Geoscience Reference
In-Depth Information
We could arrive at the same result more quickly! But the presentation
given shows more clearly the connection between the two methods
proposed.
It is easy to show that the losses are 0 for the parent material or -1
(equivalent to 100%) if
Fe
(
i
) = 0.
This method is very useful for the study of the first stages of
weathering (R Æ C). It is applied, for example, to the case of a granite
that decomposes and loses resistance and material without disintegrating
(Fig. 1.4). It is then transformed into an
isaltérite
(saprolite), so termed
because the original volume is retained.
Figure 1.6 gives the principle of the calculation.
Reasoning using index mineral (MI)
Index mineral
Horizon
studied
i
Calculation of the proportion
of the substance which would
normally have had to
accompany the resistant
index mineral if it was
concentrated in the same
proportion
Reference
horizon
(R or C)
Fig. 1.6
Calculation of the losses of material based on the presence of a resistant
index
mineral
.
If we consider that the index mineral is perfectly unweatherable, its
mass has not changed. The quantities presented above in equations (1)
and (2) are equal, from which
volor
(
i
) =
vol
(
i
)
.da
(
i
)
.MI
(
i
)
______________
da
(
r
)
.MI
(
r
)
(5)
Substituting this in equation (3) and simplifying, we get
Loss
Fe
(
i
)
=
Fe
(
i
)
.MI
(
r
)
__________
Fe
(
r
)
.MI
(
i
)
(6)
For example, if there is as much of the index mineral in
i
as in
r
, the
loss or gain of iron is directly deduced from the corresponding contents
of the element. If there is twice as much index mineral in
i
as in
r
, but
the iron content does not change, the actual loss of iron is 0.5 or 50 per
cent.
