利用者:加藤勝憲/テクスチャ (地質学)
Texture (geology) 2022-12-26T21:02:14 Klbrain トーク 投稿記録 17,506バイト +14,802 Merge from Rock microstructure following January proposal with consensus; see Talk:Rock microstructure#Proposal to merge texture (geology) with this page
地質学において、テクスチャまたは岩石の微細構造[1]は、岩石を構成する材料間の関係を指す[2]。最も広いテクスチャクラスは、結晶性（コンポーネントが相互に成長し、連動する結晶）、断片的（何らかの物理的プロセスによって断片が蓄積される）、無顕晶質（肉眼では結晶が見えない）、およびガラス状である （粒子が小さすぎて見えず、アモルファスに配置されている）[2]。構成粒子または結晶間の幾何学的側面および関係は、結晶学的テクスチャーまたは優先配向と呼ばれる。テクスチャはさまざまな方法で定量化できる [3]。
最も一般的なパラメータは結晶サイズ分布で、これにより、粒子サイズ、形状、配置、その他の特性など、岩石の物理的な外観や特徴が、目に見えるスケールと微視的なスケールの両方で作成される。
結晶テクスチャは含まphaneritic 、葉状、および斑状[2]。幻影のテクスチャは、火成岩のかみ合う結晶が肉眼で見える場所です。葉状のテクスチャは、変成岩が材料の層でできている場所である[2]。ポルフィライトテクスチャは、より大きな粒子（斑晶）がはるかに細かい粒子でできた背景の塊に埋め込まれているテクスチャである[2]。
断片的なテクスチャには、砕屑性、生物砕屑性、および火砕性が含まれる[2]。
好ましい鉱物配向は、その粒子が平らな形状（不定量）であり、それらの平面が同じ方向に配向する傾向がある変成岩のテクスチャーである[4]。
In geology, texture or rock microstructure[5] refers to the relationship between the materials of which a rock is composed.[2] The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye), and glassy (in which the particles are too small to be seen and amorphously arranged).[2] The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways.[6] The most common[要出典] parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.
Textures are penetrative fabrics of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand-specimen, and often outcrop scale. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on a hand-specimen scale and above.
Microstructure analysis [7] describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding, or alteration events.[8]
Crystalline textures include phaneritic, foliated, and porphyritic.[2] Phaneritic textures are where interlocking crystals of igneous rock are visible to the unaided eye. Foliated texture is where metamorphic rock is made of layers of materials.[2] Porphyritic texture is one in which larger pieces (phenocrysts) are embedded in a background mass made of much finer grains.[2]
Fragmental textures include clastic, bioclastic, and pyroclastic.[2]
A preferred mineral orientation, is the texture of metamorphic rock in which its grains have a flattened shape (inequant), and their planes tend to be oriented in the same direction.[4]
Nomenclature
[編集]Rock microstructure includes the texture and small-scale structures of a rock. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.
Sedimentary microstructures
[編集]Description of sedimentary rock microstructure aims to provide information on the conditions of deposition of the sediment, the paleo-environment, and the provenance of the sedimentary material.
Methods involve description of clast size, sorting, composition, rounding or angularity, sphericity and description of the matrix. Sedimentary microstructures, specifically, may include microscopic analogs of larger sedimentary structural features such as cross-bedding, syn-sedimentary faults, sediment slumping, cross-stratification, etc.
Maturity
[編集]The maturity of a sediment is related not only to the sorting (mean grain size and deviations), but also to the fragment sphericity, rounding and composition. Quartz-only sands are more mature than arkose or greywacke.
Fragment shape
[編集]Fragment shape gives information on the length of sediment transport. The more rounded the clasts, the more water or wind-worn they are. Particle shape includes form and rounding. Form indicates whether a grain is more equant (round, spherical) or platy (flat, disc-like, oblate); as well as sphericity.
Roundness
[編集]Roundness refers to the degree of sharpness of the corners and edges of a grain. The surface texture of grains may be polished, frosted, or marked by small pits and scratches. This information can usually be seen best under a binocular microscope, not in a thin section.
Composition
[編集]Composition of the clasts can give clues as to the derivation of a rock's sediments. For instance, volcanic fragments, fragments of cherts, well-rounded sands all imply different sources.
Matrix and cement
[編集]The matrix of a sedimentary rock and the mineral cement (if any) holding it together are all diagnostic.
Diagenetic features
[編集]Usually diagenesis results in a weak bedding-plane foliation. Other effects can include flattening of grains, pressure dissolution and sub-grain deformation. Mineralogical changes may include zeolite or other authigenic minerals forming in low-grade metamorphic conditions.
Sorting
[編集]Sorting is used to describe the uniformity of grain sizes within a sedimentary rock. Understanding sorting is critical to making inferences on the degree of maturity and length of transport of a sediment. Sediments become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported.[9] This results in sorting due to density. Sorting can be expressed mathematically by the standard deviation of the grain-size frequency curve of a sediment sample, expressed as values of φ (phi). Values range from <0.35φ (very well sorted) to >4.00φ (extremely poorly sorted).
Metamorphic microstructure
[編集]The study of metamorphic rock microstructures aims to determine the timing, sequence and conditions of deformations, mineral growth and overprinting of subsequent deformation events.
Metamorphic microstructures include textures formed by the development of foliation and overprinting of foliations causing crenulations. The relationship of porphyroblasts to the foliations and to other porphyroblasts can provide information on the order of formation of metamorphic assemblages or facies of minerals.
Shear textures are particularly suited to analysis by microstructural investigations, especially in mylonites and other highly disturbed and deformed rocks.
Foliations and crenulations
[編集]On the thin section and hand specimen scale a metamorphic rock may manifest a planar penetrative fabric called a foliation or a cleavage. Several foliations may be present in a rock, giving rise to a crenulation.
Identifying a foliation and its orientation is the first step in analysis of foliated metamorphic rocks. Gaining information on when the foliation formed is essential to reconstructing a P-T-t (pressure, temperature, time) path for a rock, as the relationship of a foliation to porphyroblasts is diagnostic of when the foliation formed, and the P-T conditions which existed at that time.
Lineations
[編集]Linear structures in a rock may arise from the intersection of two foliations or planar structures, such as a sedimentary bedding plane and a tectonically induced cleavage plane. The degree of lineation compared with the degree of foliation for certain strain markers in deformed rocks are commonly plotted on a Flinn diagram.
Ductile shear microstructures
[編集]Very distinctive textures form as a consequence of ductile shear. The microstructures of ductile shear zones are S-planes, C-planes and C' planes. S-planes or schistosity planes are parallel with the shear direction and are generally defined by micas or platy minerals. Define the flattened long-axis of the strain ellipse. C-planes or cissalement planes form oblique to the shear plane. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain. The C' planes are rarely observed except in ultradeformed mylonites, and form nearly perpendicular to the S-plane.
Other microstructures which can give sense of shear include
- sigmoidal veins
- mica fish
- rotated porphyroblasts
Igneous microstructure
[編集]Analysis of igneous rock microstructure may complement descriptions on the hand specimen and outcrop scale. This is especially vital for describing phenocrysts and fragmental textures of tuffs, as often relationships between magma and phenocryst morphology are critical for analysing cooling, fractional crystallization and emplacement.
Analysis of intrusive rock microstructures can provide information on source and genesis, including contamination of igneous rocks by wall rocks and identifying crystals which may have been accumulated or dropped out of the melt. This is especially critical for komatiite lavas and ultramafic intrusive rocks.
General principles of igneous microstructure
[編集]Igneous microstructure is a combination of cooling rate, nucleation rate, eruption (if a lava), magma composition and its relationships to what minerals will nucleate, as well as physical effects of wall rocks, contamination and especially vapor.
Grain texture
[編集]According to the texture of the grains, igneous rocks may be classified as
- pegmatitic: very large crystals
- phaneritic: rocks contain minerals with crystals visible to the unaided eye, commonly intrusive
- aphanitic: rapid cooling, crystal nucleation and growth is stunted, forming a uniform, fine grained rock
- porphyritic: containing phenocrysts in a fine groundmass
- vesicular: contains voids caused by trapped gas while cooling
- vitreous: glassy or hyaline without crystals
- pyroclastic: rock formed of fragments of crystals, phenocrysts and rock fragments of a volcanic origin
- equigranular: rock crystals are all the same size
参照
[編集]参考文献
[編集]- ^ Vernon, R. H. (2004). A practical guide to rock microstructure. Cambridge University Press. ISBN 0-521-81443-X
- ^ a b c d e f g h i j k l Texture & Genesis of Rocks, Introductory Geology Laboratory, Christopher DiLeonardo, Ph.D., Marek Cichanski, Ph.D., Earth & Space Sciences, De Anza College
- ^ Higgins, M. D. (2006). Quantitative Textural Measurements in Igneous and Metamorphic Petrology. Cambridge: Cambridge University Press. ISBN 0-521-13515-X
- ^ a b Essentials of Geology, 3rd Ed, Stephen Marshak
- ^ Vernon, R. H. (2004). A practical guide to rock microstructure. Cambridge University Press. ISBN 0-521-81443-X
- ^ Higgins, M. D. (2006). Quantitative Textural Measurements in Igneous and Metamorphic Petrology. Cambridge: Cambridge University Press. ISBN 0-521-13515-X
- ^ Fu, Jinlong; Thomas, Hywel R.; Li, Chenfeng (January 2021). “Tortuosity of porous media: Image analysis and physical simulation”. Earth-Science Reviews 212: 103439. doi:10.1016/j.earscirev.2020.103439.
- ^ Voznesensky, A. S.; Kidima-Mbombi, L. K. (2021-07-14). “Formation of synthetic structures and textures of rocks when simulating in COMSOL Multiphysics”. Gornye Nauki i Tekhnologii = Mining Science and Technology (Russia) 6 (2): 65–72. doi:10.17073/2500-0632-2021-2-65-72. ISSN 2500-0632.
- ^ Nelson, Stephen A.. “Sedimentary Rocks”. Tulane University - Earth & Environmental Sciences. 3 July 2012時点のオリジナルよりアーカイブ。8 April 2021閲覧。
[[Category:岩石学]]