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summing-up
1Minerals and rocks
A mineral is a naturally occurring
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solid substance with a definite
chemical composition, and a
crystalline structure, i.e. an ordered
and regular spatial arrangement of
the constituent atoms.
• Each mineral exhibits a typical
crystalline habit: a crystal is a
geometric solid with faces, edges and
vertices that arise from the
progressive growth, atom by atom, of
an elementary three-dimensional
structure with infinitesimal
dimensions (the crystal lattice).
The basic properties, related to the
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chemical composition of a mineral
(i.e. the type of atoms or ions from
which it is formed) and its crystalline
structure, are:
– crystal habit,
–hardness,
–lustre,
–cleavage,
– colour, and
–density.
2Mineral classification
Every mineral is characterised by a
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series of physical properties such as
hardness, cleavage, lustre, colour and
density.
• Around 4,000 mineral species are
known, but only 20 of these are
important constituents of the Earth’s
crust.
The most common minerals are the
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silicates, which have as a basic
structure a tetrahedron formed from
silicon and oxygen. Non-silicate
minerals, such as native elements,
sulfides, sulfates, oxides and
carbonates, are less abundant in
nature.
Silicates constitute 80% of the
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materials outcropping on the Earth’s
surface.
3Oxides, carbonates,
sulphides and sulphates
Oxides contain oxygen, combined
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(Fe), aluminium (Al), chromium
(Cr), and so on.
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Carbonates are formed from carbon
(C) and oxygen (O), bound to one or
more positive ions, such as calcium
(Ca) or magnesium (Mg).
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Sulphides and sulphates contain
sulphur (S).
4Native elements, halides,
borates and phosphates
The native elements are pure masses
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in combination with one or more
different elements.
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Borates are derived from the
combination of positive ions with the
borate ion (BO3)3-.
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Phosphates contain a phosphate ion
bound to one or more metallic
elements.
5Rocks
A rock is, in the majority of cases, a
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natural aggregate of several minerals,
sometimes even non-crystalline
substances, usually in a very compact
form. The majority of rocks are
heterogeneous, that is they are made
up of many mineral species.
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There are three lithogenic processes
which lead to the formation of rocks:
–the magmatic process transform high
temperature molten material
(magma) into magmatic or igneous
rocks;
–the sedimentary process, which takes
place on the Earth’s surface, at low
temperatures and pressures, leads to
the formation of sedimentary rocks;
–the metamorphic process, which
occurs within the Earth’s crust, at
varying temperatures and pressures,
leads to the transformation of preexisting rocks into metamorphic
rocks.
6Magmatic or igneous rocks
Magmatic rocks are formed by the
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solidification of magma, a mass of
molten rock that can form in the
crust or upper part of the mantle.
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Magmatic rocks are divided into:
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intrusive magmatic rocks; and
–
extrusive magmatic rocks.
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The various types of igneous rocks
are due to the diverse origins of the
magmas.
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The distinction between the various
types of magmas is based on their
silica content.
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The main families of magmatic rocks
are granite, diorite, gabbro,
peridotite, and alkaline rocks.
7Sedimentary rocks
Sedimentary rocks are formed
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through processes that occur on the
Earth’s surface.
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Sedimentary rocks are divided into
three groups, depending on their
origin:
–
clastic rocks that have formed as a
result of the accumulation of
fragments resulting from the
disintegration of other rocks;
–
organogenic rocks, formed by the
accumulation of materials derived
from organic activity;
–
chemical rocks, which are derived
from chemical processes, such as the
precipitation of salts.
with metallic elements such as iron
of a single element.
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Halides are formed from a halogen
(chlorine, iodine, fluorine, bromine)
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
unità 1•Minerali e rocce
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summing-up
8Metamorphic rocks
Metamorphic rocks are formed from
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pre-existing rocks that are subjected
to elevated temperatures and/or high
pressures. Metamorphic
transformations occur without the
rocks passing through the molten
state.
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Contact metamorphism occurs in
rocks that are in contact with magma
that rises in the crust.
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Regional metamorphism is
manifested in rocks that descend into
the crust and takes place where both
high temperature and pressure are
imposed over large parts of the crust.
9The Rock Cycle
Lithological processes form part of a
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cycle in which materials in the
Earth’s crust are constantly reworked
and recycled.
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The rock cycle is not perfectly
closed, but is subject to
contributions of molten material
from the mantle. In addition, the
cycle is also not closed to external
Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012
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New minerals are formed according
to the intensity of the
metamorphism. These minerals help
establish metamorphic facies, i.e. the
depth in the crust at which the
transformation took place.
losses: the atmosphere and the
hydrosphere, in fact, have
accumulated material and are
continually being “fed” by volcanic
processes.
unità 1•Minerali e rocce
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