Environmental Engineering Reference
In-Depth Information
and are predominantly represented by lapilli tuff
and breccia tuff.
Rock texture depends on the following factors:
the number and how the larger minerals are dis-
tributed within a more or less abundant matrix
constituted of smaller minerals; the small fissures;
the alteration degree of minerals; and the number
and size of pores and vacuoles.
in general, the effusive rocks show aphanitic or
microcrystalline texture (Gomes & silva, 1997),
and the pyroclastic rocks show porous to very
porous, vesicular to very vesicular, and rarely amy-
gdalar and breccioid texture.
Figure 3. location, in the island of santa Maria, of the
quarries where natural stone has been sampled.
Table 1. References, sources, trade names and exploit-
ing enterprises.
4
PeTRoGRaPhic, MineRaloGical
anD cheMical chaRacTeRiZaTion
s. Miguel Island
R1-Pedra do Porto
Formoso
herdeiros agostinho
F. Medeiros, lda
4.1
Analytical methods
The petrographic, mineralogical, and chemical
characterization of the studied natural stones was
carried out at the Departamento de Geociências da
Universidade de aveiro, and at the lneG (labo-
ratório nacional de energia e Geologia).
The petrographic analysis was carried out using
an optical microscope and polarized transmitted
light and the observation of thin sections of the
studied natural stones. Petrographic analysis pro-
vided information very useful for the interpreta-
tion of the physical-mechanical behavior of the
natural stone, and for the identification of certain
features or defects that could penalize natural
stone quality.
The mineralogical analysis was carried out using
X-Ray Diffraction (XRD) whenever due to miner-
als alteration they could not be identified by petro-
graphic analysis.
The chemical analysis was carried out using
X
-
Ray Fluorescence (XRF).
R2-Pedra da Povoação
herdeiros agostinho
F. Medeiros, lda
R3-Basalto da chã das
Gatas
herdeiros agostinho
F. Medeiros, lda
R4-Basalto do Bacharel
herdeiros agostinho
F. Medeiros, lda
R5-Pedra do Dâmaso
José Dâmaso e Filhas, lda
sta. Maria Island
R6-Pedra da Vila do
Porto
Mário Batista
R7-Pedra do anal
Mário Batista
enterprises involved in the extraction of the seven
types of natural stone being studied.
3
GeneRal anD MacRoscoPic
chaRacTeRiZaTion oF The
naTURal sTone FRoM The aZoRes
The ornamental value of an igneous rock depends
on its colour and textural homogeneity. These
characteristics determine to a large extent the rock
ornamental and commercial value.
The colour exhibited by a natural stone, to a
large extent, is associated with the colour of the
dominant minerals, with the granularity of the same
minerals, and with minerals alteration degree.
The lava rocks show gray shades, darker or
lighter, being predominantly represented by basalt,
basanite, trachybasalt, trachyandesite and trachyte.
The pyroclastic flows, associated with very
explosive eruptions of acid magma, show black to
brown shades, and are predominantly represented
by ignimbrite. o the other hand the pyroclas-
tic rocks show a wide range of shades, the most
common ones being: brown, black, red, and gray,
4.2
Petrographic and mineralogical analysis
some types of the studied natural stones show
great similarities, both in petrographic and in min-
eralogical terms:
i. Pedra do Porto Formoso (R1) and Pedra da
Povoação (R2) from the island of são Miguel
are characterized by their light gray and brown-
ish black colours and by their small pores due
to the release of gas; they show porphyritic
texture, and fenocrysts of leucite, k-feldspar,
na-ca feldspar, pyroxene and olivine have been
identified; both rocks could be classified as
leucite-basanites.
ii. Pedra do chão das Gatas (R3) and Pedra do
Dâmaso (R5), both from the island of são
Miguel, correespond to extrusive mafic igneous
