Structure (geology)
The term structure is understood in geology to denote the geometric properties of the individual rock components . This includes relative and absolute size as well as the shape of the grains and the type of grain association. The spatial arrangement of the components is not described, these properties are summarized under the term texture .
Properties of the rock components
Crystallinity
Rocks whose minerals directly from a melt or solution are crystallized, have a crystalline structure (see FIG. → crystallite ). Extremely quickly cooled (quenched) volcanic glasses without crystals have a hyaline (amorphous) structure. The term crystallinity is not used for sedimentary rocks that are built up from the debris removed from other rocks or from skeletal remains of living things. In contrast, the term "grain" for the smallest individual rock components is not only common in sedimentary rocks.
Grain size
A distinction is made between the relative and the absolute grain size. If the majority of the minerals have the same grain size, a rock is referred to as having the same grain. If there are significant differences in grain size, a rock is referred to as unevenly grained. For uneven -grained rocks, technical terms are used that depend on the formation of the rocks: Plutonites with all grain sizes between coarse and fine-grained are porphyry-like, volcanites with individual mineral inserts in a fine matrix as porphyry and metamorphic rocks with coarse components in a shale matrix referred to as porphyroblastic.
The absolute grain size relates to the diameter of the minerals. Loose rock is classified according to DIN 4022 , while solid rock is divided into fine-grain, fine-grain, medium-grain and coarse-grain rocks according to a somewhat simpler scheme.
Shape of the components
From melts or solutions crystallized or by recrystallization in the metamorphosis minerals formed either show their characteristic crystal habit and are then used as idiomorph ( intrinsically multiform hereinafter). Otherwise, their shape is determined by the space that is available and limited by neighboring grains, which is why these minerals are then called xenomorphic ( foreign shapes ). In sedimentary rocks that are built up by mechanically rearranged mineral grains, minerals can be well rounded, only rounded at the edges or angular, depending on the transport distance and hardness. In addition, for example, can coral , shells or brachiopods occur in such quantities that these fossils are the main components of a rock.
Grain association
The components of a rock can be unconnected (→ loose rock ) or firmly connected (→ solid rock ). In solid rock, a distinction is made between indirect and direct grain association. In the case of an indirect grain association, which is essentially limited to sedimentary rocks, the individual grains are cemented by a binding agent . This binding agent can consist of the same mineral substance as the grains themselves, of a foreign substance brought in from outside via pore water or, in some igneous rocks, of rock glass. The direct crystallization of rocks from solutions and melts always creates a direct grain association. The cohesion of the rock is ensured on the one hand by the interlocking of the minerals with one another and by the interface forces between the mineral surfaces.
literature
- Siegfried Matthes: Mineralogy . Springer, 1990, ISBN 3-540-51912-2 .
- Peter Kukla (Ed.): Geology - Map Studies, materials for the geological exercises . Wissenschaftsverlag Mainz, Aachen 2002, ISBN 3-86073-777-5 .
Web links
- Collection of leaflets on the geology exercise at RWTH Aachen University ( memento from July 8, 2008 in the Internet Archive ), accessed on February 19, 2010
Remarks
- ↑ In the English-language specialist literature, the terms texture and structure are used in the opposite sense