Unreferenced date December 2009 Image Almandine.jpeg thumb An almandine garnet growing as a porphyroblast in a quartzite quartzitic gneiss . The garnet measures 3 cm. Location Para ba , Brazil . Image Peridotite with pyrope porphyroblasts.JPG thumb Purple pyrope porphyroblasts in a peridotite , 1 euro coins coin of 1 euro for scale. The yellow brown matrix geology groundmass is weathering weathered olivine . Location Otr y , Western Gneiss Region , Caledonides , Norway . A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained Matrix geology groundmass . Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyroblasts in meta pelite s metamorphosed mudstone s and siltstone s are garnet s and staurolite s, which stand out in well Foliation geology foliated metapelites such as schist s against the platy mica matrix. A similar type of crystal is a phenocryst , a large crystal in an igneous rock . Porphyroblasts are often confused with porphyroclast s , which can also be large outstanding crystals, but which are older than the matrix of the rock. A rock which has many porphyroblasts is described as having a porphyroblastic texture . As porphyroblasts grow, the foliation may be preserved as oriented inclusions trapped by the porphyroblast as it overgrows them, and this is helpful for tracking changing deformation planes. In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by the matrix minerals. The relationship of porphyroblast growth to deformation is typically evaluated by comparing the shape orientation of trails of mineral inclusions in the porphyroblast to the matrix fabric. Some garnet porphyroblasts contain various curving trails of quartz and other ... Rock microstructure Category Metamorphic petrology de Porphyroblast he nl Porfyroblast ... more details
Unreferenced date April 2011 Orphan date February 2009 A poikiloblast is a porphyroblast mineral with small inclusion mineral inclusions of the previous rock in it. From the texture if any shown in the inclusion, the deformation history of the rock can be interpreted, as the inclusions are always older than the porphyroblast. Category Petrology petrology stub ... more details
Unreferenced auto yes date December 2009 Image Augen gneiss.jpg thumb 250px A Gneiss with large eye shaped feldspars Augen from German eyes are large, lenticular eye shaped mineral grains or mineral Aggregate composite aggregate s visible in some foliation geology foliated metamorphic rocks . In cross section they have the shape of an eye. Feldspar , quartz , and garnet are common minerals which form augen. Augen form in rocks which have undergone metamorphism and Shear geology shearing . The core of the augen is a porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by crystallisation of a mantle of new mineral around the porphyroblast. The mantle is formed contiguous with the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the porphyroblast within the strain shadows. During shearing, the poprhyroblast may rotate, to form a characteristic augen texture of asymmetric shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information. A metamorphic rock which is clotted with augen is often called an augen gneiss . A long wall of this augen gneiss can be felt at the Mineral and Lapidary Museum of Western North Carolina . See also Foliation Gneiss Rock microstructure Schist Shear geology Category Metamorphic petrology Petrology stub ... more details
Unreferenced date December 2009 image garnet porphyroblast.JPG thumb 350px right A mylonite showing a number of rotated porphyroclasts a clear red garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. Location the tectonics tectonic contact between the wiktionary autochthonous autochthonous Western Gneiss Region and rocks of the allochthon ous Bl h nappe on Otr y , Caledonides , Central Norway . A porphyroclast is a clast or mineral fragment in a metamorphic rock , surrounded by a groundmass of finer grained crystal s. Porphyroclasts are fragments of the original rock before dynamic recrystallisation produced the groundmass. This means they are older than the groundmass. They were Strength of materials stronger pieces of the original rock, that could not as easily deformation engineering deform and were therefore not or hardly affected by recrystallisation. They may have been phenocryst s or porphyroblast s in the original rock. Porphyroclasts are often confused with porphyroblasts. The latter are also large crystals in a finer matrix geology matrix , but they grew after deformation took place and after the matrix was formed. The timing of porphyroblast growth can be determined by examining the microstructure preserved or not within them as poikiloblasts. In strongly deformed rocks porphyroclasts are often rotation rotated by the shear stress in the rock. Their shape can be used to determine the direction of the shear. Category Metamorphic petrology nl Porfyroclast ru uk ... more details
Granoblastic is an Anhedral petrology anhedral phaneritic equi granular metamorphic rock texture. Granoblastic texture is typical of quartzite , marble and other foliation non foliated metamorphic rocks without porphyroblast s. Characteristics defining granoblastic texture include grains visible to the unaided eye, sutured boundaries and approximately equidimensional grains. The grain boundaries intersect at 120 triple junctions under ideal conditions. Variation from the ideal results from stress produced foliation during crystallization resulting in schistose textures. References http www.maden.hacettepe.edu.tr dmmrt index.html Dictionary of Mining, Mineral, and Related Terms http teachserv.earth.ox.ac.uk courses es2 metrock tex 1tex02.html Metamorphic Microstructures Category Metamorphic rocks petrology stub ... more details
Image Quartz zhaseni.gif thumb Undulose extinction of quartz in Gneiss Orthogneiss Image Quartz micro2.jpg thumb Undulose extinction or undulatory extinction is a geological term referring to the type of extinction that occurs in certain mineral s when examined in thin section under cross Polarization waves polarized light. As the microscope stage is rotated, individual mineral grains appear black when the polarization due to the mineral prevents any light from passing through. If a mineral is Plastic deformation in solids deformed plastically by dislocation processes without recovery metallurgy recovery , strain builds up within the crystal lattice causing it to warp. This means that different parts reach extinction at slightly different angles, giving the crystal an irregular, mottled look. ref http books.google.co.uk books?id 4jMq3iw XVUC&pg PA57&dq 22undulose extinction 22&lr &ei WUqISvXPEpu ygTO0f2yDg v onepage&q 22undulose 20extinction 22&f false Passchier, C.W. & Trouw, R.A.J. 2005. Microtectonics, 2nd edition, Springer, 366pp. ref Undulose extinction is very common in quartz , so much so that it is often used as a diagnostic feature of that mineral, and feldspar of various sorts, but is possible in almost any mineral. It is possible to identify the presence of a porphyroblast before the main deformation event in a metamorphic reaction, if a porphyroblast displays clear undulose extinction it is a clear sign that it existed before the main rock was created. References Reflist Commons Category Undulose extinction Category Optical mineralogy Petrology stub cs Undul zn zh en de Undul se Ausl schung es Extinci n ondulante ... more details
This page is intended to be a list of rock geology rock texture geology textural and morphology morphological terms. compactTOC5 A Cumulate rocks Adcumulate agglomerate Agglomeritic Lustre mineralogy adamantine Adamantine a type of Lustre mineralogy lustre Amygdule Amygdaloidal Anhedral petrology Anhedral Antitaxial veins Aphanitic Aplite Aplitic aplite Augen Augen textured gneiss Axiolitic texture B Botryoidal breccia ted Foliation geology Bedding fissile bedding fissility Boudinage Boudinage boudins C Cataclastic rock Cataclastic Chilled margin Clastic see also breccia Cleavage geology Cleaved crenulation Crenulated Cross bedding Cross stratification also trough cross stratification. Cumulate rock Cumulate see also layered intrusion D Decussate Devitirification devitrified Dendritic texture dendrites Schlieren Diatextite see also schlieren and migmatite E Embayed minerals see rock microstructure igneous texture Euhedral Eutaxitic Epiclastic F Fiamme Foliation geology Foliation Fissile see also Foliation geology Bedding fissility . Fossil iferous G Glomeroporphyritic Gneiss ose gneissic Granoblastic Granophyre Granophyric Granulite Granulitic Graphic texture Graphic H Holocrystalline Hyaline Hyaline texture I Imbricated Imbricate J Joint geology Jointed K L Lepidoblastic Leucocratic see also melanocratic Lineation geology Lineation , lineated Tectonite L tectonite M Cumulate rocks Mesocumulate mylonite Mylonitic N Nematoblastic O Ocelli oolite oolitic see also limestone Ophitic texture Orbicular texture Cumulate rocks Orthocumulate P Panidiomorphic Perthite Perthitic Phaneritic Phyllite Phyllitic Pisolite Pisolitic Porphyroclast ic Porphyroblast ic Porphyritic Porphyritic texture see also Porphyry geology porphyry Ptygmatic folding see migmatite Q Quench textures see obsidian , tachylyte or aphanitic R Rapakivi texture S Sand y Schistose schistosity Shear geology Shear sheared shear fabric slate Slaty cleavage texture slaty cleavage Specular Spinifex texture see al ... more details
File Fruchtschiefer, art object d.jpg thumb Artistic sculture using Theuma Fruchtschiefer on a column around 1960 File Theuma fruchtschiefer d.jpg thumb Theuma Fruchtschiefer, ground surface File Fruchtschiefer Theuma.jpg thumb right Fruchtschiefer with cordierite porphyroblast s Fruchtschiefer is a local variety of contact metamorphic rock that is derived from argillite . Formation and mineral content Fruchtschiefer is formed under very low pressure low grade metamorphic rock and temperatures of ca. 500 C. At these temperatures cordierite grows to the size of grains of corn, which is where its name comes from Frucht being German for corn as well as fruit . It contains the following minerals cordierite , muscovite , potassium feldspar , quartz , biotite and hornblende . Occurrence Fruchtschiefer occurs in Germany as contact metamorphic rock in the Harz , Ore Mountains , Odenwald and Vogtland . The best known deposits are those near Theuma and Tirpersdorf dating to the Ordovician . Rocks quarried here have been used well beyond the local region. Use The main traditional application is in the form of rough split masonry products of a wide variety. Fruchtschiefer from Theuma is also split or ground for use as facade slabs and plinth facings. Amongst the architectural components made in this material are window and door surrounds, columns and steps. Occasionally it is used for artistic or sculptural purposes. It was also used as canal lock and well covers as well as containers for galvanization galvanised baths and acids. The use of Theuma Fruchtschiefer can be traced back to the Middle Ages . Examples include slabs in the crypt of St. John s Church, Plauen St. John s Church in Plauen built 1122 , which date to the 14th century. References Georg Dehio Handbuch der Deutschen Kunstdenkm ler, Sachsen . Munich, Berlin Deutscher Kunstverlag 1990 ISBN 3 422 03016 6 C. G bert A. Steuer Karl Weiss Die nutzbaren Gesteinsvorkommen Deutschlands . Berlin Union Dt. Verl.ges 191 ... more details
of porphyroblast s to the foliations and to other porphyroblasts can provide information on the order ... a P T t pressure, temperature, time path for a rock, as the relationship of a foliation to porphyroblast ... which can give sense of shear include sigmoidal veins mica fish rotated porphyroblast s Igneous microstructure ... more details
polars of Garnet Mica Schist showing a rotated porphyroblast of garnet , mica fish and elongated minerals ... evidence of transtension includes rodding or Lineation geology stretching lineations , stretched porphyroblast ... more details
Image Mineraly.sk granulit.jpg right thumb 250px A sample of granulite facies metamorphic rock of felsic composition, with garnet porphyroblast s Granulites are medium to coarse grained metamorphic rock s that have experienced high temperature metamorphism, composed mainly of feldspar s sometimes associated with quartz and anhydrous mafic ferromagnesian minerals , with granoblastic texture and gneissose to massive structure. ref name Bowes D.R. Bowes 1989 , The Encyclopedia of Igneous and Metamorphic Petrology Van Nostrand Reinhold ISBN 0 442 20623 2 ref They are of particular interest to geologist s because many granulites represent samples of the deep continental crust . Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature others cooled while remaining at depth in the Earth. The minerals present in a granulite will vary depending on the parent rock of the granulite and the temperature and pressure conditions experienced during metamorphism. A common type of granulite found in high grade metamorphic rocks of the continent s contains pyroxene , plagioclase feldspar and accessory garnet , oxides and possibly amphibole . Both clinopyroxene and orthopyroxene may be present, and in fact, the coexistence of clino and orthopyroxene in a metamorphosed basalt defines the granulite facies. Formation Granulites form at high temperature conditions at a range of pressure conditions, typically during regional metamorphism. In some cases, the high temperatures are difficult to account for at the inferred depths at typical geothermal gradients. In extreme cases, granulites may form at temperatures in excess of 1000 degrees Celsius. Some granulites may represent the residues of partial melting, and in other cases represent Rock geology rocks that never melted, despite extremely high temperatures, because the minerals are anhydrous and therefore do not melt even at high temperature conditions. In continental crustal rocks, ... more details
in the Massif Central. The left hand side shows a porphyroblast in C S attitude with sense of movement top to the Southwest D sub 1 sub Phase . The right hand side also shows a porphyroblast in C S attitude ... more details
The Pi gut Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central in France . Its cooling age has been determined as 325 14 million years BP Mississippian Upper Mississippian , Serpukhovian . Geographical situation Image Paragneiss Nontron.jpg thumb 300px Plagioclase bearing paragneiss south of Nontron with big porphyroblast s country rocks of the granodiorite The Granodiorite was named after Pi gut Pluviers , a small town in the Arrondissement of Nontron in the northern Dordogne . Its outline is in the shape of an inverted comma pointing north with a nearly squarish main southern part turned into the NE SW direction. The main body measures 15.5 kilometers in the NE SW direction and 15 kilometers in the NW SE direction and terminates in a tapering triangular section. This section continues farther north in an almost ten kilometer long appendix that is separated from the main body only by a very thin layer of migmatite migmatitic gneiss . The granodiorite takes up about 250 square kilometers in surface area. Its lowest point has an elevation of 135 meters above sea level along the western edge the highest point in the northeast rises to about 375 meters above sea level. Geomorphologically the granodiorite forms a tabular slab, that is gently inclined to the southwest without any major topographic differences. Geological overview Image Granodiorite Transgression.jpg thumb 300px Marine transgression Transgression of liassic arkose onto the eroded Pi gut Pluviers Granodiorite near Nontron The Pi gut Pluviers Granodiorite is surrounded in the north, northeast and east by the Saint Mathieu Leucogranite and its eqivalents unit 17 on the geological map , which has a slightly younger age of 315 17 million years BP Pennsylvanian , Bashkirian . In the northwest and southeast it is bordered by various paragneiss es unit bk . Along the southwestern and western border the granodiorite is Marine transgression transgressed by lias Group lias sic a ... more details
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