The magma forms inside the Earth, in places where the conditions of pressure and of temperature allow the fusion of the rocks. Normally the magma is less dense than the encasing, enveloping rocks, and so it eventually ascends, approaching the surface. In doing so, it becomes and consolidates, forming the magmatic or igneous rocks.
When it consolidates at the surface, the resulting rock is called extrusive or volcanic magma. On the contrary, by solidifying in depth, it forms intrusive or plutonic igneous rocks. Examples are, respectively, basalt and granite.
Texture of magmatic rocks
The most abundant magmatic rocks in the earth's crust, the two examples above, can be classified according to the ability to observe the crystals that constitute them.
If it is possible to observe the naked eye the crystals, as in the case of granites, we are faced with a farenitic texture. This can be subdivided according to grain size, ranging from fine grain (<2mm) to very thick grain (> 30mm)
If it is only possible to observe the crystals using, for example, a composite optical microscope (MOC), we are faced with an aphitic texture. This is subdivided into microcrystallins (identifiable to MOC) and cryptocrystallins (not visible to MOC)
The Granite - An Ornamental Rock
The term ornamental rock designates the rocks that can be polished and used in the construction industry as flooring, walls, statues, tables and the like. Unpolished rocks used for decorative purposes are called semi-ornamental and are usually included in the above concept.
Granite is an abundant rock in the north of Portugal, mainly the gray / white color (represented by number 4 in figure 3) but in the Portuguese territory can appear of several colors and textures: Although with notorious differences, all the granites are constituted by feldspar, quartz and mica:
Feldspar - Feldspar is an important family of minerals from the group of tectosilicates, constituents of rocks that form about 60% of the earth's crust. They crystallize in the triclinic or monoclinic system and have a predominantly white color
Quartz - Quartz is the second most abundant mineral on planet earth, occupying about 12% of its entire volume. There are several varieties of quartz, some coming to be considered semiprecious stones. There are several colors but the most abundant ones are white / transparent
Mica - Mica includes several closely related minerals from the phyllosilicate group, which have highly perfect basal division. All are monoclinic crystals, with a pseudo-hexagonal tendency, and are similar in chemical composition.
Texture of Granites:
According to the size relationship between the crystals that make up the rock, the granite can be classified as:
- Equigranular, when the crystals have similar sizes;
- Inequigranular, when the crystals have different sizes. Usually, in these situations, there are larger crystals that are surrounded by smaller crystals, forming a notorious phaneritic texture;
According to the shape relationship of the crystals that make up the rock, granite can be classified as:
- Idiomorphic (equigranular or unequigranular), when most crystals have a regular geometric shape;
- Hipidiomorphic (equigranular or unequigranular), when part, about half, of the crystals have regular geometric shape;
- Alotriomórficas (equigranular or unequigranular), when the crystals have all (more than 95%) irregular geometric forms;