Characterization and comparison of pore distribution in weathered volcanic rocks by different techniques - Volcanic Rock Mechanics - page 79

in turn could depend on the modes of acquisition
and the heterogeneous nature of the rock. Pores
size distributions shows a clear linear trend on a
log-log plot ( Figure 6 ) allowing to determine the
exponent of the best fitting power law relationship.
Pore shape description, which includes circular-
ity and aspect ratio, reveals that pores are slightly
elongated with aspect ratio values ranging mainly
from 1.5 to 3.5.
To assess the influence of alteration grade on
pore size distribution we applied the fractal dimen-

Figure 7. Fractal dimension calculated from pores
size distributions obtained by different techniques (see
legend).

sion method (Turcotte, 1992; crosta et al., 2007)
to pore distribution values obtained by mercury
porosimeter, X-ray tomography, and thin section
analysis.
The values of the fractal dimension range
between 1.62 and 2.54, and they decrease pro-
gressively from intact rocks (sl1) to the fourth
grade (sl4) of alteration ( Figure 7 ) suggesting an
increase in large pore frequency. The fractal dimen-
sion corresponding to the most altered rock (sl5)
deviate from this trend.
an increment in the fractal dimension is observed
and suggests an increment of the relative frequency
of smaller pores (see also Figure 6 ). This could be
due to the fact that new small pores have been gen-
erated by hydrothermal processes, as well as, big
pores have been filled by new minerals (amorphous
silica and clay minerals) ( Figure 1 ). This hypoth-
esis is supported by thin section observations and
XRD analyses showing a strong increase in amor-
phous and semi-amorphous minerals.

5 conclUsions

Five different procedures to quantify porosity of
altered/weathered volcanic materials have been
implemented and compared. These procedures
allow the definition and quantification of total and
effective porosity, spatial pore structure and pore
size distribution. Results demonstrate the relation-
ship between alteration grade and porosity distri-
bution in the same rock lithology.
The most relevant conclusions are as follow:
• a signiicant relationship exists between porosity
and alteration/weathering grade for all the sam-
ples. Total porosity increases with grade.

Figure 6. Pore volume distributions obtained from
(a) thin section analysis, (b) mercury porosimetry,
(c) X-ray tomography.