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Minerals
and rocks 2
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3. The characteristics of minerals
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The characteristics of minerals
Minerals are natural substances, mainly solid and inorganic, characterized by a uniform and constant composition in every aspect.
They can have different compositions:
pure elements;
compounds;
isomorphic mixtures.
A specific mineralogical formula is associated with each mineral. 3
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4. Crystalline and amorphous solids
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Crystalline and amorphous solids
Most minerals are crystalline solids.
They have a polyhedral external shape and an ordered internal structure.
Some minerals have a glassy state: they are amorphous solids, without a precise external geometrical shape and with a disordered internal structure. 4
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5. How are minerals formed?
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How are minerals formed?
Minerals can be formed:
by solidification of magma or lava;
by precipitation of dissolved substances from a supersaturated solution;
through sublimation of gases or vapors.
In specific conditions, formed minerals can trasform into new minerals, without changing from the solid state. They are called metasomatic minerals. These transformations can occur on Earth’s surface or deep underground. 5
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Silicates
Minerals are divided into different classes (or families) based on similarities in chemical composition.
The most abundant minerals are silicates, formed by silicate anions SiO44– with tetrahedron geometry.
Silicates can be divided into two groups:
mafic silicates, with high amounts of metallic ions and a significant amount of iron and magnesium;
sialic silicates, with low amounts of metallic ions. 6
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Rock classifications
In nature, minerals are found in aggregates called rocks.
Depending on how they were formed, rocks are divided into
igneous (magmatic), sedimentary or metamorphic.
Rocks are identified based on their structure and mineralogical composition.
sedimentary rocks 8%
magmatic effusive rocks
40%
metamorphic rocks
27%
magmatic intrusive rocks
25% 7
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Magmatic rocks
Magma is a mixture of melted substances, especially silicates, associated with water vapor and gases. It is formed inside the Earth, by the fusion of parts of the mantle or the crust.
Intrusive magmatic rocks are formed through slow, in depth solidification of magma.
Effusive magmatic rocks are formed through rapid solidification of lava on the surface. 8
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9. Intrusive and effusive magmatic rocks
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Intrusive and effusive magmatic rocks
Intrusive magmatic rocks have a holocrystalline structure, in which minerals are visible in the form of crystals.
Granite has a holocrystalline structure.
Effusive magmatic rocks have a glassy or porphyritic structure, in which it is impossible to distinguish different minerals.
Pumice has a glassy structure. 9
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10. Composition of magmatic rocks
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Composition of magmatic rocks
You can classify magmatic rocks according to their silica content or the content of cations.
Depending on their silica content, magmatic rocks are divided
into:
felsic (or sialic) rocks;
intermediate rocks;
mafic rocks;
ultramafic rocks.
If we consider the content of cations, the most common magmas are alkali-calcic. 10
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11. Classification of magmatic rocks
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Classification of magmatic rocks
The main groups of magmatic rocks are:
granites;
diorites;
gabbros;
peridotites. 11
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Sedimentary rocks
Sedimentary rocks are derived from preexisting rocks.
They were created on Earth’s surface due to the changes induced by exogenous agents such as water, atmospheric phenomena or living organisms.
The sedimentary process can last tens of millions of years and it can include:
degradation;
transportation;
deposition;
diagenesis (transformation). 12
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13. Composition of sedimentary rocks
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Composition of sedimentary rocks
The composition of sedimentary rocks depends on the nature of the origin rock and on the erosion and deposition modalities.
Almost all sedimentary rocks are composed of layers related to distinct sedimentation events. Some sedimentary rocks have a compact structure. 13
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14. Different types of sedimentary rocks
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Different types of sedimentary rocks
They can be divided into three groups:
clastic rocks;
chemical rocks;
organic rocks. 14
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Metamorphic rocks
Metamorphic rocks are formed through in-depth variations in pressure and temperature.
They undergo changes in structure or mineralogical composition, but always remain in their solid state.
The possible effects of the metamorphic process can be:
recrystallization;
change in alignment of minerals;
fragmentation of rocks into grains. 15
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