Geoscience Reference
In-Depth Information
COMPLEMENTARY TECHNIQUES
Following arrival in the laboratory, the stone sample is
first examined in the as-received condition using the
unaided eye and low-power stereo-zoom microscope at
magnifications of up to ×60. Stone features observed at
this stage include colour, relative hardness, fabric, grain
size, open and refilled cracks, pores, and cavities, and
presence of macrofossils. The initial examination is used
to determine the most appropriate location for thin sections
(and possibly polished specimens) to be taken for further,
more detailed high-power microscopical examination. Thin
sections are prepared from slices cut across any bedding,
rift or grain. Stone features determined in thin section
include identification of constituent minerals and degree
of weathering. If opaque minerals (such as pyrite, ilmenite,
magnetite) are present, polished specimens may be
prepared to enable their examination in reflected light. An
atlas to aid the microscopical identification of opaque
minerals is provided by Marshall
et al
. (2004).
The proportions of different minerals present within a
stone may be approximately estimated by visual
comparison with standard charts (
19
). The mineral
proportions can be determined more accurately by point
counting. If the microscopical examination proves
insufficient to assign a petrographic definition of the stone,
further investigation may be necessary. Suitable techniques
include mineralogical analysis by XRD, which provides
semiquantitative estimates of mineral proportions, and XRF
analysis, which gives accurate elemental analyses.
Within the European Union, petrographic examination
of building stone is performed following a procedure
given in EN 12407 (British Standards Institution, 2007).
An American standard (WK2609) is currently undergoing
development (ASTM International, 2006). Guidance may
also be taken from the International Society for Rock
Mechanics (1977) and BS 5930 (British Standards
Institution, 1999). Petrographic examination comprises a
combination of visual and low-power examination in
hand specimen and more detailed high-power
microscopical examination in thin section (and
sometimes also polished specimen).
19
19
Comparison
chart for visual
percentage
estimation of
constituents in
thin section (after
Terry & Chilingar,
1955).
1%
3%
5%
Igneous rocks originate as molten magma far down in
the earth's mantle, rising towards the surface before they
solidify. The rock masses which are produced by igneous
activity can be classified in three groups, according to
their mode of emplacement:
•
Intrusive rocks:
dykes, sills, plugs, plutons, and
batholiths formed by consolidation of magma in
spaces within the crust.
•
Extrusive rocks:
lava flows formed by magma
erupted from volcanoes at the earth's surface.
•
Pyroclastic rocks:
agglomerate, tuff, and ignimbrite
formed by the accumulation at the earth's surface of
dust and fragments ejected by explosive volcanic
eruptions.
10%
20%
30%
40%
The minerals which crystallize from a cooling magma
depends on which elements are present in the magma. The
mineralogical composition of any igneous rock is therefore
closely related to its chemical composition, and both
characteristics are used to classify igneous rocks. In terms
of chemical composition, the principal component of all
50%
