the modal size classes. The debris produced by NaCl is almost entirely sand-sized (nearly 95% within the 200-450 um size range), with only small quantities of silt and clay. In contrast, the distributions of debris generated by MgS04 and Na ZS04 are both bimodal, although the modal values differ. As with NaCI, the bulk of the debris is in the sand size range, 70% falling within the ranges 250-450 urn and 120-250 urn for MgS0 4 and Na ZS04' respectively. Secondary peaks are in the silt size «63 urn) range and are, respectively, 24'5% and 20'0%. Both contain the full range of silt sizes (coarse, medium and fine), although amounts produced are less than those reported by Goudie & Gomez (in press; cited in Goudie (1985)) in their study of silt production by salt weathering of a quartzose sandstone.

B. J. SMITH ET AL.

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Figure 5. Photographs showing the surfaces of four experimental blocks after 60 diurnal weathering cycles using different salt solutions. Scale in em. (a) 10% Na2S04; (b) 10% MgS0 4; (c) saturated NaCl; (If) 10% NaCl. 100

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Silt particles generated through the mechanical weathering of a sandstone can derive from two sources: they may already be present in the rock and are simply released by granular disaggregation, or they may be products of fragmentation of larger grains within the rock. If the effect of simulated salt weathering was only to release already existing siltsized particles from the rock, then the particle size distribution curves of debris produced by the different salts should have been similar. This is clearly not the case: the curves indicate not only that fragmentation occurred, but also that the degree to which it occurred depended on salt type. Thin section and SEM observations proved informative with regard to the nature of fragmentation and the silt particles produced. Thin section observations Although brushing had removed most of the loose debris from block surfaces prior to thin sectioning, observations showed that many grains which had actually been removed from the main body of the specimens were nonetheless retained by a 'cement' of salt crystals. Particular attention was paid to grains which were 'floating' within this salt matrix. Grains in the NaCl- and MgS0 4-treated specimens showed no evidence of individual breakdown and retained the characteristics of those seen in the unweathered rock. In contrast, weathering by NaZS04 produced considerable modifications which involved varying degrees and forms of fragmentation. Figure 7 provides examples of fractured grains, showing the following. (i) Grains split into a series of curvilinear, wedge-shaped fragments (grains markedA, B and C in Fig. 7a, b). In each case, the fragments are of silt size and are clearly produced from the same sand-sized grain. In grains A and B, although the fragments are clearly separate, relatively little movement has occurred and the original grain outline is still quite distinct. (ii) Grains such as D in Fig. 7b which are 'cleanly' split into two fragments but with little, if any, relative movement of the two halves along the fracture plane. (iii) Grains dominated by one major fracture plane which splits the grain approximately in half, but which exhibit varying degrees of relative movement of the two halves along the plane (e.g. grains E and F, Fig. 7b, c). In some instances there is also some evidence of further fracturing of the two major components. (iv) Grains which may display the detachment of overgrowths (e.g, grain G, Fig. 7c) although, given that overgrowths could not be recognised in thin sections of unweathered rock, this observation must be regarded with some caution. (v) Grains fractured into a number of irregularly shaped angular fragments delineated by fracture planes which do not completely cut across the original sand grain (Fig. 7eI). When viewed in plain polarized light, cracks within grains were seen to be filled with microcrystalline salt crystals. Since infilling by this salt can only have occurred after crack formation, its weathering role is one of displacing fragments formed during the cracking process. Movement of fragments is evident only in grains which have been released into the salt-cemented outer layer. This might suggest that within the rock itself, grains are so constrained that movement is not possible. In the outer layer, grains usually retain their original form; ultimately, however, their release is manifested by the detachment of angular silt and fine sand particles. The fact that a variety of fracture types were observed, with no consistent pattern, would suggest either that microcrack types common to all grains do not exist or that, if they do, they are not a major control upon the fragmentation of grains by salt weathering. The first possibility would not appear likely, since the constituent quartz grains would have been subjected to the same diagenetic processes which, presumably, would have had the same effects on all grains. Regarding the second, if it were assumed that certain sets of microcracks did exist which were common to all grains, then similar fracture patterns should have emerged if grain fragmentation had occurred due to weathering by salt which

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Figure 7. Opticalmicrographs (crosspolarized light)showing fracturedquartz sandgrainsfrom the experimental block treated with 10% NaZS0 4' Arrows indicate possible directions of external stresses responsible for the observed patterns of microfracturing. For more detail, see text. had penetrated cracks. This does not seem to have been the case: the variety of fracture patterns suggests that cracking is caused not by stresses generated within grains (although this COUld well occur), but by external stresses. It should be pointed out that the occurrence of fractured grains is relatively rare. Within the NazS04-treated block, itis estimated that <5% of sand-sized grains in the outer 0'5 ern displayed visible cracking. Moreover, fractured grains appeared to concentrate in specific areas, which might suggest a relationship with pore structure variations.

SEM observations SEM observations of silt-sized material showed this to comprise three components, not all of which were generated by each of the salts used. First, there are already existing silt particles which have simply been dislodged from the parent rock by salt weathering. These are usually sub-rounded, often with irregular pitted surfaces which occasionally show limited overgrowth development (Fig. 8). This was the only type of silt particle produced through weathering by NaC!. Second, there are coarse and medium silt-sized angular fragments of quartz overgrowths, easily recognised by their characteristic smooth and well-defined crystal faces (Fig. 9), and produced by both MgS0 4 and Na ZS04 weathering. Lastly, there are coarse and medium silt-sized angular fragments which possess clean, conchoidal fracture surfaces. These were generated by both MgS0 4and Na ZS04, but were most abundant in the case of the latter. These grains can be divided into two further subgroups. First, coarse silt fragments which have several smooth fracture planes (K and M on Fig. lOa, b) as well as rounded and pitted surfaces characteristic of the original grains (J and L on Fig. lOa, b). These fragments would appearto be remnants of 'cores' of original sand grains from which a number of smaller fragments have broken away. Second, there are thin, wedge-shaped flakes (Fig. 10c, d) with fracture planes that join at very sharp

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Figure 8. SEM micrograph showing an example of a primary silt-sized grain and attached overgrowths released from the experimental blocks by granular dissaggregation. The white scale bar represents 10 urn.

Figure 9. SEM micrographs of spalled, silt-sized overgrowths showing smooth crystal faces and adhering clay (kaolinite) particles. Each white scale bar represents 10 urn.

edges. Present on the fracture planes are a series of parallel, rib-like marks (arrowed on the plates). These are known as 'wallner lines', are indicative of brittle fracture and are also visible on some of the 'core' grains. It is possible that these smaller fragments are the flakes that have broken away from the larger cores. In both instances (cores and flakes), the grains shown are of coarse or medium silt size (>20 urn) and would therefore qualify as 'loessic' in size. Furthermore, a section through one of the flakes would produce a triangular wedge similar to those seen in the thin sections (e.g. Fig. 7a, grain A). This would lend support to the contention that they are the disintegration products of sand-size quartz grains, and not fragments of secondary quartz overgrowths.

Discussion and conclusions The above results indicate that simulated salt weathering using MgS0 4 and Na zS04 can produce loess-sized quartz silt directly from a sandstone. Three principal forms of silt have been identified as deriving from this weathering: individual original silt-sized grains, fragments of quartz overgrowths, and particles created by the fracturing of sand-sized grains. Weathering with NaCl appears, however, to have produced solely the disaggregation of complete grains with overgrowths still attached. The particle size distribution of NaCl debris shown in Fig. 6 would seem, therefore, to represent approximately the size distribution of the parent sandstone.

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Figure 10. SEM micrographs of angular, silt-sized quartz fragments from experimental blocks treatedwith 10% MgS04 and 10% NaZS04' Fresh fracturesurfaces (K and M) and surfaces similar to thoseobserved on sand-sized grainsin the original rock (J and L), suggest that they formed from the breakdown oforiginal quartz sandgrains. Wallner lines(arrowed on fragments c and If) suggest that breakdown wasby brittle fracture. Each whitescale bar represents 10 urn. The release of individual grains is dependent upon the tensile strength of the original sandstone being exceeded by the stresses generated by salt weathering mechanisms. Clearly, this condition was satisfied for the sandstone used in the simulation. Tensile strengths for sandstones in general are in the range ~S MPa, while compressive strengths lie in the range 30-160 MPli (Farmer, 1968). These figures compare favourably with the pressures generated by both hydration and crystallisation of the salts used. In the case of the latter, Winkler and Singer (1972) showed that crystallisation pressures vary according to the supersaturation ratio for individual salts, but that, for example, NaZS04 (under highly evaporative conditions) may achieve a theoretical value of 196 MPa, MgS0 4 may achieve 70.8 MPa and NaCl, 373 MPa. Hydration pressures vary with both temperature and relative humidity, but Winkler and Wilhelm (1970) identified maximum hydration pressuresfor Na.SOz-« NazS04'lOHzOof48'3 MPaandfor MgS04'HzO~MgS0 4'6HzO of 41·8 MPa. A point worth noting from these figures is that NaCI has the highest crystallisation pressure and yet produced least breakdown of the experimental blocks. This might suggest that hydration, differential thermal expansion or a combination of these mechanisms in conjunction with salt crystallisation, may be the most important causes of disaggregation in this instance. At this point in time we are, however, unable to verify or refute this suggestion. Nevertheless, it would appear that under near-surface conditions of negligible normal stress, many sandstones can be disaggregated by salt action. The overgrowths shown in Fig. 9 may have been detached from the host grains in one of two ways. First, it is possible that salt solutions could have migrated into the spaces between the two and caused overgrowth detachment by crystallisation or thermal expansion during the heating phase of the diurnal temperature cycle, and/or hydration during cooling and wetting. A second possibility, and one which does not require solutions to

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enter between overgrowths and parent grains, is a situation whereby pressures applied to the outer surfaces of overgrowths could cause them to collapse onto the underlying grains. Pressure could be exerted directly by salt crystallisation, thermal expansion or hydration, or by these mechanisms functioning indirectly through an intermediary grain in contact with the overgrowth. The application of external pressures to individual grains would also seem to be responsible for the third category of silt particle, those produced by microfracturing of sand-sized grains. The microfracture patterns observed in the thin sections are very similar to those reported by Gallagher, Friedman et al. (1974) for experimentally deformed quartz sand and sandstone. Observations of microfracturing in sand grains derived from a sandstone are, however, not restricted to experimentally produced debris and, for example, Singer & Amiel (1974) described fractured grains found in soils derived from Nubian sandstone in the Golan Heights. More recently, Busche (1983) published micrographs of cracked quartz grains in Saharan silcretes which show possible fracture chains through several grains. Some of the characteristic fracture patterns associated with two-, three- and four-point loading of grains that were produced by Gallagher, Friedman et al. (1974), are shown in Fig. 11. Comparison of these 'type' patterns with those obtained in our experiment reveals certain similarities. In Fig. 7b, for example, Grain D would appear to have been fractured by two-point compressive loading, as arrowed, whereas grains A and B in Fig. 7a seem to show the characteristics of three-point loading. Some grains show evidence of shearing (grains E and F in Fig. 7b, c) which may represent fracturing of an

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unconstrained portion of a partly constrained grain or subsequent movement of fragments created by two-point loading. It is not possible to say which of these explanations holds, but the diagonal path of the microfracture in grain F might argue against a two-point loading origin (in this case at least). The above comparisons would seem to support the contention that pressures exerted by crystallisation, hydration or thermal expansion of salts in pores may not be directly responsible for grain breakage: rather, they are transmitted through grains and concentrated and intensified at grain contacts. Similarly, the observed patterns of microfractures argue against the exploitation ofpre-existing fractures within grains. Although internal fractures do occur, especially in grosses, it is unlikely that grains which have undergone aeolian, marine or fluvial transport will survive without cracking. Thus, sandstone grains are likely to be relatively free of cracks which can be exploited either by external or internal stresses resulting from the crystallisation of salts from aqueous solution. The magnitudes of the external stresses experienced by individual grains during the simulation experiment which resulted in grain fracturing, are not known. However, experiments have been carried out elsewhere which have examined the compressive strengths of quartz grains. One of the most important of these studies is that by La Rue and Schlossen (1967), which showed that mineralogical orientation is an important control on the strength of a quartz and that final crushing strengths vary between 190 and 280 MPa. In addition, they measured primary fracture stresses in the range 40-60 MPa that were capable of producing partial cracking of grains. These latter pressures are clearly within the range that can be generated by salt crystallisation and hydration. Partial cracking might be important, in that although cracks may not traverse the complete grain, they may provide a point of ingress for salt solutions, and subsequently for tensile failure. Any developing cracks could also be subject to an intensification of failure by stress-eorrosion mechanisms, which can be expected to be particularly important in the evaporative environment near the rock surface. The existence of partial cracks in quartz grains was demonstrated recently by the work of Schnutgen & Spath (1983), who show partly cracked grains (their Fig. 1) which were observed in latosols and laterities from Sri Lanka and Australia. In their case the cracks were widened at an early stage of weathering and are in most cases now filled with iron compounds. Thin-section observations from our own study showed that salt had crystallised within microfractures once these had formed (grain H in Fig. 7d, for example), and this is undoubtedly responsible for further separation of grain fragments. Our findings would, however, seem to warn against interpreting such instances as evidence of initial grain fracture by weathering mechanisms operating within grains. In this section we have outlined a number of ways in which fractures through grains can be accomplished, primarily by compressive stresses; to produce fragments of various sizes. The detailed stress patterns responsible for these fractures in any amalgamation of grains will, of necessity, be very complex, and whether compressive, shear or even tensional stresses are produced will depend upon the unique organisation of grains and salt accumulations. The point-loading concentration effect will, of course, operate only for grains held in the rock and near its surface. However, every new surface exposed by disaggregation could provide opportunities for grain fracture and silt production. Our study, and the mechanisms we have outlined, are seen as complementary to the studies conducted by Goudie, Cooke et al. (1979) and Pye & Sperling (1983). These also show that quartz silt can be generated by salt weathering mechanisms and that its production should not, therefore, be viewed as being specific to glacial and periglacial environments. The possibility of quartz grain fragmentation such as described above would depend on two factors (assuming, of course, that the requisite conditions for wetting, heating and cooling of rock surfaces are met). Most obviously, salts must be available, particularly those which are most effective in causing rock breakdown. It is well known that magnesium and sodium sulphates are powerful weathering agents and this investigation has supported this view with respect to silt production. However, any firm conclusions regarding their role in producing silt under natural conditions must await

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much more information on their direct association with salt weathering phenomena. The other control, rock type, is of equal importance. The sandstone examined here possesses physical characteristics which make it most suitable for evaluating our initial hypotheses. Whether or not such potential silt sources are present in or around hot deserts remains to be seen. The mechanisms have been proved capable. The question should now perhaps be addressed as to whether or not the requirements for their operation occur in hot deserts. The writers are indebted to the technical, secretarial and laboratory staff of the Geography Department at Queen's University, especially Trevor Molloy for preparing the photographs, Gill Alexander for drawing the diagrams and Sharon Wright for typing the manuscript. In addition, we would like to thank Dr J. M. McCrae ofthe Q. U .B. Electron Microscopy Unit for his assistance, and Natural Stone Quarries Ltd for providing information about the origins and characteristics of Darney Stone.

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