Texture and Structure of Rocks

the nature of the makeup of rocks from minerals and mineral aggregates. Texture is determined by the dimensions, shape, and interrelationships of the minerals; structure depends on the general features and spatial arrangement of the larger components of the rock (mineral aggregates).

Texture and structure of igneous rocks. The texture of igneous rocks depends on the composition of the magma and the conditions surrounding the magma’s cooling. The textures are different in intrusive, vein, and extrusive rocks. Intrusive rocks are characterized by a holocrystalline texture, in which all the rock material is crystallized. The presence of volatile components in the magma lowers the crystallization temperature and the viscosity of the magma, thereby improving crystallization. Thus, the crystallization of acidic magma at considerable depths with slow cooling and the retention of the volatile components give granular holocrystalline rocks, for example, granites. A distinction is made between holocrystalline rocks with phaneritic texture, in which the crystalline constituents can be seen with the unaided eye, and holocrystalline rocks with aphanitic texture, in which the constituents are discernible only under a microscope. Phaneritic textures are subdivided according to grain size into finegrained (crystals less than 1 mm), medium-grained (1–5 mm), coarse-grained (5–10 mm), and very coarse-grained (more than 10 mm). Texture also depends on the shape of the crystals of the component minerals. In some cases, the minerals have crystallo-graphic shapes and form idiomorphic crystals, while in others the minerals lack a distinctive form and are called allotriomorphic, or xenomorphic. A mineral may be idiomorphic in relation to some minerals and xenomorphic in relation to others. Given the idio-morphism of most minerals, the textures of intrusive rocks are described as panidiomorphic-granular (pyroxenites, peridotites, dunites). Textures resulting from a combination of major rock-forming minerals of differing degrees of idiomorphism are called hypidiomorphic-granular (granites, syenites, diorites). Rocks with anhedral textures result from crystal growth that prevents the assumption of outward crystal form. Simultaneous crystallization from a melt of feldspar and quartz produces a pegmatitic, or graphic, texture with intergrowths of the minerals. Depending on the relative size of the crystals, a distinction is made between equigranular and inequigranular textures, and in the latter type there is an additional distinction between porphyritic and por-phyro-aphanitic textures. The porphyro-aphanitic texture is one in which a fine-grained or medium-grained groundmass contains large porphyritic insets of separate minerals (porphyritic phenocrysts).

In classifying the structures of intrusive rocks, the first category applies to massive, or homogeneous, structures, in which the minerals are uniformly distributed in rock having approximately the same composition and texture throughout. Heterogeneous (taxite) structures are also very common. Banded and fluidal structures with minerals oriented in a particular fashion arise under conditions of movement of the crystallizing magma. Taxite structures may result from the nonuniform distribution of colored minerals (hornblende, biotite) or from the alternating arrangement of segments of different granularity.

Vein and extrusive rocks are characterized by a porphyritic texture, which is brought about by the rapid crystallization of the magma related to cooling and a loss of volatile components. This texture is sometimes observed around the edges of intrusive bodies. It results from the presence in rock of a dense (aphanitic) groundmass containing large mineral insets—phenocrysts. The texture of extrusive rocks not containing phenocrysts is described as aphanitic. Depending on the ratio of glass to crystals (microlites), the groundmass texture is classified as glassy (or vitrophy-ric), semicrystalline (for example, hyalopilitic texture), or micro-litic. The degree of crystallinity in extrusive rocks depends on the composition of the magma and the geological conditions of the magma’s crystallization. On the earth’s surface, lava cools rapidly with the loss of volatile components. Acidic and intermediate lavas (rhyolite, andesite lavas) form semicrystalline and glassy rocks, the glassy groundmass of which contains fine (tenths and hundredths of a mm) microlites. Basic, more liquid lavas solidify on the earth’s surface as semicrystalline rocks.

The structures of extrusive rocks are classified as massive or fluidal or as structures exhibiting flow banding, which results from a parallel arrangement of variously colored bands of volcanic glass, phenocrysts, and microlites. Depending on the quantity of gas bubbles in the lava, distinctions are made between porous, vesicular, and pumiceous structures. Amygdaloidal structures are formed upon the filling of the cavities with secondary minerals (quartz, opal, zeolites, carbonates).

Texture and structure of sedimentary rocks. The relationship between rock structure and texture and rock genesis is more pronounced in sedimentary rocks than in igneous rocks. Clastic rocks consist of detrital (clastic) grains of various sizes and shapes. The grains, which can be angular, subrounded, or rounded, sometimes lie freely without attachment to one another by any binder (cement), while in other cases the grains are to a greater or lesser extent cemented by silica (opal, chalcedony), phosphates, calcium carbonates, magnesium carbonates, and other minerals.

The structure of clastic rock, which depends on the mutual arrangement of the grains, can be random, laminar, or fluidal. With a random structure, the particles do not have an ordered arrangement. This structure is characteristic of very coarse-grained rocks, such as gravel, shingle, and sands, but is also encountered in certain fine-grained rocks. A random structure arises in regions of sedimentation characterized by an abundant and continuous supply of homogeneous detrital material or by a continuous roiling of the precipitate. With a laminar structure, the individual layers are distinguished from one another by the composition and size of the particles. A fluidal structure results from the secondary destruction of the original laminar structure of the precipitate by underwater (and terrestrial) landslides, by strong wave action, or by the collapse of the laminar structure caused by burrowing animals. Fluidal structures are rare.

The textures and structures of biogenic rocks are especially varied in the most common carbonate rocks—limestones and dolomites. Where the remains of organisms, of which the rock is chiefly composed, are well preserved, the texture will depend entirely on the type of organism. Textures of this type are described as biomorphic or whole-shell. The remains of organisms which are usually separate from one another, are gradually bound together by a cement having a different mineralogical composition or texture. This difference is seen in oyster, brachiopod, and la-mellibranch coquinas. In some cases, there is a buildup of organisms, and a growth structure results. Structures of this type are characteristic of corals, bryozoans, calcareous algae, and hydrac-tinians. The buildup of organisms produces either a flat body lying along the bottom with a gently undulating surface (stromatolite) or a small, oval mass resembling a concretion (oncolite). Bodies that grow in the shape of hills or high knolls are called bioherms. Coral reefs are usually a combination of stromatolites, oncolites, and bioherms, with a predominance of the last.

Biogenic clastic, or detrital, textures are clearly differentiated from biomorphic textures when the biogenic rock is composed of angular or rounded fragments of organisms. Detrital textures are formed in shallow areas by wave action, which destroys the shells. Predators that feed on shellfish and crush the shells of their prey play a large role in the formation of detrital textures.

Textures of recrystallization and metasomatism are characteristic of biogenic rocks. Recrystallization is accompanied by a brightening of individual segments of the rock that imparts a mottled or brecciated character (pseudobreccia). With metasomatism, part of the calcic cement and shells is replaced by dolomite or chalcedony, which process results in a patchy appearance.

The texture and structure of chemogenic rocks are characterized by the development of crystalline grains of various sizes. For sizes less than ü.0ü1 mm, the grains are not visible even under a microscope; such a texture is called amorphous or colloidal. Ma-croscopically, the rock is homogeneous and dense and has a characteristic conchoidal fracture. Grains with sizes ranging from 0.001 to 0.01 mm are discernible under a microscope (micro-grained texture), but the external appearance and conchoidal fracture of the rock are retained. For grain sizes of 0.01 to 0.1 mm, the texture is called fine-grained, but the grains are still not visible macroscopically. For grain sizes of 0.1 to 0.5 mm, the texture is called medium-grained, and of 0.5 to 1.0 mm, coarsegrained; for grain sizes greater than 1 mm, the texture is called very coarse-grained. If the grains are of different sizes, the texture is described as inequigranular.

The most common structures in chemogenic rocks are the oolitic, massive, and laminar. The oolitic structure is characterized by rounded grains or grain aggregates and is typical of carbonate rocks (limestones, dolomites), iron, manganese, and phosphate ores and bauxites. The massive structure is observed in chemogenic rocks with a homogeneous composition (dolomites, limestones, gypsums, anhydrites). The laminar structure is formed by the alternating arrangement of layers either of rock of varying mineralogical composition or of chemogenic and layer-forming rocks (anhydrites, gypsums, rock salt, potassium salts).

Texture and structure of metamorphic rocks. The structures and textures of metamorphic rocks arise during the recrystallization in the solid state of primary sedimentary and magmatic rocks. The recrystallization occurs under the action of lithostatic pressure, temperature, and abyssal solutions (fluids), often in conditions of deformation, which leads to an ordered arrangement of the mineral grains that is characteristic of gneissic and schistose structures. The textures of metamorphic rocks, which are referred to as crystalloblastic, arise as the result of the growth of minerals (crystalloblasts) in a solid or plastic medium. Irregular grains (xenoblasts) predominate; grains with crystallographic shapes (idioblasts) are formed less frequently. A distinction is made between uniformly granular (homeoblastic) and nonuni-formly granular (heteroblastic) textures. A special case of heter-oblastic texture is seen in the porphyroblastic texture, characterized by the presence of large mineral crystals (porphyroblasts) within the fine-grained mass of the rock.

The minerals in metamorphic rocks are classified according to the shape of the grain as granoblastic or granular (quartzites, marbles), lepidoblastic or foliated, a category that is characteristic of rocks containing mineral grains with a foliated form (mica schists, phyllites), or lepidogranoblastic or granular-foliated. If metamorphic rocks retain relicts of the initial rock texture, the texture receives the name of the primary texture with the prefix “blasto-,” for example, blastoporphyritic and blastopsammitic. In metamorphic rocks, relicts of the structures of the original rocks may also be retained.