Structure and classification of natural stone

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A natural conglomerate composed by one or more minerals is defined rock. Minerals with natural origin are usually solid and inorganic elements with a homogeneous chemical composition, i.e. made up of a single chemical element, well defined and recognizable.

In most cases the minerals have a crystalline structure that affects the physical, optical and chemical properties of the material, allowing a simpler and more immediate recognizability.

According to training characteristics (Genesis) stones are divided into:

  • Igneous or eruptive;
  • Sedimentary;
  • Metamorphic.

The three geological classes which are divided rocks include materials with different morphological and chemical structure and different physical (porosity, firmness and hardness) and mechanical characteristics (strength, workability and divisibility). According to the following specifications, various degrees of workability of the stone material are defined. Generally compact and hard materials are better suited to more complex processes of cutting, shaping and polishing, while the brittle and porous materials (having a lower hardness) are less suited to detail work.

Eruptive or igneous rocks: these are the most common rocks, formed by upwelling and subsequent solidification of magma from the depths of the terrestrial crust, they consist mainly of silicates. The different magma cooling speed and methodology results in a structural distinction within the rocks and then their several properties. Si distinguono pertanto:

  • Effusive rocks: these rocks form when the cooling takes place rapidly in the vicinity of the volcanic vents create fine-grained rocks, texture and finer structure (porphyry [Figure 1] [Figure2] Basalt, rhyolite, Andesite, Pumice [Figure 3]). The cooling rate of the magma is proportional and immediate consequence of the fineness of the crystals;
  • Intrusive rocks: form when magma cools more slowly and to great depths in underground chambers in the Earth's crust. They have a coarse structure and, depending on the specific chemical composition, there are Granite [Figure 4], Sieniti [Figure 5] Gabbros [Figure 6] and Dioriti.

These rocks are generally quite hard and durable, but prolonged exposure to chemical agents during rainfall and causes a meteoric degradation process that can be divided into two processes:

  • Physical breakdown (or mechanical) is fragmenting into smaller particles of original rocks without modifying the internal structure of the material;
  • Chemical weathering is the selective alteration of some components that form the structure of the material resulting in a mutation of some properties and the weakening of the rock itself. Some of these processes are the hydrolysis and oxidation.

These rocks are mainly employed in the construction industry due to their special characteristics of resistance to degradation, mechanical strength and excellent aesthetic qualities.

Sedimentary rocks: are types of rocks that derive from the deposition process. It consists in the removal of rock particles, organic matter, or by precipitation of salts by meteoric agents and by gravity. Sedimentary rocks can incorporate crystals that separate later for precipitation and evaporation of the water in which they are dissolved as a solution. Diagenesis is the set of physical and chemical processes that convert a detritus in a rock by the lithification. Sedimentary rocks can be of different types:

  • Calcareous (Limestone [Figure 7] e Dolomite);
  • Siliciclastic (Sandstone [Figure 8], Breccia);
  • Sulfates (Gypsum);
  • Mixed (clays, volcanic tuffs).

This type of rock is used within the construction industry. Limestone, for example, is used as a product for binders. Sedimentary formations often occur with characteristic features that define the different types.

Metamorphic rocks: derive from the transformtion of existing igneous or sedimentary rocks by the action of chemical, physical or mechanical agents at elevated temperatures (300-800° C) and pressure that causes recrystallization and overbuilding in the process called metamorphism. In some cases during this process the reactions may altering both the structure and the composition of the rocks.

Because of the particular way in which form these rocks are often in crystal oriented in a texture defined “scistose”, this is crucial for the possibility of being divided, in production, according to specific directions. Belong to this type of stone Slates [Figure 9], quartzite [Figure 10], Gneiss, marbles [Figure 11] and Serpentino.

These rocks are often used in the construction industry as roofing, exterior or flooring due to their excellent properties of resistance and durability and high aesthetic qualities.