Mining_Methods_UnderGround_Mining - Mining and Blasting

geOlOgy FOr Mining
to mine or tunnel through them, or both.
Some of these important characteristics,
which are also important for correct
mineral identification in the field
before chemical analysis, are hardness,
density, colour, streak, lustre, fracture,
cleavage and crystalline form.
The particle size, and the extent to
which the mineral is hydrated or other-
wise mixed with water, can be very im-
portant to the behaviour of the rock
structure when excavated. Mineral hard-
ness is commonly graded according to
the Moh 10-point scale
The density of light-coloured minerals
is usually below 3. Exceptions are
barite or heavy spar (barium sulphate
– BaSO 4 – density 4.5), scheelite (calcium
tungstate – CaWO 4 – density 6.0)
and cerussite (lead carbonate – PbCO 4
– density 6.5). Dark coloured minerals
with some iron and silicate have
densities between 3 and 4. Metallic ore
minerals have densities over 4 Gold has
a very high density of 19.3. Minerals
with tungsten, osmium and iridium are
normally even denser.
Streak is the colour of the mineral
powder produced when a mineral is
scratched or rubbed against unglazed
white porcelain, and may be different
from the colour of the mineral mass.
Fracture is the surface characteristic
produced by breaking of a piece of the
mineral, but not following a crystallographically
defined plane. Fracture
is usually uneven in one direction or
another.
Cleavage denotes the properties of
a crystal whereby it allows itself to be
split along flat surfaces parallel with
certain formed, or otherwise crystallographically
defined, surfaces. Both
fracture and cleavage can be important
to the structure of rocks containing sub-
stantial amounts of the minerals concerned.
Proper ties
Rocks, normally comprising a mixture
of minerals, not only combine the properties
of these minerals, but also exhibit
properties resulting from the way in
which the rocks have been formed, or
perhaps subsequently altered by heat,
pressure and other forces in the earth’s
Moh’s hardness Typical mineral Identification of hardness
scale
1 Talc Easily scratched with a fingernail
2 Gypsum Barely scratched with a fingernail
3 Calcite Very easily scratched with a knife
4 Fluorite Easily scratched with a knife
5 Apatite Can be scratched with a knife
6 Orthoclase Difficult to scratch with a knife, but
can be scratched with quartz
7 Quartz Scratches glass and can be
scratched with a hardened steel file
8 Topaz Scratches glass and can be
scratched with emery board/paper
(carbide)
9 Corundum Scratches glass. Can be scratched
with a diamond
10 Diamond Scratches glass and can only be
marked by itself
crust. It is comparatively rare to find
rocks forming a homogeneous mass,
and they can exhibit hard-to-predict
discontinuities such as faults, perhaps
filled with crushed material, and major
jointing and bedding unconformities.
These discontinuities can be important
in mining, not only for the structural
security of the mine and gaining access
to mineral deposits, but also as paths
for fluids in the earth’s crust which
Samples of common rock types
Amphibolite.
Dolomitic limestone.
cause mineral concentrations. In order
for mining to be economic, the required
minerals have to be present in sufficient
concentration to be worth extracting,
and within rock structures that can be
excavated safely and economically. As
regards mine development and production
employing drilling, there must be a
correct appraisal of the rock concerned.
This will affect forecast drill penetration
rate, hole quality, and drill steel
costs, as examples.
One must distinguish between microscopic
and macroscopic properties, to
determine overall rock characteristics.
As a rock is composed of grains of various
minerals, the microscopic properties
include mineral composition, grain
size, the form and distribution of the
grain, and whether the grains are loose
or cemented together. Collectively, these
factors develop important properties of
the rock, such as hardness, abrasiveness,
compressive strength and density. In
turn, these rock properties determine the
penetration rate that can be achieved,
and how heavy the tool wear will be.
In some circumstances, certain mineral
characteristics will be particularly
important to the means of excavation.
8 underground mining methods