Weathering of quartz in dune sands under subtropical conditions in Eastern Australia

Geoderma, 20 (1978) 225--237 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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W E A T H E R I N G OF Q U A R T Z IN DUNE SANDS U N D E R S U B T R O P I C A L C O N D I T I O N S IN E A S T E R N A U S T R A L I A

I.P. LITTLE*, T.M. ARMITAGE and R.J. GILKES Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, W.A. 6009 (Australia)

(Received August 9, 1977; accepted January 18, 1978)

ABSTRACT Little, I.P., Armitage, T.M. and Gilkes, R.J., 1978. Weathering of quartz in dune sands under subtropical conditions in Eastern Australia. Geoderma, 20: 225--237. Four distinct types of quartz grains are described from the sands of Fraser Island, Australia. Their character seems to reflect a combination of weathering and provenance rather than the mechanism of transport. In particular, two types present in old deposits show extreme solution pitting due to weathering. Weathered grains may also appear in younger sediments because much of these consists of re-worked older material. Many clear unweathered grains also occur in the older formations, but grain counts show that the weathered quartz grains are more frequent in the older units. From X-ray diffraction evidence and microscopic examination, one of the two highly weathered forms of quartz appears to have inherited a microcrystalline nature, whereas the other appears to have formed from grains that have inherited strains from their original environment. Grains which are not microgranular or have not inherited strains remain unweathered and are apparently stable.

INTRODUCTION Q u a r t z is o n e o f the m o s t c o m m o n minerals o f the e a r t h ' s crust. P r i m a r y low q u a r t z is t h e r m o d y n a m i c a l l y stable at n o r m a l t e m p e r a t u r e s (Winchell and Winchell, 1 9 5 1 ) . It is regarded as being p a r t i c u l a r l y stable and persistent in the s e d i m e n t a r y e n v i r o n m e n t ( J a c k s o n a n d S h e r m a n , 1 9 5 3 ) , and is o f t e n used as a stable i n d e x mineral in studies o f w e a t h e r i n g in soils (Barshad, 1965}. Krinsley and D o o r n k a m p ( 1 9 7 3 ) have p r o p o s e d t h a t the surface m o r p h o l o g y o f q u a r t z grains is indicative o f their d e p o s i t i o n a l h i s t o r y . O t h e r w o r k e r s were m o r e c a u t i o u s in associating surface m o r p h o l o g y with a p a r t i c u l a r e n v i r o n m e n t a l process ( S e t l o w a n d K a r p o v i c h , 1 9 7 2 ; B r o w n , 1 9 7 3 ) . T h e f a c t o r s w h i c h lead t o the d i s i n t e g r a t i o n o f q u a r t z have been described b y Raeside ( 1 9 5 9 ) w h o advises against its use as an i n d e x mineral in old soils. Moss and G r e e n ( 1 9 7 5 ) observed t h a t m a n y q u a r t z grains t e n d t o w e a t h e r *Permanent address: C.S.I.R.O, Division of Soils, St. Lucia, Qld. 4067, Australia.

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showing lamellae about 2 p m across, possibly corresponding to deformation lamellae inherited from the parent rock. Cataclastic deformation of chemically stable rocks results in the minerals, particularly quartz, showing strain effects such as marginal granulation and undulatory extinction (Williams et al., 1954). This publication examines the changes that have occurred in the surface morphology of quartz grains in response to intense chemical weathering. It thus provides data on modifications in surface morphology in an environment not studied by previous workers. MATERIALS A large b o d y of quartz sand derived from several sources might be considered a representative sample of quartz grains of varied provenance. Eventually a highly leaching environment must leave its additional imprint. Fraser Island in southeastern Queensland, Australia is such a body. It is situated off the Queensland coast between latitudes 24°42'S and 25°48'S, where strong leaching occurs under a subtropical environment with a mean annual rainfall of 1,330 mm and a mean annual temperature of 22°C (Commonwealth Bureau of Metereology, 1956). Thompson (1975) has described the beach ridge and dune systems of coastal areas of southern Queensland, including Fraser Island, and points to evidence for different ages. Ward and Little (1975) associate dune formation with blowing during the low sea levels of the Pleistocene ice ages and the beach ridge systems or strandplains with successive high stands of the sea during the interglacial periods. A tentative chronology has been devised b y association with strandline sequences of known age elsewhere and those of interest in this paper are listed in Table I. The dunes have a variety of forms, but they mostly have the elongate b l o w o u t shape that is characteristic of moving sands impeded by vegetation (Melton, 1940). Sharp boundaries between groups of similar dunes give evidence of periodic sand accumulation. The older geomorphic units are progressively more eroded by water scouring. The last eroded dunes lie upwind of others with more degraded outlines, and evidence is c o m m o n that younger dunes have invaded older ones which were already vegetated. Spodosols (Soft Survey Staff, 1975) were developed on all of the sand deposits except the modern beach. The depth of the A horizon over B, C horizon ranged between 1 m for Triangle Cliff dunesand to 10 m for the Awinya dunesand. There was a clear relationship between soil development as measured by the thickness of the A horizon and the age implied by the shoreline chronology. EXPERIMENTAL OBSERVATIONS

Investigation o f Fraser Island sands under the light microscope The sands investigated were moderately well sorted with a modal diameter

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TABLE I Tentative chronology for the geomorphic units sampled Name of geomorphic unit

Symbol

Modern beach Triangle Cliff Dunesand Poyungan Strandplain Yankee Jack Dunefield Ungowa Strandplain Awinya Dunefield

Qhm Qpt Qpp Qpy Qpu Qpa

Age (Ka) 0 145 400 > 400

between 0.15 and 0.30 mm. They were often stained with organic matter and iron oxides or coated with a thin layer of clay. Organic matter was removed by boiling in 10% hydrogen peroxide and the clay dispersed with 0.01 M caustic soda and 0.2% sodium hexametaphosphate and removed by washing through a fine screen. On examination under incident light with a binocular microscope at a magnification of 20X, five classes of sand grains could be discriminated: (1) Heavy minerals. Mostly ilmenite but also rutile, Zircon and tourmaline. (2} Quartz. (i) Clear, un-etched quartz (Fig. 2A). Clear quartz could be f o u n d in all samples. The grains were usually irregular in outline with prominent conchoidal fracturing, but the edges may also be well rounded. (ii) Milky quartz (Fig. 2C). C o m m o n l y present was a type of quartz which was usually opalescent. The grains were always well rounded, usually fractured internally and often contained inclusions. (iii) Saccharoidal quartz (Fig. 2E). One striking type of quartz was extremely etched and internally fractured like a well-rounded lump of sugar. Each sub-unit or crystallite appeared clear but the grain overall was nearly opaque. These were termed saccharoidal quartz. (iv) Microgranular quartz (Fig. 2G). Quite distinct again was a type comprising opaque, well-rounded often almost spherical grains with a translucent satiny surface lustre. Most of these were white, but some were colored yellow or orange possibly due to iron oxide staining, and many were bisected by fine laminae of clear quartz. Microgranular and saccharoidal quartz were fragile and could easily be shattered with a pair of tweezers, y e t they were capable of surviving the relatively vigorous preparatory treatment described. Grains from these two classes were embedded in petropoxy resin, thin sectioned, and examined under the low power of a polarising microscope. All the grains examined showed a crazing of fractures and were apparently fragmented into a mosaic of crystallites. However, in polarised light the saccharoidal grains showed undulatory extinction which was continuous across crystallite boundaries. Sometimes one part of the grain was sharply distinct from the rest, possibly indicating twinning or a composite grain. Microgranular grains consisted, how-

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Fig. 1. Micrographs of saecharoidal (A,B) and mierogranular (C,D) quartz grains. Plane .... polarized light (A,C); crossed polarizers (B,D). ever, of tiny interlocking crystailites extinguishing at different positions. Examples of these two types of grains are shown in Fig. 1.

Investigation of quartz grains under the scanning electron microscope Quartz grains were investigated under the scanning electron microscope. The mosaic-like arrangement of quartz crystallites in saccharoidat and microgranular quartz, observed under the light microscope, was reflected in the surface morphology. However it was not possible to distinguish the saccharo. idal grains from the microgranular on the basis of surface morphology alone. Some examples are shown in Fig. 2E--H. Also shown are examples o f clear quartz showing the relatively un-etched surface {Fig. 2A, B), milky quartz

229

I

1

t

i

30,u.. Fig. 2. Surface morphology of quartz grains from Fraser Island: A. clear quartz grain; B. fractured edge of clear quartz grain; C. milky quartz grain with some degree of fracturing; D. surface of milky quartz grain; E. saccharoidal quartz grain amongst grains of milky quartz; F. detail of surface of saccharoidal quartz grain; G. microgranular quartz grain; H. detail of surface of microgranular quartz grain.

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showing the more rounded outline, the beginning of fracturing or lamination and some surface etching (Fig. 2,C, D). With the scanning electron microscope used (Philips PSEM 500) there was a facility (EDAX 707) for the energy dispersive analysis of the X-rays produced from the surfaces of quartz grains. This enabled an elemental analysis of each grain to be displayed with a detection limit of a b o u t 1%. Eighteen grains from each of the four classes of quartz grains described were examined in this way. All proved to be essentially pure silica except for the milky quartz, where in addition to silicon, four grains contained appreciable aluminium and iron impurities. One contained much titanium, with some A1 and Fe as well as Si. Each analysis represents the approximate analysis of a very small area on the surface of a grain, and not that of the whole grain. In Fig. 3 some characteristic examples of the EDAX spectra, showing X-ray intensities integrated over approximately 40 sec, for the energy region 0--8 keV, are shown.

0

2 46 X-Ray Energy in KeV

8

Fig. 3. Analysis of part of the surface of several grains using the E D A X facility o f the scanning electron microscope: A. a typical quartz sample; B. an area on the surface of a milky quartz grain, showing AI, Si and metals in the region of atomic numbers 24 to 28 (5--8 keV); C. an area on the surface o f a m i l k y quartz grain showing predominantly Ti with minor Si, A] and Fe.

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X-ray examination of grains T h e e x a m i n a t i o n o f single grains b y X-ray d i f f r a c t i o n using a G a n d o l f i c a m e r a ( G a n d o l f i , 1 9 6 7 ) established the various t y p e s o f grain as essentially pure q u a r t z . In a d d i t i o n , p h o t o g r a p h s were t a k e n w i t h o u t r o t a t i o n o f each q u a r t z t y p e . The results are s h o w n in Fig. 4, and c o m p a r e d with a s t a n d a r d powder pattern for quartz.

Grain counts Counts were made of the abundance of the five classes of grain in samples

1

Fig. 4. X-ray diffraction patterns produced by various types of quartz; A. characteristic powder pattern of pure quartz from Fraser Island ; B. pattern produced by a single grain of clear quartz (sample rotated); C. pattern produced by a single grain of clear quartz (sample not rotated); milky quartz grains give similar patterns; D. pattern produced by a single grain of saccharoidal quartz showing preferred orientation of crystallites (sample not rotated); E. pattern produced by a single grain of microgranular quartz showing random orientation of crystallites (sample not rotated). Patterns B to E were taken using a Gandolfi single-grain camera and Co K~ radiation. The weak reflection adjacent to the strongest line of quartz is due to Fe K~ radiation.

232

from the geomorphic units shown in Table I. Three samples were selected from the A1, A2 and BC horizons of each of the units described, as well as a sample from the m o d e n l beach. For each sample, counts were made under the microscope for 10 fields of view, each containing about 100 grains. The mean percentage abundances (Fig. 5) were calculated for each class of grains KE_Y A1 horizon ~ 2 horizon j r C horizon lOO°

Saccharoidal Ouartz 5'

Niiky O.uartz

28%

eo.,

60

.= Heavy Ninera|s

~0,

20,

L

0

(3

0

0

0

Fig. 5. Frequency of occurrence of different types of quartz grain and heavy minerals in sands from the various geomorphic units on Fraser Island, Separate bars are shown for A1, A2, BC horizons. described, and these data were entered into a principal c o m p o n e n t analysis (Williams, 1976). The ordination resulting from this analysis is shown in Fig. 6A, and the contribution of each attribute is shown in Fig. 6B. The first c o m p o n e n t accounted for 68% of the variation and further 16% was accounted for by the second.

Analysis of waters from Fraser Island Many of the fast flowing streams draining the island to the west are spring fed and carry water which has probably been in contact with the sand bodies for a long time. Thus, the chemical composition of these waters should indicate the equilibrium concentrations of solutes in contact with the sands. Samples were taken at irregular intervals over a space of three years spanning two seasons. The results of chemical analysis for silica by the spectrophotometric molybdate blue m e t h o d are shown in Table II.

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0 •

Qhm Opt

X 4n Z~

Qpp Qpy Opu Qpo

A

• Clear Quartz

B

+10

X Milky Quartz Jr" SacclxIroidq[ QUartz 0 Microgranu[ar Quartz 0 Heavy Minerals

0

X

-s

I

+oo--~X--

/

X

+s

""

-10

+t'0

0

-5

-10

Fig. 6. Principal component analysis of grain frequencies: A. plot of the sample points on the co-ordinates provided by the first two principal components; B. plot of the attributes (types of grain) on the co-ordinates provided by the first two eigenvectors from principal component analysis (cf. Table I).

TABLE II Average content of silica in Fraser Island creek waters sampled at intervals over a period of three years Locality

Woongoolbver Ck. Bogimbah Ck. Coongul Ck. Woralie Ck. Bowarrady Ck. Bool Ck.

Silica content (ppm) mean

S.E. of mean* 1

D.F. *~

7.8 7.5 8.4 7.6 7.2 8.9

_+0.28 _+0.45 _+0.21 -+0.24 +_0.88 _+0.49

8 8 7 7 8 5

,1 Standard error of mean. ,2 Degrees of freedom.

DISCUSSION I t was i m m e d i a t e l y c l e a r f r o m e x a m i n a t i o n w i t h t h e o p t i c a l m i c r o s c o p e t h a t a variety of q u a r t z grains were p r e s e n t a n d t h a t some a p p e a r e d to be e x t r e m e l y w e a t h e r e d . ( F i g . 1). B o t h s a c c h a r o i d a l a n d m i c r o g r a n u l a r g r a i n s showed a network of internal fractures. The microgranular quartz consisted of a mosaic of crystallites extinguishing at random, demonstrating that the

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crystallites were randomly oriented. The saccharoidal quartz, on the other hand showed undulatory extinction continuous across crystallite boundaries indicating that each grain was a single unit which had undergone strain, probably during an earlier metamorphic phase. These grains have subsequently weathered to their present form in the Fraser Island environment. They therefore represent an advanced stage of weathering of a form of quartz which has inherited a pattern of strains from its original environment (Raeside, 1959). The scanning electron microscope showed that both the saccharoidal and microgranular types of quartz had extremely etched and highly weathered surfaces (Fig. 2). This contrasted with the clear quartz, which apart from some rounding of edges, and minor pitting and scarring which is probably due to fluvio--aeolian transport, appeared to have relatively fresh surfaces. The milky quartz was intermediate in that grain surfaces often showed fracturing along planes of weakness and some surface etching. The X-ray diffraction patterns showed that the four types of grain described were essentially pure quartz (Fig. 4). The EDAX spectra confirmed this b u t indicated the presence of some impurities in the milky quartz (Fig. 3). Without specimen rotation the clear and milky quartz grains gave X-ray diffraction patterns showing a few sharp spots and streaked reflections resembling a portion of a Laue photograph. The grains therefore consist of a single or small number of large crystallites which is in agreement with thinsection evidence. The photographs obtained for unrotated saccharoidal grains were essentially powder patterns but with distinct arcing of intensities. This effect resembles the texture patterns produced by wires and other oriented arrangements of crystallites (Klug and Alexander, 1954). The effect was interpreted as confirming that the mosaic of crystallites in these quartz grains developed from a larger quartz crystal and that the crystallographic axes of the crystallites have orientations distributed about those of the original crystal. This was not so in the case of microgranular quartz grains. Unrotated grains of these gave what was essentially a random powder pattern, indicating that they were made up of numerous small crystallites, randomly orientated with respect to one another. Moss and Green (1975) have described single crystal quartz grains containing intersecting planar networks of fractures which they suggest may be regions of more rapid dissolution. Such a process would eventually produce a fragile residue of oriented quartz crystaltites as occurs in the saccharoidal grains. Microgranular quartz may represent partially dissolved microcrystalline quartz of metamorphic origin. The waters of the numerous springs which drain the island were essentially saturated with SiO2 with respect to quartz (Table II) (Morey et al., 1962). This supports the concept that the surface features of microgranular and saccharoidal quartz are due to extensive solution etching. Beasley (1948) indicated t h e Mesozoic sandstones of the Moreton and Clarence basins as the source of the heavy mineral sands from Fraser Island

235

southwards. Connah and Newman (1960) support these conclusions and mention the strong northerly drift of sand along the beaches. Whitworth (1959) considered that the ultimate source of the heavy mineral sands was the pegmatite and quartz veins of the Precambrian shield, and claimed that ilmenite was a relatively recent addition from local basic igneous rocks. The above rocks would have been the source of much of the quartz now present in the coastal sand masses. The microgranular quartz could have come from the extensive metamorphic provinces of southeastern Queensland, for example the Neranleigh--Fernvale Group (Brooks et al., 1960). From Fig. 5 it can be seen that the proportion of clear quartz is lower in the older units and that the deficiency is made up by a greater proportion of milky quartz. Heavy minerals are only c o m m o n in the modern beach, and the saccharoidal grains are absent in this case. Although the highly weathered types of quartz form only a small proportion of the total quartz, the trend they show with increasing age of the unit may be important in the understanding of weathering processes in the sand bodies. The proportion of saccharoidal quartz increases with age in the dunesands which are well drained sites. The trend was not supported in the samples from the Ungowa strandplain, but this could be due to the fact that the strandplain soils are always in situations of low relief and because of impeded drainage would not permit rapid export of dissolved silica. Moreover the increase in the proportion of the saccharoidal grains was much greater in the BC horizons of the dunesands. This could be due to their complete destruction and removal from the A horizons where leaching is most intense, or the higher water contents of the BC horizons might have provided conditions more favourable for the formation of this type of grain. The microgranular quartz shows a less clear trend with age but appears to be less frequent in older units. In general, the A horizons are deficient in this form as is also the case for saccharoidal quartz. In Fig. 6A the first c o m p o n e n t in the principal c o m p o n e n t analysis tended to separate the units according to their age, particularly in the case of Qhm and Qpt. Increasing age is shown by decreasing scores for the first component. High frequencies for milky and saccharoidal quartz contribute to lower scores and high frequencies for the other three attributes contribute to higher scores, as shown by the weighting factors in Fig. 6B. The main contributors to the second c o m p o n e n t were microgranular quartz and heavy minerals, but the meaning is hidden in the explanation for the variations in frequency of the microgranular quartz. CONCLUSIONS It has been shown that the quartz dunes of the Fraser Island sand mass contain some very highly etched and solution-weathered quartz grains, consistent with a long period of leaching. A trend with age was demonstrated which could be summarised diagrammatically using principal c o m p o n e n t

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a n a l y s i s . M i l k y a n d s a c c h a r o i d a l q u a r t z t e n d e d t o i n c r e a s e in f r e q u e n c y w i t h age as c l e a r q u a r t z d e c r e a s e d . H e a v y m i n e r a l s w e r e m o s t c o m m o n in t h e y o u n g b e a c h b u t ve D" l o w a m o u n t s w e r e f o u n d in t h e o l d e r u n i t s . T h e p e r s i s t e n c e o f c l e a r q u a r t z in s o m e o f t h e o l d e s t u n i t s , h o w e v e r , c l e a r l y s h o w s t h a t t h e e x t e n t o f s u c h a l t e r a t i o n is n o t s i m p l y a f u n c t i o n o f t i m e . T h u s t h e r a t e o f w e a t h e r i n g o f q u a r t z g r a i n s is v e r y m u c h d e p e n d e n t o n t h e i r m i c r o s t r u c t u r e , w h i c h in t u r n is d e t e r m i n e d b y t h e i r o r i g i n a l e n v i r o n m e n t o f f o r m a t i o n . T h e f a c t t h a t h i g h l y e t c h e d g r a i n s a l s o o c c u r in s o m e g e o l o g i c a l l y young units can be explained by the re-working of older sediments. Clearly t h e u s e o f q u a r t z as a r e s i s t a n t , r e f e r e n c e m i n e r a l in p e d o l o g i c a l s t u d i e s is o p e n t o q u e s t i o n in s i t u a t i o n s w h e r e e x t e n s i v e l e a c h i n g m a y h a v e o c c u r r e d . REFERENCES Barshad, I., 1965. In: F.E. Bear (Editor), Chemistry of the Soil. Reinhold, New York, N.Y., pp. 1--70. Beasley, A.W., 1948. Heavy mineral beach sands of southern Queensland. Proc. R. Soc. Qld., 59: 109--140. Brooks, J.H., Bryan, W.H., Cribb, H.G.S., Denmead, A.K. and Jones, O.A., 1960. In: D. Hill and A.K. Denmead (Editors), The Geology of Queensland. Geol. Soc. Aust., V. 7, Melbourne Univ. Press, pp. 131--139. Brown, J.E., 1973. Depositional histories of sand grains from surface textures. Nature Lond., 242: 396--398. Commonwealth Bureau of Meteorology, 1956. Climate Averages, Australia. Director of Meteorology, Melbourne. Connah, T.H. and Newman, P.W., 1960. In: D. Hill and A.K. Denmead (Editors), The Geology of Queensland. Geol. Soc. Aust., V. 7, Melbourne Univ. Press, pp. 408--418. Gandolfi, G., 1967. Discussion upon methods to obtain X-ray 'powder patterns' from a single crystal. Miner. Petrogr. Acta, 13: 67--74. Jackson, M.L. and Sherman, G.D., 1953. Chemical weathering of minerals in soils. Advan. Agron., 5: 219--318. Klug, H.P., and Alexander, L.E., 1954. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. Wiley, New York, N.Y., 716 pp. Krinsley, D.H. and Doornkamp, J.C., 1973. Atlas of Quartz Sand Textures. Cambridge University Press, 91 pp. Melton, F.A., 1940. A tentative classification of sand dunes. J. Geol., 4 8 : 1 1 3 - - 1 7 4 . Morey, G.W., Fournier, R.O. and Rowe, J.J., 1962. The solubility of quartz in water in the temperature interval from 25°C to 300 ° C. Geochim. Cosmochim. Acta, 26: 1026-1043. Moss, A.J. and Green, P., 1975. Sand and silt grains: predetermination of their formation and properties by microfractures in quartz. J. Geol. Soc. Aust., 22: 485--495. Raeside, J.D., 1959. Stability o f index minerals in soils with particular reference to quartz, zircon and garnet. J. Sediment. Petrol., 29: 493--502. Setlow, L.W. and Karpovich, R.P., 1972. Glacial microtextures on quartz and heavy mineral grains from the littoral environment. J. Sediment. Petrol., 42: 864--875. Soil Survey Staff, 1975. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Agric. Handbook 436, U.S. Dept. Agric., Washington D.C., 754 pp. Thompson, C.H., 1975. Coastal areas o f southern Queensland, some land-use conflicts. Proc. R. Soc. Qld., 86: 1 0 9 - 1 2 0 . Ward, W.T. and Little, I.P., 1975. Times of coastal sand accumulation in southeastern Queensland. Proc. Ecol. Soc. Aust., 9: 313--316.

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Whitworth, H.F., 1959. The zircon rutile deposits on the beaches of the eastern coast of Australia. Techn. Rep. Dep. Mines N.S.W., 4: 7--60. Williams, H., Turner, F.J. and Gilbert, C.M., 1954. Petrography, an Introduction to the Study of Rocks in Thin Sections. Freeman, Berkeley, Calif., 405 pp. Williams, W.T. (Editor), 1976. Pattern Analysis in Agricultural Science. Pergamon Press, Melbourne, 331 pp. Winchel, A.N. and Winchell, H., 1951. Elements of Optical Mineralogy, 2. Description of Minerals. Wiley, New York, N.Y. 551 pp.