Texture (geology)
The term structure is understood in geology as a term of geometric properties of the individual rock components. These include relative and absolute size, the shape of the crystals and the type of grain Association. The spatial arrangement of the components is not described, these properties are summarized under the term texture. [A 1]
Properties of the rock constituents
Crystallinity
Rocks whose minerals are crystallized directly from a melt or solution, have a crystalline structure. Quick cooled volcanic glasses without crystals have a hyaline ( amorphous ) structure. Is not used, the term crystallinity in sedimentary rocks that are made up of the detritus of other rocks or from skeletal remains of living organisms.
Grain size
A distinction is made between the relative and the absolute grain size. Assigns the vast number of minerals have the same grain size, refers to a rock as the same grainy. There are clear differences in the grain size, a rock is called a non- grainy. For non- granular rocks technical terms are used, which are based on the creation of the rocks: Plutonite with all grain sizes between are coarse and fine- grained than porphyritic, volcanics with individual mineral phenocrysts in a fine ground mass as porphyritic and metamorphic rocks with coarse components in a schistose matrix called porphyroblastisch.
The absolute grain size refers to the diameter of the minerals. Unconsolidated rocks are classified according to DIN 4022, while hard rocks are broken down by a slightly simpler scheme in finest-grained, fine-grained, medium -grained and coarse-grained rocks.
Shape of the components
Crystallized from melts or solutions, or by recrystallization during metamorphism minerals formed either show their characteristic crystal habit and then as idiomorphic (intrinsically polymorphic ) refers. Otherwise, their shape is determined by the available, by neighboring grains limited space, which is why these minerals are then called xenomorph ( alien multiform ). In sedimentary rocks, which are constructed by mechanically rearranged mineral grains, minerals may well rounded depending on transport distance and hardness, rounded at the edges, or formed shape. In addition, for example corals, shells or brachiopods may occur in such quantities that these fossils are the main components of a rock.
Grain structure
The constituents of a rock may be unconnected (→ unconsolidated rocks ) or permanently connected (→ hard rocks ). For hard rocks, a distinction between indirect and direct grain structure. In indirect grain structure, which is essentially limited to sedimentary rocks, the individual grains are cemented together by a binder. This binder can be of the same material as the mineral grains themselves, from a through pore water supplied from the outside in some foreign substance or igneous rocks are made of rock glass. In the direct crystallization of rocks from solutions and melts is always a close grain structure. The cohesion of the rock is ensured firstly by the teeth of the minerals with each other and by interfacial forces between the mineral surfaces.