Major-element chemistry and micromorphology of Mn-oxide coatings on stream alluvium

Applied Geochemistry. Vol. 8, pp. 633--642,1993

0883-2927/93$6.00+ .00 Pergamon Press Lid

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Major-element chemistry and micromorphology of Mn-oxide coatings on stream alluvium GENE D . ROBINSON Geology-Geography Department, James Madison University, Harrisonburg, VA 22807, U.S.A. (Received 24 July 1992; accepted in revised form 5 March 1993) Abstract--The Mn-oxide coatings on alluvium of six small streams in the eastern U.S.A. have been studied using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. Low magnification surface analyses show that Mn is generally the most abundant element followed by either Fe or Si. Several percent each of Ca and AI also occur. The Mn/Fe ratios range from 0.29 to 7.69 but average 2.37. Point analyses of coating cross sections show considerable variation in Mn/Fe ratios but no evidence of regular layering or microehemical laminations such as occurs in rock varnish. Two micromorphoiogies predominate: roughly circular structures which have a cellular appearance and spherical structures. The cellular structures are remarkably similar to porous Mn-oxide precipitated by bacteria in cultures prepared by Mustoe, suggesting that Mn-oxidizing bacteria are important in forming stream Mn-oxide precipitates.

INTRODUCTION ACCUMULATIONS of Mn-oxides occur in numerous surface environments including marine nodules, found in the abyssal planes as well as continental margins; lacustrine nodules, crusts and coatings; coatings and concretions in streams and soil; crusts on proglacial sediments; coatings in fracture zones; coatings, crusts and layered deposits in caves; and coatings on rock surfaces in arid regions (rock varnish). CRERAR et al. (1976) and MARSHALL (1979) provided excellent summations of the geochemistry of Mn. Black coatings on stream alluvium are particularly common and occur under many different geological and climatic conditions including high mountains (THEOBALD et al., 1963); sub-arctic (PommR et al., 1990); humid temperate (NICHOL et al., 1967; NOWLANel al., 1983; BUCKLEY, 1989); and humid subtropical (CARPENTERet al., 1975; CERLING and TURNEB, 1982). Because of their known ability to scavenge several trace metals, they have been used as a sample medium in exploration geochemical studies (CARPENTER et al., 1975; WHITNEY, 1975; N O W L A N , 1976; ROBINSON, 1985). The origin of the various types of Mn-oxide surface accumulations is controversial. Both physicochemical and biological processes have been proposed. A biological origin is strongly indicated for some deposits. DORN and OBERLANDER (1981) and DORN (1986) argued that rock varnish forms by a two-stage process: oxidation of ambient Mn and Fe by microorganisms followed by incorporation of the oxides into wind-blown mixed-layer clays that accumulate on rock surfaces. PECK (1986) conducted culturing experiments which indicated that Mn-oxide deposits in caves are also produced by microbial precipitation. Manganese-oxidizing bacteria are also known to

occur in soils, swamps, freshwater and the ocean ( C R E w et al., 1980), and are thought to be important in precipitating Mn-oxides (FEmtts et al., 1989). Their role in the formation of marine nodules and crusts is difficult to evaluate because of the varying morphologies and chemical compositions that occur (CRONAN, 1977) and the coexistence of both Mnreducing and oxidizing bacteria on nodule surfaces (EHRLICH, 1972). BATURIN(1990) presented evidence indicating that biological processes are important in extracting Mn from sea water. He described a process in which the metal is organically precipitated to form pellets and detritus. These precipitates are eventually incorporated into the bottom sediments to form micronodules and finally macronodules. Diagenesis in sediment pore waters is generally thought to be an important source of Mn (CALVERTand PIPER, 1984). Inorganic processes may also be important in forming marine Mn-oxides (CRERARet al., 1976). Bacterial oxidation of Mn-oxide is also important in the precipitation of lacustrine nodules and coatings. Several studies have found that removal of bacteria from lake water by filtering, autoclaving, irradiation, or poisoning severely slowed or prevented the precipitation of Mn(II) (CHAPNICKet al., 1982; TIPPING, 1984; TIPPINGet al., 1984; KAWASHIMA et al., 1988). The precipitation of Mn(II) in most surface environments is slow in the absence of a catalyst (HEM, 1964a). Several inorganic catalysts are known including quartzose sand grains and ferric hydroxides, both of which are common in streams and lakes. HEM (1964a) showed that such inorganic catalysts can increase the rate of oxidation by as much as 10 x , and VAN DER WEIJDEN (1975) found that existing Mnoxide substrates can produce an even greater autocatalytic effect. The autocatalytic oxidation of

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Table 1. Sampled streams, geology and basic chemical characteristics Stream

Geology

Eh (mV)

pH

Temperature (°C)

Dissolved 0 2 (% saturation)

Mn (~g/l)

Fe (~g/l)

Mn/Fe

16

60

0.27

Cub Run

Valley and Ridge; Carbonates and calcareous shales

423

8.4

16.9

100

War Branch

Valley and Ridge; Shales and calcareous siltstones

431

7.5

18.0

100

1. l

8

0.14

Swift Run

Blue Ridge; Metamorphosed basalts and greenstones

421

7.0

18.1

98

4.0

60

0.07

Goodwin Creek

Blue Ridge; Mylonitic gneiss

415

7.2

24.0

90

12

246

0.05

Elk Run

Piedmont; Hornblende granite and syenite

402

6.8

22.0

88

252

634

0.40

Bear Garden Creek

Piedmont; Amphibolite. metafelsite

419

7.0

21.8

90

~2

966

0.(16

Mn(II) may not be very important in most nearsurface environments, however, because it is slow unless the p H is >8.5 (HEM, 1964b). DIEM and SXUMM (1984) conducted experiments clearly showing that bacteria can be much more efficient catalysts in precipitating Mn than inorganic sedimentary materials. They found that dissolved Mn(II) in sterilized lake water (aerated and maintained at p H 8.0) remained stable over seven years in the absence of any catalyst, but that most of the Mn was lost from solution after six years when buserite was initially present; however, the addition of Mn-oxidizing bacteria collected from the lake induced such rapid oxidation that almost all of the dissolved Mn precipitated in only eight days. Little research has been done on the major element chemistry, morphology, or origin of Mn-oxide precipitates in streams. CARPENTER and HAYES (1980) and CERLING and TURNER (1982) demonstrated that stream pebble coatings precipitate within a period of years and on a variety of substrates. BUCKLEY(1989) found that pebble coatings from England and Scotland had a colloform structure and were amorphous. NOWLAN et al. (1983) conducted the most thorough study designed to determine the origin of Mn stream precipitates. They argued that concretionary oxides form physicochemically at the interface between reducing sediment pore waters charged with dissolved Mn and r e , and oxidizing stream water. The present investigation is the first to study, systematically, the micromorphology and major element trends of Mn-

oxide coatings from streams and to discuss how this relates to their origin.

SAMPLE COLLECTION AND ANALYTICAL PROCEDURES

Forty-eight samples of coated pebbles averaging approximately 3-5 cm diameter were collected during June I990 from each of six small streams in Virginia (Table 1). Streams selected for sampling appeared to be well-aerated and contained dark-coated clasts. Cub Run and War Branch are located in the Valley and Ridge province (Ordovician calcareous shale and siltstone bedrock in drainage basins)~ Swift Run and Goodwin Creek in the Blue Ridge province (Precambrian and Cambrian metabasalt and mylonitic gneiss bedrock in drainage basins), and Elk Run and Bear Garden Creek in the Piedmont province (Precambrian granite and migmatite bedrock in drainage basins). In each stream, eight clasts were collected from within a 1 m2 area and an effort was made to choose pebbles which visually appeared to have the most well-developed coatings. The pH, Eh and temperature of each stream were measured using a portable meter and a platinum electrode. The dissolved O 2 content of the water was measured in the field using the azide modification of the Winkler method (SLACK and FELTZ, 1968). A water sample was also collected at each site in a 250 ml linear polyethelene bottle. After filtering (8 #m), 0.5 ml of l:l HCI was added and the samples were stored for later analysis using standard atomic absorption procedures. Although both Mn and Fe vary by more than two orders of magnitude from stream to stream (Table 1), all of the streams had well-developed dark-colored coatings. The darkest and apparently thickest coatings were from Bear Garden Creek and Cub Run, which are intermediate in Mn concentration. The least continuous coatings

Mn-oxide coatings on stream alluvium

635

Table 2. Analyses of glass standard Standardless EDXanalyses

Mn Fe Si AI Ca Mg K Na

Composition

Maximum

Minimum

Mean 4- S.D.

10.15 15.23 44.16 22.33 3.05 2.03 3.05 0.00

11.09 14.28 48.19 21.43 3.05 2.45 2.87 0.64

9.58 12.78 46.60 20.93 2.38 1.85 2.14 0.00

10.374-0.39 13.71 4- 0.52 47.28 4- 0.52 21.15 4- 0.15 2.75 4- 0.22 2.05 4- 0.18 2.53 4- 0.58 0.15 4- 0.25

Note: Glass composition (BIERMANand GILLESPIE,1991) has been converted from oxide percent and normalized to 100%, omitting Ba and Ti which were not determined in this study. Eleven analyses were made, each in a different area of the glass. are from Elk Run which has by far the highest Mn concentration; however, the patchy nature of the coating could be due to a flooding event which affected this stream a few days prior to sample collection if coatings are occasionally eroded as CARPENTERand HAYES(1980) suggest. The pebble samples were washed in deionized water to remove loose detritus and air dried, but were not treated in any other way to avoid disturbing surface morphology. After visual examination with a binocular microscope to locate well-developed areas of coating, chips averaging about 10 mm x I0 mm were cut from each sample, mounted on aluminum SEM stubs using plastic cement, and coated with gold--palladium for SEM examination and energy dispersive X-ray analysis (EDX). A 30° take-off angle, 20 keV accelerating voltage, and 75 s counting time were used for all analyses. All EDX data were obtained using a standardless program which corrects for atomic number, absorption and fluorescence (ZAF factors) before the elemental weight percent (normalized to 100%) is calculated. Surface analyses were made by scanning almost the entire area of coating exposed on each rock chip at low magnification (-75-100 mm 2). The SEM was operated in the point source mode at high magnifications to obtain analyses of discrete micromorphologies. Although efforts were made to minimize sources of error common during EDX analysis of bulk material (GoLDSTEINet al., 1981), the method is inherently limited to achieving semi-quantitative results at best. Although a prepared silicate glass similar in composition to rock varnish was used to assess the accuracy and precision of the method (Table 2), analyses of the rough coating surfaces are likely to be less accurate and less precise.

MAJOR ELEMENT CHEMISTRY As shown in Fig. 1, MN is the most abundant element in 37 of the samples, Si in 4 samples (from War Branch), and Fe in 7 samples (Swift Run and Elk Run). A l u m i n u m and Ca are also present in fairly high concentrations in all of the samples. The major element composition of the coatings is not clearly related to any of the measured chemical parameters in the streams (Table 1) or to the ratios shown in Table 3. The Mn/Fe ratios are similar to those determined in stream coatings by CARPENTERet al. (1975), CARPENTER and HAYES (1980), CERLING a n d TURNER (1982) and ROBINSON (1985), but AI/Si is lower than values reported by BUCKLEr (1989).

=UB RUN M.

WAR SWIFT RUN GOODWIN ELK RUN BEAR GAROBN BRANCH CREEK CREEK

so

~0 ~ s

i, f

f

t. 0.E.

o.i. FIG. 1. Range in major element abundance and mean concentrations from EDX analyses of eight samples in each stream.

The concentration of the major elements for each stream varies considerably from one clast to another, even though the samples were collected from the same 1 m 2 portion of each stream bed (Fig. 1). Similar variations occur in major element ratios (Table 3). H o w e v e r , analyses made in different areas of individual clasts (not iUustrated) do not vary significantly. This could be due to either: (A) clasts collected in each stream actually include coatings that precipitated in various portions of the stream and have only recently been transported to the sample location; or (B) variations in geochemical parameters not measured in this study are sufficient to cause significant variation in coating chemistry. Important differences are apparent in comparing the analytical data in the present study with that of BUCKLEY (1989). Both investigations found nonsignificant variation over the surface of individual clasts but the between-sample heterogeneity at each site is much greater for the analyses in BUCKLEY (1989) probably because they were collected from streams draining several different lithologies in the U . K . In addition, BUCKLEY(1989) collected samples over a 100 m stream course rather than at a single site

G. D. Robinson

636

Table 3. Variation in major element ratios AI/Si

Mn/Fe Stream Cub Run War Branch Swift Run Goodwin Creek Elk Run Bear Garden Creek

Maximum 3.72 3.94 7.69 1.92 5.48 6.56

Minimum

Mean

1.88 0.88 0.29 1.09 0.17 0.66

2.64 2.02 2.30 1.51 2.10 3.62

Maximum 0.66 0.32 1.08 0.78 1.11 2.70

Minimum

Mean

0.35 0.17 0.37 0.34 0.39 0.46

0.46 0.24 0.58 0.46 0.59 0.91

and used a compilation of individual point analyses instead of low-magnification surface analyses to determine major elements. BUCKLEY (1989) also • SIream C o l t m g s + LaCUStrine COrCrelKmS found that Mn/Fe averaged - 3 x higher than ratios I S h g l o w Marine C~¢retDons o k~r~ Noawas determined in the present study and AI/Fe was 13× D o I B r t Varnish greater. The high Mn/Fe ratios were from areas associated with sulfide mineralization; elevated Mn/ Fe ratios have been found previously near such mineralization (CARPENTERet al., 1975; ROBINSON, 1985). BUCKLEr (1989) suggested that the determined high AI/Si ratios were due to either coprecipitation of an aluminous phase or flocculation of colloidal Mn-A1 oxide/hydroxide particulates. Such an authigenic AI phase is unlikely in the present study SI+AI Fe because AI/Si is similar to that of clay minerals Fro. 2. Triangular plot comparing stream coatings (this occurring in the near surface environment (MILLOT. study) with other Mn-oxide precipitates. Data for lacustrine 1970). concretions and shallow marine concretions from CALVERT The dominant elements at each sample location and PmCE (1977); data for marine nodules from CRONAN and analyses of other common Mn-oxide precipitates (1977), CALVERTand PIPE~(1984) and BATUmN(1990); data for desert varnish from DORN(1989). are plotted in Fig. 2. Manganese and Ca are combined because PO~ER and ROSSMAN (1979) showed that birnessite (Na,Ca,K,MnTOi4.3H20) is the dominant manganese phase in stream pebble coatings. It is likely that Si and A1 mainly occur in included detritus previously reported in pebble coat8" ings (CARPENTERand HAVES, 1980) SOthey have been plotted together. Compared to desert varnish, the 4~ precipitates formed in water are higher in Mn and Ca 2" but lower in Si and AI. This is probably due to the genetic role clay minerals play in forming desert ib ~o ~ 4b varnish (DORN, 1986). Marine nodules and the fresh 6 water coatings of the present study plot toward the GOODWIN 4 EEK middle of the range covered by lacustrine precipi2 tates. Silicon and A1 are low in all three, but Mn exceeds Fe in the oceanic nodules and stream deb tb io ~b 2'0 m ~o ~14 posits while many analyses of lacustrine precipitates 12 have Mn/Fe ratios < 1. All the stream pebble coatings ~ELK / / Ru. I0 plot within a fairly small field except for War Branch, which plots closer to desert varnish. The samples 8. from this location appear thinner when examined in 8EAR GARDEN 6 hand specimen or with a binocular microscope and the concentration of Mn and Fe in the stream water is 2, the lowest of all streams studied. A representative sample from each stream was cut b ,'o 2o ~o 4b o'o o ,o zo ~'o normal to the coating surface for thickness measureMICRONS FROM SURFACE ments and Mn/Fe ratios (Fig. 3). Coating thicknesses FIG. 3. Variation of Mn/Fe in transects through stream coatings using EDX point analyses. The deepest data point range from 7 to 57/~m, considerably thinner than the represents the underlying rock substrate. maximum of "several millimeters" reported by Mn+Ca

e!

Mn-oxide coatings on stream alluvium

FIG. 4. Photomicrographs of pebble coatings showing common micromorphologies. Bar scale is 10 pm, except E = 1 am. A. Cub Run: colloform textures which occur when surface EDX Mn is > - 5 5 wt%; B. Cub Run: two forms of Mn-oxide typical of coatings, small roughly spherical particles and cellular structure; C. War Branch: Mn-oxide showing well-developed cellular structure. Light-colored material is fine detritus which has been deposited onto the surface; D. Goodwin Creek: botryoidal to semi-botryoidal and cellular textures occurring together; E. Goodwin Creek: association of cellular and spherical coating micromorphologies. The spherical particles appear to form from smaller particles which agglomerate into larger botryoidal masses. The cellular structures appear to accumulate in layers until a continuous coating is eventually formed; F. Swift Run: association between cellular and botryoidal coating textures. Small spherical particles can be seen accreting from a highly porous cellular matrix. Diatoms can also be seen in the process of being engulfed in coating; G. War Branch: incipient coating accreting mainly as spherical particles, although some cellular structure also occurs; H. Elk Run: incipient cellular coating on a silicate substrate. Individual cells can be seen at the left accreting together to eventually form a continuous layer of coating; I. Cub Run: Incipient coating which appears to have flocculated onto the diatom surfaces.

637

639

Mn-oxide coatings on stream alluvium Table 4. EDX point analyses of representative Mn-oxide micromorphologies Stream Cub Run Cub Run War Branch Goodwin Creek Goodwin Creek Swift Run Swift Run Elk Run Elk Run

Morphology

Mn

Fe

Si

AI

Ca

Mg

K

Na

Mn/Fe

Spherical Cellular Cellular Spherical Cellular Spherical Cellular Spherical Cellular

66.4 58.4 43.3 61.7 50.2 78.3 69.9 43.8 30.1

9.7 26.7 25.8 20.3 17.8 7.6 19.8 23.5 25.7

6.1 3.2 14.6 5.3 22.2 2.8 3.4 15.8 23.5

5.7 1.9 10.4 4.7 4.7 4.5 1.4 11.6 10.8

9.3 9.4 4.4 5.2 3.7 3.5 3.5 1.6 7.9

1.3 0.2 0.4 1.1 0.3 0.4 0.1 3.0 0.3

1.0 0.I ND 0.6 0.7 0.6 0.1 ND 1.5

0.5 0.1 1.1 1.0 0.4 2.3 1.8 0.7 0.2

6.8 2.2 1.7 3.0 2.8 10.3 3.5 1.9 1.2

Note: Analyses have been normalized to 100%; ND = not detected, detection limit, -0.08%.

BUCKLEY(1989) for the U.K. Based on Mn/Fe ratios, three internal zones can be discerned with depth in each sample: a surface or near-surface zone with moderate Mn/Fe, a central zone with high Mn/Fe, and a basal zone with low Mn/Fe. These variations are consistent with previous field observations which have found that stream pebble coatings have an Ferich base overlain by more Mn-rich material (CARPENTER and HAYES, 1980; NOWLAN et al., 1983). These authors have interpreted this as representing the greater mobility of Mn at an interface between reducing conditions in sediment pore water and oxidizing conditions in the stream. The surface zone is characterized by low Mn/Fe and high values for Si and Al (not shown). This most likely results from erosion of the upper surface of the coatings by stream abrasion as postulated by CARPENTER and HAYES (1980) and NOWLANet aL (1983). Such erosion should produce openings in the portion of the coating near the surface in which very fine detritus, high in Si and Al with low Mn/Fe, could become trapped. The Mn/Fe ratio in the two thickest coatings, from Bear Garden Creek and to a lesser extent Cub Run, fluctuates with depth. The source of this variation might be related to the annual cycle typical of small streams in mid-latitude humid climates in which high levels of dissolved metals and higher Mn/Fe ratios occur during the late autumn when most of the water flow is derived from groundwater and when dissolved 0 2 and pH are low due to decaying leaves (SLACg and FELTZ, 1968), assuming that stream Mn-oxide coatings form over the course of a few years as CARPENTER and HAYES (1980) suggested. Similar variations attributed to seasonal or climatic factors have been found in lacustrine Mn-oxide nodules (HARRISS and TROtJP, 1969), marine Mn-nodules (SOREM and FESgES, 1977), and rock varnish (DoRN, 1984).

COATING MICROMORPHOLOGY Examination by SEM showed that two morphologies predominate: roughly spherical particles and porous to cellular structures (Fig. 4). The spherical structures range from individual particles - 1 / ~ m

in diameter to large botryoidal aggregates up to 70 /~m in diameter (Fig. 4A, B, D, E and F). BUCKLEY (1989) previously reported botryoidal textures in Mn-oxide coatings on stream alluvium, and GREENSLATE (1974), SOREM and FEWKES (1977), and BURNS and BURNS (1978) published photomicrographs of similarly appearing botryoidal structures in deep sea Mn-oxide nodules. Botryoidal textures have also been shown to occur in desert varnish (DORN, 1986, 1989). Such textures have been attributed to both microbial precipitation of Mn (GREENSLATE,1974; DORN and OBERLANDER,1981) and inorganic flocculation and sedimentation of fine particulate material (SOREM and FEWKES, 1977). The cellular structures (Fig. 4B, C, E and F) are remarkably similar to the porous Mn-oxide precipitated by bacteria from a cold spring which MUSTOE (1981) successfully cultured. MUSTOE attributed the circular nature of the precipitate to masses of oxide deposited as a matrix around individual microbes. Such a pattern of Mn precipitation is common in Mn-oxidizing bacteria (FERRISet al., 1989). The cellular and spherical coating morphologies often occur together in one specimen. As Fig. 4E and F show, the coating appears to have grown directly upward by an agglomeration of minute spherical particles from a cellular base. Point analyses (Table 4) show that both forms of precipitate have higher concentrations of Mn and higher Mn/Fe ratios than coating surfaces as a whole, which is not surprising because surface analyses include inorganic and organic detritus which become engulfed during coating accretion (Fig. 4F and I). The spherical particles tend to be somewhat higher in Mn than the cellular structures. While the spherical and cellular coating structures were observed in specimens ranging from low to high Mn concentrations, the larger botryoidal agglomerates were well developed only when surface EDX analyses showed Mn to be > - 5 5 % . The stream coatings are similar to rock varnish in this respect. DORN (1986) showed that varnish with a weU-developed botryoidal texture contained significantly higher concentrations of Mn and Fe than varnishes with other textures. Incipient coatings are occasionally observed on exposed mineral substrates. It is not known whether

640

G.D. Robinson

such mineral surfaces were slow to develop coatings because they were in some way unfavorable to coating growth or, as seems more likely, these substrates represent areas where, due to variations in the stream microenvironment, coating erosion was more rapid. As shown in Fig. 4G and H, incipient coatings occur in both the spherical or cellular structural forms. Figure 4I shows a third form of incipient coating development which strongly resembles the layered textures photographed by SOREMand FEWgES (1977) in oceanic Mn-oxide nodules and attributed to the agglomeration of minute floccules. Textures resembling those produced by flocculation can also be seen in Fig. 4F. It is known that colloidal Mn can be important in streams (LAZERTEand BURLING,1990) so it is possible that coagulation of such particles may contribute to coating formation. Except for occasional quartz peaks from included detritus, the coatings did not produce any peaks during conventional XRD scans. This is a common characteristic not only of stream pebble coatings but also of many other common near-surface forms of Mn-oxide, including desert varnish (ALLEN, 1978), lacustrine nodules (CALVERTand PRICE, 1977) and fracture deposits (DOUGLAS, 1987). DIEM and STUMM (1984) noted that physicochemical oxidation of Mn(II) from oversaturated solutions typically produces crystalline 2-MnOOH while oxidation catalyzed by microbes produces amorphous Zd-manganate (IV). The X-ray amorphous nature of near-surface Mn-oxide precipitates could result from either a lack of crystallinity or the presence of extremely fine-grained particulate material (PoTtER and ROSSMAN, 1979). BUCKLEY(1989) suggested that the non-crystalline nature of stream Mn-oxide precipitates reflects a recent origin.

DISCUSSION AND CONCLUSIONS These results are significant not only because they represent the first investigation of the major-element chemistry and micromorphology of Mn-oxide coatings of stream alluvium, but also because they provide a basis for speculation concerning the origin of these common surface precipitates. The coatings have both botryoidal and cellular morphologies, but the cellular structures are more common, occurring in virtually all of the samples, while colloform surfaces are restricted to those specimens highest in Mn. Botryoidal textures have been attributed to both inorganic and bacterial precipitation and are, therefore, not definitive. Structures closely resembling the cellular textures of the coatings, however, have only been produced by bacterial precipitation. The widespread occurrence of such cellular structures in the coatings strongly suggests that Mn-oxidizing bacteria play a major rote in their genesis. CRERARet al. (1980) calculated that the common Mn-oxidizing bacteria

M e t a l l o g e n i u m , which has been identified in freshwater environments (GREGORYet al., 1980) increases the rate of oxidation by as much as seven orders of magnitude. Physicochemical precipitation does not seem to be as important in forming stream Mn-oxide precipitates as has been commonly thought. In support of such a process, NOWLAN et al. (1983) described a common field observation: coated rocks are often zoned, i.e. the lower portion of the rock submerged in sediment has no coating, the portion just above the sediment-water interface has an Fe-rich coating, and the rest of the rock has a dark Mn-rich coating, suggesting that precipitation occurs in response to a sharp Eh gradient between reducing sediment pore waters and oxygenated stream water. CARPENTERand HAYES (1980), however, showed that Mn-oxide coatings precipitate not only at the sediment-water interface but also as much as 15 cm above the stream bed. It seems unlikely that an Eh gradient could persist that far into the water column under the dynamic conditions of a stream. Instead of coated boulders, precipitates formed due to a contrast between reducing and oxygenated waters should produce flattened or girdle-shaped structures, such as is common in lakes (CALVERTand PRICE, 1977) where the cycling of metals from pore waters is thought to be important. Other considerations also suggest that a purely physicochemical mechanism is inadequate to account for stream Mn-oxide coatings. The E h - p H diagrams constructed by NOWLAN el al. (1983) showed that under determined stream water conditions, Mn should remain in solution and only Fe(OH)3 should precipitate. This is consistent with recent experimental data (SWAINet al.. 1975; SAVENKO,1989) showing that the solubility of MnO 2 exceeds the dissolved Mn concentration measured in the stream. The precipitation of Mn(II) in surface environments is known to be very slow in the absence of a catalyst (DIEM and STUMM, 1984), but CARPENTERand HAYES(1980) and CERLING and TURNER (1982) have shown that stream coatings form rapidly, with significant accumulations developing in only a few months, That the catalytic agent may be organic is suggested by the results of TIPPING (1984) and PONTER et aL (1990) who found temperature optimums for the precipitation of Mn in lake water and stream water, rather than a positive correlation between precipitation and temperature as would be expected if physicochemical processes were dominant. Some of the examined coatings have textures resembling those attributed to colloidal aggregation. LAZERTE and BURUNG (1990) showed that colloidal Mn can be important in streams with a pH of 5.5. LAXEN and CHANDLER (1983) found that turbulence increases the rate of particle coagulation and that freshly precipitated Fe-oxide tends to be associated with colloidal particles as surface coatings. These findings are consistent with a possible role for colloidal deposition in forming the pebble coatings.

Mn-oxide coatings on stream alluvium

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Acknowledgements--Financial support for this study was provided by a James Madison University Faculty Summer Research Grant. I thank Paul R. Bierman for supplying artificial glass standards used in the EDX analyses.

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