FROM A BIOTITE-HORNBLENDE ROCK

Transcription

1 Clay Minerals (1970) 8, 435. A STUDY OF WEATHERING IN A SOIL DERIVED FROM A BIOTITE-HORNBLENDE ROCK II. THE WEATHERING OF HORNBLENDE M. J. WILSON ANt) V. C. FARMER The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen (Received 1 December 1969) ABSTRACT: The weathering of hornblende in the Rehiran profile has been investigated by optical, X-ray, infrared and differential thermal methods. Optical studies of the fresh hornblende show that it contains discrete lameuar intergrowths of another amphibole which, from infrared evidence, is of an iron-rich nature. This minor phase is selectively weathered in the lower horizons and yields a clay mineral which was identified as interstratified swelling chlorite-saponite. The major hornblende component remains relatively unchanged. Single crystal photographs indicate that the formation of the clay mineral is not structurally controlled by the parent hornblende, although there is a tendency towards alignment along cleavage planes. The clay mineral becomes unstable in the upper part of the profile and could not be detected in the A horizon. Comparisons are made with the findings of other workers. INTRODUCTION The profile studied is located at Rehiran in eastern Inverness-shire and is a brown forest soil directly related to a deeply weathered biotite-hornblende rock (Wilson, 1970). The weathering of hornblende was investigated in the same manner as was that of biotite. The different horizons were sampled and weathered hornblendes from the >75/~ fraction separated by hand-picking. This material was then studied by various techniques in order to establish the general pattern of weathering and the nature of the weathering product. The latter was studied in greater detail in the clay fraction. RESULTS > 75~ fraction Optical results. In the fresh rock, thin section study at high magnification reveals that the hornblende commonly contains discrete lamellar intergrowths of another type of amphibole (Plate 1 (a) and (b)). These lamellae lie parallel to the cleavage

2 436 M. J. Wilson and V. C. Farmer planes and may be observed in basal and prismatic sections. They extinguish in unison with the enclosing hornblende (12 ~ in prismatic sections) and show pleochroism in various shades of brown contrasting with the green colour of the dominant hornblende phase. The intergrowths range from 2 to 9 ~ in thickness and similar textural relationships between co-existing actinolite and hornblende have been described by Klein (1969). Identification by optical means is precluded by their small size. In ordinary light, weathering is accompanied by a distinct colour change from a dark green in the fresh hornblende to pale green or even colourless in the weathered state. There is a similar change in pleochroism. Unaltered hornblende shows ~ dark green >fl green or yellow green >a pale golden yellow, whilst weathered hornblende shows y pale green >fl pale yellow green >a almost colourless. Weathering also induces a considerable decrease in the refractive indices and magnetic susceptibility of hornblende (Table 1). These changes are almost certainly related to loss of iron, a constituent which exerts a major influence on the hornblende minerals in general (Deer, Howie & Zussman, 1962). However, it should be emphasized that, for the Rehiran hornblende, refractive index may be representative of an average between hornblende and the exsolved phase described previously. Thus, the decrease in refractive index may be indicative of a selective decomposition of the latter phase, which would presumably be iron-rich, rather than of loss of iron throughout the structure. The weathered hornblende often shows a yellowish-brown clayey material sometimes aligned along cleavage planes, and unlike biotite, only occasional flecks of iron oxide. TABLE 1. Refractive index and magnetic susceptibility of hornblende from >75~ fraction Refractive index Depth in profile tz ), Magnetic susceptibility critical current in amperes 2--6 in "641 0" in in in Fresh rock X-ray results. Apart from the absence of some weak reflections the powder pattern yielded by the fresh hornblende was almost identical with that given on the ASTM data card No additional reflections which could possibly characterize the intergrown material could be found. X-ray diffraction traces for fresh and weathered hornblende are shown in Fig. 1. It can be seen that weathered hornblende shows a reflection at 18 A when glycerol-treated. A 14 A peak occurs when the weathered grains are untreated. These reflections indicate the presence of a smectite-like mineral which, since it is not observed in the fresh hornblende, appears to be a weathering product. It is most abundant in samples from the basal parts of the profile and could not be observed in weathered grains from the surface horizon. The

3 PLATE 1 (a) Prismatic section of Rehiran hornblende showing numerous exsolved lamellae parallel to cleavage. (b) Basal section of Rehiran hornblende showing exsolved lamellae. To face page 436

4 PLATE 2 Electron micrograph of hornblende-derived clay mineral at base of profile.

5 Weathering in soil--l/ 437 8' ' ~ ~ ~ (b) I0"0,..._.,,,~I ~ (a) I I I 1,5 I0,5 "28 FIo. 1. X-ray diffraction traces for fresh and weathered hornblende (a small amount of biotite is sometimes present at different profile depths.) (a) fresh hornblende, (b) in, (c) in, (d) in, and (e) 2--6 in clay mineral was not characterized further by heat treatment at this stage since most separates contained a certain amount of biotite. Single crystal methods were used in order to investigate the possibility of a structural relationship between the clay mineral and the weathered hornblende. A single grain from the in horizon was selected and mounted on a glass fibre. After spraying with glycerol, a photograph in a powder camera showed a strong line at 18 A confirming the presence of the clay mineral. A single crystal c-axis rotation photograph showed a normal layer line pattern, yielding a c parameter of 5"3 A for

6 438 M. J. Wilson and I1. C. Farmer TABLE 2. Clay lines observed in a c axis rotation photograph of glycerol-treated hornblende da hkl 9" " (e). ~ ~ (d) : ~ ~ - - (c) 3672~1~3659..,,,.,,.j/,J --~(b) ~^3642 (a) 1 I I P Fro. 2. Infrared spextra for fresh and weathered hornblendes at different profile depths. (a) fresh hornblende, (b) in, (c) in, (d) in, and (e) 2-6 in

7 Weathering in soil--ii 439 I I I I I ~ ] I I I I I I I I I I I I0 II W,',velength 1~o. 3. Infrared spectra for (a) Rehiran hornblende, (b) actinolite. (Ix) the hornblende, and also a series of diffuse reflections occurring as long continuous arcs. When measured these could be assigned to various orders of the glycerolexpanded basal spacing of the smectite (Table 2). The 060 line at 1,54 A for the clay mineral is also present, occurring as a diffuse arc which transects several layer lines, and indicates a trioctahedral species. From this photograph it was evident that there is no strict relationship between the crystallographic axes of the hornblende and the clay mineral weathering from it. However, the latter could still have a preferred orientation along the cleavage planes and to investigate this possibility a series of 15 ~ oscillation photographs were taken around the cleavages and also over parts of the crystal between them. It was found that the clay lines intensified when the range of oscillation included a cleavage plane but that the lines were still visible in the regions bisecting the cleavages. This indicates that the clay mineral occurs in many orientations although there is a tendency towards parallelism with the cleavage planes. Infrared results. Infrared spectra indicate significant differences between the fresh and weathered hornblendes, but detect no significant differences among the weathered samples from the various horizons. The fresh composite hornblende is characterized by four OH stretching bands (Fig. 2) whose frequencies are indicative of the coordination of OH to various cation groupings (Burns & Strens, 1966). In the weathered hornblende, bands due to OH co-ordinated to Fe] + (3623 cm-1), Fe~ + Mg (3642 cm -a) and Fe 2+ Mg2 (3656 cm -1) decrease in intensity relative to the band due to Mg3 OH groupings (3672 cm-x). These changes can be ascribed to the loss by weathering of a minor iron-rich phase, leaving a magnesium-rich phase unchanged. The iron-rich phase will contribute principally to the bands of OH groups co-ordinated to one or more Fe 2+ and

8 440 M. J. Wilson and V. C. Farmer its loss would explain the change in relative intensity of the OH bands. The constancy of the FC + : Mg ratio in the weathered hornblendes from all horizons indicates that Fe 2+ in the magnesium-rich phase is resistant to oxidation. The spectrum of the weathered hornblende in the 8-16/~ region indicates that it has a composition close to that of an actinolite (Fig. 3). Similarities between the spectra of some hornblendes and actinolite were noted by Lyon (1962), but his samples did not approach actinolite so closely. The spectrum of the fresh composite amphibole in this region is similar to, but more diffuse than, that of weathered material. No discrete bands of the minor amphibole phase could be detected, but its presence can account for the greater diffuseness of the spectrum. Differential thermal results. Differential thermal curves for weathered hornblende are featureless apart from a broad endothermic reaction characteristic of adsorbed water at 110 ~ C t~ and 2-10 t~ ]ractions X-ray results. The diffraction patterns for these fractions show that the hornblende-derived swelling mineral becomes more abundant, especially in the lower parts of the profile. In the B horizon however, the amount of this mineral decreases considerably and it disappears completely in the A horizon. TABLE 3. X-ray results (da) for homblonde~lcrived clay mineral in < 2/, fraction at base of profile Na2S204 Depth in Glycerol Heated at Heated at treated and profile Untreated treated 300~ 500~ K + saturated heated at 300~ in "3 10"3 12"5 n.d in 14" "6 10"3 12" <2 ~ traction X-ray results. X-ray diffraction shows that whilst the clay fraction from the upper horizons is predominantly of a biotite-derived nature, the clay from the lower parts of the profile is composed almost entirely of the swelling mineral associated with hornblende weathering. The relative purity of this mineral enables further characterization (Table 3 and Fig. 4). Firstly, the 060 line occurs at 1"532 A confirming the trioctahedral nature of the mineral. Heating experiments show that it is not a pure smectite. Thus, when the clay from the in horizon was heated at 300 ~ C for 2 hr there was only a partial contraction of the basal spacing to 12-6 A, indicating the presence of interstratified chlorite. However, the fact that the mineral expands to 18 A after glycerol treatment demonstrates that the chlorite component must be of the swelling type, similar to that described by Martin Vivaldi & MacEwan (1957). Had normal non-swelling chlorite been present then spacings intermediate between

9 Weathering in soil--ll I0"0 ~//Cd) I0.0 (h} 441 I 18.~2(c) J " 18.3 ~ ( b ),4, v/l'' I I [ I I I I I I I I I0 5 *20 FIG. 4. X-ray diffraction traces for 2/~ fraction at base of profile. (a)-(d) in. (a) untreated, (b) with glycerol, (c) K-saturated, (d) heated at 500~ (e)--(h) in. (e) untreated, (jr) with glycerol, (g) K-saturated, and (h) heated at 500~ (For the last trace some rehydration may have occurred.) 14 and 18 A would have been recorded. This clay is therefore more accurately described as an interstratified swelling chlorite-smectite. It is easily decomposed with 6 N HC1 but appears to be unaffected by photolytically activated acid ammonium oxalate (Endredy, 1963) or by sodium dithionite treatment (Mitchell & Mackenzie, 1954), both of which will decompose nontronite. However, heating at 300 ~ C after sodium dithionite treatment brought about a greater collapse of the basal spacing compared with the untreated clay heated at the same temperature and it is possible that the interlayer material is of an iron-rich nature. The clay from the overlying horizon at in depth is almost identical apart from a decrease in the chloritic component. This is shown by a contraction in the basal spacing to , after the untreated clay is heated at 300 ~ C. In[rared results. Infrared spectra for the hornblende-derived clay minerals (Fig. 5) show two broad bands at 3419 and 3558 cm -1 which are characteristic of chlorite.

10 442 M. J. Wilson and V. C. Farmer I I [ I 3'000 2"8"?5 2"750 2"625 ~t FIG. 5. Infrared spectra of 24, fraction at base of profile. (a) in, (b) in. Also observed are two bands at 3675 and 3706 cm -1. These may be attributed to saponite (Farmer & Russell, 1964) and they appear to intensify in the clay from the upper part of the C horizon. Differential thermal results. Differential thermal curves for the clays from the basal part of the profile are shown in Fig. 6. They are characterized by a series of endothermic reactions and are reasonably similar to curves yielded by the saponitic clays found in some meta-limestones (Wilson, Bain & Mitchell, 1968). (b) a) FIG. 6. Differential thermal curves for 24, fraction at base of profile. (a) in, (b) in.

11 Weathering in soil--ii 443 Electron microscope results. An electron micrograph of the hornblende-derived clay mineral is shown in Plate 2. The poorly-defined outline is similar to the general appearance of the smectite group of the clay minerals. DISCUSSION The results quoted have established that in the Rehiran profile, an iron-rich component present as lameuae in the fresh hornblende weathers directly to a swelling mineral, whereas the major magnesium-rich phase is relatively unchanged by weathering. Although infrared spectroscopy shows that much, if not all, of the ferrous content of this major phase survives unoxidized in the weathered hornblende, optical studies indicate a marked decrease in refractive index on weathering. This can be accounted for if the iron-rich component contributes to the overall refractive index of the fresh hornblende so that a decrease in this property is significant of the decomposition of the minor phase. This interpretation is consistent with the fact that the greatest decrease in refractive index is recorded at the base of the profile, where the hornblende-derived clay mineral is at a maximum, but thereafter remains relatively unchanged. It is of interest that for co-existing actinolite-hornblende pairs Klein (1969) found that actinolite always had the higher Mg/Mg + Fe ratio. The swelling mineral produced from the iron-rich phase is characterized by X-ray and infrared evidence as an interstratified swelling chlorite-saponite. This transformation is not structurally controlled. Single crystal photographs show that, in the early stage of weathering, there is no orientation relationship between the clay mineral and the parent hornblende. Apparently, the transformation does not involve a topotactic relationship, such as Smith (1959) described for the alteration of olivine. Rather the soluble or colloidal products arising from the decomposition of the primary mineral are retained within the cleavage planes of the major magnesiumrich phase, and subsequently crystallize to interstratified swelling chlorite-saponite. It may be conjectured that a significant factor in the breakdown of the hornblende is the oxidation of iron in the structure, rendering it unstable due to the high local charges produced. This cannot be compensated, as in the biotites, by ejection of iron into an interlamellar space. It is also possible that the iron-rich phase is rich in alkali-metal ions, so rendering it more easily weathered. In the upper part of the C horizon the amount of saponite in the interstratified material appears to increase. This could be attributed either to the instability of the chloritic component or to the more rapid formation of saponite in this part of the profile. At any rate it is evident that the clay mineral becomes quite unstable in the upper horizons, diminishing greatly in the B horizon and disappearing completely in the A horizon. Whether there are any crystalline products associated with this breakdown is unknown. It is interesting to compare the weathering of hornblende in the Rehiran profile with previous studies on the alteration of this mineral. Stephen (1952) showed that in an appinite-derived soil, hornblende weathered to chlorite which in turn altered to interstratified chlorite-vermiculite. A similar transformation was found in some

12 444 M. 3". Wilson and V. C. Farmer Japanese soils by Kanno et al. (1963) and by Kato (1965). Again Kukovskii (1958) found that in a weathered amphibolite, hornblende was replaced by hydrochlorite and Lebedev (1959) found a similar mineral in the weathered crust of hornblendebearing ultrabasic rocks in south-west Ukraine. The full weathering sequence arrived at by the latter was: hornblende > chlorite ----->-hydrochlorite > ferrihalloysite---~ferric hydroxides. On the other hand, some studies indicate a weathering product rather similar to that found in the Rehiran profile. Thus, in the weathering crust of appinite, Stephen & MacEwan (1951) record that a regularly interstratified swelling chlorite is associated with the breakdown of hornblende. Again, in a study of the diagenetic alteration of hornblende, Walker, Ribbe & Honea (1967) found a product described as an iron-rich montmorillonite. However, the diffraction trace shown for this mineral after heating at 300 ~ C does indicate the presence of interstratified chlorite which appears to be of the swelling type since glycerol treatment induces complete expansion to 18 A. This diagenetic clay mineral was found to decompose by the removal of iron which eventually crystallized as hematite. It would appear therefore that the weathering of hornblende commonly proceeds either to interstratified chlorite-vermiculite or to a swelling mineral with interstratified chlorite, Whether these different products can be related to compositional and environmental factors is at present unknown. ACKNOWLEDGMENTS The authors are indebted to Mr A. Birnie for the differential thermal curves, to Mr D. M. L. Duthie for the photomicrographs and to Dr W. J. McHardy for the electron micrograph. REFERENCES BURNS R.G. & STraiNS R.G.J. (1966). Science N. Y. 153, 890. DEER W. A., Howm R.A. & ZUSSMAN J. (1962). Rock-Forming Minerals, VoL 2: Chain Silicates. Longrnans, London. ENDmEDY A.S, DE (1963). Clay Miner. Bull. 5, 209. FARMER V.C. & RUSSELL J.D. (1964). Spectrochim. Acta 20, KANNO I., HONJO Y., ARIMURA S. & TOKUE~ME S. (1963). Bull. Kyushu agric. Exp. Stn. 11, 15. KATO Y. (1965). Soil Sci. PI. Fd. Tokyo, 11, 30. KLEIN C. (1969). Am. Miner. 54, 212. KffKovsrm E.G. (1958). Mineralog. Sb. L'vov No. 12, 448; Chem. Abstr. 1960, 54, 19322e. LEnV.DEV, Y.S. (1959). Mineralog. GeoL Zalizorudn ikh Rodovishch. Ukr. R.S.R. p. 124; Chem. Abstr. 1961, 55, 5255g. LYON R.J.P. (1962). Evaluation of Infrared Spectrophotometry for Compositional Analysis of Lunar and Planetary Soils. Stanford Research Institute, Menlo Park, California. MARTIN VIVALDI J.L. & MAcEwAN D.M.C. (1957). Clay Miner. Bull. 3, 177. Mrrcnm.L B.D. & MACKENZIE R.C. (1954). Soil Sci. 77, 153. SMITH W.W. (1959). Miner. Mag. 32, 324. S~PrmN I. & MAcEWAN D.M~C. (1951). Clay Miner. Bull. 1, 157. Sa'EprmN I. (1952). Soil Sci., 3, 219. WALg.r~ T.R., RaBBE P.N. & HONT.A R.M. (1967). Bull. geol Soc. Am. 78, WILSON M.J. (1970). Clay Miner. 8, 291. WmSON M.J., BAn~ D.C. & MrrcrmLL W.A. (1968). Clay Miner. 7, 343.