Geoscience Reference
In-Depth Information
Table 8.6
Bulk chemistry of silcretes in Australia and southern Africa (analyses by X-ray fluorescence).
Region
SiO
2
TiO
2
Al
2
O
3
Fe
2
O
3
MnO
MgO
CaO
Na
2
O
2
OP
2
O
5
SO
3
LOI
f
Inland Australia
a
Mean
94.81
2.09
0.50
0.76
0.01
0.10
0.18
0.07
0.08
0.05
no data
1.22
SD
e
n
=
72
5.06
4.06
0.77
1.50
0.01
0.09
0.20
0.16
0.09
0.08
no data
1.18
Eastern Australia
b
Mean
97.58
0.42
0.30
0.79
0.02
0.10
0.02
0.02
0.03
0.02
0.004
0.47
n
=
63
SD
1.76
0.36
0.37
1.28
0.05
0.14
0.02
0.02
0.06
0.02
0.001
0.44
Kalahari
c
Mean
91.63
0.12
1.69
0.86
0.01
0.98
0.85
0.36
0.95
0.01
no data
2.80
n
=
48
SD
4.04
0.06
0.98
0.54
0.00
0.66
1.51
0.35
0.86
0.00
no data
1.51
Cape Coastal
d
Mean
95.04
1.79
0.61
1.28
0.01
0.28
0.13
no data
0.05
0.04
no data
0.99
n
=
66
SD
2.54
0.58
0.46
1.70
0.00
0.20
0.37
no data
0.12
0.03
no data
0.67
a
Calculated from analyses in Hutton
et al.
(1972), Senior and Senior (1972), S.H. Watts (1977), Callender (1978), Senior (1978), Wopfner (1978),
O'Neill (1984), Collins (1985), Thiry and Milnes (1991), Van Dijk and Beckmann (1978), Tait (1998) and Webb and Golding (1998).
b
Calculated from analyses in Taylor and Smith (1975), O'Neill (1984), Collins (1985) and Webb and Golding (1998).
c
Calculated from analyses in Summerfield (1982), Nash and Shaw (1998), Nash, Thomas and Shaw (1994) and Nash, McLaren and Webb (2004).
d
Calculated from analyses in Frankel and Kent (1938), Bosazza (1939), Frankel (1952) and Summerfield (1983d).
e
Standard deviation.
f
Loss on ignition.
influenced by the host material mineralogy and the ratio
of matrix to detrital grains (e.g. Nash, Thomas and Shaw,
1994; Webb and Golding, 1998). Alumina content usually
reflects the presence of authigenic, illuviated or inherited
clays. Haematite and goethite may be retained during sili-
cification (Meyer and Pena dos Reis, 1985; Bustillo and
Bustillo, 1993; Armenteros, Bustillo and Blanco, 1995;
Ballesteros, Talegon and Hernandez, 1997). Glauconite
has been reported in some Kalahari silcretes and may in-
dicate suboxic, partially reducing groundwater conditions
during silicification (Nash, McLaren and Webb, 2004).
The examination of silcretes in thin section and un-
der scanning electron microscope can provide important
insights into their diagenetic history. At this scale, sil-
cretes can be seen to comprise varying proportions of
detrital minerals, silica cements and void spaces (which
may be partially or completely filled with secondary sil-
ica or other minerals). The nature of these components
reflects not only the diagenetic processes operating dur-
ing formation, but may, in part, be inherited from the host
sediment, regolith or bedrock.
Four main forms of silcrete can be identified at a mi-
croscale (Table 8.5): those that have quartzitic grain-
supported (GS-) or conglomeratic (C-) fabrics (where
clasts of >4 mm diameter are present), those where skele-
tal grains comprise >5 % of the silcrete but are dis-
persed to create a fabric of floating clasts (F-fabric sil-
cretes) and matrix-dominated varieties with
across the range of silcrete types. However, GS-fabrics are
most common in arid zone silcretes. M- and F-fabric sil-
cretes are suggested to form by pedogenesis within deeply
weathered profiles (S. H. Watts, 1977, 1978a; Summer-
field, 1979, 1983a) or via the replacement of pre-existing
calcrete matrix materials (Nash, Thomas and Shaw, 1994;
Nash, McLaren and Webb, 2004), the fabrics develop-
ing by silicification of the former matrix or by displacive
crystallisation (Butt, 1985).
Silcretes may be cemented by opal, chalcedony (a
microcrystalline fibrous form of silica comprising inter-
growths of quartz and the silica polymorph moganite; see
Heaney, 1995), cryptocrystalline silica or quartz. The ma-
trix of F- and M-fabric silcretes usually consists of micro-
quartz, cryptocrystalline or opaline silica and may include
silt- or clay-sized detrital quartz, anatase, iron oxides and
clay minerals (Frankel and Kent, 1938; Smale, 1973). GS-
fabric silcretes are commonly cemented by microquartz or
cryptocrystalline silica and may exhibit syntaxial quartz
overgrowth cements. These indicate that silica precipita-
tion occurred slowly (Thiry and Millot, 1987) and that
intergranular impurities or coatings were absent (Heald
and Larese, 1974). More ordered silica polymorphs typi-
cally occur towards the top of a profile, particularly within
pedogenic silcretes. Thiry and Millot (1987) identified a
sequence of silicification starting with amorphous opal
and progressing through chalcedony to microquartz and
megaquartz. The variety of silica within the matrix de-
pends not only upon the polymorph initially precipitated
but also the diagenetic history of the material. Silica
polymorphs may transform by dissolution and recrys-
5 % skeletal
grains (M-fabric silcretes). These fabric types are not mu-
tually exclusive. For example, F-fabrics may grade into
GS- or M-fabrics within individual profiles or even sin-
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