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CLASSES OF ROCKS
VOLCANIC
SEDIMENTARY METAMORPHIC
REGIONAL
METAMORPHIC
GRANITE
DIORITE
GABBRO
CLASTIC
BIO GENE
CHEMICAL
LOW GRADE
GRADE RATING
MEDIUM HIGH GRADE
CONTACT
METAMORPHIC
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Igneous rocks
These rock types are derived from the solidification of molten earth material (magma), which may
occur below the Earth's surface (intrusive igneous rocks) or on the Earth’s surface (extrusive
igneous rocks).
The different methods of cooling, lead to the formation of rocks with different structures that allow
them to be distinguished easily.
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An intrusive igneous rock is characterised by its lucid structure with evident presence of wellformed crystals juxtaposed next to each other. Felsic rocks are enriched in the lighter elements-the
minerals are usually light in colour, as in granite. Most mafic minerals are dark in colour.
Common mafic rocks include basalt and gabbro.
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Metamorphic rocks
Different types of rocks, subjected to temperature and pressure changes within the Earth's crust, are
subject to the phenomenon of metamorphism.
**
Metamorphism consists in the reorganization of existing minerals and / or the formation of
new minerals, more stable to the new conditions of temperature and pressure, without
changing its form.**
**(I may have misunderstood the Italian, but the last few words are incorrect as metamorphism
means “change in form”)**
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How to recognize a metamorphic rock.
Schist is characteristically foliated, meaning the individual mineral grains split off easily into flakes
or slabs and is a rock that has undergone a low-grade metamorphism. Another example is
**FILLADE**
(** Couldn’t find the English word for this rock**)
The minerals of Gneissic rocks are arranged in compact, alternate light and dark bands, (AS IN
GNEISS) where high-grade metamorphic processes form the rock.
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Sedimentary rocks
These rocks are derived from the processes of physical and chemical erosion suffered by all types of
rocks present on the Earth’s surface. The products of erosion, undergoing the processes of transport
and sedimentation, may give rise to various types of sedimentary rocks. The structures and
characteristics vary widely.
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Granite (leucograniti, monzograniti, fine-grained granite
On the archipelago of La Maddalena, granite cliffs emerge all over the island, giving it dominant
morphological characteristics. This is the monzograniti eterogranulari rosati (to mean large
grain, of whitish colour (leucocratic), inside of which there is often fine grain (micrograniti) to
medium grain, characteristic of most leucocratic.
The granites generally represent radically different compositions and fractures, which are
emphasized and highlighted in a variety of ways, often typical of one or another granite.
From deep dissected rugged cliffs, to irregular reliefs with deposits of great emerging blocks and
morphology characterised by a less deep dissection with extended and sometimes powerful covers
of material derived from the alteration of the granites.
The ground that is found generally has a light sandy colour that dominates and reflects the
mineralogical composition of the rock source. The natural distribution of coarse-grained granite is
locally interrupted by the presence of thickening biotitic and pegmatite.
First they are local ferromagnesian mineral concentrations that sometimes vanish into the granite
mass and it is not always possible to define the limits, while they sometimes have the appearance of
the included granite. Forms and dimensions are variable: from centimetres to metres, and lenticular
to spherical.
The pegmatite is coarsely crystalline granite or other high-silica rock. The mineralogical
constituents are characteristic of granite (quartz, orthoclase, microcline, plagioclase, soda-lime and
those regarded as accessories of the granite (tourmaline, zircon, garnets and fluorspar) that
sometimes cease to be accessories and become abundant and large.
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Mineral veins
The mineral veins that penetrate the granite and metamorphic formations differ in their petrography,
residual power and linear development, but are fundamentally united by a general-uniformity. On
the basis of their principal constituents, the basic veins varying from quarzodioritica to leucocratic
can be generally distinguished.
Quartz occurs in hydrothermal veins and owes their origin to the segregation of minerals from
aqueous solutions at a temperature below 370 ° c. The mineral is crystal clear and colourless.
Aplites (fine-grained acid veins) are diffusely injected both in granites and in metamorphic rock. A
fine to granulate pale pink paste with rare ferromagnesian minerals characterizes them.
Porforite veins in granite are the most important and impressive manifestations, macroscopically
characterized by a reddish-grey colour with phenocrysts of feldspar and magnetite.
Lamprophyre veins( spessartiti, camptoniti), although not reaching significant developments are
among the most frequent mineral veins. They have a homogenous grey-green colour, with small
blackish punctuations due to metallic oxides.
Morphologically mineral veins give rise to different forms depending on their composition and
strength and of the embedding cliff.
The first are predominantly acidic (aplite, granite porforite an intermediate granodioritica
composition. and stand in a less emphasized way from the embedding heap granite, which
sometimes can not be distinguished easily.
The basic mineral veins (lamprophyre) are more widespread and although the usual thickness is
sometimes limited, their sharp chromatic contrast with the embedding rock makes them easily
recognisable. The chromatic contrast with embedding cliffs are the result of a forced alteration the
embedding granite in comparison to those that are cliffs which sometimes introduce an exfoliation
characteristic.