Geoscience Reference
In-Depth Information
COMPLEMENTARY TECHNIQUES
miscellaneous minerals (graphite, apatite, chloritoid,
tourmaline, epidote, and zircon).
Slate properties are largely dependent on mineralogy.
The proportion of platy minerals controls splitting, quartz
imparts hardness and durability, while the presence of
some accessory minerals adversely affects durability
(Walsh, 2003). The colour of a slate is also determined
by its mineralogy. The most common colour is grey
which is attributed to a combination of quartz with a
small amount of graphite (carbon). Increased amounts of
graphite darken the colour to black, a colour also
associated with pyrite, which, if finely disseminated,
tends to darken slate colour. Iron oxides such as
haematite (
99
) give red and purple colours, while green
slates contain more chlorite (
100
) and occasionally
epidote. Some slates exhibit round/elliptical bleached
spots called reduction spots. These are formed around a
particle of organic matter, which acted as a reducing
agent and converted purple ferric oxide to green/yellow
ferrous oxide (
101
).
As slate is formed, by diagenetic and metamorphic
processes, the proportions and types of minerals present
change. New minerals grow as clay minerals are lost, the
chemical composition and atomic structure of phyllo-
silicates change, crystallinity increases, and the grain size
increases (Walsh, 2002). An understanding of mineralogy
gained from microscopical examination aids the
interpretation of metamorphic grade, which in turn gives
an indication of the quality of slate as a roofing material.
Within the European Union, petrographic examination
of roofing slate is performed following a procedure given
in EN 12326-2 (British Standards Institution, 2000).
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 polished specimen, which may be
supplemented by mineralogical analysis by XRD.
For initial type testing or factory production control,
petrographic examination is conducted on a single slate
randomly selected from the assessment batch. Following
arrival in the laboratory, the 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. Slate features observed at this stage include colour,
thickness, 'grain', relict bedding, open and incipient cracks,
joints and veins, nodules and inclusions, and surface
discolouration. The initial examination is used to
determine the most appropriate location for thin sections
and polished specimens to be taken for further, more
detailed high-power microscopical examination. Thin
sections are prepared from slices representing three
different orientations, comprising two perpendicular to the
cleavage (at right angles to each other) and one parallel to
the cleavage. Polished specimens are normally prepared to
enable reflected light examination of opaque minerals.
Slates are very fine-grained and relatively difficult to
resolve by optical microscopy, so it is desirable to
enhance the optical observations by SEM and associated
microanalysis. Mineralogical analysis by XRD is
particularly useful to determine the minerals that are
present, but, for routine investigations, only gives
semiquantitative estimates of proportions. The accuracy
of a quantitative determination for a particular mineral
(e.g. quartz) can be improved by adding a known amount
of that mineral ('spiking') to the sample (Walsh, 2002).
Relatively accurate elemental analyses can be obtained
by XRF analysis.
98
M
INERALOGY
The essential minerals of typical slate are quartz and
phyllosilicates (white micas and chlorite) with various
accessory minerals (
98
). The accessory minerals can
include feldspars, carbonates (calcite, dolomite,
magnesite, siderite), iron oxides (haematite, magnetite,
limonite), iron sulfides (pyrite, marcasite, pyrrhotite),
other ore minerals (rutile, anatase, ilmenite), and
98
The essential slate minerals in thin section. White
mica (white/grey/yellow), quartz (grey), and lenses of
chlorite (blue). 'Villar Del Rey' slate from Spain; XPT,
×300.
