(R 2 = 0.89) was obtained between mass specific magnetic susceptibility (X) of ... 1982; Mailer, 1986; Beckwith et al., 1990; Singer et .... interface and software.
Clays and Clay Minerals, Vo9147, No. 4, 466-473. 1999.
Q U A N T I F I C A T I O N A N D C H A R A C T E R I Z A T I O N OF M A G H E M I T E IN SOILS D E R I V E D F R O M V O L C A N I C ROCKS IN S O U T H E R N B R A Z I L ANTONIO CARLOS S. DA COSTA, I JERRY M. BIGHAM,2 FRED E. RHOTON, 3 AND SAMUEL J. TRAINA2 1Departamento de Agronomia, Universidade Estadual de Maring~, Maringfi-PR, CEP 87020-900, Brazil 2 School of Natural Resources, The Ohio State University, Columbus, Ohio 43210, USA 3 National Sedimentation Laboratory, USDA-ARS, Oxford, Mississippi 38655, USA Abstract--Many soils developed from volcanic rocks in southern Brazil exhibit spontaneous magnetization caused by the presence of fine-grained maghemite (3J-Fe203), but few attempts were made to quantify or characterize this important soil component. To that end, clays were separated from freely drained soils derived from acid (>~63% Sit2), intermediate (54-62% SiO~), and basic (-- 5 3 % SiO 2. F o r s o m e analyses, additional s a m p l e s were o b t a i n e d f r o m the A a n d B h o r i z o n s of freely drained, h i g h l y w e a t h e r e d soils (Oxisols) at o t h e r locations d e s c r i b e d b y C o s t a (1996). In all cases, the soils e x a m i n e d in this s t u d y w e r e d e v e l o p e d f r o m extrusive, i g n e o u s r o c k s p r o d u c e d b y the Parangt flood v o l c a n i s m (Bellieni et al., 1986).
Whole-soil properties P a r t i c l e - s i z e d i s t r i b u t i o n o f the soil s a m p l e s w a s d e t e r m i n e d a c c o r d i n g to t h e p i p e t t e m e t h o d ( C a m a r go et al., 1986). Total C c o n t e n t w a s m e a s u r e d b y a d r y c o m b u s t i o n p r o c e d u r e ( S o i l S u r v e y Staff, 1972), a n d soil p H w a s m e a s u r e d f r o m 1:2.5 s o i l - w a t e r m i x tures. C a t i o n e x c h a n g e c a p a c i t y ( C E C ) a n d exc h a n g e a b l e A1 w e r e d e t e r m i n e d a c c o r d i n g to the p r o c e d u r e s o f E M B R A P A (1979).
Sample fractionation and clay mineralogy T h e < 2 - m m soil m a t e r i a l s w e r e d i s p e r s e d w i t h 0.1 M N a O H a n d f r a c t i o n a t e d into s a n d (2~0.05 m m ) , silt ( 0 . 0 5 - 0 . 0 0 2 m m ) , a n d clay ( < 2 ~ m ) separates b y a c o m b i n a t i o n o f wet sieving a n d g r a v i t y s e d i m e n t a t i o n in water. T h e sand a n d silt f r a c t i o n s w e r e d r i e d at (105~ w h e r e a s the clay f r a c t i o n s w e r e flocculated w i t h 1 M NaCI, w a s h e d free o f e x c e s s salt, frozen, a n d dried in a lyophilizer. Total F e (Fet) c o n t e n t s w e r e measured by atomic absorption spectrophotometry ( A A S ) f o l l o w i n g d i s s o l u t i o n o f the clays u s i n g the m e t h o d o f B e r n a s (1968). O r i e n t e d a g g r e g a t e s o f M g a n d K - s a t u r a t e d clays (pre-treated w i t h citrate-bicarb o n a t e - d i t h i o n i t e to r e m o v e F e - o x i d e s ) w e r e p r e p a r e d b y u s i n g the filter t r a n s f e r m e t h o d ( M o o r e a n d R e y n olds, 1989). X R D p a t t e r n s f r o m the d r i e d a g g r e g a t e s w e r e r e c o r d e d f r o m 2 to 2 0 ~ u s i n g CuKo~ r a d i a t i o n f r o m a Philips P W 1316/90 w i d e - r a n g e g o n i o m e t e r fitted w i t h a t h e t a - c o m p e n s a t i n g slit, a 0.2 m m r e c e i v i n g slit, a n d a d i f f r a c t e d b e a m m o n o c h r o m a t o r . D a t a w e r e c o l l e c t e d in a step s c a n m o d e (0.02 ~ for 2 s) and t r a n s f e r r e d to a c o m p u t e r u s i n g a D a t a b o x ( M D I Inc.) i n t e r f a c e a n d software. K a o l i n i t e , halloysite, gibbsite, hydroxy-interlayered vermiculite, and smectite were identified. Q u a n t i t a t i v e a n a l y s e s for k a o l i n i t e a n d g i b b s i t e w e r e o b t a i n e d b y t h e r m o g r a v i m e t r i c analysis ( K a r a t h a n a s i s a n d Hajek, 1982).
468
Costa, Bigham, Rhoton, and Traina
Concentration o f iron oxides Iron oxides in the soil clays were concentrated for chemical and mineralogic analysis by selectively dissolving kaolinite, halloysite, and gibbsite using the boiling 5 M NaOH procedure of Norrish and Taylor (1961) as modified by K ~ n p f and Schwertmann (1982).
Selective dissolution o f iron oxides Iron oxides in the untreated clay fractions and the 5 M NaOH residues were selectively dissolved with Na-citrate-bicarbonate-dithionite (CBD) (Mehra and Jackson, 1960). Poorly crystallized Fe oxides were removed from separate samples of the untreated clays with 0.2 M acid ammonium oxalate (AAO) (McKeague and Day, 1966). The Fe and A1 contents of both the CBD (Fed, Aid) and AAO (Feo, Alo) extracts were then determined by AAS. Selective removal of maghemite from the 5 M NaOH residues was accomplished by using a modification of the procedure described by Schwertmann and Fechter (1984). Duplicate, 100-mg samples of the 5 M NaOH residues were weighed into 100-mL polypropylene tubes. The residues were washed once with Na-citrate-bicarbonate (75~ 15 min) and once with distilled water to remove any readily soluble Fe and A1. The samples were then mixed with 20 mL of 1.8 M H2SO 4 and held at 75 -+ 5~ for periods to 7.5 h. The samples were centrifuged, and the extracts were saved in pre-weighed, 25 mL polyethylene bottles for chemical analysis of dissolved Fe and A1. The solid residues were washed twice with distilled water, dried (110~ for 24 h), and saved for XRD and X analysis. This procedure effectively removed most of the maghemite and further concentrated hematite and/or goethite.
Clays and Clay Minerals
Quantitative estimates of hematite and goethite in the 5 M NaOH residues were obtained from the areas of the d(012) peak (• of hematite and the d ( l l 0 ) peak of goethite (Schwertmann and Latham, 1986). The area of the d(220) diffraction peak (• of maghemite was used also because there was overlap by any other peak. A factor of 3.5 was used for the selected hematite and maghemite peaks because the relative intensities of these diffraction peaks are - 3 0 % of the maximum.
Magnetic susceptibility Mass specific magnetic susceptibility was determined for samples of the whole soil, sand, silt, clay, and residue (5 M NaOH and 5 M NaOH + 1.8 M H2SO4) using a Bartington MS2 Magnetic Susceptibility System coupled with a MS2B sensor. This dualfrequency meter exposes the sample to a weak alternating magnetic field of --80 A m -1. The MS2B sensor has both low (0.47 kHz) and high-frequency settings (4.7 kHz) for the identification of fine-grained paramagnetic or superparamagnetic materials. Powdered sample, 10 cm 3 size, were weighed, into 20 cm 3 glass scintillation vials. The volumetric magnetic susceptibility (K) was measured at both low and high-frequency settings, and the low-frequency mass susceptibility (X~f) was calculated as follows (Dearing, 1994): X~f = (10 Kif/rn), where m is the mass (g). The presence of superparamagnetic minerals with very small particle size (
