154
Iron speciation in volcanic soils from Fogo island (Cape Verde)
a,c0DUTXHV5, aWaerenborgh
J C, a,c3UXGrQFLR0,b,cRocha F, b,cFerreira da Silva E
0RVW YROFDQLF VRLOV DUH NQRZQ DV IHUWLOH VRLOV DV
WKH\ FRQWDLQ DQ DEXQGDQFH RI LURQ ZKLFK DOO SODQWV
need. Among the different mineral phases, iron oxLGHV )Hox DUH VRLO FRQVWLWXHQWV RI JUHDW LQWHUHVW LQ
soil chemistry and relevance to plant nutrition. They
KDYH VLJQLÀFDQW HIIHFWV RQ PDQ\ SURFHVVHV VXFK DV
VRUSWLRQDQGUHGR[GXHWRWKHLUKLJKVSHFLÀFVXUIDFH
areas and reactivity. Feox DUH NQRZQ WR LQFRUSRUDWH
elements such as Cr and As, that can be a threat to
health. The Fe speciation in volcanic soils of Fogo isODQG&DSH9HUGHZKHUHWKLVHOHPHQWPD\RFFXULQ
VLJQLÀFDQW DPRXQWV LV WKH PDLQ JRDO RI WKLV ZRUN
contributing to environmental and health issues.
Fe contents, its compounds and forms in topsoils of
)RJRLVODQGZHUHVWXGLHGE\QHXWURQDFWLYDWLRQDQDO\VLV 0|VVEDXHU VSHFWURVFRS\ DQG ;UD\ GLIIUDFWLRQ
;5'7KHVRLOVZHUHFROOHFWHGIURPDOOXQLWVRIWKH
LVODQGLQGLYHUVHJHRORJLFDOIRUPDWLRQV²FDUERQDWLWH
nephelinite, limburgite, pyroclasts and historic lavas.
)LQHPDWHULDOVIURP0RWD*RPHVYROFDQLFFUDWHUWKH
PRVW UHFHQWHUXSWLRQZKLFKRFFXUUHG LQ ZHUH
DOVRVDPSOHG7KHWRWDO)HFRQWHQWYDULHVVLJQLÀFDQWO\
IURP)H2O3ZHLJKWLQDFLGVXOIDWHWRSVRLOVS+
XSWREHWZHHQDQGLQWKHRWKHUJHRORJLFDOFRQWH[WVS+ WR
In the most recent volcanic materials Fe is clearly
PRUHUHGXFHGZLWKDIUDFWLRQ)H2+WRWDO)HZLWKLQ
a
WKHUDQJH7KLV)H2+ mainly occurs in pyUR[HQHVSK\OORVLOLFDWHVDQGSXUHRUVOLJKWO\R[LGL]HG
PDJQHWLWH:LWKLQFUHDVLQJPDWHULDOVDJHWKH)H2+
WRWDO)HUDWLRJUDGXDOO\GHFUHDVHVGRZQWRLQdicating that the Fe that is still incorporated in the
VLOLFDWHV VWUXFWXUHV LV LQFUHDVLQJO\ R[LGL]HG 0DJKemite is detected instead of magnetite and hematite
appears becoming the main Fe-containing phase in
the oldest materials. These results suggest that in a
ÀUVWZHDWKHULQJVWHS)H2+LVR[LGL]HGLQWKHVLOLFDWHV
VWUXFWXUH DQG LQ PDJQHWLWH ZKLFK JUDGXDOO\ HYROYHV
to maghemite. With the eventual decomposition of
pyroxenes Fe3+LVPRELOL]HGLQKHPDWLWH$VZHDWKHUing proceeds maghemite is also replaced by hemaWLWH+\GUR[LGHVZHUHQHLWKHUGHWHFWHGE\;5'QRUE\
0|VVEDXHUVSHFWURVFRS\ZKLFKPD\EHH[SODLQHGE\
WKHKLJKDULGLW\RIWKHLVODQG,QWKHVXSHUÀFLDOPDWHULDOVLQVLGHWKH0RWD*RPHVFUDWHUZKHUHIXPDUROHV
are still active, hydroxisulphates such as jarosite are
found. Topsoils developed on nephelinites and carERQDWLWHV DUH LQ JHQHUDO PRUH R[LGL]HG WKDQ WKRVH
developed from limburgites. The former soils have
higher concentrations of rare earth elements and
VKRZ KLJKHU DPRXQWV RI KHPDWLWH 7KLV FRUUHODWLRQ
may be related to the high adsorption capacity of binary Feox6LJQLÀFDQWO\KLJK&UFRQWHQWV!˜JJ
are also found in hematite rich soils.
nstituto Tecnológico e Nuclear, EN 10, Sacavém, Portugal ([email protected])
Departamento de Geociências, Universidade de Aveiro, Aveiro, Portugal
c
GeoBioTec, Universidade de Aveiro, Aveiro, Portugal
b
9th International Symposium on Environmental Geochemistry