Rocks and minerals
Aggregates are obtained from rocks, which, in turn, are composed
of minerals (either a single mineral, or a mixture of minerals). A mineral
is naturally occurring, inorganic, has an order internal arrangement of
atoms, and has a definite composition (or range of compositions). For
example, quartz, halite, gypsum, opal, feldspar, biotite (mica), etc.
- Streak (on a porcelain plate)
- Lustre (metallic or non-metallic)
- Hardness (typically represented on Moh’s scale, where
talc has a hardness of 1 and diamond a hardness of 10)
- Specific gravity
- Crystal form
- Tenacity (brittle or ductile)
- Diaphaneity (or transparency)
- Reaction to acid (typically, HCl is used for this test)
An ore is a natural mineral that can be mined for a profit. For
example, bauxite, iron ore etc.
Types of rocks
Rocks are classified as igneous, sedimentary, or metamorphic.
95% of the outer 10 miles of the earth’s crust is composed of igneous
and metamorphic rocks, but 75% of the rocks exposed on the surface of
the earth are sedimentary.
Igneous rocks are those which form as a result of cooling from
the molten state. These are further classified as:
- Intrusive: when the molten matter cools slowly under the
earth’s surface, and results in the formation of large rocks with
typically large crystals, e.g., Granite, gabbro, pegmatite
- Extrusive: when the molten matter cools rapidly on the earth’s
surface, resulting in the formation of rocks with smaller crystals,
e.g., Basalt, andesite, rhyolite.
- Pyroclastics: these are formed due to the cementation of
extremely fine ash deposits which cool very rapidly resulting in an
amorphous rock, e.g., volcanic tuff, pumice, breccia.
Sedimentary rocks are deposited in a fluid medium due to lithification
of weathered sediments. Lithification can occur as a result of cementation
(common cements being iron oxide, calcite, or quartz), crystallization,
or compaction (due to the application of high temperature and pressure).
Shale, sandstone, and limestone make up 46, 32, and 22 % of all sedimentary
Metamorphic rocks are formed when pre-existing rocks are subjected
to heat and pressure. Recrystallization often occurs, and the resulting
rocks have typically large crystals with a well-defined cleavage. For
example, marble, gneiss, schist, phyllite, slate, etc.
Engineering considerations of rocks
- . Fine-grained siliceous materials in igneous and sedimentary
rocks can be susceptible to Alkali-silica reaction (see the link on
Durability for more details). For example, amongst igneous rocks, rhyolite,
andesite as well as the rocks containing volcanic glass are prone to
ASR. The principal active siliceous ingredients are opal (SiO2.nH2O)
and chalcedone, which is a fibrous variety of silica. Chert and greywacke
among sedimentary rocks are also highly prone to ASR. Among metamorphic
rocks, phyllite and argillite are susceptible to ASR because of the
presence of strained quartz.
- Carbonate sedimentary rocks, limestone and dolomite, are
susceptible to Alkali-Carbonate reaction when the minerals have a specific
- . Very coarse grained rocks are undesirable because of poor
- Some igneous rocks may have soluble minerals like zeolite;
also, in foundations, weathered igneous rocks should not be used. In
the case of sedimentary rocks, the source of rock is important. Some
rocks are less durable (e.g. stream gravels) and some are extremely
porous (e.g. conglomerates are weak and porous), and not good for freezing
conditions. Careful attention should also be paid to the presence of
cavities and conduits within limestone and dolomite among sedimentary
rocks, and marble among metamorphic rocks.
- Flat, flaky, and elongated pieces are obtained on crushing
metamorphic rocks. Such rocks will pose a problem if used as concrete
aggregates. Also, some metamorphic rocks show directional properties
because of their foliation.
- As dimension stone, igneous rocks are the best because of
their high resistance to weathering. Sedimentary rocks for use in dimension
stone should be carefully chosen.