CHARACTERIZATION OF IRON-MANGANESE CONCRETIONS IN …clays.org/journal/archive/volume 45/45-3-428.pdf · 2009-04-16 · Clays and Clay Minerals, Vol. 45, No. 3, 428-439, 1997. CHARACTERIZATION

Clays and Clay Minerals, Vol. 45, No. 3, 428-439, 1997.

C H A R A C T E R I Z A T I O N OF I R O N - M A N G A N E S E C O N C R E T I O N S IN K E N T U C K Y ALFISOLS WITH P E R C H E D WATER TABLES

MIN ZHANG AND A. D. KARATHANASIS

Department of Agronomy, N-122K Agr. Science-North, University of Kentucky, Lexington, Kentucky 40546-0091

Abstract--Iron-manganese concretions are common in upper sola of Alfisols in the Inner Bluegrass Region of Kentucky. Their nature and quantities appear to be related to the fluctuation of seasonal perched water tables above clayey argillic horizons. This study was conducted to examine changes in the macro-and micromorphology, chemistry and mineralogy of concretions as a function of size, color and soil depth. Total Mn and Fe contents increased, while SiO2 decreased with concretion size. Black concretions contained higher Mn, while brown concretions were higher in Fe. Crystalline Mn- and Fe-oxides fractionated with a sequential extraction procedure increased, but amorphous Mn and Fe decreased with concretion size. Goethite was the only crystalline Fe oxide mineral identified by X-ray diffraction (XRD) analysis. Manganese oxide minerals were very difficult to detect due to the diffuse nature of their XRD peaks and poor crystallinity. Examination of soil thin sections showed concretions of soil horizons overlying restrictive clayey layers to exhibit differentiated fabrics, sharp external boundaries and generally spherical shapes. Concretions found within clayey restrictive layers or above lithic interfaces usually had less structural organization, softer matrices and diffuse external boundaries due to longer term saturated conditions. Scanning electron microscopy (SEM) examinations suggested that the concretionary matrix, in spite of its density, has numerous cavities and an extensive micropore system within which dissolved plasmic Fe and Mn can diffuse and precipitate. Key Words----Elemental Composition, Fractionation, Micromorphology, Mineralogy.

INTRODUCTION

Iron and Mn concretions have been found in many soils, and especially in soils with restrictive internal drainage. The quantity, chemical composition, mineralogy and size of concretions in soils are site-spe- cific depending on the action of weathering processes for that location (Drosdoff and Nikiforoff 1940; Gal- laher et al. 1973a, 1973b; Rhoton et al. 1991; Mc- Daniel et al. 1992). Many researchers define concretions as mixtures of soil materials cemented together (Winters 1938; Radden and Porter 1962; Murthy and Mathur 1964). The cementing agents for Fe-Mn concretions are Fe and Mn oxides. Therefore, the concretionary material is characterized by a greater concentration of Fe and Mn oxides than the surrounding soil matrix (Barnhisel et al. 1969; Phillippe et al. 1972; Rhoton et al. 1993). It also differs from the surrounding soil matrix by containing less SiOv Schwertmann and Fanning (1976) found that Mn and Fe are not evenly distributed in all concretions found within a given soil horizon or even within a given concretion. Manganese content is usually related to concretion size, with larger concretions containing more Mn and lower Fe/Mn ratios. Also, dark-colored concretions are generally higher in Mn content than lighter-colored concretions (Phillippe et al. 1972), but color, shape and hardness may differ among different soils. Concretions found in the better-drained soils of loess hydrosequences in Bavaria were usually rusty- colored with a blackish interior, irregular in shape and medium in hardness. However, in soils with re-

Copyright �9 1997, The Clay Minerals Society

strictive layers, black, well-rounded and hard concretions prevailed in the upper part of the profile, where- as in deeper horizons the concretions were similar to those in the drier soils (Schwertmann and Fanning 1976).

The effect of concretion accumulation on soil chemical properties could be significant at high concentrations, depending on the chemistry and mineralogy of the oxide components. In several studies, goethite was the only Fe-oxide mineral identified by XRD in concretionary materials (Gallaher et al. 1973b; Schwert- mann and Fanning 1976; Rhoton et al. 1993). Man- ganese oxides were found to be fine-grained with rel- atively diffuse XRD peaks due to their poor crystallinity and low concentration (Tokashiki et al. 1986). Morphologically, most Mn oxides have a platy structure (bimessite and lithiophorite) or lathlike morphol- ogy (todorokite) and, if adequately crystalline, should respond to parallel orientation of XRD specimens (Ross et al. 1976). Uzochukwu and Dixon (1986) identified 2 Mn oxide minerals, birnessite and lithiophorite, by XRD in nodules of a Vertie Argiustoll in Texas using a magnetic concentration technique. Two Fe-oxide minerals, goethite and hematite, were also present in the nodular material.

Microscopic studies have shown that concretions of somewhat poorly drained soils are characterized by undifferentiated fabric and diffuse external boundaries. In contrast, concretions of well-drained soils are usually spherical with sharp external boundaries and differentiated fabric (Phillippe et al. 1972). Scanning

428

Vol. 45, No. 3, 1997 429 Fe-Mn concretions in soils with perched water tables

Table 1. Characteristics of the sites sampled.

Depth to Slope Site Pedon Landscape position bedrock (cm) gradient Classification

POFA POFA1 Upper sideslope 125 8% POFA2 Lower sideslope 90 7%

POFB POFB 1 Upper sideslope 122 9% POFB2 Lower sideslope 155 7%

STF STF1 Upper sideslope 152 6% STF2 Lower sideslope 115 6%

SRF SRF Upper sideslope >350 10%

Hagerstown, fine, mixed, mesic Typic Hapludalfs Faywood, fine, mixed, mesic Typic Hapludalfs Hagerstown, fine, mixed, mesic Typic Hapludalfs Hagerstown, fine, mixed, mesic Typic Hapludalfs Caleast, fine, mixed, mesic Mollic Hapludalfs Caleast, fine, mixed, mesic Mollic Hapludalfs Maury, fine, mixed, mesic Typic Paleudalfs

e l e c t r o n m i c r o s c o p i c e x a m i n a t i o n s o f c o n c r e t i o n s s h o w e d t h e m to be d o m i n a t e d by smal l i r regular ly shaped pores. The concre t ion fabr ic com ponen t s , for the mos t part , were m a s k e d by m as s i ve p lasmic dep- osi t ions, ind ica t ing that m u c h o f the or ig ina l ma t r ix poros i ty had p robab ly b e e n infi l led dur ing the concre- t ion fo rma t ion process (Cescas et al. 1970).

The ob jec t ives o f this s tudy were: 1) to de t e rmine changes in phys ica l , chemica l and mine ra log ica l compos i t ion o f concre t ions wi th d i f fe rent size and color found in d i f fe ren t hor i zons of pe rched wa te r table-af- fec ted soils; and 2) to e x a m i n e m i c r o m o r p h o l o g i c a l fea tures and s t ructures re la ted to thei r compos i t i ona l character is t ics .

M A T E R I A L S A N D M E T H O D S

Prof i le s a m p l e s w e r e c o l l e c t e d f r o m 4 s i tes , in- c l u d i n g 7 p e d o n s r e p r e s e n t i n g 4 soi l se r ies in the I n n e r B l u e g r a s s R e g i o n o f K e n t u c k y . C h a r a c t e r i s t i c s o f the s i tes s a m p l e d are g i v e n in Tab le 1 a n d F i g u r e 1. T h e s a m p l e d p e d o n s i n c l u d e d 4 u p p e r s i d e s l o p e p o s i t i o n s and 3 l o w e r s i d e s l o p e pos i t i ons . T h e so i l s w e r e c l a s s i f i ed as H a g e r s t o w n (fine, m i x e d , m e s i c Typ ic H a p l u d a l f s ) , F a y w o o d (fine, m i x e d , m e s i c Typ ic H a p l u d a l f s ) , C a l ea s t (f ine, m i x e d , m e s i c M o l -

l ic H a p l u d a l f s ) a n d M a u r y (fine, m i x e d , m e s i c Typ ic Pa l euda l f s ) . I r o n - M n c o n c r e t i o n s w e r e s e p a r a t e d f r o m a i r -d r i ed soi l s a m p l e s b y w e t s h a k i n g o v e r n i g h t and we t s i e v i n g o n a se t o f s i eves w i t h o p e n i n g s o f 2, 1, 0.5 a n d 0 .25 m m , r e spec t i ve ly . D e i o n i z e d w a t e r s p r a y e d o n t o the c o n c r e t i o n s r e ad i l y r e m o v e d the soi l pa r t i c les . T h e c o n c r e t i o n s w e r e s e p a r a t e d in to c o a r s e ( > 2 m m ) , m e d i u m (2 to 1 m m ) , f ine (1 to 0.5 m m ) and v e r y f ine (0.5 to 0 .25 m m ) s izes . C o a r s e a n d m e d i u m c o n c r e t i o n s w e r e s e p a r a t e d f r o m o t h e r f r a g m e n t s b y v i s u a l i n s p e c t i o n f o l l o w i n g h a n d - p i c k - ing w i t h tweeze r s , w h i l e f ine a n d v e r y f ine c o n c r e - t ions w e r e s e p a r a t e d u s i n g a s t rong m a g n e t . S o m e c o a r s e and m e d i u m c o n c r e t i o n f r a c t i o n s w e r e a l so s e p a r a t e d in to b l a c k and b r o w n c o l o r g r o u p s b y h a n d - p i c k i n g w i t h tweeze r s .

C o n c r e t i o n s u s e d for M n a n d Fe f r a c t i o n a t i o n we re g r o u n d w i t h an aga t e m o r t a r a n d pes t l e un t i l t he e n t i r e s a m p l e p a s s e d t h r o u g h a 0 . 5 - m m s ieve . T h e f r a c t i o n a t i o n o f M n a n d Fe was a c c o m p l i s h e d u s i n g a s e q u e n t i a l e x t r a c t i o n p r o c e d u r e p r o p o s e d b y S h u m a n ( 1 9 8 5 ) a n d m o d i f i e d by M c D a n i e l and B u o l (1991) . T h e s e c o n d a r y M n and Fe w e r e s e p a r a t e d in to e x c h a n g e a b l e ( E X C H ) , o r g a n i c ( O M ) , c ry s t a l - l ine M n o x i d e ( M n C R O ) , Fe c o p r e c i p i t a t e d w i t h M n

Table 2. Mean values of elemental composition and analysis of variance for different size and color Fe-Mn concretions from selected horizons of the studied pedons.

Concretion size Concretion color Analysis of variance

Coarse Medium Fine Elemental ( > 2 ram) (2-1 mm) ( 1 ~ . 5 mm Black Brown Mixed CV

oxide (n = 11) (n = I 1) (n = 11) (n = 6) (n = 6) (n = 21) Size Color (%)

SiO2 41.1 la:~ 46.89 b 51.86 c 46.588 42.33 b 47.88 a **'~ ** 6.8 A1203 14.44 a 15.01 a 14.58 a 17.30 a 14.82 b 13.89 b NS ** 11.8 Fe203 33.76 a 30.29 a 26.43 b 23.58 b 36.05 a 30.3P * ** 18.7 MnO 7.19 a 4.83 a 3.84 ~ 9.74 ~ 2.24 b 3.93 b NS * 101.3 CaO 1.35 a 1.15 ~ 2.22 ~ 1.50 ~ 2.92 ~ 1.21b NS ** 91.4 MgO 0.47 a 0.45 a 0.45 ~ 0.48 ab 0.54 a 0.43 b NS * 19.1 K20 0.958 0.92 ~ 0.88 a 1.07 ~ 1.04 ~ 0.84 b NS ** 14.1 Na20 0.32 ~ 0.27 ~ 0.35 ~ 0.54 ~ 0.38 ~b 0.23 b NS * 69.7 TiO2 0.95 ~ 0.88 ~ 0.88 ~ 0.80 b 0.84 ab 0.95 ~ NS * 14.6 P205 0.61 a 0.55 a 0.60 a 0.55 b 0.77 ~ 0.55 b NS * 28.9 Fe203/MnO 13.25 ~ 14.75 ~ 9.85 ~ 2.62 b 28.07 ~ 11.06 b NS ** 75.1

t *, ** Significant at the 0.05 and 0.01 probability levels; NS not significantly different at the 0.05 probability level. $ Within a comparison group of size or color, means of the same elemental oxide followed by the same letter are not

significantly different at the 0.05 probability level.

430 Zhang and Karathanasis Clays and Clay Minerals

Figure 1. pied.

oxides (MnCOP), amorphous (AM), Mn coprecipitated with Fe oxides (FeCOP) and crystalline Fe oxide (FeCRO) fractions. The elemental composit ion of selected concretionary material was determined by fusing a subsample (0.2 g, passed through a 10-1*m sieve) with li thium metaborate (LiBO2) (1:5 ratio), dissolving the fused sample in 4% HNO 3, and analyzing the solution for Si, A1, Fe, Mn, Ca, Mg, K, Na and Ti by atomic absorption spectrometry (AAS), and for P by colorimetry (Bartenfelder and Karathanasis 1988).

Mineralogical analyses were performed by XRD on whole concretionary materials (powder mounds) and separately on the <5-1*m fraction (glass slide) before and after a citrate-bicarbonate-dithionite treatment. The <5-1*m fractions were separated by centrifugation (Jackson 1956) following suspension in deionized water and dispersion in an ultrasonic water bath for about 2 min. The samples were spun at 300 rpm for 2.9 min using a DAMON-IEC DPR-6000 centrifuge (Interna- tional Equipment Co., Needham Heights, Massachu- setts). The XRD analysis was performed with a PW 1840 Philips diffractometer, equipped with CoKet ra- diation (40 kV, 30 mA) and a Si-diode detector (Kar- athanasis and Hajek 1982). All specimens were step- scanned from 2 to 80 ~ at 0.5 ~ increments with a counting time of 10 s per increment.

Micromorphological examinations were performed on selected soil thin sections prepared according to the procedures published by the Soil Survey Staff (1984), using a Leitz-Werner Ortholux petrographic microscope. Selected concretion specimens were also examined with a Hitachi S-800 field emission scanning electron microscope, equipped with a backscattered electron detector.

Soil topography and stratigraphy of the sites sam-

RESULTS AND DISCUSSION

Chemical Composition of Concretions

Mean values of elemental composition and analysis of variance for different size and color concretions from 5 Ap horizons and 4 Bt horizons of 6 pedons are given in Table 2. Among 10 elemental oxides, SiO2 was the highest (41.11-51.86%). Iron oxide was second (26.43-33.76%), followed by A1203 (14.44-15.01%) and MnO (3.84-7.19%). All other elemental oxides averaged <2.3%. While the A1203 content was independent of concretion size, SiO2 increased with decreasing size. The opposite was true for Fe203 and MnO, which were highest in coarser-size concretions. This is probably an indi- cation that Si and A1 in the concretions are associated with trapped soil material composed primarily of aluminosilicate minerals (more 2:1 minerals in the fine- than in the coarse-size concretions). Also, larger-size concretions appeared to be Fe-enriched

Vol. 45, No. 3, 1997 Fe-Mn concretions in soils with perched water tables

Table 3. Elemental composition of concretions from selected horizons of 2 studied pedons.

431

Pedon Horizon Color Size (mm) SiO2 AlzO 3 Fe20~ MnO CaO MgO K,O Na20 TiO2 P205 Total Fe/Mn

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

POFB1 Ap Black >2 38.69 17.72 25.99 14.28 0.88 0.57 1.45 0.57 0.94 0.31 101.40 1.8 2-1 46.03 18.28 22.96 10.73 0.57 0.43 1.07 0.66 0.84 0.31 101.88 2.1 1-0.5 53.37 16.12 24.58 5.53 0.66 0.37 0.83 0.50 0.69 0.28 102.93 4.4

Brown >2 34.15 14.56 48.97 0.89 0.44 0.47 1.t l 0.46 0.91 0.76 102.17 55.0 2-1 39.84 15.20 40.26 0.69 0.46 0.47 1.16 0.35 0.85 0.53 99.81 58.3 1-0.5 44.69 15.90 36.18 1.65 0.80 0.51 1.15 0.29 0.99 0.43 102.59 21.9

STF2 Btl Black >2 43.00 16.66 26.06 11.09 1.70 0.49 0.96 0.31 0.80 0.77 101.84 2.4 2-1 47.00 17.42 22.90 9.34 2.46 0.54 1.07 0.37 0.78 0.77 102.65 2.5 1-0.5 50.96 17.60 19.00 7.47 2.73 0.45 1.02 0.84 0.76 0.83 101.66 2.5

Brown >2 40.28 12.80 35.37 2.90 5.75 0.57 0.84 0.42 0.85 0.96 100.74 12.2 2-1 45.33 14.22 35.46 2.06 3.07 0.52 0.91 0.48 0.61 0.88 103.54 17.2 1-0.5 49.71 16.22 20.03 5.22 7.02 0.70 1.05 0.28 0.80 1.03 102.06 3.8

(h igh FezO3 /MnO ra t io ) c o m p a r e d to the f ine - s ize c o n c r e t i o n s , w h i c h w e r e M n - e n r i c h e d .

E lemen ta l compos i t i on d i f fe rences were also evident in d i f fe rent co lor concre t ions separa ted f rom A p and B t l ho r i zons of the P O F B 1 and STF2 sites, especia l ly wi th respec t to Fe203 and M n O con ten t (Table 3). A l t h o u g h all concre t ions con t a ined more Fe20 3 than M n O , b r o w n concre t ions were m u c h h ighe r in Fe203, whi le b l ack concre t ions had increased amoun t s o f M n O . These d i f fe rences were more p r o m i n e n t in coarse-s ize concre t ions . The F e / M n ra t ios in b l a c k concre t ions were re la t ively i ndependen t of size, bu t increased wi th inc reas ing concre t ion size in b r o w n concre t ions (Table 3).

The ef fec t o f conc re t ion size on Fe203, M n O and SiO2 compos i t i on o f b l ack and b r o w n concre t ions selected f rom 2 sites is ev iden t in F igure 2. For bo th color concre t ions , SiO2 increased wi th dec reas ing size, wh i l e Fe203 r e m a i n e d u n c h a n g e d or decreased w i th dec reas ing size, especia l ly in b r o w n concre t ions . In

contrast , M n O decreases wi th dec reas ing size were ev- iden t on ly in the b l ack concre t ions . These t rends indicate tha t the fo rmat ion , d e v e l o p m e n t an d accumu- la t ion of F e - M n concre t ions is a d y n a m i c and pro- g ress ive t rans loca t ion and t r ans fo rma t ion process o f Fe and M n in the soil profile.

Resul ts o f the sequent ia l f rac t iona t ion o f M n and Fe in to e x c h a n g e a b l e (EXCH) , organic (OM), c rys ta l l ine oxide (CRO) , a m o r p h o u s (AM) , coprec ip i t a ted (COP) and total s econda ry (Total-S) fo rms a m o n g d i f fe rent concre t ion s izes are repor ted in Tables 4 and 5. G e n - erally, the c rys ta l l ine M n - o x i d e s appea red to d o m i n a t e the M n f rac t ion in all size o f concre t ions . However , cons ide rab le var iabi l i ty in the i r concen t r a t ion ex is ted a m o n g d i f fe rent pedons and hor izons . W i t h the excep- t ion of the SRF site, concre t ions in B t hor i zons con- ta ined h ighe r amo u n t s of c rys ta l l ine M n than A p ho- r izons, wi th the h ighes t amo u n t s f rom the P O F B 2 site. T h e r e was no cons i s ten t t rend for c rys ta l l ine M n dis- t r ibu t ion a m o n g di f ferent concre t ion sizes, a l t hough

Figure 2. Comparison of MnO, Fe203 and SiO 2 contents in different size and color concretions. A) Concretions from pedon POFB1, Ap horizon; and B) concretions from pedon STF2, Btl horizon.


Table 4. Distribution of various forms of secondary Mn among different-size concretions in selected soil horizonst.

> 2 mm Concretions 2-1 mm Concretions 1-0.5 mm Concretions Hori-

Pedon zon EXCH O M CRO A M FeCOP E X C H O M CRO A M FeCOP E X C H O M CRO A M FeCOP

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mn (mg/kg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

POFA1 Ap 24 17 11,180 1620 990 30 11 4820 2150 970 25 22 3680 1785 965 Btl 33 27 18,900 1795 1255 24 15 6620 2005 1175 58 11 4920 3150 1125

POFA2 Ap 6 13 12,640 2270 1090 35 13 5140 I745 1010 58 17 4240 2155 1040 Btl 17 19 13,000 2005 1315 38 11 6200 2410 1260 23 9 5360 2160 1135

POFB1 Ap 6 19 7680 1230 1130 13 12 6860 1590 810 10 15 8160 2045 1335 Bt2 16 67 19,400 3690 1850 61 60 18,260 1650 1730 44 23 15,820 1990 1925

POFB2 Ap 31 11 7480 940 1025 44 15 12,060 2650 1325 65 42 9240 2450 1065 Bt2 152 71 24,080 2105 1840 194 79 17,660 1660 1585 188 45 19,980 8300 2335

STF1 Ap 58 73 13,440 4780 2485 70 31 12,700 6800 2630 93 67 8640 3295 1880 Bt2 12 20 14,920 2555 2195 18 15 14,860 6200 2740 19 16 13,240 3545 2420

STF2 Ap 52 39 13,480 3290 2560 156 111 14,300 5040 2275 282 348 10,720 2735 1875 Btl 26 20 18,120 2785 1900 23 18 19,200 5530 2465 76 11 15,840 9450 2165

SRF Ap 26 8 3200 3100 780 29 22 6780 3100 680 56 79 3360 3585 1110 Bt3 44 5 2000 275 235 34 3 1780 250 360 65 5 3300 825 370

t Key: EXCH = exchangable Mn; OM = Mn associated with organic matter; CRO = crystalline Mn oxides; AM = amorphous Mn; and FeCOP = Mn coprecipitated with Fe oxides.

m o s t o f the h i g h va lues were assoc ia ted wi th coars -

er-s ize concre t ions .

T h e c rys ta l l ine Fe -ox ide s we re also the d o m i n a n t Fe

f o r m in all s izes o f conc re t ions (Table 5). E x c h a n g e -

able, o rganic and M n - o x i d e coprec ip i ta ted Fe f o r m s

cons t i tu ted on ly a ve ry sma l l p ropor t ion o f the total

s e c o n d a r y Fe. No cons i s t en t t rends occur red wi th dif-

fe rent sizes. A m o r p h o u s Fe genera l ly inc reased wi th

dec r ea s in g conc re t i on size, bu t there was s igni f icant

site variabil i ty. Co n c re t i ons in S T F 1 and STF2 si tes

appea red to h av e h ighe r a m o r p h o u s Fe and lower c rys -

ta l l ine Fe than o ther si tes. Th i s is p robab ly due to low-

er c lay con ten t and greater wa te r t h roughf low in their

Bt hor izons .

M e a n and s t andard dev ia t ion va lues for s e c o n d a r y

M n and Fe d is t r ibut ion and ana lys i s o f va r iance a m o n g

d i f fe ren t - s ize conc re t i ons are s h o w n in Table 6. In

spi te o f the large variabil i ty, total s e c o n d a r y M n usu -

al ly inc reased wi th concre t ion size, a nd a m o r p h o u s

M n tended to a c c u m u l a t e in smal le r - s ize concre t ions .

Conve r se ly , the c rys ta l l ine M n ox ides were m o r e

p r o m i n e n t in larger-s ize conc re t ions (Table 6 and Fig-

ure 3a). S imi la r t rends were f o u n d for the d i s t r ibu t ion

o f s e c o n d a r y Fe a m o n g d i f fe ren t - s ize conc re t ions (Ta-

ble 6 and F igure 3b).

T h e s e r e l a t ionsh ips s u g g e s t that M n and Fe prec ip-

i tat ion in conc re t i ona ry mate r ia l occu r s u sua l l y at a

ve ry s low rate f r o m sa tura ted but not supe r sa tu ra t ed

soil so lu t ions . U n d e r these condi t ions , c rys ta l l ine ox-

ides d o m i n a t e over a m o r p h o u s f o r m s b e c a u s e the lat-

ter precipi ta te at a fas ter rate f r om supe r sa tu ra t ed so-

lut ions. T h e fact that m o s t c rys ta l l ine ox ides are con-

cent ra ted in larger-s ize concre t ions is p robab ly due to

the h i g h e r f r e q u e n c y o f o x i d a t i o n - r e d u c t i o n cyc l e s

Table 5. Distribution of various forms of secondary Fe among different-size concretions in selected soil horizonst.

> 2 mm Concretions 2-1 mm Concretions 1-0,5 mm Concretions Hori-

Pedon zon E X C H OM MnCOP A M CRO E X C H O M MnCOP A M CRO E X C H OM MnCOP A M CRO

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fe (mg/kg) . . . . . . . . . . . . . . . . . . . . . POFA1 Ap 16.3 169 782 17,250 175,500 7.9 150 980 21,650 147,000 5.7 113 520 20,150 190,500

Bt 16.2 152 476 12,900 148,500 2.8 201 1002 17,300 157,500 7.5 119 1058 25,550 132,000 POFA2 Ap 6.1 127 682 19,150 162,000 4.4 84 1056 16,000 143,500 5.5 114 750 20,750 113,500

Btl 5.0 172 558 18,650 176,500 4.0 142 934 19,600 146,500 2.0 102 746 19,600 93,500 POFB1 Ap 13.5 368 802 14,800 162,000 6.7 126 944 19,850 147,500 5.5 125 792 16,200 130,500

Bt2 2.2 98 620 12,350 116,000 2.0 73 682 14,250 126,500 2.2 104 790 15,750 130,000 POFB2 Ap 2.9 149 786 14,150 191,500 10.5 119 800 18,050 147,500 15.0 119 770 18,200 101,000

Bt2 1.0 157 286 12,700 131,000 3.4 141 594 15,400 150,500 1.4 61 546 47,550 89,500 STF1 Ap 7.1 102 738 32,500 142,500 2.1 99 760 48,550 98,000 1.7 65 738 25,400 65,000

Bt2 2.9 98 640 24,650 123,500 5.5 57 684 37,600 74,000 1.8 61 612 21,650 50,000 STF2 Ap 3.0 126 78 21,600 106,500 4.8 49 702 34,800 86,000 2.8 36 792 18,000 60,500

Btl 3.7 82 440 24,550 142,000 4.1 70 608 41,000 68,000 3.8 154 590 50,400 75,550 SRF Ap 9.2 161 480 27,150 139,000 i0.9 82 920 29,650 99,500 1 t.3 55 480 27,900 I01,000

Bt3 3.5 225 464 34,450 159,500 3.1 136 488 24,100 136,000 4.2 141 764 32,800 139,000

t Key: EXCH = exchangable Fe; OM = Fe associated with organic matter; MnCOP = Fe coprecipitated with Mn oxides; AM = amorphous Fe oxides; and CRO = crystalline Fe oxides.

Vol. 45, No. 3, 1997 Fe-Mn concretions in soils with perched water tables 433

Table 6. Mean, standard deviation and analysis of variance (n --- 14) for secondary Mn and Fe forms distributed among different-size concretions as determined by the sequential extraction procedure.

Concentrations (mg kg -l) Analysis of variance

Fe-Mn forms Coarse (>2 mm) Medium (2-1 mm) Fine (1-0.5 mm) Size CV (%)

EXCH-Mn'~ 35.9 +- 3 7 . 0 4 54.9 • 53.7 a 75.9 • 73.7 a NSw 105.7 O M - M n 29.1 • 23.8 a 29.7 • 31.4 a 50.7 --- 88.5 a NS 148.4 C R O - M n 14,251 • 8345 a 10,517 • 5 7 2 P 9036 • 5412 ~ NS 51.0 A M - M n 2317 • 1175 a 3058 --- 2003 a 3391 --- 2453 ~ NS 67.0 FeCOP-Mn 1475 --- 678 ~ 1501 ___ 763 a 1482 • 612 a NS 47.4

Total-S Mn 18,019 • 9591 a 15,159 • 7714 b 14,034 -+ 7901 b * 48.8

EXCH-Fe:~ 6.6 --- 5.2 a 5.2 ___ 3.0 ~ 5.1 --- 4.0 ~ NS 69.5 OM-Fe 156.1 • 71.8 ~ 109.2 • 42.9 b 97.8 • 35.8 b * 44,1 MnCOP-Fe 559 • 208 b 797 • 177 a 711 • 150 ~ ** 25,4 AM-Fe 21,918 -+ 7530 ~ 25,557 _ 10,908 a 25,707 • 10,942 a NS 40.9 CRO-Fe 148,286 • 24,442 ~ 123,428 • 31,435 b 105,107 • 37,645 b ** 26.0

Total-S Fe 171,719 -+ 23,473 ~ 149,897 __+ 22,877 b 131,628 --- 36,856 b ** 19.8

? Key: E X C H - M n = exchangable Mn; O M - M n = M n associated with organic matter; C R O - M n = crystal l in M n oxides; A M - M n = amorphous Mn; FeCOP-Mn = M n coprecipitated with Fe oxides; and Total-S M n = total secondary Mn.

:~ Key: EXCH-Fe = exchangable Fe; OM-Fe = Fe associated iwth organic matter; MnCOP-Fe = Fe coprecipitated with Mn; AM-Fe = amorphous Fe; CRO-Fe = crystall ine Fe oxides; and Total-S Fe = total secondary Fe.

w *, ** Significant at the 0.05 and 0.01 probability levels; NS not significant at the 0.05 probability level. Within a compar ison group of size, means • SD of the same forms of secondary Mn or Fe fol lowed by the same letter

are not significantly different at the 0.05 probability level.

a n d l e n g t h o f o x i d a t i o n p e r i o d s o c c u r r i n g a b o v e t h e

r e s t r i c t i v e h o r i z o n s , w h e r e t he l a r g e r - s i z e c o n c r e t i o n s

a r e c o m m o n . T h e s e c o n d i t i o n s i n d u c e m u l t i p l e l a y e r s

o f F e a n d M n p r e c i p i t a t i o n o n e x i s t i n g s m a l l e r - s i z e

n o d u l e s , r e s u l t i n g in a s i z e i n c r e a s e , w h i c h is a c c e n -

t u a t e d b y b e t t e r p o r o s i t y a n d a e r a t i o n in t h e s e h o r i -

z o n s .

M i n e r a l o g y o f C o n c r e t i o n s

X - r a y d i f f r a c t i o n a n a l y s i s o f w h o l e c o n c r e t i o n a r y

m a t e r i a l s d i d n o t s h o w d i s t i n c t p e a k s o f F e a n d M n

o x i d e m i n e r a l s d u e to i n t e r f e r e n c e f r o m v e r y h i g h

q u a r t z p e a k i n t e n s i t i e s . T h e X R D p a t t e r n s o f t h e < 5 -

ixm f r a c t i o n s p r o v i d e d b e t t e r r e s o l u t i o n , b u t o n l y g o e -

th i t e w a s d e t e c t a b l e . R e p r e s e n t a t i v e X R D p a t t e r n s o f

c o n c r e t i o n s w i t h d i f f e r e n t s i z e a n d c o l o r s e p a r a t e d

f r o m t h e A p h o r i z o n o f p e d o n P O F B 1 a re s h o w n in

F i g u r e 4. G o e t h i t e w a s i d e n t i f i e d f r o m r e f l e c t i o n s at

0 . 4 2 0 , 0 . 2 6 9 , 0 . 2 7 1 , 0 . 171 a n d 0 . 1 4 5 n m , r e g a r d l e s s

o f c o n c r e t i o n s i z e a n d color . I ts p r e s e n c e is s u p p o r t e d

b y t h e c o n c r e t i o n f r a c t i o n a t i o n da ta , w h i c h s h o w e d t h e

c r y s t a l l i n e F e o x i d e s to b e t h e d o m i n a n t F e c o m p o n e n t

( C B D e x t r a c t i o n ) . T h e a m o r p h o u s F e a m o u n t e d to

a b o u t ~/5 to l/t0 o f t h e c r y s t a l l i n e F e o x i d e s . N o c r y s -

t a l l i ne M n - o x i d e m i n e r a l s w e r e i den t i f i ed , e v e n in c o n -

c r e t i o n s w i t h t he h i g h e s t M n c o n c e n t r a t i o n ( > 2 m m

Figure 3. Compar i son of various forms of secondary M n A) and secondary Fe B) in different-size concretions.


CL421'

zb

~ 0 .2 I 0"~'1 O. 0 . ~ O.

�9 b .b Degree 2 4

I B

o ~ ~'

sso'c

y~,.s

4'o ~o

L

sso~

4~ sb - s Degr~ :~e

=b do ~ ' I ~ l m e 2 e

Figure 4. XRD patterns of different size and color concretions (<5-1xm fractions) untreated and treated with CBD from pedon POFB1, Ap horizon. A) Coarse (>2 mm) black concretions; B) medium (2-1 mm) black concretions; C) coarse (>2 mm) brown concretions; and D) medium (2-1 mm) brown concretions.

black concretions with 14.28% MnO from pedon POFB 1, Ap horizon, Table 3).

Other minerals present in the <5-jxm concretionary fraction were mica (1.015, 0.502 nm), kaolinite (0.724, 0.359 nm) and quartz (0.334, 0.182 nm). The presence of goethite (0.420, 0.269, 0.271, 0.220, 0.171, 0.151, 0.145 nm) was confirmed by the re- duction of the broad peaks in the 0.420 region following CBD treatment, which allowed a better res- olution of the 0.425 reflection of quartz (Figure 4). Upon heating to 400 and 550 ~ the 0.420, 0.418 and 0.151 nm peaks decreased, while the 0.269,

0.251 and 0.145 nm peaks were enhanced (Figure 4), indicating transformation of goethite to hematite (Tokashiki et al. 1986).

Another noticeable difference in the XRD patterns is the increased sharpness/intensity of the goethite peaks from the brown concretions that had a higher total Fe oxide content than the black concretions (Fig- ure 4 and Table 3). Goethite peak intensities also increased with crystalline Fe oxide content and increasing concretion size (Figure 4 and Table 6). Similar trends between goethite and Fe oxide content were observed for all pedons.


Figure 5. Representative micrographs of Fe Mn concretions from soil thin sections in different horizons. A) Brown concretion from pedon POFBI, BA horizon (perching water zone); B) black concretions from the same horizon as graph A; C) black concretion from pedon STFI, Bt2 horizon (restrictive layer); and D) black t'erromangan soft masses (stains), from pedon STFI, CB horizon (perched water table zone above the lithic limestone interface).

M i c r o m o r p h o l o g y of Concre t ions

Mic rog raphs of se lected F e - M n concre t ions in soil th in sect ions f rom the B A hor izon of p e d o n POFB 1, and Bt2 and CB hor izons of pedon STF1, are s h o w n in F igure 5. Concre t ions f rom pe rched wate r zones s h o w e d a more dense d i f ferent ia ted fabric, sharp external bounda r i e s and genera l ly a spher ica l shape (Fig- ures 5a and 5b). Cons t i tuen t s were usual ly a r ranged in di f fuse zones or b a n d s a round a cent ra l point , of ten a quar tz gra in or a void. Concen t r i c b a n d i n g in concre- t ions has b e e n a t t r ibuted to f requent ly a l te rna t ing ox- i d a t i o n - r e d u c t i o n periods. For the soils s tudied, these cond i t ions are p reva len t wi th in wate r pe rch ing horizons above res t r ic t ive c layey layers, w h i c h have lower clay con ten t s and ex tens ive macroporos i ty that favors concre t ion format ion .

In contrast , concre t ions wi th in c layey res t r ic t ive ho- r izons (Figure 5c) s h o w e d a less -d i f fe rent ia ted in terna l fabric, s o m e w h a t i r regular in shape, and dis t inct to dif-

fuse externa l boundar ies . This is p robab ly due to the i r dense soil mat r ix and l imi ted pore space, wh ich h inde r the d e v e l o p m e n t of dis t inct bands and sharp ex te rna l concre t ion boundar ies ,

T h e Fe a n d M n c o n c r e t i o n s f r o m p e r c h e d w a t e r t a b l e - a f f e c t e d z o n e s o v e r l y i n g l i m e s t o n e b e d r o c k in- t e r f a c e s w e r e l a c k i n g the h a r d n e s s an d o r g a n i z a t i o n f o u n d in u p p e r s o l u m h o r i z o n s . B e c a u s e o f the lon- ge r d u r a t i o n an d g r ea t e r f r e q u e n c y o f s a tu ra t i on , c o n s i d e r a b l y h i g h e r a m o u n t s o f Fe an d M n are d is - s o l v e d in t he se z o n e s an d p r o d u c e the o b s e r v e d g ray c h r o m a m a t r i x w i t h c o m m o n d i s t i nc t or p r o m i n e n t b r i g h t Fe ( f e r r ans ) an d b l a c k M n ( m a n g a n s ) sof t mas s e s . H o w e v e r , t he se soi l z o n e s are s a t u r a t e d or are m o i s t u r e - e n r i c h e d for too l o n g to a l l o w ex ten -

s ive h a r d e n i n g o f M n an d Fe ( c o n c r e t i o n f o r m a t i o n ) to occur. F i g u r e 5d i l l u s t r a t e s an F e - M n sof t m a s s w i t h a g g l o m e r o p l a s m i c f ab r i c in the C B h o r i z o n o f the STF1 p e d o n .


Figure 6. SEMs of Fe-Mn concretions from the Ap horizon, pedon POFB 1. A) A cleaved coarse (>2 ram) brown concretion showing round dense clay cores; B) quarlz and other mineral crystal grains inside a coarse brown concretion; C) a cleaved coarse black concretion showing concentric bands; and D) a magnified cross section of the concentric bands shown in Figure 5c.


Figure 7. SEMs showing the macropore and micropore structure of Fe-Mn concretions from the Ap horizon of pedon POFB 1. A) A cavity inside a concretion; B) a magnified section of the cavity shown in Figure 6a showing the cheese-like matrix and microporosity of the concretion; C) tubularly shaped Mn-enriched features coating the surface of a concretion cavity; D) clusters of accicularly shaped goethite crystals inside a concretion cavity.


Micrographs of selected concretion specimens examined by SEM are illustrated in Figures 6 and 7.Many coarse (>2 mm) concretions, especially those brown in color, contained randomly distributed quartz grains and round dense clay cores cemented by Fe and Mn oxides (Figures 6a and 6b). Both features were embedded in the concretion matrix but exhibited distinct boundaries. Although each feature appeared to occupy cavities within the matrix, they probably formed the original nucleus around which additional concret ionary material was attached. Figure 6c shows a micrograph of the interior of a c leaved coarse black concretion, spherical in shape, from which the denser core was separated during the specimen preparation process. The concentric bands or layers representing cyclic Fe-Mn precipitation periods are evident, especially in the cross section of Figure 6d.

Most concretions, in spite of their dense mor- phology, also appeared to have a sizeable macropore-rnicropore network. The macropore features observed, however, appeared to be nonconducting dead-end cavities. Considerable detail of such a cavity is shown in Figure 7a. The micropore structures (cheese-like) of the concret ionary matrix is illustrated in a magnified sector of the cavity in Figure 7b. Other infilling features of cavities found in the interior of some concretions are shown in Figures 7c and 7d.

Figure 7c illustrates Mn-enriched tubes coating the surface of a macropore cavity. This pattern may indicate microbial involvement in the Mn-precipitation process. Other cavities, such as the one illustrated in Figure 7d, were half-filled with aggregate overgrowths of acicular goethite crystals. Similar infillings of Fe and Mn are bound to occur at a smaller-size scale within the micropore system and eventually produce the dense concret ionary matrix structure. These infillings are probably the result of diffusion of dissolved plasmic Fe and Mn and sub- sequent precipitation within the pore volume of the concret ionary matrix.

S U M M A R Y AND C O N C L U S I O N S

This study conducted in the Inner Bluegrass Re- gion of Kentucky examined changes in the macro- and micromorphology, chemistry and mineralogy of Fe-Mn concretions as a function of size, color and soil depth. The chemical composi t ion of the concretions varied with concretion size and color. All of the black concretions contained higher amounts of Mn, while brown concretions were higher in Fe. To- tal Mn and Fe increased, while SiO2 decreased with concre t ion size. Similar ly , c rys ta l l ine Mn- and Fe-oxides fractionated with a sequential extraction procedure increased, but amorphous Mn and Fe decreased with increasing concretion size. With re-

gards to concretion mineralogy, goethite was the only crystall ine Fe oxide mineral identified by XRD analysis. Manganese oxide minerals could not be de- tected due to the diffuse nature of their XRD peaks and poor crystallinity. Examinat ion of soil thin sections showed that accumulation patterns and characteristics of Fe-Mn concretions in Kentucky Alfi- sols are strongly influenced by perched water tables. Concretions of soil horizons overlying restrictive clayey layers exhibited differentiated fabrics, sharp external boundaries and generally spherical shapes. Concretions found within clayey restrictive layers or above lithic interfaces usually had less structural organization, softer matrices and diffuse external boundaries due to longer-term saturation conditions. Scanning electron microscopy examinations suggested that the concret ionary matrix, in spite of its density, has numerous cavities and an extensive micropore system within which dissolved plasmic Fe and Mn can diffuse and precipitate.

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(Received 22 January 1996; accepted 29 July 1996; Ms. 2731)

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