Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich aphanitic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance; the term porphyry is from Ancient Greek and means "purple". Purple was the color of royalty, the "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase; some authors claimed. "Imperial" grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Porphyry has hardness 7 on the Mohs scale of mineral hardness, corresponding to steel and quartz. Subsequently, the name was given to any igneous rocks with large crystals; the adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts.
Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals distinguishable with the eye, as in granite. Most types of igneous rocks display some degree of porphyritic texture. Porphyry deposits are formed. In the first stage, the magma is cooled deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and final stage, the magma is cooled at shallow depth or as it erupts from a volcano, creating small grains that are invisible to the unaided eye; the term porphyry is used for a mineral deposit called a "copper porphyry". The different stages of cooling that create porphyritic textures in intrusive and hypabyssal porphyritic rocks lead to a separation of dissolved metals into distinct zones; this process, which occurs when fluids are driven off the cooling magma, is one of the main reasons for the existence in the world of rich, localized metal ore deposits such as those of gold, molybdenum, tin, zinc and tungsten.
This enrichment occurs in the porphyry itself, or in other related igneous rocks or surrounding country rocks carbonate rock. Collectively, these type of deposits are known as "porphyry copper deposits". Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhomb- shaped phenocrysts embedded in a fine-grained red-brown matrix; the composition of rhomb porphyry places it in the trachyte–latite classification of the QAPF diagram. Rhomb porphyry lavas are only known from three rift areas: the East African Rift, Mount Erebus near the Ross Sea in Antarctica, the Oslo graben in Norway, it is intrusive. Pliny's Natural History affirmed that the "Imperial Porphyry" had been discovered at an isolated site in Egypt in AD 18, by a Roman legionary named Caius Cominius Leugas. Ancient Egyptians used other decorative porphyritic stones of a close composition and appearance, but remained unaware of the presence of the Roman grade although it was located in their own country; this particular Imperial grade of porphyry came from a single quarry in the Eastern Desert of Egypt, from 600 million-year-old andesite of the Arabian-Nubian Shield.
The road from the quarry westward to Qena on the Nile, which Ptolemy put on his second-century map, was first described by Strabo, it is to this day known as the Via Porphyrites, the Porphyry Road, its track marked by the hydreumata, or watering wells that made it viable in this utterly dry landscape. Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople. After the fourth century the quarry was lost to sight for many centuries; the scientific members of the French Expedition under Napoleon sought it in vain, it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by James Burton and John Gardiner Wilkinson in 1823. As early as 1850 BC on Crete in Minoan Knossos there were large column bases made of porphyry. All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon, as well as the altars and vases and fountain basins reused in the Renaissance and dispersed as far as Kiev, all came from the one quarry at Mons Porpyritis, which seems to have been worked intermittently between 29 and 335 AD.
Porphyry was used for the blocks of the Column of Constantine in Istanbul. In countries where many cars have studded winter tires such as Sweden and Norway, it is common that highways are paved with asphalt made of porphyry aggregate to make the wearing course withstand the extreme wear from the spiked tires. List of rock textures – A list of rock textural and morphological terms Quartz-porphyry – A type of volcanic rock containing large porphyritic crystals of quartz Sarcophagi of Helena and Constantina Tyrian purple – Natural dye extracted from Murex sea snails Pictures of the Mons Porphyrites, Red Sea, Egypt. Rhomb porphyry lavas at the Wayback Machine Flash showing rhomb porphyry formation at the Wayback Machine
A phenocryst is an early forming large and conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, the adjective porphyritic is used to describe them. Phenocrysts have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization; the term phenocryst is not used unless the crystals are directly observable, sometimes stated as greater than.5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Large phenocrysts are termed megaphenocrysts; some rocks contain both megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts. Phenocrysts are more found in the lighter igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics; the largest crystals found in some pegmatites are phenocrysts being larger than the other minerals.
Rocks can be classified according to the nature and abundance of phenocrysts, the presence or absence of phenocrysts is noted when a rock name is determined. Aphyric is a term used to describe rocks that have no phenocrysts, or more where the rock consists of less than 1% phenocrysts. Porphyritic rocks are named using mineral name modifiers in decreasing order of abundance, thus when olivine forms the primary phenocrysts in a basalt, the name may be refined from basalt to porphyritic olivine basalt or olivine phyric basalt. A basalt with olivine as the dominate phenocrysts, but with lesser amounts of plagioclase phenocrysts, might be termed a olivine-plagioclase phyric basalt. In more complex nomenclature, a basalt with 1% plagioclase phenocrysts, but 4% olivine microphenocrysts, might be termed an aphyric to sparsely plagioclase-olivine phyric basalt, where plagioclase is listed before the olivine, because of its larger crystals. Categorizing a rock as aphyric or as sparsely phyric is a question of whether a significant number of crystals exceed the minimum size.
Geologists use phenocrysts to help determine rock origins and transformations, as when and whether crystals form depends on pressure and on temperature. Fumiko Shido first applied this technique to oceanic basalts, further development came from Tsugio Shibata, from W. B. Bryan. Plagioclase phenocrysts exhibit zoning with a more calcic core surrounded by progressively more sodic rinds; this zoning reflects the change in magma composition. In rapakivi granites, phenocrysts of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase. In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix; these volcanic phenocrysts show flow banding, a parallel arrangement of lath-shaped crystals. These characteristics provide clues to the rocks' origins. Intragranular microfractures and any intergrowth among crystals provide additional clues. Best, Myron. Igneous and Metamorphic Petrology. Oxford, England: Blackwell Publishing.
ISBN 978-1-4051-0588-0. Williams, Howel. Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. ISBN 978-0-7167-0206-1; the Integrated Ocean Drilling Program. Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports
A dike or dyke, in geological usage, is a sheet of rock, formed in a fracture in a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed. An intrusive dike is an igneous body with a high aspect ratio, which means that its thickness is much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters, the lateral dimensions can extend over many kilometres. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock. Dikes are high-angle to near-vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal. Near-horizontal, or conformable intrusions, along bedding planes between strata are called intrusive sills.
The term "sheet" is the general term for both sills. Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event; the world's largest dike swarm is the Mackenzie dike swarm in Canada. Dikes form as either radial or concentric swarms around plutonic intrusives, volcanic necks or feeder vents in volcanic cones; the latter are known as ring dikes. Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history. Pegmatite dikes comprise coarse crystalline granitic rocks - associated with late-stage granite intrusions or metamorphic segregations. Aplite dikes are sugary-textured intrusives of granitic composition; the term "feeder dike" is used for a dike. Magma flowed along out of the dike formed another feature.
In contrast to magmatic dikes, a sill is a magmatic sheet intrusion that forms within and parallel to the bedding of layered rock. Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock layers, they can form in two ways: When a shallow unconsolidated sediment is composed of alternating coarse grained and impermeable clay layers the fluid pressure inside the coarser layers may reach a critical value due to lithostatic overburden. Driven by the fluid pressure the sediment forms a dike; when a soil is under permafrost conditions the pore water is frozen. When cracks are formed in such rocks, they may fill up with sediments; the result is a vertical body of sediment that cuts through a dike. Batholith Ring dike Fissure vent – Linear volcanic vent through which lava erupts Laccolith Runamo – A cracked dolerite dike in Sweden, for centuries held to be a runic inscription interpreted as a runic inscription. Dike swarm Sill
Quartz is a mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral behind feldspar. Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral; the transformation from α-quartz to β-quartz takes place abruptly at 573 °C. Since the transformation is accompanied by a significant change in volume, it can induce fracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz. Since antiquity, varieties of quartz have been the most used minerals in the making of jewelry and hardstone carvings in Eurasia; the word "quartz" is derived from the German word "Quarz", which had the same form in the first half of the 14th century in Middle High German in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý.
The Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning "icy cold", because some philosophers believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the trigonal crystal system; the ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are twinned, distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive. Well-formed crystals form in a'bed' that has unconstrained growth into a void. However, doubly terminated crystals do occur where they develop without attachment, for instance within gypsum. A quartz geode is such a situation where the void is spherical in shape, lined with a bed of crystals pointing inward. Α-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221 depending on the chirality.
Β-quartz belongs to space group P6222 and P6422, respectively. These space groups are chiral. Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks; the transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked. Although many of the varietal names arose from the color of the mineral, current scientific naming schemes refer to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, has been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, smoky quartz, milky quartz, others; these color differentiation's arise from chromophores which have been incorporated into the crystal structure of the mineral.
Polymorphs of quartz include: α-quartz, β-quartz, moganite, cristobalite and stishovite. The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties; the cryptocrystalline varieties are either translucent or opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz including contrasting bands or patterns of color, are agate, carnelian or sard, onyx and jasper. Amethyst is a form of quartz that ranges from a dull purple color; the world's largest deposits of amethysts can be found in Brazil, Uruguay, France and Morocco. Sometimes amethyst and citrine are found growing in the same crystal, it is referred to as ametrine. An amethyst is formed. Blue quartz contains inclusions of fibrous crocidolite. Inclusions of the mineral dumortierite within quartz pieces result in silky-appearing splotches with a blue hue, shades giving off purple and/or grey colors additionally being found.
"Dumortierite quartz" will sometimes feature contrasting light and dark color zones across the material. Interest in the certain quality forms of blue quartz as a collectible gemstone arises in India and in the United States. Citrine is a variety of quartz whose color ranges from a pale yellow to brown due to ferric impurities. Natural citrines are rare. However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smokey appearance, it is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness. Brazil is the leading producer of citrine, with much
Rhyolite is an igneous, volcanic rock, of felsic composition. It may have any texture from glassy to aphanitic to porphyritic; the mineral assemblage is quartz and plagioclase. Biotite and hornblende are common accessory minerals, it is the extrusive equivalent to granite. Rhyolite can be considered as the extrusive equivalent to the plutonic granite rock, outcrops of rhyolite may bear a resemblance to granite. Due to their high content of silica and low iron and magnesium contents, rhyolitic magmas form viscous lavas, they occur as breccias or in volcanic plugs and dikes. Rhyolites that cool too to grow crystals form a natural glass or vitrophyre called obsidian. Slower cooling forms microscopic crystals in the lava and results in textures such as flow foliations, spherulitic and lithophysal structures; some rhyolite is vesicular pumice. Many eruptions of rhyolite are explosive and the deposits may consist of fallout tephra/tuff or of ignimbrites. Eruptions of rhyolite are rare compared to eruptions of less felsic lavas.
Only three eruptions of rhyolite have been recorded since the start of the 20th century: at the St. Andrew Strait volcano in Papua New Guinea, Novarupta volcano in Alaska, Chaiten in southern Chile. Rhyolite has been found on islands far from land. Etsch Valley Vulcanite Group near Bolzano and the surrounding area Gréixer rhyolitic complex at Moixeró range Vosges Iceland: all active and extinct central volcanoes, e.g. Torfajökull, Leirhnjúkur / Krafla, Breiddalur central volcano Papa Stour in Shetland Copper Coast Geopark in southeast Ireland various locations around Snowdonia, Wales Massif de l'Esterel, France the Thuringian Forest consists of rhyolites and pyroclastic rocks of the Rotliegendes Saxony the north west Saxony-Anhalt north of Halle Saar-Nahe Basin e.g. the Königstuhl on the Donnersberg mountain Black Forest e.g. on the Karlsruher Grat Odenwald Andes Cascade Range Cobalt, Ontario Sheep Creek, Idaho Rocky Mountains Jemez Mountains Rhyolite, Nevada was named after a rhyolite deposit that characterised the area.
Wichita Mountains within the Southern Oklahoma Aulacogen St. Francois Mountains Mount Jasper, New Hampshire Yellowstone Crater Lake, Oregon Palisade Head, a formation found at Tettegouche State Park, Minnesota; the Taupo Volcanic Zone in New Zealand has a large concentration of young rhyolite volcanoes Glass House Mountains National Park, Australia the Gondwana Rainforests of Australia World Heritage Area contains rhyolite-restricted flora along the Great Dividing Range the Flinders Peak Group in the Teviot Range in the Fassifern Valley is a rhyolite of varying colours. The Malani Igneous Suite, India; the Yandang Shan mountain chain, near the town of Wenzhou, Zhejiang province, China Tambora, Indonesia Mount Kilimanjaro, Kenya/Tanzania The name rhyolite was introduced into geology in 1860 by the German traveler and geologist Ferdinand von Richthofen from the Greek word rhýax and the rock name suffix "-lite". In North American pre-historic times, rhyolite was quarried extensively in eastern Pennsylvania in the United States.
Among the leading quarries was the Carbaugh Run Rhyolite Quarry Site in Adams County. Rhyolite was mined there starting 11,500 years ago. Tons of rhyolite were traded across the Delmarva Peninsula, because the rhyolite kept a sharp point when knapped and was used to make spear points and arrowheads. Comendite – A hard, peralkaline igneous rock, a type of light blue grey rhyolite List of rock types – A list of rock types recognized by geologists Pantellerite – A peralkaline rhyolite type of volcanic rock Thunderegg – A nodule-like rock, formed within rhyolitic volcanic ash layers University of North Dakota description of rhyolite Information from rocks-rock.com
Ilmenite known as manaccanite, is a titanium-iron oxide mineral with the idealized formula FeTiO3. It is a weakly magnetic black or steel-gray solid. From a commercial perspective, ilmenite is the most important ore of titanium. Ilmenite is the main source of titanium dioxide, used in paints, printing inks, plastics, sunscreen and cosmetics. Ilmenite crystallizes in the trigonal system; the ilmenite crystal structure consists of an ordered derivative of the corundum structure. Containing high spin ferrous centers, ilmenite is paramagnetic. Ilmenite is recognized in altered igneous rocks by the presence of a white alteration product, the pseudo-mineral leucoxene. Ilmenites are rimmed with leucoxene, which allows ilmenite to be distinguished from magnetite and other iron-titanium oxides; the example shown in the image at right is typical of leucoxene-rimmed ilmenite. In reflected light it may be distinguished from magnetite by more pronounced reflection pleochroism and a brown-pink tinge. Samples of ilmenite exhibit a weak response to a hand magnet.
In 1791 William Gregor discovered ilmenite, in a stream that runs through the valley just south of the village of Manaccan, identified for the first time Titanium as one of the constituents of ilmenite. Ilmenite most contains appreciable quantities of magnesium and manganese and the full chemical formula can be expressed as O3. Ilmenite forms a solid solution with geikielite and pyrophanite which are magnesian and manganiferous end-members of the solid solution series. Although there appears evidence of the complete range of mineral chemistries in the O3 system occurring on Earth, the vast bulk of ilmenites are restricted to close to the ideal FeTiO3 composition, with minor mole percentages of Mn and Mg. A key exception is in the ilmenites of kimberlites where the mineral contains major amounts of geikielite molecules, in some differentiated felsic rocks ilmenites may contain significant amounts of pyrophanite molecules. At higher temperatures it has been demonstrated there is a complete solid solution between ilmenite and hematite.
There is a miscibility gap at lower temperatures, resulting in a coexistence of these two minerals in rocks but no solid solution. This coexistence may result in exsolution lamellae in cooled ilmenites with more iron in the system than can be homogeneously accommodated in the crystal lattice. Altered ilmenite forms the mineral leucoxene, an important source of titanium in heavy mineral sands ore deposits. Leucoxene is a typical component of altered gabbro and diorite and is indicative of ilmenite in the unaltered rock. Ilmenite is a common accessory mineral found in igneous rocks, it is found in large concentrations in layered intrusions where it forms as part of a cumulate layer within the silicate stratigraphy of the intrusion. Ilmenite occurs within the pyroxenitic portion of such intrusions. Magnesian ilmenite is indicative of kimberlitic paragenesis and forms part of the MARID association of minerals assemblage of glimmerite xenoliths. Manganiferous ilmenite is found in granitic rocks and in carbonatite intrusions where it may contain anomalous niobium.
Many mafic igneous rocks contain grains of intergrown magnetite and ilmenite, formed by the oxidation of ulvospinel. Ilmenite occurs as discrete grains with some hematite in solid solution, complete solid solution exists between the two minerals at temperatures above about 950 °C. Titanium was identified for the first time by William Gregor in 1791 in ilmenite from the Manaccan valley in Cornwall, southwest England. Ilmenite is named after the locality of its discovery near Miass, Russia. Most ilmenite is mined for titanium dioxide production. In 2011, about 47% of the titanium dioxide produced worldwide were based on this material. Ilmenite and/or titanium dioxide are used in the production of Titanium metal. Titanium dioxide is most used as a white pigment and the major consuming industries for TiO2 pigments are paints and surface coatings and paper and paperboard. Per capita consumption of TiO2 in China is about 1.1 kilograms per year, compared with 2.7 kilograms for Western Europe and the United States.
Ilmenite can be converted into pigment grade titanium dioxide via either the sulfate process or the chloride process. Ilmenite can be improved and purified to Rutile using the Becher process. Ilmenite ores can be converted to liquid iron and a titanium rich slag using a smelting process. Ilmenite ore is used as a flux by steelmakers to line blast furnace hearth refractory. Ilmenite sand is used as a sandblasting agent in the cleaning of diecasting dies. Ilmenite can be used to produce ferrotitanium via an aluminothermic reduction. Australia was the world's largest ilmenite ore producer in 2011, with about 1.3 million tonnes of production, followed by South Africa, Mozambique, China, Ukraine, Norway and United States. Although most ilmenite is recovered from heavy mineral sands ore deposits, ilmenite can be recovered from layered intrusive sources or "hard rock" titanium ore sources; the top four ilmenite and rutile feedstock producers in 2010 were Rio Tinto Group, Iluka Resources and Kenmare Resources, which collectively accounted for more than 60% of world's supplies.
The world's two largest open cast ilmenite mines are: The Tellnes mine located in Sokndal and run by Titania AS with 0.55 Mtpa capacity and
Microcline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline contains minor amounts of sodium, it is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, pale-yellow, brick-red, or green; the chemical compound name is potassium aluminium silicate, it is known as E number reference E555. Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle is less than right angles, it is a ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase. Microcline is identical to orthoclase in many physical properties. Perthite is either orthoclase with thin lamellae of exsolved albite. Amazon stone, or amazonite, is a green variety of microcline, it is not found anywhere in the Amazon Basin, however. The Spanish explorers who named it confused it with another green mineral from that region.
The largest documented single crystals of microcline were found in Devils Hole Beryl Mine, Colorado, US and measured ~50x36x14 m. This could be one of the largest crystals of any material found so far. Microcline is used for the manufacturing of porcelain; the chemical compound name is potassium aluminium silicate, it is known as E number reference E555. It was the subject in 2018 of a Call for technical and toxicological data from the EFSA. In 2008, it was the subject of a Scientific Opinion of the Panel on Food Additives, Processing Aids and Food Contact Materials from the EFSA. List of minerals Alkali feldspars U. Texas Mindat