Geoscience Reference
In-Depth Information
Change in volume
We can also calculate the change in volume using the index mineral.
It is given by
varvol
(
i
)
=
vol
(
i
)
- volor
(
i
)
______________
volor
(
i
)
or, using equation 5 and simplifying
varvol
(
i
)
=
da
(
r
)
.MI
(
r
)
__________
da
(
i
)
.MI
(
i
)
- 1
(7)
This quantity is called
strain
by scientists.
Following an oft-quoted paper by Chadwick
et al
. (1990), numerous
scientists have used this change in volume for calculating the losses
mentioned in equation 6. But it is a roundabout means that leads to
a fruitlessly complicated expression, unfortunately reproduced from
article to article. It is as follows, using our terminology:
Loss Fe
(
i
)
=
Fe
(
i
)
.da
(
i
)
_________
Fe
(
r
)
.da
(
r
)
× [
varvol
(
i
) + 1] - 1
(8)
It is clear that one can arrive at equation (6), which is equivalent and
simpler, without too much effort by substituting in the above equation
varvol
(
i
) by its value from equation (7). In fact, the losses calculated on
the basis of an index mineral do not involve density!
In the context of this topic and considering developments that we
will conduct at many places, these equations are important.
Since the losses are calculated as a proportion of the original masses,
then on a scale of 0 to 1 or 0 to 100 per cent, we can give for each soil
profile a composite diagram, all components being shown simultaneously
(Fig. 1.7).
Graphic representation
The index mineral method is more commonly used than the isovolumetric
method. Indeed, different minerals are quite stable in soils.
Quartz has long been used (
isoquartz
reasoning) in the temperate
region. Actually it is tricky to use (Legros 1982): (i) its determination is
difficult because it is not the only silica-containing mineral in the soil;
(ii) part of the quartz, unattacked, is physically displaced; for example,
one finds coatings of quartzose silt on coarser particles in acid mountain
soils.
