For the description of rocks, external features are of great importance, reflecting the features of their structure. Such signs are divided into two groups: the first describe the structure of the rock, and the second, on which we will dwell in more detail here, relate to textural features.
The concept of the structure and texture of rocks
The structure reflects the state of the rock-forming mineral substance and is associated with the very process of crystallization and destruction of minerals, that is, with a change in the substance during the formation of the rock. Structural features include such characteristics of the rock as the degree of crystallinity, as well as the absolute and relative size of the grains that make up the rock and their shape.
The texture of a rock is a set of features that characterize its heterogeneity - in other words, how the structural elements fill the space in the rock, how they are distributed and oriented to each otherrelative to a friend. The appearance of texture is associated with the relative movement of rock components during its formation. The shape of rock fragments is also important in describing the features of its composition.
Texture classification and rock genesis
Different types of rock textures are classified according to the following criteria:
- Mutual arrangement of rock grains. There are homogeneous (massive) and heterogeneous textures. The latter, in turn, are of several types: banded, gneiss, schlieren, fluidal, etc.
- Degree of space filling. The texture can be dense or porous of one nature or another (slag, miarolitic, almond-stone, spherical).
The texture of rocks, as well as their structure, depends on the origin. According to this criterion, rocks are divided into igneous, sedimentary and metamorphic. They differ in their chemical and mineralogical composition and formation conditions. Each of them has its own textural features. Therefore, we will consider the types of textures in more detail for each class of rocks separately.
Igneous rocks
The formation of rocks of this type occurs during the solidification of magmatic melts. Depending on the conditions of this process, the emerging rocks are divided into two types. The structures and textures of the igneous rocks that belong to them differ with similar chemical and mineral composition.
- Intrusive rocks are formed as a result ofslow crystallization of magma in the deep regions of the earth's crust.
- Effusive rocks are formed by the rapid cooling of lava - magma erupted to the surface, and other volcanic products (ash).
About half of our planet's crust is made up of both types of igneous rocks.
How igneous rocks are composed
The texture of magmatites is a reflection of the dynamics of magma movement and the intensity of its physical and chemical interaction with the host stratum.
If the textures of rocks are formed simultaneously with the solidification of the magmatic melt, they are said to be syngenetic, including massive, spherical, directive, porous. The spherical texture is characterized by the presence of spherical or ellipsoidal formations in the rock; directive - by the presence of subparallel oriented grains of a flattened or elongated configuration.
In cases where there is a change in the primary breed, the resulting texture is called epigenetic. Examples include amygdalic texture (formed when bubbles and pores are filled with hydrothermal products) or breccia texture (formed when irregularly shaped fragments of another magmatite accumulate in the rock).
The origin of textures can be endogenous, associated with the processes of rock crystallization itself, or exogenous, depending on the action of external factors.
Textural features of intrusive rocks
The most common textures characteristic of intrusions are:
- massive with uniform distribution and random grain orientation (example - dunites, syenites, diorites, sometimes granite, gabbro);
- schlieren with the presence in the rock of areas of a different mineralogical composition and structure;
- banded (gneiss or directive), characterized by alternating bands with different structure or mineral composition (migmatites, sometimes granite, gabbro);
- miarolic with the presence of cavities in the rock mass formed by the faces of crystalline grains.
Textures of igneous rocks of effusive origin
Volcanic rocks most often have textures such as:
- Porous, bubbly and pumice. They have more or less numerous voids that have arisen as a result of degassing of magma when it emerges from the bowels to the surface. So, in pumice (pumicite), the porosity can reach 80%.
- Almond stone. Pores in effusive rock can be filled with chalcedony, quartz, chlorite, carbonates.
- Globular (typical for pillow lavas).
- Shaly (found in schistose igneous rocks).
- Fluid - texture in the form of a flow in the direction of lava movement. Inherent in glassy volcanic rocks.
Sedimentary rocks
There are three sources of sedimentary rock:
- redeposition of erosion products;
- precipitation from water;
- activities of various living organisms.
Accordingly, depending on the conditions and mechanism of formation, rocks of this type are divided into clastic, chemogenic and organogenic. There are also breeds of mixed origin.
The genesis of sedimentary rocks includes three stages:
- Diagenesis is the process of converting loose sediment into rock.
- Catagenesis is the stage at which the rock undergoes chemical, mineralogical, physical and structural changes. The result of catagenesis is dehydration, compaction and partial recrystallization of the rock.
- Metagenesis is a stage transitional to metamorphization. There is a maximum compaction of the rock, the transformation of the mineral composition and structure with further recrystallization until the remains of living organisms contained in the rock disappear.
The structure and texture of sedimentary rocks are determined by both primary factors that act during sedimentation (sedimentation) and secondary factors that come into effect at one stage or another of the rock genesis.
Textural features of sedimentary rocks
This type of rocks is characterized by compositional features, grouped according to two main features: intralayer and layer surface textures.
The mutual arrangement of the sedimentary rock components within the layer forms such types of textures as:
- random (typical, for example, of coarse clastic conglomerates);
- layered of various types: oblique, wavy, flysch,horizontal (most common);
- tubular or vacuolar, containing voids formed by decomposed plant remains (found in freshwater limestones);
- spotted texture of several varieties: streaky, zonal, flaky, scaly, etc.;
- patterned, characteristic of clays containing large mineral grains;
- fluidal, or turbulence texture with traces of disturbed primary orientation of structural elements.
The surface textures of the layer, resulting from short-term changes in the sedimentation environment, followed by rapid burial of the layer, are imprints left by precipitation or animals, ripple marks formed by winds, currents or waves of water flow, drying cracks and other traces.
In general, the textures of rocks of sedimentary origin are very diverse due to the high variability of the conditions under which they are formed.
Metamorphic rocks
They are formed in the thickness of the earth's crust by changing igneous and sedimentary rocks under the influence of physical (high pressures and temperatures) and chemical factors. The process of rock transformation is called metamorphism; in the case of a significant change in the chemical composition, it is customary to speak of metasomatism.
Rocks of this class are grouped according to the so-called metamorphism facies - aggregates within which they can have a different composition, but form under certain similar conditions. Structure and texture of metamorphicrocks reflect the features of the processes of recrystallization of the original sedimentary or igneous material.
Features of the addition of metamorphic rocks
The textures of metamorphosed rocks are of the following types:
- massive (found, for example, in deep zones of metamorphism and in metasomatic rocks of igneous origin that have retained their original texture);
- spotted - the result of contact-thermal metamorphism (spotted schists, hornfelses);
- almond stone (weakly metamorphosed rocks, sometimes amphibolites);
- banded (gneiss) with different mineral composition of alternating bands;
- slate is the most common texture of metamorphic rocks.
Slate texture occurs under the influence of directional pressure. It has such varieties as flaky - in cases where schistosity is complicated by very small folds - and lenticular (or spectacled, with inclusions of quartz or feldspar) textures.
In addition, metamorphic rocks often exhibit various types of deformation textures such as boudinage.
On the differentiation of concepts
It should be noted that there is no clear separation of interpretations of such closely related concepts as the structure and texture of rocks. In the structure of the rocks, there are signs that can be classified in two ways: for example, the amygdalic composition of the rock is sometimes referred to as structural characteristics. Another example is ooliticlimestones, for which it is difficult to distinguish features associated with the shape, size and structure of mineral grains - oolites.
The terminological ambiguity of these concepts is also manifested in the opposite meaning of the use of the terms "structure" and "texture" in the English tradition. In international publications, as a rule, the generalized concept of "structural and textural features" is used, without separating the features of the structure and composition of rocks.
Nevertheless, the correct description of the texture of rocks is very important for solving many problems, for example, determining the physical properties or elucidating the genesis of rocks and the dynamic conditions for their formation.
