Environmental Engineering Reference
In-Depth Information
rock such as compressive strength and modulus of
elasticity are greatly influenced by pore structure
and are presented in a companion abstract/paper.
in the following, we discuss the adopted proce-
dures to characterize total and effective porosity,
pore types and commet about their applicability
and accuracy.
identification of the original rock very difficult. in
the following we describe the petrographical char-
acterization of the altered samples.
2.1.1
Fresh lava SL1
This represents the less altered sample of the stud-
ied sequence. its major constituents are sodic pla-
gioclase (oligoclase and andesine) and potassic
feldspar with minor amounts of pyroxene and a
small amount of biotite. The sample has a por-
phyritic texture in which predominantly euhedral
phenocrysts are plagioclase with an average size of
3 mm. The matrix presents a sub-parallel arrange-
ment of micro-pyroxenes and micro-plagioclases
(pilotaxic texture). Two main types of alteration
are observed: oxidation within the boundary of
most minerals, affecting all biotite crystals, and as
stains into the macles of plagioclases; argilization
is presented around minerals as blurred stains.
2
sTUDY aRea anD MaTeRials
lava samples characterized by different degrees of
alteration were collected from the campi Flegrei
(italy). The campi Flegrei provide an excellent
setting to study the evolution of geometry and
topology of pore network and grain size and shape
in weathered volcanic rocks. campi Flegrei are
located near the city of naples and the volcanic
area belongs to the campanian province, which
represents the southernmost sector of the plio-
quaternary volcanic belt along the italian peninsula
(Washington, 1987). campi Flegrei comprises the
volcano of solfatara where the sampled outcrops
are situated. The solfatara is located in Pozzuoli
and is part of the last period of intense volcanic
activity of campi Flegrei occurred between 4.8
and 3.8 kyr B.P. (Di Vito et al., 1999).
During field work a total of 10 block samples
were collected. sampling was focused on cover-
ing the entire range of alteration and each sample
consists of a block weighting between 30 to 50 kg
and with a minimum thickness of 15 cm to allow
coring of samples at least 5.4 cm in diameter and
13 cm high. The operational procedures and initial
description of alteration followed the Bs standard
methods (Bs 5930, 1999). samples were marked
according to the origin and alteration grade. Min-
eralogical and petrographical changes in samples,
which represent different and progressively weath-
ering grades were examined by optical microscopy,
moreover, all descriptions were completed by X-ray
diffraction data.
2.1.2
Slightly weathered lava SL2
in the second grade of alteration the major con-
stituents are sanidine with minor amount of plagi-
oclase and a small amount of pyroxene and biotite.
The sample has a trachytic texture in which pre-
dominantly euhedral phenocrysts are sanidine. The
matrix is composed principally by micro-sanidines.
in this sample we can observe argilization and oxi-
dation along microfractures and within the bound-
ary of minerals. Biotite is almost totally replaced.
2.1.3
Moderately weathered lava SL3
This sample represents the third grade of altera-
tion. its major constituents are potassic feldspar
(sanidine) with minor amount of sodic plagioclase
and a small amount of pyroxene; biotite is almost
missing. Relatively large crystals of sanidine (2 mm
of ave. size) completely bounded by its character-
istic faces are surrounded by a major amount of
micro-sanidines and micro-plagioclses (porphy-
ritic texture). in this sample oxidation increases
affecting all crystals boundaries whereas biotite
and pyroxene are almost completely replaced
and all around minerals as blurred stains.
2.1
Solfatara Lava (SL)
The lava rock mass is heavily fractured, joints are
often infilled of loose material and are strongly
altered suggesting that widespread fumarolic and
thermal springs activity could be following pref-
erential pathways. Discontinuities are often of
very small size and sometimes not visible with a
naked eye. outcrop observations show that altered
lava varies significantly on a short distance espe-
cially when approaching to fumarolic activity. We
easily identified the effects produced by hydro-
thermal alteration: the rock fabric and texture in
some places are completely lost making the in situ
2.1.4
Highly weathered lava SL4
This sample represents the fourth grade of altera-
tion. althougth all minerals are altered, sanidines
prevail over pyroxenes and plagioclases. X-ray
power diffraction shows that alunite (derived from
acid alteration of potassic feldspar) is very abun-
dant. Matrix is totally replaced by argilization and
a new process of silicification can be observed into
the potassic feldspar and within the matrix. small
silica-amorphous minerals appear between the
grains.
